Study On Stabilized Heat Treatment Of TP347 Pipe And The Method To Control Welding Residual Stress By The Temperature Difference

TP347 austenitic stainless steel has good corrosion resistance,wear resistance and thermal stability,so it is widely used in the high temperature corrosion environment of the petrochemical industry.Stabilization heat treatment process is a heat treatment process to improve the intergranular corrosion resistance of the austenitic stainless steel containing stabilized elements and improve its precipitates composition.However,with the enlargement of refining equipment,the nominal diameter and wall thickness of TP347 pipeline continue to increase,and the reheat cracks appear on the pipeline after field stabilization and heat treatment.How to avoid reheat cracks and meet the purpose of heat treatment has become a major problem that troubles the intrinsic safety of large-scale pipeline.According to the above problems,this paper takes TP347 austenitic stainless steel pipe as the research object,The homogeneity test was carried out using the traditional heat treatment process,and the inner wall of the pipeline was taken as the research object.The composition of the precipitated phase and the resistance to intergranular corrosion were analyzed experimentally,then the traditional stabilization heat treatment process was evaluated.Through ABAQUS numerical analysis,the cracking reason and cracking time of thick wall pipe during heat treatment were explored,on this basis,further studies were carried out from the internal and external wall temperature difference,weld form,heating strip width,cooling mode and other aspects,and a local rapid cooling process was proposed to improve the stress distribution of the inner wall in service state.Optimize the stabilization heat treatment process of thick wall pipe effectively to avoid the reheat crack and stress corrosion.The main research contents and results of this paper are as follows:（1）According to the temperature homogeneity test of stabilization and heat treatment,for TP347 pipe with wall thickness of 50mm,when the outer wall temperature is 900℃,the inner surface temperature is only 750℃,and the inner wall cannot meet the process temperature requirements.SEM,XRD and other methods were used to analyze the microstructure composition and precipitate phase before and after heat treatment.It was found that a large amount of chromides precipitated near grain boundaries,which reduced the susceptibility to intergranular corrosion.At the same time on two different kinds of heat treatment temperature of heat treatment time of intergranular corrosion experiments were carried out,at any time after 750℃temperature processing weightlessness rate is significantly higher than 900℃,the weightlessness rate of 2.08g/m²h largest,and near the weld fusion line to produce a large number of micro cracks,crack initiation in grain boundary place first,then gradually expand to the intracrystalline,and accompanied by pitting phenomenon.It is proved that the traditional stabilization heat treatment is harmful to the intergranular corrosion sensitivity because it cannot meet the temperature uniformity of inner and outer walls.（2）Through the analysis of the crack generation method and the characteristics of the thick-walled pipeline,it is determined that the large residual stress concentration generated during welding and heat treatment is the main reason for reheating cracks.On the one hand,the large residual stress generated by the welding process On the other hand,the temperature uniformity of the heat treatment process cannot meet the requirement of generating a large temperature difference stress,and the superposition of the two increases the stress concentration;and the relationship between the axial stress and time at a point of the outer wall weld toe during the stabilization heat treatment process is analyzed to determine The stress value of the heat preservation stage is higher than the yield strength of the material,which is the most prone to cracking time of the thick-walled pipeline stabilization heat treatment.（3）Using induction heating instead of ceramic plate heating in local heat treatment of TP347 austenitic stainless steel pipe,the problem of large temperature difference between inner and outer wall is effectively solved,so that the temperature difference between inner and outer wall is controlled within 25℃during the insulation stage,and the problems of sensitization and high thermal stress of inner wall are overcome.The key process parameters affecting the heat treatment were parameterized according to the low stress criterion to determine the stabilization heat treatment process suitable for thick wall pipes:electromagnetic induction was used to replace the traditional electric heating method,the heating width was selected as 500mm,and the cooling method was adopted as slow cooling.（4）By analyzing the stress of TP347 pipeline in service state,it is found that there is a large stress concentration near the inner wall weld,and the stress corrosion problem cannot be solved.The local rapid-cooling temperature difference method was proposed to optimize the cooling process,and the stress relief effect of the inner and outer surfaces on the welded joints under different cooling rates was calculated.The greater the temperature difference between the inner and outer walls during the cooling process,the better the stress relief effect on the inner wall would be.The method was applied to the cooling link of TP347 pipeline,and the axial stress on its inner surface decreased from 171MPa to about-150MPa,with a maximum decrease of 187.7%.The minimum temperature difference?T_min,which can produce compressive stress at the welding seam of the inner wall of the cylinder by using local rapid cooling process,is investigated by means of fixed parameter variable method,and it is found that?T_min is positively correlated with thickness and diameter.By means of statistical analysis,a formula for calculating the minimum temperature difference which can produce compressive stress on the inner wall is put forward.