An austenitic alloy powder and the use thereof

Abstract

The present disclosure relates to an austenitic alloy powder and the use thereof for obtaining a heat resistant object as the object and the material will, due to the composition of the powder and its properties, have high tensile strength and excellent creep strength at elevated temperatures. Furthermore, they will also have good steam oxidation resistance, good high temperature corrosion resistance and a sufficient structural stability. The present disclosure also relates to a HIP process wherein the powder is used.

Description

[0001] An austenitic alloy powder and the use thereof

[0002] Technical field

[0003] The present disclosure relates to an austenitic alloy powder and the use thereof for obtaining a heat resistant object. The present disclosure also relates to an object manufactured from said austenitic alloy powder, more specifically aHIP:ed or a solution annealed object, which will, due to the alloying element composition of the present powder and its properties, have high tensile strength and excellent creep strength at elevated temperatures. Furthermore, the object will also have good steam oxidation resistance, good high temperature corrosion resistance and a sufficient structural stability. The present disclosure also relates to a process for manufacturing said object.

[0004] Brief description of the figures

[0005] Figure 1 discloses a SEM picture of a delivery condition object comprising the austenitic alloy powder. The object has been annealed, such as solution annealed, and has an isotropic structure and an essentially uniform grain size;

[0006] Figure 2 shows the number of Z phase precipitates per analyzed mm2of a polished section of the delivery condition object, i.e. the object has been manufactured through HIP:ing and solution annealing;

[0007] Figure 3 shows the average section area of Z-particles of a polished section of the delivery condition object, i.e. the object has been manufactured through HIP:ing and solution annealing;

[0008] Figure 4 shows a SEM picture of the said object having a uniform and fine-dispersed distribution of Z-phase particles.

[0009] Detailed description

[0010] The present disclosure relates to an austenitic alloy powder having an elementary composition of (in weight%):

[0011] C 0.03 - 0.30;

[0012] Si < 0.80;

[0013] Mn < 1.0; P < 0.03;

[0014] S < 0.03;

[0015] Cr 20.0 - 27.0;

[0016] Ni 22.0 - 32.0;

[0017] Mo < 1.0;

[0018] Co 0.5 - 3.0;

[0019] Cu 1.0 - 5.0;

[0020] Nb 0.1 - 1.0;

[0021] W 0.50 - 5.0;

[0022] Ti < 0.10;

[0023] Al < 0.05;

[0024] Mg < 0.05;

[0025] B < 0.008;

[0026] N 0.10 - 0.50;

[0027] O < 300 ppm; the balance being Fe and unavoidable impurities; wherein the austenitic alloy powder has a particle size diameter distribution within the range of > 0 but less than or equal to 600 pm.

[0028] The present disclosure also relates to an object manufactured from the powder defined hereinabove or hereinafter and said object will when in delivery conditions, i.e. after HIP:ing and solution annealing, contain nano Z-phase particles (precipitates). These particles will have an impact on the strength so that said object will have high strength at both normal temperature and higher temperatures, especially when being manufactured by a HIP (high isostatic pressing) process. Additionally, it has been found that these Z-phase particles are homogeneously distributed in said object. The Z-phase particles will have a particle size of less than 1 pm, which means that they will be nano-particles and will essentially be composed of the elements Nb, N and Cr. All numbers and sizes relating to the Z-phase particles are based on the apparent numbers and sizes of the Z-phase particles which have been obtained by measuring these particles on a cross sectional surface in SEM using the Oxford Aztec Feature software on randomly selected polished cross-sectional surfaces of the object in its final condition, i.e. in delivery condition. The object will be heat resistant meaning that it will withstand high temperatures and also have high strength and corrosion resistance at high temperatures. The powder as defined hereinabove and hereinafter is consolidated into a solid object by using hot isostatic pressing, hereinabove or hereinafter referred to as HIP or “HIP process”. However, the powder as defined hereinabove or hereinafter may be used in other techniques, such as additive manufacturing.

[0029] The elements comprised in the austenitic alloy powder and the austenitic alloy object will be described below. The list of properties mentioned for each alloying element should not be consider as being exhaustive, the elements may have other effects not mentioned. The alloying elemental ranges disclosed herein below is for both the powder and the object. Weight percentage is given as wt%, or weight%

[0030] Carbon (C) is a component effective to provide adequate tensile strength and creep rupture strength required for high temperature steel. However, if an excess carbon is added, the toughness will be reduced, and the weldability also may be deteriorated. For these reasons, the carbon content is defined by a range of from 0.03 to 0.30 wt%. According to embodiments, the content of C may be from 0.04 to 0.20 wt%. According to embodiments, the content of C may be from 0.05 to 0.10 wt%.

[0031] Silicon (Si) is effective as a deoxidizing agent and it also serves to improve oxidation resistance. However, an excess of silicon is detrimental to the weldability and in order to prevent the deterioration of ductility and toughness due to the formation of sigma phase after long term exposure, the silicon content should be <0.80 wt%. According to embodiments, the content of Si may be < 0.40 wt%, such as <0.30 wt%.

[0032] Manganese (Mn) is a deoxidizing element and is also effective to improve the hot workability. However, in order to prevent the creep rupture strength, ductility and toughness from decreasing, the manganese content should be < 1.0 wt%, such as < 0.60 wt%.

[0033] Phosphorous (P) and Sulphur (S) are detrimental to the weldability and may promote embrittlement. Therefore, the phosphorus and the sulphur content should be < 0.03 wt%.

[0034] Chromium (Cr) is an effective element to improve the corrosion resistance and oxidation resistance. In order to achieve a sufficient resistance, a chromium content of at least 20.0 wt% is needed. However, if the chromium content exceeds 27.0 wt%, the nickel content must be further increased in order to produce a stable austenitic structure and suppress the formation of sigma phase. In view of these considerations, the chromium content is restricted to a range of 20.0 to 27.0 wt%. According to embodiments, the content of Cr is from22.0 to 26.0 wt%, such as from 22.0 to 25.0 wt%.

[0035] Nickel (Ni) is an essential component for the purpose of ensuring a stable austenitic structure. The structural stability depends essentially on the relative amounts of the ferrite stabilizers such as chromium, silicon, molybdenum, aluminum, tungsten, titanium and niobium, and the austenite stabilizers such as nickel, carbon and nitrogen. In order to suppress the formation of sigma phase, the nickel content should be at least 22.0 wt%. In addition, at a specific chromium level, an increased nickel content suppresses the oxide growth rate and increases the tendency to form a continuous chromium oxide layer. However, in order to maintain the production cost at a reasonable level, the nickel content should not exceed 32.0 wt%. In view of the above circumstances, the nickel content is restricted to a range of 22.0 to 32.0 wt%. According to embodiments, the content of Ni is from 23.0 to 28.0 wt%, such as 23.0 to 26.0 wt%.

[0036] Tungsten (W) and Molybdenum (Mo)

[0037] Tungsten is added to improve the high temperature strength mainly through solid solution hardening and a minimum of 0.50 wt% is needed to achieve this effect. However, both molybdenum and tungsten promote formation of sigma phase and may also accelerate the corrosion. Tungsten is considered to be more effective than molybdenum in improving the strength. For these reasons, the molybdenum content is held low, < 1.0 wt%, such as be < 0.50 wt%, such as < 0.30 wt%. In order to have a solid solution hardening effect, the tungsten content should be higher than 0.50 wt%. However, the content should not exceed 5.0 wt% for avoiding unwanted intermetallic phases. The tungsten content is therefore in the range of 1.5 to 4.0 wt%.

[0038] Cobalt (Co) is an austenite-stabilizing element. The addition of cobalt may improve the high temperature strength through solid solution strengthening and the suppression of sigma phase formation after long exposure times at elevated temperatures. However, in order to maintain the production cost at a reasonable level, the cobalt content should be in the range of 0.5 to 3.0 wt%, such as 1.0 to 2.0 wt%.

[0039] Titanium (Ti) may be added for the purpose of improving the creep rupture strength through the precipitation of carbonitrides, carbides and nitrides. However, an excessive amount of titanium can decrease the weldability and the workability. For these reasons, the content of titanium is <0.1 wt%.

[0040] Copper (Cu) is added in order to produce copper rich phase, finely and uniformly precipitated in the matrix, which may contribute to an improvement of the creep rupture strength. However, an excessive amount of copper results in a decreased workability. In view of these considerations, the copper content is defined to a range of 1.0 to 5.0 wt%. According to embodiments, the Cu range is from 1.5 to 3.5 wt%.

[0041] Aluminium (Al) and Magnesium (Mg)

[0042] Aluminium and magnesium are effective for deoxidization during manufacturing. However, an excessive amount of aluminium may accelerate the precipitation of sigma phase and an excessive amount of magnesium may deteriorate the weldability. For these reasons, if added, the content of aluminium is < 0.05 wt%, such as 0.003 to 0.05 wt% and the content of magnesium is < 0.05 wt%, such as 0.003 to 0.05 wt%.

[0043] Niobium (Nb) is generally accepted to contribute to improving the creep rupture strength through the precipitation of carbonitrides and nitrides. However, an excessive amount of niobium can decrease the weldability and the workability. In view of these considerations the niobium content is restricted to a range of 0.10 to 1.0 wt%. According to embodiments, the content of Nb is 0.30 to 0.70 wt%.

[0044] Boron (B) contributes to improve the creep rupture strength partly due to the formation of finely dispersed M23(C,B)e and the strengthening of the grain boundary. Boron may also contribute to improve the hot workability. However, an excessive amount of boron may deteriorate the weldability. In view of these considerations, if added, the boron content is restricted to a range of < 0.008 wt% , such as 0.002 to 0.008 wt%.

[0045] Nitrogen (N) is known to improve the elevated temperature strength, the creep rupture strength and to stabilize the austenite phase. However, if nitrogen is added in excess, the toughness and ductility will be reduced. For these reasons, the content of nitrogen is defined to a range of 0.10 to 0.50 wt%. According to embodiments, the content of N is from 0.20 to 0.40 wt%.

[0046] Oxygen (O) is considered to be a negative element as it will have an impact on the welding properties but also on ductility and toughness. Hence, the maximum content of oxygen is 300 ppm, such as less than 150 ppm.

[0047] When the terms “max” or “<” are used, the skilled person knows that the lower limit of the range is 0 wt% unless another number is specifically stated.

[0048] The remainder is iron (Fe) and normally occurring impurities as discussed above. The term “impurities” means elements that are considered to be impurities meaning that they are allowed to be present but only in such amount that the properties are not affected. Thus, impurities are elements and compounds which have not been added on purpose but cannot be fully avoided as they normally occur as impurities in e.g. the raw material or the additional alloying elements used for manufacturing of the steel. The impurities may be present in a range of < 1.0 wt%, such as 0.50 wt%.

[0049] According to embodiments, present austenitic alloy powder and said object may comprise the following elements in weight% (wt%):

[0050] Balance Fe and unavoidable impurities

[0051] Further, the present powder or object as defined hereinabove or hereinafter may consist or comprise of all the elements mentioned herein in the different ranges as mentioned herein.

[0052] The present disclosure also relates to an austenitic alloy object which can be used in high temperature applications and which are made of the powder as defined hereinabove or hereinafter through a HIP process as defined hereinabove or hereinafter and therefore comprises or consists the elements in the ranges as disclosed hereinabove or hereinafter. According to embodiments, said austenitic alloy object is a solution annealed object.

[0053] Additionally, a cross-section of an object as defined hereinabove or hereinafter, which crosssection been obtained through a process as defined hereinabove or hereinafter has a Z-phase precipitation density (1 / mm2) of at least 30 000. It is believed that this precipitation density of at least 30 000 / mm2will have a positive impact on the creep strength. Furthermore, optionally the object as defined hereinabove or hereinafter may have Z-phase particles having an average number cross section area of less than 0.15 pm2but not 0, by having this value it is believed that even higher creep strength will be obtained. The Z-phase particles are primary particles.

[0054] The powder as defined hereinabove or hereinafter may be manufactured by using inert gas atomization, wherein the molten metal is poured via a nozzle at which the stream of molten metal is disintegrated by a high pressure, high velocity inert gas into a spray of rapidly solidifying metal droplets. The melting may take place in a VIM (vacuum induction melting) furnace chamber or in an open furnace. The atomizing gas may be e.g. nitrogen or argon. Due to the rapid cooling, the powder particles are free from macro-segregations. The powder particle size range typical for a powder suitable for HIP may be in the range of less than or equal to 600 pm.

[0055] The feedstock material may consist of virgin raw material of elements, alloys and / or scrap metal. The feedstock material may also consist of AOD or VIM produced pre-alloyed feedstock.

[0056] The present disclosure relates to a process wherein the powder is consolidated into solid dense material by using Hot Isostatic Pressing, (HIP). The process comprises the steps of a) providing a form defining at least a portion of the shape of said object; providing an austenitic alloy powder as defined hereinabove or hereinafter; b) filling at least a portion of said form with said powder; c) subjecting said form to hot isostatic pressing at a predetermined temperature, a predetermined isostatic pressure and for a predetermined time so that the powder particles bond metallurgically to each other and all interparticle voids are closed whereby a solid body is formed d) annealing the solid body at a predetermined temperature during a predetermined time; according to embodiments, the annealing is solution annealing; e) subsequently quenching the annealed body.

[0057] The form could also be called mould or capsule and may be formed from a e.g. low-carbon steel, the powder is poured thereinto. The air between the powder particles inside the powder- filled mould may then be evacuated. The mould is then sealed, typically by welding after which the powder-filled and evacuated capsule is placed inside the HIP vessel.

[0058] The form is subjected to hot isostatic pressing at a predetermined temperature, a predetermined isostatic pressure and for a predetermined holding time. In this step, the powder will consolidate into a solid part through the combined influence of heat and external pressure acting upon the capsule. The external pressure may be applied by an inert gas pressure, such as argon pressure introduced in the vessel and the pressure is increased further by raising the temperature in the vessel. The predetermined holding time may be from 10 minutes to 3 hours. The predetermined pressure may be from 900 to 1500 bar and the predetermined temperature may be from 1100 to 1270 °C, such as 1100 to 1200 °C. Thus, the temperature and the pressure in the HIP vessel as well as the holding time will be chosen such that the voids between the powder particles are closed and interparticle solid-state diffusion bonding will occur.

[0059] The consolidation process may occur entirely in solid state, i.e. the temperature of the process is set so that no apparent liquification occurs inside the powder volume.

[0060] In order to dissolve unwanted phases which may have been formed during cooling down after HIP, the consolidated solid body is annealed, such as solution annealed, and subsequently quenched in liquid, such as water or oil. The annealing process may be performed for 10 to 60 minutes at a temperature of 1100 to 1250 °C.

[0061] The obtained object will have a homogeneous composition and isotropic properties throughout the whole body due to the rapid cooling of the powder particles in the atomization process and the solid state consolidation obtained by the HIP process.

[0062] The present disclosure is further described by the following non-limiting examples.

[0063] Example

[0064] The capsule used for HIP:ing of the powder had the rough outer dimensions 178x69x49mm. Three different HIP -temperatures were utilized: 1150°C, 1200°C and 1250°C. The HIP pressure used was about 100 MPa and the time used was about 2 to 4 h.

[0065] Table 1 show the composition of the powder used in the Example.

[0066] The bodies of consolidated powder were subsequently solution annealed and then quenched and had the rough dimensions 150x63x23 mm. The solution annealing was performed in a range of 30 to 50 min in 1100 to 1250°C and thereafter water quenched.

[0067] Sections of the material were polished to OPS surface finish (0.025 pm oxide polish surface finish) and the documentation of the microstructure was performed both in Light Optic Microscope (Leica Reichert MEF4M, 500X) and SEM (Zeiss Sigma VP). Furthermore, quantification of the Z phase precipitation number, size and morphology distributions was performed using Oxford Aztec Feature . Feature analysis was performed in the Zeiss VP FEG-SEM with software from Oxford Instruments. Number, dimensions and morphology of the bright contrast precipitates (backscatter detector mode) were measured over a certain area on a polished section.

[0068] The calculation was performed accordingly:

[0069] Number density = the number of Z-phase particles observed divided by the total analyzed area on which the actual feature counting was performed.

[0070] Particle size = the apparent section area of an identified Z-phase particle.

[0071] Figure 1 discloses a SEM picture of the material as can be seen the material has an isotropic structure and an essentially uniform grain size.

[0072] Figure 4 discloses a SEM picture of the material having a uniform and fine-dispersed distribution of Z-phase particles.

[0073] The number density of Z-phase precipitates is shown in Figure 2.

[0074] The average Z-phase particle size is shown in Figure 3.

[0075] Table 1 The composition used, the balance is Fe and unavoidable impurities.

[0076] Test performed on the obtained object

[0077] Table 2 show the test result of the mechanical testing at room testing

[0078] Table 2 Room temperature tensile test results Table 3 Creep test results

[0079] Creep testing was performed at 700°C material temperature on the HIP:ed and quench annealed material. The tensile stress levels were chosen based on the results of creep testing of seamless tube material of similar composition and the desired life lengths were min. 1000 h at 220 MPa and 3000 h at 185 MPa. The creep test results are presented in Table 3. The creep rupture lifetime for most specimens exceeded the desired life lengths.

Claims

CLAIMS1. An austenitic alloy powder having an elementary composition of (in percentages by weight):C 0.03 - 0.30;Si < 0.80;Mn < 1.0;P < 0.03;S < 0.03;Cr 20 - 27;Ni 22 -32;Mo < 1.0;Co 0.5 - 3.0;Cu 1.0 - 5.0;Nb 0.1 - 1.0;W 0.50 - 5.0;Ti < 0.10;Al < 0.05;Mg < 0.05;B < 0.008;N 0.10 - 0.50;O < 300 ppm; the balance being Fe and unavoidable impurities; wherein the powder has a particle size diameter distribution of a range of >0 to less than or equal to 600 pm.

2. The austenitic alloy powder according to claim 1, wherein the content of C is in the range of from 0.04 to 0.20 weight%, such as 0.05 to 0.10 weight%.

3. The austenitic alloy powder according to claim 1 or 2, wherein the content of Si is less than 0.40 weight%; such as less than 0.30 weight%.

4. The austenitic alloy powder according to any one of claims 1 to 3, wherein the content of Mn is i less than 1 weight%; such as less than 0.60 weight%.

5. The austenitic alloy powder according to any one of claims 1 to 4, wherein the content of Cr is in the range of 22.0 to 26.0 weight%, such as 22.0 to 25 weight%.

6. The austenitic alloy powder according to any one of claims 1 to 5, wherein the content of Ni is in the range of 23.0 to 28.0 weight; such as in the range of 23.0 to 26.0 weight%.

7. The austenitic alloy powder according to any one of claims 1 to 6, wherein the content of Co is in the range of 1.0 to 2.0 weight%.

8. The austenitic alloy powder according to any one of claims 1 to 7 wherein the content of Cu is in the range of 1.5 to 3.5 weight%.

9. The austenitic alloy powder according to any one of claims 1 to 8, wherein the content of Nb is in the range of 0.30 to 0.70 weight%.

10. The austenitic alloy powder according to any one of claims 1 to 9, wherein the content of W is in the range of 1.5 to 4.0 weight%.

11. The austenitic alloy powder according to any one of claims 1 to 10, wherein the content of N is in the range of from 0.20 to 0.40 weight%12. An austenitic alloy object having an elementary composition according to any one of claims 1 to 10, wherein the object has a Z-phase precipitation number density (1 / mm2) of at least 30000.

13. The austenitic alloy object according to claim 12, wherein the Z-phase particle has a number average cross section area of less than 0.15 pm2but not 0.

14. The austenite alloy object according to claim 12 or claim 13, wherein said object is a HIP:ed object.The austenite alloy object according to claim 12 or claim 13, wherein said object is an annealed object, such as a solution annealed object. A process of manufacturing an austenitic alloy object comprising the steps of: a. providing a form defining at least a portion of the shape of said object; providing a powder according to any of claims 1 to 11; b. filling at least a portion of said form with said powder; evacuate the form and seal it tight from the outside atmosphere c. subjecting said form to hot isostatic pressing at a predetermined temperature, a predetermined isostatic pressure and for a predetermined time so that all inter-particle voids are closed, and a solid, dense body is formed by solid-state diffusion bonding of the powder particles; d. annealing the solid body at a predetermined temperature during a predetermined time, such as solution annealing the solid body; e. subsequently quenching the annealed body.

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