Microstructure Transformations in Tempering Steels during Continuous Cooling from Hot Forging Temperatures

Abstract

Apart from reducing the processing energy, hardening and tempering of near-net shape forged components from their forging heat primarily promises shortened conventional process sequences. In this case, the continuous cooling transformation diagrams (CCT diagrams) found in the literature can only be used as a rough approximation of microstructural transformations during the heat-treatment. The reasons for this are that firstly, the deformation influences the transformation kinetics and secondly, the deformation temperatures are comparatively high. Therefore, both deformation CCT diagrams and, for reference purposes, CCT diagrams without deformation were determined for a selection of heattreatable steels (34CrMo4, 42CrMo4, 52CrMo4, 51CrV4, 34NiMo6) at the heating temperature of 1200 8C and deformation levels of 0.3 and 0.6.

The combined effect of chemical composition and cooling rate on transformation temperatures of hypoeutectoid steels

“…It was not only a favourite dilatometry test (see e.g. [9,[24][25][26][27]), but also laboratory rolling tests [4], or experiments using intermittent compression [28] or torsion deformation [25,29,30]. Non--isothermal tests can then even determine an important non-recrystallization temperature simultaneously with phase transformation temperatures in the course of cooling [29].…”

“…The acceleration effect of deformation is manifested especially at the beginning of these transformations. Deformation increases the number of lattice defects, which promotes the diffusion of all atoms in the solid solu- tion and leads to faster nucleation and grain growth of the new phase [9,10]. Phase transformations based on the diffusion mechanism use nucleation and subsequent growth of new grains especially on the original grain boundaries.…”

Formation of Microphases and Constituents from Remaining Austenite Decomposition in API X80 Steel Under Different Processing Conditions

“…When the hardenability is increased by carbon enrichment of austenite is possible to use slow cooling rates to achieve a martensitic microstructure, avoiding the formation of ferrite, pearlite and bainite from remaining austenite decomposition 22,24 . However, some retained austenite may be present in microstructure after the final cooling 25,26 .…”

“…In general, the higher the carbon content and the cooling rate, the higher the hardness of secondary phases formed 4,12,40 . When the carbon enrichment of austenite reaches high contents, the martensite can be obtained by applying of low cooling rates 24 . This was observed for the sample A7-300 (Table 3 and Figures 4h and 6f).…”

Effect of hot rolling history and cooling rate on the phase transformation of plain carbon steel

“…As for the physical methods, results of dilatometric tests performed at different, but always constant, cooling rates are used for the study of the phase transformation temperatures [8,9]. However, in industrial operating conditions, the cooling rate of material gradually changes -with the *Corresponding author: e-mail address: ivo.schindler@vsb.cz decreasing temperature it decreases, which can be further complicated by the evolution of the exothermic phase transformations [10,11], leading even to a local increase in temperature. Therefore, it proved very useful to combine the knowledge of the DCCT diagrams with the data provided by real cooling curves.…”