Prediction of properties distribution of 7B50 alloy thick plates after quenching and aging by quench factor analysis method

Abstract

In the present work, continuous cooling curves were accurately measured by the modified Jominy specimen of 7B50 alloy during water-spray quenching tests. Besides, the time–temperature–properties (TTP) curves of this alloy were obtained during isothermal treatments. Based on the accurate cooling curves and TTP curves, the hardness distribution along the thickness direction of 7B50 alloy thick plates was predicted by quench factor analysis method. It is found that the quench sensitive temperature range of 7B50 alloy is 240–410 °C, the nose temperature is 335 °C, and the incubation period at the nose temperature is about 0.87 s. When 7B50 alloy was isothermal treated at 180–400 °C after solid solution treatment (470 °C for 1 h followed by 483 °C for 2 h), the exponent (n) in the Johnson–Mehl–Avrami equation is close to 1 until transformed fraction of new precipitates is up to 60%, indicating that new precipitates first grow into rodlike shape and then coarsen or thicken. When the distance is less than 65 mm from the spray quenching surface of the modified Jominy specimen, the deviation between the predicted and measured hardness is less than 2.7%, confirming the quench factor analysis method as the feasible way to predict the hardness distribution along the thickness direction of 7B50 alloy thick plates. When the distance from the spray quenching surface is 25 mm, the average cooling rate in quench sensitive temperature range is 9.93 °C·s−1, while the quench factor (τ) is 9.89 and the corresponding predicted hardness is HV 185.1 equivalent to 97.3% of the maximum measured hardness of 7B50 alloy in T6 temper.