Refinement of microstructure and grain during heat treatment of steel

Grain refinement has always been an effective method to improve the strength and toughness of metal materials in the field of metal materials. The metal structure is composed of many grains. The more grains per unit area, the finer the grains. On the contrary, the coarser the grains. When plastic deformation occurs under the action of external stress, If the grains are very small and there are many grains per unit area, the stress can be shared to each lattice, so that the stress will not be very concentrated, and the larger the grain boundary area of each grain is, the more tortuous it is, and it is not easy to produce crack propagation. In this way, the “coarse” tissue is much worse than the “fine” tissue at room temperature. Hall Petch formula can explain this conclusion.

In the formula σ S is the yield strength, σ 0 represents the lattice slip stress produced by the solute and the second phase, K is the strengthening parameter, and D is the effective grain size. The microstructure of 2Cr13 after heat treatment is tempered sorbite or martensite, so its effective grain size is these two grain sizes.

Some studies have shown that the size of ferrite has a direct impact on strength and toughness. Insoluble ferrite in steel is dispersed in the form of fine particles, which effectively inhibits the growth of grains and refines the grains. There are several methods:

  1. Increase the undercooling degree: the nucleation rate and grain growth rate will change with each other. The change rates are different. When the undercooling degree becomes larger, the grains become smaller.
  2. Modification: adding nucleation modifier and adsorption modifier to change the grain growth.
  3. Vibration and stirring.
  4. For cold deformed metals, it is realized by changing the annealing temperature and controlling the degree of deformation.