Abstract
The effect of a cooling rate on the S690Q quenched and tempered steel welded joint coarse grain heat affected zone microstructure was investigated using a dilatometer with controlled heating and cooling fixture. Steel samples were heated to a peak temperature of 1350 °C and cooled at the different cooling time Dt8/5. A dilatometric analysis and hardness measurements of the simulated thermal cycle coarse grain samples were done. Transformation start and finish temperature were determined using dilatation vs. temperature data analysis. The microstructure of the sample with a cooling time 5 s consists of martensite, whereas at cooling time 80 s a bainitic microstructure was observed. The investigated steel cooling cycle using simulation approach makes possible to determine the range of an optimum CG HAZ cooling time for the welding.
Highlights
The quenched and tempered low-alloy steels, usually contain less than 0.25% carbon and less than 5% of alloying elements
The results suggest that coarse grained heat affected zone (CG HAZ) hardness tends towards the base metal hardness when t8/5 cooling time is greater than 80 s
The present work describes a simplified approach to the prediction of CG HAZ hardness and microstructural characteristics in quenched and tempered welded steel
Summary
The quenched and tempered low-alloy steels, usually contain less than 0.25% carbon and less than 5% of alloying elements. The temperature in the weld coarse grained heat affected zone (CG HAZ) can reach above 1100 °C 6. The initial tempered martensite microstructure of the as delivered steel undergoes austenite transformation into a CG HAZ. Weld cooling rates that are too high because of inappropriate welding parameters (low welding heat input) can result in HAZ hardening, while too high inter-pass temperature during welding in conjunction with high heat input can lead to softening of the HAZ 3. This work aimed to obtain the cooling time range used for the welding procedure, by application of thermal weld CG HAZ simulation and the corresponding microstructure analysis.
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