| Abstrakt: |
Isothermal diagram of decomposition of overcooled austenite of R350LHT steel has been plotted on the basis of dilatometric, metallographic, and durometric studies. While comparing thermal kinetic and isothermal diagrams of decomposition of overcooled austenite of R350LHT steel, it has been established that the thermal kinetic diagram plotted during continuous cooling is displaced downwards and rightwards in comparison with the isothermal diagram. This result completely agrees with the known regularities. During the studies, the critical points of R350LHT steel have been determined: Ac1 = 711°C; Ms = 196°C. Using the isothermal diagram of decomposition of overcooled austenite of R350LHT steel, the temperature of minimum resistance of overcooled austenite has been determined equaling to 500°C. Under the isothermal conditions, the pearlitic structures are formed in the temperature range from 700 to 600°C. A mixture of pearlitic and bainitic structures is formed at 550°C. In the range from 500 to 250°C, the bainitic structures are formed: at 500–400°C, the upper bainite is formed; at 350°C, the mixture of upper and lower bainite is formed; at 300–250°C, the lower bainite is formed. The increase in the hardness of transformation products upon decrease in the holding temperature from 246 HV (at 700°C) to 689 HV (at 250°C) is observed nearly in the overall studied temperature range of isothermal decomposition of overcooled austenite. However, at 500°C, the hardness slightly drops, which could be attributed to the occurrence of residual austenite in the course of bainitic transformation. [ABSTRACT FROM AUTHOR] |
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