Heat Treatments

Internal tensions can arise in cast pieces, e.g. due to uneven cooling and concentrations of volume during coagulation. As the name suggests, the purpose of this heat treatment is to reduce the internal tensions in the material. This barely changes the structure, strength and/or hardness of the material.

Material that must be machined may still contain internal tensile forces from the previous production process. These tensions make it difficult to predict how the steel will behave. During processing, the internal tensions can result in undesired dimension deviations or deformations (changes in shape). Low-tension annealing is sometimes called tension-free annealing. However, it is not possible to remove every bit of tension from the steel; a certain amount of residual tension always remains. Therefore, low-tension annealing is a better name for this heat treatment.

With low-tension annealing, the temperature of the product is increased only to such a level that the structure of the steel does not change. Low-tension annealing of steel is generally performed in the temperature range of 580°C to 620°C. The annealing temperature is greatly dependent on the material’s thermostability. If the material is kept at this temperature for a longer period of time (usually several hours), the internal tensions decrease. During this annealing process, it is important that heating and cooling occur slowly so that no new tensions occur as a result of temperature differences in the casting. Cooling must be performed twice as slowly as the heating up.

A material gets its martensitic structure through hardening: a hard and brittle structure. Cracks may form. To decrease this risk, hardened pieces are given an additional heat treatment immediately after hardening before they have cooled down completely. This heat treatment is also called tempering. This makes the material less brittle and the internal tensions caused by quenching decrease. Reducing internal tensions is extremely important. After hardening, a product can look good, but may fail immediately upon use due to tensions still present in the material.

The level of the tempering temperature is determined by the composition of the steel and the desired properties after tempering. For most types of steel, the tempering temperature is between 150°C and 350°C. In general, the lower the tempering temperature, the greater the toughness and the lower the hardness. Therefore the proper tempering temperature must be chosen for the intended purpose. Tempering at temperatures of around 500°C or higher is called high tempering or refining.

If steel undergoes treatment such as forging, casting or welding, for example, the material can get extremely hot. As a result of this, uneven, rough structures can occur that result in diminished mechanical properties. Normal annealing (also called normalising) is a method of annealing designed to give the steel the same structure everywhere again. This results in fine grains and improved mechanical properties. Internal tensions are also reduced to a minimum. This annealing process takes place in steel types with a carbon percentage that is lower than around 0.7%.

With normal annealing, you warm up the product until only austenite is present (300 to 700°C). A minimum of 30 minutes is generally used for the duration time in the furnace, +2 minutes for each millimetre of wall thickness. Cooling then follows in still air. The rate of cooling is slightly greater than with low-tension annealing. As long as one is still in the range of austenite, grain growth can still occur, resulting in rough grains. This can counteract the grain-refining effect of normal annealing.

Since a phase transition takes place twice with this type of annealing, i.e. from ferrite to perlite to austenite and back, reduction in grain size will occur twice. This occurs because multiple growth nuclei occur for each crystal during the transition phase. This gives rise to a fine-grained perlite structure that is characteristic for the particular steel. After normal annealing, the material has the same structure overall.

Soft annealing and refining are annealing treatments that make steel softer. The purpose of soft annealing is to give the steel a structure that can be machined better and is deformed better. It also reduces the risk of cracking during any hardening treatments. Annealing causes the tensile strength and hardness to decrease by approximately 10 to 25%; by contrast, the toughness increases slightly. A structure of globular carbides is obtained in a crystal ferrite grid. Because of the globular carbides, the burr breaks off better during machining; therefore, the metal surface will be smoother than when machining with a non-annealed steel. The steel is heated to a temperature just below the austenite range, followed by very slow cooling.

Another way is to have the temperature fluctuate around 720°C, such that only a small part of the structure is transposed into austenite. This type of annealing treatment is used primarily with steel types with high carbon content. The duration of the annealing varies from 5 hours to 24 hours and is followed by slow cooling in the furnace.