Heat treating (or heat treatment) is a group of processes used to alter the properties of a material. Although the most common application for heat treatment processes are metallurgical, they are also used for the manufacture of other materials, such as glass.
Heat treatment involves heating or cooling a material to achieve the desired properties and outcomes, such as hardening or softening. Heat treating processes include annealing, case hardening, precipitation strengthening, tempering, carburising, normalising and quenching. Different heat treating methods should be chosen depending on the desired outcomes.
Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering physical and mechanical properties intentionally, heating and cooling often occur incidentally during other manufacturing processes, such as hot forming or welding.
Heat treatment can be time-consuming and costly but is also a mandatory operation specified in many application codes and standards, as well as an essential variable on welding procedure qualification specifications, which means it is vital to get it right.
Heat Treatment Methods
There is a range of heat treatment techniques available to industry, each with their own particular characteristics and uses, as follows:
Annealing:
The annealing process involves heating the material to a high temperature, where recrystallisation and/or a phase transformation take place, followed by slow cooling, often in the heat treatment furnace itself. This process is frequently used to soften a metal after hardening, for example once it has been cold worked. Annealing not only offers soft microstructures, but also reduces the yield and tensile strength and, in the case of ferritic steels, usually reduces toughness.
Normalising:
Only used with ferrous metals, the metal is heated to 30-50°C above the upper transformation temperature before being allowed to cool back to room temperature. This reduces the grain size and improves strength and toughness.
Quenching:
This method uses water, oil or a blast of air to rapidly cool a ferritic steel. In this process the metal is heated to a temperature above its upper transformation temperature to create a high strength, fine grained martensite before cooling. Once quickly cooled, the steel should also be tempered (see below).
Tempering:
This process involves heating a ferritic steel to a temperature below the lower transformation temperature. This process reduces hardness, lowers the tensile strength and improves both ductility and toughness. Most normalised steels are tempered ahead of welding, while all quenched steels need to be quenched and tempered before use.
Solution Treatment:
This high temperature process takes elements and compounds into a solution where they are retained by rapid cooling from the solution treatment temperature. The amount of time for this to occur varies, but it can reduce the strength of a joint or improve corrosion resistance. With some alloys the treatment can be followed by a lower temperature process to reform the precipitates in a controlled manner (see age or precipitation hardening, below).
Ageing or Precipitation Hardening:
This low temperature process is used to produce the correct size and distribution of precipitates, increasing the yield and tensile strength. It is usually preceded by a solution treatment (see above). Using the process over longer periods of time and/or higher temperatures will create an increase in size of the precipitate and a reduction in both hardness and strength. Unlike with iron based alloys, most heat treatable alloys do not experience ferrite transformation but harden by precipitation. This is a typically slow process, depending on the critical temperatures, often referred to as ‘age hardening.’
Stress Relief:
This stress relieving process reduces the residual stresses created by weld shrinkage. By raising the temperature of the metal, the yield strength decreases allowing the residual internal stresses to be redistributed by creep of the weld and parent metal. Harmful thermal gradients are avoided by controlled cooling from the stress relief temperature.
Post Heat:
This process is carried out immediately following welding ferritic steels, increasing the preheat by around 100°C and maintaining this temperature for 3 or 4 hours. This assists with the diffusion of hydrogen in the weld and/or heat affected zones, reducing the risk of hydrogen-induced cold cracking. This process is typically only used in instances where hydrogen cold cracking is a major concern, such as with crack sensitive steels and very thick joints.
Post Weld Heat Treatment (PWHT):
Not to be confused with heat treatments performed after welding (see above), PWHT is a specific term that covers both stress relief and tempering. These treatments may also include the ageing of aluminium alloys, solution treatment of austenitic stainless steel, and hydrogen release. PWHT reduces the risk of brittle fracture by reducing the residual stress and improving toughness. It also reduces the risk of stress corrosion cracking and is a mandatory requirement in many codes and specifications, particularly when dealing with thicker joints or where stress corrosion is a possibility. It does, however, have little benefit for fatigue performance unless the stresses are mostly compressive.