Effects Of Peak Temperatures And Alloy Elements On Microstructure And Strength Of Welding Heat Affected Zone For The Pipeline

The effects of welding peak temperature and alloying elements on themicrostructures evolution was studied in this dissertation by analyzing the microstructuresand strength in the HAZ of six kinds of domestic X80and X100pipeline steelrespectively.The microstructure of all the four regions in the HAZ was observed by usingGleeble-3500simulator.45KJ/cm welding heat was input, and the microstructures of theHAZ, which were obtained at the peak temperature of1350℃,1200℃,1000℃,900℃,800℃and730℃respectively, were observed respectively. The results shows that, withthe increasing of peak temperature, the microstructures of test steel experienced achanging process which turned the base material firstly to fine polygonal ferrite andfinally to acicular ferrite, binate ferrite microstructures, and the M/A components werealso turned from a small number of massive forms distribution to the a strip, massiveuneven distribution and, finally, to a lath distribution in the ferrite matrix. The strength ofthe test steel has an apparent softening in the fine grain area and the intensity reached thehighest value at730℃and1350℃.Measured by thermal expansion, tested steeltransformation starting point were tested in the cooling process, the results show that asthe peak temperature increased, the phase transition temperature of pipeline began todecrease.The observation of the tested steel showed that, experiencing the same peaktemperature, the more solution of the alloy elements such as Mn, Mo, Cr, Si, Cu and Ni,the finer and more uniform the HAZ microstructures could be and the more higherdislocation density microstructures could be obtained. In the meantime, more solution ofalloy elements could also make the transformation start at lower temperature, whichleaded to getting small grains and high dislocation density microstructures easily.Therefore, these microstructures transformation made the steel which contained highlevels of solute elements have higher strength. Besides the solid solution elements, themicro-alloying elements (Ti, Nb, V) in the pipeline steel also restrained austenite growing, reduced the phase transition temperature, produced a precipitation, and promoted theformation of high dislocation density microstructures. Grain refinement is the mainstrengthening mechanisms of the four kinds of strengthening mechanisms of the HAZ ofpipeline steel, especially in the fine-grained area. The differences of the sum of thedislocation strengthening and the precipitation strengthening between each CGHAZ wererelatively obvious.