Coarse grains in Forged steel microstructure and possible remedies

Coarse grain refers to larger than desirable grain size present in the metal's microstructure. This is usually discovered in the initial ultrasonic non-destructive testing of the steel bar, as large grains cause ultrasonic waves to scatter.

The presence of coarse grains reduces the performance of the steel as it lowers the strength of the steel and induces a cracking tendency upon heat treatment. Therefore, coarse grain must be treated before the final application of the steel.

What causes coarse grain to develop?

Excessively high temperatures while forging, insufficient reduction ratios, or improper cooling rates post forging may lead to coarse-grain formation. An incorrect post-forging annealing process can also play a role in this, as improper cooling may lead to abnormal grain growth.

What are possible remedies?

The most common method used to treat coarse grains is normalizing the steel. This is a method where controlled cooling of steel is carried out in air after heating it 30-50°C above the critical point to homogenize the microstructure. Faster cooling generally produces finer grains as rapid cooling limits the time for grain growth, locking in a finer structure.

Steel may also be reforged to a smaller size to achieve a greater overall reduction ratio.

Grain refining elements such as aluminum, titanium, vanadium, etc., may also be added to the alloy to achieve a more refined microstructure.