Secondary hardening and fracture behavior in alloy steels containing Mo, W, and Cr
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I.
INTRODUCTION
SECONDARY hardening steels, in which the ultrahigh strength can be obtained by a fine dispersion of M2C-type carbides, have attracted a significant attention for high-performance applications. The alloying elements Mo and W form the carbides of M2C type, accompanying the dissolution of cementite of M3C type during tempering (aging) in the range of 500 7C to 650 7C.[1,2,3] In contrast, since alloying element Cr alone does not form M2C carbides but M7C3 or M23C6 carbides, hardening with precipitates of the latter carbides could only occur by Cr additions greater than about 9 wt pct.[4,5] However, even the lower Cr contents combined with Mo and/or W additions can contribute to the formation of M2C carbides in which Cr is also dissolved. Since Mo forms M2C carbides at lower temperatures, where a relatively high dislocation density is sustained, an effectively fine dispersion of M2C carbides can be accomplished, as compared to W, which forms M2C carbides at higher temperatures, where the dislocation recovery is well advanced because of its slower diffusion.[1,2,3] Hence, a weak hardening occurs in the W steel compared with a strong hardening in the Mo steel. After tempering in the secondary hardening range, however, embrittlement can occur. This embrittlement is referred to as secondary hardening embrittlement (SHE). Secondary hardening embrittlement can be classified into two types; intergranular and transgranular SHE, according to fracture mode. It has been suggested that the intergranular SHE is caused by impurity segregation resulting in easy intergranular fracture[6] and that transgranular SHE is associated with coarse boundary carbides leading to easy transgranular cleavage fracture.[7] In recent years, Kwon and co-workers[8–15] reported that the intergranular SHE is an
overaged condition occurred, as compared to the transgranular SHE in the underaged condition, for the Mo, Mo-W, and W-Ni steels. Hence, the fracture behavior in SHE can be greatly affected by aging condition. On the other hand, the Cr addition seems to be necessary in the high Co-Ni secondary hardening ultrahigh strength alloy steels with superior toughness, such as HY180,[16,17] AF1410,[18,19] and AerMet 100,[20,21] even though Cr could weaken the extent of secondary hardening. However, there are a few studies on the effect of Cr addition on hardening[22,23] and fracture behavior.[24] The purpose of this study was to analyze the roles of alloying elements Mo, W, and Cr, which directly are dissolved in M2C carbides, in the hardening and fracture behavior of the 4Mo(2Mo2Mo), 6W(3W3W), 2Mo3W, 2Mo2Cr, and 3W2Cr steels.
II.
EXPERIMENTAL PROCEDURE
Ingots weighing about 15 kg were prepared by vacuum induction melting and were then hot forged to plates of 14mm thickness. Standard Charpy V-notch impact specimens were machined from those plates. Chemical compositions of experimental alloys are presented in Table I. Impact specimens were austenitized at 1200 7C, a high temperature where almost all alloying elements can be dissolved in t
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