Delayed quenching or bainitic isothermal quenching
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Reviewed by A3TS on July 29, 2026
PRINCIPLE:
HARDENING BY QUENCHING is in all cases obtained by cooling from the austenitic state, in continuous conditions (direct quenching) or discontinuous with intermediate isothermal maintenance (isothermal or deferred or staged quenching).
Direct martensitic hardening (should be shown on a TRC curve)
Delayed martensitic hardening
Delayed bainitic quenching
DIRECT QUENCHING
Provided that cooling is sufficiently rapid, this process results in a martensitic structure (the hardest); the transformation occurs sequentially from the surface toward the center of the workpiece according to the cooling gradient (the surface layer cools first, followed by the underlying layer). This process sometimes leads—due to the accumulation of stresses resulting from the superimposition of transformations—to residual tensile stresses, deformations, and even cracks.
DELAYED QUENCHING
It produces the following based on the isothermal holding temperature:
A martensitic structureforms if the material is held at a temperature slightly above the martensitic transformation start temperature (Ms) for a short time and before the start of the bainitic transformation.
A bainitic structurewill form if the temperature is maintained above Ms for a sufficient amount of time to allow the austenite to transform completely.
The bainitic structure is a fine-grained structure, similar to martensite, and achieves hardness levels comparable to those of tempered martensite obtained through the two-step process of quenching and tempering. Depending on the holding temperature, several variants of bainite form. The main ones are lower bainite and upper bainite.
Bainitic quenching is performed by isothermal holding, usually in a salt bath at a temperature between 230 and 450°C, depending on the grades being treated and the desired results.
BENEFITS
Delayed martensitic hardening
The fast part of the cooling with high rate gradient takes place in the austenitic state, thus without transformation, thus without stress.
The transformation occurs once the temperature has become uniform throughout the entire cross-section of the part (for parts with low mass), allowing for simultaneous transformation across the entire cross-section and avoiding stresses caused by overlapping transformations; thus, delayed quenching is effective for minimizing distortion and is particularly recommended for delicate parts.
Bainitic quenching
Capable of achieving, in a single heat treatment, the same hardness and tensile strength as those obtained through quenching and tempering (HRC = 35 to 55).
Less distortion than martensitic quenching, even delayed martensitic quenching.
At equivalent hardness, the ductility and fatigue strength of heat-treated steel are superior to those of quenched and tempered steel.
These treatments are usually performed after heating in a controlled atmosphere that is in equilibrium with the steel's carbon content.
The bainitic heat-treatment specification is becoming increasingly common for safety components, in order to take advantage of their impact resistance and resistance to mechanical fatigue.
APPLICATIONS
Delayed martensitic hardening
It is applied to parts with complex geometries to minimize deformation, particularly in bearing cages and rings.
Bainitic quenching
For carbon and low-alloy steels (such as spring steels), bainitic quenching is used to achieve better ductility while maintaining the same strength as that produced by quenching and tempering.
For example, this process is used in automotive safety components such as seat belt buckles and anchor rails, springs, body clips, and spring washers.
LIMITATIONS
Treatments involving immersion in salt baths require particularly thorough cleaning procedures to prevent salt buildup.
The positioning of parts in the processing assembly to prevent salt carryover.
The delayed quenching times must be observed to prevent the formation of undesirable microstructures such as bainite.
The bath must be carefully monitored and maintained to preserve its nominal composition and ensure the removal of sludge.
Health, safety, and environmental requirements are significant. Splashing and fumes may occur, and there is a risk of explosion in the event of overheating (nitrate and nitrite salt baths are unstable above 550°C). Sludge reprocessing must be properly monitored and carried out by specialized companies.
Performing isothermal bainitic quenching in vacuum furnaces is challenging because of the need to cool the charge very rapidly at the start of quenching and the temperature inhomogeneity within the charge during the holding period.
The use of low-alloy steels with high hardenability requires very long holding times during isothermal bainitic quenching.
Implementation TECHNOLOGIES
Batch or continuous atmosphere furnaces followed by quenching in a nitrate-nitrite salt bath. The nitrate-nitrite salt baths allow the charge to cool rapidly during the quenching phase. The temperature during the subsequent isothermal holding phase is very uniform.
Isothermal bainitic quenching tests were conducted in vacuum furnaces with gas-assisted cooling. The encouraging results make it possible to reduce the need for complex cleaning processes.
