Laser-arc Hybrid Welding Of Martensitic Stainless Steel For Endless Rolling

Endless rolling is the key technique to increase the producting efficiency of steel sheet by welding two sheets online.Due to low cost and morderate performace,martensitic stainless steel?MSS?has been widely used in modern industry.However,the welding of the MSS,including laser welding and arc welding,cannot meet the rolling requirement because of the bad weld toughness.How to improve the toughness of MSS weld is then one of the most important challenges for welding research.By integrating the advanteges of both laser welding and arc welding,laser-arc hybrid welding would be potential to increase the toughness of MSS weld,and meet the rolling requirement,but relevant study has not be carried out.In order to achieve the endless rolling of the MSS,some key technqueis and mechanisms about toughness improvement of AISI420 MSS wled were studied in detail.The main results are listed as follows.In order to increase the rolling stability and roller life,the effects of welding parameters on the reinforcement of hybrid weld was studied in terms of response surface method.The quadratic regression models of top reinforcement?R_t?,bottom reinforcement?R_b?and the thickenss ratio of total reinforcement to base metal?R_p?were obtained according to least squares method,whose fitting degrees are 0.928,0.893 and 0.965,respectively.The reinforcement parametes can well predicted by these modles.Besides,according to the requirement of endless rolling,the parameters were optimized by multi-objective optimization,which are laser power 4.5 kW,welding speed 2.5-3.5 m/min,wire feeding rate 2-5 m/min.The effects of welding parameters and Ni content on the microstructure and toughness of the weld were studied.It was found that the coarse-grained heat affected zone?CGHAZ?is the poorest area because of the formation of coarse martensite and a large amouth of carbides,while the lower part of fusion zone?LaserZ?is the secondary weakest area.The Erichsen toughness agrees the microstructure of CGHAZ well,both of which are divided into three stages with the increase of heat input.According to their relationship,an efficient grain diameter of CGHAZ,D_e was proposed to propose a new Hall-petch relationship between CGHAZ microstructure and Erichsen toughness.The Erichsen value is larger than4.5 mm when the D_e is below 11?m,at which the fracture location changes from the weld to the base metal?BM?.After the toughness of the CGHAZ was improved under optimized heat input,increasing Ni content of filling wire can further increase the toughness by increasing the austenite content within the fusion zone.When the Ni content increases from13%to 20%,the fracture location of Erichsen cupping sample changes from weld fusion line to the BM,but the crack of LaserZ at 45%rolling reduction cannot be suppressed.When the Ni content increases to 66%,the rolling crack of LaserZ at 45%rolling reduction is also suppressed.The results indicate that the general criterion,which only employs Erichsen result to predict whether the weld meets the rolling requirement or not,is not suitable for the MSS.The content of retained austenite(C_RTA)and the content of high misorientation martensite(C_HM)must be taken into account,which should be higher than 11.6%and 59.5%,respectively.The toughness improvement is dependent on two factors.Firstly,the retained austenite induces the DARA effect,softens the martensite by absorbing moving dislocations,and then suppresses the crack initiation and propagation by reducing stress concentration.Secondly,the high misorientation martensite increases the crack propagation resistance and decreases the propagation rate.The effects of online induction heat treatment on weld microstructure and rolling toughness were studied to increase the process window.The heat treatment parameters were optimized as heating temperature about 650°C and holding time longer than 5s,under which the rolling crack at 75%reduction is suppressed.The effects of heat treatment on rolling toughness can be divided into two stages with the increase of heating temperature.When the heating temperature is at the range of 350-650°C,the toughness is only dependant on the content of reversed austenite(C_RVA),and is improved with the increase of the heating temperature.The rolling crack was thoroughly suppressed when the C_RVAVA reaches 24.8%.At this stage,the reversed austenite softens the martensite by absorbing dislocations,and then suppresses the crack initiation.When the heating temperature is at the range of 650-850°C,the weld toughness is dependant both on C_RVAand C_HM,and is decreased with the increase of the heating temperature.The decreasing C_RVAweakens the softening ability of austenite,while the decreasing C_HMpromotes the crack initiation and increases the propagation rate.It causes a 0.95 mm penetrating crack at 850°C.The effects of rollding reduction on the evolution of the microstructure and work hardening of hybrid welded were discussed.It was found that the heattreated weld can suffer the rolling reductioin of 75%,while the weld without heat treatment only suffers the reduction of 25%.The effects of rolling reduction can be divided into two stages.When the reduction is no larger than 25%,the dislocation multiplication occurring during rolling forms the dislocation wall in the austenite grains,and promotes the austenite-martensite transformation to induce the work hardening.When the reduction is larger than 25%,the increase of deformation temperature decreases the nucleation ability of martensite by increasing the stacking fault energy,and a part of dislocations in the austenites react to decrease the dislocation density by the effect of muti-slip.Both of the effects slow down the formation rate of martensite,and stablize the work hardening.