Materials Transactions, Vol. 53, No. 6 (2012) pp. 1203 to 1205
© 2012 The Japan Institute of Metals
EXPRESS RAPID PUBLICATION
Effect of Molybdenum-Rich Carbides on Wear Behavior
of Powder Metallurgy Steel
Fang Liu and Ke-Chao Zhou
State Key Lab for Powder Metallurgy, Central South University, Changsha 410083, P. R. China
Effect of molybdenum-rich carbides on wear behavior of the powder metallurgy (P/M) steel was studied. Wear-resisting property of the
couple part was also studied. The friction pair materials have low wear ratio. M2C carbides and M6C carbides exit in the P/M steels. Crisp M2C
carbides decrease wear-resisting property of the materials obviously. While fine grainy M6C carbides are beneficial to good wear-resisting
property of the materials. [doi:10.2320/matertrans.M2012063]
(Received February 17, 2012; Accepted April 4, 2012; Published May 16, 2012)
Keywords: powder metallurgy steel, carbide, wear behavior, wear-resisting property
1.
Introduction
Wear occurs in a wide variety of items and results in severe
economic loss. On the other hand, the requests for more
excellent property materials are ever increasing.1,2) Addition
of strong carbide-forming element by powder metallurgy
(P/M) process techniques is one way to obtain the materials with both high wear resistance and good mechanical
properties.3,4) The P/M steels with high molybdenum
element content (8.0­10.0 mass% Mo) are examined in this
paper. These types of steels are used as spacing adjustment
pieces in a new friction system with a high power engine.
The couple part in this friction system is a cast iron
(international standards). The aim of this paper is to evaluate
the effect of the molybdenum-rich carbides on the wear
behavior of the P/M steels. Special emphasis is given to the
wear-resisting property of the friction pair materials.
2.
Properties of the frication pair materials.
Properties
Hardness (HRC)
Flextural Strength (MPa)
Impact Toughness (J·cm¹2)
P/M steel
Cast iron
50­54
59­61
692­745
2.0­2.8
910­950
8.0­10.0
Experimental Procedure
The studied materials in this paper consisted of two series
of the P/M steels (8.0­10.0 mass% Mo, 1.3­1.4 mass% C
and balance Fe) and the cast irons (international standard,
0.90­1.20 mass% Cr, 0.15­0.25 mass% Mo, 0.17­0.37
mass% Si, 0.50­0.80 mass% Mn). The mixed powders of
the P/M steels were cold pressed, sintered, annealed and heat
treatment. The cast irons were heat treatment. The properties
of two series of materials were shown in Table 1.
The wear tests were carried out on a standard ball-on-flat
sliding wear test machine. The microstructures of the
materials and the phase identification of the P/M steels were
examined by scanning electron microscopy (SEM), X-ray
diffraction (XRD) and energy dispersive spectroscopy (EDS)
analysis respectively.
3.
Table 1
Results and Discussion
3.1 XRD results of the surface
Figure 1 is the XRD patterns of the P/M steels. There are
two carbide types, i.e. M2C carbides and M6C carbides in the
materials. With the extension of wear time, M6C carbides
content remains unchanged while M2C carbides content
drops markedly.
Fig. 1 XRD patterns of the P/M steels. (a) Before wear test. (b) Wear test
of 60 min. (c) Wear test of 120 min.
3.2 Wear resistance
Figure 2 shows the wear ratios of the friction pair materials
with the different wear test time. The wear ratio rises slowly
when wear time prolongs. The iron casts have a lower wear
degree than the P/M steels.
3.3 Microstructure of the original surfaces
Figure 3 and Table 2 are the microstructure and EDS
results of the P/M steels before wear test. Two carbide types,
white needle carbides and white block carbides, are present in
the material. High molybdenum contents of the white needle
carbides (Fig. 3, Point A, B and C) are observed at 76.58,
1204
F. Liu and K.-C. Zhou
Fig. 2 Wear ratios of the friction pair materials with the different wear test
time.
Fig. 4
SEM image of the cast iron before wear test.
(a)
(b)
Fig. 3 SEM image of the P/M steel before wear test.
Table 2
EDS results of the P/M steels worn surfaces before the wear test.
Point
Type
Fig. 3, A
Fig. 3, B
Element content (at%)
Mo
Fe
C
M2C
M2C
76.58
73.89
12.95
14.88
10.47
11.23
Fig. 3, C
M2C
72.36
17.99
9.65
Fig. 3, D
M6C
52.77
37.87
9.36
Fig. 3, E
M6C
56.25
33.63
10.12
Fig. 3, F
M6C
57.42
33.59
8.99
Fig. 5 SEM images of the worn surfaces in the P/M steels. (a) Wear test of
60 min. (b) Wear test of 120 min.
73.89 and 72.36 at%. The molybdenum contents of the
white block carbides (Fig. 3, Point D, E and F) are also high
(52.77, 56.25 and 57.42 at%). However, iron content of the
white block carbides (37.87, 33.63 and 33.59 at%) are
obviously different from those of the white needle carbides
(12.95, 14.88 and 17.99 at%). According to the XRD pattern
(Fig. 1(a)) and EDS results (Table 2), the carbide types can
be identified that the white needle carbides are M2C carbides
and the white block carbides are M6C carbides.
Figure 4 displays the microstructure of the cast iron before
wear test. This material has very high density and a uniform,
even microstructure.
3.4 Microstructures of the worn surfaces
Figure 5 indicates SEM images of the worn surfaces in the
P/M steels with the wear test of 60 and 120 min, respectively.
The degree of pores in the worn surface is higher than that
of the surface before wear test. The porosity of the surface
before wear test is about 4.2%. When wear time prolonged to
60 and 120 min, the porosity increases to be 6.5 and 8.9%
respectively. With the extension of wear time, M6C carbides
are neither cracked nor broken up even in the exposed part,
so this type of carbide is useful to the wear-resistance of
the materials. While M2C carbides distribute along crystal
Effect of Molybdenum-Rich Carbides on Wear Behavior of Powder Metallurgy Steel
(a)
ninety-six percent of the carbides, which are around the big
bores, are M2C carbides.
Figure 6 displays SEM images of the worn surfaces in the
cast irons with the wear test of 30 and 120 min, respectively.
There are clearly homogeneous and unidirectional
grooves, parallel appearing in the surface. No phase makes
major influence on the wear behavior of the cast irons. With
the prolonging wear test time, the dimensions of the grooves
present in the cast irons seem to be wider and deeper.
4.
(b)
1205
Conclusion
(1) The two friction pair materials in the friction system
have low wear ratio. The iron casts have a lower wear
degree than the P/M steels.
(2) The white block carbides (M6C) are useful to the wearresistance property of the materials. The white needle
carbides (M2C) decrease the wear-resistance property of
the materials.
Acknowledgements
The authors acknowledge the financial support from the
National Natural Science Fund for Innovation Group of
China (No. 50721003) and the Natural Science Fund of
Hunan (No. 11JJ3044). The authors also would like to thank
Mr. Zhang, Mr. Tian and Mr. Chu for their help in material
preparation and characterization.
Fig. 6 SEM images of the worn surfaces in the cast irons. (a) Wear test of
30 min. (b) Wear test of 120 min.
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