Study On Welding And Performance Of 5Mn Steel On Offshore Platform

In this thesis,the weldability of 5Mn steel for 690MPa-class offshore platform,the evolution of microstructure and mechanical properties of welded joints under different heat input,and related welding processes were studied,and the welding theory guidance and test for the application of 5Mn steel in marine engineering structures were provided.This paper firstly evaluates the weldability and cold cracking susceptibility of the test steel through the highest hardness test in the heat affected zone,the rigid restraint crack test of the butt joint,and the welded SH-CCT diagram;Based on the above conclusions,the microstructure evolution and mechanical properties of 50 mm thick 5Mn steel butt joints were studied under three heat input conditions(12KJ/cm,15KJ/cm,18KJ/cm).Concluded as follow:(1)The results of the highest hardness test and rigid restraint test show that the50 mm thick 5Mn steel HAZ has a relatively obvious tendency of tissue hardening and cold cracking.But preheating at 200°C during the actual welding process can prevent the cold cracking of the HAZ.The SH-CCT diagram of the test steel was plotted by thermal simulation test.The evolution rule of the CGHAZ was simulated as the increase of t8/5:PM?LM?LM+LB+GB?LB+GB.The hardness of simulated CGHAZ gradually decreased with the increase of t8/5.In the actual welding process,the preheating temperature needs to be increased to 200°C to prevent the cold cracking of the HAZ.(2)With three heat inputs,the loss of alloying elements in the weld increases with increasing heat input;The effective inclusions that promote AF nucleation in welds decrease with the increase of heat input,and with the change of inclusion size and composition,the way to promote AF nucleation changes.The weld consists mainly of AF and the content of GBF and FSP decreases with increasing heat input.Inter-channel organizations mainly consist of AF,GBF,and GB.CGHAZ is made up of large lath martensite,and FGHAZ undergoes a tempering process to form a fine uniform lath martensite.With the increase of heat input,the strength of welded joints decreases,plasticity becomes better,and the two sides become inversely related;The hardness distribution of welded joints is complicated,the weld seam and HAZ hardness have a tendency to decrease with the increase of heat input,and there is obvious hardening in CGHAZ,while the softening zone is not obvious;The overall low-temperature impact energy of the weld is high and the weld toughness is good,while the low-temperature impact energy of the fusion zone and the HAZ is lower.The fracture of the weld is a typical uniformly distributed dimple,while the HAZ fracture consists of crystallographic intergranular fractures and quasi-cleaved facets of the river pattern.