| 2.25Cr-1Mo is a Cr-Mo alloyed bainitic steel. It has been used in the area of power station, nuclear energy, oil and chemical. It is the main material for pressure vessel of hydrogenation, 2.25Cr-1Mo-0.25V,3Cr-1Mo-0.25V etc. are this kinds of Cr-Mo material. It has high harden ability and cool cracking sensitivity in the welding. For this reason, if we can't control the welding process in some ways at some time in manufacture, the quality of welding joint is affected in some grade. For example, the HAZ of welding joint is hardened, Microstructure of HAZ is overage crassness, the impact of HAZ is decreased, etc.The HAZ of welding joint is extraordinary unsubstantial area in the welding, which is affected by a great deal of factor, so it is difficult to control. It is indicated that some accident of production take place due to the defects of HAZ. The petrochemical is newly industry product since October 2005 in China Erzhong Group Corporation. so we stride forward the area of producing heavy pressure vessel. From that time, large numbers of pressure vessel is produced in our workshop. In order to control the quality of welding joint and ensure the security of the heavy pressure vessel in future, it is necessary to research deeply the property and structure of the HAZ about welding joint. It is very important significance to work out the welding process, and to control the micro-structure and to control the property of 2.25Cr-1Mo steel. It is extraordinary significance that this kind of material is used in project application and theory of welding is abundance in this paper.Firstly, Microstructure and the property of steel of 2.25Cr-1Mo that is used daily in our workshop are introduced in the paper. And we have analyzed the chemistry, Microstructure of fracture photos, mechanical properties and impact fracture about this steel, which is of benefit to master the compositional properties of HAZ. It is introduced the general property of HAZ, such as hardness, brittleness, toughness and intenerate. Because the HAZ of 2.25Cr-1Mo steel is affected by the welding thermal cycle, its grain-boundary structure and the property come out serious asymmetric.Secondly, in order to know and master the rule of heating about HAZ, we discuss the parameter of welding thermal cycle including Maximum Temperature Tm, Time over Metal structure tH, Instantaneous Cooling Speed Wc, Cooling time etc. in the paper. We have drawn the curve of Welding Thermal Cycle about HAZ, according to the parse formula of experience. It is been known deeply that the process of heating and welding thermal cycle about the HAZ.Thirdly, the area of welding joint's HAZ is very straitens in actually submerged arc wilding's process. It is about from 2mm to 4mm and it is too straitens to study its property. In the paper, we do it through American GLEEBLE-1500D. It is a machine of welding thermal simulation and can enlarge the small area according to the welding thermal cycle. So we know about the microstructure and the impact appearance and mechanical property and the fracture photos by the testing.The results indicate HAZ's microstructure of 2.25Cr-1Mo steel is mainly Martensite and Bainite. If the cooling speed is charge, the scale of two kinds of microstructure is inflected. When the t8/5 is from 20s to 30s, the microstructure and impact toughness is better that other range. In this scope, the mainly microstructure are up- Bainite and down-Bainite. The heat input of SAW is from 26KJ/cm to 29KJ/cm.Finally, the property of weld testing for steel of 2.25Cr-1Mo is introduced, the results indicate, we must preheat before welding the kinds of steel and do PWHT after welding. Cracking will come out in the welding if we don't preheat. In order to improve the joint, we have to preheat 100 degree for steel of 2.25Cr-1Mo. The welding is accepted if we do PWHT and the property of welding joint will improve. It is necessary that we do preheat and PWHT for this kind of weling joint.
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