STAINLESS STEEL POWDER FOR PRODUCING DUPLEX SINTERED STAINLESS STEEL

Abstract

Embodiments of the present invention may provide a new stainless steel powder suitable for manufacturing of duplex sintered stainless steels. Embodiments of the present invention may also relate to a method for producing the stainless steel powder, the duplex sintered stainless steel as well as methods for producing the duplex sintered stainless steel.

Claims

1. A stainless steel powder comprising: up to 0.1% of C, 0.5-3% of Si, up to 0.5% of Mn, 20-27% of Cr, 3-8% of Ni, 1-6% of Mo, up to 3% of W, up to 0.1% N, up to 4% of Cu, up to 0.04% of P, up to 0.04% of S, unavoidable impurities up to 0.8%, optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, rest Fe.

2. A stainless steel powder according to claim 1 comprising: up to 0.06% of C, 1-3% of Si, up to 0.3% of Mn, 23-27% of Cr, 4-7% of Ni, 1-3% of Mo, 0.8-1.5% of W, up to 0.07% N, 1-3% of Cu, up to 0.03% of P, up to 0.03% of S, unavoidable impurities up to 0.8%, optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, rest Fe.

3. A stainless steel powder according to claim 1 comprising: up to 0.03% of C, 1.5-2.5% of Si, up to 0.3% of Mn, 24-26% of Cr, 5-7% of Ni, 1-1.5% of Mo, 1-1.5% of W, up to 0.06% N, 1-3% of Cu, up to 0.02% of P, up to 0.015% of S, unavoidable impurities up to 0.8%, optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, rest Fe.

4. A stainless steel powder according to claim 1 wherein the stainless steel powder is ferritic.

5. A stainless steel powder according to claim 1 wherein the stainless steel powder is produced by water atomization.

6. A stainless steel powder according to claim 1 wherein the stainless steel powder is produced by gas atomization.

7. A stainless steel powder according to claim 1 wherein the particle size of the powder is between 53 microns and 18 microns such that at least 80% of the particles are less than 53 microns and at most 20% of the particles are less than 18 microns.

8. A stainless steel powder according to claim 1 wherein the particle size of the powder is between 26 microns and 5 microns such that at least 80% of the particles are less than 26 microns and at most 20% of the particles are less than 5 microns.

9. A stainless steel powder according to claim 1 wherein the particle size of the powder is between 150 microns and 26 microns such that at least 80% of the particles are less than 150 microns and at most 20% of the particles are less than 26 microns.

10. A stainless steel powder according to claim 1 wherein the powder is a prealloyed powder.

11. A method for producing a stainless steel powder by water atomization comprising the steps of: providing a molten metal of having a chemical composition corresponding to the chemical composition of the stainless steel powder according to claim 1, subjecting a stream of the molten metal to water atomization, recovery of the obtained stainless steel powder.

12. A sintered duplex stainless steel having a chemical composition according to claim 1 and wherein the microstructure of the sintered duplex stainless steel is characterized by austenite phase precipitated in ferrite phase.

13. A sintered duplex stainless steel according to claim 12 wherein the Ni equivalent (Ni.sub.eq) is such that 5<Ni.sub.eq<11 and the Cr equivalent (Cr.sub.eq) is such that 27<Cr.sub.eq<38 and wherein Cr.sub.eq and Ni.sub.eq are calculated according to the formulas:
Cr.sub.eq=Cr+2Si+1.5Mo+0.75W
Ni.sub.eq=Ni+0.5Mn+0.3Cu+25N+30C and, wherein Cr, Ni, etc. are the level of each element in the alloy in weight %.

14. A sintered duplex stainless steel according to claim 12 wherein the pitting resistance equivalent number (PREN) is 28<PREN<33 and wherein PREN is calculated according to the formula:
PREN=Cr+3.3Mo+16N and, wherein Cr, Mo and N are the level of each element in the alloy in weight %.

15. A sintered duplex stainless steel according to claim 12 wherein the microstructure of the sintered duplex stainless steel contains 30-70% austenite.

16. A sintered duplex stainless steel according to claim 12 wherein the microstructure is characterized by being free from sigma phases and nitrides.

17. A method for producing a duplex sintered stainless steel comprising the steps of: providing a stainless steel powder according to according to claim 1, optionally mixing the stainless steel powder with a lubricant and optionally other additives, subjecting the stainless steel powder or the mixture to a consolidation process forming a green component, subjecting the compacted green component to a sintering step in an inert or reducing atmosphere or in vacuum at a temperature between 1150? C. to 1450? C., preferably at a temperature between 1275? C. to 1400? C. for a period of time of 5 minutes to 120 minutes, subjecting the sintered component to a cooling step down to ambient temperature.

18. A method for producing a duplex sintered stainless according to claim 17 wherein the consolidation process includes: uniaxial compaction at a compaction pressure of up to 900 MPa in a die to form a green component, ejecting the obtained compacted green component from the die.

Description

FIGURE LEGENDS

[0128] FIG. 1 shows the microstructure of invented sintered stainless steel, austenite and ferrite phases are present in equal proportions in as sintered condition, black spots are porosity.

[0129] FIG. 2 discloses a comparison of ultimate tensile strength (UTS) and corrosion properties of the invented sintered stainless steel compared to 300 and 400 alloys, (SAE grades).

[0130] FIG. 3 shows a comparison of mechanical properties of the invented sintered stainless steel at different sintering conditions

EXAMPLES

Example 1

[0131] A stainless steel powder, having a particle size below 325 mesh, i.e. 95 wt % of the particles passed 45 ?m sieve, was mixed with 0.75 wt % of Acrawax as a lubricant. The chemical analysis of the stainless steel powder was 0.01 wt % C, 1.52 wt % Si, 0.2 wt % Mn, 0.013 wt % P, 0.008 wt % S, 24.9 wt % Cr, 2.0 wt % Cu, 1.3 wt % Mo, 1.0 wt % W, 0.05 wt % N, balance Fe.

[0132] The obtained powder mixture was pressed in a uniaxial press and compacted into transverse rapture strength (TRS) bars, according to ASTM B528-16 at a compaction pressure of 750 MPa. The pressed TRS bars were then sintered in 100% hydrogen atmosphere at 1343? C. with ramp rate of 7? C./minute for 45 minutes. This was followed by furnace cooling at rate 5? C./minute. The samples were then mounted and polished for microstructure examination. The polished samples were then electro-etched with 33% NaOH at 3V for 15 sec. Electro-etch with NaOH reveals the ferrite phase as tan, austenite as white (unaffected) and sigma phases in dark orange at grain boundaries within ferrite matrix. The microstructure observed is as shown in FIG. 1. The microstructure shows approximately 50/50 mixture of ferrite (tan) and austenite (white). There is no sign of any sigma phase (dark orange) in the microstructure. The black spots are porosity in the sample.

Example 2

[0133] Various stainless steel powders according to embodiments of the invention, and as comparative samples, were produced by water atomizing. The chemical composition of the stainless steel powders are shown in table 1. Stainless steel melts having various chemical compositions were melted in an induction furnace, the molten metal was subjected to water stream to obtain steel powder. The obtained powders was then dried and screened to ?325 mesh. The screened powder was ?45 microns i.e. 95 wt % of the powder particles were less than 45 microns. The powders were then mixed with 0.75 wt % of the lubricant Acrawax.

[0134] In order to test the mechanical properties i.e. ultimate tensile strength (UTS), yield strength (YS) and elongation, TS samples (dog bone) per ASTM B925-15 were pressed with a compaction pressure of 750 MPa. The bars were then sintered as mentioned in Example 1. The sintered bars were then tested for mechanical properties per ASTM E8/E8M-16a. Metallographic examination was also conducted in order to establish the ratio between austenite and ferrite in sintered samples. The test results are shown in table 2 in comparison with published data from samples of known duplex stainless steels in wrought, (DSS 329 Wrought), and gas atomized and hipped conditions (DSS 329 PM GA).

[0135] Table 2 shows that the stainless steel powders according to the present invention can be used for producing sintered duplex stainless steel having desired mechanical properties.

TABLE-US-00001 TABLE 1 chemical compositions of various stainless steel powders, there production method and type of process for producing sintered samples. Chemical analysis [% by weight] Sample Type C Si Mn S P Cr Ni Mo W Cu O N Other Comparative DSS 329 Wrought 0.08 1.00 23-28 2.5-5 1-2 0.08 Wrought steel Comparative DSS 329 Water 0.20 0.75 1.00 23-28 2.5-5 1-2 0.05 0.08 PM WA atomized powder, HIP Comparative DSS Gas 0.03 1.00 2.00 0.020 0.030 22.0-23.0 .sup.4.5-6.5 3.0-3.5 0.75 0.14-0.20 2205 atomized PM GA powder HIP Premix XSS DP1 Water 0.03 2.00 0.10 0.006 0.008 25 5.5 1.3 1 2 0.2 0.06 PM WA atomized Premix powders.sup.4, compacted and sintered Invention XSS DP1 Water 0.01 1.52 0.20 0.013 0.008 24.9 5.5 1.3 1 2 0.15 0.05 PM WA atomized Prealloy powder, prealloyed compacted and sintered Invention XSS Water 0.03 1.97 0.10 0.007 0.012 23.4 5.1 1.2 0.9 1.9 0.13 0.05 DP1-1 atomized powder, prealloyed compacted and sintered Invention XSS Water 0.03 2.12 0.20 0.007 0.012 26.1 5.2 1.3 0.9 3 0.15 0.02 DP1-2 atomized powder, prealloyed compacted and sintered Invention XSS Water 0.03 1.94 0.20 0.008 0.013 25.1 5.6 1.2 0.8 2 0.15 0.02 0.58 Nb DP1-3 atomized powder, prealloyed compacted and sintered Comparative XSS Water 0.03 2.14 0.20 0.009 0.015 22.3 5.2 1.3 0.9 1.9 0.16 0.06 0.6 Sn DP1-4 atomized powder, prealloyed compacted and sintered .sup.4Premix of 316L, 434L and elemental powders of Si, W and Cu.

TABLE-US-00002 TABLE 2 mechanical properties and metallographic structure for sintered samples produced from stainless steel powders according to table 1. Mechanical Properties Sintering time TS YS TRS Elongation % austenite in Sample Type [minutes] [Mpa] [Mpa] [Mpa] [%] ferrite matrix Comparative DSS 329 Wrought steel Annealed 725 550 25 ~50 Wrought Comparative DSS 329 PM WA Water atomized powder, HIP 45 523 460 180 7 0 Comparative DSS 2205 PM GA Gas atomized powder, HIP 45 578 427 200 11 ~50 Comparative XSS DP1 PM WA Water atomized powders.sup.5, compacted 45 720 700 220 2.5 ~35 Premix and sintered Invention XSS DP1 PM WA Water atomized powder, prealloyed 45 776 617 278 8.6 ~60 Prealloy compacted and sintered Invention XSS DP1-1 Water atomized powder, prealloyed 45 727 504 275 11.0 ~50 compacted and sintered Invention XSS DP1-2 Water atomized powder, prealloyed 45 809 745 265 2.5 ~50 compacted and sintered Invention XSS DP1-3 Water atomized powder, prealloyed 45 843 691 257 6.5 ~45 compacted and sintered Comparative XSS DP1-4 Water atomized powder, prealloyed 45 749 743 218 0.5 ~10 compacted and sintered .sup.5Premix of 316L, 434L and elemental powders of Si, W and Cu.

[0136] An embodiment of the invented powder with composition as in Example 1 was also sintered at various temperatures and atmospheres below, to show the effect on mechanical properties. Such data is plotted in FIG. 3. [0137] A. 2500? F. for 45 minutes in hydrogen gas [0138] B. 2450? F. for 45 minutes in hydrogen gas [0139] C. 2450? F. for 60 minutes in hydrogen gas [0140] D. 2300? F. for 60 minutes in hydrogen gas [0141] E. 2250? F. for 60 minutes in hydrogen gas [0142] F. 2250? F. for 60 minutes in dissociated ammonia

Example 3

[0143] In order to perform corrosion test, TRS bars as in Example 1 were produced along with bars for 316L and 434L as representatives from austenitic and ferritic grades. The samples were then tested for corrosion in 5% NaCl solution at room temperature per ASTM B895-16. The corrosion was compared by the hours takes for onset of corrosion on the samples. The comparative data is plotted in FIG. 2 along with the UTS and YS for these samples. The diameter of the bubbles in the FIG. 3 represents the number of hours taken for the start of the corrosion on the samples. The corrosion test for the invented powder was discontinued after 3700 hours as there was no sign of corrosion and it already exceeded 3 times that of 316L samples.