ABSTRACT:

Toughness and microstructure of HSLA welds reheated by successive welds was studied. Toughness changes can be classified into four categories by attained peak temperature. The causes of embrittlement are increasing of GBF, MAC, and effective grain size and the main governing factors depends on the peak temperature. The low C, Ni-TiB type weld metal shows higher toughness in all reheated regions.

1. INTRODUCTION

As energy development advances into deeper seas and Arctic areas, the toughness requirements for weldments has increased in severity from -40·C to -60 or -80·C.l) Especially through-notch CTOD properties, which are dominated by local embrittle zone2), are required for offshore structures. In order to get high CTOD properties, all regions of the weld metal in the weldments should be toughened. Further, fabricators demand large heat-input welding for efficiency. It is well known that the most preferable microstructure for toughness is the acicular ferrite (AF) in the as-welded region and there are many reports on the condition of AF formation3,4, S). As the TiB bearing system gives the AF under wider welding conditions and the weld metals provide excellent CTOD properties, many kinds of welding materials have been developed for offshore structures, etc. based on the Ti-B bearing system6, 7,8). But toughness of weld metal including the region reheated by successive welds with large heat-input is not always high compared with the aswelded region as shown in Fig.I. Then as shown in Fig.2 a brittle fracture initiated from the reheated region in the CTOD specimens. Therefore the toughness of reheated region should be increased to improve the CTOD properties in the joint weld metal. However the attained peak temperature is changed continuously and the most embrittle parts are not always specified in the reheated region. There are few reports on the toughness of reheated weld metal.

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