Abstract
The Charpy impact behaviour of three normalised steels with 18, 38, and 100% pearlite volume fraction, respectively, has been compared before and after tempering in the temperature range 600–680°C for periods up to 15 h. For the ferrite–pearlite steels containing lower amounts of pearlite (18 and 38%), tempering over long periods at 680°C was accompanied by a large reduction in both the lower yield strength and ultimate tensile strength and by an increase in impact transition temperature (ITT). There was a balance in impact behaviour resulting from tempering of these steels between the decrease in ITT relating to the decrease in yield strength and an increase in ITT resulting from the coarsening of the grain boundary carbides. For the steel containing the lowest amount of pearlite, the coarsening of the grain boundary carbides was a major influence on the fracture process and led to an increase in ITT. Increasing the pearlite content from 18 to 38% resulted in a reduction in the influence of carbides on fracture and a smaller increase in transition temperature. For the fully pearlitic steel, impact changes on tempering were controlled solely by the large decrease in yield strength, so that a large decrease in transition temperature occurred.
MST/829
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