The hazards and influencing factors of overheating defects caused by heat treatment of steel using high-frequency induction heating equipment

The application of high-frequency induction heating equipment is becoming more and more widespread. It can heat treat a variety of workpieces. However, during the operation process, affected by many factors, the workpiece may produce some defects, such as overheating, etc. . Many people don’t know much about overheating defects. Today, the editor will tell you about the dangers of steel overheating and its influencing factors.

1. The essence and harm of steel overheating

The heating temperature of steel is too high, or the holding time at the initial forging temperature is too long, causing the austenite grains to grow rapidly due to aggregation and recrystallization. As a result, the strength of the forging is reduced, and the plasticity and toughness are also deteriorated. This phenomenon It is called overheating of steel.

Overheating can easily cause naphthalene-like fractures, stone-like fractures, or naphthalene-like fracture defects. The naphthalene-like fracture is characterized by fish scale-like bright spots, transgranular fracture, stone-like fracture with obvious coarse crystals, no metallic luster on the surface, gray color, and intergranular fracture.

In addition, if the degree of deformation is small and the final forging temperature is high, abnormal structures will appear during cooling after forging. For example, when overheated hypoeutectoid steel is cooled, the austenite grains decompose to form a Widmanstatten structure. In eutectoid steel, the cementite precipitated during cooling forms a stable network structure. After tool and die steel (or high alloy steel) is overheated, it often presents a network of primary carbides, etc., which all lead to the strength, plasticity and impact of the steel. Reduced toughness. Although the bad structure of forgings caused by overheating of the blank can be eliminated by secondary forging or heat treatment to refine the grains, this increases the production cycle and cost. However, steel types without phase transformation cannot use heat treatment to change the superheated structure, such as chromium-nickel austenitic steel. When heating this type of steel, care should be taken to prevent overheating, especially for high-temperature alloy forgings that have strict requirements on grain size, and should be heated more carefully.

2. Factors affecting steel overheating

The superheating of steel depends on the heating temperature and heating time. In particular, the heating temperature has a great influence on austenite grain coarsening. When using high-frequency induction heating equipment for heat treatment, the higher the temperature, the more serious the large grains will engulf the small grains with the same orientation, so the larger the grains will be. Usually, the temperature at which grains begin to grow rapidly when a metal is heated is called the critical temperature for grain growth. It serves as the upper limit for controlling the initial forging temperature. If it exceeds this temperature, the metal is considered to be overheated. This should be avoided when formulating heating specifications and on-site production operations.

After reading the above description, I believe everyone has a certain understanding of overheating defects. Therefore, during the heat treatment process, we should be very serious and careful to avoid unnecessary defects.