Effect of silicon on CGHAZ toughness and microstructure of microalloyed steels
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I.
INTRODUCTION
THE increasingly severe environment of offshore petroleum sites has spurred metallurgists to improve structural steels. In order to be used, the steels must satisfy stringent requirements for the base metal and, particularly, in the weldments. Cold cracking and low toughness levels are two major problems induced by welding operations. IL21 These two phenomena may occur in the coarse-grained heat-affected zone (CGHAZ) following short or long cooling times, respectively. At intermediate cooling times, i.e., intermediate welding energy and/or plate thickness, i~1 welding may be conducted safely. The extension of this safe welding domain governs the development of new steels. During welding, structural changes and phase transformations occur in the heat-affected zone (HAZ), which determine its mechanical properties 1~.2iand, more particularly, the loss of toughness in the CGHAZ formed at the highest welding energies. Figure 1 illustrates the parameters that affect the CGHAZ toughness. This synoptical diagram can be divided in three main parts. The connection (I) is well defined by means of Rosenthal equations established in 1946J 3i The study of the relationship (III) shows that among the parameters likely to act on the CGHAZ toughness, retained austenite (yr) exerts a predominant and harmful effect. 14,-s,61However, the retained austenite is at least partially transformed into martensite during testing. The mechanical destabilization of retained austenite depends on its carbon content, on its morphology, and on the test temperature. t6'71 This martensite-austenite constituent (MAC) gives rise to concentration and triaxiality of stresses at the interface in the ferritic matrix. This stress-raising R. TAILLARD and J. FOCT, Professors, are with the Physical Metallurgy Laboratory, U . R . A . C . N . R . S . No. 234, University of Lille, 59655 Villeneuve d'Ascq, France. P. VERRIER and T. MAURICKX, formerly Graduate Students, Physical Metallurgy Laboratory, University of Lille, are Research Engineers, Centre de Recherches et de D6veloppement M6tallurgiques, Sollac, 59381 Dunkerque, France. Manuscript submitted October 25, 1993.
METALLURGICAL AND MATERIALS TRANSACTIONS A
effect leads to the brittle fracture by cleavage of the matrix of ferrite. 16,7,81 The aim of the present study was to investigate the relationship between chemical composition and microstructure (II) and, especially, the effect of silicon on the CGHAZ microstructure and toughness of microalloyed steels. Finally, we propose a steel composition that exhibits very high toughness at low temperatures.
II. M A T E R I A L S A N D EXPERIMENTAL PROCEDURE A. Base Metal
Steels A through D were laboratory castings, while alloy E was industrially manufactured. The 12-mm-thick plates of steels A through D, on the one hand, and the 50-mm-thick plate of steel E, on the other hand, were obtained by distinct controlled rolling and accelerated cooling processes. Alloys A through D were produced as a 60-kg heat that was cast in four ingots. The silicon
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