43 Quench Tempering

Quench Tempering is the process for making material harder. This method has been known for hundreds of years but was only perfected in the 20th century. The metal is heated to a specific temperature and rapidly cooled (quenched) in a bath of water, brine, oil, or air to increase its hardness

Tempering is most often performed on steel that has been heated above its upper critical temperature and then quickly cooled, in a process called quenching. The quenched steel, at or very near its hardest possible state, is then tempered to slowly decrease the hardness to the desired level. The hardness of the quenched steel depends on both cooling speed and on the composition of the alloy. Steel with a high carbon content will reach a much harder state than steel with a low carbon content. Likewise, tempering high-carbon steel to a certain temperature will produce steel that is considerably harder than low-carbon steel that is tempered at the same temperature. The amount of time held at the tempering temperature also has an effect. Tempering at a slightly elevated temperature for a shorter time may produce the same effect as tempering at a lower temperature for a longer time. Tempering times vary, depending on the carbon content, size, and desired application of the steel, but typically range from a few minutes to a few hours.

Tempering quenched steel at very low temperatures, between 66 and 148 °C (151 and 298 °F), will usually not have much effect other than a slight relief of some of the internal stresses and a decrease in brittleness.

Tempering at higher temperatures, from 148 to 205 °C (298 to 401 °F), will produce a slight reduction in hardness, but will primarily relieve much of the internal stresses. In some steels with low alloy content, tempering in the range of 260 and 340 °C (500 and 644 °F) causes a decrease in ductility and an increase in brittleness, and is referred to as the “tempered martensite embrittlement” (TME) range. Except in the case of blacksmithing, this range is usually avoided. Steel requiring more strength than toughness, such as tools, are usually not tempered above 205 °C (401 °F). Instead, a variation in hardness is usually produced by varying only the tempering time. When increased toughness is desired at the expense of strength, higher tempering temperatures, from 370 to 540 °C (698 to 1,004 °F), are used.

Tempering at even higher temperatures, between 540 and 600 °C (1,004 and 1,112 °F), will produce excellent toughness, but at a serious reduction in strength and hardness. At 600 °C (1,112 °F), the steel may experience another stage of embrittlement, called “temper embrittlement” (TE), which occurs if the steel is held within the temperature range of temper embrittlement for too long. When heating above this temperature, the steel will usually not be held for any amount of time and quickly cooled to avoid temper embrittlement.

One drawback of using this method by itself is that the metal can become more brittle. This treatment is therefore typically followed by a tempering process which is another heating process at a lower specific temperature to stress relieve the material and minimize the brittleness problem. The temperature chosen for the tempering process directly impacts the hardness of the work piece. The higher the temperature in the tempering process, the lower the hardness.

The benefits of the quench tempering process is the restructuring of the crystalline nature of the metal. Metals have a specific structure at the molecular level and the material used most in the machine shop, the steels, have a structure that repeats itself throughout the material. Iron alloy structure is different depending on the temperature they are heated to. Their structure (left) is called Body Centered Cubic when the steel is allowed to cool at a slow pace.

Their structure is Face Centered Cubic (right) when the temperature of the iron alloy is more than approximately 1400 degrees Fahrenheit. So, heat-treating consists of freezing the crystalline structure obtained at high temperature by cooling the iron alloy very fast. The tempering process permits a better re-diffusion of the carbon atoms in the material and imparts ductility to the workpiece (although it reduces the hardness of the material).

Derived from Tempering (metallurgy) – Wikipedia accessed and available 29 February 2024 and The Virtual Machine Shop (20111)  http://www.jjjtrain.com/vms/eng_heat_treat/eng_heat_treat_05.html retrieved from the Wayback Machine 17 January 2024.