The effects of heat input and mechanical constraints on the MAG and laser weldability of 316L stainless steel
Abdo, Tarig (2018)
Diplomityö
Abdo, Tarig
2018
School of Energy Systems, Konetekniikka
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2018111648352
https://urn.fi/URN:NBN:fi-fe2018111648352
Tiivistelmä
Stainless steel is the dominant material in many industries such as petrochemicals, paper and pulp, cutlery and food processing units, in addition to offshore and corrosive environment applications. While welding represents the essential joining method for metals under demanding conditions, weldability of austenitic stainless steel must be guaranteed. High coefficient of thermal expansion remains a welding challenge for austenitic stainless steel. Whereas, it promotes the occurrence of the distortion in the welded structures. In this study, angular and longitudinal distortion on thin plates of austenitic stainless steel have been investigated. Ten samples of 316L stainless steel with 3 mm thickness were welded in butt-square joints using MAG and fiber laser welding processes. Therefore, angular and longitudinal distortions were measured by laser- 2D highly sensitive device, and then optical micrograph used to reveal the microstructure and carbides precipitation.
Fibre laser welding has produced smaller fusion zone and smaller heat affected zone (HAZ) compared to MAG welding. Therefore, relatively smaller distortion has been generated for the laser-welded samples. Laser welding speed of 2.2 m/min, the power of 2.5KW, and the focal position of 3mm represent the optimum parameters to prevent distortion in the 3mm plate of 316L stainless steel. In MAG welding, test results revealed a proportional relation between welding heat input (KJ/mm) and angular distortion. Raising the heat input from 0.3 to 0.472 KJ/mm increases the angle of distortion four times and increases the bending on the welded plate three times from 1.2 mm to 3.6mm. Constraints, which applied in form of a mechanical clamp does not prevent the occurrence of distortion.
316L showed Ferrite-Austenite solidification mode and an insignificant tendency to hot cracking due to moderate content of ferrite structure estimated as 10% of the total structure. SEM micrography beside the EDS test revealed a proportionality between heat input and carbides formations on the grain boundaries of HAZ, which indicates that degree of sensitization (DOS) to intergranular corrosion is higher when heat input increases and welding speed decreases. FE model has built with ANSYS to simulate the experiment and verify the model based on experimental results, the simulation model showed a significant deviation in the distortion results. Disability of the FE software to introduce the completely welding parameters justifies the variation between numerical and experimental results.
Fibre laser welding has produced smaller fusion zone and smaller heat affected zone (HAZ) compared to MAG welding. Therefore, relatively smaller distortion has been generated for the laser-welded samples. Laser welding speed of 2.2 m/min, the power of 2.5KW, and the focal position of 3mm represent the optimum parameters to prevent distortion in the 3mm plate of 316L stainless steel. In MAG welding, test results revealed a proportional relation between welding heat input (KJ/mm) and angular distortion. Raising the heat input from 0.3 to 0.472 KJ/mm increases the angle of distortion four times and increases the bending on the welded plate three times from 1.2 mm to 3.6mm. Constraints, which applied in form of a mechanical clamp does not prevent the occurrence of distortion.
316L showed Ferrite-Austenite solidification mode and an insignificant tendency to hot cracking due to moderate content of ferrite structure estimated as 10% of the total structure. SEM micrography beside the EDS test revealed a proportionality between heat input and carbides formations on the grain boundaries of HAZ, which indicates that degree of sensitization (DOS) to intergranular corrosion is higher when heat input increases and welding speed decreases. FE model has built with ANSYS to simulate the experiment and verify the model based on experimental results, the simulation model showed a significant deviation in the distortion results. Disability of the FE software to introduce the completely welding parameters justifies the variation between numerical and experimental results.