Reseach On Cold Cracking Susceptibility And Prediction For Pipeline Steel X80

With the national economy development, the demands for pipeline steels are being increased dramatically. Driven by cost considerations, gas & oil transportation companies have readily adopted higher strength pipeline steels to permit gas & oil transportation at higher pressure. As a result, recent construction of pipeline has shown a trend towards higher operation pressure with greater wall thickness and higher strength pipe. As a new generation of high strength pipelines steel, the development and application of X80 pipeline steel has been thought much.Diffusible hydrogen is main factors leading to welding cold cracking. H-MAT 221 was used to measure diffusible hydrogen in deposited metal after welding and at 50℃under different welding conditions, which were identified by HR and HR50 in the paper. Based on the experimental dates,the effects how welding parameters namely heat input, preheated temperature to influence the behavior of diffusible hydrogen was studied. An equation of hydrogen thermal diffusible coefficient-M and heat input, preheated temperature, average weld thickness, original diffusible hydrogen content was established.An orthogonal regression scheme is planned in order to study the correlation between cold cracking susceptibility of pipeline steel X80 and heat input, preheated temperature, diffusible hydrogen. First of all, the thermal cycle curves were measured and according to the gotten thermal cycle curves, the thermal cycle parameters such as tH, t8/5, t100 are determined. On the basis of experimental results, regression equitations about thermal cycle parameters tH,t8/5,t100 has been created separately. It is shown that heat input and preheated temperature have the influence on the thermal cycle parameters tH,t8/5,t100, among which the heat input is main influence of the parameters tH,t8/5,t100. In addition, compared with tH,t8/5, t100 is affected much more heavily by preheated temperature. Microstructure of CGHAZ undergone by different welding conditions was investigated. The experimental results show that welding parameters and welding thermal cycle parameter have influence on the types and proportions of microstructure in CGHAZ.Maximum hardness in HAZ were measured for the purpose of assessing cold cracking susceptibility of two kinds X80 pipeline steels (Nb-MoX80, Nb-CrX80), and with the help of the experimental results, relations between Hmax and heat input, preheated temperature were studied. At last, critical stresses under different welding conditions were determined by implant test and engineering-oriented and theoretical equations were established by multivariate regression. With the help of the regressions equations, the predication about cold cracking susceptibility of pipeline steel Nb-Mo X80 would be realized.