Effect Of Mn/N Ratio On HAZ Microstructure And Properties Of Low-Ni Duplex Stainless Steel Welded At Different Peak Temperature

The change of the content ratio of Mn and N elements in duplex stainless steel will change the two phase ratio and the type and distribution of precipitation phases.As an important process parameter in the welding process,the peak temperature directly affects the change of two phase ratio in the heat affected zone（HAZ）,which will affect the manchanical properties and corrosion properties of HAZ.In this paper,some Ni elements in duplex stainless steel were replaced by adjusting the ratio of manganese and nitrogen,and used the Gleeble-3800 thermal simulation machine to simulate the heat-affected zone of experimental steels with different Mn/N ratios when the peak temperature is 725-1325℃,by characterizing the structure and precipitates of HAZ,testing mechanical and corrosion properties,and studying the influence of Mn/N ratio on microstructure evolution and mechanical and corrosion properties of HAZ under different peak temperature conditions.and provide reference for optimizing welding performance from the design of components and peak temperature。The results of the organizational evolution analysis show that with the increase of Mn/N ratios,the content of ferrite phase in the HAZ gradually increased,from slender ferrite gradually coarsened to bulk ferrite.At low peak temperature,austenite phase mainly transformed into PTA,when the peak temperature rises to 1325℃,it becomes Widmanite austenite,grain boundary austenite and intragranular austenite.With the increase of Mn/N ratio,the morphology of Widmanite austenite changes from fine peak to dendritic and feathery.When the peak temperature is 1325℃,the precipitation phase of 2205 duplex stainless steel（Mn/N ratio is 3.06）is granular Cr₂N precipitated along the austenite grain boundary;when the Mn/N ratio increases to 17.80,the short rod-like Cr₂N increases and mainly exists in the ferrite phase.The Cr₂N precipitation in the austenite phase of the experimental steel with Mn/N ratio of 3.28gradually decreases with the increase of the peak temperature.When the peak temperature is825℃,the round Cr₂N particles are precipitated from the austenite grain boundaries and within the phase.When the Mn/N ratio increases to 65.95,the precipitation phase was mainly bulkyσphase at the two-phase boundary.The results of impact toughness analysis show that with the increase of Mn/N ratio,the transformed austenite in the HAZ of the experimental steel was decreasesed,the fracture morphology changes from dimple to cleavage,and the overall impact toughness was decreasesed.When the peak temperature is 825℃,the impact energy of 2205 duplex stainless steel（Mn/N ratio of 3.06）reaches the highest value of 232 J,the fracture cross-sectional area becomes larger,the equiaxed dimples are densely distributed,the hardness difference between the two phases was decreasesed,and the impact toughness performance was the best;when the Mn/N ratio increases to 3.28 and 17.80,more Cr₂N precipitation will reduce the impact toughness of the experimental steel,and the peak value of the impact energy of the experimental steel will decrease.When the peak temperature is 825℃,the experimental steel with Mn/N ratio of 65.95 has the minimum impact energy（7.9 J）,which is related to the increase of the hardness difference between the two phases of HAZ and the increase of the radiation zone area,and the fracture morphology is brittle fracture.The results of the pitting corrosion performance analysis show that with the increase of Mn/N ratio,the increase of ferrite content leads to the decrease of pitting corrosion resistance of the experimental steel;pitting corrosion mostly occurs at the boundary between the ferrite phase and the two-phase phase,and the pitting gradually becomes larger.2205 duplex stainless steel（Mn/N ratio of 3.06）has a higher ferrite phase pitting equivalent（PREN）value and better pitting resistance.When the peak temperature is 825℃,the pitting potential value of the experimental steel HAZ with Mn/N ratio of 3.28 decreases to 91.5 m V,which is related to the precipitation of more Cr₂N which promotes the formation of chromium-poor areas and induces pitting corrosion nucleation.When the Mn/N ratio increases to 17.80,the content of Mo element in the ferrite phase of the experimental steel decreases at 1325℃,and the increase of the ferrite phase reduces the pitting corrosion resistance.When the Mn/N ratio increases to 65.95,the ferrite phase is severely corroded,which is related to the precipitation of Widmanite austenite and intragranular austenite from the ferrite phase,which consumes a large amount of Cr element to form a chromium-depleted area.The results of intergranular corrosion performance analysis show that with the increase of Mn/N ratio,the intergranular corrosion resistance of experimental steel HAZ gradually was decreased,due to the grain refinement,the corrosion degree of the fine-grained heat-affected zone of the experimental steels was more serious than the critical heat-affected zone and the coarse-grained heat-affected zone with different Mn/N ratios.2205 duplex stainless steel（Mn/N ratio of 3.06）has a lot of dislocation outcrops in the ferrite phase,which promotes the nucleation of intergranular corrosion,and the Cr₂N was precipitated at the two-phase boundary will increase the tendency of corrosion;the Mn/N ratio increases to 3.28,the Cr₂N precipitation phase were increased,and intergranular corrosion ravines were easily formed near it.When the Mn/N ratio increased to 17.80,the grains in the experimental steel FGHAZ were refined,and the ferrite and austenite grain boundaries were corroded;When the Mn/N ratio is increased to65.95,the intergranular corrosion sensitivity of the experimental steel is the highest,and the ferrite content is the highest.Too high Mn content will greatly reduce the corrosion equivalent,and the corrosion degree will be aggravated,so the corrosion resistance is the worst.