Effect of Heat Treatment on the Microstructure and Morphology of Steel for Cold Rolling Rolls

During ultrasonic inspection of as-forged blanks of large forgings represented by rolls, a disorderly signal resembling overgrown weeds is generated during surface wave detection, which is defined as grass-like echo. Grass-like echo defects lead to a reduction in the local mechanical properties of rolls, making it prone to form microcracks at such locations, which can result in roll spalling during subsequent service. At present, the analysis of the causes of such grass-like echoes generally recognizes two influencing factors: one is the inhomogeneous grain size and uneven carbide distribution in the forgings, and the other is the presence of microcracks, inclusions and voids inside the forgings.

Prior to roll quenching and tempering, appropriate heat treatment processes can improve the grass-like echo defects of roll blanks. Aiming at the grass-like echo defects detected by ultrasonic testing in 70Cr5Mo steel roll blanks, this paper adopts post-forging heat treatment processes such as normalizing and spheroidizing annealing to refine the microstructure morphology of 70Cr5Mo steel blanks, obtain favorable microstructural features, and reduce or eliminate grass-like echoes in the blanks, so as to ensure no adverse effects on inspection and service performance.

Experimental Material

The chemical composition of 70Cr5Mo steel is listed in the table.

Conclusion

Through normalizing tests at 1070 ℃ and process tests with isothermal holding at 700 ℃ and 770 ℃ for different durations, the following conclusions are drawn:

(1) Owing to its high alloy element content, 70Cr5Mo steel exhibits strong austenite stability after heating. The microstructures obtained by air cooling and oil cooling are basically identical, both consisting of quenched martensite + carbides + retained austenite. Normalizing at high temperature for a short duration helps mitigate the inhomogeneous grain size, resulting in fine and uniform microstructures after normalizing.

(2) For 70Cr5Mo steel, a large amount of alloy carbides exist, which readily act as nucleation sites for spherical carbides during slow cooling after heating. In other words, this material shows a strong tendency to form spheroidized pearlite.

(3) The cooling rate exerts a significant effect on the microstructure during slow cooling after austenitization. Tests indicate that relatively uniform and fine spherical carbides can be obtained at a furnace cooling rate of 70–100 ℃/h.

(4) Severe chemical segregation during the smelting of 70Cr5Mo steel tends to induce local lath-like or flake-like ferrite during subsequent post-forging heat treatment (e.g., isothermal annealing), causing microstructural inhomogeneity and grass-like echoes in surface wave testing. A combined pretreatment process of normalizing + isothermal spheroidizing annealing can effectively improve the grass-like echo behavior of 70Cr5Mo steel. The recommended process is as follows: normalizing—holding at 1070 ℃ for a certain time followed by air cooling → isothermal spheroidizing—holding at 970 ℃ → rapid cooling to 770 ℃ and holding for a certain time → furnace cooling to 500 ℃ then air cooling.