SE545185C2 - An austenitic alloy object - Google Patents

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

An austenitic alloy object.

Technical field The present disclosure relates to an austenitic alloy object which is heat resistant object. The present disclosure also relates to an object which has manufactured from HIP, a H1P:ed object, which will, due to the composition of the 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 HIP process for manufacturing said object _ Brief description of the figures Figure 1 discloses a SEM picture of tan object comprising the austenitic alloy powder. The object has been annealed. and has an isotropic structure and an essentially uniform grain size; Figure 2 shows the number of Z phase precipitates per analyzed mmz of a polished section of the object; Figure 3 shows the average section area of Z-panicles of a polished section of the object; Figure 4 shows a SEM picture of the object having a uniform and fine-dispersed distribution of Z-phase particles. Detailed description The present disclosure relates to an austenitic alloy powder having an elementary composition of (in percentages by weight): C 0.03 - 0.30; Si 5 0.80; Mn 5 1.0; S 0.03; S S 0.03; Cr 20.0 - 27.0; Ni 22.0 - 32.0; 1 21302401 O-BESK-E OLF submitted 2021-09-21Mo _<_ 1.0; Co 0.5 - 3.0; Cu 1.0 - 5.0; Nb 0.1 - 1.0; W 0.50 - 5.0; Ti s 0.10; Al S 0.05; Mg S 0.05; B s 0.008; N 0.10 ~ 0.50; O _<_ 300 ppm; the balance being Fe and unavoidable impurities; wherein the austenitic alloy powder has a particle size distribution of (ø) >0 but less than or equal to 600 um.

An object manufactured from the powder deñned hereinabove or hereinafter will when in delivery conditions contain nano Z-phase particles (precipitates). These particles will have an impact on the strength so that the object will have a high strength at both normal temperature and higher temperatures, especially when being inanufactured by a HIP process. Additionally, it has been found that these Z-phase particles are homogeneously distributed in the object. The Z-phase particles will have a particle size of less than l um, which means that they will be nano-particles and these particles 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 paiticles 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. The object will be a heat resistance object meaning that it will withstand 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.

O-BESK-E OLF submitted 2021-09-21The eïements comprised in the austenitic alioy powder and the austenitic alloy object will be described below. The list of properties mentioned for each element should not be consider as being exhaustive, the elements may have other effects not mentioned.

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%. The content of C may in the powder, and the object also be from 0.04 to 0.20 wt%. The content of C in the powder, and the object from 0.05 to 0.10 wt%.

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 tenn exposure, the Silicon content should be 50.80 wt%. The content of Si of the powder, and the object may also be 5 0.40 wt%, such as 50.30 wt%.

Manganese (Mn) is a deoxidizing element and is also effective to improve the hot workability. Hou/ever, in order to prevent the creep rupture strength, ductility and toughness from decreasing, the rnanganese content should be 5 1.0 wt%, such as 5 0.60 wt°/2>.

Phosphorous (P) and Sulphur (S) are detrimental to the weldability and may promote embrittlement. Therefore, the phosphorus and the sulphur content should be 5 0.03 wt°/í>.

Chromium (Cr) is an effective element to improve the corrosion resistance and oxidation resistance, In order to achieve a sufñcient 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 the considerations, the chrontium content is restricted to a range of 20.0 to 27.0 wt°/o, such as 22.0 to 25.0 wt%.

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 O-BESK-E OLF submitted 2021-09-21austenite 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 vvt%. 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 reasonabie 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%, such as 23.0 to 28.0 wt%, such as 23.0 to 26.0 wt% Tungsten (W) and Molybdennm (Mo) 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 molybdenurn content is held low, 5 1.0 wt%, such as be S 0.50 wt%, such as S 0.30 wt%. In order to have a solid solution hardening effect, the tungsten content shoušd be higher than 0.50 wt%. However, the content should not exceed 5.0 wt% for avoiding unwanted interrnetallic phases. The tungsten content in the powder and the object will be 1.5 to 4.0 wt%.

Cobalt (Co) is an austenite-stabilizing element. The addition of cobalt may improve the high temperature strength through solid solution strengthening and suppression of sigma phase formation after long exposure times at elevated temperatures. I-Iowever, 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% Titanium (Ti) may be added for the purpose of irnproving the creep rupture strength through the precipitation of carbonitrides. carbides and nitrides. However, an excessive amount of títanium can decrease the weldability and the workability. For these reasons, the content of títaniurn is 50.1 wt%.

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 worlcability. In view of these considerations, the copper content is defined to a range of 1.0 to 5.0 wt%, such as 1.O-BESK-E OLF submitted 2021 -09-21to 3.5 wt%.

Aluminium (Al) and Magnesium (Mg) Aluminium and magnesium are effective for deoxidization during manufacturing. However, an excessive amount of aluminium may accelerate the precipitation of the sigma phase and an excessive amount of magnesium may deteriorate the weldability. For these reasons, if added, the content of aluminium is 5 0.05 wt%, such as 0.003 to 0.05 wt% and the content of magnesium is 5 0.05 wt%, such as 0.003 to 0.05 wt%.

Niobium (Nb) is generally accepted to contribute to improving the creep rupture strength through the precipitation of carbonitrides and nitrides. I-Iowever, 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°/o, such as 0.30 to 0.70 wt°/ Boron (B) contributes to improve the creep rupture strength partly due to the formation of tinely dispersed Mz3(C,B)6 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 5 0.008 wt% , such as 0.002 to 0.008 wt%.

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°/o, such as 0.20 to 0.40 wtWo.

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. I-Ience, the maximum content of oxygen isppm, such as less than 150 ppm.

When the terms "max" or "å" are used, the skilled person knows that the lower limit of the range is 0 wt% unless another number is speciñcally stated.

The rernainder is iron (Fe) and norrnally occurring irnpurities as discussed above. The term "impuritíes" means elements that are considered to be impurities meaning that they are O-BESK-E OLF submitted 2021-09-allowed to be present in the steel but only in such amount that the properties of the steel are not affected. Thus, irnpurities are elements and compounds which have not been added on purpose but cannot be fully avoided as they normally occur as impurities in eg. the raw material or the additional alloying elements used for manufacturing of the steel. The impurities may be present in a range of 3 0.50 wt°/<>. The present austenitic alloy powder and said object may comprise the following elements in weight% (wt%): Elements Broad Intermediate Narrow C 0.03 - 0.30 0.04 - 0.20 0.05 ~ 0.10 Si S 0.80 S 0.40 S030 Mn S 1.0 S 0.60 S 0.60 P S 0.03 S 0.03 S 0.03 S 0.03 S 0.03 S 0.03 Cr 20.0 - 27.0 20.0 - 27.0 22.0 - 25.0 Ni 22.0 - 32.0 23.0 - 28.0 23.0 - 26.0 B S 0.008 S 0.008 S 0.008 Co 0.5 - 3.0 1.0 - 2.0 1.0 - 2.0 Mo S 1.0 S 0.50 S 0.30 W 0.50 - 5.0 1.5 - 4.0 1.5 ~ 4.0 Ti S 0.1 S 0.1 S 0.1 Al S 0.05 S 0.05 S 0.05 Mg S 0.05 S 0.05 S 0.05 Cu 1.0 -5.0 l.5-3.5 1.5 ~3.5 Nb 0.10 - 1.0 0.30 -0.70 0.30 - 0.70 N 0.10 ~ 0.50 0.20 - 0.40 0.20 -- 0.40 O S 300 ppm S 300 ppm S 300 ppm Balance Fe and unavoidable impurities Further, the present powder or object as defined hereinabove or hereinafter may eonsist or compríse of all the elements rnentioned herein and in the different ranges as mentioned herein. 6 21302401 O-BESK-E OLF submitted 2021-09-21The 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 eiements in the ranges as disclosed hereinabove or hereinafier.

Additionally, a cross-section of the object as defined hereinabove or hereinafter which has been obtained through the process as defined hereinabove or hereinafter has a Z-phase precipitation density (l/mmz) of at least 30 000. It is believed that this precipitation density of at least 30 000 rnmz will have a positive impact on the creep strength. Furthermore, optionally the austenitic object may have an average number cross section area of less than 0.15 un? but not 0 as without being bound to any theory it is believed that an even better creep strength will be obtained. The Z-phase particles are primary particles 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 meltiiig 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.

The feedstock material may consist of Virgin raw material of elements, alloys and/or scrap metal. The feedstoek material may also consist of AOD or VIM produced pre-alloyed feedstock.

The present disclosure relates to a process wherein the powder is consolidated into solid dense material by using Hot Isostatic Pressing, (HIP). 'Ihe process comprises the steps of a) providing a form deñning 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; e) subjecting said form to hot isostatic pressing at a predetemiined temperature, a predetennined isostatic pressure and for a predeterrnined time so that the O-BESK-E OLF submitted 2021-09-21powder particles bond metallurgically to each other and all interparticle voids are closed whereby a. solid body is formed d) annealing the solid body in a predeterniined temperature during a predetermined time; e) subsequentiy quenching the annealed body. The form could also be called mould or Capsule and may be formed from a eg. 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.

The form is subjected to hot isostatíc pressing at a predetermined temperature, a predetennined isostatic pressure and for a predetermined time. ln this step, the powder will consolidate into a solid part through the combined influence of heat and extemal pressure actíng 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 predeterrnined time may be from 10 minutes to 3 hours. The predetemiined pressure may be from 900 to 1500 bar and the predeterrnined 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 interpaiticle solid-state diffusion bonding will occur.

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.

In order to dissolve unwanted phases which may have been formed during Cooling down after HIP, the consolidated solid body is annealed and subsequently quenched in liquid, such as water or oil. The anneaiing may be performed for 10 to 60 minutes at a temperature ofto 1250 "C and thereafter subsequently quenched.

The obtained object will have a homogeneous composition and isotropio properties throughout the whole body due to the rapid Cooling of the powder particles in the atomization process and the solid state consolidation of the HIP process.

The present disclosure is further described by the following non-limitiiig examples.O-BESK-E OLF submitted 2021-09-21Example The capsule used for HlPzing of the powder had the rough outer dimensions 178x69x49mm. Three different HIP-temperatures were utilized: l150°C, l200°C and 1250°C. The HIP pressure used was about 100 MPa and the time used was about 2 to 4 h.

Table 1 show the composition of the powder used in the Example.

The bodies of eonsolidated powder were subsequently quench-annealed and had the rough dimensions 150x63x23 mm. The annealing was performed in a range of 30 to 50 min into 1250°C and thereafter water quenched.

Sections of the material were polished to OPS surface finish (0025 pm oxide polish surface finish) and the documentation of the microstruoture was performed both in Light Optic Microscope (Leica Reíchert 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, dirnensions and morphology of the bright Contrast precipitates (EDS or back-scatter deteetor mode) were measured over a certain area on a polished section. The calculation was performed accordingly: Number density = the number of Z-phase partioles observed divided by the total analyzed area on which the actual feature counting was performed. Particle size = the apparent section area of an identified Z-phase particle.

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.

Figure 4 discloses a SEM picture of the material having a unifonn and ñne-dispersed distribution of Z-phase particles.

'Ihe number density of Z-phase precipitates is shown in Figure The average Z-phase particle size is shown in Figure O-BESK-E OLF submitted 2021-Table 1 The conzposition used, the balance is Fe and unavoídable impurities.

C Si Mn P S Cr Ni 0.06 0.19 0.49 0.017 00007 22.6 24.7 Co Cu Nå) W B N 1.4 2.8 0.49 3.5 0.004 0.T est performed on the obtained obiect Table 2 show the test result of the mechanical testing at room testing Table 2 Room :ernperature tensíle test results .else :ssete ett assets; e i 2:. t:tfs: se; så? "fl Table 3 Creep test results HIP temp. __* 1tso°c 1zoo°c 1 1250°c Stress level 220 MPa (desired life length > 1000 h) Specimen 1 2 1 2 1 2 Creep life 1162 h 1187 h 1208 1221 1201 1171 Stress level 185 MPa (desired life length > 3000 h) Specirnen 1 2 1 2 1 Creep life 3496 2755 3 148 3247Creep testing was performed at 700°C material temperature on the fllPzed and quench annealed material. The tensile stress levels were chosen based on the results of creep testing of seamless tube material of similar eomposition 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 exceedecl the desired life Iengths.

Claims (14)

1.An austenitic alloy object having an elementary composition of, in percentages by weight: C Si Mn 0.03 - 0.30; 5 0.80; 5 1.0; S 0.03; S 0.03; 20 - 27; 22 -32; _<_ 1.0; 0.5 - 3.0; 1.0 - 5.0; 0.1 - 1.0; 0.50 - 5.0; 5 0.10; 5 0.05; 5 0.05; 5 0.008; 0.10 - 0.50; 5 300 ppm; the balance being Fe and unavoidable impurities; wherein the object has a Z-phase precipitation number density (I/mmï) of at least The austenitic alloy object 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%.

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

3.The austenitic alloy object 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%.

4.O-PKRAV-E OL submitted 2022-07-

5. The austenitic alloy object according to any one of claims 1 to 4, wherein the content of Cr is in the range of 22.0 to 25.0 weight%.

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

8. The austenitic alloy object 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 object 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 object according to any one of claims l to 9, wherein the content of W is in the range of 1.5 to 4.0 weight%.

11.1. The austenitic alloy object 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. The austenitic alloy object according to any one of claims 1 to 11, wherein the Z-phase particle has a number average cross section area of less than 0.15 pmz but not

13. The austenite alloy object according to any one of claims 1 to 11, wherein said object is a H1P:ed object.

14. A process of manufacturing an austenitic alloy object according to any one of claims 1 to 13 comprising the steps of: a. providing a form defining at least a portion of the shape of said object; and providing a powder which has a particle size distribution of (d) >21302401 O-PKRAV-E OL submitted 2022-07-but less than or equal to 600 pm and having an element composition according to any of claims 1 to 11; b. filling at least a portion of said form with said powder; and evacuate the form and seal it tight from the outside atrnosphere; c. subjecting said form to hot isostatic pressing at a predetennined temperature, a predetennined isostatic pressure and for a predetennined time so that all inter~partie1e voids are closed, and a solid, dense body is formed by solid-state diffusion bonding of the powder partícles; d. annealing the solid body at a predetermined temperature during a predeterrnined time; e. subsequently quenching the annealed body.