A comparison of toughness of
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I.
INTRODUCTION
II.
EXPERIMENTAL
A B O U T 20 years ago it was discovered tlj that lowalloy high-strength steel austenitized at a temperature several hundred degrees higher than conventionally used showed a significant increase in plane strain fracture toughness (Kic), and since this discovery, numerous investigations have been made. However, with the improvement in K~c, a reduction in the Charpy V-notch impact energy has been shown, t2-51 and this behavioral discrepancy has made it difficult to control the temperature in heat-treatment process and to evaluate the toughness of materials. Recent research t61 has concluded that some microstructural aspects related to intensive self-tempering, appropriate distribution of retained austenite, and an increase in chemical homogeneity might increase K~c inside a coarse grain structure at a high austenitization temperature. The most comprehensive explanation for this has been given by Ritchie et al. t31in AISI 4340 alloy steel. He attributes the higher Klc of coarse grain steel to its longer characteristic distance, over which the normal stress (tryy) must exceed the cleavage fracture stress (o':). This implies that the distance of the cleavage initiation site from the precrack tip in a coarse grain sample should be longer than that in a fine grain sample. However, some experiments t7,8~ on the toughness of C-Mn steel have shown that the distance of the initiation site from the precrack tip seems to be random. Therefore, it is necessary to further investigate toughness data, especially that on steels without a quenching and tempering process. In this article, we report on the toughness of C-Mn weld steel at a low temperature, with notched and precracked specimens.
C. Experimental Procedures
J.H. CHEN, Professor and President, and C. YAN, Lecturer, are with the Welding Research Institute, Gansu University of Technology, Lanzhou, Gansu 730050, People's Republic of China. Manuscript submitted April 15, 1991.
The tensile properties and work-hardening exponent, N, were measured over a temperature range of - 1 9 6 ~ to - 3 0 ~ The COD tests were carried out by threepoint bending at - 1 9 6 ~ and - 1 1 0 ~ The 4PB tests
A. Materials The material used was C-Mn base steel with a thickness of 24 mm. The weld metals were deposited in a wide V-groove using a basic electrode which has a similar composition to the base steel, as shown in Table I. For the sake of changing grain size in weld metals, two welded plates were heat-treated by austenitizing for 1 hour at a temperature of 900 ~ and 1100 ~ respectively; this was followed by air-cooling. The structures austenitized at 900 ~ and 1100 ~ will be referred to hereafter as FG and CG structures, respectively. As shown in Figure l, in both microstructures, there were only two phases, i.e., a white ferrite matrix and black pearlite colonies. The sizes of ferrite in FG (Figure l(a)) and CG (Figure l(b)) were l0 to 15 /~m and 30 to 4 0 / x m respectively. The coarse grain shows remarkably larger ferrite compared to that of FG. Both FG
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