DEVELOPMENT OF A DUAL-PHASE PRECIPITATION-HARDENING PM STAINLESS STEEL

Chris Schade & Tom Murphy

Hoeganaes Corporation Cinnaminson, NJ 08077

Alan Lawley & Roger Doherty

Drexel University Philadelphia, PA 19104

ABSTRACT

Stainless steels can now be fabricated by the pressing and sintering of water atomized powder. PM grades embrace: ferritic, austenitic, martensitic, duplex (ferritic + austenitic), dual-phase (ferritic + martensitic), and precipitation hardening (martensitic). Development of dual-phase PM stainless steels reflects the growing need for higher strength, coupled with ductility and toughness. In the present study, a new low-cost PM stainless steel has been developed which combines the advantages of a dual-phase (ferrite + martensite) microstructure with precipitation hardening. Unlike other precipitation hardening alloys, ductility and impact toughness increase significantly upon aging, notwithstanding attendant increases in hardness and strength. The mechanical properties of the new alloy are evaluated in terms of composition and microstructure. INTRODUCTION In the competition between wrought and PM stainless steels, PM materials are at an extreme disadvantage due to the deleterious effect of porosity on mechanical properties such as tensile strength, ductility and impact toughness. Furthermore, the use of increased alloy levels in PM stainless steels is both costly and counter-productive due to the negative effect on compressibility. The addition of graphite, which is used for increasing mechanical properties in ferrous PM, is detrimental to the corrosion resistance of stainless PM steels and reduces ductility. In order to achieve improved mechanical properties and enhance corrosion resistance in PM stainless steels, it is necessary to explore non-traditional strengthening mechanisms. It has been shown that utilizing a dual phase microstructure can lead to increased strength in a PM stainless steel. 1-3 The microstructure is a combination of ferrite and martensite (Figure 1). The ferrite allows for a higher sintered density, improving ductility and toughness, while the martensite imparts strength and hardness. The levels of martensite and ferrite can be balanced by adjusting the content of the austenite stabilizers (nickel and copper) and the ferrite stabilizers (chromium, silicon and molybdenum). One of the most

common dual phase PM stainless steels, SS-409LNi (Table I), is commonly used for exhaust flanges. The ferritic microstructure of SS-409L is altered by admixing nickel which promotes the formation of high temperature austenite during sintering, and which transforms to martensite during cooling.

(a) (b) Figure 1. Representative microstructures: (a) press + sinter PM dual-phase stainless steel; sintered density 7.20 g/cm3 (b) press + sinter PM 17-4 PH; sintered density 6.70 g/cm3. Precipitation hardening stainless steels are not defined by their microstructure, but rather by the strengthening mechanism. These grades can have austenitic, semiaustenitic or martensitic microstructures and can be hardened by aging at moderately elevated temperatures, 480 oC to 620 oC (900 oF to 1150 oF). The strengthening effect is due to the formation of intermetallic precipitates from elements such as copper or aluminum. Aluminums high affinity for nitrogen and oxygen in PM stainless steels necessitates strict atmosphere control during sintering and, for this reason, copper is the most commonly used element for precipitation hardening. These alloys generally have high strength and high apparent hardness while exhibiting superior corrosion resistance compared with martensitic stainless steels. This improved corrosion resistance is derived from the fact that the carbon levels are low and the martensite is formed from additions of nickel and copper. The low carbon martensite that is formed is weaker but more ductile than the martensite formed in alloys such as SS-410-90HT (carbon bearing), but the strength of these alloys is developed by aging.

Table I: Composition of Stainless Steel PM Alloys (w/o).

One of the most common precipitation hardening stainless steel grades in both the wrought and PM industries is 17-4 PH (Table I). This grade has a martensitic

Alloy C P Si Cr Ni Cu Mn Mo Cb17-4PH 0.018 0.025 0.85 17.1 4.00 3.55 0.15 0.03 0.25409LNi 0.013 0.01 1.00 11.3 1.30 0.04 0.12 0.05 0.56

DP2 0.015 0.014 0.84 11.6 1.03 0.29 0.10 0.22 ---SS-410-90HT 0.200 0.012 0.81 12.0 0.14 0.01 0.11 0.05 ---

microstructure and its strength and hardness can be improved by aging treatments.4-7 The general corrosion response of this alloy is superior to that of standard martensitic stainless steels due to the higher chromium level. There are many applications in which stainless steels with only moderate corrosion resistance (compared with 17-4 PH) but excellent mechanical properties are required. A widely used alloy is SS-410-90-HT. This is a PM 410L stainless steel in which graphite is added to the atomized powder and the alloy is sintered in a nitrogen-rich atmosphere. When carbon and nitrogen are added to 410L stainless steel, the microstructure becomes martensitic, thereby increasing both strength and hardness. This grade of stainless steel is used in applications where high strength and hardness are required. The disadvantage of using carbon and nitrogen is that they degrade corrosion resistance and also reduce impact strength and ductility. Many PM fabricators select this alloy because there are few alternative compositions. ALLOY DEVELOPMENT The purpose of this work was to develop an alternative composition that makes use of the two strengthening mechanisms. The combination of a dual phase microstructure and precipitation hardening should allow for the development of a low cost, high strength alloy with moderate corrosion resistance. The development of this dual phase precipitation hardening (DPPH) alloy can improve PMs competitive advantage over wrought materials. Previous work by the authors focused on developing a dual phase microstructure in a nominal 12 w/o chromium-containing stainless steel.3 This was accomplished by balancing the ferrite stabilizers (chromium, silicon and molybdenum) and austenite stabilizers (carbon, nitrogen, nickel and copper) such that the microstructure consisted of ferrite and martensite (DP2 in Table I). The copper in this alloy was intended only to stabilize the austenite and was at too low a level for any significant precipitation. Therefore a new set of alloys was made with copper ranging from 1 w/o to 4 w/o.

Table II: Composition of Experimental DPPH Alloys (w/o)

Alloy C P Si Cr Ni Cu Mn Mo

Alloy A 0.011 0.007 0.65 11.81 1.05 0.04 0.10 0.35Alloy B 0.015 0.005 0.82 11.71 1.22 0.35 0.12 0.31Alloy C 0.013 0.012 0.83 12.11 1.06 0.99 0.07 0.38Alloy D 0.014 0.007 0.83 12.65 0.97 2.13 0.05 0.33Alloy E 0.017 0.010 0.78 12.13 1.05 2.55 0.06 0.35Alloy F 0.016 0.008 0.73 12.77 1.08 3.06 0.15 0.35Alloy G 0.015 0.011 0.92 11.82 1.06 3.48 0.08 0.36Alloy H 0.016 0.012 0.80 12.17 1.07 3.95 0.06 0.34

ALLOY PREPARATION AND TESTING The powders used in this study were produced by water atomization with a typical particle size distribution 100 w/o

RESULTS AND DISCUSSION Development of Dual Phase Microstructure The compositon of the DPPH alloys was based on the dual phase chemistry previously developed. This was a nominal 12 w/o Cr-1 w/o Ni-.35 w/o Mo alloy. For this study the copper was varied up to 4 w/o. Since copper is an austenite stabilizer, it promotes the formation of martensite in the sintered microstructure. For this reason metallography was performed to confirm the level of ferrite and marteniste in the final microstructure. This was to determine at what level of copper a dual phase microstructure still existed. Quantitative metallography was performed to measure the actual ferrite content of the PM alloys.

0

10

20

30

40

50

0 1 2 3 4 5

Ferr

ite C

onte

nt (v

/o)

Copper Content (w/o) (a) (b)

Figure 2. Sintered properties of DPPH as a function of copper content: (a) v/o ferrite, and (b) sintered

density.

Figure 2(a) shows the ferrite level as a function of the copper content. At low copper levels ferrite percentages exceed 40 v/o and at high copper levels the microstructure is predominately martensitic. There is a range of copper levels between 2 w/o and 4 w/o over which the ferrite level is relatively stable at 30 v/o. In Figure 2(b) the sintered densities of the alloys are shown. Since higher copper levels lead to poor compressibility, the sintered density decreases as the copper level increases, with a dramatic drop-off in density at 4 w/o Cu. For these reasons the study focused on DPPH P/M compositions with copper concentrations < 3 w/o. Most of the studies of dual phase steels have been on wrought low alloy steels. In these studies the properties of the dual phase steel have been related to the microstructure in terms of the levels of martensite and ferrite. The tensile strength and other properties have been shown to vary linearly with the volume fraction of the phases by the law of mixtures.8-9 Intuitively, the tensile strength and hardness increase with the level of martensite, while the ductility and impact toughness vary proportionally with the level of ferrite. In order to optimize the PM alloy, the materials were sintered to a density of 6.60

6.6

6.8

7

7.2

7.4

0 1 2 3 4 5

Sint

ered

Den

sity

(g/c

m3 )

Copper Content (w/o)

g/cm3 at temperatures ranging from 1120 C to 1260 C (2050 F to 2300 F). In this temperature range the alloy is in the two-phase austenite and ferrite region. Upon cooling the austenite transforms to martensite. By varying the sintering temperature the levels of each phase change. In general, higher sintering temperatures and low copper levels favor the formation of ferrite. Figure 3 shows the mechanical properties as a function of v/o ferrite and the copper level.

50

60

70

80

90

100

345

414

483

552

621

690

0 10 20 30 40 50

3 w/o Cu 2 w/o Cu1 w/o Cu0.35 w/o Cu

Yiel

d St

reng

th (p

si x

103 )

Ferrite (v/o)

Yiel

d St

reng

th (M

Pa)

(a) (b)

65

70

75

80

85

90

95

0 10 20 30 40 50

3 w/o Cu2 w/o Cu1 w/o Cu0.35 w/o Cu

App

aren

t Har

dnes

s (H

RB

)

Ferrite (v/o) (c) (d) Figure 3: Mechanical properties of DPPH PM stainless steel versus v/o ferrite Sintered density = 6.60 gm/cm3.

Similar to the low alloy dual phase wrought steels, as the v/o of ferrite increases the tensile strength and apparent hardness decrease while the ductility (measured by the elongation) increases. For a given v/o ferrite, increasing the copper concentration increases the strength and apparent hardness. This is due to the solution strengthening effect of the copper in the matrix. The microindentation hardness measurements in Figure 3(d) support this conclusion with the higher copper contents having the higher

250

300

350

400

450

500

0 10 20 30 40 50

3 w/o Cu2 w/o Cu1 w/o Cu0.35 w/o Cu

Mar

tens

ite M

icro

inde

ntat

ion

Har

dnes

s (H

V25)

Ferrite (v/o)

0

1

2

3

4

5

0 10 20 30 40 50

3 w/o Cu2 w/o Cu1 w/o Cu0.35 w/o Cu

Elon

gatio

n (%

)Ferrite (v/o)

microindentation hardness. Table III gives a summary of the mechanical properties of the DPPH alloys.

Table III: Mechanical Properties of DPPH PM Stainless Steels (As Sintered and Aged Conditions).

Development of Precipitation Hardening Component Strengthening, as a result of precipitation hardening, takes place in three steps:10

(1) Solution treatment, in which the alloy is heated to a relatively high temperature, allows any precipitates or alloying elements to form a supersaturated solid solution. Typical solution treatment temperatures are in the range of 982 oC to 1066 oC (1800 oF to 1950 oF).

(2) Quenching, in which the solution treated alloy is cooled to create a supersaturated solid solution. The cooling can be achieved using air, water or oil. In general, the faster the cooling rate the finer the grain size which can lead to improved mechanical properties. Regardless of the method of cooling, the cooling rate must be sufficiently rapid to create a supersaturated solid solution.

(3) Precipitation or age hardening, in which the quenched alloy is heated to an intermediate temperature or held at room temperature for a period of time. During aging, the supersaturated solid solution decomposes and the alloying elements form small precipitate clusters. The precipitates hinder the movement of dislocations and consequently the metal resists deformation and becomes harder and stronger.

In the DPPH PM alloys copper is the element involved in precipitation formation. Similar to 17-4 PH, the DPPH alloys can be aged after sintering to enhance their strength and hardness. Figure 4 shows an aging profile (by temperature) for a range of copper levels in the DPPH PM stainless alloys. Both the yield strength and apparent hardness behave as expected for an alloy in which precipitation hardening is occurring. As the

Impact Apparent Hardness Elongation

AISI ft.lbs.f (J) (HRA) (103 psi) (MPa) (103 psi) (MPa) (%)Alloy A Sintered 73 98 52 95 654 69 475 4.3

Alloy A Aged 76 102 53 95 654 70 482 6.6Alloy B Sintered 49 66 53 95 654 70 482 4.2

Alloy B Aged 54 72 53 96 660 73 502 6.6Alloy C Sintered 56 75 51 89 612 69 475 4.6

Alloy C Aged 56 75 53 100 688 79 544 7.1Alloy D Sintered 61 82 52 105 722 81 557 3.2

Alloy D Aged 69 92 57 121 832 100 688 5.1Alloy E Sintered 36 48 58 117 805 95 654 2.2

Alloy E Aged 40 54 62 142 977 117 805 4.6Alloy F Sintered 30 40 54 113 777 87 599 2.8

Alloy F Aged 36 48 60 137 943 116 798 3.7

UTS 0.20% Offset Yield

alloy is heated, precipitates are formed impeding the movement of dislocations causing an increase in strength and apparent hardness. Above 800 oF ( 427 oC) the alloy starts to increase in strength and hardness, reaching a maximum in both properties at approximately 538 oC (1000 oF). Above this temperature the precipitates start to coarsen and the hardness and strength decrease. The change in ductility of the DPPH alloys is more complicated. There occurs a ductility trough in the 2 and 3 w/o copper alloys at 482 oC (900 oF), while the ductility of the 1 w/o copper is relatively constant at this temperature. For a given set of processing conditions, as the copper level is increased there will be an increase in the level of martensite in the dual phase alloy. Since both carbon and nitrogen are ferrite stabilizers, they segregate to the martensite. At low aging temperatures the carbon and nitrogen form carbides and nitrides in the matrix, strengthening the alloys.

70

75

80

85

90

0 200 400 600 800 1000 1200

1 w/o Cu2 w/o Cu3 w/o Cu

483

506

529

552

575

598

6210 133 266 400 533

Yiel

d St

reng

th (p

si x

103 )

Aging Temperature (oF)

Yiel

d St

reng

th (M

Pa)

Aging Temperature (oC)

(a) (b)

(c) Figure 4: Aging profile of DPPH PM stainless steel as a function of copper concentration.

0

1

2

3

4

5

0 200 400 600 800 1000 1200

1 w/o Cu2 w/o Cu3 w/o Cu

0 133 266 400 533

Elon

gatio

n (%

)

Aging Temperature (oF)

Aging Temperature (oC)

35

40

45

50

55

60

0 200 400 600 800 1000 1200

1 w/o Cu2 w/o Cu3 w/o Cu

0 133 266 400 533

App

aren

t Har

dnes

s (H

RA

)

Aging Temperature (oF)

Aging Temperature (oC)

However, as the aging temperatures are increased, the elements diffuse to grain boundaries and embrittlement occurs. This is the reason for the decrease in elongation at 482 oC (900 oF). This effect is more pronounced as the martensite level of the alloy increases (1 w/o Cu versus 3 w/o Cu). Normally the ductility of an alloy decreases as the strength and apparent hardness increase. The DPPH alloys are remarkable in that, as the strength and hardness increase, the ductility also increases. At 538 oC (1000 oF), when all the DPPH alloys reach their maximum strength and apparent hardness, the elongation of the alloys increases and in fact exceeds the as sintered levels. The higher the v/o ferrite in the dual phase microstructure, the larger the increase in ductility. Comparison of Dual Phase Precipitation Hardening Alloy with 17-4 PH PM applications mandating strength and hardness with moderate corrosion resistance have experienced limited options.

50

60

70

80

90

100

110

0 200 400 600 800 1000 1200345

414

483

552

621

690

7590 133 266 400 533

Yiel

d St

reng

th (p

si x

103 )

Aging Temperature ( oF)

DPPH w/o Cu

17-4 PH Yiel

d St

reng

th (M

Pa)

Aging Temperature ( oC)

(a) (b)

50

52

54

56

58

60

62

0 200 400 600 800 1000 1200

0 133 266 400 533

App

aren

t Har

dnes

s (H

RA

)

Aging Temperature (oF)

DPPH 2 w/o Copper

17-4 PH

Aging Temperature (oC)

(c) (d)

Figure 5: Mechanical properties of DPPH (2 w/o Cu) compared with 17-4 PH.

1

2

3

4

5

6

0 200 400 600 800 1000 1200

0 133 266 400 533

Elon

gatio

n (%

)

Aging Temperature (oF)

Aging Temperature ( oC)

17-4 PH

DPPH 2 w/o Cu

10

15

20

25

30

35

40

45

50

20

30

40

50

60

0 200 400 600 800 1000 1200

0 133 266 400 533

Impa

ct E

nerg

y (ft

.lbs.

f)

Aging Temperature ( oF)

Impa

ct E

nerg

y (J

oule

s)DPPH 2 w/o Cu

17-4 PH

Aging Temperature ( oC)

Adding carbon (in the form of graphite) to some of the ferritic grades such as SS-410L and SS-430L can provide a martensitic microstructure with strength and hardness but the alloys are extremely brittle and exhibit poor corrosion resistance due to the formation of chromium carbides. 17-4 PH, a significantly more costly alloy, offers high strength and hardness with excellent corrosion resistance but with limited ductility. Fgure 5 shows a mechanical property comparison of 17-4 PH and the 2 w/o Cu DPPH alloy. Despite being a leaner alloy in terms of chromium, nickel and copper contents, the strength, apparent hardness and ductility of the DPPH alloy are all superior to the corresponding properties of 17-4 PH. Under identical processing conditions (pressing and sintering) the DPPH alloys, because of their leaner chemistry, achieve a higher sintered density. This is one reason for the superior mechanical properties. The other factor contributing to the superior strength of the DPPH alloys may be the role that phase boundaries play in impeding dislocation motion.12 Not only do dual phase steels contain boundaries between grains of the same phase but also boundaries between different phases. These boundaries act as barriers to dislocation motion and and result in higher work hardnening than in a conventional single phase alloy. These barriers become more effective as the hardness of the two phases differs. The unique feature of this alloy is not only its excellent strength but the fact that, as the strength increases the ductility also increases. At aging temperatures exceeding 482 oC (900 oF), work hardening occurs because of the formation of copper precipiates in both the martensite and the ferrite, but there is also a tempering of the martensite in the dual phase structure. The net effect is an increase in both tensile strength and in ducility.

(a) (b) (c)

Figure 6. Representative appearance of salt spray specimens: (a.) SS-410L-90HT, (b.) 2 w/o Cu DPPH, (c.) 17-4 PH. Magnification 75% of actual size.

To evaluate the corrosion resistance of the 2 w/o Cu DPPH alloy, it was compared with 17-4 PH and SS-410-90HT, both high strength and high hardness alloys. These three alloys were compacted at 690 MPa (50 tsi) and sintered at 1260 oC (2300 oF) in 100 v/o hydrogen. Salt spray testing, performed according to ASTM Standard B 117-03, was conducted for 240 h. The results can be seen in Figure 6; this test is intended as a general guideline as to the performance of the alloys. As expected, due to the high carbon content SS-410-90HT performed poorly. The 2w/o Cu DPPH alloy exhibited moderate corrosion

resistance and the highly alloyed 17-4 PH performed the best. Since the mechanism for corrosion varies by application, specific testing must be undertaken to ensure satisfactory performance of the alloy under a given set of conditions.

Fatigue tests were performed on PM 17-4 PH and two of the new PM DPPH grades (Alloy D and Alloy F). The results of these tests, in terms of the 90% survival limit, are compared with other PM stainless steel fatigue data by Shah et al. 13 in Figure 10. The latter study compared the fatigue strength of various stainless steels as a function of tensile strength. The 17-4 PH (6.98 g/cm3) exhibited fatigue strength comparable with that of SS-410L-90-HT. The excellent fatigue response of the two PM DPPH alloys appears to be related to their high tensile strength. In general, fatigue crack propagation rates in PM steels are high and the fatigue limit is dictated by crack initiation rather than crack propagation. Thus, resistance to crack initiation increases as the tensile strength increases. Both the DPPH alloys (D and F) have high tensile strengths, and therefore, high fatigue endurance limits. It appears that the addition of copper, along with the addition of nickel and molybdenum, leads to harder martensite, which, in this case has a positive effect on fatigue strength.

Figure 7. Fatigue endurance limit (90% Survival) as a function of tensile strength for various PM stainless steels.

0

5

10

15

20

25

30

35

40

45

50

40 50 60 70 80 90 100 110 120 130 140

Tensile Strength (psi x103)

Fatig

ue E

ndur

ance

Lim

it (p

si x

103 )

-5

45

95

145

195

245

295

3450 100 200 300 400 500 600 700 800 900

Tensile Strength (MPa)

Fatiq

ue E

ndur

ance

Lim

it M

Pa)

410L

17-4PH

410HT

420

409LNi-HC409LNi

409LE

430N2

434N

430L

434L

DPPH ALLOY F

DPPHAlloy D

DUPLEX

Dual Phase

CONCLUSIONS

A lean precipitation hardening grade of stainless steel which utilizes a dual phase microstructure and copper for precipitation hardening has been developed for applications that require high strength and toughness, but with moderate corrosion resistance.

The DPPH alloys exhibit a unique combination of high strength, high toughness

and high fatigue resistance. This is attributed to their microstructure, which is dual-phase, and precipitation hardened.

The DPPH alloys exhibit an improvement in ductility with an increase in strength

and hardness.

The mechanical properties of the DPPH alloys exceed those of 17-4 PH.

The salt spray corrosion resistance of the DPPH alloys falls between that of SS-410L-90HT and 17-4 PH.

The DPPH alloys are cost effective when high strength, coupled with moderate

corrosion resistance, are mandated.

REFERENCES 1. S.O. Shah, J.R. McMillen, P.K. Samal and L.F. Pease, Mechanical Properties of

High Temperature Sintered P/M 409LE and 409LNi Stainless Steels Utilized in the Manufacturing of Exhaust Flanges and Oxygen Sensor Bosses, 2003, SAE Paper No. 2003-01-0451. SAE International, Warrendale, PA.

2. P.K. Samal, and J.B. Terrell, Mechanical Properties Improvement of P/M 400 Series Stainless Steel via Nickel Addition, Advances in Powder Metallurgy and Particulate Materials, compiled by C.L. Rose and M.H. Thibodeau, Metal Powder Industries Federation, Princeton, NJ, 1999, vol. 3, part 9, pp. 15-28.

3. A. Lawley, E. Wagner, and C.T. Schade, Development of a High-Strength-Dual-Phase P/M Stainless Steel, Advances in Powder Metallurgy and Particulate Materials 2005, compiled by C. Ruas and T. Tomlin, Metal Powder Industries Federation, Princeton, NJ, 2005, part 7 pp.78-89.

4. K.A. Green, PIM 17-4PH Actuator Arm for Aerospace Applications, Advances in Powder Metallurgy and Particulate Materials, ibid reference no. 2, vol. 5, pp. 119-130.

5. M.K. Bulger and A.R. Erickson, Corrosion Resistance of MIM Stainless Steels, Advances in Powder Metallurgy and Particulate Materials, compiled by A. Neupaver and C. Lall, Metal Powder Industries Federation, Princeton, NJ, 1994, vol. 4 pp. 197-215.

6. J.H. Reinshagen, and J.C. Witsberger, Properties of Precipitation Hardening Stainless Steel Processed by Conventional Powder Metallurgy Techniques, ibid. reference no. 5, vol. 7, pp. 313-323.

7. A. Lawley, R.Doherty, P. Stears and C.T. Schade, Precipitation Hardening Stainless Steels, Advances in Powder Metallurgy and Particulate Materials 2006, compiled by W. R. Gasbarre and J.W. von Arx, Metal Powder Industries Federation, Princeton, NJ, 2006, part 7 pp.141-153.

8. A.K.Jena and C. Chaturvedi, On the Effect of the Volume Fraction of Martensite on the Tensile Strength of Dual-phase Steel, Materials Science and Engineering, No. 100, 1988, pp.1-6.

9. A. Bag, K.K, Ray and E.S. Dwarakadasa, Influence of Martensite Content and Morphology on Tensile and Impact Properties of High-Martensite Dual Phase Steels, Metallurgical and Materials Transactions A, 1999 vol. 30A, pp. 1193 1202.

10. L.H. Van Vlack, Elements of Materials Science and Engineering, Sixth Edition,1989, Addison Wesley Publishing Company, Reading, MA, pp.304-309.

11. K.J. Irvine, D.J. Crowe, M.A. Cantab and F.B. Pickering, The Physical Metallurgy of 12% Chromium Steels, Journal Iron and Steel Institute, August 1990, pp. 386- 405.

12. E. Werner and H.P. Stuwe Phase Boundaries as Obstacles to Dislocation Motion, Materials Science and Engineering, 1984-1985, vol. 68 pp. 175-182.

13. S.O. Shah, J.R. McMillen, P.K. Samal and L.F. Pease, Mechanical Properties of High Temperature Sintered P/M 409LE and 409LNi Stainless Steels Utilized in the Manufacturing of Exhaust Flanges and Oxygen Sensor Bosses, 2003 SAE Paper No. 2003-01-0451, SAE International, Warrendale, PA.