The strengthening of metal means increasing the strength of the metal material by means of alloying, plastic deformation, heat treatment, and the like. The actual strength of a metal is only a few tenths of the theoretical strength, or even a few thousandth. In order to improve the strength of the metal, commonly used strengthening methods include deformation strengthening, solid solution strengthening, second phase strengthening, and precipitation strengthening.
Typical metal material tensile curve
1. Deformation strengthening
As the degree of deformation increases, the strength and hardness of the material increase, and the phenomenon of decreased plasticity and toughness is called deformation strengthening or work hardening.
As the plastic deformation progresses, the dislocation density increases, resulting in an increase in interaction during dislocation motion, an increase in dislocation motion resistance, and an increase in deformation resistance, thereby increasing the strength of the metal.
The degree of deformation increases and the dislocation density increases continuously. According to the formula Δσ=αbGρ1/2, the intensity is proportional to the one-half of the dislocation density (ρ). The larger the Bob’s vector (b) of the dislocation, the more significant the strengthening effect. .
It is usually strengthened by cold deformation (extrusion, rolling, shot peening, etc.). Deformation strengthening is an effective method to strengthen metals, especially for materials that cannot be strengthened by heat treatment; it can also make the metal evenly deformed and improve the safety of parts or components during use.
Deformation strengthening also causes troubles in the production and use of materials. The deformation increases the strength and plasticity, and requires recrystallization annealing to increase the production cost.
Dislocation plugging diagram
2, solid solution strengthening
The essence of solid solution strengthening is to dissolve the alloying elements into the matrix phase to form a solid solution. The internal lattice distortion caused by the difference in atomic radius and the lattice change between the two causes the strength and hardness of the metal to increase, and the plasticity and toughness decrease.
The mechanism of solid solution strengthening is that the solute atoms distorted the lattice of the solid solution and hindered the dislocations on the slip surface. Second, the Kodak air mass formed by the solute atoms that were segregated on the dislocation line. The pinning effect increases the resistance of the dislocation motion; the third is that the segregation of the solute atoms in the stacking fault zone hinders the movement of the extended dislocation.
In the range of solid solution solubility, the larger the mass fraction of alloying elements, the greater the strengthening effect; the larger the difference between the size of solute atoms and solvent atoms, the more significant the strengthening effect; the strengthening effect of solute elements forming interstitial solid solution is greater than that of forming solid solution Element; the greater the difference in the valence electron number between the solute atom and the solvent atom, the greater the strengthening effect.
The method commonly used for solid solution strengthening is alloying, that is, adding alloying elements.
Solid solution schematic
3. Second phase enhancement
Second phase strengthening generally refers to the various compound particles. By dispersing the second phase particles by various means, the dislocation motion inside the alloy can be hindered, thereby improving the yield strength and the tensile strength. The alloys currently used in the industry are mostly multiphase or multiphase alloys whose microstructure is such that a second phase (excess phase) is distributed on the solid solution matrix.
There are three main forms of the second phase in the steel, namely, mesh, flake and granular. The mesh shape, especially the continuous network Fe3C precipitated along the grain boundary, reduces the mechanical properties of the steel, the plasticity and toughness drop sharply, and the strength also decreases.
When the second phase is in the form of a sheet, the smaller the interlayer spacing, the higher the strength, and the better the ductility and toughness. When the second phase is a granular distribution, the finer the particles, the more uniform the distribution, the higher the strength of the alloy, and the greater the number of second phases, the greater the damage to plasticity;
When precipitated along the grain boundary, the grain boundary strength is lowered regardless of the form, and the mechanical properties of the steel are lowered. The second phase prevents the movement of dislocations, whether flake or granular.
The second phase strengthening method is usually to add alloying elements and then heat treat or plastically process the morphology and distribution of the second phase.
4, Fine grain strengthening
Fine grain strengthening: As the grain size decreases, the strength and hardness of the material increase, and the phenomenon that the plasticity and toughness are improved is called fine grain strengthening. Refining the grains can simultaneously improve the strength and improve the ductility of the steel, and is a better method for strengthening the material.
The finer the grain of the alloy, the greater the number of internal grains and grain boundaries. Fine grain strengthening strengthens the material by utilizing the irregularity of the arrangement of atoms on the grain boundary and the high atomic energy. According to the Hall-match relationship, the smaller the average diameter of the grains, the higher the yield strength of the material.
The methods for refining crystal grains mainly include: increasing the degree of subcooling during the crystallization process, changing the quality, treating the vibration and stirring, increasing the nucleation rate and refining the grains. The cold deformed metal refines the grains by controlling the degree of deformation and the annealing temperature. The grain is refined by a normalizing and annealing heat treatment method; a strong carbide is formed into the steel to form an element or the like.
When the grain size is smaller than the critical dimension dc, an inverse Hall-Pecch phenomenon occurs, that is, the intensity decreases as the grain size decreases.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)