Microstructure and local brittle zone phenomena in high-strength low-alloy steel welds
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I.
INTRODUCTION
RECENTL Y, very low toughness values have been noticed in the heat-affected zones (HAZ's) of high-strength low-alloy (HSLA) steels welded with mUltipass submerged arc welding (SAW) proceduresy-6 J Metallographic analyses of HSLA steel welds reveal significantly different regions in HAZ microstructures. For example, in single-pass welds, there are four characteristic regions in the HAZ determined by the peak temperature, to which the region was exposed during the weld thermal cycle:[5,7] a coarse-grained region, a fine-grained region, an intercritical region, and a subcritical region. In multipass welds, these regions undergo multiple thermal cycles, resulting in inhomogeneous and complicated microstructures. An idealized sketch of a heat-affected multipass weld is shown in Figure 1. The coarse-grained region can be roughly categorized into four zones according to the reheating temperature as follows: (a) a subcritically reheated coarse-grained zone (SRCG HAZ), the zone reheated below ACb (b) an intercritically reheated coarsegrained zone (ICRCG HAZ), the zone reheated bet~een Ac, and AC3' (c) a supercritically reheated coarse-gramed zone (SCRCG HAZ), the zone reheated above AC3 and below about 1200 °C, and (d) an unaltered coarse-grained zone (UACG HAZ), the zone that is not reheated or reheated above 1200 0c. Although many studies have been conducted on HSLA steel welds, few reports are available on the factors that govern the toughness of the welded joints. It is now generally accepted that the coarse-grained region has the lowest toughness. This local brittle zone (LBZ) becomes a serious problem since the low-temperature toughness
B.C. KIM, Research Assistant. S. LEE, Assistant Professor, N.J. KIM. Associate Professor. and D.Y. LEE. Associate Professor, are with the Department of Materials Science and Engineering, Pohang Institute of Science and Technology. Pohang 790, Korea. Manuscript submitted August 30, 1989. METALLURGICAL TRANSACTIONS A
is severely reduced due to the presence of the unfavorable microstructural features in the LBZ, such as large prior austenite grain size, upper bainite, martensitic islands, and microalloy precipitatesY-8 J Among the above microstructural features in the coarse-grained HAZ, the martensitic island is the main factor causing deterioration of toughness because of its high hardness and crack susceptibility, [5,6,9J although the relative ranking of the microstructural features depends on the chemical compositions and the local thermal cycles including post-weld heat treatments. The present work describes the specific roles of the microstructural factors, such as martensitic islands, in the process of microvoid and microcrack initiation in a 500 MPa class HSLA steel. Specifically, by examining the initiation of microvoids and microcracks in sectioned tensile specimens, the micromechanism of fracture processes is identified and related to the observed Charpy test results.
II.
EXPERIMENTAL
A. Material and Welding
The material used in this study is a 44-mm-thick
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