Study On Toughness And Fracture Behavior Of Different Weld Metal For NiCrMoV Steel

NiCrMoV steel has good strength and toughness,so it can be applied to manufacture components with high strength and toughness,especially for low pressure welded rotor components.Low pressure welded rotor will produce large impact load due to its large size when starting and stopping,and the welded joints become the focus of fracture failure due to the uneven microstructure.In order to ensure the safe operation of welded components,it is necessary to evaluate the reliability of welded joints,especially to study the fracture toughness and impact toughness.In this paper,the fracture toughness and impact toughness of submerged arc welded joint of NiCrMoV steel were tested under different flux conditions,especially the mechanism of the influence of cooling rate during post-weld heat treatment on impact toughness was studied,which provided experimental support and theoretical basis for better understanding of fracture mechanism of weld,selection of welding materials and optimization of welding process.Firstly,the microstructures of BB24 and OK flux weld metals were characterized in detail by OM and SEM.It was found that the microstructures of equiaxed grains in the middle of BB24 flux weld were composed of granular bainite?GB?,polygonal ferrite?QPF?,acicular ferrite?AF?,tempered martensite or tempered martensite island?Tempered M-A/M?.Compared with BB24 flux,the microstructure of OK flux weld is uniform and fine,the acicular ferrite increases in the weld,and the ferrite bundles of acicular ferrite interlock with each other,which inhibits the growth of other microstructures in the interior of grain,so the intragranular microstructure is refined.Hardness test shows that the welds manufactured by the two kinds of flux have the same hardness characteristics.Secondly,the fracture toughness and impact toughness of two kinds of weld metal were tested and studied,and the intrinsic correlation between toughness and structure was clarified.The results of fracture toughness test for two kinds of flux weld metal show that the J_IC values of BB24 and OK flux weld metal are 190.62 kJ/m²and 306.19 kJ/m²,respectively.Compared with BB24 flux,Ti content in the inclusions formed by OK flux increases.The average diameter of inclusions in weld metal is 0.6 micron,the quantity density is 3.5 x10³/mm²,which is higher than that of BB24 flux.According to Jye-Long's nucleation model,the nucleation ability of acicular ferrite in OK flux weld metal is stronger,the acicular ferrite interlaces with each other,which improves the resistance to crack propagation.The crack path was observed by SEM.It was found that the crack propagated along the tempered structure,and the crack deflected after encountering the protruding martensitic block microstructure for BB24 flux weld.More secondary cracks and crack deflection were found in the corresponding crack paths of OK flux weld,and the crack deflection was as high as 90 degrees.EBSD was used to analyze the grain distribution along the crack path.A large number of large angle grain boundaries were found in OK flux weld metal.The structure was mainly fine block,and the orientation difference distribution was uniform.The crack path was composed of many uniform small deflections,and the peak strain was distributed to many regions.In BB24 flux weld metal,small angle grain boundaries are dominant,long parallel strips exist,the orientation difference between grains is uneven,and there is a large orientation difference locally,which leads to the sudden deflection of cracks.The impact toughness of two kinds of flux weld metals at different tempering cooling rates?5?/h,20?/h and 40?/h?was tested.The results show that the impact toughness increases with the increase of cooling rate.Cooling rate has little effect on impact toughness of OK flux weld,but has great influence on impact toughness of BB24 flux weld.At the same cooling rate,impact toughness of OK flux weld metal is higher than that of BB24 flux weld metal.The amount of carbides in tempered M-A/M decreases with the increase of cooling rate.The crack path was observed by SEM.It was found that the cooling rate had a great influence on the crack growth process.When the cooling rate was 5?/h,M-A/M decomposed greatly.Tempered M-A/M formed a continuous chain.After being subjected to external loads,the crack mainly propagated along tempered M-A/M,resulting in reduced impact toughness.When the cooling rate is 20?/h,tempered M-A/M does not form a continuous chain.Under the external load,the voids mainly initiate at the edge of carbide and intragranular martensite island.When the crack meets the intragranular martensite island,it deviates from the original direction of the crack and tends to propagate along the interface between the island and the matrix,which increases the deviation of the crack path.In addition,the chances of crack propagation in intragranular ferrite matrix increase,and the ferrite matrix has good plastic deformation ability,which improves the resistance of crack propagation.When the cooling rate is 40?/h,the number of voids in tempered M-A/M decreases further.A large number of voids nucleate at the edge of the intragranular islands and ferrite matrix.The distance of crack propagation in the intragranular matrix increases further.At the same time,the crack cannot pass through after encountering the martensite islands in the crystals.It tends to deflect the crack and the crack path is more complex,which leads to great improvement of the impact toughness.Finally,the influence of loading speed on the crack growth process is discussed.OM and SEM observations show that more voids are generated in the impact process than in the quasi-static loading process.The voids mainly germinate around the inclusion and the edge of the bulge structure,and the voids produced by quasi-static loading of fracture toughness show smooth characteristics while the voids produced by impact load mostly show long strip characteristics.The impact fracture presents lamellar morphology with a large number of dimples and deep voids,while the fracture surface of fracture toughness is relatively flat,and the micro-morphology is a large number of dense dimples.The difference between the distribution of voids and the fracture surface is mainly due to the large inertia force produced at the moment of impact loading,which leads to excessive local deformation and general yield of ligaments.Therefore,the impact toughness values cannot fully reflect the fracture toughness,but there is a certain correlation between them.