Study on heat affected Zone in welding of low alloy high strength steel
The manufacture of low-alloy high-strength steel ASTMA572GR.65 steel (hereinafter referred to as GR.65 steel) for UHV tower can not only increase the overall strength of the tower, reduce the use of steel, but also reduce the total cost of the project, which has been more and more in use. However, the steel has a larger tendency to harden, a higher hardness, and a larger tendency to crack (especially cold crack) after welding, which affects the quality of the tower.
For the research on welding heat affected zone, many literatures generate welding heat affected zone samples by welding heat affected zone simulator based on actual welding process parameters. Although this method can enlarge welding heat affected zone for convenience of research, some literatures also point out that there are errors between sample parameters obtained by welding heat affected zone simulator and those obtained in actual welding. Based on the above considerations, welding experiments were conducted to study the welding heat affected zone of GR.65 steel. The effects of welding heat input (E) on the microstructure and mechanical properties of the welding heat affected zone were studied under different preheating temperatures (TP), providing theoretical and experimental references for crack control of GR.65 steel in UHV welding tower welding.
The experimental material was hot-rolled GR.65 steel with a size of 300mm×200mm×10mm, and its chemical composition (mass fraction, %) was as follows: 0.203C, 0.043Si, 1.26Mn, 0.006P, 0.001S, 0.046Nb, 0.019Ni, 0.012Cr, 0.29Al, 0.018Cu. 45° L-shaped bevel with a blunt edge of 2mm. The welding material is Φ1.2mm ER55-G welding wire. The welding method is CO2 gas shielded welding, the welding equipment model is YM-500KR, and the single pass butt welding is adopted. After welding, it is air-cooled at room temperature and then tested for impact and hardness.
When the welding line energy of GR.65 steel is 10kJ/cm, the width of the welding heat affected zone is about 4mm, and it will reach about 9mm when it is 40kJ/cm. The grain size of the coarser zone is very different under different wire energy input. The average grain size of the coarser zone when the wire energy is 40kJ/cm is almost twice that of 10kJ/cm. This indicates that GR.65 steel has limited inhibitory effect on austenite grain at high linear energy, while at low linear energy, due to nailing effect of Nb(C,N), austenite grain growth will be hindered. At the low welding energy (10kJ/cm), preheating before welding can refine the hardened microstructure such as martensite and M-A block compared with no preheating, which has a favorable effect on the toughness of coarse-grained zone. However, under high line energy (40kJ/cm), preheating before welding will not only make the coarse-grained zone more coarse-grained structure, but also more easily produce the microstructure with relatively poor mechanical properties such as upper bainite, which seriously reduces the toughness of the coarse-grained zone, which needs attention in welding.
