STAINLESS STEEL AISI GRADES FOR PM APPLICATIONS Chris Schade Hoeganaes Corporation Cinnaminson, NJ 08077 John Schaberl Ancor Specialties Ridgway, PA 15853 Alan Lawley Drexel University Philadelphia, PA 19104 ABSTRACT Applications requiring stainless steels are growing at a rate of about 5% annually. Opportunities for using powder metallurgy (PM) exist, but additional grades not covered by MPIF Standard 35 are required. The American Iron and Steel Institute (AISI) has standards for a broad range of stainless steels that can be used in many applications, but the compositions of these grades must be modified to be conducive to manufacture by conventional PM techniques. Several of these grades have been produced as standard press and sinter powders. The physical properties, mechanical properties and microstructures of these various grades are reviewed to serve as a guideline for PM parts manufacturers and potential applications of these grades are addressed. INTRODUCTION MPIF Standard 35 1 lists the most common grades of stainless steel used by PM parts manufacturers. These include austenitic grades such as 303L, 304L and 316L, and ferritic grades such as 409L, 410L, 430L and 434L. However, with the continued growth of stainless steel there exists many opportunities for specialized stainless steel grades not covered by MPIF Standard 35. These include applications requiring enhanced physical properties, corrosion resistance, weldability and machinability. There are additional grades covered by the American Iron and Steel Institute (AISI) that can be manufactured by conventional press and sinter powder metallurgy (PM), Figure 1. The AISI designation for these alloys is well known with the number series 200 and 300 referring to austenitic stainless steels and the 400 series covering the ferritic and martensitic stainless steels. Letter designations attached to the end of the number series indicate modifications to the composition. 2 Many societies such as the Society for Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) use the AISI specification with the latter adding physical property specifications. 3 SAE and
Transcript
STAINLESS STEEL AISI GRADES FOR PM APPLICATIONS
Chris Schade
Hoeganaes Corporation Cinnaminson, NJ 08077
John Schaberl
Ancor Specialties Ridgway, PA 15853
Alan Lawley
Drexel University Philadelphia, PA 19104
ABSTRACT
Applications requiring stainless steels are growing at a rate of about 5% annually. Opportunities for using powder metallurgy (PM) exist, but additional grades not covered by MPIF Standard 35 are required. The American Iron and Steel Institute (AISI) has standards for a broad range of stainless steels that can be used in many applications, but the compositions of these grades must be modified to be conducive to manufacture by conventional PM techniques. Several of these grades have been produced as standard press and sinter powders. The physical properties, mechanical properties and microstructures of these various grades are reviewed to serve as a guideline for PM parts manufacturers and potential applications of these grades are addressed. INTRODUCTION MPIF Standard 351 lists the most common grades of stainless steel used by PM parts manufacturers. These include austenitic grades such as 303L, 304L and 316L, and ferritic grades such as 409L, 410L, 430L and 434L. However, with the continued growth of stainless steel there exists many opportunities for specialized stainless steel grades not covered by MPIF Standard 35. These include applications requiring enhanced physical properties, corrosion resistance, weldability and machinability. There are additional grades covered by the American Iron and Steel Institute (AISI) that can be manufactured by conventional press and sinter powder metallurgy (PM), Figure 1. The AISI designation for these alloys is well known with the number series 200 and 300 referring to austenitic stainless steels and the 400 series covering the ferritic and martensitic stainless steels. Letter designations attached to the end of the number series indicate modifications to the composition.2 Many societies such as the Society for Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) use the AISI specification with the latter adding physical property specifications.3 SAE and
ASTM have worked together to create the unified numbering system (UNS) for metals and alloys, which is recognized globally and can be used as a cross-reference internationally. 4 Other references covering both wrought and cast grades of stainless steel are available.5-6
Figure 1. Available stainless steel alloy systems.3
There are hundreds of commercially available stainless steel compositions, fabricated by multiple processing steps which modify their properties. Fortunately these stainless steels can be classified into several distinct categories. These include austenitic, ferritic, martensitic, precipitation hardening, and duplex stainless steels. For convenience the development of additional PM grades of stainless steel will adhere to these categories.
ALLOY PREPARATION AND TESTING The powders used in this study were produced by water atomization with a typical particle size (100 w/o) <150 µm (–100 mesh) and with 38 to 48 w/o <45 µm (-325 mesh). All the alloying elements were prealloyed into the melt prior to atomization, unless otherwise noted. Admixed copper, molybdenum and nickel powders were used to make some compositions and are so designated in the tables of chemical composition. The stainless powders were mixed with 0.75 w/o Acrawax C lubricant. Samples for transverse rupture (TR) and tensile testing were compacted uniaxially at 690 MPa (50 tsi). All the 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), unless otherwise noted. Prior to mechanical testing, green and sintered density, dimensional change (DC), and apparent hardness, were determined on the tensile and TR samples. Five tensile specimens and five TR specimens were tested for each composition. The densities of the green and sintered steels were determined in accordance with MPIF Standard 42 and tensile testing followed MPIF Standard 10. Impact energy specimens were tested in accordance with MPIF Standard 40. Apparent hardness measurements were conducted on tensile, TR and impact specimens, following MPIF Standard 43. Rotating bending fatigue (RBF) 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 of 7,000-8000 rpm at R equal -1 using four fatigue machines simultaneously. Thirty specimens were tested for each alloy composition, utilizing the staircase method to determine the 50% survival limit and the 90% survival limit for 107 cycles (MPIF Standard 56). Metallographic specimens of the test materials were examined by optical microscopy in the polished and etched (glyceregia) conditions. Etched specimens were used for microindentation hardness testing, per MPIF Standard 51. Salt spray testing on TR bars was performed in accordance with ASTM Standard B 117-03. Five TR bars per alloy (prepared as previously described) were tested. The percent area of the bars covered by red rust was recorded as a function of time. The level of corrosion was documented photographically.
RESULTS AND DISCUSSION Ferritic Stainless Steels For PM applications, the ferritic stainless steels are by far the most widely used grades, reflecting their application in the automotive industry. Examples are ABS sensor rings and muffler exhaust flanges. Chromium is the major alloy constituent of the ferritic grades along with minor additions of other ferrite stabilizers such as silicon and niobium (Table I). In general, the 400 series, ferritic stainless steels contain 11 to 27 w/o Cr, are magnetic, have moderate ductlity and corrosion resistance and are relatively weak at high temperatures.7-8 In order to form the passive oxide layer a minimum of about 11 w/o Cr is required.
Table I: Composition of PM Ferritic Stainless Steels (w/o)
AISI UNS C S P Si Cr Ni Cu Mn Mo Nb
409L1 S409400.03 Max.
.030 Max.
.040 Max.
1.0 Max.
10.5 11.7
.50 Max.
.50 Max.
1.0 Max.
.50 Max.
.40
.60
410L1 S410000.03 Max.
.030 Max.
.040 Max.
1.0 Max.
11.5 13.0
.50 Max.
.50 Max.
1.0 Max.
.50 Max. ---
416L S416030.03 Max.
.150
.300.040 Max.
1.0 Max.
11.5 13.0
.50 Max.
.50 Max.
1.0 Max.
.50 Max. ---
430L1 S430000.12 Max.
.030 Max.
.040 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
1.0 Max.
.50 Max. ---
434L1 S434000.03 Max.
.030 Max.
.040 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
1.0 Max.
0.75 1.25 ---
436 S436000.03 Max.
.030 Max.
.040 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
1.0 Max.
0.75 1.25
.40
.60
439 S430350.03 Max.
.030 Max.
.040 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
1.0 Max.
.50 Max.
.40
.60
441 S441000.03 Max.
.030 Max.
.040 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
1.0 Max.
.50 Max. 1.00
446 S446000.03 Max.
.030 Max.
.040 Max.
1.0 Max.
23.0 27.0
1.0 Max.
.50 Max.
1.0 Max. --- ---
446 S446260.03 Max.
.030 Max.
.040 Max.
1.0 Max.
25.0 27.5
1.0 Max.
.50 Max.
1.0 Max.
0.75 1.50
.05
.20 1 Covered by MPIF Standard 35
The early use of these grades was limited by the amount of carbon and nitrogen in the alloys. With higher levels of carbon and nitrogen, the ductile to brittle transition can occur at low temperatures. However, with the advent of argon-oxygen-decarburization (AOD), lower values of nitrogen and carbon have been acheieved and the ductility of these grades has been greatly enhanced.9 The effect of carbon and nitrogen can furthur be reduced by the addition of niobium which combines with the interstitial elements to prevent sensitization. Niobium is also a ferrite stabilizer which helps to prevent the formation of martensite in the alloys. In general, the oxidation resistance and mechancial properties (Table II) increase as the chromium level increases. The addition of other alloying elements to the base compositions can enhance certain properties. For example, in the case of 434L, when molybdenum is added, the resistance of the alloy to corrosion by road salt is increased.
Niobium is added to several stainless steel grades to prevent the formation of chromium carbides which leads to intergranular corrosion (409L,436 and 439). This is particularly important when welding ferritic stainless steels, since the formation of chromium carbides is rapid and difficult to avoid. Sulfur can be added to enhance the machinability of ferritic stainless steels. In AISI 416L, sulfur is prealloyed prior to atomization, and the element combines with manganese during solidification to form managnese sulfides that assist in machining. This technique has been used in the PM grade of 303L for many years.
Table II: Mechanical Properties of PM Ferritic Stainless Steels
uperferritics have been developed for increased oxidation or scaling resistance. In
Sgeneral, the higher the chromium content the higher the oxidation rsistance. Additions ofmolybdenum and niobium can enhance oxidation resistance even further. Figure 2 shows that the oxidation resistance of S44626 (containing molybdenum) and S44100 (containing 1 w/o niobium) approach that of high chromium-nickel grades suchand Hasteloy X (a superalloy).
0
5
10
15
20
800 850 900 950 1000 1050 1100 1150 1200
Hastelloy XSuper Ferritic310L441409Cb
% o
f Ini
tial W
eigh
t
Temperature (oC) Figure 2. Weight gain (in air) as a function of temperature for selected oxidation resistan loys. t al
Austenitic Stainless Steels The AISI 300 austenitic series stainless steels contain nickel and chromium and have excellent corrosion resistance in diverse environments. The properties of austenite are generally described as nonmagnetic, with a relatively low yield strength, high ductility and excellent impact toughness. Austenitic stainless steels behave in a manner similar to that of low carbon steels but with enhanced high temperature strength and oxidation resistance. Depending on chemical composition, these stainless steels can resist scaling up to 1095 oC (2000 oF). Conversely, austenitic stainless steels can be used in low temperature applications where their high toughness levels are compatible with cryogenic applications. Based on Table III, there exists a wide range of 300 series stainless steels suitable for a variety of applications. This table also includes stainless steel grades commonly used by the PM industry and detailed in MPIF Standard 35, namely 303L, 304L and 316L. With the increased use of PM stainless parts an exploration of other grades listed in Table III would appear to be timely.
Table III: Composition of PM Austenitic Stainless Steels (w/o)
AISI UNS C S Si Cr Ni Cu Mo Nb
302B S302150.03 Max.
.030 Max.
2.0 3.0
17.0 19.0
8.0 10.0
.50 Max.
.50 Max. ---
303L1 S303000.03 Max.
.150
.3001.0
Max.17.0 19.0
8.0 10.0
.50 Max.
.50 Max. ---
304L1 S304030.03 Max.
.030 Max.
1.0 Max.
17.0 19.0
8.0 10.0
.50 Max.
.50 Max. ---
304Cu S304300.12 Max.
.030 Max.
1.0 Max.
17.0 19.0
8.0 10.0
3.0 4.0
.50 Max. ---
309Cb S309400.03 Max.
.030 Max.
1.0 Max.
22.0 24.0
12.0 16.0
.50 Max.
.50 Max.
.40
.60
310S S310080.03 Max.
.030 Max.
1.5 Max.
24.0 26.0
19.0 22.0
.50 Max.
.50 Max. ---
316L1 S316030.03 Max.
.030 Max.
1.0 Max.
16.0 18.0
10.0 14.0
.50 Max.
2.00 3.00 ---
316Cb S316400.03 Max.
.030 Max.
1.0 Max.
16.0 18.0
10.0 14.0
.50 Max.
2.00 3.00
.40
.60
317L S317030.03 Max.
.030 Max.
1.0 Max.
18.0 20.0
11.0 15.0
.50 Max.
3.00 4.00 ---
321L S321000.03 Max.
.030 Max.
1.0 Max.
17.0 19.0
9.0 12.0
.50 Max.
.50 Max.
.40
.60
904L N089040.02 Max.
.030 Max.
1.0 Max.
19.0 23.0
23.0 28.0
1.0 2.0
4.0 5.0 ---
1 Covered by MPIF Standard 35 There is a growing need to weld PM austenitic stainless steel parts to other structures. In doing so the normal grades of stainless steel (304L and 316L) are susceptible to
sensitization, particularly in areas adjacent to the weld. Sensitization is the process by which chromium combines with carbon to form chromium carbides. The chromium is removed from areas close to the grain boundaries and leaves these areas depleted of chromium, with attendant susceptibility to intergranular corrosion. The formation of chromium carbides is enhanced by temperature and generally occurs in austenitic stainless steels at temperatures between 480 oC and 815 oC (900 oF and 1500 oF). The cooling rate resulting from the welding process is generally slow which increases the likelihood of chromium carbide formation. Increased carbon levels due to insufficient lubricant burn-off can also increase the chance of sensitization. In order to avoid postweld heat-treatment, stabilized grades of austenitic stainless steels have been developed. In order to prevent the chromium from forming carbides, strong carbide-forming elements have been added to the austenitic grades of stainless steel. In AISI grades 309Cb, 316Cb and 321L, titanium and niobium were added for this reason. In PM products, the use of niobium is preferred because water atomization oxidizes the titanium. Table IV lists the mechanical properties of these niobium-stabilized austenitic PM grades. In general, due to the addition of niobium and the formation of carbides, the ductility and impact toughness of these grades are slightly lower than those of the non-stabilized grades. However, in general, this decline in mechanical properties is small, and can be compensated for by an increase in the level of other elements (such as nickel and chromium). Due to the formation of the carbides, the creep resistance of these grades of stainless steel is improved.
Table IV: Mechanical Properties of PM Austenitic Stainless Steels
Other alloying elements, such as molybdenum, can be added to the austenitic stainless steels to improve corrosion resistance. Molybdenum, when added at levels between 2 w/o and 4 w/o improves resistance to oxidation, pitting and crevice corrosion. The addition of molybdenum also tends to improve both room and high temperature properties such as tensile strength and creep. The mechanical properties of AISI 317L are cited in Table IV and the corrosion resistance is illustrated in Figure 3. Currently PM fabricators sinter austenitic grades in a hydrogen/nitrogen atmosphere to increase strength (MPIF Standard 35: grades SS-304N and SS-316N). In so doing, significant chromium nitride formation occurs, which is detrimental to the overall corrosion resistance of the alloy. The addition of molybdenum is beneficial if both corrosion resistance and strength are required.
(a) (b) (c) Figure 3. Representative appearance of salt spray specimens: (a.) 304L, (b.) 316L, (c.) 317L. More highly alloyed 300 series stainless steels are available that are designed to resist oxidation at high temperatures while maintaining a high degree of tensile strength and creep resistance. These alloys rely on the formation of the chromium oxide film for protection from corrosion, but the additional nickel and silicon in these alloys helps to form a more ductile scale, which increases its adherence to the base metal. The adherent scale is particularly important when service conditions involve cyclic temperatures. The properties of several of these PM grades (302B, 304L and 310L) are listed in Table IV and the relative oxidation resistance of these grades is shown in Figure 4.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
600 700 800 900 1000 1100 1200
304L302B310L
% o
f Ini
tial W
eigh
t
Temperature (oC)
Figure 4. Oxidation of 300 series stainless steels.
As with the other categories of stainless steels, there exists “super austenitics” where increased levels of nickel, copper, and molybdenum provide superior or specialized corrosion resistance. However, for the PM grades of these alloys the increased alloy content has a negative impact on powder compressibility and therefore on overall density. In consequence, care has to be taken to ensure that the increased corrosion resistance and enhanced mechanical properties gained by the increase in alloy content are not offset by a reduction in achievable density. Type 904 stainless steel is considered a “super austenitic” stainless steel. Martensitic Stainless Steels
Martensitic steels in the 400 series are similar to the ferritic stainless steels in that they contain chromium in the range of 11 w/o to 18 w/o but also contain other elements such as nickel Table V. The martensitic stainless steels are magnetic and are generally used in applications where hardness and/or wear resistance is required. When heat-treated they can achieve high strength and when tempered, they can exhibit some ductility. Essentially, these steels achieve mechanical properties comparable with those of a heat-treatable low alloy steel but with enhanced corrosion resistance, although their corrosion resistance is the lowest of any of the stainless steel categories. MPIF standard 35 recognizes SS-410-90HT as a martensitic alloy. For this grade, the sintering atmosphere contains a high level of nitrogen, and the alloys form high temperature austenite, which transforms to martensite on cooling.
Table V: Composition of PM Martensitic Stainless Steels (w/o)
AISI UNS C S Si Cr Ni Cu Mo Nb
414M S414000.03 Max.
.030 Max.
1.0 Max.
12.0 15.0
1.25 2.50
.50 Max.
.50 Max. ---
414M S414260.03 Max.
.030 Max.
1.0 Max.
12.0 15.0
4.0 7.0
1.5 2.0
1.5 2.0 ---
415M S415000.03 Max.
.030 Max.
1.0 Max.
11.5 14.0
3.5 5.5
.50 Max.
.5 1.0 ---
420M S42000.15 .30
.030 Max.
1.0 Max.
12.0 14.0
.50 Max.
.50 Max.
.50 Max. ---
440BM S440030.75 0.95
.030 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
.75 Max. ---
440CM S440040.95 1.20
.030 Max.
1.0 Max.
16.0 18.0
.50 Max.
.50 Max.
.75 Max. ---
410LCu J91151 0.15 Max.
.030 Max.
1.5 Max.
11.5 14.0
1.0 Max.
3.0 5.0
.50 Max. ---
M Designates material made from a mix of a base powder and additives such as nickel, graphite, copper and molybdenum. Other AISI martensitic grades of stainless steel, such as 420,440A, 440B and 440C, can be processed by adding graphite to ferritic grades of stainless steels such as 410L and 430L. Table VI gives the properties of 420L, 440B and 440C made by this approach. The level of carbon added to the alloy dictates the mechanical properties of the martensitic stainless steel. The higher the carbon content, the larger the extent of
chromium carbide formation, and the higher the strength and apparent hardness of the alloy.
Table VI: Mechanical Properties of PM Martensitic Stainless Steels
AISI 415 = Powd
major drawback to carbon-containing martensitic stainless steels is their relatively poor
n
re
er Mix, AISIPA = Prealloy. M
Acorrosion resistance and ductility. Low carbon-containing martensitic stainless steels can be produced by adding nickel, molybdenum and copper to form martensitic stainless steels with improved toughness and corrosion resistance. Table V cites the compositioof several martensitic stainless steels made by adding nickel, copper and molybdenum. Nickel and copper are austenite formers, while molybdenum improves properties via solid solution strengthening; this element is responsible for improving high temperatuproperties. While the apparent hardness of these alloys is slightly inferior to that of heat-treated carbon-containing martensitic stainless steels, other mechanical properties such astensile strength, toughness and ductility are superior. These grades of stainless steel also exhibit superior corrosion resistance which reflects the absence of carbide formation and hence sensitization, as shown in Figure 5.
Figure 5. Representative appearance of salt spray speci s 40B, (b.) 4 ) 410
As with other categories of stainless steels, super-martensitic stainless steels can be adding high levels of nickel, copper and molybdenum. For PM alloys the formed by
prealloyed materials are usually low in compressibility but can exhibit superior corrosresistance due to their high alloy content. Precipitation-Hardening Stainless Steels
ion
ot defined by their microstructure, but rather
to
l compositions and mechanical properties of several
xist
on Hardening Stainless Steels (w/o)
o Nb
17-4PH S174000.03 Max.
.030 Max.
1.0 Max.
15.0 17.0
3.0 5.0
3.0 5.0
.50 Max.
.15
.45
410LCu J91151 .15 .30
.030 Max.
1.0 Max.
12.0 14.0
.50 Max.
.50 Max.
.50 Max. ---
633M S350000.07 0.11
.030 Max.
1.0 Max.
16.0 17.0
4.0 5.0
.50 Max.
2.50 3.25 ---
Precipitation hardening stainless steels are nby strengthening mechanism. These grades may have austenitic, semi-austenitic 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 the formation of intermetallic precipitates from elements such as copper or aluminum. These alloys generally have high strength and high apparent hardness while exhibiting superior corrosion resistance compared with martensitic stainless steels. Heat treatmentscan be used to vary the properties of the alloys and involve short times (1 h) at temperatures ranging from 480 oC to 620 oC (900 oF to 1150 oF). The aging treatment cantake place in either air or in nitrogen, depending on the surface appearance required. However, these alloys should not be subjected to welding or in service temperatures above the heat-treatment temperature because strength can be lost due to overaging. The AISI designation for these alloys is the 600 series of stainless steels, but most aremore commonly known by their alloy name, for example, 15-5PH, 17-4PH and17-7PH. The aluminum-containing precipitation hardening alloys are difficult to process by the PM route due to their high affinity for nitride formation and the difficulty in reducing aluminum oxide during sintering. Table VII and VIII give the chemicaprecipitation hardening alloys produced by conventional PM techniques. 17-4PH is a martensitic grade in which ductility and toughness are generally higher than in the carbon-containing martenstitc grades. The mechanical properties of 17-4PH can be increased by 15% by aging at 538 oC (1000 oF) for 1 h. Applications for this alloy ein the food, chemical and aerospace industries.
633 is a semcorrosion resistance compared with martensitic precipitation hardening alloys. These alloys are used for parts requiring high strength at moderately elevated temperatures. Depending on the aging treatment, the ductility and toughness of this alloy can approac
those of the austenitic stainless steels. The microstructure of the alloy is a mixture of austenite, martensite and small quantities of ferrite.
Table VIII: Mechanical Properties of PM Precipitation Hardening Stainless Steels
alloying e en troduced based on UNS J91151 (a cast grade). 10 This alloy has only 13 w/o chromium
Table IX: Composition of PM Duplex/Dual Phase Stainless Steels (w/o)
Max. Max. Max. 28.0 5.0 Max.1.00 2.00 ---
Duplex- 2205 S322050.03 Max.
.030 Max.
1.0 Max.
21.0 23.0
4.5 6.5
3.0 5.0
2.50 3.50 ---
Duracor/3Cr12 S41003.15 .30
.030 Max.
1.0 Max.
10.5 12.5
1.50 Max.
.50 Max.
.50 Max. ---
Usage of the precipitation-hardening alloys is generally limited by the high cost of the
lements. Recently, a lower cost PM precipitation hardening alloy has beinand utilizes the precipitation of copper to provide a low cost-high strength alloy with moderate corrosion resistance. Table VIII shows that the mechanical properties approach those of 17-4PH, while still maintaining a level of corrosion resistance that is better than that of the high carbon martensitic grades. Duplex Stainless Steels
AISI UNS C S Si Cr Ni Cu Mo Nb
Duplex- 329M S329000.08 .030 1.0 23.0 2.5 .50
Technically, duplex steels are stainless steels that contain two phases.3 Duplex stainless teels are more-accurately defined as alloys containing a mixed microstructure of ferrite nd austenite. New alloys being developed that contain mixtures of ferrite and martensite
Impact Density DensityApparent Hardness Elongation
saare generally termed dual-phase.11 Compositions of PM Duplex/Dual Phase stainless steels are listed in Table IX. A major advantage of these stainless steel grades is that eachphase imparts improved properties to the alloy.
(a) (b) Figure 6. Representative microstructures of (a) duplex stainless steel: (b) dual-phase stainless steel. Duplex stainless steels are ferritic stainless steels (Figure 6(a)) containing chromium and molybdenum to which austenite formers (primarily nickel) have been added to ensure that austenite is present at room temperature. Duplex stainless steels have several advantages over the austenitic grades including high strength, acceptable toughness, and superior corrosion resistance, particularly to chloride stress corrosion cracking. The mechanical properties of a duplex stainless steel (2205) are shown in Table X. Table X: Mechanical Properties of PM Duplex/Dual Phase Stainless Steels
Dual phase stainless steels vary in composition but are generally non-austenitic (Figure 6(b)) and magnetic, containing 11 w/o Cr. The chemistry of the alloy is balanced by ferrite formers and austenite formers. The austenite transforms to martensite upon cooling resulting in a mixture of ferrite and martensite. Because of the low cost of the alloy it is used as a replacement for plain carbon steels where increased corrosion resistance is needed. The martensite in the alloy allows the material to be used in applications requiring strength and wear resistance. The properties of a PM version of this stainless steel (S41003) are cited in Table X. FATIGUE BEHAVIOR OF PM STAİNLESS STEELS Fatigue tests were performed on some of the high strength PM alloys developed. The results of these tests,in terms of the 90% survival limit, are compared with those of other stainless steel fatigue data by Shah et al. 12 in Figure 7. The latter study study compared the fatigue strength of various stainless steels as a function of tensile strength.
0
5
10
15
20
25
30
35
40
45
50
40 50 60 70 80 90 100 110 120 130 140
Tensile Strength (KSI)
Fatig
ue E
ndur
ance
Lim
it (K
SI)
-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
410LCuMartensitic
410LCuAged
DUPLEX
Dual Phase
Figure 7. Fatigue endurance limit (90% survival) as a function of tensile strength.
The excellent fatigue response of these alloys is attributed 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 by crack propagation. Resistance to crack initiation increases as the tensile strength increases. All the PM alloys included in Figure 7 have high tensile strengths, and therefore high fatigue endurance limits. It appears that the addition of copper, nickel, and molybdenum, results in harder martensite, which has a positive effect on fatigue strength. CONCLUSIONS
• Many AISI grades of stainless steel can be made via conventional water atomization and press and sinter PM. These grades are not currently covered by MPIF Standard 35, but provide a range of properties and corrosion resistance that can lead to increased opportunities for PM parts producers.
• These PM grades can be made as prealloys, or admixed nickel, copper and
molybdenum powders can be added to the base stainless steel.
• In ferritic grades, higher levels of chromium, niobium and sulfur can lead to improved mechanical properties, corrosion resistance and machinability.
• The addition of carbon to low chromium PM alloys results in martensitic stainless
steels with increased strength and apparent hardness. Additions of nickel, copper
and molybdenum produce low carbon martensitic stainless steels with increased toughness and corrosion resistance.
• Niobium, when added to PM austenitic stainless steels, improves weldability.
Increases in the molybdenum content of austenitic stainless steels can increase strength and enhance corrosion resistance. Increases in chromium, nickel and silicon levels enhance oxidation resistance.
• Several precipitation hardening alloys with a range of mechanical properties,
microstructure and attendant corrosion resistance can be produced by conventional PM processes.
6. H. Cobb, Steel Products Manual- Stainless Steels, 1999, Iron and Steel Society, Warrendale, PA.
7. T.R. Albee, P. DePoutiloff, G. Ramsey and G.E. Regan, “Enhanced Powder Metal Materials for Exhaust System Applications,” SAE Paper No. 970281, SAE International, Warrendale, PA, USA.1997.
8. T. Hubbard, K. Couchman and C. Lall, “Performance of Stainless Steel PM Materials in elevated Temperature Applications,” SAE Paper No. 970422, SAE International, Warrendale, PA, USA.1997.
9. R.J. Causton, T. Cimino-Corey, and C.T. Schade “Improved Stainless Steel Process Routes,” Advances in Powder Metallurgy and Particulate Materials – 2003, compiled by R. Lawcock and M. Wright, Metal Powder Industries Federation, Princeton, NJ, 2003, part 2 pp.1-13.
10. 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.
11. 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.
12. 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,” SAE Paper No. 2003-01-0451, SAE International, Warrendale, PA 15096, USA, 2003.
