Heat treatment overview – The most important methods at InProCoat
Heat treatment of steel adjusts the workpiece properties such as surface hardness, strength, temperature resilience and ductility, and thus clearly extends the service life of metal components. We use, among others, the following methods for this:
- Annealing (solution annealing, normalising, stress relief heat treatment)
Let us introduce the individual heat treatment procedures to you in more detail below.
Tempering of steel is a combination of hardening and heat treatment. The tempering temperature is usually above 550 °C. This form of heat treatment serves to clearly improve toughness at the same tensile strength. The highest toughness is achieved by tempering a strictly martensitic hardness structure.
Annealing is only suitable for materials that can be tempered. This includes steel as well as NF metals such as titanium alloys. The tempering capacity of a material depends on whether it is able to form martensite or bainite structures. Beyond this, tempering capacity depends on grain size of the structure, which influences the temperature-dependent conversion processes.
Steel must have a carbon content of 0.2 to 0.3 percent for classic tempering. For steels with other properties, surface hardening is the better choice.
Annealing is the initial heating, heating through and subsequent cooling off of workpieces to achieve special workpiece properties.
The initial heating phase serves to raise the steel to the holding temperature. During the subsequent retention phase, a temperature adjustment in the workpiece and balancing of physical and chemical processes in the material will take place. In the final cooling phase, the temperature of the steel is reduced to ambient temperature again.
Depending on the intended target, this type of heat treatment uses different methods.
One of the most important methods for heat treatment is normalising, giving the steel an even, fine-grained structure with homogeneous properties. This method is applied particularly for carbon steels or low-alloyed steel types. Their structures are normalised after hot-rolling or casting. The hardness that can be achieved by normalising is according to the properties of the steel and the cooling speed. Generally, hardnesses of 100 to 250 HB can be achieved.
During annealing, the steel is heated to a temperature that corresponds roughly to its hardening temperature. This causes new austenite grains to grow. They are much smaller than the original ferrite grains. Cooling leads to the production of new, finer ferrite grains.
Hot forming of steel leads to structure changes due to the heat influence and uncontrolled cooling. This particularly applies to austenitic steels, where high temperature in the range of 500 to 800 °C may produce carbide precipitation at the grain thresholds. Due to deviating electrochemical properties, these precipitations lead to inter-crystalline corrosion. They therefore must be dissolved again by annealing.
Solution annealing, also called diffusion annealing, is performed at relatively high temperatures of 1050 to 1300 °C. The process can take up to two days. It leads to even arrangement of foreign atoms in the metal grid. The formation of phases, and therefore also the properties of the steel, are determined by the choice of cooling speed.
Stress relief heat treatment
Stress relief heat treatment of steel is mostly performed in a temperature range of 480 to 680 °C; the holding time can be one to two hours. This heat treatment method removes most internal tensions of the workpiece that previously resulted from processing or mechanical forming. This reduces undesired distortion of the final mechanical fine processing. Stainless steel with niobium and tantalum shares is not suitable for stress relief heat treatment.
If a steel workpiece needs to have good forming and machining properties rather than being resilient to high mechanical forces, soft annealing is the optimal heat treatment method. In order to reduce hardness and strength, annealing at temperatures of 650 to 750 °C reduces present cementite or perlite precipitations.
Heat treatment at InProCoat
Our company has a high-performance, extraordinarily precise bogie hearth furnace, where we can perform various methods of heat treatment at temperatures of 200 to 1050 °C. Regular TÜV monitoring ensures that all methods are performed properly and that optimal results are achieved.
We can process parts with unit weights of 25 tonnes according to any specifications. We meet all requirements of the AD leaflet HP 7/1-3.
We will gladly use our long experience and comprehensive knowledge to find the right heat treatment method for your project. Contact us and let us provide individual advice.