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Tempering process is heating hardened steel to a lower critical temperature, annealing at that temperature, and then cooling very slowly. The tempering temperature depends on the strength (or hardness) and bonding required for the application. Typically, martensite is decomposed by heating during tempering, resulting in a decrease in hardness and strength but an increase in flexibility and toughness. What is heat treatment? How does tempered steel or heat treat? What is upper critical temperature or normalized steel?

What are the purposes of tempering steel? 

Heat treating is one of the essential thing in CNC. Below we write about the effects of tempering steel (or quenched steel). This results in the following: 

1. removal of internal stresses created during hardening.

2. Restoration of elasticity and flexibility at the expense of hardness and strength.

3. Improvement of dimensional stability through decomposition of residual austenite.

4. Improving magnetic properties, converting non-magnetic austenite into magnetic products.

Hardened structure

Hardened steel has a complex structure that typically includes the following:

  • Highly supersaturated martensite – The mantle 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 can be strongly twinned as the carbon content of the steel increases.
  • 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, preneutectoid cementite in super-eutectoid steel or vanadium carbide in high-speed steel (18/4/1) to control grain size.
  • Carbon sorting – Carbon is segregated into low-energy sites, such as dislocations or vacancies, or clusters along a plane in layered martensite or along a plane in double-plane martensite. Segregation can occur during quenching between Ms and room temperature or at room temperature during storage, or even heating to about 100°C during tempering. At about 0.2% carbon in the steel, the defect sites are nearly saturated with carbon, while the remaining carbon in the steel (if present) remains in the normal interstitials. 0.20% carbon is also the point at which martensite tetragonality can be detected, i.e., if the carbon in the steel drops to 0.20%, the martensite is BCC. Otherwise, it is BCT.

Stages of hardening

Tempering of carbon steel is carried out in four separate but overlapping stages:

1. First quenching stage – Up to 200°C – Precipitation of carbides e(ε) by tetragonal reduction of martensite.

2. Second tempering stage – 200° to 300°C – Decomposition of residual austenite.

3. Third stage tempering – 200° to 350° C – Produces cementite rods or plates with competitive martensite loss and electron carbide dissolution.

4th tempering stage – 350° to 700° C – Densification and spheroidization of cementite, recovery, and recrystallization of ferrite.

What is tempering classification? 

Tempering is generally divided into three categories depending on the range of tempering temperatures. Tempering fully or partially relieves internal stresses created during hardening – these are more completely removed at higher temperatures, such as 1.5 hours at 550°C.

Low-temperature tempering (250 ℃ for 1-2 hours)

Low-temperature tempering is carried out to reduce brittleness without losing too much hardness. The hardened martensitic two-phase structure increases strength while improving elasticity and reducing internal stresses. The hardness of hardened ordinary carbon steel (0.6~1.3%C) is Rc 58~63. This treatment is commonly applied to standard carbon steel and low-alloy steel tools, and the main developmental properties are high machinability, wear resistance, and some toughness. Increasing the tempering temperature in this range will reduce hardness to some extent, but increase ductility, thus better relieving internal stresses. Low-temperature tempering is carried out in oil baths (up to 250°C – silicone oil), salt baths, or air-circulating furnaces (since below 500°C heat transfer through the air is prolonged). Low-temperature tempering is also used for carburized and case-hardened parts, such as carburizing, cyaniding, or carbonitriding.

Temper color

In ancient times and today, the tempering temperature achieved for ordinary carbon steel and low-alloy steel parts was determined by the surface color developed on a 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 color of the surface can judge the tempering temperature after quenching in the muffle furnace. The tempered color is due to the formation of a fragile transparent layer of iron oxide. Light interference occurs in this thin surface layer, which, depending on the thickness of the coating, takes on a tempered color. This method of determining the tempering temperature by color is based on the fact that each temperature has a certain thickness of the oxide layer that produces a specific color.

Medium-temperature tempering (350°C to 500°C)

In this tempering range, a „pro style” microstructure is formed, showing the development of high-yield strength with good ductility and hardness in the HRC 40-50 range. Quenching in water after tempering in the 400-450°C range increases the strength limit, resulting in compressive stress in the surface layer. Due to the high limits of flexibility and strength, this range is mainly used for two types of coil springs laminated springs and dies. Care must be taken to avoid embrittlement at 350°C.

High-temperature tempering (500-650 ℃)

The higher the tempering temperature of ordinary carbon and low-alloy steel, the higher the flexibility developed. This tempering range creates a „sorbitol” structure in the steel, providing machine parts with the best strength and toughness. Structural steel 0.3-0.5% carbon usually has a higher tempering temperature. This 1-2 hours treatment can almost completely release the residual stresses created during the hardening process.

Tempering effect

Carbon plays a very important role in tempering steel. Martensitic hardness after tempering depends mainly on the carbon content of the steel, and, therefore also on the martensitic morphology from flakes to highly twisted plates. Ms and Mf temperatures decrease as the carbon content of the steel increases, i.e., the chance of self-heating decreases, and the number of residual austenite increases.

Effect of tempering steel on mechanical properties

In the first stage of tempering steel with a carbon content of more than 0.2%, martensite reduces its tetragonality and the hardness of the steel. Still, there is also precipitation of ε-carbide, which increases the hardness of the steel, and is proportional. Therefore, up to a tempering temperature of 200°C, the hardness of steel usually decreases continuously, but only slightly, depending on the result of these two effects. In fact, in high-carbon steels with a carbon content of 1.2%, the hardness increases somewhat in the temperature range up to 200°C due to the relatively high volume proportion of carbides formed, which not only compensates for the loss of hardness caused by the reduction of the tetragonality but also compensates for the slight increase in overall hardness. 

During tempering in the third stage, significant softening occurs due to a sharp decrease in hardness due to the dissolution of ε-carbides in the matrix and the complete loss of tetragonality of martensite. However, cementite at this stage contributes to some increase in hardness, but the overall effect softens. At this stage, the steel consists of ferrite and refined grains of cementite.