What is Heat treatment and How does it works?
How Does Heat Treatment of Metals Work?
Heat treatment of metals (heat treatment) is a set of operations of heating, holding and cooling metals and metal alloys.
Heat treatments are carried out in order to change the structure and, accordingly, the mechanical properties of metals and alloys in a given direction. Heat treatment is used to purposefully change the structure of the heat treated materials, namely, the phase composition and redistribution of components, the size and shape of crystalline grains, the type of defects, their number and distribution, which ultimately makes it quite easy to obtain the required properties of materials.
Role of External Factors During Heat Treatment
It should be noted that a change in the structure and, accordingly, such properties are possible not only under the influence of heat treatment, but also under the influence of other external factors, for example, during chemical, mechanical, radiation, electromechanical and other types of heat treatment processes.
It is important to remember that the desired properties of metals and alloys depend not only on their structure, but also on their chemical composition, which is formed during metallurgical and foundry processes.
What will be the influence of the chemical composition During Heat Treatment?
During heat treatment, the chemical composition remains unchanged, only sometimes, with an incorrectly chosen composition of the protective medium and the temperature-time parameters of heat treatment, the chemical composition on the surface of workpieces and products changes as a result of the heat treatment processes of decarburization, carburization or oxidation. In this article, the influence of the chemical composition on the change in various properties of metals and alloys is not considered.
The tasks of heat treatment are the elimination of internal stresses in heat treated metals and alloys, the improvement of machinability by cutting or pressure, the increase in mechanical properties and operational properties, etc.
Heat treatment is applied to blanks, semi-finished products and finished products. The range of products is extremely wide – from fasteners to the largest castings and forgings of static and dynamic petrochemical equipment.
Heat Treatment Process Steps
Heat treatment is applied to steels, cast irons and alloys based on non-ferrous metals. So, for example, in chemical engineering, the main share of proper heat treatment process will be when steels is made up of carbon steels St3, steel 20, etc. (> 50%), the share of low-alloyed steels such as 09G2S, 16GS, 12KhM is ~ 20%, and corrosion-resistant steels – 26-28%. Among cast iron, ductile and ductile cast irons are most often used. The most common non-ferrous alloys are those based on nickel, titanium, aluminum and copper.
It must be understood that for the same material, i.e. material with the same chemical composition, due to the implementation of various heat treatment modes, it is possible to obtain several different structures with completely different properties.
Improvement of mechanical properties with the help of heat treatment makes it possible to use alloys of simpler compositions more widely. Heat treatment can increase the allowable Stress relieving, reduce the weight of heat treated parts and mechanical properties, increase their reliability and durability.
Heat treatment of metals and alloys is usually carried out in cases where:
Polymorphic transformations in metal;
limited and variable (increasing with temperature) solid state solubility of one component in another;
change in the structure under the influence of cold plastic deformation.
The modern classification of types of heat treatment is based on the concept of phase and structural changes in the alloy.
Heat treatment is subdivided into thermal, thermomechanical and chemo-thermal.
Actually heat treatment includes the following main types: annealing of the 1st kind, annealing of the 2nd kind, Age Hardening, tempering. Normalization falls out of the general classification somewhat, because. has features when applied to steels of varying degrees of alloying.
What materials can be heat treated?
The presented types of heat treatment of metals are not suitable for various types of alloys and non-ferrous metals. For example, when working with copper, recrystallization annealing is carried out. Bronze heat treatment up to 550 degrees. They work with brass at 200 degrees. Aluminum is initially hardened, then annealed and aged.
Heat treatment of metal is considered a necessary heat treatment processes in the manufacture and further use of structures and parts for industrial equipment, machines, aircraft, ships and other equipment. The material becomes stronger, more durable and more resistant to corrosion processes. The choice of heat treatment process depends on the metal or alloy used.
The types of Heat Treatment
Steel tempering is often used in mechanical engineering, as well as in the manufacture of heat treated parts for various purposes from steel billets. Usually used with hardening because it helps to reduce the internal stress of the material. This makes the raw material much stronger, removes the brittleness that may appear when exposed to elevated temperatures.
Another purpose of the application is to increase the impact strength. The material becomes less rigid, which means that it will be difficult to damage it with some strong external mechanical properties.
The tempering technology is divided into three types:
1. Short. The technology is used to create a martensitic metal structure. The main goal is to significantly increase the viscosity of the raw material while maintaining its hardness.
The maximum Heat treating temperature is up to 250 °C. Usually it is not more than 150 °C. With such heat treatment, the steel will need to be kept for about an hour and a half. Cooling is carried out inside oil or air, which also helps to harden the workpiece or finished product.
Most often, low tempering is used when creating a measuring tool or various types of cutting products.
2. Average. The difference lies in the increase in the maximum temperature to 500 °C. Usually parts are processed at Heat treating up to 340 °C. Air cooling is used.
The main task of middle tempering is to transform martensite into troostite. This provides an increase in viscosity against the background of a decrease in hardness. The technology is useful if it is planned to produce heat treated parts that operate under heavy loads.
3. High. One of the most successful means to reduce high levels of internal tension. The product is heated to high temperatures, which helps to create and increase viscosity and plasticity without loss of strength. Although the heat treatment techniques is difficult to use for critical parts, it is optimal. Heating range – 450-650°С.
The method is used to stabilize the internal structure of the material and increase its homogeneity. It also helps to greatly reduce stress levels. The technological heat treatment process involves heating to high temperatures, holding and long, slow cooling. There are several main approaches used in industry:
- It is also called diffusion annealing. This is a process of heat treatment of steel in the temperature range from 1000 to 1150 °C. In this state, the raw material is kept for 8 hours. For some steel grades, the time is increased to 15. The cooling temperature is controlled. The workpiece can be pulled out of the furnace only when it reaches 800°C. Further, the temperature naturally decreases in the air.
- This is the low annealing required after deformation. The main task is to make the material much stronger by changing the shape of the grain in the internal structure. The temperature range is 100-200 °C. Compared to homogenization, the exposure time is greatly reduced – up to two hours. Slow cooling takes place inside the oven.
- Isometric impact. Suitable for alloy steels only. In the created state, austenite gradually disintegrates. The temperature depends on the natural maximum for a particular metal grade. The limit must be exceeded by 20-30°C. Cooling depends on two stages – fast and slow.
- Internal and Stress relieving. The technique is suitable after the part is machined, welded or processed using casting. The maximum Heat treating temperature is 727°C. This heat treatment process has the longest holding time of any annealing process at 20 hours. The workpiece will cool very slowly.
- If you need to achieve a fine-grained material structure with a predominance of pearlite and ferrite. The heat treatment techniques are suitable for different types of blanks – from stamped and cast to forged. The heat treatment here is the same as for isometric annealing – heat treatment is performed up to the melting point and another 30-50°C above it. Cooling is carried out up to 500°C. The secret of a quality operation is to control cooling rates. It is indicated on the basis of 60 minutes. For carbon steel, cooling should be less than 150°C, and for alloy steel – 50°C.
- The main task of carrying out incomplete annealing is the transfer of pearlite into a ferrite-cementite structure. The technology is suitable for parts that were created by electric arc welding. In this case, the temperature is 700°C, and the holding time is 20 hours. After slow cooling, the workpiece can be used – its strength and protection against damage will increase significantly.
How Hardening reduce the compressive strength and ductility?
Quenching and tempering of steel are among the most common heat treatment modes.
This exposure option is needed to increase important material indicators – from hardness and maximum elasticity to wear protection and surface hardness. With the help of hardening, it is possible to reduce the compressive strength and ductility.
This heat treatment process format is one of the oldest. It is based on the rapid cooling of the metal heat treatment to high temperatures. The Heat treating limit differs depending on the type of alloy. It is necessary to take into account at what temperature the internal crystal lattice begins to change.
Depending on the grade of steel, several basic parameters change:
- Cooling medium. The easiest way is to dip in water. Additional useful mechanical properties make it possible to obtain the use of technical oils, gases of an inert type and solid solution with a high salt content.
- cooling rate. It varies depending on the initial degree of warming up. The temperature of the water, brine or gas may also vary.
- heat treatment. Selected depending on the limits needed to change the internal structure. For many types of raw materials, this figure is about 900°C.
Why Normalization is suitable for both alloy and low carbon steels?
The normalization process is necessary in order to change the structure and create a fine grain inside the metal. This option is suitable for both alloy and low carbon steels.
The main advantage of the technology allows you to bring the surface hardness up to 300 HB. You will be able to use hot-rolled blanks, as well as increase strength, fracture protection and toughness. This simplifies the post-processing process.
Air cooling is used as the cooling medium. Maximum heat treatment temperatures – no more than 50 ° c in excess of the limit established for the material.
What is Cryogenic heat treatment?
The basics of heat treatment of cryogenic steel are the significant cooling of previously hardened blanks. The main purpose of use is to stop martensitic transformation temperatures.
As in the case of the other means listed, the workpiece will need to be gradually heat treatment to standard temperature.
What is Chemical-theraml treament?
During processing, the outer layer of the material is transformed. This allows you to increase hardness, protect raw materials from corrosion and further increase wear resistance.
The following methods can be used in the heat treatment process:
- Also called carburization. The surface is saturated with carbon. First, heat treatment is carried out, areas that are not planned to be treated are coated with protective compounds. The procedure is carried out in the range of 900-950°C.
- Unlike carburizing, nitrogen is used instead of carbon. To do this, a heat treatment for ammonia environment is created. The temperature range is 500-520°C.
- Both carbon and nitrogen are used in different proportions depending on the temperature. The process is possible both in gaseous and liquid media.
- Chrome plating. One of the types of metallization. It is named after the main substance with which the material is saturated (chromium). Improves strength, corrosion resistance, appearance of the part.
The technology is selected based on the features and characteristics of a particular type of alloy.
The advantages of Heat Treatmentermal treatment?
Heat treatment of metal blanks is a mandatory process when it comes to the manufacture of structures for long-term use. This technology has a number of advantages:
Increased wear resistance of metal.
Finished parts last longer, the number of defective blanks is reduced.
Improves resistance to corrosion heat treatment process.
Metal structures after heat treatment withstand heavy loads, their service life increases.
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