Novel iron-chromium-based alloy for laser cladding

Abstract

In order to provide a laser-clad surface having a low risk of cracking, whether macroscopic or microscopic, in the laser-clad surface, an iron-chromium-based alloy is provided, consisting of, by weight, 20.5-28.0 wt% of chromium (Cr), up to 5.0 wt% of nickel (Ni), 0.5-2.5 wt% of silicon (Si), 0.50-1.5 wt% of boron (B), 0.15-2.0 wt% of molybdenum (Mo), and the balance of iron and unavoidable impurities, based on the total weight of the alloy. 0.10 to 0.90 wt.% of manganese (Mn), 0.01 to 0.20 wt.% of carbon (C), up to 1.5 wt.% of niobium (Nb), up to 0.2 wt.% of copper (Cu), up to 1.0 wt.% of cobalt (Co), the balance being iron (Fe) and unavoidable impurities, not more than 0.3 wt.%.

Description

Technical Field

[0001] In the field of iron-chromium-based alloys, a series of novel iron-chromium-based alloys suitable for laser cladding are disclosed, which have minimal crack formation during the laser cladding process.

[0002] Background

[0003] In recent years, laser cladding has largely replaced hard chromium plating of worn components exposed to saline environments, such as piston rods of hydraulic roof supports in the mining industry, because the service life that such laser-clad worn components can have is increased by up to 5 times compared to previous hard chromium plating when using laser cladding. For example, during the last decade, laser cladding using AB's 401 (Fe-18Cr-2.5Ni-0.5Mo-0.15C) has been used in the coal mining industry to coat piston rods for hydraulic roof supports and has become the current market-leading alloy for laser cladding both globally and especially in the APAC region.

[0004] Currently, only large piston rods are laser-clad, i.e., most components with a diameter of 300 mm and a length of 1300 mm. This accounts for approximately 15 - 20% of the piston rods produced, including new products and refurbished products. Due to the cost and technical issues of laser-clad worn components, the remaining 85% are hard chromium plated. For example, currently used piston rods usually have too small a diameter to adequately dissipate the high heat input under the existing production conditions of the current laser cladding process. However, due to the benefits such as service life, the future goal of OEMs is to laser-clad 100% of the worn components produced, including piston rods, without being restricted by the current size.

[0005] Furthermore, OEMs hope to reduce the coating cost by increasing productivity, such as by using higher cladding speeds and new nozzles, reducing the coating thickness and / or minimizing post-welding processes, such as machining. The current industrial standard coating thickness after deposition is approximately 1.2 mm, but the industrial goal is to reduce it to less than 0.8 mm, preferably less than 0.5 mm or even less than 0.3 mm. These other requirements pose new demands on the alloys and powder particle sizes used in the laser cladding process because higher cladding speeds and lower coating thicknesses result in higher cooling rates of the coating and potentially higher internal stresses, and further affect the alloy welding behavior, final coating microstructure, etc.

[0006] To increase the cladding speed while reducing the coating thickness, smaller coating particles are required during the laser cladding process compared to processes that require thicker coatings, because smaller particles melt faster. In the art, the working range of the particle size distribution for laser cladding can be 10 - 150 μm. However, high-speed laser cladding requires a more narrowly defined particle size distribution, and the current industry target distribution range aims to find a particle size distribution in the range of 10 - 110 μm.

[0007] Unfortunately, when attempting to produce coatings thinner than currently commercially available, it was found that existing iron-chromium alloys falling within the required size distribution range were not satisfactory in the test trials conducted by the inventors of the present case, because it was found that the increased cooling rate associated with thinner coatings led to crack formation and an unstable microstructure. Additionally, when using commercially available iron-chromium powders for thin coating cladding, the resulting hardness of the coatings showed unsatisfactory dispersion in terms of the hardness / wear resistance and corrosion of this type of coating based on existing iron-chromium powders.

[0008] Accordingly, the present invention is driven by this current need for a new stable alloy suitable for high-speed / high-productivity laser cladding processes, which can be used to produce thin (<0.3 mm) and substantially crack-free coatings having a stable microstructure and a hardness in the 400 - 550 Vickers range, while having the same corrosion resistance and machinability as the best protective iron-chromium alloys currently available on the market, such as 401.

[0009] In the field of the present invention, alloy powders for laser cladding repair of mining hydraulic columns are known, for example, from CN113046625. The alloy contains 15 - 17 wt% Cr, 1.5 - 2.0 wt% Ni, 1.5 - 2.0 wt% Co, 0.8 - 1.2 wt% Mo, 0.0 - 0.4 wt% Mn, 0.1 - 0.2 wt% Nb, 0.07 - 0.14 wt% C, 0.06 - 0.12 wt% N, 0.03 - 0.06 wt% Ce, 0.6 - 1.0 wt% B, 0.8 - 1.2 wt% Si, with the balance being Fe.

[0010] However, the high cobalt content causes health and safety problems for operators using alloy powders of the prior art, and thus special protective measures are required during the manufacturing process. Accordingly, another object of the present disclosure is to avoid cobalt exceeding the level of unintentional inclusions, which is solved by the iron-chromium alloys detailed herein.

[0011] The use of alloying particles for laser cladding with a particle size of 15 - 53 μm at 50 m / min in obtaining a 1.5 mm cladding layer is described in CN111097908. The alloy consists of 17.5 - 19.5 wt% Cr, 1.7 - 2.3 wt% Ni, 0.8 - 1.2 wt% Si, 0.9 - 1.2 wt% B, 0.4 - 0.6 wt% Mo, ≤0.3 wt% Mn, 0.15 - 0.23 wt% C, and the balance being iron (Fe). The resulting surface hardness (HV) is 658 HV.

[0012] The use of alloying particles for laser cladding with a particle size of 15 - 175 μm is described in CN111809177 in obtaining a 1.4 mm cladding layer. The alloy consists of 18 - 19 wt% Cr, 3.6 - 4 wt% Ni, 1.1 - 1.3 wt% Si, 0.9 - 1.1 wt% B, 1.5 - 1.7 wt% Mo, 0.2 - 0.3 wt% Mn, 0.15 - 0.20 wt% C, 0.5 - 0.55 wt% Nb, 0.1 - 0.15 wt% Co, 0.1 - 0.15 wt% V, and the balance being iron (Fe) and having 0.06 - 0.08 wt% nitrogen inclusions from the atomization of the alloy melt used to form the alloying particles. The resulting surface hardness (HV) is 700 HV.

[0013] According to the inventors in the two subsequent prior art documents CN111097908 and CN111809177, the resulting coatings are highly corrosion - resistant at the selected levels of iron - chromium balance, thus confirming the discovery of the applicant in respect of its commercial product 401, which has the same chromium content as the two subsequent disclosures. Further, the observed hardness level of this prior art matches that of the applicant's commercial product 401.

[0014] The problem with the alloys proposed in the two subsequent references is that when high - speed laser cladding is used for coating, very thick coatings must be produced to avoid crack formation in the deposited coatings because the high hardness of the prior art coatings makes them highly prone to releasing internal hardness stresses by cracking, which is thus compensated for by increasing the layer thickness. A similar problem was observed for the applicant's own product 401 (Fe - 18Cr - 2.5Ni - 0.5Mo - 0.15C).

[0015] This observation formed the starting point for the inventors of the present case to search for cobalt - free iron - chromium - based alloys that do not suffer from the same drawback in terms of crack formation during the laser cladding process, i.e., having to compensate for crack formation by increasing the cladding thickness, thus using more material and increasing the coating cost.

[0016] The inventors of the present case have surprisingly found that the above-mentioned drawbacks can be alleviated in a simple manner detailed herein by increasing the chromium content beyond that known from the prior art, wherein in the restricted region of the increased chromium content, a favorable bcc / fcc balance of the resulting alloy is formed, which maintains the corrosion resistance known from the prior art but surprisingly allows high-speed laser cladding without crack formation in a coating with a thickness of about 100 - 350 μm, associated with a reduction in the hardness of the coating to about 400 - 450 HV, which is still fully acceptable for the proposed uses.

[0017] This result is unexpected because the prior art shows that chromium increases hardness and thus increases the stress cracking sensitivity, as even observed in the prior art coatings for lower chromium concentrations. Summary of the Invention

[0019] According to the present disclosure and the present invention, the object of the present disclosure is solved in a first aspect and its embodiments by providing an iron - chromium - based alloy which consists by weight, based on the total weight of the alloy, of:

[0020] Chromium (Cr): 20.5 - 28.0 wt%,

[0021] Nickel (Ni): at most 5.0 wt%,

[0022] Silicon (Si): 0.5 - 2.5 wt%,

[0023] Boron (B): 0.50 - 1.5 wt%,

[0024] Molybdenum (Mo): 0.15 - 2.0 wt%,

[0025] Manganese (Mn): 0.10 - 0.90 wt%,

[0026] Carbon (C): 0.01 - 0.20 wt%,

[0027] Niobium (Nb): at most 1.5 wt%,

[0028] Copper (Cu): at most 0.2 wt%,

[0029] Cobalt (Co): at most 1.0 wt%,

[0030] The balance being iron (Fe) and unavoidable impurities, not exceeding 0.3 wt%.

[0031] Other aspects and embodiments are detailed herein in the specification and claims. Brief Description of the Drawings

[0033] Figure 1: Hardness HV0.2 of alloys A1 - A8, A10 - A12, A20 - A24.

[0034] Figure 2 : Hardness of alloys A1 - A8 and A10 - A12 plotted against the calculated volume fractions of boride and austenite.

[0035] Figure 3 : Hardness of alloys A26 - A34 containing 0.5 - 1.0 wt% Nb.

[0036] Figure 4 : Qualitative estimation of the number of microcracks in the coatings of alloys A1 - A12, A16, A17, and A20 - A24.

[0037] Figure 5 : Qualitative estimation of the number of microcracks against the melting range calculated for the last 10% of the melt using Shiel simulation.

[0038] Figure 6 : Influence of boron content on the number of hot cracks in the coating (qualitative estimation).

[0039] Figure 7 : Coating examples clad with alloy A3 with a limited number of defects such as pores and microcracks and alloy A5 with a large number of defects at 30 m / min using a HighNo 4.0 nozzle.

[0040] Figure 8 : Coating examples clad with alloy A3 with a limited number of defects such as pores and microcracks and alloy A5 with a large number of defects at 100 m / min using a HighNo 4.0 nozzle.

[0041] Figure 9 : Microstructure examples of coatings clad with alloys A10 and A7 at 30 m / min and 100 m / min respectively using a HighNo 4.0 nozzle.

[0042] Figure 10 : Microstructure of the coating of A10 coated at A) 30 m / min and B) 100 m / min at a higher magnification.

[0043] Figure 11 : Alloy A1, A) LOM overview, B) SEM EBSD map, and C) Euler map.

[0044] Figure 12 : Alloy A7, A) LOM overview, B) SEM EBSD map, and C) Euler map.

[0045] Figure 13: Alloy A10, A) LOM overview, B) SEM EBSD images, and C) Euler diagrams.

[0046] Figure 14 : Exemplary samples for corrosion degree rating after 7 days in the NSS chamber.

[0047] Figure 15 : Boron corrosion grade after 7 days in NSS.

[0048] Figure 16 : Chromium corrosion grade after 7 days in NSS.

[0049] Figure 17: ThermoCalc results for 3 different Cr concentrations.

[0050] Detailed Description

[0051] According to the present disclosure and the present invention, the object of the present disclosure is solved in a first aspect and its embodiments by providing an iron-chromium-based alloy which consists, by weight, based on the total weight of the alloy, of:

[0052] Chromium (Cr): 20.5 - 28.0 wt%,

[0053] Nickel (Ni): at most 5.0 wt%,

[0054] Silicon (Si): 0.5 - 2.5 wt%,

[0055] Boron (B): 0.50 - 1.5 wt%,

[0056] Molybdenum (Mo): 0.15 - 2.0 wt%,

[0057] Manganese (Mn): 0.10 - 0.90 wt%,

[0058] Carbon (C): 0.01 - 0.20 wt%,

[0059] Niobium (Nb): at most 1.5 wt%,

[0060] Copper (Cu): at most 0.2 wt%,

[0061] Cobalt (Co): at most 1.0 wt%,

[0062] The balance is iron (Fe) and unavoidable impurities, not exceeding 0.3 wt%.

[0063] The inventors of the present case surprisingly found that the laser cladding coating based on the alloy disclosed in the present case can be used with a cladding speed of 30 - 100 m / min and 0.5 - 1.5 m at a thickness of 100 - 350 μm 2The deposition rate of / h is produced on a laboratory scale. The coating of the alloy of the present invention produced does not contain cold cracks and exhibits a hardness of 400 - 450 HV and a corrosion resistance of >> 96 h in NSS.

[0064] Chromium (Cr) with iron forms the main body of the alloy of the present invention, where chromium is the main component responsible for corrosion protection, and the other elements disclosed herein mainly contribute to the properties of the powder of the alloy of the present invention for laser cladding. Advantageously, when adjusted with other elements in accordance with the present disclosure, the working range of chromium is quite wide, with 20.5 - 28.0 wt% chromium in the alloy. However, it is found that the best performance of chromium is between 23 - 24 wt%, where the performance increases from the above limits to the observed optimal concentration range. Therefore, in an embodiment of the alloy of the present invention, an iron - chromium - based alloy is detailed herein, where chromium (Cr) is present in an amount of 21 - 27 wt%, 21.5 - 26 wt%, 22 - 25 wt%, 22.5 - 24.5 wt%, preferably 23 - 24 wt%, more preferably 23.2 - 23.8 wt% or 23.4 - 23.6 wt%.

[0065] When using the alloy of the present invention to prepare a laser - clad surface, it is found that nickel (Ni) together with chromium can be used for corrosion protection. However, when the nickel concentration rises above 5 wt%, the required surface hardness will be affected, thus limiting the increase in nickel content. But since nickel is an expensive additive compared to chromium, it is desirable to keep the nickel content as low as possible. It is found in the experiments that nickel can be absent or present only at the level of unavoidable impurities while still achieving the objectives of the present disclosure, yet the best results are found when nickel is present in an amount of 1 wt% and above. Therefore, in an embodiment of the alloy of the present invention, an iron - chromium - based alloy is detailed herein, where nickel (Ni) is present in an amount of up to 5 wt%, 0.5 - 5 wt%, 1 - 5 wt%, 1.5 - 4.5 wt%, 2.0 - 4.0 wt%, 2.15 - 3.85 wt%, 2.25 - 3.75 wt%, 2.35 - 3.65 wt%, 2.50 - 3.50 wt%, 2.65 - 3.35 wt% or preferably 2.75 - 3.25 wt%.

[0066] It was further found in the experiments that when operating with raw materials having low levels of other residual contaminants, the main impurity present in the atomized alloy is oxygen (O) due to the high chromium content. It is generally found that oxygen is introduced as a major unavoidable impurity during the atomization process, particularly during water atomization. The concentration of oxygen in the laboratory experiments does not exceed 0.3 wt% based on the total mass of the alloy, but in the initial experiments under production conditions, it was found that the oxygen is at most 0.6 wt% based on the total mass of the atomized alloy. Therefore, in an embodiment of the present invention, oxygen (O) as an unavoidable impurity in the atomized alloy can be present up to 0.6 wt%, but preferably at a lower level, such as preferably up to 0.55 wt%, up to 0.5 wt%, up to 0.45 wt%, up to 0.4 wt%, up to 0.35 wt%, or more preferably up to 0.3 wt% or less.

[0067] In an embodiment of the present disclosure, copper (Cu) can be present in the alloys of the present disclosure. Since it has been found that the presence of copper in the alloys of the present invention generally disadvantages the avoidance of crack formation during the laser cladding process, copper cannot be present in an amount exceeding 0.2 wt% Cu. Therefore, in an embodiment of the present disclosure, an iron-chromium-based alloy is detailed herein, in which copper (Cu) is present up to 0.2 wt%, up to 0.15 wt%, or in which copper (Cu) is present up to 0.1 wt% or 0.05 wt%, but preferably in which copper is present only as an unavoidable impurity, preferably below the detection level.

[0068] It was found in the experiments that up to 1.0 wt% cobalt (Co) can be used as an innocuous filler in the alloys of the present invention. However, due to health and safety reasons in the laser cladding process and when handling iron powders containing cobalt, the alloys of the present invention preferably do not rely on cobalt for their properties. Therefore, in a preferred embodiment of the alloys of the present invention, an iron-chromium-based alloy is detailed herein, in which cobalt (Co) can be present up to 0.2 wt%, preferably can be present up to 0.1 wt%, but preferably cobalt, if present, is present only as an unavoidable impurity, preferably below the detection level.

[0069] In experiments, it was found that niobium (Nb), when present during laser cladding, advantageously reduces crack formation and for high concentrations of boron (B) and / or carbon (C), niobium is a necessary additive for preventing cracks during laser cladding. Furthermore, although niobium may thus not be present in the alloys of the present invention or may only be present as an unavoidable impurity, in embodiments of the alloys of the present invention, an iron-chromium-based alloy is detailed herein, in which niobium (Nb) is present in an amount from 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt%, 0.5 wt%, 0.55 wt%, 0.60 wt%, 0.70 wt%, 0.8 wt%, 0.9 wt% or 1.0 wt%, up to 1.4 wt%, 1.3 wt%, 1.2 wt%, 1.1 wt%, 1.0 wt%, 0.9 wt% or 0.8 wt%, preferably 0.40 - 1.2 wt%, 0.45 - 1.1 wt% or 0.50 - 1.0 wt%.

[0070] In its embodiments, an iron-chromium-based alloy is detailed herein, in which if the content of carbon (C) exceeds 0.15 wt%, the content of boron (B) exceeds 1.1 wt% or the combined content of carbon and boron exceeds 1.20 wt%, niobium (Nb) is present in an amount of 0.30 - 1.5 wt%, preferably 0.5 - 1.25 wt%, more preferably 0.6 - 1 wt%.

[0071] The elements silicon (Si), boron (B), molybdenum (Mo), manganese (Mn) and carbon (C) must be present in the alloys of the present disclosure, and their presence has been found to be necessary for providing the necessary adjustment of the corrosion resistance of laser cladding or iron, chromium and, if present, nickel. Certain optimal concentrations of the above elements can be obtained from experiments as detailed below.

[0072] It was found that silicon (Si) must be present in the alloys of the present disclosure in an amount of 0.5 - 2.5 wt%. However, in embodiments of the alloys of the present invention, an iron-chromium-based alloy is detailed herein, in which silicon (Si) is present in an amount of 0.75 - 2.45 wt%, 1.0 - 2.4 wt%, 1.25 - 2.35 wt%, preferably 1.4 - 2.3 wt%, 1.5 - 2.3 wt%, 1.6 - 2.3 wt%, more preferably 1.7 - 2.3 wt%, 1.8 - 2.2 wt% or more preferably 1.9 - 2.1 wt%. It was found that when the amount of silicon exceeds 1.4 wt%, crack formation is minimized.

[0073] It was found that boron (B) must be present in the alloys of the present disclosure in an amount of 0.5 - 1.5 wt%. However, in embodiments of the alloys of the present invention, an iron-chromium-based alloy is detailed herein, in which boron (B) is present in an amount of 0.6 - 1.4 wt%, 0.7 - 1.3 wt%, 0.8 - 1.2 wt%, 0.9 - 1.1 wt% or preferably 0.95 - 1.05 wt%.

[0074] It has been found that molybdenum (Mo) must be present in the alloys of the present disclosure in an amount of 0.15 - 2.0 wt%. However, in embodiments of the alloys of the present invention, an iron - chromium - based alloy is detailed herein, in which molybdenum (Mo) is present in an amount of up to 1.9 wt%, up to 1.8 wt%, up to 1.7 wt%, up to 1.6 wt%, up to 1.5 wt%, preferably up to 1.4 wt%, up to 1.3 wt%, up to 1.2 wt%, up to 1.1 wt%, up to 1.0 wt%, up to 0.90 wt%, up to 0.80 wt%, more preferably up to 0.70 wt%, up to 0.60 wt% or more preferably up to 0.50 wt%.

[0075] In its embodiments, an iron - chromium - based alloy is detailed herein, in which molybdenum (Mo) is present in an amount of from 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt%, 0.50 wt%, 0.55 wt%, 0.60 wt%, 0.65 wt% or 0.70 wt%.

[0076] In its embodiments, an iron - chromium - based alloy is detailed herein, in which molybdenum (Mo) is present in an amount of 0.20 - 1.3 wt%, 0.25 - 1.1 wt%, 0.3 - 0.90 wt%, 0.35 - 0.70 wt% or 0.40 - 0.60 wt%.

[0077] In its further embodiments, an iron - chromium - based alloy is detailed herein, in which molybdenum (Mo) is present in an amount of 0.3 - 1.8 wt% or 0.4 - 1.7 wt%, preferably 0.5 - 1.6 wt% or 0.6 - 1.5 wt%, more preferably 0.6 - 1.5 wt% or 0.65 - 1.45 wt%, most preferably 0.7 - 1.4 wt%, 0.75 - 1.35 wt% or 0.8 - 1.3 wt%.

[0078] It has been found that manganese (Mn) must be present in the alloys of the present disclosure in an amount of 0.1 - 0.9 wt%. However, in embodiments of the alloys of the present invention, an iron - chromium - based alloy is detailed herein, in which manganese (Mn) is present in an amount of from 0.2 wt% or 0.3 wt%, preferably 0.35 wt% or 0.40 wt% or more preferably 0.45 wt% or 0.50 wt%; and up to 0.85 wt%, 0.80 wt%, 0.75 wt%, 0.70 wt%, 0.65 wt%, 0.60 wt%, 0.55 wt% or 0.50 wt%. In its embodiments, manganese (Mn) is preferably present in an amount of 0.30 - 0.80 wt%, 0.35 - 0.7 wt% or 0.40 - 0.60 wt%.

[0079] The inventors of the present case surprisingly found that when the total concentration of molybdenum (Mo) and manganese (Mn) is in the range of 0.6 - 1.8 wt%, preferably 0.7 - 1.5 wt%, more preferably 0.8 - 1.3 wt% or most preferably 0.9 - 1.1 wt%, the alloy of the present invention has optimal properties.

[0080] It was found that the presence of carbon (C) in combination with boron in the alloys disclosed herein, as detailed herein, is necessary for obtaining the appropriate hardness of the laser cladding coating. However, it was observed that carbon, being a light element, has already reached its effective molecular weight at a concentration by weight which is additionally at the level of inevitable impurities of the carbon contained in the raw materials in the current context. However, for optimal results, carbon should be present in the alloys disclosed herein at 0.01 - 0.20 wt%, preferably carbon (C) from 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.10 wt%; and up to 0.19 wt%, 0.18 wt%, 0.17 wt%, 0.15 wt%, 0.14 wt%, 0.13 wt%, 0.12 wt%, 0.11 wt% or 0.10 wt%, preferably 0.05 wt% to less than 0.15 wt%.

[0081] In a preferred embodiment of the alloy of the present invention, an iron - chromium - based alloy is disclosed herein, which consists by weight based on the total weight of the alloy of:

[0082] Chromium (Cr): 20.5 - 28.0 wt%,

[0083] Nickel (Ni): 2.35 - 3.55 wt%,

[0084] Silicon (Si): 1.35 - 2.5 wt%,

[0085] Boron (B): 0.7 - 1.2 wt%,

[0086] Molybdenum (Mo): 0.3 - 1.8 wt%,

[0087] Manganese (Mn): 0.35 - 0.90 wt%,

[0088] Carbon (C): 0.01 - 0.20 wt%,

[0089] Niobium (Nb): up to 1.5 wt%,

[0090] Copper (Cu): up to 0.2 wt%,

[0091] Cobalt (Co): up to 1.0 wt%,

[0092] The balance is iron (Fe) and unavoidable impurities, not exceeding 0.3% by weight. In its preferred embodiment, carbon (C) is less than 0.15% by weight.

[0093] In a particularly preferred embodiment of the alloy of the present invention, an iron-chromium-based alloy is disclosed herein, which consists of, by weight, based on the total weight of the alloy:

[0094] Chromium (Cr): 22 - 28% by weight,

[0095] Nickel (Ni): 2.5 - 3.5% by weight,

[0096] Silicon (Si): 1.7 - 2.3% by weight,

[0097] Boron (B): 0.9 - 1.1% by weight,

[0098] Molybdenum (Mo): 0.3 - 1.8% by weight,

[0099] Manganese (Mn): 0.35 - 0.70% by weight,

[0100] Carbon (C): 0.01 - 0.20% by weight,

[0101] Niobium (Nb): at most 1.5% by weight,

[0102] Copper (Cu): at most 0.2% by weight,

[0103] Cobalt (Co): at most 1.0% by weight,

[0104] The balance is iron (Fe) and unavoidable impurities, not exceeding 0.3% by weight. In its preferred embodiment, carbon (C) is less than 0.15% by weight. In its further preferred embodiment, chromium (Cr) is 22 - 25% by weight.

[0105] In the alloy of the present invention, the total content of unwanted impurities that are not oxygen should not exceed 0.4% by weight. Nitrogen, which is an unwanted impurity from powder atomization, should not exceed 0.15% by weight, and other unwanted impurities that are not oxygen should not exceed 0.3% by weight.

[0106] In an embodiment of the present invention, up to 0.3% by weight of iron can be replaced by one or more of titanium, vanadium, aluminum, or tungsten as unavoidable impurities without affecting the performance of the alloy of the present invention, which makes material procurement cheaper. Preferably, their content is kept as low as possible, such as less than 0.2% by weight, or more preferably less than 0.1% by weight. However, and most preferably, these elements are not present in the alloy of the present invention in amounts below the content of insignificant impurities, because the material properties are improved when these elements are substantially absent.

[0107] Generally, when the alloy of the present invention has not been formulated into powder by water atomization, the content of other impurities is usually less than 0.1% by weight, and only oxygen and nitrogen are introduced into the alloy when the alloy is atomized into powder subsequently.

[0108] In one aspect of the present invention, an iron-chromium-based alloy in powder form is detailed herein.

[0109] In its embodiments, an iron-chromium-based alloy in powder form is detailed herein, wherein oxygen (O) as an inevitable impurity does not exceed 0.6% by weight based on the total weight of the alloyed powder.

[0110] Therefore, according to the embodiments of the present disclosure, the total content of inevitable impurities should not exceed 0.8% by weight based on the total weight of the iron-chromium-based alloy of the present invention, but preferably does not exceed 0.75% by weight, 0.7% by weight, 0.65% by weight, 0.6% by weight or 0.5% by weight based on the total weight of the iron-chromium-based alloy of the present disclosure. More preferably, only oxygen (O) and nitrogen (N) exist as inevitable impurities in a content separately exceeding 0.05% by weight, wherein oxygen (O) as an inevitable impurity should be contained in a content of at most 0.3% by weight and nitrogen (N) as an inevitable impurity should be contained in a content of at most 0.15% by weight.

[0111] In a preferred embodiment of the present invention, the iron-chromium-based alloy in powder form comprises at least 80% by weight of the iron-chromium-based alloy powder contained in the sieve fraction of the iron-chromium-based alloy powder having a size distribution of 1-100 μm, 2.5-90 μm, 5-80 μm, preferably 10-75 μm or 15-70 μm or more preferably 20-60 μm measured by sieving according to ASTM B 214.

[0112] In a preferred embodiment of the present invention, the iron-chromium-based alloy in powder form comprises at least 80%, at least 85% by weight, preferably at least 90% by weight, or more preferably at least 95% by weight of the iron-chromium-based alloy powder having a size distribution of 2.5-100 μm measured by sieving according to ASTM B 214. More preferably, it comprises at least 80%, at least 85% by weight, preferably at least 90% by weight, or more preferably at least 95% by weight of the iron-chromium-based alloy powder having a size distribution of 10-80 μm measured by sieving according to ASTM B 214, or even more preferably comprises at least 80%, at least 85% by weight, preferably at least 90% by weight, or more preferably at least 95% by weight of the iron-chromium-based alloy powder having a size distribution of 20-60 μm measured by sieving according to ASTM B 214.

[0113] If it is necessary to measure particles with a size less than 20 μm, the laser diffraction ASTM B822 method can be used to determine the particle size.

[0114] In another aspect of the present invention, a composition for forming an iron-chromium based alloy according to any aspect of the iron-chromium alloy detailed herein is detailed, the composition consisting, by weight based on the total weight of the composition, of:

[0115] Chromium (Cr): 20.5 - 28.0 wt%, Nickel (Ni): at most 5.0 wt%,

[0116] Silicon (Si): 0.5 - 2.5 wt%,

[0117] Boron (B): 0.50 - 1.5 wt%,

[0118] Molybdenum (Mo): 0.15 - 2.0 wt%,

[0119] Manganese (Mn): 0.10 - 0.90 wt%,

[0120] Carbon (C): 0.01 - 0.20 wt%,

[0121] Niobium (Nb): at most 1.5 wt%,

[0122] Copper (Cu): at most 0.2 wt%,

[0123] Cobalt (Co): at most 1.0 wt%,

[0124] The balance is iron (Fe) and unavoidable impurities, not exceeding 0.3 wt%.

[0125] In a preferred embodiment of the composition of the present invention, the composition is adjusted in its composition to match any of the iron-chromium based alloys disclosed herein.

[0126] Typically but not necessarily, the elements of the composition are provided directly as elemental metals. However, in some embodiments, one or more components of the composition are pre-alloyed before being added to the composition of the present invention, for example in the form of scrap metal from a recyclable source.

[0127] To form the alloy of the present invention, the composition of the present invention is heated to above the melting point of its main constituent elements and thus the alloy of the present invention formed.

[0128] In another aspect of the present invention, the use of a powder according to any embodiment detailed herein in coating a surface by means of a laser cladding method is detailed.

[0129] In another aspect of the present invention, an iron-chromium based alloy is detailed which is formed from an iron-chromium based alloy according to any one of its embodiments detailed herein in a laser cladding method.

[0130] In another aspect of the present invention, a surface coating composed of an iron-chromium-based alloy according to any one of its embodiments detailed herein is described in detail herein.

[0131] In another aspect of the present invention, a shaped article is described in detail herein, which comprises a surface coating composed of an iron-chromium-based alloy according to any one of its embodiments detailed herein.

[0132] In another aspect of the present invention, a method for producing a coated surface or article is described in detail herein, which has the following steps:

[0133] - Providing a powder according to one or more of its embodiments detailed herein in a form or formulation suitable for laser cladding;

[0134] - Conducting a laser cladding process using the powder;

[0135] - Obtaining the desired surface coating or the desired article. Examples

[0136] Example 1 - Manufacturing the alloy and the impurities contained therein:

[0137] In accordance with the present disclosure and the present invention, the suitability of the following iron-chromium alloys in powder form for solving the purposes of the present disclosure was tested, see Tables 1 and 2.

[0138] The alloys according to Tables 1 and 2 were produced by co-melting the components on a test scale of about 10 kg furnaces. Some tests were repeated in a large-scale 200 kg furnace. After alloying, the alloys recorded in Tables 1 and 2 in powder form for testing in laser cladding experiments were atomized using one of gas atomization (GA), water atomization (WA), or high-pressure water atomization (HPWA).

[0139] Measurement of Particle Size Distribution:

[0140] The particle size distribution was measured using a Ro-TAP sieve shaker or laser diffraction.

[0141] Using a Ro-Tap sieve shaker, the powder particles were shaken through a stack of metal sieves with different apertures by an oscillating motion. Then the weights of the powder on each sieve were weighed using a calibrated balance and the fraction powder weights were normalized with respect to the total powder weight. Sieve analysis using a Ro-Tap sieve was conducted according to ASTM B 214.

[0142] Laser diffraction analysis was conducted using an analyzer from Sympatec. The measurement was conducted according to ASTM B822.

[0143] Impurities Detected in the Raw Material:

[0144] The impurities found in the raw materials include Cu, Co, Al, S, and P.

[0145] In tests not recorded herein, it was found that when copper (Cu) exceeded 0.2 wt% of the iron-chromium-based alloy, solidification cracks began to form in the laser cladding surface. Therefore, if copper (Cu) is present in the alloy of the present invention, the content of copper should not exceed 0.2 wt% based on the total weight of the alloy, preferably not exceed 0.1 wt%. However, most preferably, copper is present only as an unavoidable impurity. In the tests recorded herein, there was substantially no copper (Cu), i.e., below the analytical detection limit.

[0146] In tests not recorded herein, it was found that cobalt (Co) can be present in the iron-chromium-based alloy up to 1.0 wt% without affecting the properties of the laser cladding surface coated with the alloy of the present disclosure. Therefore, it is possible to include up to 1.0 wt% cobalt (Co) as a non-affecting filler in the alloy of the present disclosure. However, this is highly undesirable because the carcinogenic potential of cobalt-containing powders makes it undesirable to include more than 0.2 wt% of cobalt (Co) as a filler for health and safety reasons. Most preferably, cobalt is present only as an unavoidable impurity. In the tests recorded herein, there was substantially no cobalt (Co), i.e., below the analytical detection limit.

[0147] Aluminum (Al) is present as an unavoidable impurity up to 0.1 wt% based on the total mass of the iron-chromium-based alloy of the present disclosure in the starting materials initially tested but not recorded herein without affecting the alloy of the present invention. Starting materials having only 0.05 wt% aluminum as an unavoidable impurity are preferably used for the tests of the present invention. However, in the tests recorded herein, there was substantially no aluminum (Al), i.e., below the analytical detection limit.

[0148] In the alloys recorded herein, phosphorus and sulfur were detected as unavoidable impurities at contents below 0.05 wt% respectively.

[0149] The alloy atomized by one of gas atomization (GA), water atomization (WA) or high-pressure water atomization (HPWA) contains up to 0.5 wt% oxygen (O) as an unavoidable impurity and up to 0.15 wt% nitrogen (N) as an unavoidable impurity. The combined content of oxygen and nitrogen as unavoidable impurities generally does not exceed 0.3 wt% based on the total weight of the iron-chromium-based alloy of the present disclosure, where combined contents of 0.25 wt%, 0.20 wt%, 0.15 wt% or 0.10 wt% can be obtained.

[0150] Thus, according to an embodiment of the present disclosure, the total content of inevitable impurities should not exceed 0.8 wt% based on the total weight of the iron-chromium-based alloy of the present invention, but preferably does not exceed 0.75 wt%, 0.7 wt%, 0.65 wt%, 0.6 wt% or 0.5 wt% based on the total weight of the iron-chromium-based alloy of the present disclosure. More preferably, only oxygen (O) and nitrogen (N) are present as inevitable impurities in an amount exceeding 0.05 wt% individually, where oxygen (O) should be contained as an inevitable impurity in an amount up to 0.3 wt% and nitrogen (N) should be contained as an inevitable impurity in an amount up to 0.15 wt%.

[0151] Example 2 - Alloy in powder form examined in laser cladding

[0152] The alloys in powder form recorded in Table 1 were prepared by water atomization of melts having the alloy compositions recorded in the table. Nitrogen and oxygen are impurity inclusions generated by the water atomization process. All the powder samples recorded therein were made from melts by water atomization except for the high-pressure water atomized sample A11 and the gas atomized sample A41. After atomization, the powder was dried and sieved into a fraction with a size between 20 - 63 μm, which was considered suitable for subsequent laser cladding tests. In the table, n.d. is not detected, and * adjacent to the sample number indicates that the sample is compared with the alloy of the present invention.

[0153] Sample A35 is a comparison where a low-chromium alloy was tested. This alloy did not perform well in terms of the objectives of the present disclosure, for example, as evaluated by the formation of microcracks and / or macrocracks as defined below. When the chromium content becomes lower than the limits detailed herein, macrocracks will start to form during laser cladding, which leads to coating rupture.

[0154] Overall, a coating with a thickness of 100 - 350 μm was produced on a laboratory scale using a cladding speed of 30 - 100 m / min and a deposition rate of 0.5 - 1.5 m 2 / h. The coatings of the alloys of the present invention produced were free of cold cracks and showed a hardness of 400 - 450 HV; and corrosion resistance in NSS >> 96 h.

[0155] Table 1: Tested alloy compositions - Fe (balance)

[0156]

[0157] Example 3 - Alloy in powder form containing niobium examined in laser cladding

[0158] The alloys recorded in Table 2 in powder form were prepared by water atomizing a melt having the alloy composition recorded in the table. Nitrogen and oxygen are impurity inclusions generated by the water atomizing process. All the powder samples recorded therein were manufactured from the melt by water atomizing. After atomizing, the powder was dried and sieved into a fraction with a size of 20 - 63 μm, which was regarded as suitable for subsequent laser cladding tests. In this table, n.d. means not detected, and * adjacent to the sample number indicates that the sample is compared with the alloy of the present invention.

[0159] For the alloy of the present invention recorded in Example 2 in the laser cladding test, it was found that although the alloy generally performed in line with the specifications, when carbon or boron exceeded 0.15 wt% (C) or 1.1 wt% (B) respectively, or the combination of the two exceeded 1.2 wt%, the number of macroscopic cracks or solidification cracks formed during the laser cladding process increased compared with other alloys of the present invention.

[0160] In subsequent tests of the alloys recorded in Table 2, it was found that niobium (Nb) was suitable for suppressing crack formation in high-content (i.e., exceeding the above-given content limits) carbon and / or boron alloys, such as cracks visible to the naked eye, while being a neutral additive for other concentrations of carbon, boron, or the combination of carbon and boron.

[0161] Table 2: Alloy compositions tested with niobium - Fe (balance)

[0162]

[0163] Test Setup and Method

[0164] High - speed laser cladding

[0165] In the context of the present invention, high - speed laser cladding refers to a laser cladding process operating at a cladding speed exceeding 1 m / min. In the tests recorded below, cladding speeds of 30 m / min and 100 m / min were used respectively.

[0166] The powders of the alloys recorded in Tables 1 and 2 were (high - speed) laser - clad according to the test settings and parameters detailed in Table 3. A single - layer and double - layer coating were produced using a HighNo4 nozzle or a 6 - jet GTV nozzle. Low - carbon steel rods with a diameter of 50 mm and a length of 200 mm were used for the tests using the HighNo 4.0 nozzle and low - carbon steel rods with a diameter of 80 mm were used for the tests using the 6 - jet GTV nozzle.

[0167] To evaluate the coating properties of the steel rods, such as microstructure and hardness, 30 - mm - long single - layer coatings (also known as clad layers in the art) produced at two cladding speeds of 30 m / min and 100 m / min respectively were evaluated.

[0168] To evaluate the corrosion performance of the steel bars, 90-mm-long double-layer coatings were produced at two cladding speeds of 30 m / min and 100 m / min, the same as those for the single-layer coatings.

[0169] Table 3: Test parameters used in laser cladding

[0170]

[0171] Coating performance evaluation - method

[0172] Samples Coated with HighNo4.0 Nozzle:

[0173] The cracks of 30-mm-long single-layer coatings produced using a High No 4.0 nozzle were tested using a dye penetrant.

[0174] The samples were cut perpendicular to the cladding direction using the standard method for metallographic sample preparation, and the molds were ground and polished.

[0175] The Vickers hardness HV0.2 was measured in the coating cross-section using a 200-g load. Seven indentations were made and the average value and dispersion were calculated.

[0176] The samples were etched in a 4% nitric acid ethanol solution to highlight the coating and better distinguish it from the substrate. The coating quality was judged as follows:

[0177] - The number of holes and slag with diameters of 50 - 25 μm was counted in an area of approximately 35 mm × 0.25 mm. - The number of "hot cracks", also known as microcracks, was qualitatively estimated by examining a 35-mm ×

[0178] 0.25-mm coating area at a magnification of 5x.

[0179] - The number of microcracks was rated from 1 to 5 according to Table 4.

[0180] Table 4: Rating scheme for evaluating the severity of microcracks in laser cladding coatings

[0181] Rating Standard 5 Very Many 4 Many 3 Medium 2 Some 1 Few

[0182] The microstructure of the coatings was further studied by optical microscopy (LOM) and SEM. For LOM analysis, the samples were etched in Vilella (94 ml EtOh + 5 ml HCl + 1 g picric acid). EBSD-SEM analysis was performed on some selected samples. The unetched samples polished with colloidal SiO2 (OP-U from Struers) for 20 minutes were used for EBSD-SEM analysis.

[0183] Perform neutral salt spray test (NSS test) on a 90 mm long double-layer clad layer according to the current version (2022) of ASTM B117 under the test conditions recorded in Table 5.

[0184] Table 5: Test conditions for neutral salt spray test (NSS)

[0185] Temperature 35℃±2℃ <![CDATA[For the average collection rate of a horizontal collection area of 80 cm 2 > 1.5 ml / h ± 0.5 ml / h Concentration of NaCl 50 g / l ± 5 g / l pH 6.5-7.2

[0186] Grind the sample to a surface roughness R a of approximately 0.8 - 1 μm before the NSS test. There is no possibility of controlling material removal during grinding and the sample is ground until a smooth and uniform surface is obtained. The surface smoothness is judged by visual inspection.

[0187] Keep the sample in the NSS chamber for 7 days and analyze it after 24 hours, 48 hours, and 168 hours. Qualitatively rate the sample according to the rating criteria given in Table 6 using the current version (2022) of ISO10289.

[0188] Table 6: Qualitative rating scale for evaluating corrosion resistance according to ISO 10289

[0189] Rating Standard 10 No Defects in the Sample 9 Very Few Surface Defects 8 Few Surface Defects 7-6 Moderate Surface Defects 5 Severe Surface Defects

[0190] Samples Coated with 6-Jet GTV Nozzle:

[0191] Test for cracks in the samples coated with GTV nozzles using dye penetrant. Measure the hardness HV0.3 in the coating cross-section and judge the coating quality using LOM.

[0192] Results and Discussion

[0193] Coating Performance Evaluation

[0194] Calculated thermodynamic properties - Samples A1 - A9

[0195] Calculate the thermodynamic properties of alloys A1 - A9 in Table 1 (using the pre-alloying target values according to Table 7 for the alloy composition, rather than the actual composition determined after alloying) with the aim of obtaining a deeper understanding of the coating properties. Additionally, calculate the phase amount and composition of the alloy at a temperature 200 °C below the solidus to evaluate the theoretical level of the alloy's stability against process variations under equilibrium conditions.

[0196] Calculate the melting range (difference between the solidus and liquidus) ΔT to estimate the alloy's sensitivity to solidification cracking. Also capture the segregation tendency of the alloy by calculating the Scheil solidification interval (SSI) of the last 10% of the melt.

[0197] As is known in the art, some alloying elements have a strong tendency to segregate, which results in a large melting range. The larger the melting range, the more likely the alloy is to exhibit solidification cracking or "hot cracks". However, all of the alloys recorded in Table 7 behaved as expected.

[0198] For all alloys except A2 (marked with *) calculated under the assumed bcc matrix, the PREN was calculated using the chemical composition of austenite at 200 °C below the solidus temperature using the following equation:

[0199] PREN = 100 (W(FCC,CR) + 3.3W(FCC,MO))

[0200] The parameters used for the calculation are listed below.

[0201] · The grain size was set to 10 μm.

[0202] · The subcritical annealing temperature was set at 200 °C below the solidus temperature.

[0203] Table 7: Calculated thermodynamic properties of alloys A1 - A9 using nominal chemical compositions

[0204]

[0205] Coating Performance

[0206] HighNo 4.0 nozzle, cladding speeds of 30 m / min and 100 m / min

[0207] Cladding Speed 30 m / min

[0208] Tables 8 and 9 summarize the test results of various coatings prepared using a HighNo 4.0 nozzle at a cladding speed of 30 m / min, with no niobium (Nb) present in the alloy (Table 8) and with niobium (Nb) present in the alloy (Table 9).

[0209] Notably, all of the alloys of the present disclosure were clad on the sample surface without the formation of macroscopic cracks, with the exceptions of alloys A7, A8, and A35. It was found that the failure of alloys A7 and A8 to prevent the formation of macroscopic cracks was related to a high total content of carbon and boron - a total of more than 1.2 wt% carbon and boron, which, however, as recorded in Table 9, could be compensated for by the addition of niobium. Alloy A35, with a chromium content of 18.70 wt%, was only for comparison with the alloys of the present invention, as it was found that when chromium was present outside the limits detailed herein, macroscopic crack formation could not be suppressed by other means within the limits detailed for the contents of other components related to the alloys of the present invention.

[0210] Table 8: Coating properties of alloys - no Nb

[0211]

[0212] Table 9: Coating properties of the alloy - with Nb

[0213]

[0214]

[0215] Cladding Speed 100 m / min

[0216] The tests carried out at a cladding speed of 100 m / min reproduced the tests at a cladding speed of 30 m / min, showing that the alloys of the present invention are also suitable for very fast high-speed laser cladding. It was again found that alloys A7 and A8 had a large number of macroscopic cracks (test score of 5), which were again completely compensated by the addition of niobium. The surface coated with alloy A35 also received a test score of 5, which could not be compensated in other ways by adjusting the other elements of the alloys of the present invention. Some other alloys showed slightly worse scores than at 30 m / min, which emphasizes the need for individual optimization of the cladding speed for a given alloy.

[0217] Coating properties - GTV nozzle, cladding speed 50 m / min

[0218] In a smaller experimental study, alloys A1 - A11 (minus A9) were tested at a cladding speed of 50 m / min using a GTV nozzle (see Table 10). The results proved to be comparable to those obtained with the HighNo 4.0 nozzle at a cladding speed of 100 m / min.

[0219] Table 10: GTV nozzle

[0220]

[0221] Discussion

[0222] Hardness

[0223] Figure 1 Shows the hardness HV0.2 of alloys A1 - A8, A10 - A12 and A20 - A24 clad using the HighNo 4.0 nozzle at 30 m / min and 100 m / min and using the GTV nozzle at 50 m / min. In Figure 1 the striped bars of alloys A7 and A8 reflect that these two alloys showed macroscopic cracks after cladding.

[0224] It can be observed that for the same alloy chemical composition, the hardness HV0.2 is within the same range and is independent of the cladding speed and nozzle used. The hardness of the cladding layer produced using the GTV nozzle is slightly lower, which is likely due to the lower solidification rate. Additionally, the hardness HV0.2 varies within approximately 350 HV0.2 - 500 HV0.2. Although these hardness variations are significant, they can be explained by the large variations in the carbon and boron contents in the alloys studied, consistent with the alloys having the lowest carbon and boron contents showing the lowest hardness and those having the highest carbon and boron contents showing the highest hardness.

[0225] In Figure 2 the hardness of alloys A1 - A8 and A10 - A12 was plotted against the calculated volume fractions of borides and austenite at 200 °C below the solidus temperature to examine the correlation between the simulated microstructures and the observed coating hardness. In Figure 2 the coating hardness was plotted against the calculated volume fractions of borides and fcc at 200 °C below the solidus with the remaining phase being Bcc. Calculations were made using the nominal compositions of the alloys. The volume fraction of borides was studied for the range of chemical compositions and it increases with increasing boron content.

[0226] A significant variation in the fcc content was observed in the calculated results. In the alloys, it was expected that the fcc - packed phase would transform to martensite during cooling and this contributes to the hardness. Thus, alloys with a higher initial fcc content were expected to be harder. However, no clear correlation could be established between the volume fraction of austenite and the measured hardness.

[0227] To suppress the risk of forming macroscopic cracks, it was decided to add 0.5 - 1.0 wt% Nb. Niobium is a strong carbide former. If primary carbides form in the melt, the austenite matrix will be depleted in carbon and “softer” martensite is expected to form.

[0228] Figure 3 Shows the hardness of alloys A26 - A34 containing 0.5 - 1.0 wt% Nb clad at 30 m / min and 100 m / min using the HighNo 4.0 nozzle. The dotted staple refers to the coatings of the macroscopic crack alloys A31 and A32 with the highest C and B contents (C = 0.17 wt% and B = 1.30 wt%), which showed cracks when clad at 100 m / min using the HighNo 4.0 nozzle. Alloys A26 with C = 0.16 wt% and B = 1.15 wt% and A17 with C = 0.16 wt% and B = 1.17 wt% did not crack. Alloys A7 and A8 (without Nb) with chemical compositions similar to A26 and A27 showed cracking. Thus, adding Nb is beneficial for suppressing crack formation, especially at high carbon and / or boride contents.

[0229] Analysis of the microstructure of the coatings

[0230] Holes and Slag:

[0231] Pores were found in all coatings and when cladding with the HighNo 4.0 nozzle, the pore size was typically <50 μm. The number of pores and slag in the investigated coatings was counted, but no correlation was found between the alloy chemical composition, such as the Si and O contents, and the number of pores, see Tables 7 - 10.

[0232] Hot Cracks:

[0233] A qualitative estimate of the number of microcracks in the coatings was made for alloys A1 - A12, A16, A17 and A20 - A24. The results are shown in Figure 4 in.

[0234] In Figure 5 the number of microcracks was plotted against the solidification range calculated for the last 10% of the melt using the Sheil simulation. This gives an indication of the alloy segregation tendency. Except for A3, the melting ranges of all alloys were very similar, showing a similar segregation tendency in the melt. No obvious correlation was found between the melting range and the number of microcracks in the coatings.

[0235] By plotting the number of microcracks against the boron content as done in Figure 6 it was observed that the number of microcracks was highest for boron contents below 0.9%. No relationship was found between the number of microcracks and Si or C, see Table 11. Figure 6 It is shown that the boron content should not be less than 0.9 wt%, preferably not less than 0.95 to minimize the number of hot cracks and the maximum B content should not exceed 1.5 wt%.

[0236] Since carbon must be allowed to vary between 0.05 - 0.15 wt% for cost - effective selection of raw materials and the capabilities of the production process, there is a risk of forming macro - cracks if both carbon and boron approach the upper specification limits simultaneously, which would need to be mitigated by adding niobium (Nb).

[0237] Table 11: Coating properties of alloys A1 - A11. GTV nozzle, cladding speed 50 m / min

[0238]

[0239]

[0240] Microstructure:

[0241] The microstructure of the coatings in the as - deposited and unetched state was examined to check for porosity, oxides and microcracks. AsFigure 4 and 5 The number of microcracks shown varies in the coating depending on the chemical composition. Overall, the samples clad at 30 m / min show fewer and smaller defects than those clad at 100 m / min.

[0242] Typical microstructures of coatings with few pores and cracks and coatings with numerous microcracks and pores are shown in Figure 7 and 8 . In Figure 7 a, the coating of alloy A3 at 30 m / min is shown, presenting good coating quality with few visible pores. In Figure 7 b, the coating of alloy A5 at 30 m / min is shown, presenting a poorer quality coating with several microcracks and pores visible in it, while in Figure 8 the same results for the same alloys A3 and A5 at 100 m / min are shown, also see Table 7 - 10.

[0243] Samples were etched in Vilella to examine the coating microstructure. All coatings showed a very fine microstructure that could not be further resolved by LOM. The alloys clad at 30 m / min had a uniform microstructure except for A7 (see Figure 9 , which shows two examples of the microstructure of coatings using alloys A10 and A7, clad at 30 m / min and 100 m / min respectively using a HighNo 4.0 nozzle). A7 showed a tendency to form a layered structure, where the bright etched regions were harder (HV ~ 600) than the dark etched regions (HV ~ 450). Based on thermodynamic analysis, this alloy formed the highest amount of fcc, indicating that the observed fine layering might be related to the segregation between the fcc phase and other further alloy phases in the coating. The tendency of alloy A7 to form a layered structure was greatest when a clad speed of 30 m / min was used.

[0244] Figure 10 Shows the microstructure of the coating of A10 for both 30 m / min and 100 m / min at higher magnifications. Furthermore, the microstructure could not be resolved by LOM at the highest magnifications.

[0245] Based on thermodynamics, significant changes in fcc and bcc were expected in the alloys studied. However, the properties of the alloys were still within the required target parameters. Since the microstructure could not be resolved by LOM, SEM EBSD analysis was performed on two alloys (one alloy with a large amount of austenite stabilizer such as A7, one alloy with a large amount of ferrite stabilizer) and one alloy with a target chemical composition clad at 100 m / min.

[0246] Figures 11 - 13Overview of the coating microstructures of alloy A1 ( Figure 11 ), A7 ( Figure 12 ), and A10 ( Figure 13 ) are shown separately. In Figures 11 - 13 , the figures marked A are overviews of the coating microstructures observed by LOM, the figures marked B are SEM EBSD maps, and the figures marked C are Euler diagrams. The EBSD maps show that the microstructure of A1 ( Figure 11 B and 11C) consists of columnar primary grains of the bcc phase, A7 ( Figure 12 B and 12C) consists of more equiaxed primary grains of bcc and fcc mainly present in the overlapping region and close to the substrate, while for A10 ( Figure 13 B and 13C), the amounts of bcc and fcc and the size of the primary grains are between those of A1 and A7. For alloy A1, the SEM EBSD map ( Figure 11 B) mainly shows a bcc structure, where the black dots are unresolvable structures. The Euler diagram ( Figure 11 C) shows elongated primary grains. For alloy A7, the SEM EBSD map ( Figure 12 B) shows a bcc structure (red) and an fcc structure (blue), where the black dots are unresolvable structures. The Euler diagram ( Figure 12 C) shows equiaxed primary grains. For alloy A10, the SEM EBSD map ( Figure 13 B) shows a bcc orientation (red) and an fcc orientation (blue), where the black dots are unresolvable structures. The Euler diagram ( Figure 13 C) shows equiaxed primary grains.

[0247] By observing the microstructures of A1 and A7 at higher magnifications, it can be observed that the bcc structure in A1 shows a small number of defects indicated by the light gray pattern of the contrast bands. This indicates that the microstructure consists of ferrite and eutectic structures. In contrast, for A7, the contrast bands show regions with a small number of defects (light gray in the contrast band map), which consist of ferrite and eutectic structures and are located in the central part of the track, and regions with a larger number of defects (dark gray in the contrast band map), which are located in the overlapping part between the two tracks and most likely consist of martensite, retained austenite, and eutectic structures.

[0248] The columnar shape of the primary grains in A1 makes the alloy more sensitive to the formation of hot cracks. The differences in the size and geometry of the primary grains can explain why alloy A1 is more prone to forming hot cracks than alloy A7.

[0249] Corrosion tests

[0250] All the studied alloys were tested for corrosion. The results are summarized in Table 12. For the criteria used to evaluate the samples, see Figure 7The reference pictures and Tables 4-6 in

[0251] Table 12: Qualitative ratings of alloys tested for 7 days in NSS. The alloys were clad at 30 m / min and 100 m / min using a HighNo 4.0 nozzle:

[0252]

[0253]

[0254] * Not tested due to too much grinding

[0255] ** Macroscopic cracks

[0256] In Figure 14 Exemplary samples showing the corresponding grades are presented, where the corrosion level improves in quality from Grade A to Grade E after 7 days in the NSS chamber. Grade A = 0, macroscopic cracks in the coating; Grade B = Alloy A5 coated at 30 m / min, Grade = 5, severe corrosion; Grade C = Alloy A21 coated at 30 m / min, Grade = 6 - 7, moderate corrosion; Grade D = Alloy A26 coated at 30 m / min, Grade = 8, slight corrosion; and Grade E = Alloy A31 coated at 30 m / min, Grade = 9, very slight corrosion.

[0257] Some of the alloys studied showed poor corrosion resistance in neutral salt spray. By plotting the corrosion rate against the boron ( Figure 15 ) and chromium ( Figure 16 ) contents respectively, it can be observed that the alloys with the highest corrosion level showed the lowest boron content and the largest amount of hot cracks, indicating that hot cracks affect the corrosion resistance of the alloy. For alloys graded 8 and 9, almost no corrosion spots were detected on the surface. The spots appeared after the first day and generally did not grow after a one-week test.

[0258] ThermoCalc Results

[0259] The inventors of the present case were guided by the consideration that depending on the composition, the fcc phase may transform into martensite during cooling. A high fraction of martensite increases the residual stress in the material and the risk of cracking. Similarly, as a guiding principle, the inventors of the present case believe that too high a level of bcc-phase iron also increases the risk of hot cracking.

[0260] To examine the appropriate chromium range based on the above guiding considerations, the inventors of the present case examined the theoretically predicted phase behavior of three iron-chromium alloy compositions. Calculations were performed using the commercially available database TCFE9 with the software Thermo-Calc. The compositions were A) Fe-20.8Cr-2.8Ni-1.6Si-1.3B-0.4Mo-0.66Mn-0.16C, B) Fe-22.5Cr-3Ni-2Si-1B-0.4Mo-0.6Mn-0.09C (the present invention), C) Fe-29Cr-3Ni-2Si-1B-2Mo-0.5-0.09C, see Figure 17.

[0261] A) Fe-20.8Cr-2.8Ni-1.6Si-1.3B-0.4Mo-0.66Mn-0.16C:

[0262] Figure 17A Shows the equilibrium calculation of the above composition (Fe-20.8Cr-2.8Ni-1.6Si-1.3B-0.4Mo-0.66Mn-0.16C), which was subsequently shown to be unsuitable for high-speed laser cladding due to cracking problems in the above-recorded tests (see alloy A7).

[0263] It is noteworthy that the bcc phase does not exist in the structure in the shown phase diagram.

[0264] B) Fe-22.5Cr-3Ni-2Si-1B-0.4Mo-0.6Mn-0.09C:

[0265] Figure 17B Shows the equilibrium calculation of the above composition (Fe-22.5Cr-3Ni-2Si-1B-0.4Mo-0.6Mn-0.09C), which was proven to be suitable for high-speed laser cladding in the R & D work (see alloy A10).

[0266] It can be seen that the material is not expected to fully transform into fcc during cooling, but shows a suitable balance between the bcc and fcc phases. The maximum equilibrium fraction of the fcc phase is about 0.45.

[0267] C) Fe-29Cr-3Ni-2Si-1B-2Mo-0.5-0.09C:

[0268] Figure 17C Shows the equilibrium calculation of the above composition (Fe-29Cr-3Ni-2Si-1B-2Mo-0.5Mn-0.09C). It can be seen that no FCC is expected for this composition and the matrix is entirely ferrite (bcc). Although high corrosion resistance is thus achieved with high chromium and molybdenum, however, this increases the fraction of the bcc phase, which is too soft for the required hardness for the intended use of the present invention. Therefore, this alloy was not tested in any of the tests.

[0269] Overall, the alloys disclosed in the present invention explore a window of opportunity where the detrimental effect of chromium (and molybdenum) on the necessary hardness for the intended application is offset by the reduced crack formation obtained and the resulting ability to form thinner coatings in the laser cladding process, while maintaining corrosion resistance and acceptable hardness.

[0270] Design Considerations and Conclusions

[0271] Final optimization tests of the alloy chemical composition were carried out in terms of cost and performance. The results are recorded in Table 13.

[0272] In the tests, nickel, which is considered an expensive alloying element for stabilizing austenite, was considered. During cooling, austenite transforms into martensite which contributes to the hardness of the coating. To verify whether the addition of Ni contributes to the coating hardness, an alloy (A25) with the target chemical composition and without pre-alloyed Ni was atomized and high-speed laser cladded. The coating hardness was not significantly affected, indicating that the addition of nickel is not critical for the coating hardness. The number of microcracks in the coating was greater compared to the alloy with the target chemical composition. The reason may be the formation of columnar ferrite grains during solidification.

[0273] In the tests, similar to nickel, carbon was further considered to contribute to the coating hardness. Carbon stabilizes austenite which transforms into martensite during cooling and forms carbides. Due to the high solidification rate of the high-speed laser cladding process, it is not clear the effect of carbon on the coating hardness. These alloys showed hardness close to that of the alloy with the optimized chemical composition. Since both alloys with and without carbon and alloys with and without nickel showed similar hardness, it can be assumed that borides and the fine grain structure are the main reasons for the coating hardness.

[0274] In the tests, chromium, which is responsible for the corrosion resistance of the alloy, was further considered. However, chromium stabilizes ferrite and further increasing the target chromium content from 23 wt% to 25 wt% may result in lower coating hardness. Therefore, an alloy (A36) with 25 wt% chromium and the remaining elements at the target chemical composition was studied. The hardness and microstructure of the coating were comparable to those of the alloy with the target chemical composition and the corrosion resistance was improved.

[0275] In the tests, molybdenum, which is known from the literature to improve pitting corrosion resistance, was further considered. However, since molybdenum is a ferrite stabilizer, the addition of molybdenum may lead to a reduction in coating hardness. Therefore, an alloy (A40) with a target molybdenum content of 1.5 wt% was studied. The addition of molybdenum did not significantly affect the coating hardness.

[0276] Table 13: Coating properties of alloys with different chemical compositions, cladded at 30 m / min and 100 m / min using the HighNo4.0 nozzle showing optimized cladding performance

[0277]

[0278]

[0279] Thus, in summary and although all of the inventive alloys detailed herein are suitable for high speed cladding, it has been found that alloys falling within the limits given in Table 14 below are particularly effective and meet the objectives of the present invention.

[0280] Table 14: Optimized alloy composition without niobium (Nb)

[0281] Chemical Composition Set Point Minimum Maximum Fe Balance wt% wt% Cr 23.5 22.00 25.00 B 1.00 0.90 1.10 Mo 0.50 0.30 1.80 Ni 3.00 2.50 3.50 C 0.10 0.00 0.15 Mn 0.50 0.30 0.70 Si 2.00 1.70 2.30

[0282] When niobium forms part of the inventive alloy, the carbon and boron contents can be higher, as detailed above.

[0283] Conclusion

[0284] Although the present invention has been described in detail for purposes of illustration, it is to be understood that such details are solely for that purpose and that changes can be made herein by those skilled in the art upon implementing the claimed subject matter by studying the drawings, this disclosure, and the appended claims.

[0285] The term "comprising" as used in the claims does not exclude other elements or steps. The indefinite articles "a" or "an" used in the claims do not exclude a plural form. Reference signs used in the claims should not be construed as limiting the scope.

Claims

1. An iron-chromium-based alloy, which consists of the following by weight based on the total weight of the alloy: chromium (Cr): 20.5-28.0% by weight, nickel (Ni): at most 5.0% by weight, silicon (Si): 0.5-2.5% by weight, boron (B): 0.5-1.5% by weight, molybdenum (Mo): 0.15-2.0% by weight, manganese (Mn): 0.10-0.90% by weight, carbon (C): 0.01-0.20% by weight, niobium (Nb): at most 1.5% by weight, copper (Cu): at most 0.2% by weight, cobalt (Co): at most 1.0% by weight, the balance being iron (Fe) and unavoidable impurities, not exceeding 0.3% by weight.

2. The iron-chromium-based alloy according to claim 1, wherein copper (Cu) is present at most 0.1% by weight, preferably wherein copper is present as an unavoidable impurity.

3. The iron-chromium-based alloy according to any one of the preceding claims, wherein cobalt (Co) is present at most 0.2% by weight, preferably at most 0.1% by weight, more preferably wherein cobalt is present as an unavoidable impurity.

4. The iron-chromium-based alloy according to any one of the preceding claims, wherein niobium (Nb) is present at 0.30% by weight, 0.35% by weight, 0.40% by weight, 0.45% by weight, 0.5% by weight, 0.55% by weight, 0.60% by weight, 0.70% by weight, 0.8% by weight, 0.9% by weight or 1.0% by weight; and up to 1.4% by weight, 1.3% by weight, 1.2% by weight, 1.1% by weight, 1.0% by weight, 0.9% by weight or 0.8% by weight, preferably 0.40-1.2% by weight, 0.45-1.1% by weight or 0.50-1.0% by weight.

5. The iron-chromium-based alloy according to any one of the preceding claims, wherein if the content of carbon (C) exceeds 0.15% by weight, the content of boron (B) exceeds 1.1% by weight or the combined content of carbon and boron exceeds 1.20% by weight, niobium (Nb) is present at 0.30-1.5% by weight, preferably 0.5-1% by weight.

6. The iron-chromium-based alloy according to any one of the preceding claims, wherein chromium (Cr) is present at 21-26% by weight, 22-25% by weight, 22.5-24.5% by weight or preferably 23-24% by weight.

7. The iron-chromium-based alloy according to any one of the preceding claims, wherein nickel (Ni) is present at 2.15-3.85% by weight, 2.25-3.75% by weight, 2.35-3.65% by weight, 2.50-3.50% by weight, 2.65-3.35% by weight or preferably 2.75-3.25% by weight.

8. The iron-chromium-based alloy according to any one of the preceding claims, wherein silicon (Si) is present at 0.75-2.45% by weight, 1.0-2.4% by weight, 1.25-2.35% by weight, 1.5-2.3% by weight, 1.6-2.25% by weight, preferably 1.7-2.2% by weight, 1.8-2.15% by weight or more preferably 1.9-2.1% by weight.

9. The iron-chromium-based alloy according to any one of the preceding claims, wherein boron (B) is present in an amount of 0.7-1.3 wt%, 0.8-1.2 wt%, 0.9-1.1 wt% or preferably 0.95-1.05 wt%.

10. The iron-chromium-based alloy according to any one of the preceding claims, wherein molybdenum (Mo) is present in an amount of at most 1.9 wt%, at most 1.8 wt%, at most 1.7 wt%, at most 1.6 wt%, at most 1.5 wt%, at most 1.4 wt%, at most 1.3 wt%, at most 1.2 wt%, at most 1.1 wt%, at most 1.0 wt%, at most 0.90 wt%, at most 0.80 wt%, preferably at most 0.70 wt%, at most 0.60 wt% or more preferably at most 0.50 wt%.

11. The iron-chromium-based alloy according to any one of the preceding claims, wherein molybdenum (Mo) is present in an amount of from 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt%, 0.50 wt%, 0.55 wt%, 0.60 wt%, 0.65 wt% or 0.70 wt%.

12. The iron-chromium-based alloy according to any one of the preceding claims, wherein molybdenum (Mo) is present in an amount of 0.20-1.3 wt%, 0.25-1.1 wt%, 0.3-0.90 wt%, 0.35-0.70 wt% or 0.40-0.60 wt%.

13. The iron-chromium-based alloy according to any one of the preceding claims, wherein manganese (Mn) is present in an amount of from 0.35 wt%, 0.40 wt%, 0.45 wt% or 0.50 wt%; and up to 0.85 wt%, 0.80 wt%, 0.75 wt%, 0.70 wt%, 0.65 wt%, 0.60 wt%, 0.55 wt% or 0.50 wt%; preferably 0.30-0.70 wt% or 0.40-0.60 wt%.

14. The iron-chromium-based alloy according to any one of the preceding claims 1-14, wherein carbon is present in an amount of 0.01-0.20 wt%, preferably carbon (C) is present in an amount of from 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.10 wt%; and up to 0.19 wt%, 0.18 wt%, 0.17 wt%, 0.15 wt%, 0.14 wt%, 0.13 wt%, 0.12 wt%, 0.11 wt% or 0.10 wt%; preferably 0.05-0.15 wt%.

15. The iron-chromium-based alloy according to any one of claims 1-14 preceding, which is in powder form.

16. The iron-chromium-based alloy in powder form according to claim 15, which is formed by an atomization process, preferably a water atomization process.

17. The iron-chromium-based alloy in powder form according to claim 16, wherein oxygen (O) introduced as an inevitable impurity in the atomization process does not exceed 0.6 wt% based on the total weight of the alloyed powder.

18. An iron-chromium-based alloy in powder form according to any one of claims 15-17, wherein the iron-chromium-based alloy in powder form comprises at least 80% by weight of the iron-chromium-based alloy powder contained in the sieve fraction of the iron-chromium-based alloy powder having a size distribution measured by sieving according to ASTM B214 of 1-100 μm, 2.5-90 μm, 5-80 μm, preferably 10-75 μm or 15-70 μm or more preferably 20-60 μm.

19. An iron-chromium-based alloy in powder form according to any one of claims 15-18, comprising at least 80% by weight, at least 85% by weight, preferably at least 90% by weight, or more preferably at least 95% by weight of the iron-chromium-based alloy powder having a size distribution measured by sieving according to ASTM B 214 of 2.5-100 μm.

20. Use of a powder according to any one of claims 15-19 in coating a surface by means of a laser cladding method.

21. An iron-chromium-based alloy formed from an iron-chromium-based alloy according to any one of claims 1-14 in a laser cladding method.

22. A surface coating composed of an iron-chromium-based alloy according to any one of claims 1-14.

23. A shaped article comprising a surface coating composed of an iron-chromium-based alloy according to any one of claims 1-14.

24. A method for producing a coated surface according to claim 22 or an article according to claim 23, having the following steps: - providing a powder according to one or more of claims 15-19 in a form or formulation suitable for laser cladding; - performing a laser cladding process using the powder; - obtaining the surface coating or the article.

25. A composition for forming an iron-chromium-based alloy according to any one of claims 1-14, the composition consisting by weight based on the total weight of the composition of: chromium (Cr): 20.5-28.0% by weight, nickel (Ni): at most 5.0% by weight, silicon (Si): 0.5-2.5% by weight, boron (B): 0.50-1.5% by weight, molybdenum (Mo): 0.15-2.0% by weight, manganese (Mn): 0.10-0.90% by weight, carbon (C): 0.01-0.20% by weight, niobium (Nb): at most 1.5% by weight, copper (Cu): at most 0.2% by weight, cobalt (Co): at most 1.0% by weight, the balance being iron (Fe) and unavoidable impurities not exceeding 0.3% by weight.

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