Thermal treatment (TT) largely influences material properties, its structure and mechanical parameters. These properties determine product/tool quality.
In thermal treatment, specific technological procedures must be strictly observed. Essentially, thermal treatment means heating of the steel to defined temperature, with subsequent controlled cooling.
The temperatures and periods below are only for guidance. Exact values depend on specific chemical structure of the material as well as product size.
Annealing is aimed at reduction of material hardness or removing internal stress resulting from preceding operations, e. g. turning, forming and/or welding. The common feature of annealing procedures is long dwell time at specific temperature and subsequent slow cooling.
As the name suggests, this type of annealing is aimed at reduction of internal stress (resulting from preceding operations, e.g. rough processing) or reduction of internal stress in the casting or welding joints. Intermediate annealing is a frequently used method
Owing to comparatively low temperatures, not any structural changes occur at this type of heat treatment. The product will be heated to a temperature below Ac1 (as a rule, 600-650°C), dwell time approx. 1 hour, subsequent cooling in air.
Aimed at reduction of hardness, improving product machinability.. Semi-finished shapes will be heated to a specific temperature ( 650-720°C, depending on chemical composition), dwell time up to 4 hours, subsequent cooling in the kiln at a speed of 10 – 20°C/1 hour. The material structure after soft annealing is the initial condition for final thermal treatment.
Used for forged pieces or castings (in most cases, these units have not a homogenous structure and large grain size). This structure has negative impact on steel properties.
Hence, normalising annealing is used for obtaining fine-grained, homogenous structure with uniform carbide distribution.
Procedure: Heating of steel to 30°C above Ac3 (i.e. 800°C-920°C, depending on chemical structure), dwell time approx. 1 hour, subsequent cooling in air.
Involves the highest temperatures of all annealing procedures. Aimed at levelling out the differences in chemical composition occurred during the casting process. Homogenisation annealing includes heating to temperature 1000-1200°C, dwell time and subsequent cooling in air.
Hardening influences material properties (i.e. increasing the hardness and reduction of toughness). Aimed at structural change (i. e. forming of martensite and/or bainite) and obtaining unbalanced material condition. Heating to austenization temperature, i.e. 30°C above Ac1 or Ac3 temperatures ( corresponding to 800°C-920°C, depending on chemical structure). This temperature is required for dissolution of carbides and obtaining higher carbon content in austenite.
After heating to the specific temperature and dwelling time, rapid cooling (at so called supercritical speed). The cooling must be extremely rapid to allow transformation of austenite to martensite.
Water, oil, saline bath or air are used for hardening (in all cases depending on chemical structure and steel dimensions). An appropriately hardened steel must have minimum 50% of martensite in the middle of its cross section.
Tempering is a heat treatment process which always follows immediately on hardening. After hardening, material structure is not stable (high martensite content, internal stress). Hence, the material has low toughness and is brittle. Tempering results in modification of the martensitic structure to achieve required material toughness and hardness.
The tempering temperature depends on which material properties should be obtained, achieving balance between hardness and toughness.
In any case, this temperature must be lower than Ac1, with dwell time and subsequent cooling in a suitable environment. Dwell time must be long enough to assure uniform heating of the entire material structure. The higher tempering temperature, the lower the resulting hardness and higher toughness of the material.
Generally, two scales of tempering temperatures are used: 150-300°C for low-temperature tempering (for tools intended for cold processing) and 350-700°C for high- temperature tempering (heavy duty steel as well as steel intended for hot processing).
Multiple tempering procedures are implemented to achieve more efficient decomposition of the remaining austenite. This is why alloy steels are mostly tempered twice, high-speed steel is tempered three times and more.
Steel tempering at low temperatures results in formation of oxidised steel surface layer , thus changing steel colour, due to interference of light (from yellow to blue).
Chemical heat treatment of steel results in modification of chemical composition of the surface and sub-surface material layer. Aimed at obtaining high surface hardness and abrasion resistance, while maintaining tough core material of the product.
Chemical thermal treatment procedures are distinguished by chemical elements used for treatment of the steel surface.
Essentially, these elements can penetrate into the crystalline surface structure. The following elements are used for this purpose: Carbon (case-hardening), Nitrogen (nitriding), Case-hardening using nitrogen (combination of carbon and phosphorus), Boriding (boron), Chromisation (chromium) and other elements.
It is the most widely used method. In the course of case-hardening, saturation of the steel surface with carbon at temperatures higher than Ac3 (approx. 920°C) occurs.
The saturation process can take place in gaseous, liquid or solid environment. Case-hardened layers are very hard (up to 800 HV) and highly abrasion-resistant. The saturation depth ranges 0.5-1.5mm. Concentration of carbon in case-hardened layers ranges 0.7 – 1.1%.
Case-hardening is used, primarily, for construction and carbon steels. The material should have extreme core toughness. This procedure is used, mostly, for gears, shafts, pins etc. Case-hardening requires thermal treatment (hardening and tempering at low temperature).
Nitriding of the steel surface causes forming of very hard nitride layer. This process takes place at temperatures approx. 500°C and lasts up to 60 hours.
Thanks to formation of fine nitrides, the nitrided layer is as hard as 1200HV and extremely resistant to wear and fatigue. The corrosion resistance is also increased, to a certain degree. Thickness of the nitrided layer ranges 0.2-0.6mm.
As a rule, nitriding is used for high-grade steel, containing 0.3-0.4% of carbon. Nitriding can also be carried out in gaseous or liquid environment. Nitrided layer contains as much as 12% of nitrogen.
Compared with case-hardening, the main advantage of nitriding is the ability of the nitrided surface to maintain hardness even at higher temperatures.