Influence of Heating Rate on the Transformation Temperature Change in Selected Steel Types

Abstract

The paper is focused on the polymorphic transformations of selected steel types. Generally, when creating CCT diagrams, the transformation temperature values Ac1 and Ac3 are determined under at very low heating rates (max. 0.1 °C•s -1 ). The only exceptions are so-called in-situ CCT diagrams, which are determined based upon the real temperature cycles. However, in the case of dynamic processes such as welding technology or heat treatment with magnetic induction heating, cannot be used information from conventional CCT diagrams. Incorrectly selected temperatures lead either to incomplete austenitizing during heat treatment or to a different development of phase transformations in HAZ of welds. Especially, when these processes are carried out without soaking temperature. That is why as the main aim of this paper was to determine the heating rate influence for steels S460MC, S690QL and X23CrMoV12-1 on the shift of their transformation temperatures Ac1 and Ac3. The intensity of the heating rate effect on the transformation temperature depends on the steel structure and amount of alloying elements. Consequently, steels with different structures, different type of processing and mechanical properties were deliberately selected to make the assessment as complex as possible.

Microstructural Changes and Determination of a Continuous Cooling Transformation (CCT) Diagram Using Dilatometric Analysis of M398 High-Alloy Tool Steel Produced by Microclean Powder Metallurgy

“…In the case of actual experiments, the temperatures of Ac 1 and Ac 3 vary slightly depending on the heating rate. This issue is dealt with in more detail [ 70 ]. From the diagram, we can determine the measured temperatures of Ac 1 = 960°C and Ac 3 = 1070°C.…”

“…In the case of actual experiments, the temperatures of Ac 1 and Ac 3 vary slightly depending on the heating rate. This issue is dealt with in more detail [70] Based on our measurements and simulation, a fully martensitic microstructure can be achieved at cooling rates approaching 100 °C/s, since at a cooling rate of 10 °C/s we noted a step change in the expansion curves, indicating the formation of the bainitic phase, but the simulation with the help of software JMatPro does not predict an amount of bainite greater than 0.02%. The Ms curve, which was constructed based on dilatometric measurements, has a decreasing tendency, while at a cooling rate of 100 °C/s, the temperature is Ms = 243 °C, and subsequently, with a gradually decreasing cooling rate, the Ms temperature drops to a value of Ms 195 °C at a cooling rate of 0.5 °C/s.…”

Analysis of heat treatment parameters on the properties of selected tool steels M390 and M398 produced with powder metallurgy

“…[9] The heating of the sample is provided by induction heating, the advantage of which is the possibility of precise power control to achieve the optimal heating rate. [11,12,13]…”

“…We used two methods to reduce the amount of residual austenite, the first was to use subzero cooling after quenching to -78°C and then the samples were tempered [13]. In the second case, the amount of residual austenite decreases during the tempering itself.…”

Softening Effect in the Heat-Affected Zone of Laser-Welded Joints of High-Strength Low-Alloyed steels

“…When compared to arc welding technologies, HAZ of laser-welded joints have different properties because of their higher heating and cooling rates and lower heat input. While a higher heating rate increases A c1 and A c3 temperature values, decreasing the width of the CGHAZ, FGHAZ, and ICHAZ subzones [20,21], a higher cooling rate promotes the development of harder and stronger structures in these areas. The lower heat input has also a partial effect on the properties of the structures in the SCHAZ area, which, due to the narrowed temperature elds and lower soaking times, are less tempered as a result of the welding process [22].…”