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.
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 TREMPING, provided that cooling is sufficiently rapid, leads to the martensitic structure (the hardest), the transformation occurs successively from the surface to the core of the part according to the cooling gradient (the skin is cooled first, then the sub-layer). This process sometimes leads to deformations due to the accumulation of stresses linked to the superposition of transformations.
THE DIFFERENTIAL TREMP produces according to the isothermal holding temperature:
A martensitic structure if the holding is carried out slightly above the temperature of the beginning of the martensitic transformation Ms for a time lower than the beginning of the bainitic transformation, after this holding, the deviation from the temperature of martensitic transformation is sufficiently low that a simple exit to air leads to the transformation.
A bainitic structure if it is held above Ms for a sufficient time to completely transform the austenite.
The bainitic structure is a fine structure like martensite and reaches hardness levels similar to that of tempered martensite (quenching and tempering).
What are the benefits of delayed quenching?
The fast part of the cooling with high rate gradient takes place in the austenitic state, thus without transformation, thus without stress.
The transformation is carried out when the temperature has become homogeneous in the whole section of the part (for not very massive parts) allowing a simultaneous transformation in the whole section avoiding the constraints due to the superimpositions of the transformations, thus the delayed quenching is favorable to minimize the deformations and particularly recommended for the delicate parts.
BAINTICIAN QUENCHING is carried out by isothermal maintenance in a salt bath at a temperature of between 230 and 450°C, depending on the grades treated and the desired results. In addition to the advantages of delayed quenching, it offers specific qualities:
Ability to obtain by a single treatment the equivalent in hardness and tensile strength of a hardening and tempering treatment (HRC = 35 to 55).
Less deformation than martensitic hardening (even delayed) (bainitic transformation takes place without developing stresses in the steel's crystal lattice).
With equivalent hardness, the ductility of the treated steel is superior to that of a tempered steel, hence the applications on parts that must have a good resistance to shocks: (see comparative tensile curves).
These treatments are carried out after heating in a controlled atmosphere in equilibrium with the carbon content of the steel, carburizing or carbonitriding.
The bainitic hardening specification is becoming more and more important for safety parts in order to benefit from the impact resistance that is particularly important to guarantee for this type of part.
APPLICATIONS OF BAINITIC QUENCHING
APPLICATIONS OF BAINITIC QUENCHING
On carbon and low-alloy steels (spring steel type), bainitic hardening is used to produce a better ductility with the same characteristics as those produced by hardening and tempering.
This application can be found on safety parts in the passenger compartment of a car: skirts, seatbelt rails. Many parts with a spring function: bodywork clips, spring washers and stops. (see table below)
It is also used for bearings and diesel injection components made of 100Cr6 steel, where a lower bainite structure is required, with a hardness very close to that of martensite, and which offers a better distribution of the stresses induced by the treatment, totally excluding residual austenite and thus guaranteeing perfect dimensional stability over time.
Spheroidal graphite cast irons (SGC), when they are not made to have this bainitic structure at the mould outlet, undergo a bainitic quenching treatment.
At the present time, there is no other way to effectively carry out the cooling stage, the temperatures of which vary from 200 to 450°C, other than by keeping them in a salt bath consisting of a mixture of sodium or potassium nitrate-nitrite. The wash water from the treated parts is generally recovered for purification in a thermal evaporator. The salts are then recovered and recycled in the quenching bath.
It should be noted that the parts treated in this way must be properly washed or neutralized and protected in order to avoid their corrosion by the residual salt action.
Source DURFERRIT Thermal Treatment N° 316
Implementation
Main equipment (furnace, reactor, line, machine...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
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