Solid solution held chromium carbides for thermal spraying and method of making the same

Abstract

In the present disclosure there is detailed a chromium-based powder consisting by total weight of powder of: Iron (Fe) : 15 – 18 wt%, Carbon (C) : 4.7 – 5.5 wt%, Tungsten (W) : 2.2 – 3.4 wt%, Molybdenum (Mo): 1.4 – 2.3 wt%, Boron (B) : 0.3 – 1.0 wt%, Silicon (Si) : 0.1 – 1.0 wt%, Niobium (Nb) : 0.35 – 0.60 wt%, the balance being chromium (Cr) and unavoidable impurities not exceeding 0.3 wt%, wherein nickel (Ni) and / or cobalt, if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt%, which powder, when used in a HVOF and / or a HVAF process, provides primarily chromium and tungsten carbides in solid solution in a BCC-phase iron- chrome stainless-steel alloy. As the matrix alloy is free of nickel and cobalt it provides a sustainable yet cost beneficial replacement for conventional binders for carbide products in particular for uses where nickel and / or cobalt contamination is undesired.

Description

[0001] TITLE OF INVENTION

[0002] Solid solution held chromium carbides for thermal spraying and method of making the same.

[0003] TECHNICAL FIELD

[0004] In the field of powder technology for HVOF and HVAF applications there is suggested a range of solid solution held chromium carbides for thermal spraying together with method of making the same and uses thereof.

[0005] BACKGROUND

[0006] Tungsten / chromium Carbide powders are hard materials (hardness »1000 HV) , used for dense coatings. They can be sprayed using High-Velocity Oxygen-Fuel (HVOF) and High- Velocity Air-Fuel (HVAF) spray methods onto various metallic parts and structures to improve the surface resistance of these parts and structures to severe wear and corrosion. Conventional carbide powders consist of hard WC or CraC2 particles in a binder matrix, for which the CoCr powders are used. Different alloying elements, e.g., Ni, Fe, Ti, etc., may be added to the matrix to improve mechanical, corrosion, and spray-ability properties, c.f. e.g., J. Garcia et al. (J. Garcia, V. Collado Cipres, A. Blomqvist and B. Kaplan, "Cemented carbide microstructures: a review," International Journal of Refractory Metals and Hard Materials, vol. 80, pp. 40-68, 2019.)

[0007] In the art, other inventors have delved into the general technical field of the present inventions, c.f. e.g. US 8659934, JR 2003253405, US 2022 / 389549, EP 4112222, or S. Houdkova et . al., J. Thermal Spray Tech, Sept. 13, 2017. In the recent years, use of some elements, such as Co and Ni in particular, has become of concern due to their harmful characteristics to the environment and health and there have been attempts to replace these elements by less harmful elements in the binder, c.f., e.g., M. Walbriihl et al. (M Walbriihl, D. Linder, K. Agren and A. Borgenstam, "Diffusion modeling in cemented carbides: Solubility assessment for Co, Fe and Ni binder systems," International Journal of Refractory Metals and Hard Materials, vol. 68, pp . 41-48, 2017) .

[0008] It is known that conventional binders can be replaced by high-alloyed steels, or atomized powders, c.f., e.g., J. Garcia et al. Herein, the present inventors introduce new alloys that suitable for use as hard face coatings prepared by thermal spraying from an atomized powder. In the type of material introduced, instead of conventional carbide / binder concept, hard carbides precipitate in solid solution as a coherent phase during cooling after melting in the herein detailed matrix BCC-phase iron-chromium alloy, as the solid solution .

[0009] Since the detailed alloys are essentially free of hazardous species, i.e., Ni and Co, they are excellent candidates for all industries wherein sustainability and trace mineral contamination is a concern. Food industries for example are promoting a new class of "new food" where coating made from the present alloys can be used for packaging materials. Also, they are strong candidates for coating different components in aerospace or power plants, e.g., sliding tubes, sliding pistons, axles, bolts, bushings, flanges, etc., as well as having potential uses for coating grinding rolls, crushers, and calender rolls in the relevant industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1: Phase diagram for the molten alloy in the temperature as a function of carbon-content calculated using Thermo-Calc.

[0011] Figure 2: EM-cross section of pre-alloyed, gas atomized powder according to the invention. A) powder particle, B) enlarged section of particle.

[0012] Figure 3: Material consumption versus coverage rate for different spray techniques and parameters, with comparative .

[0013] Figure 4: Cross section of the coating achieved by JP spraying, in different magnifications. A) scalebar 100 pm, B) scalebar 50 pm.

[0014] Figure 5: Cross section of the coating achieved by DJ spraying, in different magnifications. A) scalebar 100 pm, B) scalebar 50 pm.

[0015] Figure 6: NSS tested coupons after A) 168 hours for JP- sprayed and B) after 504 hours for DJ-sprayed s amp les.

[0016] Figure 7: Gas permeability test results for coated samples.

[0017] Figure 8: Cavitation test results versus material consumption for coated samples.

[0018] Figure 9: Hardness measurements for sprayed samples using different techniques and parameters.

[0019] Figure 10: Roughness measurements for sprayed samples using different techniques and parameters.

[0020] Figure 11: Young modulus measurements for sprayed samples using different techniques and parameters.

[0021] Figure 12: Material consumption for spraying versus wear resistance for sprayed samples.

[0022] Figure 13: SEM image and EDX map of elements showing different types of carbides in a BCC-matrix. It is to be understood, that the embodiments shown in the figures are for illustration of the present invention and cannot be construed as being limiting on the present invention. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure.

[0023] DETAILED DESCRIPTION

[0024] In a first aspect of the present invention and embodiment thereof, a BCC-phase iron-chrome stainless-steel alloy consisting by total weight of alloy of:

[0025] Iron (Fe) 44.0 - 47.0 wt % ,

[0026] Silicon (Si) 5.0 - 7.0 wt%,

[0027] Molybdenum (Mo) 0.60 - 0.80 wt%,

[0028] Tungsten (W) 0.40 - 0.60 wt%,

[0029] Carbon (C) 0.01 - 0.10 wt % ,

[0030] Boron (B) 0.001 - 0.010 wt %

[0031] Niobium (Nb) 0.005 - 0.010 wt % the balance being chromium (Cr) and unavoidable impurities not exceeding 0.20 wt%, wherein nickel (Ni) and / or cobalt (Co) , if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt%. It is preferable that the amount of unavoidable impurities does not exceed 0.15 wt% if any of nickel (Ni) and / or cobalt (Co) is present, and does not exceed 0.1 wt% if any of nickel (Ni) and / or cobalt (Co) are essentially absent, i.e. nickel and / or cobalt being present to a level of trace elements only.

[0032] In a preferred aspect thereof, the BCC-phase iron-chrome stainless-steel alloy of the invention is present as a powder. In particularly preferred embodiments thereof, the stainless-steel powder comprises at least 80% by weight of the stainless-steel powder contained within a sieved fraction of the stainless-steel powder having a size distribution from 1 pm to 100 pm, preferably from 2.5 pm to 75 pm, or even more preferably from 5 pm to 50 pm, as measured by sieving in accordance with ISO-14232-1-2017-E, Particle distribution by sieving, and / or preferably comprises at least 85% by weight, at least 90% by weight, or more preferably at least 95% by weight of the stainless-steel powder having a size distribution from 2.5 pm to 100 pm as measured by sieving in accordance with ISO-14232-1-2017-E, Particle distribution by sieving .

[0033] In embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, iron (Fe) is present in the alloy from 44.5 wt% to 46.5 wt%, preferably from 45.0 wt% to 46.0 wt%, more preferably from 45.4 wt% to 45.9 wt% .

[0034] In further embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, silicon (Si) is present in the alloy from 5.3 wt% to 6.7 wt%, preferably from 5.5 wt% to 6.3 wt%, more preferably from 5.7 wt % to 6.1 wt % .

[0035] In further embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, molybdenum (Mo) is present in the alloy from 0.62 wt% to 0.78 wt%, preferably from 0.65 wt% to 0.75 wt%, more preferably from 0.63 wt% to 0.73 wt%.

[0036] In further embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, tungsten (W) is present in the alloy from 0.42 wt% to 0.58 wt%, preferably from 0.45 wt% to 0.55 wt%, more preferably from 0.47 wt% to 0.53 wt%.

[0037] In further embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, carbon (C) is present from 0.02 wt% to 0.09 wt%, preferably from 0.03 to 0.07 wt%, or more preferably from 0.04 wt% to 0.06 wt% .

[0038] In further embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, boron (B) is present from 0.002 wt% to 0.009 wt%, preferably from 0.003 wt% to 0.007 wt%, more preferably from 0.004 wt% to 0.006 wt% .

[0039] In further embodiments of the BCC-phase iron-chrome stainless-steel alloy according to any embodiment herein, niobium (Nb) is present from 0.006 wt% to 0.009 wt%, preferably from 0.007 wt% to 0.008 wt%.

[0040] In general, the elements carbon, boron, and niobium were found to stabilize the formation of the BCC-phase and consequently are needed in the alloys even if only in trace amounts. Contrary thereto, nickel and cobalt were found to be neutral elements in the present stainless steels, even if present in higher trace amounts than carbon, boron or niobium. The elements nickel and cobalt are herein eliminated not for their function, but for their undesirable environmental properties in accordance with the underlying contemplations of the present inventions.

[0041] Manufacturing the BCC-phase iron-chrome stainless-steel alloy can be done by providing in an embodiment of the present invention, a composition consisting by total weight of composition of:

[0042] Iron (Fe) 44.0 - 47.0 wt % ,

[0043] Silicon (Si) 5.0 - 7.0 wt%,

[0044] Molybdenum (Mo) 0.60 - 0.80 wt%, Tungsten (W) 0.40 - 0.60 wt%, Carbon (C) 0.01 - 0.10 wt % , Boron (B) 0.001 - 0.010 wt % Niobium (Nb) 0.005 - 0.010 wt % the balance being chromium (Cr) and unavoidable impurities not exceeding 0.20 wt%, wherein nickel (Ni) and / or cobalt (Co) , if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt%; wherein the composition is subsequently melted thereby permitting alloying of the elements. It is preferable that the amount of unavoidable impurities does not exceed 0.15 wt% if any of nickel (Ni) and / or cobalt (Co) is present, and does not exceed 0.1 wt% if any of nickel (Ni) and / or cobalt (Co) are essentially absent, i.e. on the level of trace elements only.

[0045] As is obvious from the above description, the concentrations of the individual elements of the composition can be adjusted identically to their desired final concentration in the alloy, within the above given respective elemental concentrations for the alloy, in accordance with the above given ranges for the concentrations of the respective elements in the resulting stainless steel.

[0046] As detailed in the below experimental section, the stainless- steel alloys of the present invention are useful as solid solution matrix alloys for various carbides, in particular for chromium and tungsten carbides. The alloys of the invention can be manufactured as powders for HVOF and / or HVAF spray coating by pre-alloying the elements of the alloy, atomi z ing, e . g . , water-atomi z ing, the pre-alloyed elements , whereby a pre-alloyed stainless-steel powder containing the alloys of the invention is obtained .

[0047] An advantage of the alloys of the present invent ion is that they can be mixed with carbides of a suitable powder si ze for HVOF and / or HVAF spray coating and spray coated together directly onto a suitable surface . Likewise , other thermal spray methods beyond HVOF thermal spraying and HVAF thermal spraying, have been tested and found useful with the present alloy powders , such as plasma spraying, laser cladding, high speed laser cladding, and / or ultra-high speed laser cladding .

[0048] Consequently, there is herein detailed the use of a powder composition comprising a pre-alloyed BCC-phase iron-chrome stainless-steel alloy powder in an embodiment as herein detailed above , the powder further comprising a metal carbide powder, for the coating of a surface by means of a thermal spray method .

[0049] And further, in a further aspect and embodiments thereof , there is herein detailed a powder composition comprising a pre-alloyed BCC-phase iron-chrome stainless-steel alloy powder in an embodiment as herein detailed above , the powder further comprising a metal carbide powder .

[0050] In particularly preferred embodiments thereof , the powder composition comprising a pre-alloyed BCC-phase iron-chrome stainless-steel powder and a metal carbide powder as detailed above the powder composition comprises at least 80% by weight of the powder composition contained within a sieved fraction of the powder composition having a si ze distribution from 1 pm to 100 pm, preferably from 2 . 5 pm to 75 pm, or even more preferably from 5 pm to 50 pm, as measured by sieving in accordance with I SO-14232-1-2017-E , Particle distribution by sieving, and / or preferably comprises at least 85% by weight , at least 90% by weight , or more preferably at least 95% by weight of the powder composition having a si ze distribution from 2 . 5 pm to 100 pm as measured by sieving in accordance with I SO-14232-1-2017-E , Particle distribution by s ieving .

[0051] It is a maj or advantage of the alloys of the present invention that these can be formulated as pre-alloyed binder powders and desired amounts of metal carbide powders added, e . g . , by in-mixing . This not only broadens the range and the composition of metal carbide powders , which can be added, making the hard-coatings independent on the formation chemistry of the constituent powders , but also it allows for lower deposition temperatures , since rather than providing coating temperatures necessary for metal carbide formation, it is now only necessary to provide energy for melting prealloyed binder powder and not the metal carbides .

[0052] Accordingly, there is herein disclosed a powder composition consisting by total weight of the composition of from 10 wt% to 90 wt% of a powdered BCC-phase iron-chrome stainless-steel alloy according to any of the embodiments thereof detailed herein and from 10 wt% to 90 wt% of at least one powdered metal carbide .

[0053] Following this and the further embodiments detailed herein, the use of the disclosed powder composition for the coating of a surface by means of a thermal spray method becomes much more adaptive and simplified as optimi zation of the metal carbide content can be experimentally verified following well-known experimental procedures of systematical changes to the concentrat ions of the const ituent stainless steel and the selected metal carbides .

[0054] The actual concentration and composition of the metal carbide powder in the final pre-alloyed BCC-phase iron-chrome stainless-steel alloy powder composition will normally be at the discretion of an end-user . However, in embodiments of the powder composition consisting of a pre-alloyed BCC-phase iron-chrome stainless-steel alloy and at least one powdered metal carbide , the at least one metal carbide is selected from one or more powders of a nickel carbide , a chromium carbide , a vanadium carbide , a tungsten carbide (WC) , a molybdenum carbide , a silicon carbide , a manganese carbide , an aluminum carbide , a titanium carbide , a niobium carbide , a tantalum carbide , a hafnium carbide , or a z irconium carbide , preferably is selected from one or more of a nickel carbide , a chromium carbide , a vanadium carbide , a tungsten carbide (WC) , a molybdenum carbide , a silicon carbide , a manganese carbide , or an aluminum carbide , more preferably is selected from one or more of a nickel carbide , a chromium carbide , a vanadium carbide , a molybdenum carbide , or a tungsten carbide (WC) , most preferably a chromium carbide or a tungsten carbide (WC) . Most preferred are chromium carbides and / or tungsten carbides , but when the powders are formed by pre-alloying a chrome-based powder as detailed below, chromium carbides will dominate .

[0055] For the uses to which the present of the BCC-phase ironchrome stainless-steel alloys with at least one chromium carbide (which are the commercially intended products for the present applicant ) the present inventors have established that chromium carbide shall be present in the BCC-phase ironchrome stainless-steel alloy powder compositions of the present invention from 40 wt% to 90 wt%, preferably from 50 wt% to 80 wt%, or more preferably from 60 wt% to 70 wt%.

[0056] However, in many situations it is more preferable to allow the alloys of the invention to form jointly with the carbides from melt during thermal spray coating as will be detailed herein below.

[0057] To this purpose, it is preferable to prepare a chromium-based powder consisting by total weight of powder of:

[0058] Iron (Fe) 15 - 18 wt%

[0059] Carbon (C) 4.7 - 5.5 wt %

[0060] Tungsten (W) 2.2 - 3.4 wt %

[0061] Molybdenum (Mo) 1.4 - 2.3 wt %

[0062] Boron (B) 0.3 - 1.0 wt%

[0063] Silicon (Si) 0.1 - 1.0 wt%

[0064] Niobium (Nb) : 0.35 - 0.60 wt% the balance being chromium (Cr) and unavoidable impurities not exceeding 0.3 wt%, wherein nickel (Ni) and / or cobalt (Co) , if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt%.

[0065] In the most preferred embodiments of the present invention, the chromium-based powder is a pre-alloyed powder, preferably pre-alloyed from a melt. When the powder is pre-alloyed before powder formation, such as pre-alloyed from melt, e.g., done by gas or water atomization, the abovementioned BCC- phase iron-chrome stainless-steel alloy will form as a solid solution matrix for co-precipitated carbide phases in solid solution. As discussed below, such pre-alloyed powders are directly useable in thermal spray processes with high transfer ratios of the elemental constituents to the surfaces coated by thermal spray coating processes. Preferred embodiments of the chromium-based powder are detailed herein below.

[0066] In embodiments of the chromium-based powder according to invention, the amount of iron (Fe) is from 15.5 wt% to 17.5 wt%, preferably from 16.0 wt% to 17.0 wt%.

[0067] In embodiments of the chromium-based powder according to invention, the amount of carbon (C) is from 4.9 to 5.3 wt%, preferably from 5.0 to 5.2 wt%.

[0068] In embodiments of the chromium-based powder according to invention, the amount of tungsten (W) is from 2.5 wt% to 3.1 wt%, preferably from 2.7 wt% to 2.9 wt%.

[0069] In embodiments of the chromium-based powder according to invention, the amount of molybdenum (Mo) is from 1.6 wt% to 2.1 wt%, preferably from 1.7 wt% to 2.0 wt%.

[0070] In embodiments of the chromium-based powder according to invention, the amount of boron (B) is from 0.4 to 0.9 wt%, preferably from 0.5 wt% to 0.8 wt%.

[0071] In embodiments of the chromium-based powder according to invention, the amount of silicon (Si) is from 0.3 wt% to 0.9 wt%, preferably from 0.5 wt% to 0.8 wt%. In general, enough silicon must be present in the chromium-based powders for the subsequent forming of the abovementioned alloys of the invention .

[0072] In embodiments of the chromium-based powder according to invention, the amount of niobium (Nb) is from 0.40 wt% to 0.55 wt%, preferably from 0.45 wt% to 0.50 wt%. When the chromium-based powder detailed herein i s a prealloyed powder, metal carbides , primarily chromium and tungsten carbides , will be in sol id solution in a BCC-phase iron-chrome stainless-steel alloy according to any of the herein detailed embodiments of this steel .

[0073] In preferred embodiments of the chromium-based powder whether pre-alloyed or mixed, the chromium-based powder preferably comprises at least 80% by weight of the chromium-based powder contained within a sieved fraction of the chromium-based powder having a si ze distribution from 1 pm to 100 pm, preferably from 2 . 5 pm to 75 pm, or even more preferably from 5 pm to 50 pm, as measured by sieving in accordance with I SO- 14232-1-2017-E , Particle distribution by sieving, and / or preferably comprises at least 85% by weight , at least 90% by weight , or more preferably at least 95% by weight of the chromium-based powder having a si ze distribution from 2 . 5 pm to 100 pm as measured by sieving in accordance with I SO- 14232-1-2017-E , Particle distribution by sieving . Thereby the powders of the present invention will have optimal si zes for the intended use in thermal spray processing methods .

[0074] Accordingly, there is herein detailed the use of a chromium- based powder according to any embodiment of the present invention for the coating of a surface by means of a thermal spray method .

[0075] An advantage of the powders of the present invention is that when used in a thermal spray method, such as for HVOF and / or HVAF spray coating and spray coated together direct ly onto a suitable surface , the BCC-phase iron-chrome stainless-steel alloy according to any embodiment of the herein detailed invention will form, together with a desired amount of metal carbide , the latter being dependent on the stochiometric ratios of the initial powder . Likewise , other thermal spray methods beyond HVOF thermal spraying and HVAF thermal spraying, have been tested and found useful with the present alloy powders , such as plasma spraying, laser cladding, high speed laser cladding, and / or ultra-high speed laser cladding .

[0076] Further, there i s herein detailed a shaped object having a coating comprising a BCC-phase iron-chrome stainles s-steel alloy according to any embodiment of the herein detailed invention .

[0077] Further, there is herein detailed a method for the production of a shaped object having the steps :

[0078] - provision of a chromium-based powder according to any embodiment herein detailed in a form or formulation that is suitable for thermal spraying;

[0079] - carrying-out of a thermal spraying process using the chromium-based powder;

[0080] - obtaining of the shaped object .

[0081] EXPERIMENTAL

[0082] Preparation of pre-alloyed and gas atomized powders

[0083] In preparation to the herein detailed experiment s , powder mixtures as detailed herein above and having the below given specific compositions were pre-alloyed, and either stored as a pre-alloyed ingot for subsequent gas atomi zation, or directly gas atomi zed from the pre-alloy melt . In this way, each powder particle was cooled fast enough for the atomi zed powder to be cons idered as a separate bulk material , in which carbides can nucleate and grow upon cooling as intermetallics . These intermetallics form a coherent part of the material and no fusion was needed to create or bind them . Powder si zes were determined in accordance with I SO-14232 - 1-2017-E Particle distribution by sieving and are stated as 95% of the powder mass falling inside the given si ze exclusion interval . The measured si ze exclusion ranges for the respective atomi zed powders of the present experiments were : 3 wt% > 53pm > 45pm; 30 wt% > 45pm > 38pm; 55 wt% > 38 pm > 15pm; 2 wt% > 15pm .

[0084] HVOF-coating experiments

[0085] Coating properties were tested for different HVOF spray parameters . In the present work, two different jet guns were used for thermal spraying of the samples : Diamond j et gun (DJ) , and high-pressure jet gun ( JP ) . The fuel for the DJ consisted of O2+H2 + air and for the JP it was a mixture of oxygen and argon .

[0086] The high-pressure jet gun ( JP-5000 ) is designed for liquidfuel (kerosene ) and oxygen operation . Fuel and oxygen are fed into the gun, atomi zed by the coaxial stabili zer, and ignited in the combustion chamber, resulting in a supersonic flame . Spray powder from the powder feeder is fed radially into the supersonic flame through two powder ports positioned directly after the combustion chamber . The spray stream is accelerated through a converging / diverging noz z le several times the speed of sound . The spray particles are heated to a molten or semi-molten state and propelled at high velocity, impacting the coating surface in a plastic state . Table 1 provides an overview of the spray parameters used in the present experiments . Table 1: Spray parameters for high-pressure jet gun (JP-5000)

[0087] The DiamondJet, DJ-process, uses oxygen, fuel gas and air to produce a high-pressure annular flame, which provides uniform heating of the axially introduced powder spray material. The gas stream is accelerated through a converging / diverging nozzle to supersonic speeds. The gas stream propels the powder particles towards the substrate. Individual particles deform plastically upon impact, tenaciously bonding the coating to the substrate. Table 2 details the spray parameters used for the present experiments.

[0088] Different spray parameters were tested, e.g., power feeder, nozzle size, combustion pressure, etc., c.f., Tables 1 and 2. Parameters were adjusted in a way that a dense coating with optimal deposition efficiency is achieved. Separate coupons were sprayed with the same parameters for different tests . Table 2 : Spray parameters for Diamond jet gun

[0089] Hardness , roughness , gas permeability, Young' s modulus , and wear resistance were measured for each spray parameter .

[0090] After spraying, coated samples were analyzed with optical microscope to measure the coating thickness and porosity and additionally examined using electron microscopy .

[0091] Another set of coupons were placed in the natural salt spray chamber and evaluated after specific periods of time to study the corrosion properties . Corrosion resistance of the material was also evaluated based on gas permeability and cavitation tests , c . f . Figure 8 .

[0092] Finally, coated samples were analyzed using SEM / EDX analysis to find a better understanding of the final product . Calphad-based calculation

[0093] Figure 1 shows the phase diagram for the presently developed alloys as calculated using Thermo-Calc software , steel database TCFE12 .

[0094] Composition range marked with vertical stippled lines in Figure 1 shows composition ranges of for the carbon (C) content for this powder, which results in carbides as hard phase , precipitated in a BCC matrix . Small amounts of Cr- boride precipitates also form which can have a positive effect on hardness .

[0095] The starting pre-alloyed powders ' compositions were selected such that the formed BCC-phase , which is predominately an iron-chromium-silicon alloy, forms a soft solid solution matrix having optimi zed elastic properties for the resulting coatings from thermal spraying while being able to contain the formed carbide phases strongly in solid solution .

[0096] In the shown calculations , the BCC-phase forms as a eutectic composition having the composition given below in Table 3 .

[0097] Table 3 : Composition of BCC-phase from Thermo-Calc

[0098] As can be seen from the resulting equilibrium composition, essentially all niobium, boron, and carbon has been consumed in the formation of the various carbide phases distributed in the matrix al loy, although at least the amount of carbon remains sufficient for interaction with the main al loy elements of iron (Fe ) , silicon ( Si ) , molybdenum (Mo ) , tungsten (W) , and chromium (Cr) as the balance , whereas niobium (Nb) and boron (B) appear to be present only in amounts commensurate with both of these elements being present in the matrix alloy as unavoidable impurities based on the starting materials used .

[0099] Microstructure analysis

[0100] Examination of pre-alloyed powder

[0101] Figure 2 A) and B) show (EM) the same cross section of the atomi zed powder at two resolutions (B enlarged) . Dark gray carbides (M23C6 and M7C3 according to the phase diagram in Figure 1 ) can be seen in a light-color matrix of solid solution BCC . Small amounts of Cr-boride precipitates also form which improve hardness .

[0102] Sprayed coupons

[0103] Sprayed coupons were cut and analyzed by a LEICA DM6 M light optical microscope , to measure the coating thicknes s and porosity .

[0104] Figure . 3 shows indexes correlated to deposition efficiency and coating' s thickness for different spraying methods and parameters in accordance with Tables 1 and 2 . The results are also compared with an existing product , Amperit® 588 .

[0105] Most of the trails fall in the high-product ivity / high- efficiency area of the plot ( compatible with, and in some cases better than the existing products ) which shows that this product is cost-beneficial through which a dense coating can be achieved . The JP-technique seems to have higher productivity than Amperit® 588 , while the DJ-technique has a higher efficiency . Figures 4 and 5 show cross sections of the coating using JP and DJ techniques respectively. JP method seems to be able to create denser coatings (at about 0.2 - 0.9 area% porosity) , while the porosity level of DJ sprayed sample appears higher (at about 3 area%) . However, since the porosity content is measured by image analysis, which is based on the contrast of dark spots versus light bulk in the micrographs, the certainty of these values is not very high, as it is possible that the dark spots are small oxide particles. Nevertheless, it should be noted that even 3% porosity is completely acceptable for this type of coatings.

[0106] Corrosion Properties

[0107] Salt Spray Testing:

[0108] Corrosion properties of the coating was studied using natural salt spray (NSS) test by natural salt spray (NSS) test using a SC1000 Weiss GmbH machines according to standard ASTM B117. Samples were placed in the chamber and inspected after specific times, i.e., 168, 336, 504, 672, 840 and 1008 hours.

[0109] Figure 6 shows the tested coupons after A) 168 hours for JP- sprayed samples, at which time severe corrosion became observable, and after B) 504 hours for DJ sprayed samples, when signs of corrosion were only then becoming observable. Accordingly, the corrosion resistance of the present coatings is compatible or better than currently marketed products, such as e.g., Amperit® 588, which complies with a minimum of 168 hours exposure to the test conditions before severe corrosion becomes observable. Gas Permeability:

[0110] Gas permeability test results were used as further tool for providing an index useful in evaluating corrosion resistance of the coated samples. Figure 7 shows these results for the two different spray techniques measured using a GPT-03 machine in accordance with standard ISO 4022.

[0111] The coating seems rather dense but at higher experimental pressures the gas penetration accelerates.

[0112] Cavitation Tests:

[0113] Cavitation test was run on the coated samples using a KLN Type 587 machine in accordance with standard ASTM G32-85. Results are shown in Figure 8.

[0114] It is interesting that although DJ sprayed coatings are less dense (also confirmed by gas permeability and EM) , they show a better corrosion resistance in the NSS test.

[0115] Overall, the present experiments confirm the corrosion resistance of the present coatings, ranking at on par with or better than commercialized products, but in general being intermediate corrosion resistance coatings.

[0116] Mechanical Properties

[0117] Mechanical properties of the coatings were measured. Results for hardness, roughness, Young's modulus and wear resistance are shown in Figures 9 to 12.

[0118] Hardness Vickers was measured using a Struers Dura Scan machine according to standard ISO 6507,1-4:2018. The present product shows a general hardness of 900-1000 HV, c.f. Figure 9, which makes it a good candidate for the intended applications .

[0119] Roughness of coated samples were measured based on standards DIN EN ISO 4287 and ASME B46.1, using a MarSurf PS10 machine. The results presented in Figure 10 show an average value of 5-7 Ra for these compositions, which is slightly higher than expected for this property among different carbides (c.f., J. Garcia et al . ) .

[0120] Young' s modulus represents elastic properties of material and is correlated to hardness. The measurements were done using a LA-wave V2-1 Fraunhofer IWS apparatus. The average value of 100-130 GPa (Figure 11) is expected for a hard material and makes it a good candidate for applications where heavy loading is applied.

[0121] Wear resistance of the coated samples was measured according to ASTM G65 (Figure 12) using a built-in machine. Volume loss is shown versus material consumption for spraying in this figure .

[0122] It shows that all coatings have excellent wear resistance, i.e., very low volume loss for medium powder consumption in comparison with Amperit® 588. This makes the present product significantly beneficial from cost point of view and a strong candidate for application where wear properties play an important role. SEM Analysis

[0123] A Hitachi SU6600 Scanning Electron Microscope was used to analyze coated samples . Figure 13 shows the cross section together with EDX map for high-content elements , us ing voltage of 15kV . The results are aligned with the thermodynamic calculation, showing different types of carbides in a BCC matrix .

[0124] CLOS ING COMMENTS

[0125] Although the present invention has been described in detail for purpose of illustration, it is understood that such detail is solely for that purpose , and variations can be made therein by those skilled in the art in practicing the claimed subject matter, from a study of the drawings , the disclosure , and the appended claims .

[0126] The term " compris ing" as used in the claims does not exclude other elements or steps . The indefinite article "a" or "an" as used in the claims does not exclude a plurality . A unit may fulfill the functions of several means recited in the claims . A reference sign used in a claim shal l not be construed as limiting the scope .

Claims

CLAIMS1. A BCC-phase iron-chrome stainless-steel alloy consisting by total weight of alloy of:Iron (Fe) 44.0 - 47.0 wt % ,Silicon (Si) 5.0 - 7.0 wt%,Molybdenum (Mo) 0.60 - 0.80 wt%,Tungsten (W) 0.40 - 0.60 wt%,Carbon (C) 0.01 - 0.10 wt % ,Boron (B) 0.001 - 0.010 wt %Niobium (Nb) 0.005 - 0.010 wt % the balance being chromium (Cr) and unavoidable impurities not exceeding 0.2 wt%, wherein nickel (Ni) and / or cobalt (Co) , if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt% in total.

2. A BCC-phase iron-chrome stainless-steel alloy according to claim 1, wherein iron (Fe) is present in the alloy from 44.5 wt% to 46.5 wt%, preferably from 45.0 wt% to 46.0 wt%, or more preferably from 45.4 wt% to 45.9 wt%.

3. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim, wherein silicon (Si) is present in the alloy from 5.3 wt% to 6.7 wt%, preferably from 5.5 wt% to 6.3 wt%, or more preferably from 5.7 wt% to 6.1 wt % .

4. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim, wherein molybdenum (Mo) is present in the alloy from 0.62 wt% to 0.78 wt%, preferably from 0.65 wt% to 0.75 wt%, or more preferably from 0.63 wt% to 0.73 wt%5. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim, wherein tungsten (W) is present in the alloy from 0.42 wt% to 0.58 wt%, preferably from 0.45 wt% to 0.55 wt%, or more preferably from 0.47 wt% to 0.53 wt% .

6. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim, wherein carbon (C) is present in the alloy from 0.02 wt% to 0.09 wt%, preferably from 0.03 to 0.07 wt%, or more preferably from 0.04 wt% to 0.06 wt% .

7. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim, wherein boron (B) is present in the alloy from 0.002 wt% to 0.009 wt%, preferably from 0.003 wt% to 0.007 wt%, or more preferably from 0.004 wt% to 0.006 wt%.

8. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim, wherein niobium (Nb) is present from 0.006 wt% to 0.009 wt%, preferably from 0.007 wt% to 0.008 wt% .

9. A BCC-phase iron-chrome stainless-steel alloy according to any previous claim as a powder.

10. A BCC-phase iron-chrome stainless-steel alloy according to claim 9, wherein the stainless-steel powder comprises at least 80% by weight of the stainless-steel powder contained within a sieved fraction of the stainless-steel powder having a size distribution from 1 pm to 100 pm, preferably from 2.5 pm to 75 pm, or even more preferably from 5 pm to 50 pm, as measured by sieving in accordance with ISO-14232-1-2017-E, Particle distribution bysieving, and / or preferably comprises at least 85% by weight , at least 90% by weight , or more preferably at least 95% by weight of the stainless-steel powder having a si ze distribution from 2 . 5 pm to 100 pm as measured by sieving in accordance with I SO-14232-1-2017-E , Particle distribution by sieving .11 . A BCC-phase iron-chrome stainless-steel alloy according to claim 9 or claim 10 , wherein the stainles s-steel powder was formed by melting the BCC-phase iron-chrome stainless-steel alloy followed by atomization, preferably followed by water atomi zation .12 . A powder composition consisting by total weight of the composition of from 10 wt% to 90 wt% of a powdered BCC- phase iron-chrome stainless-steel alloy according to any of the claims 1 to 11 and from 10 wt% to 90 wt% of at least one powdered metal carbide .13 . A powder composition consisting of a powdered BCC-phase iron-chrome stainless-steel powder and at least one powdered metal carbide powder according to claim 12 , wherein the powder composition comprises from 40 wt% to 90 wt% , preferably from 50 wt% to 80 wt% , or more preferably from 60 wt% to 70 wt% of the at least one powdered metal carbide .14 . A powder composition consisting of a pre-alloyed BCC- phase iron-chrome stainless-steel alloy and at least one powdered metal carbide according to claim 12 or claim 13 , wherein the at least one powdered metal carbide is selected from one or more powders of a nickel carbide, a chromium carbide , a vanadium carbide , a tungsten carbide (WC) , a molybdenum carbide , a silicon carbide , amanganese carbide, an aluminum carbide, a titanium carbide, a niobium carbide, a tantalum carbide, a hafnium carbide, or a zirconium carbide, preferably is selected from one or more of a nickel carbide, a chromium carbide, a vanadium carbide, a tungsten carbide (WC) , a molybdenum carbide, a silicon carbide, a manganese carbide, or an aluminum carbide, more preferably is selected from one or more of a nickel carbide, a chromium carbide, a vanadium carbide, a molybdenum carbide, or a tungsten carbide (WC) , most preferably a chromium carbide and / or a tungsten carbide (WC) .

15. A powder composition according to any of the claims 12 to 14, wherein the at least one metal carbide is either a chromium carbide and / or a tungsten carbide (WC) .

16. A powder composition comprising a pre-alloyed BCC-phase iron-chrome stainless-steel powder and at least one powdered metal carbide according to any of the claims 12 to 15, wherein the powder composition comprises at least 80% by weight of the powder composition contained within a sieved fraction of the powder composition having a size distribution from 1 pm to 100 pm, preferably from 2.5 pm to 75 pm, or even more preferably from 5 pm to 50 pm, as measured by sieving in accordance with ISO-14232-1-2017- E, Particle distribution by sieving, and / or preferably comprises at least 85% by weight, at least 90% by weight, or more preferably at least 95% by weight of the powder composition having a size distribution from 2.5 pm to 100 pm as measured by sieving in accordance with ISO- 14232-1-2017-E, Particle distribution by sieving.

17. Use of a powder composition according to any of the claims 12 to 16 for the coating of a surface by means of a thermal spray method.

18. A composition for the formation of a BCC-phase ironchrome stainless-steel alloy according to any of the claims 1 to 11 consisting by total weight of composition of :Iron (Fe) 44.0 - 47.0 wt % ,Silicon (Si) 5.0 - 7.0 wt%,Molybdenum (Mo) 0.60 - 0.80 wt%,Tungsten (W) 0.40 - 0.60 wt%,Carbon (C) 0.01 - 0.10 wt % ,Boron (B) 0.001 - 0.010 wt %Niobium (Nb) 0.005 - 0.010 wt % the balance being chromium (Cr) and unavoidable impurities not exceeding 0.2 wt%, wherein nickel (Ni) and / or cobalt, if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt%; wherein when the composition is subsequently melted the BCC-phase iron-chrome stainless-steel is formed by alloying of the elements.

19. A chromium-based powder consisting by total weight of powder of :Iron (Fe) 15 - 18 wt%,Carbon (C) 4.7 - 5.5 wt % ,Tungsten (W) 2.2 - 3.4 wt % ,Molybdenum (Mo) 1.4 - 2.3 wt % ,Boron (B) 0.3 - 1.0 wt%,Silicon (Si) 0.1 - 1.0 wt % ,Niobium (Nb) 0.35 - 0.60 wt% the balance being chromium (Cr) and unavoidable impurities not exceeding 0.3 wt%, wherein nickel (Ni) and / or cobalt(Co) , if present, are only present as unavoidable impurities in amounts not exceeding 0.10 wt% in total.

20. A chromium-based powder according to claim 19, wherein the amount of iron (Fe) is from 15.5 wt% to 17.5 wt%, preferably from 16.0 wt% to 17.0 wt%.

21. A chromium-based powder according to claim 19 or claim 20, wherein the amount of carbon (C) is from 4.9 to 5.3 wt%, preferably from 5.0 to 5.2 wt%.

22. A chromium-based powder according to any of the claims 19 to 21, wherein the amount of tungsten (W) is from 2.5 wt% to 3.1 wt%, preferably from 2.7 wt% to 2.9 wt%.

23. A chromium-based powder according to any of the claims 19 to 22, wherein the amount of molybdenum (Mo) is from 1.6 wt% to 2.1 wt%, preferably from 1.7 wt% to 2.0 wt%.

24. A chromium-based powder according to any of the claims 19 to 23, wherein the amount of boron (B) is from 0.4 to 0.9 wt%, preferably from 0.5 wt% to 0.8 wt%.

25. A chromium-based powder according to any of the claims 19 to 24, wherein the amount of silicon (Si) is from 0.3 wt% to 0.9 wt%, preferably from 0.5 wt% to 0.8 wt%.

26. A chromium-based powder according to any of the claims 19 to 25, wherein the amount of niobium (Nb) is from 0.40 wt% to 0.55 wt%, preferably from 0.45 wt% to 0.50 wt%.

27. A chromium-based powder according to any of the claims19 to 26 as a pre-alloyed powder.28 . A chromium-based powder according to claim 27 , wherein the chromium-based powder was formed by melting the chromium-based powder followed by atomi zation, preferably followed by water atomi zation .29 . A chromium-based powder according to either claim 27 or claim 28 , wherein metal carbides are in solid solution in a BCC-phase iron-chrome stainless-steel alloy according to any of the claims 1 to 11 .30 . A chromium-based powder according to any of the claims 27 to 29 , wherein the chromium-based powder comprises at least 80% by weight of the chromium-based powder contained within a sieved fraction of the chromium-based powder having a si ze distribution from 1 pm to 100 pm, preferably from 2 . 5 pm to 75 pm, or even more preferably from 5 pm to 50 pm, as measured by sieving in accordance with I SO-14232-1-2017-E , Particle distribution by sieving, and / or preferably comprises at least 85% by weight , at least 90% by weight , or more preferably at least 95% by weight of the chromium-based powder having a si ze distribution from 2 . 5 pm to 100 pm as measured by sieving in accordance with I SO-14232-1-2017-E , Particle distribution by sieving .31 . Use of a chromium-based powder according to any one of claims 27 to 30 for the coating of a surface by means of a thermal spray method .

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