Hardening is a phenomenon in which the strength and hardness of a metal product increases. Changes in properties are achieved through plastic deformation. Metal hardening occurs at a high temperature, the value of which is not sufficient for recrystallization of the workpiece. This phenomenon can be both harmful and beneficial.
Hardening is a technological process that pursues the same goals as cold hardening. The main difference is that the latter phenomenon can occur as a result of conscious or unconscious actions. For example, during machining by cutting with high speed and cutting depth, the surface acquires excess strength, which increases the fragility of the product. Cold hardening is only useful strain hardening, the use of which is deliberate.
Hardening is
Hardening is a conscious technological process that is carried out specifically to improve the strength characteristics of the metal. If hardening of the top layer by plastic deformation is carried out deliberately to increase resistance to cracking and to prevent metal fatigue, then this is cold hardening. Work hardening is also called strain hardening.
Deformation compaction occurred as a result of operation - this is work hardening
What is metal hardening?
Unlike cold hardening, cold hardening is a more capacious concept. The hardening process can have both positive and negative effects. This is any manifestation of strain hardening of crystalline materials - beneficial and harmful, intentional or unintentional. If undesirable harmful hardening occurs, for example, when cutting ductile soft metals and alloys, then parts made of these materials are subject to further heat treatment. As a result of recrystallization, under the influence of high temperature, the undeformed grain structure is restored. Useful hardening is created specifically under the influence of cold plastic deformation. In this case, both terms are appropriate: cold hardening and hardening.
What's the difference when to use?
In technical documentation, including GOSTs, American and international standards (ANSI and ISO), the term peening is absent. “Usefully riveted” metal products are usually called “cold-hardened”. For example, GOST 18907-73 – cold-worked rods. All deformation-hardened aluminum is called cold-worked. The cold-worked state of aluminum is designated by the letter N. GOST 21631 for sheets of aluminum and aluminum alloys: cold-worked sheets (N); semi-hardened (H2).
Hardening is a phenomenon that is aimed at changing properties due to plastic deformation. Hardening of the metal occurs at a temperature whose value is not sufficient for recrystallization of the workpiece. In this case, a change occurs in the internal structure and phase composition of the material. Microscopic defects (dislocations) are formed in the crystal lattice, which emerge on the surface of the deformed product.
Cold hardening is used when it is not possible to harden the surface of the material by hardening and heat treatment. This applies to some aluminum alloys, pure copper and steel with a carbon content of less than 0.25%. Cold-pressing is important for sheet material. As a result of the cold deforming operation (cold rolling, drawing, knurling, shot peening, etc.), dislocations are restored in the crystal structure, which strengthen the metal.
Re-hardening
This is a phenomenon characterized by the destruction of the crystal lattice of a material.
The process is accompanied by peeling and peeling of metal particles, which reduces the performance of the surface. Usually, overhardening occurs when the technological requirements of mechanical processing of products are violated. The reason is excess pressure in the contact area between the tool and the workpiece.
This process is irreversible: the properties of the metal cannot be restored even with the help of heat treatment.
Methods for achieving cold hardening and useful hardening
On an industrial scale, cold hardening is performed using special targeted operations to achieve metal hardening. Also, favorable hardening of the material is achieved as a result of plastic deformation during cold working of the metal by pressure. The first method includes such types of peening as centrifugal ball and shot peening. The latter includes:
- Cold forging. Special equipment is used for it: pneumatic hammers, steam-air hammers, hydraulic presses;
- Cold rolling. This is the most common cold hardening method, which is designed for various long workpieces (rails, pipes, profiles for metal structures). Rolling can be used to produce sheet metal;
- Cold pressing. This type allows you to obtain maximum surface deformation without destruction;
- Cold drawing. This option is intended for cold-pressing large volumes of wire;
- Reduction. This method of cold hardening deserves special attention. We will talk about it in detail in one of the following articles in the “advice from the expert” section.
Softening
A process in which there is a reduction in the stress required for plastic flow of a material. As a rule, this phenomenon can be observed in hardened steels during high-temperature tempering.
Thermal softening parameters depend on the degree of hardening. The negative consequences of this phenomenon must be taken into account in any operations with metal products. For example, car body parts are made by stamping and joined using spot welding, the use of which is local. When performing body repairs, it is necessary to use equipment that has the smallest heat-affected zone. Otherwise, heating above the recrystallization temperature will lead to softening of the element.
Mechanical properties
During the cold hardening process:
- The hardness of the metal product increases;
- Increases resistance to mechanical stress;
- Resistance to dynamic loads and resistance to plastic deformations with the opposite sign decreases (Bauschinger effect);
- Indicators of plasticity, such as relative residual elongation and narrowing, are lost.
Due to this, the yield strength of the metal is reduced - the maximum stress on the product at which it begins to deform plastically. If the degree of load does not exceed the permissible limit, then the metal returns to its original state after the influence of external forces ceases. Stress above the ductility limit is unacceptable for cold-worked steel, which is used in the construction of load-bearing structures of various buildings and structures.
Types of cold hardening
There are two main types of cold hardening, which differ in the processes that occur during its formation in the material. If new phases in the metal, characterized by a different specific volume, are formed as a result of phase changes, then this phenomenon is called phase hardening. If the changes that occurred in the crystal lattice of the metal occurred due to the influence of external forces, they are called strain hardening.
Deformation hardening, in turn, can be centrifugal ball or shot peening. To perform cold hardening of the first type, the surface to be treated is impacted by balls, initially located in the internal slots of a special rim. When the rim rotates (which is performed as close as possible to the surface being processed), the balls, under the influence of centrifugal force, are thrown to its periphery and have an impact on the part. The formation of hardening in shot blasting plants occurs due to the impact on the surface being treated of a flow of pellets moving through the internal chamber of such equipment at a speed of up to 70 m/s. Cast iron, steel or ceramic balls can be used as such pellets, the diameter of which can be 0.4–2 mm, for hardening.
Scheme of traditional strain hardening and graph of increasing material hardness
In order to understand why cold hardening or cold hardening leads to hardening of the metal, you should understand the processes that occur in the material when performing such procedures. During cold plastic deformation, which occurs under the influence of a load whose magnitude exceeds the yield strength of the metal, stresses arise in its internal structure. As a result, the metal will be deformed and will remain in this state even after the load is removed. The yield strength will become higher, and its value will correspond to the magnitude of the stresses formed in the material. To deform such a metal again, it will be necessary to apply much greater force. Thus, the metal will become stronger or, as experts say, will go into a hard-worked state.
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During cold deformation of the metal, which occurs as a result of the action of appropriate pressure (in the process, for example, cold hardening), the dislocations that make up the internal structure of the material begin to move. Even one pair of moving defect lines formed in a crystal lattice can lead to the formation of more and more similar locations, which ultimately increases the yield strength of the material.
Change in the structure of the surface layer as a result of cold deformation
The internal structure of the metal, when it is deformed during cold hardening or hardening, undergoes serious changes. In particular, the configuration of the crystal lattice is distorted, and the spatial position of crystals, which are randomly oriented, is ordered. This ordering leads to the fact that the axes of the crystals, in which they have maximum strength, are located along the direction of deformation. The more active the deformation is, the more crystals will take on a similar spatial position. There is a misconception that the grains that make up the internal structure of the metal are crushed when it is deformed. In fact, they are only deformed, and their surface area remains unchanged.
From all of the above, we can conclude that during the process of cold hardening or cold hardening, the crystal structure of steel or other metal changes, as a result, the material becomes harder and stronger, but at the same time more brittle. Work-worked steel is therefore a material that has been specifically subjected to plastic deformation to improve its strength characteristics.
Wrought aluminum alloys
Depending on the chemical composition, deformable aluminum alloys are divided into several groups, and based on methods for increasing mechanical properties - into two categories:
- Strain-hardening alloys. A lot of deformable aluminum alloys improve their properties by cold hardening (working hardening). These include alloys of aluminum with manganese, and aluminum with manganese and magnesium. Strengthening is achieved due to plastic deformation of the product. Plastic deformation (hardening) occurs together with various annealing options to achieve the required characteristics. Such alloys are called strain-hardening, as well as thermally non-hardening. They have good strength, weldability, ductility, and corrosion resistance.
- Thermally hardenable alloys. They change their composition and acquire high mechanical properties only after heat treatment, which includes: intense heating for hardening, rapid cooling to room temperature, compaction by aging. Alloys that are thermally hardened can be either wrought or cast.
Why is aluminum ductile?
The ability to undergo large plastic deformations is one of the most useful properties of metals. Metals with a face-centered cubic lattice, which includes aluminum, usually exhibit good ductility - they can be easily deformed into various complex shapes. Typically metals are made up of large numbers of individual grains or crystals, meaning they are polycrystalline. A typical grain or crystal of aluminum after hot and cold working and then annealing has a diameter of, say, 40 microns, and the unit cell of the atomic crystal lattice is only about 0.4 nm = 0.0004 microns. So each grain contains many millions of such elementary cells - about 10 15 pieces.
Where are cold-worked products used?
Metal parts with strain hardening have excellent performance characteristics. They are indispensable in structures that are subject to significant mechanical loads, adverse effects (sudden temperature changes, pressure fluctuations, etc.) Cold-worked steel is used in various industries: mechanical engineering, construction, shipbuilding, the automotive industry, etc. Cold-worked aluminum sheets (AMtsN) and (AMtsN2) are used for ship cladding, in the creation of building structures, in food production, and in the automotive industry for the manufacture of radiators, frames, and rivets. Today, thanks to its strength properties and the ability to significantly increase the efficiency of metalworking, cold hardening has a wide range of applications. Hardening of metal is an effective way to strengthen it.
Equipment used
The procedure for surface strain hardening is used in various industries that place increased demands on surface resistance to cracking.
There is a wide selection of metal hardening equipment. Dimensions and technical characteristics depend on the size of the processed products and production volumes.
Useful hardening on an industrial scale is performed on machines with a high level of automation. Shot blasting machines are mainly used.
Temperature tempering of the metal is used to remove cold hardening. This helps to activate processes that return the material to its original state.
Metal hardening is a process that, along with cold hardening, is actively used in the manufacture of components and assemblies in various industries. Have you encountered strain hardening of a surface? Do you think the cold hardening process will be in demand in the foreseeable future? Write your opinion in the comments block.
