Study on the effects and mechanisms of induction heat treatment cycles on toughness of high frequency welded pipe welds

The low toughness of the high frequency welded pipe welds seriously affects the performance of the welded pipe. Two induction heat treatment cycles of quenching + tempering (Q&T) and similar normalizing (SN) were put forward in this paper to study the toughness of the high frequency welded pipe welds. The effect of heating rate on the phase transition temperatures (Ac1, Ac3) of the raw material of the high frequency welded pipe was studied and the reasonable ranges for heat treatment parameters were determined. It was found that the austenite transformation temperatures increase with the increase of heating rate. For the heating rate of 200 ℃/s., Ac1 and Ac3 were determined as 769 ℃ and 920 ℃, respectively, and then the specific heat treatment parameters for Q&T and SN were determined. The mechanical properties of the base metal, as-welded, Q&T and SN treated welds were obtained by tensile and impact tests. The optimum heat treatment parameters corresponding to Q&T and SN respectively were determined based on the principle of a good matching of strength and toughness of the welds. Compared with those of the as-welded welds, the tensile strength of the optimum SN treated welds decreased from 843 MPa to 827 MPa, while the absorbed energy of the Charpy V-notched specimen at room temperature increased from 20 J to 42 J, which is larger than that of the base metal. The tensile strength of the optimum Q&T treated welds increased from 843 MPa to 861 MPa, and the absorbed energy of the Charpy V-notched specimen at room temperature increased from 20 J to 36 J, which is slightly lower than that of the base metal. The reasons for improving the toughness of the welds were also analyzed from the aspects of microstructure, texture, inclusion, grain and grain boundary. It is found that the toughness of the high frequency welded welds is affected by the combined effects of microstructure, the density of cleavage planes oriented for easy fracture, the grain number and size, the fraction of high-angle grain boundaries. The toughness of the welds is good when the density of cleavage planes oriented within 10° to the fracture surface is low, the total grain number, the fraction of fine grains, and the fraction of high-angle grain boundaries are high.