Tempering is the process of heating hardened steel to a lower critical temperature, annealing at this temperature, and then cooling, usually very slowly. The tempering temperature depends on the strength (or hardness) and toughness required for a given application. Typically, martensite decomposes by heating during tempering, which results in a decrease in resistance, hardness, and strength, but an increase in ductility and toughness. What does this heat treatment process involve? Is tempering about hardening? We write about this and other elements below!
What are the goals of steel tempering?
Below we write about the effects of steel tempering. In such a case, the following occurs:
1. Removal of internal stresses that occurred during hardening.
2. Restoration of elasticity and ductility at the expense of hardness and strength.
3. Improvement of dimensional stability through the decomposition of residual austenite.
4. Improvement of magnetic properties, transformed from non-magnetic austenite into magnetic products.
Hardened structure
Hardened steel has a complex structure, which usually includes:
- Highly supersaturated martensite - The lath type or plate type has a high dislocation density of about 1012 cm/cm3 for the former and a lower dislocation density for the latter, but it can be heavily twinned with the increase of carbon content in the steel.
- Residual austenite - the amount of which depends on the carbon content and alloying elements (also on the ambient temperature). Carbon steels containing less than 0.5% carbon retain less than 2% austenite, 6% at 0.77% C, but more than 30% at 1.25% C.
- Undissolved carbides - For example, pre-eutectoid cementite in over-eutectoid steel or vanadium carbide in high-speed steel (18/4/1) to control grain size.
- Coal sorting - Coal is divided into low-energy sites, such as dislocations or vacancies, or into clusters along the plane in layered martensite or along the plane in twin-plane martensite. Segregation can occur during tempering between Ms and room temperature or at room temperature during storage, and even heating to about 100°C during annealing. At about 0.2% carbon in steel, defect sites are almost saturated with carbon, while the remaining carbon in the steel (if present) remains in normal interstices. In fact, 0.20% carbon is also the point at which the tetragonality of martensite can be detected, i.e. if the carbon in the steel drops to 0.20%, the martensite is BCC, otherwise it is BCT. For the above reasons, temper brittleness is significant.
Stages of tempering
During tempering, the entire process is divided into several elements and cooling rates. Tempering of carbon steel takes place in four separate but overlapping stages:
1. The first stage of tempering – Up to 200°C – Precipitation of carbides e(ε) as a result of tetragonal reduction of martensite.
2. The second stage of tempering – 200° to 300°C – Decomposition of residual austenite.
3. The third stage of hardening – 200° to 350° C – Produces cementite rods or plates with competitive loss of martensite and dissolution of electron carbide.
4. The fourth stage of tempering – 350°C to 700°C – Densification and spheroidization of cementite, recovery and recrystallization of ferrite.
What is tempering classification?
Tempering generally falls into three categories depending on the range of tempering temperatures. Tempering completely or partially alleviates internal stresses that arise during hardening – they are more completely removed at higher temperatures, e.g. 1.5 hours at 550°C.
Low-temperature tempering (250 ℃ for 1-2 hours)
Low tempering is carried out to reduce brittleness without losing too much hardness. The tempered martensitic two-phase structure increases strength while improving flexibility and reducing internal stresses. The hardness of tempered ordinary carbon steel (0.6～1.3%C) is Rc 58～63. This treatment is commonly used for ordinary carbon steel tools and low-alloy steel, and the main developmental properties are high machinability, wear resistance, and some ductility. Increasing the tempering temperature in this range will somewhat reduce hardness, but increase ductility, thus better alleviating internal stresses. Low-temperature tempering is carried out in oil baths (up to 250°C – silicone oil), salt baths or air circulation furnaces (because below 500°C heat transfer through air is very slow). Low-temperature tempering is also applied to carburized and case-hardened elements, such as carburized, cyaniding or carbonitriding.
Color of Temperament
In ancient times and today, the tempering temperature achieved for elements made of ordinary carbon steel and low-alloy steel was determined by the color of the surface developed in the color scale. These colors appear on clean steel surfaces when the temperature rises above 220°C. The steel surface should be clean so that the tempering temperature can be assessed based on the surface color after annealing in a muffle furnace. The tempered color results from the formation of an extremely thin transparent layer of iron oxide. In this thin surface layer, there is light interference, which, depending on the thickness of the layer, takes on a tempered color. This method of determining the tempering temperature by color is based on the fact that each temperature has a specific thickness of the oxide layer, which gives a specific color.
Tempering at medium temperature (350°C to 500°C)
Tempering medium means that in this tempering range, a microstructure of "troostite" is formed, showing the development of a high yield point with good ductility and hardness in the HRC 40-50 range. Water quenching after tempering in the range of 400-450°C increases the yield strength, which causes compressive stress in the surface layer. Due to large limitations in plasticity and strength, this assortment is mainly used for two types of coil springs and laminated springs and dies. Care should be taken to avoid brittleness at a temperature of 350°C.
High-temperature annealing (500-650 ℃)
Tempering at high temperatures causes the higher the tempering temperature of ordinary carbon steel and low-alloy steel, the higher the developed ductility. This tempering range creates a "sorbitol" structure in the steel, providing machine parts with the best combination of strength and durability. Structural steel 0.3-0.5% carbon usually has a higher tempering temperature. This treatment for 1-2 hours can almost completely release residual stresses created during the hardening process.
Tempering Effect
Carbon plays a very important role in tempering steel. The hardness of martensite after tempering mainly depends on the carbon content in the steel, and therefore also on the morphology of the martensite from flakes to tightly twisted plates. The Ms and Mf temperatures decrease with the increase in carbon content in the steel, i.e. they reduce the chance of self-heating, and also increase the amount of residual austenite.
Impact of steel tempering on mechanical properties
In the first stage of tempering steel with a carbon content above 0.2%, martensite reduces its tetragonality and decreases the hardness of the steel, but there is also precipitation of ε-carbide, which increases the hardness of the steel, and is proportional. Form. Therefore, up to the tempering temperature of 200°C, the hardness of the steel usually decreases continuously, but only slightly, depending on the final result of these two effects. In fact, in high-carbon steels with a carbon content of 1.2%, the hardness slightly increases in the temperature range up to 200°C due to the relatively large volume fraction of formed carbides, which not only compensates for the loss of hardness caused by the reduction of the tetragon, but also compensates for a slight increase in overall hardness. During tempering in the third stage, there is a significant softening due to a sharp decrease in hardness as a result of the dissolution of ε-carbides in the matrix and the complete loss of tetragonality of martensite, although cementite at this stage contributes to a certain increase in hardness, but the overall effect softens. At this stage, the steel consists of ferrite and fine grains of cementite.
As we can see, the heat improvement process is significant for machining. Nevertheless, it is worth knowing its course, tempering brittleness, or eutectoid transformation temperatures.