Tempered Steel
Application Area
Tempered Steels are used where steels such as S235, S355, etc., would result in excessively large dimensions. They are ideal for all highly stressed components in gearboxes, engines, chassis, etc., such as gears, crankshafts, engine shafts, screws, shift forks, etc.
Description
Tempered Steels are engineering steels that, due to their chemical composition, are suitable for hardening and, when tempered, exhibit good toughness at a given tensile strength.
Through tempering, engineering steels achieve the following property combinations:
- Higher yield strength Re, Rp0.2 for higher permissible stresses
- Increased toughness (notched impact energy Av) for significant deformation before fracture
To achieve this toughness, the microstructure must not contain too much cementite. Therefore, carbon content is limited to 0.25 to 0.6%.
Properties
A selected combination of yield strength and toughness.
Welding: Not suitable (C35 and C45 are only conditionally weldable).
Machining: Heat treatable steels can also be machined after tempering.
Tempering
Tempering is a heat treatment process that involves quenching engineering steels followed by tempering at higher temperatures. Objective: To achieve toughness while increasing yield strength. Steels with higher yield strength and good toughness can generally be produced using two heat treatment processes:
- Tempering by annealing:
Tempering by annealing involves hardening (martensitic structure) followed by tempering (heating) at 450°C to 600°C, where maximum values of toughness and strength are achieved.
1. Quenching in water.
2. Tempering treatment (annealing), optimizing strength and toughness.
- Tempering by isothermal transformation:
Quenching an austenitic structure to a temperature between Ms (start of martensite formation) and Ar1 (cooling transformation point), followed by holding at this temperature (isothermal process). During this time, the entire austenite transforms. The steel can then be cooled to room temperature without further changes. This process is also known as bainitic tempering.