CHOICE OF QUENCHING FLUIDS
Written in the framework of the A3TS commission "Fluids and quenching systems
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.
Guide to choosing a quenching fluid
The choice of a quenching fluid is conditioned by a certain number of constraints, ranging from the metallurgical requirements of the part design (metal to be treated, geometry, etc.) to the conditions of implementation (installation), taking into consideration the technical and economic criteria set beforehand.
The main properties sought for the quenching fluid are:
cooling in optimum conditions with respect to the metallurgical characteristics of the material to be treated.
avoid the risk of tapping
minimize deformations
be inert with respect to the surfaces to be cooled.
Characteristics sought on treated parts
- Mechanical and metallurgical
Once the steel is defined, the drasticity (cooling curve) of the quenching fluid will be decisive for the choice. Depending on the case, we can choose
Water for quenching
non-alloyed steels
aluminium alloys
titanium alloys
of certain copper alloys
Water/polymer emulsions for quenching
low-alloy steels
aluminium alloys
Mineral or synthetic oils for quenching
Alloyed steels with good hardenability (medium drastic oil)
steels with low hardenability (oil with accelerated drasticity)
Air foror the quenching of "self-hardening" steels
Gases foror the quenching of alloy steels and special alloys previously heated under vacuum or in an atmosphere
- Geometric
The choice of fluid should also take into account the complexity of the geometry of the part to be treated, the more severe the drasticity of it will be, the greater the risk of deformation and / or tapping of hardening will increase, especially in the context of the use of a cold hardening oil.
If the hardenability allows it, a cold, non-accelerated hardening oil should be used, or preferably a semi-hot or hot hardening oil with a medium or slightly accelerated hardness.
- Selection of the quenching fluid
First of all, it is necessary to know the characteristics (given by the steel manufacturer) of the metal to be treated. Then, you need to know what you want to obtain, i.e. the surface and core hardness. It is also important to know the operations undergone before (residual stresses related to the shaping of the part) and after heat treatment (leaching, tempering, shot blasting, grinding, hard turning, etc.).
A wrong choice of quenching fluid or the drift of certain parameters during production can lead to additional costs resulting from incorrect cooling compared to the specifications. This could result in additional operating costs due to non-conformities
geometrical (requiring reworking),
metallurgical (insufficient hardness of the parts to be reprocessed)
and in the worst case, scrap parts.
To obtain the desired metallurgical characteristics, continuous or staged cooling laws (TRC or TTT curves) are used. This is a determining factor for metallurgists who use quench cooling to give a given part the desired properties while minimizing the risk of defects such as tapping or deformation.
For the latter to be optimal, the concentration of the solid solution must be practically the same as at the solution temperature, which does not necessarily imply that the cooling is very rapid.
During accelerated cooling, the structural transformations take place under non-equilibrium conditions: we therefore deviate from what the phase diagram predicts (or equilibrium diagram).
For this purpose, a diagram called TRC diagram (Transformations in Continuous Cooling) is used. The critical quenching speeds that guarantee the best compromise between characteristics and deformations, as well as the transformation points, are generally specific to each steel.
The necessary cooling must be fast enough to form the hard constituents such as martensite and/or bainite (lower or upper) to a predefined depth (surface and core hardness) in order to avoid the formation of soft constituents such as pearlite and/or ferrite, or even bainite when 100% martensite is sought.
The parameters that influence the metallurgical results are in principle the composition of the steel, but for a given alloy, it is possible to act on the quenching fluid and its parameters in service, such as agitation, bath temperature, product concentration in the case of polymers, etc. This is what is known as the quenching severity of an installation (see below).
For any given steel composition, we have a critical cooling rate to obtain maximum hardness with minimum stresses. At this critical rate, the austenite formed at high temperature turns into martensite avoiding the pearlite/bainite nose.
- Surface condition
Obtaining metallurgical characteristics with, if possible, a good surface finish (no iron oxide formation, deposits, stains, etc.) avoids the cost of reworking or finishing by tribofinishing. The hardening fluid will be chosen for its ability not to alter the surface of the hardened part.
The fluid used should therefore be designed to have
a good resistance to thermal shocks and oxidation, thus avoiding its decomposition into sticky and colored elements that make post-treatment washing operations difficult.
an absence of additives coloring the parts or making a deposit that makes subsequent treatment operations difficult (electrolytic surface treatments, electroplating, etc.).
easy washing before tempering to avoid the formation of hard deposits during the reheating of the part during tempering, which can be a nuisance for later operations or the simple presentation of the part. For example, a bad washing after quenching with oil will generate polluting fumes for the environment and clogging of the furnace.
The chosen fluid must therefore take into account the washing process, ease of degreasing and release (in the case of a quenching oil) in order not to quickly saturate the washing product (lye or solvent) to ensure the longevity of the bath and limit waste for destruction.
Consideration of the process-installation couple
For the practitioner, the ideal would be to use a single quenching fluid adapted to all his heat treatments and installations.
Unfortunately, this desirable versatility is rarely met, and apart from the selection criteria related to metallurgical, geometrical and surface finish requirements, the selection of the quenching fluid will have to take into account certain specificities of treatments or installations. Hereafter, we will describe some cases.
- Influence of the treatment
Conventional quenching treatment in an independent quenching tank
Generally, the parts are heated without a protective atmosphere and are quenched in a tank independent of the austenitizing furnace. This type of treatment is most often practiced on raw forged parts where the requirements of surface condition and least deformation are limited, we will use in this case, preferably, a quenching oil or a polymer if these fluids meet the desired metallurgical requirements and safety and environmental problems ... For parts whose surface must not be altered or deformation limited for finishing operations we will use a furnace with a built-in quenching tank or a pass-through furnace (see below).
Hardening after case hardening or carbonitriding
Case hardened steel is a surface enriched alloy which makes its structure duplex, i.e. the core is made of a mild base steel with low hardening and hardenability, and the surface is made of a carbon-rich steel (and nitrogen in the case of carbonitriding) with progressive hardening and hardenability depending on the depth of the enriched layer and its composition.
For thin parts, fluids with a medium cooling rate can be used (cold or semi-hot quenching oils, quenching polymers, gases).
For parts with large cross-sections requiring a faster speed, accelerated oils, quenching polymers are used.
Generally speaking, since carbonitrided parts are often small or medium-sized (screws, pins) and are not reworked after quenching, the quenching fluid chosen should not alter the surface.
As many carbonitrided parts are made of free-cutting steels or steels with irregular hardenability, the use of a fluid with accelerated drasticity is desirable (accelerated oil, polymer). If this is not possible, an adaptation of the carbonitriding range is necessary.Cementation under atmosphere in an incorporated tank
The fluid used must have a low vapour pressure (generally linked to a high flash point) to avoid pollution of the atmosphere by the vapours of the fluid and the clogging of the measuring probes. Depending on the type of "specific" installation, gas quenching is an alternative to oils or polymers.Cementation in salt baths
These installations are tending to disappear due to hygiene and environmental problems, as the cementation salts are most often cyanide. In this particular case, the use of so-called "compound" oils should be avoided, whose fatty substances, by saponifying, make washing operations difficult.Vacuum or low pressure carburizing
Depending on the configuration of the installation, either a specific synthetic oil (with low vapor pressure), or a pressurized gas (nitrogen or helium) is used. The fluid used must not affect the surface condition of the hardened parts
- Influence of the quenching device
Incorporated quenching tank
The choice of quenching fluid for tanks integrated in the same metal structure as the laboratory or the heating chamber of the treatment installation must take into account specificities such as:
o Type of loads or treated parts: most often, the loads are massive and the volume of quenching fluid actually used for the metal/fluid heat exchange can be small. This results in significant heating of the quenching fluid immediately after the load is immersed. It is not uncommon for the temperature of the quenching fluid to rise by 30 to 50°C after the charge is immersed.
The quenching of vertically immersed "tube" type parts causes significant vaporization.
o Treatment atmosphere: too much emission of vapors from the quenching fluid will contaminate the laboratory atmosphere and distort the results of CO-CO2 measurements.
o Quenching bath not visible: the fact that the tank is incorporated into the furnace makes it difficult to visually check the fluid bath and the risks of incidents such as fires (for quenching oils) inherent to pollution or malfunction (blocked elevator) are significant.
o The chosen quenching fluid must have characteristics specific to its use in the installation (suitable viscosity, flash point, volatility, etc.). For example, for a quenching oil, the flash point should be as high as possible in relation to its maximum heating temperature, in general the following formula should be used: flash point > 50°C in relation to the maximum heating temperature (not the working temperature).
The use of so-called "washable" oils should be avoided, as the emulgator disperses the water in the oil mass, making it difficult to detect and thus increasing the risk of fire.Pass-through or belt furnaces
Most often, this type of furnace is followed by an open-air quenching tank connected to the heating chamber by a drop channel or a chute immersed in the quenching fluid. In this type of installation, small parts such as springs, screws and bolts are treated.
The quenching fluid must have both a low viscosity to limit consumption by entrainment with the parts, and a high flash point (in the case of an oil) limiting the emission of vapors which, by rising through the chute, would pollute the atmosphere of the furnace and dirty the parts.
In general, the quenching fluid used is required to obtain white parts.
- Control of the hardening power of a plant
The use of the corner test piece, according to the NFT 60179 standard, will allow to characterize the global severity of quenching of an installation by taking into account all the parameters of heating (furnace) and cooling (tank, agitation and fluid).
Other types of specimens are also used in industry (2-step specimens).
User/supplier relationship for quenching fluid
The following tables help define the quenching problem and inform the supplier to recommend the most suitable fluid.
Synoptic table to help in the choice of a quenching fluid (adapted from a CETIM document)
Selection Guide - User / Supplier
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