Abstract: The morphologies of the weld crack of the X90 SAWH pipe as made with the two-step(tack and finish welding)process are studied.Furthermore, the causes for the said crack as occurring during the said two-step welding process are also investigated.The results show that the transversal crack as developed in the inner weld bead of the pipe is the delayed cold crack which expands in the weld bead structure in the form of transgranular and intergranular coexistence.The said cold crack is caused due to the reasons as follows.The X90 steel contains a number of alloy elements such as Mn, Cr, Mo, Ni and Cu, and thus possesses rather high strength, which leads to the welding residual stress of the inner weld bead as higher than the critical stress of the crack.And the hardness of both sides of the inner weld bead is so asymmetrically distributed that the stress concentration and distribution around the inner weld joint are unbalanced.Furthermore, it is not so easy for the diffusible hydrogen as exists in the inner/outer weld bead overlap area to escape, and due to so high a content, the hydrogen gathers in the spots of the weld defects like gas hole and entrapped slag, and in turn the crack is developed, and meanwhile lots of hydrogen flakes formed.And finally, the primary dentrite boundary in the weld bead acts as the "channel" for expansion of the cold crack.