Coarse grains will significantly reduce the performance of forgings and affect their service life. Below we will introduce in detail the causes of coarse grain formation in large forgings.

1. The original steel ingot size of large forgings is large and the crystallization speed is slow, resulting in coarse cast structure. The unit where the author works has forged steel ingots weighing 460 tons, with a maximum size of 3873mm and a length of 4671mm, and the demolding time exceeds 3 days.

2. The forging process takes a long time, and the heating cycle in and out of the furnace is repeated many times, resulting in uneven deformation. The forging cycle of large forgings is generally long and is affected by factors such as forging workload, forging temperature, and operator proficiency. The completion time of different parts of large forgings is not consistent, and the parts that are completed first often need to undergo multiple high-temperature heating processes. At the same time, the temperature difference between the surface and the core of the forging is also large. Although the surface of the forging has reached the final forging temperature, the core is still at a high temperature of more than 1000℃, resulting in coarse and uneven grains in the forging.

3. The austenite grain inheritance phenomenon of some large forging steels is serious. Generally speaking, with the refinement of grains, the yield strength and fatigue strength of forgings are improved, and they also have higher plasticity and impact toughness. The grain size of forgings changes continuously with the heating temperature, forging deformation and the choice of heat treatment method. Refining the grain size requires comprehensive consideration of the entire production process, selection of reasonable process parameters, and effective process control.