Welding heat effect of low alloy high strength steel
The application of low alloy high strength steel ASTMA572GR.65 (hereinafter referred to as GR.65 steel) in the manufacture of 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 used more and more. However, the hardening tendency and hardness of the steel are higher, and the cracks (especially cold cracks) after welding tend to be larger, which affects the quality of the tower.
For the study of weld heat affected zone, many literatures based on the actual welding process parameters, through welding thermal simulator to generate weld heat affected zone samples, although this method can be enlarged to facilitate research, but there are also literatures pointed out that the sample parameters obtained by welding thermal simulator and actual welding. There are errors in the sample parameters. Based on the above considerations, the welding heat affected zone (HAZ) of GR.65 steel was studied by welding experiments. The effects of welding heat input (E) on the microstructure and mechanical properties of HAZ were studied under different preheating temperatures (TP). The theoretical and experimental results were provided for crack control of GR.65 steel in UHV welding tower. Reference resources.
The experimental material is hot-rolled GR.65 steel with a size of 300 mm *200 mm *10 mm. Its chemical composition (mass fraction,%) is 0.203C, 0.043Si, 1.26Mn, 0.006P, 0.001S, 0.046Nb, 0.019Ni, 0.012Cr, 0.29Al, 0.018Cu. A 45 degree L groove is adopted, and the edge of the groove is blunt edge 2mm. The welding material is ER55-G type 1.2mm wire. The welding method is CO2 gas shielded arc welding, the type of welding equipment is YM-500KR, and single way butt welding is adopted. After welding, the air is cooled at room temperature, and then impact and hardness tests are carried out.
When the welding energy of GR.65 steel is 10 kJ/cm, the width of HAZ is about 4 mm, while the width of HAZ is about 9 mm at 40 kJ/cm. The grain size of coarse grain zone varies greatly with different input energy of welding line. The average grain size of coarse grain zone is almost twice as large as that of 10 kJ/cm when the input energy of welding line is 40 kJ/cm. This shows that the restraining effect of GR. 65 steel on austenite grain is limited at high wire energy, but restraining effect of Nb (C, N) pinning on austenite grain growth at small wire energy. Under the small welding line energy (10kJ/cm), the pre-heating before welding can refine the lath martensite, M-A block and other hardened microstructure, which is beneficial to the toughness of coarse grain zone, while under the large welding line energy (40kJ/cm), the pre-heating before welding can not only make the coarse grain zone structure coarser, but also more easily produce upper bainite and other mechanical properties than before welding. Poor microstructure seriously reduces the toughness of coarse-grained zone, which needs attention in welding.