Heat effects of preprecipitation stages on tempering of carbon martensites
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segregation is energetically more favorable. Therefore clustering, as a preprecipitation stage for 77 (e) carbide formation, is not expected to occur for carbon contents less than about 0.2 wt pct and indeed, to our knowledge, for such carbon contents no experimental evidence for clustering has been presented. Furthermore, it is likely that significant clustering at room temperature takes place only after longer aging times. 5'7 Van Rooyen and Mittemeijer 12 measured the heat evolution occurring on tempering water-quenched (WQ) and water-quenched and subsequently liquid nitrogen quenched (WQ/LNQ) iron-carbon alloys. The Ms temperatures of these alloys are high enough to allow carbon segregation during quenching. 4 Indeed, the results indicate no preprecipitation-heat production if the carbon content is less than about 0.18 wt pct (=0.85 at. pct). For larger carbon contents the heat released on tempering the WQ/LNQ specimens was larger than that observed from the WQ specimens. This difference in heat effect can be ascribed to carbon clustering which occurred in the WQ specimens during holding at room temperature. For the tempering of carbon martensites the total heat evolution due to both preprecipitation processes may now be estimated as a function of carbon content by combining the heat evolution of iron-nickel-carbon alloys containing less than about 0.2 wt pct C, as observed on aging by Mogutnov et al, II with the difference in heat evolution between WQ/ LNQ and WQ iron-carbon alloys, as observed on tempering by van Rooyen and Mittemeijer. ~2The result is presented in Figure 1: Up to about 0.85 at. pct C the heat effect as observed by Mogutnov et al, lz which is ascribed to segregation, has been adopted (the heat evolution observed by these authors from pure iron after quenching was ascribed to the release of quenching stresses u and therefore it has been subtracted from the heat data). For carbon contents larger than about 0.85 at. pct C the heat effect observed by van Rooyen and Mittemeijer, ~2 which is ascribed to clustering,
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Q3
0.6
0.9
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at %C E.J. MITTEMEIJER, Project Leader, and F.C. VAN DOORN, Student, are associated with the Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands. Manuscript submitted November 15, 1982.
976--VOLUME 14A, MAY 1983
Fig. 1--The total heat of preprecipitation for the tempering of carbon martensites, - A H (per mol martensite), due to segregation and clustering, as obtained from data by Mogutnov et al H and van Rooyen and Mittemeijer, ~2respectively. The total heat of preprecipitation as obtained for a steel employed by Oketani et al~3 is also indicated.
METALLURGICALTRANSACTIONS A
has been added to the heat of segregation as observed at 0.85 pct C by Mogutnov et al. It should be recognized that this estimate is subject to the assumptions (see the above discussion): (i) the heat effect observed by Mogutnov et al 1~for carbon contents less than abo
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