New iron-chromium-based alloys for laser cladding.

Abstract

An iron chromium based alloy is provided to provide a laser clad surface with a low risk of cracking of the laser clad surface, whether by macro or micro cracking. The iron-chromium-based alloy contains, by weight, 20.5 to 28.0% chromium (Cr), 5.0% or less nickel (Ni), 0.5 to 2.5% silicon (Si), 0.5 to 1.5% boron (B), 0.15 to 2.0% molybdenum (Mo), 0.10 to 0.90% manganese (Mn), 0.01 to 0.20% carbon (C), 1.5% or less niobium (Nb), 0.2% or less copper (Cu), and 1.0% or less cobalt (Co), with the remainder consisting of iron (Fe) and unavoidable impurities not exceeding 0.3%.

Description

[Technical Field]

[0001] Within the field of iron-chromium based alloys, a series of novel iron-chromium based alloys are disclosed that are suitable for laser cladding with minimal crack formation during the laser cladding process. [Background technology]

[0002] In recent years, laser cladding has increasingly replaced hard chrome plating on wear parts exposed to salt environments (e.g., piston rods for hydraulic ceiling supports in the mining industry) because laser cladding can extend the life of the laser-clad wear parts by up to five times compared to traditional hard chrome plating. For example, laser cladding using Hoganas AB's Rockit® 401 (Fe-18Cr-2.5Ni-0.5Mo-0.15C) has been adopted to coat hydraulic ceiling support piston rods in the coal mining industry over the past decade, making it the current market-leading alloy for laser cladding worldwide, especially in the APAC region.

[0003] Currently, only large piston rods, typically 300 mm in diameter and 1300 mm in length, are laser clad. This represents approximately 15–20% of all piston rods produced, including new products and refurbishments. The remaining 85% are hard chrome plated due to the cost and technical challenges of laser cladding wear parts. For example, currently used piston rods are typically too small in diameter to adequately dissipate the high heat input of current laser cladding processes under current production conditions. Nevertheless, due to benefits such as longer life, OEMs' future goal is to laser clad 100% of all produced wear parts, including piston rods, without the current size limitations.

[0004] OEMs are also trending toward reducing coating costs by improving productivity (e.g., using higher cladding speeds and new types of nozzles, reducing coating thickness, and / or minimizing post-weld processes (e.g., machining)). Currently, the industry standard coating thickness after deposition is on the order of 1.2 mm, but the industry goal is to reduce this to less than 0.8 mm, preferably less than 0.5 mm, or even less than 0.3 mm. These additional requirements impose new demands on the alloy and powder particle size used in the laser cladding process, as higher cladding speeds and lower coating thicknesses result in higher cooling rates and potentially higher internal stresses in the coating material, further affecting the alloy's welding behavior, the final coating microstructure, etc.

[0005] To reduce coating thickness and simultaneously increase cladding speed, smaller coating particles must be used during the laser cladding process compared to processes where thicker coatings are desired, because smaller particles dissolve faster. While the art suggests that particle size distributions for laser cladding can range from 10 μm to 150 μm, high-speed laser cladding requires a more narrowly defined particle size distribution, with the current industry target being a particle size distribution in the 10 μm to 110 μm range.

[0006] Unfortunately, existing iron-chromium alloys that fall within the desired size distribution range have proven unsuitable when attempting to produce thinner coatings than those currently available commercially, as test experiments conducted by the inventors have shown that the increased cooling rates associated with thinner coatings result in crack formation and an unstable microstructure. Additionally, when thin cladding is performed using commercially available iron-chromium powders, the hardness of the resulting coating layer exhibits insufficient consistency in hardness / wear resistance and corrosion compared to coatings based on existing iron-chromium powders.

[0007] The present invention is therefore motivated by this current need for new robust alloys suitable for high speed / high productivity laser cladding processes, which can be used to produce thin (less than 0.3 mm), essentially crack-free coatings with stable microstructures and hardnesses in the Vickers hardness range of 400-550, while at the same time having corrosion resistance and processability comparable to the best commercially available protective iron-chromium alloys for laser cladding, such as, for example, Rockit® 401.

[0008] In the field of the present invention, an alloy powder for laser cladding repair of hydraulic pillars for mining is known, for example from Patent Document 1, which 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, and the remainder is Fe.

[0009] However, the high cobalt content poses health and safety concerns for workers using the prior art alloy powders, and therefore requires special precautions during production. As a result, avoiding cobalt above the level of unintentional contamination is a further objective of the present disclosure and is solved by the iron-chromium alloys described in detail herein.

[0010] Patent document 2 details the use of alloy particles in laser cladding performed at 50 m / min using particle sizes of 15 μm to 53 μm to obtain a 1.5 mm cladding layer, the alloy consisting of 17.5 to 19.5 wt% Cr, 1.7 to 2.3 wt% Ni, 0.8 to 1.2 wt% Si, 0.9 to 1.2 wt% B, 0.4 to 0.6 wt% Mo, up to 0.3 wt% Mn, 0.15 to 0.23 wt% C, and the balance iron (Fe). The resulting surface hardness (HV) was 658 HV.

[0011] Patent Document 3 details the use of alloy particles in laser cladding, performed using particle sizes of 15 μm to 175 μm to obtain a 1.4 mm cladding layer, with the alloy consisting 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 iron (Fe), with 0.06-0.08 wt% nitrogen due to atomization of the alloy melt used. The resulting surface hardness (HV) was 700 HV.

[0012] According to the inventors of the two latter prior art documents, US Pat. No. 5,629,999 and US Pat. No. 5,629,999, the resulting coatings are highly resistant to corrosion at selected levels of iron-chromium balance, thereby confirming the findings of the applicant with their own commercial product, Rockit® 401, which has a similar chromium content as those disclosed in both latter documents. Also, the observed hardness levels of the prior art are consistent with the applicant's commercial product, Rockit® 401. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Chinese Patent Application Publication No. 113046625 [Patent Document 2] Chinese Patent Application Publication No. 111097908 [Patent Document 3] Chinese Patent Application Publication No. 111809177 Summary of the Invention [Problem to be solved by the invention]

[0014] A problem with the alloys suggested in the two latter patents is that when coating using high-speed laser cladding, very thick coatings must be produced to avoid crack formation within the deposited coating layer. This is because the high hardness of the prior art coatings makes them susceptible to cracking, which relieves internal hardness stresses, and this is therefore compensated for by increasing the layer thickness. A similar problem was observed with the applicant's own product, Rockit® 401 (Fe-18Cr-2.5Ni-0.5Mo-0.15C).

[0015] This observation led the inventors of the present invention to search for cobalt-free iron-chromium based alloys that do not suffer from the same drawback of crack formation during laser cladding and do not require increased cladding thickness to compensate for crack formation, thereby increasing material usage and coating costs.

[0016] Surprisingly, the inventors of the present invention have discovered that the above-mentioned drawbacks can be alleviated in a simple way by increasing the chromium content compared to that known in the prior art, as will be explained in detail herein, which leads to the formation of a favorable bcc / fcc balance in the resulting alloy in the limited area of ​​increased chromium content, which, while maintaining the corrosion resistance known in the prior art, surprisingly allows high-speed laser cladding without crack formation in coating layers with thicknesses of about 100 μm to about 350 μm, which is associated with a reduction in the hardness of the coating layer to about HV 400-500, but which remains perfectly acceptable for the proposed application.

[0017] This result is unexpected because the prior art indicates that chromium increases hardness and therefore susceptibility to stress cracking as observed in prior art coatings even at low chromium concentrations. [Means for solving the problem]

[0018] According to the present disclosure and invention, the object of the present disclosure is to provide, in a first aspect and embodiments thereof, a composition comprising, of the total weight of the alloy: 20.5 wt% to 28.0 wt% chromium (Cr); 5.0 wt. % or less of nickel (Ni); 0.5 wt% to 2.5 wt% silicon (Si); 0.50 wt% to 1.5 wt% of boron (B); 0.15 wt% to 2.0 wt% molybdenum (Mo); 0.10 wt% to 0.90 wt% manganese (Mn); 0.01 wt% to 0.20 wt% carbon (C); 1.5 wt. % or less of niobium (Nb); 0.2% by weight or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), The problem is solved by providing an iron-chromium-based alloy, the balance of which consists of iron (Fe) and unavoidable impurities not exceeding 0.3 wt %.

[0019] Further aspects and embodiments are detailed herein in the description and claims. [Brief explanation of the drawings]

[0020] [Figure 1] Hardness HV0.2 for alloys A1 to A8, A10 to A12, and A20 to A24 [Figure 2] Hardness of alloys A1-A8 and A10-A12 plotted against calculated volume fractions of borides and austenite [Figure 3] Hardness of alloys A26 to A34 containing 0.5 to 1.0 wt% Nb [Figure 4] Qualitative assessment of the number of microcracks in coatings of alloys A1-A12, A16, A17, and A20-A24 [Figure 5] Qualitative evaluation of the number of microcracks versus the melt extent calculated using Shiel simulation for the last 10% of the melt. [Figure 6] Effect of boron content on the number of hot cracks in coatings (qualitative evaluation) [Figure 7] Examples of coatings clad using a HighNo 4.0 nozzle at 30 m / min: A) Alloy A3 has a limited number of defects as pores and microcracks, B) Alloy A5 has a large number of defects. [Figure 8] Examples of coatings cladded using a HighNo 4.0 nozzle at 100 m / min: A) Alloy A3 has a limited number of defects as pores and microcracks, B) Alloy A5 has a large number of defects. [Figure 9] Example microstructures of coatings of alloys A10 and A7 clad using a HighNo 4.0 nozzle at 30 m / min and 100 m / min, respectively. [Figure 10] Coating microstructure of A10 at high magnification coated at A) 30 m / min and B) 100 m / min [Figure 11] Alloy A1, A) LOM overview, B) SEM EBSD map, and C) Euler map [Figure 12] Alloy A7, A) LOM overview, B) SEM EBSD map, and C) Euler map [Figure 13] Alloy A10, A) LOM overview, B) SEM EBSD map, and C) Euler map [Figure 14] An exemplary sample evaluated for corrosion after 7 days in the NSS chamber [Figure 15] Boron corrosion ranking after 7 days in NSS [Figure 16] Chromium corrosion ranking after 7 days within NSS [Figure 17] Thermocalc results for three different Cr concentrations DETAILED DESCRIPTION OF THE INVENTION

[0021] According to the present disclosure and invention, the object of the present disclosure is to provide, in a first aspect and in its embodiments, a composition comprising, relative to the total weight of the alloy: 20.5 wt% to 28.0 wt% chromium (Cr); 5.0 wt. % or less of nickel (Ni); 0.5 wt% to 2.5 wt% silicon (Si); 0.50 wt% to 1.5 wt% of boron (B); 0.15 wt% to 2.0 wt% molybdenum (Mo); 0.10 wt% to 0.90 wt% manganese (Mn); 0.01 wt% to 0.20 wt% carbon (C); 1.5 wt. % or less of niobium (Nb); 0.2% by weight or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), The problem is solved by providing an iron-chromium-based alloy, the balance of which consists of iron (Fe) and unavoidable impurities not exceeding 0.3 wt %.

[0022] Surprisingly, laser clad coatings based on the disclosed alloys exhibit excellent thermal conductivity at cladding speeds of 30-100 m / min and 0.5-1.5 m / min. 2 The inventors have found that coatings of this alloy can be produced on a laboratory scale with thicknesses of 100-350 μm using deposition rates of 100 / hr. The produced coatings are free of cold cracks and exhibit hardnesses of 400-450 HV and corrosion resistance of NSS>96 hours.

[0023] Chromium (Cr) and iron form the bulk of the alloy, with Cr being the primary corrosion inhibitor, and the other elements disclosed herein primarily contribute to the properties of the alloy powder used in laser cladding. Advantageously, the range of chromium use, when adjusted with other elements according to the present disclosure, is quite broad, ranging from 20.5 wt% to 28.0 wt% chromium in the alloy. However, optimum chromium performance is found at 23 wt% to 24 wt% chromium, with performance increasing from the aforementioned limits toward this observed optimum concentration interval. Thus, in embodiments of the present alloy, iron-chromium-based alloys are detailed herein, with chromium (Cr) present at 21 wt% to 27 wt%, 21.5 wt% to 26 wt%, 22 wt% to 25 wt%, 22.5 wt% to 24.5 wt%, preferably 23 wt% to 24 wt%, and more preferably 23.2 wt% to 23.8 wt%, or 23.4 wt% to 23.6 wt%.

[0024] Nickel (Ni), along with chromium, has been found to be useful for corrosion protection when preparing surfaces to be laser clad using this alloy. However, as the nickel concentration increases beyond 5 wt. %, the desired surface hardness decreases, limiting further increases in nickel content. However, nickel is a costly additive compared to chromium, so it is desirable to keep the nickel content as low as possible. Experiments have shown that the objectives of this disclosure can be achieved even when nickel is absent or present only at levels of unavoidable impurities. However, optimal results have been found when nickel is present at 1 wt. % or greater. Thus, in the present alloy embodiments, iron-chromium-based alloys are detailed herein in which nickel (Ni) is present at 5 wt% or less, 0.5 wt% to 5 wt%, 1 wt% to 5 wt%, 1.5 wt% to 4.5 wt%, 2.0 wt% to 4.0 wt%, 2.15 wt% to 3.85 wt%, 2.25 wt% to 3.75 wt%, 2.35 wt% to 3.65 wt%, 2.50 wt% to 3.50 wt%, 2.65 wt% to 3.35 wt%, or preferably 2.75 wt% to 3.25 wt%.

[0025] Experiments have further demonstrated that oxygen (O) is the primary impurity present in atomized alloys due to the high chromium content when working with starting materials with low residual contaminants. It has been found that oxygen, as a primary unavoidable impurity, is generally introduced during atomization, particularly during water atomization. While the oxygen concentration in laboratory experiments did not exceed 0.3 wt.% based on the total mass of the alloy, initial experiments under production conditions have found oxygen concentrations of up to 0.6 wt.% based on the total mass of the atomized alloy. Therefore, in embodiments of the present invention, oxygen (O) as an unavoidable impurity in atomized alloys may be present at levels up to 0.6 wt.%, but is preferably present at a lower level, such as 0.55 wt.%, 0.5 wt.%, 0.45 wt.%, 0.4 wt.%, 0.35 wt.%, or more preferably 0.3 wt.% or less.

[0026] In embodiments of the present disclosure, copper (Cu) may be present in the alloys of the present disclosure. The presence of copper in the alloys has generally been found to be detrimental to the avoidance of crack formation during laser cladding, and therefore copper cannot be present in an amount greater than 0.2 wt.% Cu. Thus, in embodiments of the present disclosure, iron-chromium-based alloys are detailed herein in which copper (Cu) is present at not more than 0.2 wt.%, not more than 0.15 wt.%, or not more than 0.1 wt.%, or not more than 0.05 wt.%, but preferably copper is present only as an unavoidable impurity, preferably below the detection limit.

[0027] Experiments have shown that up to 1.0 wt.% cobalt (Co) can be used in the present alloy as a non-detrimental filler. However, for health and safety reasons in the laser cladding process, when working with iron powders containing cobalt, it is desirable that the present alloy not be dependent on cobalt for its properties. Therefore, in preferred embodiments of the present alloy, cobalt (Co) may be present at up to 0.2 wt.%, preferably at up to 0.1 wt.%, but preferably, if present, the iron-chromium-based alloy detailed herein is present only as an unavoidable impurity, preferably below the detection limit.

[0028] Experiments have shown that niobium (Nb), when present, beneficially reduces crack formation during laser cladding, and that when boron (B) and / or carbon (C) are present in high concentrations, niobium is a necessary additive element to prevent cracking during laser cladding. Thus, while niobium may be absent from the present alloys or may be present only as an unavoidable impurity in the present alloy embodiments, iron-chromium-based alloys are detailed herein in which niobium (Nb) is present in an amount of at least 0.30 wt.%, at least 0.35 wt.%, at least 0.40 wt.%, at least 0.45 wt.%, at least 0.5 wt.%, at least 0.55 wt.%, at least 0.60 wt.%, at least 0.70 wt.%, at least 0.8 wt.%, at least 0.9 wt.%, or at least 1.0 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 0.9 wt.%, or at most 0.8 wt.%, preferably between 0.40% and 1.2 wt.%, between 0.45 wt.% and 1.1 wt.%, or between 0.50 wt.% and 1.0 wt.%.

[0029] In one embodiment, the iron-chromium-based alloy detailed herein has a carbon (B) content of 0.30 wt% to 1.5 wt%, preferably 0.5 wt% to 1.25 wt%, and more preferably 0.6 wt% to 1 wt%, when the carbon (C) content exceeds 0.15 wt%, or the boron (B) content exceeds 1.1 wt%, or when the total carbon and boron content exceeds 1.20 wt%, niobium (Nb) or the combined carbon and boron content exceeds 1.20 wt%.

[0030] The alloys described herein contain the essential elements silicon (Si), boron (B), molybdenum (Mo), manganese (Mn), and carbon (C), the presence of which has been found to be necessary to provide the necessary adjustment of the laser cladding or corrosion resistance properties of iron, chromium, and, when present, nickel. Experimentation has allowed the derivation of specific optimum concentrations of the aforementioned elements, as detailed herein below.

[0031] It has been found that silicon (Si) is necessarily present in the alloys of the present disclosure in amounts between 0.5% and 2.5% by weight. However, in embodiments of the present alloys, iron-chromium-based alloys are detailed herein in which silicon (Si) is present in amounts between 0.75% and 2.45% by weight, 1.0% and 2.4% by weight, 1.25% and 2.35% by weight, 1.4% and 2.3% by weight, 1.5% and 2.3% by weight, 1.6% and 2.3% by weight, more preferably between 1.7% and 2.3% by weight, 1.8% and 2.2% by weight, or more preferably between 1.9% and 2.1% by weight. It has been found that amounts of silicon greater than 1.4% by weight minimize crack formation.

[0032] Boron (B) has been found to be necessarily present in the alloys of the present disclosure at 0.5 wt.% to 1.5 wt.%. However, in the alloy embodiments detailed herein, iron-chromium-based alloys are present at 0.6 to 1.4 wt.%, 0.7 to 1.3 wt.%, 0.8 to 1.2 wt.%, 0.9 to 1.1 wt.%, or preferably, 0.95 to 1.05 wt.% boron (B).

[0033] Molybdenum (Mo) has been found to be necessarily present in the alloys of the present disclosure at 0.15 wt% to 2.0 wt%. However, in the alloy embodiments detailed herein, iron-chromium-based alloys are present with not more than 1.9 wt%, not more than 1.8 wt%, not more than 1.7 wt%, not more than 1.6 wt%, not more than 1.5 wt%, not more than 1.5 wt%, preferably not more than 1.4 wt%, not more than 1.3 wt%, not more than 1.2 wt%, not more than 1.1 wt%, not more than 1.0 wt%, not more than 0.90 wt%, not more than 0.80 wt%, more preferably not more than 0.70 wt%, not more than 0.60 wt%, or more preferably not more than 0.50 wt%.

[0034] In one embodiment thereof, detailed herein is an iron-chromium-based alloy wherein molybdenum (Mo) is present at 0.20 wt.% or more, 0.25 wt.% or more, 0.30 wt.% or more, 0.35 wt.% or more, 0.40 wt.% or more, 0.45 wt.% or more, 0.50 wt.% or more, 0.55 wt.% or more, 0.60 wt.% or more, 0.65 wt.% or more, or 0.70 wt.% or more.

[0035] In one embodiment thereof, detailed herein is an iron-chromium-based alloy in which molybdenum (Mo) is present at 0.20 wt% to 1.3 wt%, 0.25 wt% to 1.1 wt%, 0.3 wt% to 0.90 wt%, 0.35 wt% to 0.70 wt%, or 0.40 wt% to 0.60 wt%.

[0036] In further embodiments thereof, detailed herein are iron-chromium based alloys in which molybdenum (Mo) is present in an amount of 0.3 wt.% to 1.8 wt.%, or 0.4 wt.% to 1.7 wt.%, preferably 0.5 wt.% to 1.6 wt.%, or 0.6 wt.% to 1.5 wt.%, more preferably 0.6 wt.% to 1.5 wt.%, or 0.65 wt.% to 1.45 wt.%, and most preferably 0.7 wt.% to 1.4 wt.%, 0.75 wt.% to 1.35 wt.%, or 0.8 wt.% to 1.3 wt.%.

[0037] It has been found that manganese (Mn) is necessarily present in the alloys of the present disclosure at 0.1 wt% to 0.9 wt%. However, in embodiments of the present alloys, iron-chromium-based alloys are detailed herein in which manganese (Mn) is present at 0.2 wt% or more, or 0.3 wt% or more, preferably 0.35 wt% or more, or 0.40 wt% or more, or more preferably 0.45 wt% or more, or 0.50 wt% or more, and 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% or less. In those embodiments, manganese (Mn) is preferably present at 0.30 wt% to 0.80 wt%, 0.35 wt% to 0.7 wt%, or 0.40 wt% to 0.60 wt%.

[0038] Surprisingly, the inventors have discovered that the alloy has optimum properties 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%.

[0039] The presence of carbon (C) in the alloys of the present disclosure has been found to be necessary in combination with boron to obtain adequate hardness of the laser clad coating, as detailed herein. However, carbon, being a light element, has been observed to already reach effective molecular weights in weight concentrations, which are currently at the level of the carbon unavoidable impurities present in raw materials. However, for optimum results, carbon should be present in the alloys of the present disclosure from 0.01 wt.% to 0.20 wt.%, and preferably carbon (C) is present in an amount of at least 0.02 wt.%, at least 0.03 wt.%, at least 0.04 wt.%, at least 0.05 wt.%, at least 0.06 wt.%, at least 0.07 wt.%, at least 0.08 wt.%, at least 0.09 wt.%, at least 0.10 wt.%, and at most 0.19 wt.%, at most 0.18 wt.%, at most 0.17 wt.%, at most 0.15 wt.%, at most 0.14 wt.%, at most 0.13 wt.%, at most 0.12 wt.%, at most 0.11 wt.%, or at most 0.10 wt.%, preferably at least 0.05 wt.% but less than 0.15 wt.%.

[0040] In one preferred embodiment of the alloy, by weight of the total alloy: 20.5 wt% to 28.0 wt% chromium (Cr); 2.35 wt% to 3.55 wt% nickel (Ni); 1.35 wt% to 2.5 wt% silicon (Si); 0.7% to 1.2% by weight of boron (B); 0.3 wt% to 1.8 wt% molybdenum (Mo); 0.35 wt% to 0.90 wt% manganese (Mn); 0.01 wt% to 0.20 wt% carbon (C); 1.5 wt. % or less of niobium (Nb); 0.2% by weight or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), Disclosed herein is an iron-chromium-based alloy, the balance of which consists of iron (Fe) and incidental impurities not exceeding 0.3 wt. %. In one preferred embodiment, the carbon (C) is less than 0.15 wt. %.

[0041] In one particularly preferred embodiment of the alloy, by weight of the total alloy: 22% to 28% by weight of chromium (Cr); 2.5 wt% to 3.5 wt% nickel (Ni); 1.7 wt% to 2.3 wt% silicon (Si); 0.9 wt% to 1.1 wt% of boron (B); 0.3 wt% to 1.8 wt% molybdenum (Mo); 0.35 wt% to 0.70 wt% manganese (Mn); 0.01 wt% to 0.20 wt% carbon (C); 1.5 wt. % or less of niobium (Nb); 0.2% by weight or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), Disclosed herein is an iron-chromium-based alloy, the balance of which consists of iron (Fe) and not more than 0.3 wt% of unavoidable impurities. In a preferred embodiment, carbon (C) is less than 0.15 wt%. In a more preferred embodiment, chromium (Cr) is 22 wt% to 25 wt%.

[0042] In the alloys of this invention, the total level of unwanted non-oxygen impurities must not exceed 0.4 wt. %. Nitrogen, as an unwanted impurity due to powder atomization, must not exceed 0.15 wt. %. Other unwanted non-oxygen impurities must not exceed 0.3 wt. %.

[0043] In embodiments of the present invention, less than 0.3 wt.% iron can be substituted with one or more of the inevitable impurities titanium, vanadium, aluminum, or tungsten without affecting the properties of the alloy, thereby reducing material procurement costs. Preferably, their contents are kept as low as possible, such as less than 0.2 wt.%, or more preferably less than 0.1 wt.%. Most preferably, however, none of these elements are present in the alloys of the present invention below the level of an insignificant impurity, since their near absence improves the material properties.

[0044] Generally, if the alloys of the present invention were not formulated as powders by water atomization, the content of further impurities would generally be less than 0.1 wt. %, with oxygen and nitrogen only being subsequently introduced into the alloy when the alloy was atomized into powder.

[0045] In one aspect of the present invention, an iron-chromium-based alloy in powder form is detailed herein.

[0046] In one embodiment thereof, detailed herein is an iron-chromium-based alloy in powder form, wherein oxygen (O) as an unavoidable impurity does not exceed 0.6 wt. % of the total weight of the alloy powder.

[0047] Therefore, according to embodiments of the present disclosure, the total content of unavoidable impurities should not exceed 0.8 wt.%, based on the total weight of the iron-chromium-base alloy, and preferably should 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-base alloy according to the present disclosure. More preferably, oxygen (O) and nitrogen (N) are present as unavoidable impurities only in amounts exceeding 0.05 wt.%, with oxygen (O) being present only in amounts not exceeding 0.3 wt.%, and nitrogen (N) being present only in amounts not exceeding 0.15 wt.% as an unavoidable impurity.

[0048] In a preferred embodiment of the present invention, the powdered iron-chromium-based alloy contains 80% by weight or more of the iron-chromium-based alloy powder contained in a sieved fraction of the iron-chromium-based alloy powder having a size distribution, as measured by sieving in accordance with ASTM B 214, of 1 μm to 100 μm, 2.5 μm to 90 μm, 5 μm to 80 μm, preferably 10 μm to 75 μm, or 15 μm to 70 μm, or more preferably 20 μm to 60 μm.

[0049] In a preferred embodiment of the present invention, the iron-chromium-based alloy in powder form comprises 80% by weight or more, 85% by weight or more, preferably 90% by weight or more, or more preferably 95% by weight or more of iron-chromium-based alloy powder having a size distribution of 10 μm to 80 μm as measured by sieving in accordance with ASTM B 214. More preferably, the iron-chromium-based alloy in powder form comprises 80% by weight or more, 85% by weight or more, preferably 90% by weight or more, or more preferably 95% by weight or more of iron-chromium-based alloy powder having a size distribution of 2.5 μm to 100 μm as measured by sieving in accordance with ASTM B 214, or even more preferably comprises 80% by weight or more, 85% by weight or more, preferably 90% by weight or more, or more preferably 95% by weight or more of iron-chromium-based alloy powder having a size distribution of 20 μm to 60 μm as measured by sieving in accordance with ASTM B 214. If particles with a size less than 20 μm need to be measured, the laser diffraction ASTM B822 method can be used to establish the particle size.

[0050] In a further aspect of the present invention, there is provided a composition for forming an iron-chromium-base alloy according to any aspect of the iron-chromium alloy detailed herein, comprising, by weight of the total composition: 20.5 wt% to 28.0 wt% chromium (Cr); 5.0 wt. % or less of nickel (Ni); 0.5 wt% to 2.5 wt% silicon (Si); 0.5% by weight to 1.5% by weight of boron (B); 0.15 wt% to 2.0 wt% molybdenum (Mo); 0.10 wt% to 0.90 wt% manganese (Mn); 0.01 wt% to 0.20 wt% carbon (C); 1.5 wt. % or less of niobium (Nb); 0.2% by weight or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), Compositions are detailed herein, with the balance consisting of iron (Fe) and not more than 0.3 wt. % incidental impurities.

[0051] In a preferred embodiment of the present composition, the composition is tailored to be compatible with any of the iron-chromium-based alloys disclosed herein.

[0052] Typically, although not necessarily, the individual elements of the composition are provided directly as elemental metals, however, in some embodiments, one or more of the components of the composition are pre-alloyed, for example, in the form of scrap metal from a recyclable source, before being added to the compositions of the present invention.

[0053] To form the alloys of the present invention, the compositions of the present invention are heated to a temperature above the melting point of their major constituent elements and the alloys of the present invention formed therefrom.

[0054] In a further aspect of the present invention, detailed herein is the use of a powder according to any embodiment detailed herein for coating a surface by laser cladding.

[0055] In a further aspect of the present invention, detailed herein is an iron-chromium-based alloy formed by a laser cladding process from an iron-chromium-based alloy according to any of the embodiments thereof detailed herein.

[0056] In a further aspect of the present invention, detailed herein is a surface coating comprising an iron-chromium-based alloy according to any of the embodiments thereof detailed herein.

[0057] In a further aspect of the present invention, detailed herein is a molding comprising a surface coating consisting of an iron-chromium-based alloy according to any of the embodiments thereof detailed herein.

[0058] In a further aspect of the invention there is provided a method of producing a coated surface or object, comprising the steps of: providing a powder according to one or more of the embodiments detailed herein in a form or shape suitable for laser cladding; performing a laser cladding process using the powder; obtaining a desired surface coating or a desired object; The method is detailed herein, including: [Example]

[0059] Example 1: Production of alloys and impurities contained therein In accordance with the present disclosure and invention, the following iron-chromium alloys in powder form were tested to assess their suitability for solving the objectives of the present disclosure (see Tables 1 and 2):

[0060] The alloys listed in Tables 1 and 2 were produced by co-melting of the components in a test furnace on an approximately 10 kg scale. Some tests were repeated in a larger 200 kg furnace. Alloys in powder form, such as those reported in Tables 1 and 2 for testing in laser cladding experiments, were alloyed and then atomized using one of the following methods: gas atomization (GA), water atomization (WA), or high-pressure water atomization (HPWA).

[0061] Particle size distribution measurement Particle size distribution was measured using a Ro-TAP sieve shaker or laser diffraction.

[0062] Using a Ro-TAP sieve shaker, powder particles were shaken by a vibrating motion through a stack of metal sieves with different openings. The weight of the powder on each sieve was then measured using a calibrated balance, and the fractional powder weight was normalized to the total powder weight. Sieve analysis using Ro-TAP sieves was performed in accordance with ASTM B 214.

[0063] Laser diffraction analysis was performed using a Sympatec analyzer. Measurements were performed according to ASTM B822.

[0064] Impurities detected in raw materials Impurities found in the raw materials include Cu, Co, Al, S, and P.

[0065] In experiments not reported herein, it was found that when copper (Cu) exceeded 0.2 wt. % in iron-chromium-based alloys, solidification cracks began to form on the laser cladding surface. Therefore, in this alloy, if copper (Cu) is present, the copper content should not exceed 0.2 wt. %, and preferably should not exceed 0.1 wt. % based on the total weight of the alloy. However, it is most preferred that copper be present only as an unavoidable impurity. In the experiments reported herein, copper (Cu) was essentially absent, i.e., below the analytical detection limit.

[0066] In experiments not reported herein, it was found that cobalt (Co) can be present in iron-chromium-based alloys up to 1.0 wt. % without affecting the properties of laser cladding surfaces coated with the alloys of the present disclosure. Thus, although the alloys of the present disclosure can contain up to 1.0 wt. % cobalt (Co) as a filler without any effect, it is highly undesirable to include more than 0.2 wt. % cobalt (Co) as a filler due to the carcinogenicity of cobalt-containing powders, which is undesirable for health and safety reasons. Most preferably, cobalt is present only as an unavoidable impurity. In the experiments reported herein, cobalt (Co) is essentially absent, i.e., below the analytical detection limit.

[0067] Aluminum (Al) was originally present in the raw materials tested, but was included as an incidental impurity at a maximum of 0.1 wt.% based on the total mass of the iron-chromium-based alloy of the present disclosure, and is not reported in this report because it does not affect the alloy of the present invention. Preferably, raw materials having only 0.05 wt.% aluminum as an incidental impurity were used for the experiments, but in the experiments reported herein, aluminum (Al) is essentially absent, i.e., below the analytical detection limit.

[0068] Phosphorus and sulfur, as unavoidable impurities in the alloys reported herein, were detected at levels below 0.05 wt.% each.

[0069] The alloys atomized by either gas atomization (GA), water atomization (WA), or high-pressure water atomization (HPWA) contained a maximum of 0.5 wt. % oxygen (O) as an unavoidable impurity and a maximum of 0.15 wt. % nitrogen (N) as an unavoidable impurity. Generally, the combined content of oxygen and nitrogen as unavoidable impurities did not exceed 0.3 wt. % based on the total weight of the iron-chromium-based alloy of the present disclosure, with combined contents of 0.25 wt. %, 0.20 wt. %, 0.15 wt. %, or 0.10 wt. % being obtained.

[0070] Therefore, according to embodiments of the present disclosure, the total content of unavoidable impurities should not exceed 0.8 wt.%, based on the total weight of the iron-chromium-base alloy, and preferably should 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-base alloy according to the present disclosure. More preferably, oxygen (O) and nitrogen (N) are present as unavoidable impurities only in amounts exceeding 0.05 wt.%, with oxygen (O) being present only in amounts of 0.3 wt.% or less as an unavoidable impurity and nitrogen (N) being present only in amounts of 0.15 wt.% or less as an unavoidable impurity. [Example]

[0071] Example 2 Alloys in powder form tested for laser cladding The alloys reported in Table 1 in powder form were prepared by water atomization of a melt having the alloy composition as reported in the table. Nitrogen and oxygen are impurity inclusions resulting from the water atomization process. All powder samples reported herein were produced by water atomization from the melt, except for sample A11, which was high-pressure water atomized, and sample A41, which was gas atomized. After atomization, the powders were dried and sieved to a size fraction of 20 μm to 63 μm, which was deemed suitable for subsequent laser cladding experiments. In the table, nd stands for not detected, and an * next to the sample number indicates that the sample is compared to the alloys of the present invention.

[0072] Sample A35 was compared to tested low chromium alloys that performed poorly with respect to the goals of the present disclosure, for example, when evaluated by the formation of microcracks and / or macrocracks as defined later in this specification. When the chromium content was lowered below the limits detailed herein, macrocracks began to form during laser cladding, leading to failure of the coating.

[0073] Overall, coatings with thicknesses of 100-350 μm are achieved with cladding speeds of 30-100 m / min and 0.5-1.5 m2 Coatings of this alloy were produced on a laboratory scale using a deposition rate of 1000 / hr. The coatings produced were free of cold cracks, exhibited hardnesses of 400-450 HV, and corrosion resistance of NSS>>96 hours.

[0074] [Table 1] [Example]

[0075] Example 3: Alloys in powder form containing niobium tested by laser cladding The alloys reported in Table 2 in powder form were prepared by water atomization of a melt having the alloy composition as reported in the table. Nitrogen and oxygen are impurity inclusions resulting from the water atomization process. All powder samples reported herein were produced by water atomization from the melt. After atomization, the powders were dried and sieved to a size fraction of 20 μm to 63 μm that was deemed suitable for subsequent laser cladding experiments. In the table, nd stands for not detected, and an * next to the sample number indicates that the sample is compared to the alloy of the present invention.

[0076] For the alloys of the present invention reported in Example 2 of the laser cladding experiments, it was found that while the alloys generally performed to specification, when either carbon or boron, or both, exceeded 0.15 wt. % (C) or 1.1 wt. % (B), respectively, or a total of both, exceeded 1.2 wt. %, an increased number of macrocracks or solidification cracks formed during laser cladding compared to other alloys of the present invention.

[0077] Subsequent testing of the alloys reported in Table 2 has shown that niobium (Nb) is suitable for suppressing crack formation, including visible cracks, in alloys with high carbon and / or boron contents (i.e., above the content limits given above), while at other concentrations of carbon, boron, or a combination of both carbon and boron, niobium is a neutral additive.

[0078] [Table 2]

[0079] Experimental setup and methods High-Speed ​​Laser Cladding In the context of the present invention, high speed laser cladding refers to a laser cladding process operating at a cladding speed of more than 1 m / min. In the experiments reported herein below, cladding speeds of 30 m / min and 100 m / min, respectively, were used.

[0080] Powders of the alloys reported in Tables 1 and 2 were (high-speed) laser clad according to the experimental setup and parameters detailed in Table 3. One- and two-layer coatings were produced using a HighNo 4 nozzle or a 6-jet GTV nozzle. A low-carbon steel rod with a diameter of 50 mm and a length of 200 mm was used for the experiments with the HighNo 4.0 nozzle, and a low-carbon steel rod with a diameter of 80 mm was used for the experiments with the 6-jet GTV nozzle.

[0081] For the evaluation of the coating properties of the steel rods, such as microstructure and hardness, evaluations were carried out on 30 mm long single layer coatings (also called clads in the art) produced using two cladding speeds of 30 mm / min and 100 m / min, respectively.

[0082] For the evaluation of the corrosion properties of steel rods, two-layer coatings of 90 mm length were produced using the same two cladding speeds as the one-layer coatings: 30 m / min and 100 m / min.

[0083] [Table 3]

[0084] Coating property evaluation - methods Sample coated using HighNo 4.0 nozzle A 30 mm long single layer coating produced using a HighNo 4.0 nozzle was tested for cracks using a dye penetrant.

[0085] Samples were cut perpendicular to the cladding direction, mold ground and polished using standard methods for metallographic sample preparation.

[0086] The hardness of the coating cross section was measured using a Vickers hardness of HV0.2 with a load of 200g. Seven indentations were made and the average value and variance were calculated.

[0087] To highlight the coating and to better distinguish it from the substrate, the samples were etched in Nital 4%. The quality of the coating was judged as follows: The number of pores and slugs with diameters between 50 and 25 μm was counted within an area of ​​approximately 35 mm × 0.25 mm. The number of "hot cracks", also known as microcracks, was assessed qualitatively by examining a 35mm x 0.25mm coating area at 5x magnification. · The number of microcracks was ranked from 1 to 5 according to Table 4.

[0088] [Table 4]

[0089] The microstructure of the coatings was further investigated by optical microscopy (LOM) and SEM. For LOM analysis, samples were etched with Villela's reagent (94 ml ethanol + 5 ml hydrochloric acid + 1 g picric acid). For some selected samples, EBSD-SEM analysis was performed. For EBSD-SEM analysis, unetched samples were polished for 20 minutes using colloidal silica (OP-U from Struers).

[0090] Neutral salt spray tests (NSS tests) were carried out on 90 mm long two-layer cladding according to ASTM B117 current edition (2022) according to the test conditions reported in Table 5 .

[0091] [Table 5]

[0092] Prior to the NSS test, the samples were ground to a surface roughness Ra of approximately 0.8–1 μm. During grinding, it was not possible to control the material removal, and the samples were ground until a smooth, uniform surface was obtained. Surface smoothness was determined visually.

[0093] Samples were stored in the NSS chamber for 7 days and analyzed after 24, 48, and 168 hours. Samples were qualitatively ranked according to ISO 10289 current edition (2022) using the ranking criteria given in Table 6.

[0094] [Table 6]

[0095] Sample coated using a 6-jet GTV nozzle Samples coated using the GTV nozzle were tested for cracks using a dye penetrant. Hardness HV0.3 was measured on a cross section of the coating, and the quality of the coating was judged using LOM.

[0096] Results and Discussion Coating property evaluation Calculated Thermodynamic Properties - Samples A1-A9 To gain a more detailed understanding of the coating properties, the thermodynamic properties of alloys A1-A9 in Table 1 were calculated (using the pre-alloying target values ​​according to Table 7, rather than the actual compositions experimentally determined after alloying). The phase contents and compositions of the alloys at 200°C below the solidus temperature were also calculated to assess the theoretical level of alloy stability to process variations under equilibrium conditions.

[0097] The melting interval (difference between solidus and liquidus) ΔT was calculated to assess the alloy's susceptibility to solidification cracking. The alloy's tendency to segregate was also captured by calculating the Scheil solidification interval (SSI) of the last 10% of the melt.

[0098] As is known in the art, some alloying elements have a strong tendency to segregate, resulting in wider melting intervals. The longer the melting interval, the more susceptible the alloy is to solidification cracking, or "hot cracking," but all of the alloys reported in Table 7 performed as expected.

[0099] PREN is calculated using the chemical composition of austenite at 200°C below the solidus temperature for all alloys except A2 (marked with *), which is calculated assuming a body-centered cubic matrix, using the following formula:

number

[0100] The parameters of the calculation are listed below. · Grain size set to 10μm. The intercritical annealing temperature was set at 200° below the solidus.

[0101] [Table 7]

[0102] Coating Properties HighNo 4.0 nozzle, cladding speed 30m / min and 100m / min Cladding speed 30m / min Tables 8 and 9 summarize the experimental results of various coatings prepared using a HighNo 4.0 nozzle at a cladding speed of 30 m / min in the presence (Table 8) and absence (Table 9) of niobium (Nb) in the alloy.

[0103] Notably, all of the alloys disclosed herein, except for alloys A7, A8, and A35, clad the sample surfaces without macrocrack formation. The failure of alloys A7 and A8 to prevent macrocrack formation was found to be correlated with their high combined boron and carbon contents, exceeding 1.2 wt.%, which can be compensated for by adding niobium, as shown in Table 9. Alloy A35, which has a chromium content of 18.70 wt.%, is used only for comparison with the alloys of the present invention. This is because it was found that when chromium is present outside the limits estimated herein, adjusting the contents of other elements associated with this alloy within the limits detailed herein does not suppress macrocrack formation.

[0104] [Table 8]

[0105] [Table 9]

[0106] Cladding speed 100m / min Experiments performed at a cladding speed of 100 m / min replicated the 30 m / min cladding speed experiments, demonstrating the suitability of this alloy for very high-speed laser cladding. Alloys A7 and A8 were again found to have numerous macrocracks (test score of 5), which were again fully compensated for by the addition of niobium. Surfaces coated with alloy A35 also achieved a test score of 5, which could not be compensated for by adjusting the other elements of the alloy. Some of the other alloys showed slightly worse scores than at 30 m / min, highlighting the need to individually optimize the cladding speed for a given alloy.

[0107] Coating characteristics - GTV nozzle, cladding speed 50m / min In a small-scale experimental study, alloys A1 to A11 (except A9) were tested using a GTV nozzle at a cladding speed of 50 m / min (see Table 10). The results were found to be comparable to those of a HighNo 4.0 nozzle at a cladding speed of 100 m / min.

[0108] [Table 10]

[0109] Consideration hardness Figure 1 shows the hardness HV0.2 of alloys A1-A8, A10-A12, and A20-A24 clad at 30 m / min and 100 m / min using a HighNo 4.0 nozzle, and at 50 m / min with a GTV nozzle. In Figure 1, the striped bars of alloys A7 and A8 reflect that these two alloys exhibited macrocracks after cladding.

[0110] It can be observed that for the same alloy chemistry, the hardness HV0.2 is in the same range, regardless of the cladding speed and nozzle used. The hardness of cladding produced using the GTV nozzle is slightly lower, likely due to the slower solidification rate. Furthermore, the hardness HV0.2 generally varies from 350 to 500. These hardness variations are large, but can be explained by the large variation in carbon and boron content of the alloys investigated, which is consistent with the alloys with the lowest carbon and boron contents having the lowest hardness and the alloys with the highest carbon and boron contents having the highest hardness.

[0111] In Figure 2, the hardness of alloys A1-A8 and A10-A12 is plotted against the calculated volume fraction of boride and austenite at 200°C below the solidus temperature to examine the correlation between the simulated microstructure and the observed coating hardness. In Figure 2, the coating hardness versus the volume fraction of boride and fcc was calculated at 200°C below the solidus, with the remaining phase being bcc. Calculations were performed using the nominal composition of the alloys. The volume fraction of boride in the range of chemical compositions investigated increased with increasing boron content.

[0112] Significant variations in fcc content were observed in the calculated results. In the alloys, the fcc filler phase transforms to martensite during cooling, which is expected to contribute to hardness. Therefore, alloys with a higher initial fcc content are expected to be harder. However, a clear relationship between the austenite volume fraction and the measured hardness could not be established.

[0113] To reduce the risk of macrocrack formation, it was decided to add 0.5-1.0 wt% Nb. Niobium is a strong carbide former. When primary carbides form in the melt, the austenite matrix becomes carbon-depleted and "softer" martensite is expected to form.

[0114] Figure 3 shows the hardness of alloys A26–A34 containing 0.5–1.0 wt% Nb, clad at 30 and 100 m / min using a HighNo 4.0 nozzle. The dotted staples indicate coatings with macrocracks. Alloys A31 and A32, with the highest C and B contents (C = 0.17 wt% and B = 1.30 wt%), cracked when clad at 100 m / min using a HighNo 4.0 nozzle. Alloys A26, with C = 0.16 wt% and B = 1.15 wt%, and Alloy A17, with C = 0.16 wt% and B = 1.17 wt%, did not crack. Alloys A7 and A8, with chemical compositions similar to A26 and A27 (but without Nb), did exhibit cracking. Therefore, the addition of Nb is beneficial in suppressing crack formation, especially at high carbon and / or boride contents.

[0115] Analysis of the coating microstructure Pores and slag Pores were found in all coatings, and in cladding with the HighNo 4.0 nozzle, the pore size was typically less than 50 μm. The number of pores and slag in the investigated coatings was counted, but no correlation could be found between the chemical composition of the alloy (e.g., Si and O content) and the number of pores (see Tables 7-10).

[0116] Hot Crack A qualitative assessment of the number of microcracks in the coating was performed for alloys A1-A12, A16, A17, and A20-A24. The results are shown in Figure 4.

[0117] In Figure 5, the number of microcracks was plotted against the solidification extent calculated for the last 10% of the melt using Sheil simulations to obtain an indication of the alloy's segregation tendency. The melting extent was very similar for all alloys except A3, indicating similar segregation tendencies in the melt. No clear correlation was found between the melting extent and the number of microcracks in the coating.

[0118] By plotting the number of microcracks against the boron content, as done in Figure 6, it could be observed that the number of microcracks was highest at boron contents less than 0.9%. No relationship was found between the number of microcracks and Si or C (see Table 11). Figure 6 shows that to minimize the number of hot cracks, the boron content should not be below 0.9 wt%, preferably not below 0.95 wt%, and the maximum B content should not exceed 1.5 wt%.

[0119] For cost-effective selection of raw materials and capability of the manufacturing process, carbon must be allowed to vary between 0.05 and 0.15 wt.%, so if both carbon and boron approach the upper specification limits simultaneously, there is a risk of macrocrack formation, and niobium (Nb) must be added to mitigate this risk.

[0120] [Table 11]

[0121] Microstructure The microstructure of the unetched coatings was examined for pore counts, oxides, and microcracks. Depending on the chemical composition, the coatings exhibited different numbers of microcracks, as shown in Figures 4 and 5. Overall, samples clad at 30 m / min exhibited fewer small defects than samples clad at 100 m / min.

[0122] Typical microstructures of coatings with few pores and cracks, and those with numerous microcracks and pores, are shown in Figures 7 and 8. Figure 7a shows a coating of alloy A3 at 30 m / min, indicating good coating quality with few visible pores, while Figure 7b shows a coating of alloy A5 at 30 m / min, indicating a poor quality coating with several microcracks and pores. Meanwhile, Figure 8 shows similar results for the same alloys A3 and A5 at 100 m / min (see also Tables 7-10).

[0123] The samples were etched with Vilela's reagent to check the coating microstructure. All coatings exhibited a very fine microstructure that could not be further resolved by LOM. The microstructure was uniform for all alloys except A7, which was clad at 30 m / min (see Figure 9, which shows two examples of the microstructure of coatings clad at 30 m / min and 100 m / min for alloys A10 and A7, respectively, using a HighNo 4.0 nozzle). A7 showed a tendency to form a layered structure, with the light etched areas (HV ~ 600) being harder than the dark etched areas (HV ~ 450). Based on thermodynamic analysis, this alloy formed the greatest amount of fcc, suggesting that the observed fine stratification may be related to the separation of the coating between the fcc phase and other alloy phases. The tendency for alloy A7 to form a layered structure was greatest when using a cladding speed of 30 m / min.

[0124] Figure 10 shows the microstructure of the A10 coating at high magnification at both 30 and 100 m / min, and at the highest magnification, LOM was unable to resolve the microstructure.

[0125] Based on thermodynamics, significant fcc-bcc variation is expected for the alloys investigated. Nevertheless, the alloy properties remain within the desired target parameters. Because the microstructure could not be resolved by LOM, SEM EBSD analysis was performed on two alloys (A7 alloy with high austenite stabilizing elements and one with high ferrite stabilizing elements) and on alloys of the target chemistry clad at 100 m / min.

[0126] Overviews of the coating microstructures are shown in Figures 11–13 for alloys A1 (Figure 11), A7 (Figure 12), and A10 (Figure 13), respectively. In Figures 11–13, the figures labeled A are overviews of the coating microstructures observed by LOM, labeled B is the SEM EBSD map, and labeled C is the Euler map. The EBSD maps indicate that the microstructure of A1 (Figures 11B and 11C) was composed of columnar primary grains of the bcc phase, while A7 (Figures 12B and 12C) was composed of more equiaxed primary grains of bcc and fcc phases present primarily in the overlap region and near the substrate. In alloy A10 (Figures 13B and 13C), the amount of bcc and fcc phases and the size of the primary grains were intermediate between those of A1 and A7. For alloy A1, the SEM EBSD map (Figure 11B) shows a predominantly bcc structure, with the black dots representing uncharacterized structures. The Euler map (Figure 11C) showed elongated primary grains. For alloy A7, the SEM EBSD map (Figure 12B) showed bcc (red) and fcc (blue) structures, with the black dots representing unresolved structures. The Euler map (Figure 12C) showed equiaxed primary grains. For alloy A10, the SEM EBSD map (Figure 13B) showed bcc (red) and fcc (blue) alignments, with the black dots representing unresolved structures. The Euler map (Figure 13C) showed equiaxed primary grains.

[0127] By viewing the microstructures of A1 and A7 at higher magnification, we observed that the bcc structure of A1 exhibited a small number of defects, indicated by the light gray pattern of contrast bands. This indicates that the microstructure consisted of ferrite and eutectic structures. In the case of A7, instead, the contrast bands indicated areas with fewer defects (light gray in the contrast band map) and areas with more defects (dark gray in the same map). The light gray area consisted of ferrite and eutectic structures located in the center of the tracks, while the dark gray area was located in the overlap between the two tracks, likely consisting of martensite, retained austenite, and eutectic structures.

[0128] The columnar primary grains of A1 make the alloy more susceptible to hot crack formation. The difference in primary grain size and geometry may explain why alloy A1 is more prone to hot crack formation than alloy A7.

[0129] Corrosion Testing All alloys investigated were tested for corrosion. The results are summarized in Table 12. See the reference photograph in Figure 7 and Tables 4-6 for the criteria used to evaluate the samples.

[0130] [Table 12]

[0131] Figure 14 shows exemplary samples in increasing order of corrosion rating from A to E after 7 days of exposure in the NSS chamber. A = rating 0, multiple macrocracks in the coating; B = alloy A5 coated at 30 m / min, rating 5, severe corrosion; C = alloy A21 coated at 30 m / min, rating 6-7, moderate corrosion; D = alloy A26 coated at 30 m / min, rating 8, slight corrosion; and E = alloy A31 coated at 30 m / min, rating 9, very slight corrosion.

[0132] Some of the alloys investigated showed insufficient corrosion resistance in neutral salt spray. By plotting the corrosion rate against the boron (Figure 15) and chromium (Figure 16) content, respectively, it can be observed that the alloys with the highest corrosion rate exhibit the lowest boron content and the largest number of hot cracks, indicating that hot cracks affect the corrosion resistance of the alloys. In the case of alloys ranked 8 and 9, few corrosion spots were detected on the surface. The spots appeared already after the first day and usually did not become larger after one week of testing.

[0133] ThermoCalc results The inventors developed this invention based on the consideration that, depending on the composition, the fcc phase may transform into martensite during cooling. A large amount of martensite increases the residual stress in the material and increases the risk of cracking. Similarly, the inventors considered that, as a guideline, too high a level of bcc iron also increases the risk of hot cracking.

[0134] To determine the appropriate chromium range based on the above guidelines, we investigated three iron-chromium alloy compositions for theoretically predicted phase behavior. Calculations were performed using Thermo-Calc software with the commercially available database TCFE9. 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 (present invention); and C) Fe-29Cr-3Ni-2Si-1B-2Mo-0.5-0.09C (see Figure 17).

[0135] A)Fe-20.8Cr-2.8Ni-1.6Si-1.3B-0.4Mo-0.66Mn-0.16C FIG. 17A shows the equilibrium calculation for the above composition (Fe-20.8Cr-2.8Ni-1.6Si-1.3B-0.4Mo-0.66Mn-0.16C), which was subsequently shown in the experiments reported above (see Alloy A7) to be unsuitable for high-speed laser cladding due to cracking problems.

[0136] What is noteworthy about the illustrated phase diagram is the absence of a bcc phase in the structure.

[0137] B)Fe-22.5Cr-3Ni-2Si-1B-0.4Mo-0.6Mn-0.09C FIG. 17B shows the equilibrium calculation for the above composition (Fe-22.5Cr-3Ni-2Si-1B-0.4Mo-0.6Mn-0.09C), which in development work was found to be suitable for high-speed laser cladding (see Alloy A10).

[0138] As can be seen, this material is not predicted to completely transform to fcc upon cooling, but does exhibit a good balance between the bcc and fcc phases. The maximum equilibrium ratio for the fcc phase is approximately 0.45.

[0139] C)Fe-29Cr-3Ni-2Si-1B-2Mo-0.5-0.09C Figure 17C shows the equilibrium calculation for the above composition (Fe-29Cr-3Ni-2Si-1B-2Mo-0.5Mn-0.09C). As can be seen, for this composition, no fcc phase is predicted; the matrix is ​​entirely ferritic (fcc). Consequently, although high corrosion resistance is achieved with high chromium and molybdenum contents, this increases the proportion of bcc phase, which is too soft for the desired hardness for the current intended application. As a result, the alloy was not tested in any experiments.

[0140] Overall, the presently disclosed alloys explore the desirable opportunity to offset the adverse effects of chromium (and molybdenum) on the hardness required for the intended application by gaining reduced crack formation and the resulting ability to form thinner coatings in laser cladding procedures, while maintaining corrosion resistance and usable hardness.

[0141] Design Considerations and Conclusions Final optimization tests of the alloy chemistry were carried out in terms of cost and properties, and the results are reported in Table 13.

[0142] The test took into account nickel, an expensive alloying element that stabilizes austenite. During cooling, the austenite transforms into martensite, which contributes to the hardness of the coating. To confirm whether the addition of nickel contributes to the hardness of the coating, a pre-alloyed alloy (A25) containing the target chemistry but without nickel was atomized and subjected to high-speed laser cladding. The hardness of the coating was not significantly affected, indicating that the addition of nickel is not critical to the hardness of the coating. The number of microcracks in the coating was larger compared to the alloy with the target chemistry. The reason for this may be the formation of columnar ferrite grains during solidification.

[0143] The tests further considered that carbon, like nickel, contributes to the coating's hardness. Carbon stabilizes austenite, which transforms to martensite during cooling, forming carbides. Due to the high solidification rate of the high-speed laser cladding process, the effect of carbon on the coating's hardness is unclear. The alloy exhibited hardness close to that of the alloy with the optimized chemical composition. Since both alloys with and without carbon and with and without nickel exhibited similar hardness, it can be assumed that borides and a fine grain structure are the main contributors to the coating's hardness.

[0144] Testing further considered chromium, which contributes to the alloy's corrosion resistance. However, because chromium stabilizes ferrite, further increasing the target chromium content from 23 wt% to 25 wt% could decrease the coating's hardness. Therefore, an alloy containing 25 wt% chromium and the target chemical composition of the remaining elements was investigated (A36). The coating's hardness and microstructure were comparable to those of the alloy containing the target chemical composition, and corrosion resistance was improved.

[0145] Molybdenum was further considered in the tests, as it was known from the literature that it improves pitting corrosion resistance. However, since molybdenum is a ferrite stabilizing element, the addition of molybdenum may decrease the hardness of the coating. Therefore, an alloy with a target molybdenum content of 1.5 wt.% was investigated (A40). The addition of molybdenum did not significantly affect the hardness of the coating.

[0146] [Table 13]

[0147] In conclusion, therefore, and in accordance with the present invention, it has been found that while all of the alloys detailed herein are suitable for high speed cladding, alloys falling within the limits given under Table 14 have been found to be particularly effective and compliant for the purposes of the present invention.

[0148] [Table 14]

[0149] When niobium forms part of the alloy of the present invention, the carbon and boron contents may be higher, as detailed hereinabove.

[0150] Conclusion Although the invention has been described in detail for purposes of illustration, it will be understood that such detail is for that purpose only and that, from the drawings, the disclosure, and the appended claims, those skilled in the art can make variations in the invention in practicing the claimed subject matter.

[0151] Where used in the claims, the term "comprising" does not exclude other elements or steps. Where used in the claims, the indefinite articles "a" or "an" do not exclude a plurality. Any reference signs used in the claims shall not be construed as limiting the scope thereof.

Claims

1. An iron-chromium-based alloy, comprising, by weight of the total alloy: 20.5 wt% to 28.0 wt% chromium (Cr); 5.0 wt. % or less of nickel (Ni); 0.5 wt.% to 2.5 wt.% silicon (Si); 0.5% to 1.1% by weight of boron (B); 0.15 wt.% to 2.0 wt.% molybdenum (Mo); 0.10 wt.% to 0.90 wt.% manganese (Mn); 0.01 wt.% to 0.15 wt.% carbon (C); 0.2 wt. % or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), the balance consisting of iron (Fe) and unavoidable impurities including niobium, the unavoidable impurities not exceeding 0.3 wt.%; An iron-chromium-based alloy having a total carbon and boron content not exceeding 1.20% by weight.

2. 2. The iron-chromium-based alloy of claim 1, wherein copper (Cu) is present in an amount of not more than 0.1 wt. %, preferably copper being present as an unavoidable impurity.

3. 3. An iron-chromium based alloy according to claim 1 or 2, wherein cobalt (Co) is present in an amount of not more than 0.2 wt%, preferably not more than 0.1 wt%, more preferably cobalt is present as an unavoidable impurity.

4. 4. An iron-chromium based alloy according to any one of claims 1 to 3, wherein chromium (Cr) is present in an amount of 21 wt% to 26 wt%, 22 wt% to 25 wt%, 22.5 wt% to 24.5 wt%, or preferably 23 wt% to 24 wt%.

5. 5. An iron-chromium based alloy according to any one of the preceding claims, wherein nickel (Ni) is present in an amount between 2.15wt% and 3.85wt%, between 2.25wt% and 3.75wt%, between 2.35wt% and 3.65wt%, between 2.50wt% and 3.50wt%, between 2.65wt% and 3.35wt%, or preferably between 2.75wt% and 3.25wt%.

6. 6. An iron-chromium based alloy according to any one of the preceding claims, wherein silicon (Si) is present in an amount from 0.75wt% to 2.45wt%, from 1.0wt% to 2.4wt%, from 1.25wt% to 2.35wt%, from 1.5wt% to 2.3wt%, from 1.6wt% to 2.25wt%, preferably from 1.7wt% to 2.2wt%, from 1.8wt% to 2.15wt%, or more preferably from 1.9wt% to 2.1wt%.

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

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

9. 9. An iron-chromium based alloy according to any one of claims 1 to 8, wherein molybdenum (Mo) is present in an amount of at least 0.20 wt%, at least 0.25 wt%, at least 0.30 wt%, at least 0.35 wt%, at least 0.40 wt%, at least 0.45 wt%, at least 0.50 wt%, at least 0.55 wt%, at least 0.60 wt%, at least 0.65 wt%, or at least 0.70 wt%.

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

11. Manganese (Mn) 0.35 wt.% or more, 0.40 wt.% or more, 0.45 wt.% or more, or 0.50 wt.% or more; and 0.85% by weight or less, 0.80% by weight or less, 0.75% by weight or less, 0.70% by weight or less, 0.65% by weight or less, 0.60% by weight or less, 0.55% by weight or less, or 0.50% by weight or less; An iron-chromium based alloy according to any one of the preceding claims, preferably present in an amount of from 0.30 to 0.70 wt.%, or from 0.40 to 0.60 wt.%.

12. The carbon is present in an amount of 0.01% to 0.15% by weight. Preferably, the carbon (C) is 0.02 wt% or more, 0.03 wt% or more, 0.04 wt% or more, 0.05 wt% or more, 0.06 wt% or more, 0.07 wt% or more, 0.08 wt% or more, 0.09 wt% or more, or 0.10 wt% or more; and 0.15% by weight or less, 0.14% by weight or less, 0.13% by weight or less, 0.12% by weight or less, 0.11% by weight or less, or 0.10% by weight or less; An iron-chromium based alloy according to any one of the preceding claims, preferably present in an amount of from 0.05% to 0.15% by weight.

13. The iron-chromium-based alloy according to any one of claims 1 to 12, in powder form.

14. 14. An iron-chromium based alloy in powder form according to claim 13, formed by an atomization process, preferably by a water atomization process.

15. 15. The iron-chromium-based alloy in powder form according to claim 14, wherein oxygen (O) as an unavoidable impurity introduced in the atomization process does not exceed 0.6 wt. % of the total weight of the alloy powder.

16. 16. The iron-chromium-based alloy in powder form according to any one of claims 13 to 15, wherein the iron-chromium-based alloy in powder form comprises 80% by weight or more of iron-chromium-based alloy powder contained in a sieved fraction of the iron-chromium-based alloy powder having a size distribution, as measured by sieving in accordance with ASTM B 214, of 1 μm to 100 μm, 2.5 μm to 90 μm, 5 μm to 80 μm, preferably 10 μm to 75 μm, or 15 μm to 70 μm, or more preferably 20 μm to 60 μm.

17. 17. An iron-chromium-based alloy in powder form according to any one of claims 13 to 16, comprising 80% by weight or more, 85% by weight or more, preferably 90% by weight or more, or more preferably 95% by weight or more of iron-chromium-based alloy powder having a size distribution of 2.5 μm to 100 μm as measured by sieving in accordance with ASTM B 214.

18. Use of the powder according to any one of claims 13 to 17 for surface coating by laser cladding.

19. An iron-chromium-based alloy formed by a laser cladding method from the iron-chromium-based alloy according to any one of claims 1 to 12.

20. A surface coating comprising the iron-chromium-based alloy according to any one of claims 1 to 12.

21. A molded article comprising a surface coating made of the iron-chromium-based alloy according to any one of claims 1 to 12.

22. 22. A method for producing a coated surface according to claim 20 or a molded article according to claim 21, comprising the steps of: Providing a powder according to one or more of claims 13 to 17 in a form or shape suitable for laser cladding; performing a laser cladding process using the powder; obtaining the surface coating or the molding; A method comprising:

23. A composition for forming an iron-chromium-based alloy according to any one of claims 1 to 12, comprising, by total weight, 20.5 wt% to 28.0 wt% chromium (Cr); 5.0 wt. % or less of nickel (Ni); 0.5 wt.% to 2.5 wt.% silicon (Si); 0.5% to 1.1% by weight of boron (B); 0.15 wt.% to 2.0 wt.% molybdenum (Mo); 0.10 wt.% to 0.90 wt.% manganese (Mn); 0.01 wt.% to 0.15 wt.% carbon (C); 0.2 wt. % or less of copper (Cu); and 1.0 wt. % or less of cobalt (Co), the balance consisting of iron (Fe) and unavoidable impurities including niobium, the unavoidable impurities not exceeding 0.3 wt.%; A composition having a total carbon and boron content not exceeding 1.20 wt.%.

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