technical trends
Seeking high
strength in PM
stainless steels
Joe Capus looks at recent Höganäs research that suggests
that PM stainless steel formed using liquid-phase
sintering can compete with wrought stainless steel when
it comes to strength and cost.
H
eat-treatable, highstrength stainless steels,
such as stainless cutlery
— surgical instruments,
and stainless steel razor blades – are
not commonly produced by press-andsinter PM. Even near-full-density MIM
is mostly limited to 316L and 17-4PH
stainless compositions. However, recent
research at North American Höganäs
(NAH) have highlighted the possibilities of achieving strengths on a par with
wrought high-strength stainless while
taking advantage of the net-shape manufacturing capabilities of PM.
The first of these studies took a close
look at the influence of residual carbon
content on the mechanical properties of
PM 17-4PH stainless steel in both the
as-sintered and heat-treated conditions.
17-4PH stainless was first developed in
wrought form more than fifty years ago,
and belongs to a group of martensitic
precipitation-hardening steels that can
achieve high strength and toughness.
These steels are generally austenitic at
the solution-annealing temperature but
transform to martensite on cooling.
Hardening is accomplished by reheating
or ageing at a moderate temperature
around 500-600°C.
17-4PH has a nominal composition
of 17% chromium, 4% nickel, 4% copper, and 0.3% niobium. The composition is carbon-free and balanced to
facilitate the desired phase transformations. Both carbon and nitrogen can
30
MPR September/October 2010
affect this balance and interfere with
full hardening properties. This alloy
composition has been successfully
employed for many years in aerospace
applications, including the space shuttle. As fine powder, 17-4PH has also
found successful applications in MIM,
where sintering to near-full density
allows mechanical strength and corrosion resistance properties to approach
those of the wrought material. In
fact, 17-4PH and 316L stainless steels
have become the most popular alloys
employed in injection moulding.
Press-and-sinter route
Prasan Samal and colleagues at NAH
and its parent company have looked
into the prospect of developing highstrength 17-4PH material by the conventional press-and-sinter route. A major
hurdle was that these complex, highlyalloyed powders are not the most compressible materials. Another factor was
the influence of residual carbon in the
powder on the structure and mechanical properties. Nevertheless, the work
showed that despite the density limitations of the press-and-sinter processing,
attractive combinations of strength and
ductility could be obtained with the
right heat-treatment cycle.
Laboratory tests were made using a
commercially-available grade of wateratomised 17-4PH powder, nominally
minus 100 mesh with a minus 325 mesh
(45 microns) content of 47%. This was
mixed with 1% lithium stearate and
small amounts of graphite to produce
test blends containing zero, 0.06%,
0.09%, and 0.12% graphite. With these
levels of graphite addition, the residual
carbon contents obtained in the sintered
samples largely represent the residual
carbon levels resulting from inadequate
delubrication that can occur in commercial sintering operations. Test bars
compacted at 55 tsi (760 MPa) were sintered, after de-lubricating in hydrogen,
Table 1. Mechanical properties and carbon contents of sintered PM 17-4PH,
solution-annealed and aged at 552°C. After Prasan Samal et al. (Ref.1).
Carbon, %
UTS (MPa)
Y.S. (MPa)
Elong. (%)
Hardness (HRB)
0.021
1032
946
3.2
95
0.035
949
854
4.0
93
0.055
934
843
1.8
92
0.062
894
809
1.6
86
0026-0657/10 ©2010 Elsevier Ltd. All rights reserved.
7.2 g/cm3 attractive properties are
obtainable in a PM processed 17-4PH,
provided that the residual carbon level
is no higher than 0.04% and the material is aged at 552°C.
Liquid-phase sintering
Figure 1. Effect of residual carbon on UTS and elongation for PM 17-4PH stainless steel,
aged at 552°C. (After Prasan Samal et al.1)
in a batch furnace at 1275°C for 45
min, also in pure hydrogen. Samples
were cooled at 2°C/sec after sintering.
Compacted densities of 6.30 g/cm3
rose to 7.12-7.22 g/cm3 after sintering,
with the higher carbon content samples
showing the lower densities, postulated
as due to the increased percentage of
acicular martensite as carbon rose.
Influence of heat
treatment
To determine the influence of postsinter heat-treatment, a portion of
the as-sintered samples were solutionannealed at 1065°C for 90 min in a
low-pressure hydrogen atmosphere (per
SAE Aerospace Materials Specification
AMS 2759/3D), followed by cooling at 2°C/sec, then ageing at either
482°C for 2 hrs or 552°C for 4 hrs in
a nitrogen atmosphere. Tensile test
results (standard dog-bone test-pieces)
showed the influence of treatment and
carbon content.
In the as-sintered condition, tensile
and hardness properties were little
affected by carbon levels, with UTS
averaging between 856 and 874 MPa,
elongation between 2.5 and 3.1%, and
hardness at 83-89 HRB. A stronger
influence of carbon content was seen
after solution treatment and ageing,
with both strength and elongation
declining as carbon increased. The
maximum strength was found after ageing at 482°C but the elongation levels
were poor, averaging between 0.9 and
1.5%. A better combination of tensile
properties was found after ageing at the
higher temperature (552°C), with UTS
averages falling from 1032 to 894 MPa
as the carbon level increased from 0.021
to 0.062% (see Table 1). The lower carbon samples also gave superior ductility,
with average elongation of 3.1-4.0%
for 0.021% and 0.035% carbon, versus 1.6 -1.8% for 0.055% and 0.062%
carbon (Figure 1). Although strengths
obtained in these samples were lower
than for wrought or even MIM 17-4PH,
the values were higher than found in
alternative press-and-sinter PM ferritic,
martensitic or austenitic stainless steel
materials. The overall results showed
that with sintered densities of at least
In a subsequent project, Samal and
colleagues pursued the study of highstrength PM martensitic stainless steel
in an exploration of densification by
liquid-phase sintering. In this study,
the composition/alloy system was
modelled on the hardenable martensitic
wrought AISI 440C composition. (In
the wrought 440 series, based on 17%
Cr-1% Mo, the 440A material contains
0.60-0.75%C, 440B has 0.75-0.90%C,
while the 440C alloy has a carbon range
of 0.95-1.20%). In its wrought form,
440C stainless has applications in the
production of cutlery, food-processing/
canning equipment, surgical and
dental instruments, scissors, springs,
valves, gears, and so on. Due to its
hardness and low formability in the
annealed condition, wrought 440C
has limitations in the production of
components. However, the near-netshape processing capabilities of PM and
MIM opens the door to cost-effective
production in a variety of applications
requiring high wear and corrosion
resistance. The proviso, of course, is the
achievement of full or near-full density.
Samal and colleagues devised a programme to test the production of nearfull-density 440C by the liquid-phase
sintering of a mix based on a low-carbon
powder of similar alloy composition.
Graphite and ferro-boron were added to
434L stainless steel powder, which has
the same 17Cr-1Mo composition but no
carbon (see Table 2). The mix composition by weight was 97.9% annealed 434L,
1.35% fine graphite, 0.25% ferro-boron,
and 0.50% lithium stearate. Standard
test-pieces were compacted at 55 tsi
(760 MPa) to maximize the green density
and minimize shrinkage during sintering.
Both delubrication and sintering were
done in pure hydrogen.
Table 2. Composition and Properties of 434L Base Powder, wt.%. (After Prasan Samal et al.2)
Alloy
Fe
Cr
Mo
Si
Mn
P
S
C
N
O
AD, g/cm3
% <44 µm
434L
Bal
16.8
0.94
0.82
0.13
0.015
0.011
0.024
0.035
0.22
2.85
53
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MPR September/October 2010
metal-powder.net
Table 3. Density and Mechanical Properties of Sintered and Heat-treated PM 440C Stainless Steel. (After Prasan Samal et al.2)
Material
Metallurgical
condition
440C – This study As-sintered
Sintered
density, g/cm3
Hardness HRC
U T S, MPa (KSI)
Yield strength,
MPa (KSI)
Elong, %
7.70
39-42
738 (107)
730 (106)
1.0
440C – This study
Heat treated –
As Quenched
7.70
56-58
NA
NA
NA
440C – This study
Heat treated &
Tempered
7.70
56-59
752 (109)
710 (102)
2.8
Wrought 440C
Heat treated &
Tempered
7.75
57-60
896-1930
(130-280)
620-1860
(90-270)
1.0-2.0
MIM 440C
Heat treated &
Tempered
7.50
43
620 (90)
410 (59.5)
2.0
Figure 2. Microstructure of as-sintered PM 440C, etched in glyceregia.
(After Prasan Samal et al.2)
An initial sintering test at 1260°C
did not achieve the targeted density
(of 7.60 g/cm3), but further sintering
tests at 1271°C successfully reached
the target of >98% theoretical density
(>7.60 g/cm3). Both ‘dog-bone’ and TRS
test-pieces were produced. The cooling
rate from the sintering temperature to
538°C averaged 0.70°C/sec. Samples were
tested as-sintered and after a subsequent
heat-treatment to maximize hardness.
The latter involved soaking at 1052°C
for 20 min in vacuum, followed by airquenching and sub-zero cooling to below
minus 75°C to convert any remaining
austenite to martensite. One set of ‘dogbone’ tensile test-pieces were tempered
at 163°C for 30 min. Microstructural
examination of the as-sintered material
showed fine primary carbides within
grains and secondary carbides along
metal-powder.net
Figure 3. Microstructure of heat-treated (as-quenched) PM 440C,
showing carbide precipitates in a coarse martensitic matrix. Etched
in glyceregia. (After Prasan Samal et al.2)
grain-boundaries in a fine martensitic
matrix (Figure 2). After heat-treatment,
the as-quenched microstructure showed
References
[1] P. K. Samal, N. Nandivada, I.
Hauer, “Properties of 17-4PH
Stainless Steel Produced via
Press and Sinter Route”,
Advances in Powder Metallurgy
& Particulate Materials – 2008,
Part 7, pp.109-120.
[2] P. K. Samal, J. C. Valko,
J. D. Pannell, “Processing
and Properties of PM 440C
Stainless Steel”, Advances
in Powder Metallurgy &
Particulate Materials – 2009,
Part 7, pp. 112-121.
fine primary carbides in a coarser martensitic matrix (Figure 3). The mechanical properties are summarised in Table 3.
Results indicated that the liquid-phasesintered 440C had hardness and tensile
properties on a par with wrought 440C,
even with a slightly lower density (7.70
vs 7.75 g/cm3), and compared favourably with MIM 440C. Shrinkage during
sintering was found to be between 5.0
and 5.5%.
A positive result
Despite the obvious problem of controlling dimensional tolerances with this
large degree of shrinkage, the authors
believe that liquid-phase sintering of
PM 440C stainless offers an attractive
economic alternative to wrought and
MIM processes.
September/October 2010 MPR
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