How many of the four processes that make metal stronger do you know?
2023-01-13
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Definition
The solid solution of alloying elements in the base metal causes a certain lattice distortion, thereby increasing the strength of the alloy.
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Principle
Atoms of solutes dissolved in the solid solution cause lattice distortion, which increases the resistance to dislocation movement and makes it difficult to slide, thereby increasing the strength and hardness of the alloy solid solution. This phenomenon of metal hardening due to the dissolution of some dissolved elements to form a solid solution is called solid solution hardening. With an appropriate concentration of dissolved atoms, the strength and hardness of a material can be increased, but its impact strength and ductility decrease.
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Influencing factors
The higher the atomic fraction of dissolved atoms, the greater the strengthening effect, especially when the atomic fraction is very low, the strengthening effect is more significant. The greater the difference in the size of the atoms of the solute and the base metal, the greater the strengthening effect.
Introduced dissolved atoms have a stronger strengthening effect of the solid solution than substituting atoms, and since the lattice distortion of interstitial atoms in body-centered cubic crystals is asymmetric, their strengthening effect is greater than that of face-centered cubic crystals; however, the solubility of the interstitial atoms in the solid state is very limited, so the actual hardening effect is also limited.
The greater the difference in the number of valence electrons between the dissolved atoms and the matrix metal, the more obvious the effect of solid solution hardening, that is, the yield strength of the solid solution increases with increasing concentration of valence electrons.
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Degree of solid solution hardening
Mainly depends on the following factors:
(1) Difference in size of matrix atoms and solute atoms. The greater the difference in size, the more the original crystal structure is disturbed and the more difficult it is for dislocations to slide.
(2) Number of alloying elements. The more alloying elements added, the greater the strengthening effect. If too many too large or too small atoms are added, the solubility will be exceeded. This is due to another hardening mechanism, hardening by the dispersed phase.
(3) Embedded dissolved atoms have a greater strengthening effect on the solid solution than substituting atoms.
(4) The greater the difference in the number of valence electrons between the dissolved atoms and the base metal, the greater the effect of strengthening the solid solution.
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Effect
Higher yield strength, tensile strength and hardness compared to pure metal;
In most cases, ductility is lower than that of pure metal;
The electrical conductivity is much lower than that of puremetals;
Creep resistance, or loss of strength at elevated temperatures, can be improved by solution strengthening.
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Definition
As the degree of cold deformation increases, the strength and hardness of metallic materials increase, but the ductility and toughness decrease.
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Introduction
A phenomenon in which the strength and hardness of metallic materials increase when they are plastically deformed below the recrystallization temperature, while ductility and toughness decrease. Also known as cold hardening. The reason is that during plastic deformation of the metal, the grains slip, the dislocations are entangled, the grains are elongated, broken and fibrous, and residual stresses arise inside the metal. The degree of hardening is usually expressed by the ratio of the microhardness of the surface layer after processing to that before processing and the depth of the hardened layer.
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Explanation in terms of dislocation theory
(1) Intersections occur between dislocations, and the resulting steps prevent the movement of dislocations;
(2) A reaction occurs between dislocations, and the formed immobile dislocations prevent the movement of dislocations;
(3) Dislocations proliferate, and increasing the density of dislocations further increases the resistance to dislocation movement.
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Danger
Strain hardening makes it difficult to further process metal parts. For example, in the process of cold rolling of thick steel, it will be more and more difficult to roll it, therefore, in the process of processing, it is necessary to arrange intermediate annealing, and its work hardening should be eliminated by heating. Another example is to make the surface of the workpiece brittle and hard during cutting, thereby accelerating tool wear and increasing cutting force.
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Benefits
Can improve the strength, hardness and wear resistance of metals, especially pure metals and some alloys that cannot be improved by heat treatment. For example, cold-drawn high-strength steel wire, cold-rolled spring, etc. use cold deformation to improve their strength and elastic limit. Another example is the caterpillars of tanks, tractors, jaws of crushers and turnouts of railways, etc., which also use hardfacing to increase their hardness and wear resistance.
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Role in mechanical engineering
Through processes such as cold drawing, rolling and shot peening (see Surface hardening), the surface strength of metal materials, parts and components can be greatly improved;
After a part is loaded, the local stress in some parts often exceeds the yield strength of the material, causing plastic deformation. Due to work hardening, the constant development ofelastic deformation is limited, which can increase the safety of parts and assemblies;</p >
When a metal part or part is stamped, its plastic deformation is accompanied by hardening, so that the deformation is transferred to the non-hardened part surrounding it. After repeated alternating actions, it is thus possible to obtain cold-formed parts with uniform cross-sectional deformation;
Can improve the cutting performance of mild steel and facilitate chip separation. But work hardening also introduces difficulties in the further processing of metal parts. For example, cold-drawn steel wire, due to hardening, further drawing consumes a lot of energy and even breaks, so it must be annealed in the middle to avoid work hardening before drawing. Another example is that in order to make the surface of the workpiece brittle and hard during the cutting process, the cutting force is increased when re-cutting, and tool wear is accelerated.
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Definition
The method of improving the mechanical properties of metallic materials by grinding grains is called fine-grain hardening. In industry, the strength of materials is increased by grinding grains.
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Principle
Usually, metals are polycrystals, consisting of many grains. The grain size can be expressed as the number of grains per unit volume. The higher the number, the finer the grains. Experiments have shown that fine-grained metals at room temperature have greater strength, hardness, ductility and impact strength than coarse-grained metals. This is because the plastic deformation of small grains under the action of an external force can be distributed over a larger number of grains, the plastic deformation is more uniform, and the stress concentration is less; in addition, the finer the grains, the greater the grain boundary area, and the more tortuous grain boundaries are more unfavorable for crack propagation. Therefore, in industry, the method of increasing the strength of materials by grinding grains is called fine-grained hardening.
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Effect
The finer the grain, the smaller the number of dislocations (n) in the cluster of dislocations, the lower the stress concentration and the higher the strength of the material;
The law of fine-grain hardening, the more grain boundaries, the finer the grains, according to the Hall-Patch relationship, the smaller the average value (d) of the grains, the higher the yield strength of the material.
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Method of grain improvement
Increased hypothermia;
Handling damage;
Vibration and agitation;
For cold-worked metals, grains can be reduced by controlling the degree of deformation and the annealing temperature.
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Definition
SundayWhen used with single-phase alloys, multi-phase alloys have a second phase in addition to the matrix phase. With a uniform distribution of the second phase in the matrix phase in the form of fine particles, a significant strengthening effect will take place. This amplifying effect is called second phase amplification.
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Category
For the movement of dislocations, the second phase contained in the alloy has the following two situations:
(1) Reinforcement with non-deformable particles (bypass mechanism).
(2) Reinforcing action of deformable particles (cutting mechanism).
Both dispersion and dispersion hardening are special cases of the second stage of hardening.
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Effect
The main reason for hardening of the second phase is the interaction between them and dislocations, which prevents the movement of dislocations and increases the deformation resistance of the alloy.
The most important factors affecting strength are the composition, structure and surface condition of the material itself; the second is the stress state, such as the rate of application of the force, the method of loading, whether it be simple stretching or multiple. strength, everything will show differences. In addition, the geometry and size of the sample and the test medium also have a great influence, sometimes even decisive, for example, the tensile strength of ultra-high-strength steel in a hydrogen atmosphere can decrease exponentially.
There are no more than two ways to harden metal materials: increasing the strength of the interatomic bond of the alloy, increasing its theoretical strength and obtaining a solid crystal without defects, such as whiskers. It is known that the strength of iron whiskers is close to the theoretical value, and it is believed that this is due to the fact that NCs do not contain dislocations or contain only a small number of dislocations that cannot propagate during deformation.
Unfortunately, when the diameter of the mustache is larger, the strength drops sharply. Another hardening method is to introduce a large number of crystal defects into the crystal, such as dislocations, point defects, heterogeneous atoms, grain boundaries, fine particles or inhomogeneities (for example, segregation), etc. These defects prevent the movement of dislocations. and also significantly improve the strength of the metal.
This proved to be the most effective way to increase the strength of metals. For engineering materials, as a rule, due to the complex effect of amplification, the best complex performance is achieved.
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