The Influencing Factors And Control Of Toughness Of Heat-resistant Steel Weld Metal

2.25Cr-1Mo-0.25 V steel is a heat-resistant steel modified from Cr-Mo steel,which has better high temperature strength,creep strength and corrosion resistance.However,the microstrucute and the mechanical properties of the 2.25Cr-1Mo-0.25 V weld metals often affected by different welding processes when the corresponding filler metal is designed with the same chemical composition.It will cause that the low-temperature impact toughness of the 2.25Cr-1Mo-0.25 V weld metal can not satisfy the minimum standards requirement of engineering,further cause the problem of lower impact energy and cleavage fracture.In this paper,the relationship between the microstructure and impact toughness in low temperature is systematically analyzed in order to solve the problem of low value and unstable low-temperature impact toughness in2.25Cr-1Mo-0.25 V weld metal by submerged arc welding(SAW).The total absorbed energy was measured with Charpy impact experiment at-30? for the as-welded and post-weld heat treatment(PWHT)specimens.The factors affecting the stability of impact toughness of the weld metal were confirmed and improvement ways were proposed by characterization of the microstructures and analysis of the impact toughness.Based on experimental results and related literature,two suggestions of toughness improvement were revealed.The first method is the adjustment of the chemical elemental composition.The second method is the redesigning of the welding and post-weld heat treatment process,such as cooling rate,heat input,etc.The main results are shown below:(1)The absorbed energy of as-welded specimen of 2.25Cr-1Mo-0.25 V weld metal at-30? is in the range of 6-7J,and improved to 30-36 J by introducing a PWHT with peak temperature of 705? and holding time of8 hours at peak temperature of 705?.By comparing the impact fracture surface and microstructure of the as-welded and PWHT specimens,the main factors affecting the toughness of 2.25Cr-1Mo-0.25 V weld metal are revealed: the type of microstructure,coarse ferrite,grain size and prior austenite size.On one hand,the type of microstructure is changed from granular bainite to ferrite with temper heat treatment,which improved the toughness.On the other hand,it is found that the coarse prior austenite in columnar region of weld metal lead to the formation of coarse ferrite,which results in the early cleavage fracture and low impact toughness.(2)The impact toughness of the 2.25Cr-1Mo-0.25 V weld metal can be improved by changing cooling rate and heat input,but the impact toughness is not stable.Finally,by adding a pre-tempering treatment at650? for 40 hours before tempering,impact absorb energy was improved from 30-36 J to the 126J-167 J.The toughness of 2.25Cr-1Mo-0.25 V weld metal was stable and effectively improved.(3)The absorbed energy of low-temperature impact toughness can be improved from 30-36 J to 95-157 J by increasing the content of manganese from 1.04wt% to 1.27wt% in 2.25Cr-1Mo-0.25 V weld metal.The toughness can satisfy the industrial standard of impact toughness because acicular ferrite effectively preventing crack propagation and improving toughness.