The Research On Impact Toughness In Overheated Zone Of Pressure Vessel Steel Welding Heat Affected Zone

Welding is one of the main methods of metal processing technology,which plays an important role in industrial production and national defense construction,and the welding quality must be paid attention to in the process of product manufacturing.Many engineering structural failures and major catastrophic accidents are caused by embrittlement of the weakest region of the heat-affected zone of welded joints.The coarse grain heat affected zone(the overheated zone)of the heat-affected zone is often the area where embrittlement and the weakest toughness occur.Domestic steel manufacturers use the toughness of the heat-affected zone to measure the quality of welded joints when conducting welding assessment,while ignoring the impact toughness of the weakest region of the welded joints.It has caused a deep suspense and hidden danger for the safe operation of steel equipment.Qualified evaluation of impact toughness of the weakest regions should be established.Therefore,in this paper,the low-temperature pressure vessel steels 09MnNiDR and 08Ni3DR are selected as test materials.The relationship between the fraction of the weakest region of the actual welded joint in the front of notch tip in Charpy V-notch specimen and impact toughness,and microscopic factors of affecting impact toughness of welding heat affected zone were systematically studied,which mainly includes the following two aspects:The microstructure and impact toughness of the various zones for the welded joints can be evaluated by preparing the notch in base metal,weld metal,coarse grain heat affected zone,fine grain heat affected zone,intercritical heat affected zone,and subcritical heat affected zone notch.Based on this,the weakest toughness region in the welded joints and the influence of the weakest region on the impact toughness of the overall welded joints are studied;the effects of different peak temperatures and different heat inputs on the microstructure and performance of the heat affected zone were studied by thermal simulation technique.The microscopic factors affecting the toughness of the heat affected zone are summarized in details.The results show that:(1)For actual welding,the weakest regions of 09MnNiDR submerged arc welding joints and 08Ni3DR shielded metal arc welding joints are coarse grained heat affected zones,which microstructure is composed of coarse granular bainite,lath bainite,and side lath ferrite.The low temperature impact absorbed energy of the coarse grain heat affected zone are 20 J(experimental value)and 27 J(non-experimental linear fitting value).The main cause of the deterioration of the toughness of the coarse-grained heat-affected zone of the welded joint is the appearance of granular bainite,lath bainite,side slab ferrite and grain coarsening.(2)In two kinds of actual welded joints,with the fraction of the weakest region in the front of the notch tip increases,the corresponding impact toughness decreases.When the weakest region completely penetrates the entire front of the notch tip,the impact absorbed toughness of weakest region decreases to 92.7% of the base metal.which fully reflects the influence of the weakest region of welded joints on the impact toughness.If the impact toughness of the complete weakest region cannot be obtained under the actual welding conditions,the impact toughness corresponding to the fraction of the width of the coarse grain heat affected zone can be indirectly calculated through the fitted functional relationship.(3)Under thermal simulated technology,with the increase of the peak temperature,the hardness of the heat affected zone of 09MnNiDR steel gradually increased,and the microstructure is gradually changed from small block ferrite to fine block ferrite + granular bainite to coarse granular bainite + lath bainite.With the increase of the peak temperature,the impact toughness of the heat affected zone of 09MnNiDR steel gradually decreases.Subcritical heat affected zone(650 ?)has the best impact toughness,coarse grained heat affected zone(1320 ?)is the weakest region of 09MnNiDR steel heat affected zone.Coarse grain size,microstructural transformation,and different proportion of high angle grain boundaries makes great difference in impact toughness in each zone of the heat affected zone.(4)With the increase of welding heat input,the microstructure of 09MnNiDR coarse grained heat affected zone gradually changed from coarse granular bainite + lath bainite to granular bainite + massive ferrite?the hardness gradually decreased and impact toughness increases at first then decrease.The research shows that the prior austenite grain size is an important factor affecting the impact toughness change of the coarse grain heat affected zone under different heat input.(5)The results of actual welding joints and thermal simulated experiments also reveals that in the 09MnNiDR ferritic steel,the critical event is controlled by cracks propagation of grain(bainite colony)size across the grain / grain boundaries.The size of the coarsest grains(bainite colony)characterizes the size of the critical event.