Process and background information
Unlike the classic hardening of, for example, quenched and tempered steels in oil or water, which creates martensite, the purpose of the process is to create the bainite to which the process owes its name. It develops from austenite under isothermal conditions or continuous cooling below the temperature necessary for pearlite formation.
Bainite is categorised as lower bainite or upper bainite, based on the temperature range of bainite formation. Upper bainite consists of a mixture of needle-shaped ferrite and films of carbides that are arranged in parallel. In the case of lower bainite, the carbides are formed at an angle of 60° to the ferrite, which is arranged in plates. Based on the transformation conditions, bainite is further sub-categorised as inverse, granular, or long needle bainite.
- The steel is heated to a temperature between 790 - 950 °C, leading to the formation of austenite in the microstructure. This process is referred to as austenitising.
- Next, the material to be hardened is quenched in a hot bath, e.g. in a salt melt. An isothermal transformation requires a constant temperature between 220 °C and 400 °C. The exact temperature depends on the alloy and the specific position of the bainite area in the time-temperature conversion diagram. It should be greater than the martensite start temperature.
- Until the austenite has been converted to bainite as completely as possible throughout the entire workpiece, the steel remains in the quenching bath. This may take minutes or hours, depending on the temperature, the steel composition, and the dimensions of the component.
- The process concludes with cooling to room temperature. Because of the low residual stresses in the resulting microstructure, there is no need for tempering.
The advantages at a glance
Bainitising is used to specifically adjust specific properties of steels and cast iron, and it offers the following advantages:
- Increased strength and hardness at maximum toughness
- Minimum warping (especially in thin-walled workpieces)
- Greater fatigue strength (compared to quenching and tempering in oil)
- Reduced wear and greater resistance (e.g. also against hydrogen-induced embrittlement during a coating treatment)
Applications and suitable materials
Bainitising is suitable for a wide range of applications. It is particularly suitable for thin-walled components that are exposed to high loads and require minimum warping.
One field of application is the automotive industry , where bainitising is used, for example, for screws and fasteners or for sheet metal parts in such safety-critical elements as seatbelt systems or seat adjusters. These elements require maximum ductility and high load capacity before breakage is to be expected.
Other applications include nails, springs, crankshafts made of cast iron or, in general, all components made of metal sheets and strip coils with a small cross-section.
Steels with a medium or high carbon content and a hardness of 35 to 55 HRC, as well as ductile iron castings, are well suited for bainitising. Examples of suitable materials can be found in the material table.
Areas of application
Bainitising is a key process for the preparation of steel in a multitude of industries. These include:
- Wind turbines
- Semi-finished metal products
- Automotive industry
- Safety engineering
- Agricultural machinery
Continuous treatment system:
Heating capacity: 500 kg/h
Belt width: 900 mm
Heated length of the furnace: 5.4 m or 7.20 m
Suitable for bulk material with up to approx. 300 g per part
Length of the parts: <200 mm
Would you like to commission bainitising from us?
We need the following information from you:
- Material designation
- Required hardness (HRC) and tolerance
- Maximum permissible warping
- Hardening temperature/material data sheets and empirical values
If a test area is stipulated, please send us a corresponding drawing, and add a note in your order.