Heat Treatment Process

Heat Treatment Processes
for Product Forming
Heat treatment to shape the product involves normalizing (air cooling and tempering) processes which remove the residual stress from the previous processes (e.g. cold forming) and / or metallurgical improvement and soften the material.
Annealing Process: It is the process that involves heating the metals to the temperature below the solidification curve, keeping them at that temperature and cooling back. In other words, the material is always solid.
Annealing processes:
a. Soft annealing: It is mostly applied for reducing the hardness of steel, improving the machining properties or reducing the internal stresses of cast and forged parts.
b. Coarse-grain annealing: It is mostly applied for improving the machining properties of low-chrome metals/ steels.
c. Diffusion annealing: It is applied for the homogeneous diffusion of soluble components in the internal structure.
d. Normalization: It is applied for reducing the grain size, obtaining a homogeneous structure, improving mechanical properties, and scatting the carbide network that is in the grain boundary in metals/ steels above the eutectic point. After the normalization process, the forged, rolled, coarse structures are corrected and the material can be restored to properties that can be gained any time (such as pulling strength, ductility).
e. Stress relief annealing: It is applied to cast, forged, welded parts and cold formed materials. It is aimed at reducing the internal stresses of the material that are created during production.
f. Recrystallization: It is applied for the purpose of restoring the crystal structure of the forged and rolled materials to pre-processing conditions.
Heat Treatment Processes Adding Appropriate Properties to Metal Parts
Heat treatment processes aimed at adding appropriate properties to metal parts are carried out to achieve the desired properties with metal parts under working conditions when the heat treatment is completed or nearly completed. The heat treatment processes applied in accordance with the purpose of use vary depending on technical methods.
a. Hardening: This process is applied for the purpose of improving mechanical properties, increasing the hardness and improving the wearing resistance.
b. Tempering (Annealing): This process is applied for eliminating high stresses and brittleness and adding a solid structure to the material.
c. Martempering: This is the intermittent tempering process that is performed slightly above the martensite formation point for the purpose of reducing the cracking and dimension change risks that may arise during hardening (quenching).
d. Austempering: This is the intermittent tempering process that is performed above the martensite formation point for the purpose of obtaining bainitic phase in the micro-structure to increase ductility.
e. Surface hardening: This is the process of hardening a certain zone or layer from the surface to the core that is applied when solid structure is needed against impacts and a solid hard surface against wearing of components. For this purpose, many different processes are applied. 
These processes are classified in two main groups:
e.1. Thermochemical process: It is applied to almost all steel groups particularly including low carbon steels. Steels gains a different chemical composition and micro structure in the surface when nitrogen, carbon and/or borone is sent at a temperature of 500-1000 oas interstitial atom. Details of these processes are provided below: 
e.1.1. Carburizing (cementation): It is based on the principle of introducing carbon as interstitial atom. A hard surface resistant to wearing and a solid core are obtained after the process. The process is performed at 800-1500 0C.
e.1.2. Carbonitriding: It is performed by introducing ni-trogen with carbon as primary in terstitial atom. The process is performed at 800-900 0C.
e.1.3. Nitriding and nitrocarburizing: It is performed by introducing carbon (nitrocarburizing) with nitrogen (nitriding) as primary interstitial atom.
The process is performed at 400-610 0C .
e.1.4. Boronizing: It is based on the principle of introducing borone as interstitial atom. The process is performed at 850-1000 0C.
e.2. Thermal processes: The hardening is obtained by changing only the micro structure without changing chemical composition of the material during the heat treatment process. It is applied to steel materials with at least 0.35% C.
e.2.1. Surface hardening with induction: Metallic material is placed in the middle or against an electrical magnetic field and it is heated at 800-900 oC upon which it is cooled gradually.
e.2.2. Surface hardening with flame: Metallic material is heated to 800-900 oC with flame and subsequently it is cooled suddenly or gradually.
e.2.3. Surface hardening with laser: Metallic material is heated to 900-1400 oC with laser and subsequently it is cooled suddenly or gradually.
There are limits for the application of heat treatment processes in the heat treatment industry:
a. In general, heat treatment is the last process applied for eliminating the stress of the carbon or low alloy parts. Therefore, mechanical properties of the treated materials should not be affected adversely.
b. Stress relief treatments between cutting processes can be applied to pre-treated materials. Stress relief effect is applied in slightly reduced mode in order to prevent loss of mechanical properties.
c. Many austenite stainless steel materials require rapid cooling after stress relief or solution treatment at high temperature. In these cases, slight bending or permanent stresses are unavoidable. Dimensions of the part that requires stress relief, treatment or normalization depend on the specifications and capacities of heat treatment machines. Providing machine-equipment of suitable size is important for large parts.
Certain requirements may arise during heat treatment applications carried out in the heat treatment industry.
a. Many stress relief processes are applied under open atmosphere and there are also protective environments. In open atmosphere, alloys change colour and an oxide layer is formed depending on the type of alloy and temperature. Therefore cleaning is required after the process.
b. In general, normalization is applied to semi-finished steel parts under open atmosphere and oxide layer formation or decarburizing problem does not arise as they are subsequently cleaned through machining.
In addition, sometimes, a protective environment may be required. For example, during normalization before partial surface hardening of parts with final dimensions.
c. Annealing processes can be applied under protective environments such as open atmosphere or salt, controlled gas atmosphere or vacuum. Some annealing processes may require selection of an environment that provides long-lasting protection.
d. There is always bending/ buckling risk when high temperature processes are applied on delicate, thinwalled vessels and large, heavy parts. Special supports and tools are required during heat treatment in order to prevent this problem.
Source: Istanbul Chamber of Industry

Date : 18.05.2018