Tempering
What is tempering process?
Temperingis a method applied to give the steel the mechanical properties needed. With this process, the hardness, strength, and toughness values of the material are increased.
This method basically consists of 3 steps.
As a first step, the material is heated until it reaches the austenite field, taking into account the iron-carbon equilibrium diagram. As a result of holding the steel at this temperature, the cage structure changes and an austenitic structure is formed. This stage is carried out under protective gas so that undesirable decarburization and oxidation that will occur in the material at high temperature is prevented.
The steel whose lattice structure has changed passes to the second step, the cooling stage, after this step. It is rapidly cooled by immersion in oil in accordance with the cooling curve (TTT) diagrams according to the steel properties and the desired hardness value. As a result of this cooling process, the steel structure transforms from austenite to dense martensite. As a result of this process, the material gets the maximum hardness it can get, but the material structure is exposed to intense stresses.
After the cooling process, the material is put into the tempering process. In this last step, the material is reheated. Here, the temperature and time parameters in the process vary according to the material type, desired hardness, and material cross-section. As a result of the tempering process, the brittleness of the material decreases, and the stresses in the material are removed. With the tempering process, the hardness values of the steel decrease, and the toughness value increases.
After the process, the steels are cooled with nitrogen or helium gas at high pressure up to 20 bar pressure in another chamber waiting cold. With this cooling process, martensitic transformation is completed. Since there is no cooling process in liquid environments such as oil or salt baths, tempering can be started directly without any washing or cleaning process.
After martensitic transformation, the process is completed by applying the tempering process after the cooling process in order to remove the brittle structure and obtain a tougher structure and remove the stress that occurs during the transformation.
Tempering Process at Önerler
Atmosphere controlled chamber type furnaces are used for the tempering process in our company.
With various furnaces with charging capacities of 600 - 1000 - 2000 kg, we offer our customers the opportunity of flexible charging.
All of our 5 furnaces in total are of German origin and the IPSEN brand, which is a world leader in its field.
With the Carb-o Prof program in our furnaces, parameters such as gas flow, temperature, protective atmosphere composition, oil temperature, cooling rate, which are critical for the process, are controlled and thus high repeatability and traceability is provided.
Thanks to the Rapid-quench system, materials gain the maximum hardness value that can be achieved by cooling faster.
Since the establishment of our company in 1967, we have been offering the best quality to our customers with our expert staff, state-of-the-art equipment and experience in the improvement process we have been doing for 50 years.
Benefits of The Processes
High wear resistance
High strength
High fatigue resistance
High hardness
- High toughness
- High impact resistance
- High stability
A steel with low hardness can be processed easily and quickly and then brought to the desired strength and hardness values by applying the treatment process.
This allows the manufacturer to produce quickly while reducing material and processing costs.
In addition, cutting edge and material cracking that may occur during the processing of a hard material are prevented.
Steels Applicable to the Process
The treatment process can be applied to almost all steels containing 0,3% and above carbon. In the table below, the steels used extensively in the market and treated with the treatment process and the maximum hardness values that can be obtained after the treatment process are given in Rockwell.
1035 50-52 HRC
1040 50-52 HRC
1045 50-52 HRC
1050 57-59 HRC
1060 60-62 HRC
4130 49-51 HRC
4140 58-60 HRC
5140 57-59 HRC
4340 58-60 HRC
50CrV4 57-59 HRC
100Cr6 63-65 HRC
1.2714 56-58 HRC
1.2842 63-65 HRC
1.2080 63-65 HRC
1.2550 60-62 HRC
Problems That May Occur After The Procedure
One of the most troublesome problems in the treatment process is the change in the size of the materials, that is, distortion. The tempering process is a thermal process. The material is heated to high temperatures and cooled very quickly, and in this case, the material naturally expands and then shrinks rapidly, and when the material reaches the maximum hardness it can reach, the stresses in the material increase excessively. Even if the material is heated homogeneously, the shape of the material (especially thin-thick sections), the inhomogeneous stresses that occur in the material as a result of the processes performed before the heat treatment can increase the distortion and even crack the material.
After the martensitic transformation in the tempering process, the hardness is at the maximum and the internal stresses are at the highest level. If steel in this situation contains residual stresses due to processes before heat treatment, there may be more distortion or cracking in the material.
If a steel with high internal stress needs to be treated, then the internal stresses of the steel should be relieved by applying stress relief annealing beforehand. Despite all this, if the risk of distortion or cracking in the material is high, vacuum hardening should be considered.
The manufacturer must correctly determine the cleaning criteria on the part surface before the process. Even if we use washing baths before and between the process in our company, it is a more dirty process compared to other processes due to the treatment process structure. After the process, stains that may occur due to oil and heat can be observed on the surface of the material. In cases where this situation does not meet the cleaning criteria, methods such as surface cleaning by vibration can be applied after the process.