Estimation of the Temperature in the Stirred Zone and Cooling Rate of Friction Stir Welding of EH46 Steel from TiN Precipitates

Measuring the peak temperature in the contact region of the tool/workpiece in friction stir welding (FSW) is difficult using conventional methods such as use of thermocouples or a thermal imaging camera, hence an alternative method is required to tackle this problem. The objective of the present work was to estimate more accurately, for the first time, the peak temperature and cooling rate of FSW from precipitation of TiN in friction stir welded steel samples. Microstructures of nine friction stir welded samples of high strength shipbuilding steel of EH46 grade were examined closely by SEM-EDS to detect the TiN precipitates. Thermal heat treatments using an accurate electrical digital furnace were also carried out on 80 unwelded EH46 steel samples over a range of temperatures and cooling rates. Heat treatments were to create a basis to understand TiN precipitation behavior under various heating and cooling regimes for the studied alloy. Heat treatment showed that TiN particles can precipitate at a peak temperature exceeding 1000°C and the size of TiN precipitates particles increases with decreasing cooling rate. In a temperature range between 1100-1200°C the TiN precipitates were accompanied by other elements such as Nb, S, Al and V. Pure TiN particles were found after the peak temperature exceeded 1250°C with limited precipitation after reaching a peak temperature of 1450°C. The comparison between the friction stir welding samples and the heat treatments in terms of types and sizes of TiN precipitates suggests that the welding peak temperature should have been in the range of 1200-1350°C with a cooling rate in the range of 20-30 K/sec. The current work represents a step change in estimating the friction stir welding temperature and cooling rate which are difficult to determine using thermocouples and thermal imaging camera.