Evaluation Of Welding Process Of Boiler Header And Numerical Simulation Of Welded Joint

In this thesis considering the actual production situation,the welding process of the materials(15CrMo?20)used by the boiler header was studied.The temperature field and stress field of the welded joint were analyzed,and the strength of the connection between the cylinder and the header was evaluated.In accordance with the " The standard of NB/T 47016-2011 pressure equipment welding test pieces for the mechanical properties ",we tested the mechanical properties of welded joints.The results showed that the mechanical properties of the welded joints(15CrMo/15CrMo)formed by the two welding processes meet the requirements.The microstructure and hardness of the two kinds of welded joints were tested,there was no significant difference.The mechanical properties of the welded joints(15CrMo/20)also meet the requirements.And we found that preheating the base metals before welding was a good way to avoid some welding defects.The grain size of the weld was small.The test of hardness showed that the hardness of 15 CrMo was slightly harder than 20 steel,and the hardness peak appeared in the heat affected zone.It can be seen that the temperature of the specified nodes under the welding process 2 was greater than the temperature of the welding process 1,which was consistent with the heat input of two welding processes.The transverse residual tensile stress peaks of the two welding processes were in the vicinity of the heat affected zone,which was consistent with the test of tensile strength.The 15CrMo/20 welded joints were simulated and suggested that the temperature field distribution on both sides was not symmetrical.The isotherm's area on 20 steel was bigger than on 15 CrMo,and this was consistent with the thermal conductivity of the materials.In the stress field,the transverse residual tensile stress distributed near the heat affected zone.Either the safety factor was 1.5 or 2,the evaluation of the strength of the header was found to be less than the limiting value,it indicated that the connection strength of the cylinder and the header is safe.We setted the safety factor was 2,and it was found that when the pressure was increased to 17 MPa,the stress value was close to its limiting value,but still within the permissible range.When the pressure was increased to 18 MPa,the stress exceeded its limiting value.Therefore,the limiting pressure was 17 MPa.