The 72 h salt spray corrosion surface morphology of 20mncr5 steel, an automobile gear test material after normalizing and isothermal annealing, is shown in Figure. It can be seen that after 72 h salt spray corrosion, a large number of corrosion points appear on the surface of 20mncr5 steel, the automotive gear test material after normalizing treatment, and the corrosion points show signs of expanding to the surrounding. At the same time, there are a small amount of white corrosion products, and the corrosion degree is relatively heavy.
The corrosion point of the sample after isothermal annealing treatment is very few and the size is much smaller than that after normalizing. At the same time, no white corrosion products are found and the corrosion degree is light, indicating that the salt spray corrosion resistance of 20mncr5 steel for automobile gear treated by isothermal annealing is strong and far better than that treated by normalizing.
When the automobile gear test material 20mncr5 steel is normalized, the structure transformation of the steel is carried out in the process of continuous cooling, resulting in uneven structure; During isothermal annealing, the isothermal temperature is generally 560 ~ 650 ℃, which makes the structure uniform after heat treatment. This is also the reason why the corrosion resistance of the material in NaCl solution after isothermal annealing is much better than that of normalizing treatment.