Effect of microstructure on strength and toughness of heat-treated low alloy structural steels
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I.
INTRODUCTION
IT has been recognized that phase transformation products such as martensite and bainite play a dominant role in determining the mechanical properties of heat-treated low alloy structural steels. Commercial, heat-treated low alloy structural steels have martensitic and bainitic structures with a lath type substructure. ~-6The lath type structure is composed of packets of parallel laths within the prior austenite grain. The packet consists of two components: (1) a matrix of laths separated by low-angle boundaries and (2) sets of other laths that are separated from the matrix by high angle boundaries. ~.2.6The matrix of laths having the low-angle boundaries will be called "a block" throughout this paper. Many investigators have shown that prior austenite grain size plays a role in controlling the strength and toughness of steels having lath martensitic and bainitic structure. For example, a Hall-Petch type relationship was observed to exist between the prior austenite grain size and the steel's strength. 7'8 More recently, however, it was shown that the yield stress and DBTT of martensitic and bainitic steels depend strongly on the packet diameter. 4'9-~5 On the other hand, Owen ~6 has suggested that the block size is the primary microstructural unit controlling the yield stress of FeNi-C alloys. Matsuda et al. ~7have shown that DBTT of low carbon tempered martensitic and bainitic steels depends on the covariant packet size where a covariant packet is synonymous to the block previously mentioned. Nevertheless, Smith and Hehemann~8 have found that the yield stress varies inversely with the lath width in AISI 4340 steels having martensitic and lower bainitic structure. Most recently, Naylor ~9 has indicated that the lath width is the basic parameter controlling the yield stress and DBTT of Fe-Mn and Fe-Mn-Cr low carbon steels having lath martensitic or bainitic structures. In contrast with the previous investigations, Brozzo et al. 5 have demonstrated that the yield stress of 2 pet Mn-3 pet Cr low-carbon bainitic steels does not change significantly with variation in prior austenite grain size or covariant packet size. YOSHIYUKI TOMITA and KUNIO OKABAYASHI are with the Department of Metallurgical Engineering, College of Engineering, University of Osaka Prefecture, 4-804 Mozu-Umemachi, Sakai, Osaka 591. Japan. Manuscript submitted June 28, 1985. METALLURG[CALTRANSACTIONS A
The above survey of literature serves to show the complex effects of microstructure on the mechanical properties. It is therefore the purpose of the present paper to study the effect of transformation structures on the strength and toughness of several low alloy structural steels and to find the dominant microstructural factor that controls the steel's properties.
II.
EXPERIMENTAL
The steels used in this investigation were low alloy structural steels with different levels of Ni, Cr, and Mo and carbon contents ranging from 0.12 to 0.42 wt pct. The chemical compositions of the steels are given in Table 1. The steels were received a
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