Effect of heat on microstructural, mechanical and electrochemical evaluation of tungsten inert gas welding of low-nickel ASS

Abstract

Purpose The purpose of the study is to evaluate Cr-Mn ASS weld using different heat inputs for its microstructure, mechanical properties and electrochemical behavior. The microstructural examination used optical and scanning electron microscopy. It was observed that ferrite content decreases with increasing heat input. The length of dendrites, inter-dendritic space and volume of lathy ferrite increase with increasing heat input. The increasing heat input caused grain coarsening near the fusion boundary and produced wider heat-affected zone (HAZ). It also decreases hardness and tensile strength. This is attributed to formation of more δ ferrite in the weld. The electrochemical evaluation suggested that the δ ferrite helps in improving the pitting potential in 3.5 per cent NaCl solution saturated with CO2. Whereas in 0.5-M H2SO4 + 0.003-M NaF solution, higher passivation current density was observed because of dissolution of dferrite. The interphase corrosion resistance decreased with increasing heat input. Design/methodology/approach The Cr-Mn austenitic stainless steel or low-nickel ASS was procured in form of 3-mm sheets in rolled condition. The tungsten inert gas welding was performed at three different heat inputs (100 A, 120 A and 140 A), argon as shielding gas with a flow rate of 15 L/min. Different welded regions were observed using optical microscope and scanning electron microscope. Electrochemicals test were performed in solutions containing 3.5 per cent NaCl with saturated CO2 solution and 0.5 M sulfuric acid + 0.003 M NaF at a scan rate of 0.1667 mV/s at room temperature (30 °C ± 1 °C) using a potentiostat. Findings The test steel Cr-Mn ASS is suitable with the selected electrode (308 L) and it produces no defects. Vermicular ferrite and lathy ferrite form in welds of various heat inputs. The increase in heat input reduces the formation of lathy ferrite. The width of HAZ and un-mixed zone increases with increase in heat input. The weld zone of low heat input (LHI) has the highest hardness and tensile strength because of higher δ ferrite content and small grain size in the weld zone. The hardness at high heat input (HHI) is found to be lowest because of grain coarsening in the weld. With increase in δ ferrite, the pitting resistance increases. In 0.5-M sulfuric acid + 0.003-M NaF, the increase in ferrite content reduces the passivation current density. Interphase corrosion resistance increases with increase in δ ferrite content as higher per cent degree of sensitization was observed in LHI welds as compared to medium heat input and HHI welds. Originality/value This work focuses on welding of ASS by tungsten inert gas welding at different heat inputs. Welding is a critical process for joining metals in most of the fabrication industries and proper heat input is required for getting desired microstructure in the weld metal. This would highly affect the strength and corrosion behavior of the alloy. This paper would give an understanding of how the change in heat input by tungsten inert gas welding affects the microstructural and corrosion behavior in the weld metal.

Effect of Heat Input on the Microstructural, Mechanical, and Corrosion Properties of Dissimilar Weldment of Conventional Austenitic Stainless Steel and Low-Nickel Stainless Steel

“…Therefore, the LHI weldment exhibits better corrosion resistance than HHI weldment. These results are consistent with the results obtained by EIS Nyquist and Bode plots and also are consistent with the findings of other authors [25,26].…”

“…It can be observed that the dendritic length and interdendritic arm spacing increase with increase in heat input. The faster cooling rate in the LHI process might help in suppressing the dendritic size and cell spacing formation as reported by authors [25,26]. Therefore, the increase in dendritic length and interdendritic arm spacing in the HHI process can be ascribed to a slower cooling rate.…”

“…Hence, these results indicate that LHI FZ depicts more corrosion resistance than HHI FZ. This can be attributed to fine-grained structure in the LHI FZ [26].…”

“…The ultimate tensile strength (UTS) of LHI and HHI weldments was measured as 582 and 567 MPa, respectively. This higher UTS for LHI weldment can be attributed to a faster cooling rate, which resulted in higher δ-ferrite content and finer ferrite morphology with less interdendritic spacing [25,26].…”

“…Also, it can be noticed that the microhardness value of LHI FZ is higher as compared to HHI FZ. This is due to a faster cooling rate, which resulted in a fine structure with less interdendritic spacing [25,26]. In PMZ of both the 316L austenitic SS and 201 austenitic SS side, high microhardness values can be observed in both LHI and HHI processes.…”