engleski

Effect of the heating rate in the tempering of steel grade EN 100 V1

The effect of tempering time and tempering temperature on the properties of hardened (and tempered) steel is usually presented by the Hollomon-Jaffe parameter expressed as P = T (C + logt). The expression does not take into account the heating rate at which a given tempering temperature is reached. A need to investigate how the heating rate at which a tempering temperature is reached affects both the tempering process and the properties has arisen from the fact that the tempering of an workpiece made of hardened steel is carried out at a finite heating rate at which a tempering temperature is reached. The heating rate as an additional factor in the process should not be neglected, particularly in a tempering process involving high heating rates (with or without keeping the workpiece at a tempering temperature). The paper deals with the effect of the heating rate at which a tempering temperature is reached on the development of the tempering process and on the obtained hardness in the hardened and tempered condition of non-alloyed tool steel grade EN 100 V1 (W.No. 1.2833). Two starting conditions of steel were investigated: the standard quenched condition and the quenched and deep cryogenically treated condition. The joint effect of the prior state and the heating rate on the temperature at the beginning of a particular stage in tempering and on the obtained properties was investigated by a dilatometric method (differential dilatation analysis), hardness tests, and XRD analysis. The test samples were tempered in a dilatometer at varying heating rates, tempering times and tempering temperatures between 200 and 690 ° C. It was found that the heating rate at which a tempering temperature is reached has a marked effect on the beginnings of microstructural transformations which are typical of tempering. If higher heating rates are involved, tempering processes start at higher temperatures. The joint effect of the heating rate, tempering temperature and tempering time at a given tempering temperature is given by the effective tempering parameter P* = T*(C + log t*). The results of conducted tests were analysed by means of a regression analysis to formulate equations expressing the dependence of microhardness and dimensional changes (dilatation) of test samples on the effective tempering parameter P*. The practical application of the obtained equations can be found in the preparation of tempering processes for hardened non-alloyed and low-alloyed tool steels as well as for structural steels which offer the possibility of reducing the duration of tempering.

carbon tool steel; heating rate; tempering parameter; tempering; phase transformations; differential dilatation analysis; XRD analysis

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