Microstructure Evolution and Surface Cracking Behavior of Superheavy Forgings during Hot Forging

Abstract EN

In recent years, superheavy forgings that are manufactured from 600 t grade ingots have been applied in the latest generation of nuclear power plants to provide good safety.

However, component production is pushing the limits of the current free-forging industry.

Large initial grain sizes and a low strain rate are the main factors that contribute to the deformation of superheavy forgings during forging.

In this study, 18Mn18Cr0.6N steel with a coarse grain structure was selected as a model material.

Hot compression and hot tension tests were conducted at a strain rate of 10−4·s−1.

The essential nucleation mechanism of the dynamic recrystallization involved low-angle grain boundary formation and subgrain rotation, which was independent of the original high-angle grain boundary bulging and the presence of twins.

Twins were formed during the growth of dynamic recrystallization grains.

The grain refinement was not obvious at 1150°C.

A lowering of the deformation temperature to 1050°C resulted in a fine grain structure; however, the stress increased significantly.

Crack-propagation paths included high-angle grain boundaries, twin boundaries, and the insides of grains, in that order.

For superheavy forging, the ingot should have a larger height and a smaller diameter.