Nitrogen alloying boost for PM stainless steels

Nitrogen alloying boost for PM stainless steels

,bPMZTEC’95 i Nitrogen alloying boost ‘c for PM stainless steels I 0.5 0.6 I 0.7 I 0.8 0.9 1.0 1.1 1.2 1 1.8 1.4 Nitrogen (WI%) FlGUR...

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,bPMZTEC’95 i Nitrogen alloying boost

‘c

for PM stainless steels

I

0.5

0.6

I

0.7

I

0.8

0.9

1.0

1.1

1.2

1

1.8

1.4

Nitrogen (WI%) FlGURE 7: Yield strength

of high nitrogen PM stainless steels produced by pressurized gas atomization

itrogen has been used as an alloy addition in wrought stainless steels plus hot extrusion. for many years. Its prime function has been to substitute for nickel as a less expensive austenite stabilizer. Commercially available nitrogen-alloyed stainless steels include the AISI 200 series. Recent developments in high-pressure processing techniques have made it possible to produce commercial high-nitrogen stainless steels with nitrogen levels over l%, far above the limited solubility of nitrogen in liquid ferrous alloys at atmospheric pressure. In a presentation based on laboratory experimental alloys, John Simmons of the US Bureau of Mines Albany Research Center in Albany, Oregon, compared the mechanical properties of highnitrogen austentic stainless steels based on AISI 201 (Fe17Cr-lOMn-5Ni-0.2C) prepared by ingot metallurgy and by powder metallurgy (PM). The wrought alloys were prepared

N

using a high-pressure induction melting furnace followed by hot rolling, and the PM alloys using pressurlsed gas atomization followed by hot extrusion. The high

80

85

nitrogen concentrations in the powder alloys were achieved by pressuring the melt chamber to 10 atmospheres and by adding ferro-chromium nitride (FeCrN) to the melt just prior to atomization. Nitrogen levels up to 1.35% were obtained in the modified AISI 201 powders (about 5 times the atmospheric solubility) The PM alloys showed yield strength and ultimate tensile strength values equal to or greater than those of materials processed by ingot metallurgy at equal nitrogen levels (Figure 1). High tensile ductilities were obtained despite the presence of oxide contamination of the powder by refractory material from the melting and atomizing equipment. On a seemingly more conventional note, but with poten-

90

9s

100

% Full dense

0 304L

+ 316L w410L *43OL

04341

FlGURE2: Effect of density on the ultimate tensile strength of PM stainless steels.

Our powder press systems for tomorrow

MPR JulylAugusi-

1995

tially high significance to PM practitioners, Howard Sanderow (Management & Engineering Technologies, Dayton, Ohio) reviewed the results of the second stage in a test programme designed to provide the database for future revision of properties standards. The programme mvolves the testing of 10 austrnitic and ferritic PM stainless steel compositlons. In the first part, presented earlier in the year at t.he 1995 SAE Congress in Detroit (see Metal Powder Report, May 1995), Dah-Wei Yuan and colleagues from Concurrent Technologies Corp provided details of corrosion tests that had been completed on t.he PM stainless steels in a variety of density and sintering conditions. This second paper is a progress report on the development of mechanical properties, but only deals with tensile, impact and hardness values. In combination with further properties test,ing yet to be completed, this data will he used in the preparation of revised material propert irs for inclusion in MPIF Standard 35. In line with industry practice. several pro-

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Tungsten Tungsten

prietary alloy grades were included, as well as sintering at conventional and higher temperatures in dissociated ammonia and vacuum. The results confirmed and enlarged on several currently accepted views. Thus, mechanical properties of PM stainless steels are not determined solely by density (Figure 2), but also depend strongly on the sintering temperature and atmosphere higher sintering temperatures being especially beneficial for ductility and impact resistance (Figure 3). Sintering in a nitrogen-based atmosphere increased both strength and hardness but reduced ductility and impact resistance. The specialized grades having improved corrosion resis‘316LSC’ and tance, e.g. ‘316PLUS’ manufactured by SCM and Ametek, respectively, were found to satisfy the minimum strength and elongation requirements of MPIF Standard 35 when produced with typical density and sintering conditions suggested in the standard of regular stainless st,eel grades.

General

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90

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I00

Fulldense

704L +316L n 4lOL

*37OL 0434L t 303L +CI.ow,dloysteel o LAS-HT.5 FlGURE

of density on

3: Effect

the elongation of PM stainless steeis.

Joseph M. Capus

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