Materials Science Forum Vols. 475-479 (2005) pp. 4223-4226 online at http://www.scientific.net © 2005 Trans Tech Publications, Switzerland

High Pressure Wear Characteristics of Alloyed Ductile Cast Iron

- Effect of Cu, Mn, Si and Mo

W.H. Bang1'

8, C.S. Kang1

·b, J.H. Park2·c and K.H. Oh1·d

1School of Materials Science and Engineering, Seoul National University, San 56-1, Shinrim-Dong, Kwanak-Ku, Seoul, 151-742, Korea.

2Research Institute of Industrial Science and Technology, San32, Hyoja-dong, Nam-gu, Pohang, 790-784, Korea.

8 Usk2@snu.ac.kr, bkangcs@snu.ac.kr, cpjhyun@rist.re.kr, dkyuhwan@snu.ac.kr

Keywords: High-pressure condition, Wear, Ductile cast Iron, Hardness, Toughness

Abstract. Under a high-pressure condition, the wear characteristics of alloyed and unalloyed ductile cast iron were investigated. Mn, Cu, Si and Mo were chosen as alloying elements, and their effects on mechanical properties of hardness and toughness were correlated with wear properties. In most cases, high-pressure wear of alloyed and unalloyed DCI were sequentially developed by mild wear region followed by severe wear. The transition of wear mode was indicated by the drastic increase of friction coefficient and wear rate. The observation of worn surface at mild-wear region suggested that wear had been concentrated at graphite phases, whereas severe wear mode damaged both of graphite and matrix. It was found that DCI alloyed with Si-Mo exhibited the most prominent resistance to the beginning of severe wear while the toughness ofDCI was also a crucial factor.

Introduction

Ductile cast iron(DCI, nodular or spheroidal graphite cast iron) has attractive attentions because of its mechanical properties and castability. The general grade provides tensile strength upper than -400MPa, and spheroidal graphite prohibits brittleness. Combining with its excellent castability, ductile cast iron is used for the wide range of engineering mechanisms that require structural stability and complicated configuration.[! , 2]

The ductile cast iron also exhibits prominent wear resistance. Under a frictional environment, graphite films consisted of hexagonal carbon rings come out from matrix. They act as lubricant between the contacting surfaces of DCI and counter material, and hence, suppress the severely adhesive wear. For its resistance to severe wear, DCI is often applied to the high-pressure wear environment such as guides in rolling system, gears, liners, etc.[3]

In present work, the effects of alloying elements on the high-pressure wear characteristic of DCI were investigated. The procedure and micromechanisms of the wear of DCI were characterized by frictional characteristic and wear-damaged surface. And evaluated wear properties were correlated and discussed with the conventional mechanical-properties of hardness and toughness.

Experiment

Specimen preparation Alloyed(Cu, Mn, Si, Mo) and unalloyed ductile cast irons were melted in induction furnace and poured in Y -shaped sand mould. Spherodizing of graphite was achieved by plunging of commercial Mg-Si alloy. Carbon equivalent content(CE, C%+ 1!3(Si%+P%)) was controlled to be about 4.5. Wear test Pin-on-disc wear test(pin-shaped DCI on the counter material of 300Hv steel) was performed at the normal contact-pressure of3MPa and wear speed of0.38m/sec, which caused the high-pressure wear condition in iron-based hard facing alloy.[ 4] Friction coefficient was recorded during testing. Wear

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(d) Si(4.5wt%) (e) Si(4.3wt%), Mo(0.5wt%) (f) Grain Boundary of Fig. 1-(e)

Figure 1. Microstructure of unalloyed and alloyed ductile cast iron

Table 1. The microstructure, hardness and toughness of unalloyed and alloyed DCI

Hardness(Ha) V-notch Charpy energy Goule)

Unalloyed Mn(0.5wt%) Cu(0.9wt%) Si(4.5wt%) Si(4.3wt%)-Mo(0.5wt%)

180 198 247 199 214

2.99 1.87 2.5 1.42 1.35

rate was evaluated by the weight loss of specimen after experiment. The worn surfaces of specimens were observed by SEM(JSM5600, JEOL) for the inference of wear mechanism.

Result and discussion

The optical micrographs of alloyed and unalloyed DCI are given in Fig. 1. In cases of unalloyed and Mn-alloyed DCI, microstructures were typical Bull' s eyes surrounded by pearlite. The addition of copper and silicon altered the matrix into almost pearlite and ferrite, respectively. Mo in the Si-alloyed DCI gave rise to pearlite formation along the grain boundaries at which Mo was highly segregated from EDS analysis.

The hardness and toughness ofDCI are summarized in Table 1. It was reported that ,the fraction of pearlite in matrix generally determined the strength ofDCI. But highly ferritized DCI alloyed with Si or Si-Mo exhibited higher hardness than those of unalloyed or Mn alloyed DCI. This is due to the solid solution hardening by Si in matrix.[2]

The wear development of unalloyed DCI is presented in Fig. 2, in terms of friction coefficients and worn surface. Fig. 2-(a) shows that the wear of unalloyed DCI under high-pressure condition was developed by two steps, initial stage oflow friction coefficient followed by the region of high friction coefficient. The hole-like damaged area in Fig. 2-(b) indicates that wear occurred mainly by the fall off of graphite phases at low friction coefficient range. It was measured that wear rate was about 2mg/Km(weight loss per distance) in this region. At high friction coefficient range, Fig. 2-(c) shows that wear severely occurred at the overall surface of matrix. The wear rate in this condition was

Materials Science Forum Vols. 475-479 4225

o.a

0"0o ..__ ~--'300'--~6 ...... 00~-90.L.0~-1--'200'--..........J

Wear distance (mm)

(a) friction coefficient during wear test (b) at low friction coefficient (c) at high friction coefficient

Figure 2. Change of friction coefficient during wear test and wear-damaged surface at the each oflow and high friction coefficient regime (white arrows indicate wear direction)

measured to be over 200mg!Km. Alloyed specimens also exhibited similar trends in the morphology of worn surface and wear rate. But DCI alloyed with Si only exhibited high friction coefficient and showed severely worn surface even at the short wear distance of several meters.

Lim et al.[6] reported that transition from mild to severe wear was deeply related with the heating of wearing surface. They suggested that friction force elevated the temperature in proportion to wear distance, and consequently, contact surface with lowered hardness became severely damaged. However, in this experiment, the transition of wear mode occurred at various wear distance that might cause different temperatures at wearing surfaces.

Relationships between the wear distances for the transition to severe wear( at Fig. 2) and hardness and toughness are presented in Fig. 3. From Fig. 3-( a), wear distance for the commencement of severe wear was not clearly concerned with the hardness of DCI. But solid line in Fig. 3-(b) suggests toughness of DCI could be strongly influential to the severe wear. Although the Si-alloyed DCI had exhibited the poorest resistance to the onset of severe wear, the addition of Mo notably delayed the severe wear ofDCI alloyed with Si.

1200 ..:-.! Cll 900 .§. ~ 600 c

~ ·- 300 0

unalloyed •

Sl-Mo •

~ Mn Cu

• "I 0~~~~~~·~~--~~~----' 180 200 220 240 260

Hardness (H8

)

(a) correlation with hardness

1200 .si-Mo

-.. ~ 900

.§. Cll 600 u c ~ 300

c Sl

0 • 1.5 2.0 2.5 3.0

Charpy energy (ooule)

(b) correlation with V -notched charpy energy

Figure 3. Relationship between the transition distance of friction coefficient and hardness and toughness (Solid line is fitting without the result ofDCI alloyed with Si-Mo)

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It has been generally recognized that the wear resistance of a cast iron is proportional to the hardness dependent on pearlite fraction or hardening alloys.[5] However, under a high-pressure condition at this study, unalloyed DCI exhibited prominent resistance to the onset of severer wear even with the lowest hardness and intermediate pearlite fraction.

Instead of hardness, the fracture toughness ofDCI seemed to be more influential to the beginning of severe wear that caused the anomalous wear rate. It is not feasible at this time to capture the full details on this correlation oftoughness and evolution of severe wear, but following description on the appearance and procedure of severe wear could suggest micromechanisms and relationship of Fig. 3-(b).

As it is shown in Fig. 2-(b ), the mild wear ofDCI was mainly progressed by the escape of graphite. But the elongation of hole-like damaged area to wear direction(Fig. 2-(b)) denotes that the deformation of matrix has been progressively developed in this regime. It was highly possible that the elongated region of matrix became the nucleation site for matrix fracture, and then, severe wear occurred at overall matrix with the indication of high friction coefficient. The alloying elements ofCu, Mn and Si reduced the fracture toughness of DCI. Therefore, it seems that severe wear could occur more easily at these alloyed-DCis.

Although the DCI alloyed with Si showed the most poor wear characteristic under high-pressure condition, the addition ofMo to this alloy system prominently delayed the beginning of severe wear. In the previous report of Yang et al.[7], Mo in iron reduced the chemical reactivity of free surface at which severe adhesive wear became difficult. Clear examination on this micromechanism is further needed.

Conclusion

The effect of alloying elements on the high-pressure wear characteristic of DCI was investigated. Even though the alloying elements of Cu, Mn and Si had hardened DCI, the onset of severe wear occurred at shorter wear distance than unalloyed DCI. This seems to be related with higher toughness of unalloyed DCI, which could delay the fracture of wearing surface during wear test. Although the addition ofMo had not notably modified the mechanical properties ofSi-alloyed DCI, it delayed the beginning of severe wear during high-pressure wear testing. Further study is required for finding micro-chemical or -mechanical effect ofMo on wear resistance.

References

[1] C.R. Loper and J.Y. Park: Met. Mater.-Int. Vol9 (2003), p. 327

[2] C.F. Walton and J.T. Opar: Iron Castings Handbook (Iron Castings Society, Ohio 1981)

[3] A.F. Spengler: The Ductile Iron Process (Miller and Company, Illinois 1982)

[4] I. Jarudi and L.C. Zhang: J. Mater. Sci. Vol. 37 (2002), p. 3935

[5] A.D. Sarkar: Friction and Wear (Academic Press, New York 1980)

[6] S.C. Lim and M.F. Ashby: Acta Metall. Vol.35 (1997). P.l

[7] D.H. Yang, Q.J. Xue, X.S. Zhang and H.Q. Wang: Wear Vol. 173 (1994), p.129