How to improve the strength of steel?

The strength of steel refers to the deformation and fracture performance of metal materials under the action of external force, which generally includes tensile strength, bending strength and compressive strength. The more resistant steel is to external forces, the stronger the steel will be. So how can we improve the strength of steel?

Solution Strengthening

The solid solution of alloying elements in the matrix metal causes certain lattice distortion and increases the strength of the alloy. Lattice distortion increases the resistance of dislocation movement and makes it difficult to slip, thus increasing the strength and hardness of the alloy solid solution. This phenomenon of strengthening a metal by dissolving into a solute element to form a solid solution is called solid solution strengthening.

The strength and hardness of the material are increased with the proper concentration of solute atoms, but the toughness and plasticity are decreased. The higher the atomic fraction of solute atom is, the greater the atomic size difference between solute atom and matrix metal is, and the stronger the strengthening is. 

The interstitial solute atoms have a greater solution strengthening effect than the substitutive atoms, and the strengthening effect of interstitial atoms is greater than that of face-centered cubic crystals because the lattice distortion of interstitial atoms in body-centered cubic crystals is asymmetric. However, the solid solubility of interstitial atoms is very limited and the actual strengthening effect is also limited. The larger the difference in the number of valence electrons between the solute atom and the substrate metal is, the more obvious the solution strengthening is, that is, the yield strength of the solid solution increases with the increase in the concentration of valence electrons.

Work Hardening

With the increase of cold deformation, the strength and hardness of metal materials increase, but the plasticity and toughness decrease. Cold work hardening is the phenomenon that the strength and hardness of metal materials increase while the plasticity and toughness decrease during plastic deformation below the recrystallization temperature. Because the metal in the plastic deformation, grain slip, dislocation causes grain elongation, fragmentation and fibrosis, the metal internal residual stress. Work hardening is usually expressed by the ratio of the microhardness of the surface layer after machining and before machining and the depth of the hardening layer.

Work hardening can improve the cutting performance of low carbon steel and make the chip easy to separate, but it brings difficulties to the further machining of metal parts. For example, in the process of the cold-rolled steel plate and cold-drawn steel wire, the energy consumption of drawing is increased and even is broken, so it must be through intermediate annealing to eliminate work hardening. In the cutting process to make the surface of the workpiece brittle and hard, increase the cutting force and accelerate tool weariness, etc.

It improves the strength, hardness and wear resistance of steels, especially for those pure metals and some alloys whose strength cannot be improved by heat treatment. Such as cold drawn high strength steel wire and cold coil spring, is the use of cold processing deformation to improve the strength and elastic limit. The track of tank, tractor, and the turnout of railway also use work hardening to improve its hardness and wear resistance.

Fine-grain Strengthening

The method of improving the mechanical properties of metal by refining grain is called fine grain strengthening. We know that a metal is a polycrystal composed of many grains, and the size of the grains can be expressed by the number of grains per unit volume. The more the number, the finer the grains. The experiments show that the fine grain metal has higher strength, hardness, plasticity and toughness than the coarse grain metal at normal temperature. This is because the fine grains can be dispersed in more grains when plastic deformation occurs under external force, so the plastic deformation is more uniform and the stress concentration is small.

In addition, the finer the grain is, the larger the grain boundary area is, and the more tortuous the grain boundary is, the more disadvantageous the crack propagation is. Therefore, the industrial method to improve the material strength by refining grain is called fine grain strengthening. The more grain boundaries are, the smaller the stress concentration is, and the higher the yield strength of the material is. Methods to refine the grain include: increasing the degree of supercooling;

Metamorphic treatment;

Vibration and agitation;

Cold-deformed metals can be refined by controlling the degree of deformation and annealing temperature.

Second Phase Strengthening

In addition to the matrix phase, the second phase exists in the multiphase alloy compared with the single-phase alloy. When the second phase is distributed uniformly in the matrix phase as finely dispersed particles, the strengthening effect will be significant. This strengthening is called second phase reinforcement. For the dislocation movement, the second phase of the alloy has the following two conditions: (1) reinforcement by an indeformable particle (a bypassing mechanism). (2) The strengthening effect of deformable particles (a cutting mechanism).

The dispersion strengthening and precipitation strengthening both belong to the special cases of the second phase strengthening. The main reason for the strengthening of the second phase is the interaction between them and the dislocation, which hinders the dislocation motion and increases the deformation resistance of the alloy.

In general, the most important thing that affects the strength is the composition of the metal itself, the organizational structure and the surface state, followed by the stress state, such as the speed of the after force, the loading method, the simple stretching or repeated stress, they will show different strength; In addition, the shape and size of the metal and the test medium also have an effect, sometimes even decisive, such as the tensile strength of ultra high strength steels may be reduced exponentially in a hydrogen atmosphere.

There are two main ways to improve the strength: one is to improve the interatomic bonding force of the alloy to improve its theoretical strength, and to produce a complete crystal without defects such as whiskers. The strength of the known iron whiskers is close to the theoretical value, which can be assumed to be due to the lack of dislocations in the whiskers or to the fact that they contain only a small number of dislocations that cannot proliferate during deformation. However, when the diameter of the whisker is large, the strength will decrease sharply. Secondly, a large number of crystal defects are introduced into the crystal, such as dislocation, point defects, heterogeneous atoms, grain boundaries, highly dispersed particles or inhomogeneity (such as segregation), etc. These defects hinder the dislocation movement and significantly improve the metal strength. This proved to be the most effective way to increase the strength of the metal.

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