Cast iron heat treatment can mainly be divided into the following types:
(1) Stress relief annealing heat treatment; (2) Graphitizing heat treatment; (3) Heat treatment for changing the matrix structure. This chapter briefly introduces the theoretical basis and process characteristics of the above heat treatment processes.

Section 1: Stress relief annealing heat treatment
Stress relief annealing is to hold the casting at a certain temperature and then cool it slowly to eliminate the casting residual stress in the casting. For gray cast iron, stress relief annealing can stabilize the geometric dimensions of the casting and reduce deformation after machining. For white cast iron, stress relief annealing can prevent the casting from deforming or even cracking due to vibration or environmental changes during storage, transportation, and use.

I. Generation of casting residual stress
During solidification and subsequent cooling, the casting undergoes volume contraction or expansion. This volume change is often constrained by external factors and between different parts of the casting itself, and cannot proceed freely, thus generating casting stress. If the casting stress disappears after the cause of the stress is eliminated, this stress is called temporary casting stress. If the casting stress still remains after the cause is eliminated, this stress is called casting residual stress.

During solidification and subsequent cooling, due to different wall thicknesses and cooling conditions, the temperature and degree of phase transformation vary in different parts of the casting, resulting in different volume changes in each part. If the casting alloy is already in an elastic state at this time, mutual constraints will occur between the various parts of the casting. Casting residual stress is often the stress generated by these differences in temperature and degree of phase transformation.

II. Theoretical basis of stress relief annealing
Research shows that casting residual stress is proportional to the temperature difference between parts during cooling and the elastic modulus of the casting alloy. For a long time in the past, it was believed that casting alloys have an elastic-plastic transition temperature during cooling, and that the elastic-plastic transition temperature of cast iron was around 400°C. Based on this understanding, the heating temperature for stress relief annealing should be 400°C. However, practice has shown that this heating temperature is not ideal. Recent research indicates that alloy materials do not have an elastic-plastic transition temperature; even alloys in a solid-liquid coexistence state still possess elasticity.