normalizing process of steel

normalizing process of steel

一、Normalizing, also known as normalising

It is a heat treatment process that involves heating hypoeutectoid steel to a temperature 30-50°C above Ac3, and hypereutectoid steel to a temperature 30-50°C above Accm, followed by air cooling after holding at that temperature.

二、Purposes of Normalizing

① As a preparatory heat treatment for low and medium carbon structural steels, it can appropriately increase hardness and improve machinability.

② It eliminates the network-like secondary cementite in hypereutectoid steels and prepares the structure for spheroidizing annealing.

③ Normalizing can refine grains, homogenize the structure, and eliminate internal stress. To a certain extent, it also has the effect of improving strength, toughness and hardness, so it can be used as the final heat treatment for ordinary structural parts.

Microstructure after Normalizing: Hypoeutectoid steel consists of F + S, eutectoid steel consists of S, and hypereutectoid steel consists of S + secondary cementite (with the latter being discontinuous).

It is worth noting that for some steels with a very low critical cooling rate, cooling in air can transform austenite into martensite. This treatment is not classified as normalizing but is called air-cooled quenching. On the contrary, for large-section workpieces made of steels with a relatively high critical cooling rate, even quenching in water cannot produce martensite, and the quenching effect is close to that of normalizing.

三、Precautions for Normalizing

1. Strictly control the heating temperature to avoid excessive grain growth caused by overheating or insufficient microstructural transformation due to low temperature.
2. Ensure sufficient holding time to guarantee full and uniform microstructural transformation inside the workpiece.
3. The cooling process must be carried out in still air, avoiding interference such as blowing wind or rain to prevent abnormal cooling rates.
4. Large-section workpieces should be stacked properly to avoid mutual shielding between workpieces and ensure uniform cooling of all parts.