Tempering Conditions for Spot Welds of Hardenable Steel Sheets

概要

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This report gives detail of a new method to determine tempering conditions for spot welds of hadenable steel sheets. Up to the present, selection of tempering conditions is normally based on the temperature measurement. Then, the ductility of the weld must be checked by cruciform mechanical test. In practical application a slight variation in welding and tempering conditions have a significant effect on weld ductility. Thus, the accurate indication of optimum schedule can not be expected. The principle of the new method developed is as follows : electrical resistance of steel rises with temperature and then the rate of its rise drops because of austenite transformation. When the weld has been properly tempered, its center is heated above the level of austenitising temperature. Consequently, the resultant resistance in the weld during tempering incleases first and then decreases slowly or steeply depending on temper current level. Thus, by analysing the electrical resistance characteristic, the optimum temper schedule can be selected. To check the validity of this method, fatigue strength test and hardness survey were carried out ; the results are in fairly good agreement with temper schedule selected.
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