MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Fe-C-Cu POWDER-FORGED CONNECTING ROD
Linna BAI1,Fuping LIU1,2,Sui WANG1,Feng JIANG1(),Jun SUN1,Liangbin CHEN1,WANG,Fengyuan2
1 State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China 2 School of Automobile and Transportation, Qingdao Technological University, Qingdao 266520, China
Cite this article:
Linna BAI,Fuping LIU,Sui WANG,Feng JIANG,Jun SUN,Liangbin CHEN,WANG,Fengyuan. MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Fe-C-Cu POWDER-FORGED CONNECTING ROD. Acta Metall Sin, 2016, 52(1): 41-50.
Powder-forged (P/F) connecting rods have been widely used due to their advantages of high strength, less machining, light weight, and consistency etc.. Currently, P/F connecting rods were only supplied by GKN in Britain and Metaldyne in US in commercial quantities. In this work, the microstructure and mechanical properties of the P/F Fe-C-Cu automobile engine connecting rods (H16) were designed and manufactured domestically, and the factors affecting the fatigue performance were systematically analyzed. The Measured results indicate that the density of the connecting rod is greater than 7.80 g/cm3. Microstructure observation showed that there were no oxide penetrations or network near or at rod surface and the surface decarburization layer is thinner than 70 mm. Anisotropy at different locations inside the P/F connecting rod was revealed. Furthermore, the bending of connecting rods was found to affect the fatigue performance significantly. The microstructure and the surface shot-peening condition had certain influence on the sites of fatigue crack initiation. Most importantly, the fatigue strength of H16 P/F connecting rod was found to be superior to that of the wrought steel-forged connecting rod (C70), and similar to that of P/F connecting rods designed and manufactured by entities.
Fig.1 Blanks (a) and finished products (b) of H16 powder-forged (P/F) connecting rods
Fig.2 Schematics of sampling locations and testing specimens for the H-shaped cross section (a) and the horizontal section (b) of rod shanks (arrows in Figs.2a and b indicate viewing directions); tensile test specimens extracted from the shank rib and shank web (c) and a fatigue test specimen of the connecting rod blank (d)
Fig.3 OM image of a polished and un-etched up surface cross section of the H16 P/F connecting rod shank, observed from the metallurgical mount shown in Fig.2a
Fig.4 Names of H-shaped cross-sectional areas and morphology and distribution characteristics of MnS (a); OM images of the middle part of rib (b), the inner corner (c), the middle part of web (d), the middle part of horizontal section (e), the lower corner (f) and the outer corner (g) (Fig.4e was observed from the metallurgical mount shown in Fig.2b and the others from that shown in Fig.2a. Dotted line in Fig.4f shows the flow direction, and arrows in Fig.4g show the distribution direction of MnS)
Fig.5 OM images of the decarburized layer near surface (a) and the inner area (b) of the H16 P/F connecting rod
Fig.6 SEM image of the H16 P/F connecting rod (a), distribution of copper by EDS (b), and its high magnified SEM image (c) and TEM images (d~f) (Arrows indicate copper-rich precipitates that mainly distribute in pearlite and aggregate around the cementite)
Material
Hardness / HRC
Tensile strength / MPa
Yield strength / MPa
Elongation / %
2Cu6C[2]
27
945
605
12
3Cu5C[2]
31
1000
710
13
C70[8]
26
950~1050
550
>10
H16
29
1038
703
11
Table 1 Comparison of Mechanical properties among various connecting rods
Specimen No.
N / cyc
s / MPa
ft / kN
fc / kN
fm / kN
fa /kN
1
1138600
398.1
41.4
-62.1
-10.4
51.8
2
6026900
380.8
39.6
-59.4
-9.9
49.5
3
>107
4
5598300
5
6219400
363.5
37.8
-56.7
-9.5
47.2
6
3517600
7
>107
8
>107
9
5726400
10
7871700
11
>107
346.2
36.0
-54.0
-9.0
45
12
>107
13
>107
Table 2 Fatigue properties of the H16 P/F connecting rod blanks under stress ratio R=-1.5
Fig.7 Stair-case method figure of the H16 P/F connecting rod blanks under R=-1.5
Si / MPa
i
fi
ifi
i2fi
398.1
2
1
2
4
380.8
1
2
2
2
363.5
0
4
0
0
Total
N/A
7
4
4
Table 3 Data analysis of stair-case fatigue tests of the H16 P/F connecting rod blanks
B' / mm
R
N / cyc
s / MPa
0.15
-1.4
5.47×105
385
1.48×106
-1.5
＞107
346
＞107
0.35
-1.4
5.84×104
385
2.23×104
-1.5
9.16×104
346
1.30×105
Table 4 Fatigue properties of the H16 P/F connecting rod blanks with different extents of bending
Fig.8 Macro-fractographies of the wholely shot-peening H16 P/F connecting rod (a) and partly shot-peening P/F connecting rod blanks with B'=0.15 (b) and B'=0.35 (c) (Circles show the crack initiation areas)
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