Abstract
Ledeburitic die steel containing 2.55% carbon, 11.8% chromium, 0.65% tungsten has been subjected to isothermal transformation studies in order to illustrate how the transformation characteristics are altered by maximum heating temperature. The data obtained by the use of microscopic, hardness and dilatometric tests for three austenitizing temperatures at 900°, 970° and 1050° for 10 minutes, have been brought together in the form of a TTT-diagram. In this work the influence of undissolved carbides on the transformation was mainly investigated. The pearlite transformation is enhanced by the nucleation of undissolved carbides, but the bainite is not. Consequently, in the pearlite reaction, the retardation effect of alloying elements in the austenite does not appear in all cases. On the other hand, in the bainite reaction, as the austenitizing temperature is higher, the remarkable retardation of transformation attributed to the enrichment of carbon and alloy contents in the austenite occurs and moreover the temperature range of bainite formation becomes smaller. As the austenitizing temperature is higher, the Ms point becomes lower and it has the values of 210° at 900°, 170° at 970° and 70° at 1050°.
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