Challenges in Processing of P/M Chromium Manganese Low-Alloy Steels
Robert J. Causton1 and Bruce A. Lindsley2
1
2
Hoeganaes Corporation, Buzau, Romania
Hoeganaes Corporation, Cinnaminson, NJ 08077, USA
ABSTRACT
Extending the application of P/M steels into larger or more highly stressed components
requires the development of steel compositions of higher hardenability. Ideally, new lowalloy steels should offer the same or higher compressibility than existing P/M steels and
possess compositions that can be surface hardened to produce the higher wear and contact
fatigue resistance necessary in many highly stressed components. This paper illustrates how
Ancorsteel 41AB, a chromium-manganese P/M alloy steel, meets some of these
requirements.
INTRODUCTION
Many components suitable for conversion to P/M processing require a combination of high
density and high strength plus resistance to wear and fatigue. Chromium and manganese offer
the designers of P/M steels and sintered parts significant theoretical advantages over copper,
molybdenum and nickel in developing P/M steels to meet these requirements. Despite these
advantages, it has proven difficult for powder and parts producers to attain the use of
chromium and manganese P/M low-alloy steels in high volume applications. This paper
discusses the properties of Ancorsteel 41AB, a P/M low-alloy steel that employs chromium
and manganese as alloying elements in addition to molybdenum and nickel.
OXIDE STABILITY
The primary reason stated for the difficulties in producing and processing chromium and
manganese P/M steels is the stability of their oxides as shown in the Ellingham and
Richardson diagrams (Ref.1). The problems of the oxidation of chromium and manganese
affect both powder production and sintering of P/M parts (Ref. 2).
Table I: Qualitative Ranking of Alloying Elements in Pre-Alloy Additions
Ranking
Hardenability
Affitinity for
Compressibility
Oxygen
High
Manganese
Manganese
Molybdenum
Chromium
Chromium
Manganese
Molybdenum
Nickel
Chromium
Copper
Molybdenum
Nickel
Low
Nickel
Copper
Copper

Ancorsteel is a registered trademark of Hoeganaes Corporation
To meet the needs for improved performance, powder and parts producers try to develop alloy
systems that offer the greatest benefit and can be processed easily (Table I). Pre-alloyed
molybdenum is employed in high performance alloy systems and copper is used as a premix
or diffusion bonded ingredient to gain the benefits of partial liquid phase sintering and ferrite
strengthening without adverse effects on compressibility. Nickel is used in limited amounts
in pre-alloyed powders but larger amounts are added as premix additives or diffusion bonded.
However, the slower diffusion in the solid state leads to inhomogeneous nickel distribution,
which may not be acceptable in quenched and tempered parts.
Pre-alloying is the simplest method to introduce chromium and manganese as alloy additions.
However, they act as solid solution hardeners to reduce compressibility. They also tend to
increase the oxygen content of atomized and annealed powders (Ref.3), further reducing
compressibility, Figure 1.
Green Density (g/cm 3)
6.94
0.75% lubricant, 550 MPa
6.92
6.90
6.88
6.86
6.84
6.82
6.80
6.78
0.00
0.10
0.20
0.30
0.40
Oxygen Content (%)
0.50
0.60
Figure 1. Effect of oxygen content on compressibility of experimental pre-alloyed powders
The majority of published P/M literature concludes that high temperatures and low dew points
are required to reduce chromium and manganese oxides and produce P/M low-alloy steels that
possess the anticipated mechanical properties and hardenability (Table II).
Table II: Dew-Point Required for Sintering P/M steels in Hydrogen Atmosphere
T (ºC)
1120
1300
CuO
NiO
≅100
≅100
FeO
90
90
MoO2
70
90
Cr2O3
-30
-20
MnO
-50
-40
SiO2
-70
-60
Al2O3
-130
-110
The combination of temperature and dew-point indicated are close to or beyond the limits of
sintering for P/M steels even in high temperature sintering furnaces. More recent research
work has indicated that the “classical approach” is incomplete. It fails to examine the potential
benefits of reduction of metal oxides by carbon (Ref. 4) or the lower activity of chromium and
manganese when in solution compared to their standard state (Ref. 5).
Both of these changes offer the possibility that chromium and manganese low-alloy steels
may be sintered at lower dew-point and somewhat lower temperature than indicated in
Table II, and possess properties superior to more conventional P/M steels. This will be
illustrated by the properties of Ancorsteel 41AB.
ANCORSTEEL 41AB
Ancorsteel 41AB is the first of a series of alloy steels developed to possess a superior
combination of compressibility, hardenability and fatigue performance to conventional P/M
steels. The composition of this nickel, chromium, manganese and molybdenum containing
low-alloy steel is given in Table III (Ref. 5). The combination of alloying elements possesses
good compressibility, sintered properties and better hardenability than developed by higher
contents of the individual alloying elements alone.
Table III: Composition of Ancorsteel 41AB
Mo (%)
Ni (%)
0.85
1.0
Cr (%)
0.75
Mn (%)
0.85
7.3
Green Density (g/cm 3)
7.2
7.1
7.0
6.9
41AB
FL-4605
6.8
6.7
6.6
6.5
300
400
500
600
700
Compaction Pressure (MPa)
800
Figure 2. Compressibility of Ancorsteel 41AB compared to FL-4605 (0.5% Zinc stearate)
The high compressibility shown in Figure 2 is achieved by introducing molybdenum as a prealloy where it has small effects on compressibility and offers greatest benefit on hardenability.
Nickel is added as a fine powder, so as not to reduce compressibility. As it enters solution in
the iron it has beneficial effects on growth during sintering and appears to provide synergistic
effects upon the hardenability of molybdenum. Chromium and manganese are introduced as
ground high carbon ferro-alloys. By add-mixing the Ni, Cr and Mn in this way, the
compressability of 41AB is much higher than the lower alloyed FL-4605. The combination of
chromium and manganese at relatively low levels should provide greater benefits on
hardenability, while avoiding the problems encountered if they were introduced as pre-alloys.
TENSILE PROPERTIES
The combination of alloy design and good compressibility leads to good sintered tensile
properties without heat treatment. Naturally, high temperature sintering increases tensile
strength, although acceptable static properties and hardness may be obtained at lower
sintering temperatures. In tests conducted with a sintering atmosphere of 90% nitrogen / 10%
hydrogen at dew-points of –40°C utilizing convection cooling for sinter hardening, increasing
density from 6.7 to 7.25 g/cm3 increased yield strength from about 500 to about 700 MPa
(Figure 3). Increasing sintering temperature to 1260°C increased yield strength significantly,
to a maximum of roughly 1000 MPa. At a sintering temperature of 1120°C, increasing
density increased ultimate tensile strength (UTS) from about 700 to 10000 MPa, while high
temperature sintering increased UTS about 150 MPa at all densities. The increases in static
strength at higher sintering temperatures are likely due to increased alloy diffusion during
sintering. Sintered carbon and oxygen contents were 0.55 wt% and 250 ppm, respectively.
1300
Strength (MPa)
1150
1000
YS 1120C
UTS 1120C
YS 1260C
UTS 1260C
850
700
550
400
6.6
6.8
7.0
7.2
3
Sintered Density (g/cm )
7.4
Figure 3. Strength of Ancorsteel 41AB sintered at 1120 °C and 2300 °C in the sinter
hardened condition
The alloy chemistry is relatively resistant to tempering (Ref. 2). There is little change in
hardness or tensile strength for tempering temperature in the range 150 to 550°C. This feature
can be useful as the increased “power density”, greater torque transmission and loadings in
smaller components of modern systems leads to higher operating and coolant temperatures.
HARDENABILITY
Adding chromium and manganese together produce excellent hardenability, and benefit heat
treatment of larger sections and parts production by sinter-hardening. The improved
hardenability with the addition of Cr and Mn to the 1%Ni, 0.85%Mo alloy in powder forged
Jominy bars can be seen in Figure 4. In addition, the chromium in the alloy enables
components to be hardened by techniques such as plasma nitriding or carbo-nitriding that
form a hard wear resistant surface.
Jominy Hardenability of CrMnNiMo P/F Steels
o
0.4% graphite, 1290 C
700
1%Ni, 0.85%Mo, 0.75%Cr, 0.85%Mn
Hardness (Hv)
600
500
1%Ni, 0.85%Mo, 0.85%Mn
400
300
1%Ni, 0.85%Mo, 0.75%Cr
200
1%Ni, 0.85%Mo
100
0
0
20
40
60
80
100
Distance from Quenched End (mm)
120
Figure 4. Effect of Cr and Mn on Jominy Hardenability of P/F Steels.
Median Fatigue Strength (MPa)
600
500
Sinter
Sinter-Harden
HT Sinter
Q&T
Carbonitride
CN 41AB
400
300
200
MPIF Standard 35 Fatigue = 0.38*UTS
100
300
500
700
900
1100
UTS (MPa)
1300
1500
Figure 5. Fatigue strength of P/M steels, including carbo-nitrided 41AB (CN 41AB)
The median fatigue strength of the carbo-nitrided Ancorsteel 41AB, with a core carbon of 0.2
to 0.25% is significantly higher than that of conventional P/M steels of similar strength. The
high hardenability of the P/M Cr-Mn steel enables the core carbon content to be adjusted
during premixing and sintering to provide a tempered martensite microstructure with the
combination of strength and toughness required to support the wear resistant surface during
service.
DISCUSSION
In this work, test pieces were sintered in both research and production sintering furnaces.
Under research conditions, it was possible to obtain low sintered oxygen contents at
somewhat lower temperature and higher dew-points than anticipated from the P/M literature.
Research test pieces frequently have smaller cross sections than commercial parts and
sintering cycles enable parts to be held at temperature for periods of 30 min in furnaces that
employ relatively higher gas flows. These combine to provide somewhat “ideal” sintering
conditions in which: all parts of a test piece reach equilibrium with the sintering temperature
and atmosphere; there is time for diffusion on a micro scale within the particles and macroscale within the pores and parts; and reaction by-products are purged from the furnace
atmosphere and pores within the part.
It is more difficult to attain these “ideal” conditions in the production of commercial parts.
Commercial sintering cycles frequently provide shorter time and temperature and relatively
lower gas flows of lower hydrogen content gas. It is more difficult for reactions and diffusion
to reach equilibrium conditions, leading to less reduction of oxides and greater segregation of
alloying elements within a part. These problems are encountered during sintering of
conventional P/M steels. They are more severe when sintering chromium and manganese
alloys particularly as equilibrium between alloying elements and oxide can move from
oxidizing, to reducing and back to oxidizing as the part is heated to and cooled from sintering
temperature. Control of the sintering cycle is vital to production of P/M parts for high
performance applications and may account for the difficulties in converting parts to Cr-Mn
P/M steels.
CONCLUSIONS
Chromium and manganese low-alloy steels such as Ancorsteel 41AB possess a superior
combination of properties to conventional P/M steels that employ copper and nickel as
alloying additions. In this work, reduction of chromium and manganese oxide and attractive
sintered properties were developed at dew points of -10/-15°C.
The combination of chromium and manganese provides a more efficient alloy design in terms
of compressibility, hardenability and processing than larger amounts of the individual
elements. Chromium-manganese alloy steels are very suitable for applications requiring
surface modification by carbo-nitriding or plasma nitriding.
REFERENCES
1. Thermodynamics of Substances of Interest In Iron and Steelmaking, F. Richardson and I.
Jeffes, JISI, Nov. 1948, p.261.
2. Surface Hardenable P/M Steels, W.B. James and R.J. Causton, Advances in Powder
Metallurgy and Particulate Materials, Vol. 5, 1992, p.65.
3. Application of High Performance and Systems-Alloy Systems, A.B. Davala, et al,
Advances in Powder Metallurgy and Particulate Materials, Vol. 13, 1998, p.181.
4. A Superior Sinter Hardenable Material, M.C. Baran et al, Advances in Powder Metallurgy
and Particulate Materials, Vol. 7, 1999, p.185.
5. Fundamentals of High Temperature Sintering: C. Lall, IJPM, Vol. 27, No. 4, p.315.
6. Degassing and Deoxidation Processes During Sintering of Unalloyed and Alloyed PM
Steels, H. Danninger et al, Powder Metallurgy Progress, Vol 2, no. 3, 2002, p.125.