Determination of Heat Treatment Conditions of Spot welds in Anti-Corrosive High Tensile Steels

Abstract

This report gives details of a new method to determine post heat (tempering) conditions for the spot welds in anti-corrosive high tensile steel sheets.
Up to the present, selection of post heat conditions is normally based on the temperature measurement of the nugget. Since this can not be expected to give accurate indication of optimum conditions, the ductility of the weld must be checked by cruciform tension strength, micro-examination and hardness. In practical application a slight variation in welding and post heat conditions can have a singificant effect on weld ductility. Thus, as it is difficult to check the post heat effect by such a method the conditions selected are to be impracticable.
The principle of the new method developed in this report is as follows : Electrical resistance of steels rises with temperature but the rate of its rise drops remarkably because of austenite transformation. When the weld periphery has been properly tempered, the center is heated above the level of austenitising temperature. Consequently, the resultant resistance in the weld during post heating increases first and then decreases slowly or steeply depending on post heat current level. Thus, by analysing the resistance characteristics, the optimum post heat conditions can be selected. Practically, since the resistance corresponds to voltage drops, post heat conditions to give the required heat treatment can be readily obtained from the records on an oscilloscope. Short post heat time and narrow allowable range of post heat time will be obtained for higher current while long post heat time and wide allowable range will be for small current.
To check the validity of this method, fatigue strength test and hardness survey were carried out; the results are in fairly good agreement with post heat conditions selected.
Plotted on the log-log scale, the post heat variable comes out in a straight line. This relation can be formulated by empirical equations.
The determination of proper quench time was the most time-consuming operation but it can be made simply by analysing the voltage drop records.
The rate of rise in electrical resistivity with temperature is measured and the average temperature in the weld during post heating is calculated.