Study On Microstructure And Properties Of Heat Affected Zone Of X80 Pipeline Steel After Multiple Thermal Cycles

It is one of the key factors to ensure the safe and reliable service of oil and gas transportation pipelines that how to ensure the uniform microstructure and excellent properties of heat affected zone?HAZ?of pipeline steel after welding thermal cycles of"high temperature,non-uniform and rapid heating--short-term heating preservation--rapid cooling".In this paper,the fine X80 pipeline steel samples of miniature hollow cylinders was taken as the research objects.The research methods and detection methods were used by welding thermal simulation techniques DIL805A/D that is a kind of high temperature phase transformation instrument with high precision,optical microstructure analysis technology?OM?,electron microscopy analysis technology?SEM,EBSD and TEM?,microhardness testing?HV-1000?,etc.We finally got the laws of microstructural evolution and mechanical properties in heat affected zone of X80pipeline steel after multiple welding thermal cycles with different peak temperatures,cooling rates and thermal cycles,and established continuous cooling transformation diagram of simulated welding heat affected zone?i.e.,SHCCT curves?with high accuracy.There is the Arrhenius relationship between austenite grain size and peak temperature of Reheated coarse-grained HAZ?RCGHAZ?of X80 pipeline steel under the condition of the same cooling rate?35°C/s?and different peak temperatures.The microstructure of Unaltered Coarse-Grained HAZ?UCGHAZ?is mainly BF?The microstructure of UCGHAZ is mainly BF+GB when welding peak temperature is 1100°C?.The microstructure of Reheated UCGHAZ?RUCGHAZ?is mainly GB+QF when the welding peak temperature is 950¹050°C in the second welding thermal cycle.While welding peak temperature is down to 850°C and 900°C,the microstructure of RUCGHAZ is mainly GB+BF,and a"necklace"structure of martensite-austenite?M-A?constituents exists at the original austenite grain boundary.With the welding peak temperature increasing,the value of microhardness of UCGHAZ increases,while the value of microhardness of RUCGHAZ reduces first and then increases.At the same peak temperature?1 thermal cycle T_m=1350°C,2 thermal cycles T_m=1200°C,3 thermal cycles T_m=1000°C?,the changing trend of microstructure of X80 pipeline steel is as follows under the condition of different cooling rates from 0.5°C/s to 200°C/s,PF+QF+P'?QF+GB?GB?GB+BF?BF?BF+LM?UCGHAZ and RUCGHAZ??LM?only CGHAZ?.There is an excellent correlation between the value of microhardness and cooling rate,which largely obeys a power function equation,i.e.,HV₁=a+bexp?-kv??a,b,k are constants?.Mathematical equations of Bainite transformation temperature-cooling rate are established as T=a-bln?v+c??a,b,c are constants?.Moreover,it can also be observed that the kinetic curve shifts to lower temperature range with increasing the cooling rate.With an increase of bainite transformation?f_b?volume fraction,the activation energy of bainite transformation decreases,and the dominating mechanism of bainite transformation in CGHAZ and UCGHAZ are two-dimensional growth and one-dimensional growth.The new grain of RUCGHAZ is mainly nucleate nucleated at crystal edges,and grow following two-dimensional mode.In addition,with the increase of welding thermal cycles,the critical rate of transformation increases from proeutectoid ferrite to bainite,and the main orientation plane of grain changes from{111}to{001}.The microstructure transformation zone in the SHCCT curve of X80pipeline steel also changes with the change of welding thermal cycles.Not only has an intense variant selection occurred for microstructure transformation in RUCGHAZ,but also the effective grain size is small,which is beneficial to restrict cracks propagation.Even if the volume fraction of M-A constituents increases with the increasing of thermal cycles and the formation of grain boundary M-A,intergranular M-A as well as interconnected M-A in the various morphology,such as slender,bulky or mixture.However,the volume fraction of M-A constituents is lower in each welding thermal cycles?the value does not exceed 0.3%?,and thus to suppress their detrimental effect on toughness.Overall,the toughness will be improved with the increasing of welding thermal cycles.