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Abstract
The practical strength of line pipe steel has extended to X80 or greater. The line pipes of X80 grade have been commercialised successfully and now the development and evaluation of X100 and X120 grade line pipes are being conducted.
The effect of boron addition on the toughness and microstructure of seam weld metal was investigated in tensile strength range from 700 to 1100 MPa. Two types of weld metal were employed in this study. Type A weld metal was produced by boron added flux. The boron content in type A weld metal was around 30 ppm. Type B weld metal was produced by boron free flux. The boron content in type B weld metal was 15 ppm or less. In a tensile strength range less than 800 MPa, type B weld metal had lower absorbed energy than type A because of grain boundary ferrite formation. In a tensile strength range of 800 MPa or more, type B weld metal had greater absorbed energy than type A. In this tensile strength range, some of the acicular ferrite is replaced with bainite or bainite/martensite and the absorbed energy decreases with increase of tensile strength. And the type B weld metal had more amounts of acicular ferrite and greater absorbed energy than type A in this tensile strength range.