When steel is heated using medium frequency induction heating power, its austenite grains grow.

Many people have asked our technical staff how the structure of steel changes when it is heat treated using medium frequency induction heating power. To be honest, I don't know much about it. I only figured it out after asking our technical staff. Today, the editor will tell you how the austenite grains change when steel is heated by an intermediate frequency heating power supply.

The steel is heated using a medium frequency induction heating power supply. When heated to a temperature higher than A1, austenite nucleates at the ferrite-carbide boundary. At this time, the nucleation rate of austenite is always very high, and the initially formed austenite grains are very fine.

If the temperature is further increased or maintained at this temperature for a long time, the austenite grains will grow. Corresponding to the thermodynamic trend of reducing free energy in the entire system, the surface area of the grains is reduced.。

The mechanism of grain growth is the migration of large-angle boundaries. Therefore, grain growth is controlled by the diffusion channels of atoms through large-angle boundaries.

The austenite grain size formed at a certain temperature will of course not change after cooling. Steel of the same grade will have different grain growth tendencies under different smelting conditions. The grain growth tendency of steel is divided into two types: intrinsically fine-grained and intrinsically coarse-grained steel. The grain growth of intrinsically fine-grained steel is not obvious when heated to 950-1000°C, but will grow violently at higher temperatures. On the contrary, for intrinsically coarse-grained steel, the grains grow rapidly at a temperature slightly higher than Ac1.

The grain growth tendency of steel depends on the chemical composition of the steel and the deoxidation conditions from a metallurgical perspective. Aluminum deoxidized steel is an intrinsically fine-grained steel. The AIN particles formed in the steel hinder the growth of austenite grains. But after these particles are dissolved (>1000-1050℃), the crystal grains will grow rapidly. In the Ac1-Accm temperature range of hypereutectoid steel, the growth of austenite grains is restricted by undissolved carbide particles. In hypoeutectoid steel, the grain growth of austenite in the Ac1-Ac3 temperature range is hindered by ferrite.

In hypoeutectoid steel, as the carbon content increases, the tendency of grain growth increases. In hypereutectoid steel, the tendency of grain growth is reduced due to the hindrance of residual cementite.

Alloying elements, especially carbide-forming elements (the most influential ones are Ti, V, Zr, Nb, W and Mo) hinder austenite grain growth. This is due to the formation of alloy carbides that are insoluble in austenite and hinder grain growth. The two elements that have a greater impact are Ti and V. After elements such as Mn, P, and S are dissolved into austenite, they can accelerate the diffusion of iron atoms and promote the growth of austenite grains.

The original structure and heating conditions of the steel will also affect the austenite crystal. The smaller the interlaminar spacing of flaky pearlite, the greater the austenite nucleation rate and the finer the initial grains. The flaky pearlite structure is coarser than the starting grains of austenite formed by the spherical structure. The reason is that a large number of grains with the same orientation are formed on the surface of flaky cementite, and when they grow up, they easily combine with each other to form a large grain. When the heating temperature is significantly higher than the critical temperature, the grains gradually grow and the influence of the original structure gradually disappears.

The austenite grain size continues to grow as the heating temperature increases or the holding time increases. At each temperature, there is a stage in which grains accelerate to grow. When they reach a certain size, the growth trend gradually weakens. The greater the heating rate, the shorter the residence time of austenite at high temperature and the finer the grains.

Understanding the structural changes of the workpiece when heated is of great help to our heat treatment operations. This article briefly introduces the phenomenon of austenite grain growth when steel is heated. If you want to know about changes in other structures, you can consult our technical staff.