DEVELOPMENT OF A HIGH-STRENGTH-DUAL-PHASE P/M STAINLESS STEEL

Chris Schade Hoeganaes Corporation Cinnaminson, NJ 08077

Alan Lawley

Emeritus Professor Department of Materials Science and Engineering

Drexel University Philadelphia, PA

19104 215-895-2326 (T) 215-895-6760 (F)

alan.lawley@drexel.edu

Eva Wagner Drexel University

Philadelphia, PA 19104

ABSTRACT

Applications requiring high strength stainless steels are growing at a fast pace. Typical alloys used for these applications are either highly alloyed materials such as 17-4PH or materials that require a secondary heat treatment such as SS-410HT. A new dual-phase stainless steel has been developed as a lower cost option. The microstructure of the dual-phase stainless steel consists of a mixture of ferrite and martensite, the proportions of which are dictated by the chemical composition of the alloy. This unique microstructure results in high strength and hardness, while maintaining ductility. The mechanical properties of this new alloy are compared with those of competing materials such as 17-4PH, SS-409LE and SS-410HT. Potential applications for this new material are reviewed.

INTRODUCTION Dual-phase stainless steels have been developed in the wrought steel industry to replace carbon and galvanized steels. The purpose of dual-phase stainless steels is to provide the corrosion resistance of the ferritic stainless steels, while maintaining the strength, toughness and ductility of the carbon steels.1 Dual-phase stainless steels are commonly used in applications where the mechanical properties of carbon steels are acceptable, but the corrosion resistance is marginal. Typical appliations for this grade of steel in plate form, include coal cars, coal handling equipment, electrical transmission towers, floor plate, tubing, and buckets for front-end loaders. The unique properties of dual-phase stainless steels are a consequence of the microstructure. Unlike duplex stainless steels, which exhibit a microstructure of austenite and ferrite, dual-phase stainless steels have a microstructure consisting of martensite and ferrite. It is the combination of these two phases that results in this unique combination of properties, Figure 1.

(a) (b) Figure 1. Representative microstructure of a dual-phase stainless steel: (a) Durracor cast and and hot-rolled plate. Note the banded microstructure due to rolling. (b) press + sinter P/M dual-phase stainless steel; sintered density 7.20 g/cm3.

BACKGROUND To predict the microstructure of the dual-phase stainless steels, it is necessary to understand the attendant phase equilibria. The pseudo-phase diagram in Figure 2 shows the dependence of phase stability on the ferrite factor (Km ), to be defined, and temperature. At the normal sintering temperatures for stainless steels (~ 1260 oC (2300 oF)), the microstructure consists of a mixture of ferrite and austenite. Upon cooling to room temperature, the austenite transforms to martensite. The proportions of austenite and ferrite can be determined by the lever rule. The addition of other elements to the iron-chromium system broadens the ferrite + austenite region, and makes it difficult to determine the phases present accurately.2 Since the transformation of austenite to martensite in ferritic stainless steels results in weld embrittlement, several empirical methods and tools, such as the Schaeffler diagram, the Delong diagram, and the ferrite factor, have been developed to predict the microstructure. These predictions can also be used in the development of the microstructure in dual-phase stainless steels. The most commonly used tool is the ferrite factor (Km).

Figure 2: Phase relations in the iron-chromium system as a function of the ferrite factor (Km) and temperature.

Several authors developing dual-phase-stainless steels have used the ferrite factor.2-4. This factor accounts for the influence of the alloying elements in stabilizing austenite or ferrite; the empirical equation for the ferrite factor Km is:

Km = Cr + 6Si + 8Ti + 4Mo + 2Al – 2Mn – 4Ni – 40(C+N) – 20P – 5Cu (1)

where Cr, Si, etc. are the levels of each element present in the alloy in weight %. Chromium, silicon, titanium, molybdenum, and aluminum stabilize the ferrite. Manganese, nickel, carbon, nitrogen, phosphorus and copper promote the formation of high temperature austenite, which transforms to martensite during cooling. In established wrought grades of stainless steel, the optimum ferrite factor has been determined to lie between 8 and 13. However, it is difficult to compare ferrite factors for alloys of substantially different base compositions.5-7 In general, a low Km (< 6) reflects a stainless steel with a predominantly martensitic microstructure, whereas a high Km (> 15) is indicative of a stainless steel with a predominantly ferritic structure. Ferrite factors between these values result in mixed microstructures of martensite and ferrite. It is this mixed (or dual-phase) microstructure that promotes the unique combination of high strength and toughness, while maintaining ductility and fatigue strength. ALLOY DEVELOPMENT Representative chemical compositions of wrought dual-phase-stainless steels are shown in Table I. In order to develop a P/M dual-phase stainless steel, changes have to be made to the overall chemical composition. A review of the role and influence of pertinent elements, in relation to P/M dual-phase stainless steels, follows: Carbon, Sulfur, Oxygen and Nitrogen- The levels of these elements are generally kept as low as possible in order to improve compressability and sinterability. Nitrogen stabilizes the austenite. Silicon- In order to produce a powder that is low in oxygen, the silicon level in the melt, prior to atomization, needs to be relatively high (~0.85 w/o). Silicon adds strength to the alloy, and also stabilizes the ferrite. Chromium- This element is a ferrite stabilizer and imparts corrosion resistance. This is accompanied by small increases in strength, hardenability, and wear resistance. Nickel- The primary influence of nickel is to promote the formation of high temperature austenite. In addition, nickel is known to improve toughness, impact resistance and corrosion resistance. The element can be used at moderate levels in P/M steels without dramatically decreasing compressibility. Manganese- This element is generally kept at low levels in stainless steel, due to the formation of a porous oxide on the powder surface. The porous oxide leads to a high level of oxygen on the powder surfaces, which impedes sintering. Manganese also enhances work hardening capacity, and decreases the compressibilty of the powder. Copper- Copper increases corrosion resistance, while providing solid solution strengthening. If it is kept at low levels, copper does not dramaticlly impair the compressibility of the powder.

Molybdenum- Hardenability and high temperature strength are enhanced by molybdenum. The element also aids high-temperature oxidation resistance. Research on wrought dual-phase-stainless steels has shown that molybdnum increases impact toughness.1,5

Table I: Composition of Representative Wrought Dual-Phase Stainless Steels (UNS Designation 41003). (Weight %)

TypeUNS41003 C S N P Si Cr Ni Cu Mn Mo Ti

3CR12 0.026 0.015 0.014 0.023 0.44 11.26 0.68 0.06 1.16 0.02 0.35Durracorr 0.025 0.008 0.030 0.040 0.70 11.60 1.00 0.03 1.50 0.25 ---

Based on this review, and the fact that the best combinations of properties in the wrought grades of stainless steels were achieved with ferrite factors between 8 and 13, a series of P/M dual-phase stainless steel alloys were fabricated. All the powders were produced by water atomization with a typical particle size <150 µm (–100 mesh) with 38 to 48% <45 µm (-325 mesh). All the alloying elements were prealloyed into the melt prior to atomization. Table II shows the chemical composition of the dual-phase (DP) alloys, along with those of two standard stainless grades (410L and 430L).

Table II: Composition of P/M Dual-Phase Stainless Steels (Weight %)

Alloy C S O N P Si Cr Ni Cu Mn Mo Km

DP1 0.004 0.003 0.04 0.0483 0.014 0.60 11.7 1.52 0.27 0.11 0.22 6.1DP2 0.004 0.014 0.10 0.0319 0.014 0.85 11.8 1.03 0.29 0.13 0.22 10.2DP3 0.005 0.015 0.20 0.0445 0.014 0.84 11.6 1.03 0.03 0.10 0.34 11.3DP4 0.003 0.013 0.24 0.0133 0.013 0.80 12.0 0.52 0.48 0.10 0.52 13.3DP5 0.013 0.006 0.16 0.0139 0.012 0.81 12.0 0.58 0.01 0.11 0.32 14.2410L 0.011 0.010 0.14 0.0246 0.014 0.85 12.5 0.03 0.02 0.11 0.03 15.6430L 0.011 0.010 0.26 0.0270 0.014 0.85 16.5 0.03 0.02 0.11 0.03 19.5

ALLOY PREPARATION AND TESTING The prealloyed powders were mixed with 0.75w/o Acrawax C lubricant. Samples for transverse rupture (TR) and tensile testing were uniaxially compacted at 690 MPa (50 tsi). All test pieces were sintered in a high temperature Abbott continuous-belt furnace at 1260 °C (2300 °F) for 45 min in hydrogen with a dewpoint of –40 oC (-40 °F). Prior to mechanical testing, green and sintered density, dimensional change (DC), and apparent hardness, were determined on the tensile and TRS samples. Five tensile and five transverse rupture specimens (TRS) were tested at each composition. The densities of the green and sintered steels were determined in accordance with MPIF Standard 42, while tensile testing followed MPIF Standard 10. Impact energy specimens were tested according to MPIF 40. Apparent hardness measurements were conducted on tensile,TRS and impact specimens according to MPIF 43. Rotating bending contact fatigue specimens were machined from test blanks that were pressed at 690 MPa (50 tsi) and sintered at 1260 oC (2300 oF). The dimensions of the test blanks were 12.7 mm x 12.7 mm x 100 mm. RBF tests were performed using rotational speeds in the range 7,000-8000 rpm at R = -1 using four machines simultaneously. Thirty specimens were

tested for each alloy and the staircase method was utilized to determine the 50% survival limit and the 90% survival limit for 107 cycles (MPIF 56). Metallographic specimens of all the test materials were examined by optical microscopy in the polished and etched conditions. Etched specimens were used for microhardness testing as per MPIF 51. Two corrosion tests were performed. Salt spray testing on TRS bars was performed according to (ASTM B 117-03). Five TRS bars (prepared as previously described) per alloy were tested. The percent area of the bars covered by red rust was recorded over time. Since ferritic steels are frequently used in conditions where they experience high-temperature oxidation (such as exhaust flanges) a test have been devised to measure the oxidation during cycling from room temperature to 1200 °C (2192 oF). The technique developed was very similar to previous tests in which sintered impact bars (sintered) were cycled from room temperature to 676 °C (1249 oF) in air.8-10 Each heating cycle lasted two to four hours, and this was repeated as much as 400 times with changes in mass recorded. In this study, sintered cross-sections of tensile bars were used in a thermal gravimetric (TG) unit to measure the mass gain over time. Each of the four alloys was heated from room temperature to 1200 °C (2192 oF) in air. This cycle was repeated several times, and the weight gain per cycle monitored. RESULTS AND DISCUSSION Table III cites the green density, sintered density, transverse rupture strength, apparent hardness, yield strength, ultimate tensile strength, elongation, and ferrite factors, for the P/M experimental alloys, and two standard P/M ferritic grades.

Table III: Mechanical Properties of P/M Dual-Phase Stainless Steel.

Green Density

Sintered Density

ApparentHardness Elongation

Alloy Km (g/cm3) (g/cm3) (MPa) (103 psi) (HRB) (MPa) (103 psi) (MPa) (103 psi) (%)DP1 6.2 6.69 7.03 1578 229 84 687 100 538 78 2.3DP2 10.2 6.68 7.34 1914 278 88 736 107 564 82 3.1DP3 11.3 6.62 7.38 1730 251 83 674 98 509 74 4.0DP4 13.3 6.58 7.35 1201 175 66 444 64 284 41 8.5DP5 14.2 6.59 7.31 1274 185 68 478 69 297 43 10.0410L 15.6 6.66 7.3 1430 208 54 377 55 213 31 16.2430L 19.5 6.61 7.16 905 132 54 358 52 202 29 15.3

TRS UTS 0.20% OFFSET

Km in the experimental dual-phase P/M alloys was varied by changing the levels of nickel, copper, and molybdenum. Quantitative metallography was performed to measure the actual ferrite content of the P/M alloys. Figure 3 shows the relationship between the amount of ferrite present and Km. There is an increase in ductility and a decrease in strength, as the level of ferrite in the microstructure increases, Table III. Apparent hardness appears to be directly related to the amount of martensite in the microstructure and is maximized at Km values between 6 and 11. Nevertheless, based on Figure 3, there is very little ferrite in the microstructure with Km values <9. Notwithstanding, the presence of prealloyed nickel, copper, and molybdenum, the dual-phase P/M steels exhibit a level of compressibility close to that of 410L. It was expected that the

increase in the level of alloying elements in the prealloy would harden the powders, with an adverse effect on green density; this does seem to be the case, Table III. There also appears to be a synergistic effect of prealloyed nickel, molybdenum and copper on sintered density; with the exception of DP1, the sintered density of the dual-phase stainless steels exceeded 7.3 g/cm3.

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Figure 3. Relationship between measured ferrite content and Km in experimental P/M dual-phase

stainless steels

From the results of the static mechanical tests, it was determined that alloy DP2 was the most promising, in terms of its combination of physical and mechanical properties. The purpose of developing the dual-phase P/M stainless steel was to provide a high-strength material that had good corrosion resistance. In order to determine the relative success of the alloy, it was necessary to compare its properties with those of existing stainless steels, which are typically used in high-strength applications. These stainless grades include 17-4PH, 409LNi, and 410L with a graphite addition. The compositions of these three alternative materials, and the DP2, are listed in Table IV.

Table IV: Chemical Composition of High-Strength Stainless Grades. (Compositions in w/o)

Alloy C S O N P Si Cr Ni Cu Mn Mo Cb

410+ Graphite 0.100 0.008 0.30 0.0180 0.013 0.85 12.5 0.09 0.08 0.15 0.02 ---17-4PH 0.018 0.010 0.30 0.0200 0.025 0.85 17.0 4.00 3.55 0.15 0.03 0.25409LNi 0.013 0.004 0.19 0.0160 0.01 1.00 11.3 1.30 0.04 0.12 0.05 0.56

DP2 0.015 0.007 0.21 0.0090 0.014 0.84 11.6 1.03 0.29 0.10 0.22 ---

When graphite is added to 410L stainless steel, the microstructure becomes martensitic, with increasing strength and hardness. This grade of stainless steel is used in applications where high strength is required. The disadvantage of using graphite is that it negatively impacts the corrosion resistance. 17-4PH is a precipitation-hardening, martensitic stainless steel that combines high strength and hardness with excellent corrosion resistance. The alloy is age-hardened to achieve

maximum strength. This compressibility of this material is compromised somewhat, compared with the other alloys studied due to its high alloy content. 409LNi is a nickel-modified 409Cb stainless steel, which exhibits excellent mechanical properties, and is commonly used for automotive exhaust flange applications.11-13 In this study, nickel powder (Inco-123) was admixed with a 409Cb powder. The addition of nickel has been shown to improve the high temperature strength of the stainless steel.

Table V: Physical and Mechanical Properties of High Strength Stainless Steels.

Sintered Density D.C. Apparent

Hardness Elongation

Alloy (g/cm3) (%) (MPa) (103 psi) (HRB) (MPa) (103 psi) (MPa) (103 psi) (%)410L + graphite 7.05 -2.17 1248 181 79 633 92 358 52 4.9

17-4PH 6.67 -1.80 1421 206 86 757 110 627 91 1.4409LNi 7.01 -2.10 1255 182 79 612 89 482 70 1.8

DP2 7.15 -2.99 1641 238 91 819 119 612 89 2.50.75% Acrawax C lubricant and sintered; sintered at 1260oC (2300oF) in 100% hydrogen. Note: 17-4PH was heat-treated at 482 oC (900 oF) for 30 min.

TRS UTS 0.20% OFFSET

The static mechanical properties of these three alloys are compared with those of DP2 in Table V. From Table V, it is seen that DP2 is superior, in both strength and apparent hardness, to the other alloys. One reason for this is the higher sintered density of DP2. 17-4PH, because of its high alloy content, is inherently less compressible and this translates into a low sintered density. An additional heat treatment is required for 17-4PH in order to maximize strength and hardness. 409LNi generally requires longer times at temperature than DP2 to achieve acceptable diffusion of the admixed nickel. The advantage of the prealloyed nickel, copper and molybdenum in DP2 is evident when the alloys are sintered under similar conditions. Due to the high compressibility of 410L with graphite, the alloy has a high sintered density, and acceptable properties. However, in order to achieve consistent carbon levels, tight control in the furnace is necessary. Graphite additions also reduce the corrosion resistance of the stainless steel.

Table VI: Dynamic Properties of High Strength Stainless Steels. Sintered Density

ApparentHardness

Material (g/cm3) (HRB) (J) (ft.lbf) (MPa) (103 psi)410L + graphite 7.05 79 39 29 330 47.8

17-4PH 6.67 86 26 19 244 35.4409LNi 7.01 79 32 24 249 36.0

DP2 7.15 91 65 48 315 45.6

Impact Energy Fatigue Strength (90% Survival)

The dynamic properties of the four alloys are cited in Table VI. It has been found that in wrought dual-phase stainless steels, the addition of molybdenum has a dramatic effect on dynamic properties. Davison et al. have shown that adding molybdenum, and eliminating titanium in the 3CR12 chemistry, increases impact toughness.5 Titanium can lead to the formation of titanium carbides and nitrides, which reduce impact energy. This may be one explanation why DP2 significantly outperforms 409LNi. This grade contains niobium, which is more conducive to the atomizing process than is titanium. The ferrite content of DP2 also enhances the impact properties, compared with 17-4PH and 410L + graphite, both of which have martensitic microstructures.

Since sintered density has been proven to be an important factor in fatigue response, one may question the fatigue strength of DP2 in respect to the other alloys. Although DP2 and the other alloys were sintered under identical conditions, DP2 achieved a higher sintered density. Shah et al. compared the fatigue strengths of various stainless steels, as a function of tensile strength, Figure 4.13 Data from this study are included in Figure 4 for 409LNi, at a density of 7.20 g/cm3 (similar density to that of the DP2 alloy).

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Figure 4. Fatigue endurance limit as a function of tensile strength for various P/M stainless alloys. The excellent fatigue response of the DP2 alloy appears to be related its high tensile strength. In general, fatigue propagation rates in P/M 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 410L + graphite and the DP2 alloy have high tensile strengths and therefore higher fatigue endurance limits. Notwithstanding, having a high tensile strength, the 17-4PH has a lower fatigue endurance limit due to its low sintered density (high level of porosity). Micro-indentation hardness measurements of the martensite (along with the corresponding microstructures) are shown in Figure 5. The micro-indentation hardness of the martensite in the 410L + graphite was 372 HV 50 gf. The 410L with graphite had the highest fatigue endurance limit however, the martensite was formed with the use of carbon and the alloy is fairly brittle and had lower impact energy than the DP2. The martensite grains in the DP2 had a micro-indentation hardness of 318 HV 50 gf, while the 409LNi had a martensite micro-indentation hardness of 250 HV 50 gf. It appears the addition of copper (along with the nickel) leads to a harder martensite. The micro-indentation hardness of the martensite in the 17-4PH was lower (245 HV 50 gf). The combination of the strong martensite and the ductile ferrite allows the DP2 alloy to have a high strength, but superior ductility and impact toughness than the other alloys.

(a) (b)

(c) (d) Figure 5: Microstructure of alloys cited in Table V; (a) 410L + Graphite (microhardness of martensite 372 HV 50 gf); (b) 17-4PH (microhardness of martensite 245 HV 50 gf); (c) 409LNi (microhardness of martensite 250 HV 50 gf); (d) DP2 (microhardness of martensite 318 HV 50 gf). In the wrought stainless steel industry, dual-phase-stainless steels were developed as a replacement for weathering steels. The corrosion resistance was substantially improved when the 12 w/o chromium steel was used. The dual-phase P/M stainless steels will compete with ferritic grades of stainless steel. There are many applications that have less-severe corrosion conditions, and, in such cases, ferritic stainless steels are preferred over the more expensive austenitic stainless steels. The four stainless steels were subjected to salt spray testing (ASTM B 117-03). Five TRS bars were tested from each alloy composition. The results are given in Table VII.

Table VII: Results of Salt Spray Testing (ASTM B 117-03).

Material Red Rust After 24 h (%)

410L + graphite 84.0017-4PH 1.00409LNi 20.00

Dual Phase 2 5.00

Although the sintered density of the 17-4PH alloy was much lower than the other three alloys the high alloy content (17 w/o chromium and 4 w/o nickel) resulted in superior corrosion resistance, with an average of only 1% of the TRS bars showing red rust after 24 h exposure. The dual-phase alloy, and the 409LNi alloy, performed at a similar level, as expected, since each alloy has a similar chromium and nickel content. The addition of small amounts of copper and molybdenum may lead to slightly better corrosion resistance for the DP2. The 410L with graphite had the highest level of red rust after 24 h. This is not surprising, since the addition of carbon leads to sensitization and makes the chromium less effective in passivation. Many of the P/M grades such as 410L and 409LNi are used in flange applications. A described in the experimental procedure, cross-sections of the sintered tensile specimens were heated in air from room temperature to 676 °C (1249 oF). This cycle was repeated several times. Figure 6 shows the mass gain per cycle. The alloys behaved similarly, with a high initial rate of oxidation. This was followed by saturation to a constant rate of oxidation.

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Figure 6. Mass gain per cycle. TGA

As in the salt spray testing, despite its lower density, 17-4PH exhibited the lowest initial mass gain, and saturated at the lowest rate of oxidation. DP2 exhibited the highest initial rate of oxidation, but it saturated at a lower number of cycles than the other alloys. It also exhibited a slightly lower oxidation rate than 409LNi and 410L with carbon. This may be due to the additions of molybdenum and copper; these elements are known to improve high temperature oxidation. The corrosion testing highlights a final aspect of the DP2 alloy. Despite having a much leaner chemistry than the 17-4PH and a lower nickel content han the 409LNi the corrosion resistance of the DP2 alloy performs comparable to these materials. The excellent mechancial properties are developed without need for secondary treatments (such as heat treating). Both these aspects make the alloy a cost effective solution when a high strength-moderate corrosion resistant material is needed.

CONCLUSIONS

• The chemical composition and physical properties of a new dual-phase P/M stainless steel have been optimized by evaluating a series of prealloys produced by water atomization.

• The new alloy DP2 has a dual (mixed) microstructure of martensite and ferrite.

Quantitative metallography has shown the alloy contains approximately 5 to 8%, by volume, ferrite in a martensitic matrix.

• The static properties of the DP2 show improvements over competing alloys, in

particular higher tensile strength and hardness.

• The fatigue and impact properties of DP2 are superior to those of existing alloys such as 17-PH, 409LNi, and 410L with graphite.

• Corrosion properties of DP2 show it to be a viable candidate for replacing ferritic

stainless steels.

REFERENCES 1. F.B.Fletcher, B.N. Ferry, and D.G. Beblo “High Performance Corrosion-Resistant

Structural Steel,” International Symposium on High Performance Steels for Structural Applications, Edited by Riad Asfahani, Cleveland, Ohio, USA, 1995. ASM International, Materials Park, OH.

2. R.H. Kaltenhauser, “Improving the Engineering Properties of Ferritic Stainless Steels,” Metals Engineering Quarterly, Vol. 11, No. 2, 1971, pp. 41-47.

3. A. Ball, and J.P. Hoffman, “Microstructure and Properties of a Steel Containing 12% Cr,” Metals Technology, September 1981, Vol. 8, Part 9, pp.329-338.

4. R.D. Knutsen, and R. Hutchings, “Occurrence of Non-metallic Inclusions in 3CR12 steel and their Effert on Impact Toughness, Materials Science and Technology, February 1999, Vol. 4, pp. 127-135.

5. R.M. Davison and P.J. Grobner., “General Purpose Maintenance-Free Constructional Steel of Superior Processability,” U.S. Patent No. 4,608,099. August 26, 1986.

6. J.B. Hill., “Ferritic Stainless Steel,” U.S. Patent No. 4,964,926. October 23, 1990. 7. T.B. Beeeton, R.J. Mostert, E.B. Navarro, P.R. Rosinger and R.P. Badenhorst, “Special

Rolled Steels,” U.S. Patent No. 4,881,991. November 21,1989. 8. T.M. Cadle, and J. Mandel, “How Powder Metallurgy Exhaust System Flanges Can

Respond to Recent Commercial, Market and Legislative Pressures,” 1996, SAE Paper No. 960384. SAE International, Warrendale, PA 15096, USA.

9. T.R. Albee, P. dePoutiloff, G.L. Ramsey and G.E. Regan, “Enhanced Powder Metal Materials for Exhaust System Applications,” 1997, SAE Paper No. 970281. SAE International, Warrendale, PA 15096, USA.

10. T. Hubbard, K. Couchman, and C. Lall, “Performance of Stainless Steel P/M Materials in elevated Temperature Applications,” 1997, SAE Paper No. 970422. SAE International, Warrendale, PA 15096, USA.

11. P.K. Samal and E. Klar, “Nickel-Containing Strengthened Sintered Ferritic Stainless Steels,” U.S. Patent No. 5,976,216. November 2, 1999.

12. 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.

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,” presented 2003, SAE Paper No. 2003-01-0451. SAE International, Warrendale, PA 15096, USA.