Staged quenching is based on cooling in stages. It includes a traditional initial heating phase, to reach a given temperature, but requires controlled cooling to be divided into several stages. This process offers a number of advantages, including reduced risk of breakage or deformation, compared with direct cooling.
Two possible cooling modes for a quenching operation
Hardening is one of the oldest heat treatment processes for steel. This method involves heating steel to a precise temperature, generally between 900 and 1,000 degrees Celsius, before rapidly cooling it. In the case of steels, the heating temperature is called the austenitizing temperature, and ranges from 800 to 900 degrees Celsius, or even 1,200°C for high-speed steels. At this temperature, the crystallographic structure of the metal is transformed into austenite.
After the metal has been heated, a quenching operation involves a cooling phase, which can take place in one of two ways: continuously or in stages. In the first case, the part is heated to the specified temperature and cooled all at once. With continuous cooling, the metal comes down to room temperature in a matter of minutes or even seconds. Staged cooling, on the other hand, involves inserting a step into the process. Instead of going directly from austenitizing temperature to room temperature, we add, for example, a longer step at 400 degrees Celsius (depending on the steel's chemical composition). This method is not suitable for all steels, but has several advantages.
Step quenching operation
There are two main technologies for step quenching: salt baths and vacuum furnaces. The former is the simplest, but also the least environmentally friendly, as it uses liquid salts that must be controlled to protect the environment. Vacuum quenching, on the other hand, is far more eco-friendly, as it relies solely on the use of electricity, vacuum and a neutral gas (nitrogen or argon), guaranteeing completely clean final parts with little deformation.
Why choose stepped quenching?
Step quenching, sometimes referred to as isothermal step quenching, therefore involves controlling the cooling steps of the steel. There are two main reasons for using this method: firstly, step cooling reduces the temperature gradient between the surface and the core of the part. Indeed, the temperature difference between these two zones can be more or less significant, depending on the mass of the part or the duration of the treatment, for example. In some cases, this gradient can reach 500 degrees Celsius, in which case staged cooling helps to even out and reduce this value across the entire part, leading to less deformation.
The second advantage of stepped quenching can be seen in the finished part. Poorly managed continuous cooling can, if too rapid, cause deformations or cracks in geometries with many sharp angles. This is also known as " quench tapping". Step cooling greatly reduces the risk of breakage and deformation.
Bainite formation: the other advantage of stepped quenching
Staged quenching, with stepwise cooling, is also a good way of obtaining bainite. A quenching operation based on the process described above can be modified to suit certain parameters. For example, martensitic hardening can be achieved by promoting the transformation of austenite into martensite. Martensite has excellent hardness, but is more brittle than bainite, which can also be produced by bainitic hardening. In fact, stepped quenching offers the possibility of producing bainite, which is less brittle than martensite, even if it remains less hard than the latter.
Staged hardening is used particularly in the tooling sector. As it guarantees extremely resistant parts, with no risk of breakage and controlled deformation, it is an ideal method for long-term tooling efficiency, and reduces the need for reworking after heat treatment.
Staged quenching takes its name from its cooling method: following the heating operation, the metal is cooled in an isothermal stage, as opposed to the continuous method. This can take place in a vacuum or using the salt bath technique. Step cooling reduces the temperature gradient between the surface and the core of the part, and limits the risk of deformation, breakage and hardening taps. This method also offers the possibility of modifying the structure of the steel to obtain martensite or bainite, for example, depending on the properties required.
