Four processes to make metal materials stronger,Let’s take a look with the titanium tube manufacturer.
1. Solid solution strengthening
definition
A phenomenon in which alloying elements dissolve in the matrix metal, resulting in a certain degree of lattice distortion and thus an increase in alloy strength.
principle
The solute atoms dissolved in the solid solution cause lattice distortion, which 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 incorporating certain solute elements to form a solid solution is called solid solution strengthening. When the concentration of solute atoms is appropriate, the strength and hardness of the material can be increased, but the toughness and plasticity of the material are decreased.
Influencing factor
The higher the atomic fraction of the solute atom, the greater the strengthening effect, especially when the atomic fraction is very low. The larger the size difference between the solute atom and the base metal, the greater the strengthening effect.
Interstitial solute atoms have greater strengthening effect than replacement atoms, and because the lattice distortion of interstitial atoms in body-centered cubic crystals is asymmetric, its strengthening effect is greater than that of face-centered cubic crystals. However, the solid solubility of interstitial atoms is very limited, so the actual strengthening effect is also limited.
The larger the difference in the number of valence electrons between the solute atoms and the base metal, the more obvious the effect of solid solution strengthening, that is, the yield strength of the solid solution increases with the increase of valence electron concentration.
The degree of solution strengthening
Mainly depends on the following factors:
(1) Size difference between matrix atoms and solute atoms. The larger the size difference, the more disturbed the original crystal structure will be, and the more difficult dislocation slip will be.
(2) alloying element quantity. The more alloying elements added, the greater the strengthening effect. If you add too many atoms that are too big or too small, the solubility will be exceeded. This brings us to another reinforcement mechanism, dispersed phase reinforcement.
(3) Interstitial solute atoms have greater solution strengthening effect than replacement atoms.
(4) The larger the difference in valence electron number between solute atom and matrix metal, the more significant the strengthening effect of solid solution.
effect
Yield strength, tensile strength and hardness are stronger than pure metal;
In most cases, the ductility is lower than that of pure metals;
Conductivity is much lower than pure metal;
Creep resistance, or strength loss at high temperatures, can be improved by solid solution strengthening.
2. Work hardening
definition
With the increase of cold deformation degree, the strength and hardness of metal materials increased, but the plasticity and toughness decreased.
introduction
The phenomenon in which the strength and hardness of a metal material increase while the plasticity and toughness decrease during plastic deformation below the recrystallization temperature. Also known as cold hardening. The reason is that when the metal is in plastic deformation, the grain slips, the entanglement of dislocation occurs, so that the grain elongates, breaks and fibrosis, and the residual stress is generated inside the metal. The degree of work hardening is usually expressed by the ratio of microhardness of the surface layer after machining to that before machining and the depth of the hardening layer.
Explain from the perspective of dislocation theory
(1) Cross-cutting occurs between dislocations, resulting in cutting steps that hinder dislocation motion;
(2) Reactions occur between dislocations, forming fixed dislocations that hinder dislocation motion;
(3) Dislocation proliferation occurs, and the displacement resistance increases further with the increase of dislocation density.
harm
Work hardening makes further processing of metal parts difficult. For example, in the process of cold rolling steel plate will be rolled harder so that rolling does not move, so it is necessary to arrange intermediate annealing in the process of processing, by heating to eliminate its work hardening. For example, the workpiece surface is brittle and hard in the cutting process, thus accelerating tool wear and increasing cutting force.
benefit
It can improve the strength, hardness and wear resistance of metals, especially for those pure metals and some alloys that 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 its strength and elastic limits. For example, tank, tractor track, crusher jaw plate and railway turnout are also the use of work hardening to improve its hardness and wear resistance.
In mechanical engineering
After cold drawing, rolling and shot peening (see surface strengthening), the surface strength of metal materials, parts and components can be significantly improved;
After the parts are stressed, the local stress of some parts often exceeds the yield limit of the material, causing plastic deformation. Because the work hardening limits the further development of plastic deformation, it can improve the safety of parts and components.
When a metal part or member is pressed, its plastic deformation is accompanied by strengthening, causing the deformation to be transferred to its surrounding unworked hardened part. The cold stamping parts with uniform cross section deformation can be obtained through such repeated alternating action;
It can improve the machinability of low carbon steel and make the chips easy to separate. But work hardening also brings difficulties to the further processing of metal parts. Such as cold drawing steel wire, due to work hardening to further drawing energy dissipation, even broken, so must be annealing, eliminate work hardening after drawing. For example, in the cutting process to make the workpiece surface brittle and hard, increase the cutting force when cutting, accelerate tool wear.
3, fine crystal strengthening
definition
The method of refining grain to improve the mechanical properties of metal materials is called fine grain strengthening, which is used in industry to improve the strength of materials by refining grain.
principle
Usually a metal is a polycrystal composed of many grains, the size of the grain can be expressed by the number of grains per unit volume, the more the number, the finer the grain. The results show that fine grained metal has higher strength, hardness, plasticity and toughness than coarse grained metal at normal temperature. This is because the fine grains can be dispersed in more grains due to plastic deformation caused by external forces. The plastic deformation is more uniform and the stress concentration is smaller. In addition, the finer the grain size, the larger the grain boundary area, the more tortuous the grain boundary, the more unfavorable to crack propagation. Therefore, the method of refining grain to improve material strength is called fine grain strengthening in industry.
effect
The smaller the grain size, the smaller the number of dislocation (n), the smaller the stress concentration and the higher the strength of the material.
Strengthening law of fine grain strengthening: the more grain boundary, the finer grain, according to the Hall – Pecchi relation, the smaller the average grain (d), the higher the yield strength of the material.
Method of refining grain
Increase the degree of supercooling;
Metamorphic treatment;
Vibration and agitation;
For cold deformed metals, grain can be refined by controlling deformation degree and annealing temperature.
4. The second phase of reinforcement
definition
Compared with single-phase alloys, there is a second phase in addition to the matrix phase. When the second phase is evenly distributed in the matrix phase as fine dispersed particles, the strengthening effect will be significant. This reinforcement is called second phase reinforcement.
classification
For the motion of the dislocation, the second phase contained in the alloy has the following two conditions:
(1) Strengthening effect of non-deformable particles (bypass mechanism).
(2) Strengthening effect of deformable particles (cutting mechanism).
Dispersion strengthening and precipitation strengthening are special cases of second phase strengthening.
effect
The main reason of the second phase strengthening is their interaction with dislocation, which hinders the dislocation movement and improves the deformation resistance of the alloy.
summarize
The most important factors affecting the strength are the composition, structure and surface state of the material itself. Secondly, the stress state, such as the speed of adding force, loading mode, simple tensile or repeated force, will show different strength; In addition, the sample geometry and size and the test medium also have great influence, sometimes even decisive, for example, the tensile strength of ultra-high strength steel in hydrogen atmosphere may be reduced exponentially.
There are only two ways to strengthen metal materials. One is to improve the interatomic binding force of the alloy, improve its theoretical strength, and make a complete crystal without defects, such as whiskers. Whisker strength of iron is known to be close to the theoretical value, which can be attributed to the absence of dislocations in the whisker, or to the inclusion of only a few dislocations that cannot proliferate during deformation. Unfortunately, when the diameter of the whisker is larger, the strength drops sharply. Another strengthening approach is to introduce a large number of crystal defects into the crystal, such as dislocation, point defects, heterogeneous atoms, grain boundaries, highly dispersed particles or inhomogeneity (such as segregation), which hinder the dislocation movement and significantly improve the strength of the metal. This has proved to be the most effective way to strengthen metals. For engineering materials, it is generally through comprehensive strengthening effect to achieve better comprehensive performance.