Cleavage initiation in the intercritically reheated coarse-grained heat-affected zone: Part I. Fractographic evidence
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I.
INTRODUCTION
THE balance of high strength and good toughness in steel plate can be upset by the thermal cycles experienced during welding, producing areas of poor toughness known as local brittle zones (LBZs). One of the most potent of these LBZs in modern low-carbon structural steels has been found to be the intercritically reheated coarse-grained heat-affected zone (IC CG HAZ). The main cause of the loss in toughness is the presence of MA constituent (high-carbon martensite with some retained austenite). A number of mechanisms by which the MA constituent could affect toughness have been proposed in the literature. The purpose of this article is to investigate in detail the role of the MA constituent and the part it plays in the embrittlement mechanism. This has been done through the examination of a number of steels selected to provide different distributions and morphologies of MA particles in the IC CG HAZ structure. II.
LOCATION AND MICROSTRUCTURE OF THE IC CG HAZ
The HAZ from a single-pass weld consists of four distinct zones formed as a result of the thermal cycle experienced during welding (Figure l(a)). The region of lowest toughness is generally associated with the CG HAZ. tl-4] During multipass welding, these zones are modified by the subsequent thermal cycles forming localized and discontinuous zones (Figure l(b)). For multipass welds in low-carbon structural steels, it is often found that the IC CG HAZ displays the lowest toughness, tS-~] and it is therefore important to understand the failure micromechanisms controlling this loss in toughness. Obviously, the microstructure of the CG HAZ is C.L. DAVIS, Ph.D. Student, and J.E. KING, University Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB23QZ, United Kingdom. Manuscript submitted July 19, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
strongly dependent on the base plate chemistry and the thermal cycle experienced during welding. The most important thermal cycle parameter for a fixed peak temperature (taken as 1350 ~ for the CG HAZ) is the cooling rate. The cooling rate is usually specified as the time taken to cool from 800 ~ to 500 ~ (Ats/5) and can be related to welding heat input (E, KJ/mm). Where the peak temperature is less than 800 ~ an equivalent cool time is used to generate the same portion of the cooling curve that would have been produced for a cool from above 800 ~ using Ats/5. For welding of thick plate, At8/5 is approximately proportional to E:
At8~5 = kE where k is in the range 4 to 5 seconds per KJ/mm for conventional structural steels, t~z] In the CG HAZ, the high peak temperature results in extensive grain growth, so that on cooling, ferrite nucleation is suppressed and lower temperature transformation products, such as bainite and martensite, may form. Most standard welding procedures on high-strength low-alloy (HSLA) steels result in a CG HAZ comprised of upper bainitet7'~3-~61 or martensite/lower bainite, t]4,16,'7] However, with the introduction of more adva
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