The use of salts for quenching
Written in the framework of the "Fluids and quenching systems" committee
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.
I. Description and Classification:
Quenching salts consist of a mixture of sodium nitrate/potassium nitrite salts; sodium nitrate/potassium nitrate or only sodium nitrate forming a eutectic with a melting point between 140°C and 330°C, depending on the mixture. They are generally used in the temperature range 180-550°C.
The choice of salts and the working temperature will have an impact on the viscosity of the fluid used. The more fluid the salt is, the less entrainment there is, for the choice of the product, it is therefore preferable to work with the salt that will have the lowest viscosity, in the working conditions.
The quenching salts are used in externally heated crucible furnaces equipped with a stirring system. The installations can also include a submerged cooling circuit to limit the temperature rise of the salt bath in the case of intensive production. A coil in which water circulates is placed at the bottom of the vessel.
It is also possible to find a salt pump which allows the emptying of the tank in case of necessity (example: observation of the state of the tank). It will be necessary to take care to slightly overheat the bath to avoid that it solidifies during the extraction)
Quenching salts are used for :
Hardening of parts sensitive to deformation, case-hardened or austenitized parts in salt baths as well as in a protective atmosphere (batch or belt furnaces),
bainitic tempering,
tempering of hardened parts (example of a case in aeronautics).
Heated parts should not reach temperatures above 980°C when the load is immersed in the nitrate/nitrite salt baths, in order to avoid oxidation of the baths.
Once immersed in the salt bath, rinsing is usually done with water in an installation of tanks connected in reverse cascade (usually 3) with agitation and heating.
The loaded water from the first rinsing tank is sent to a storage tank which feeds an evaporator. The dry residues obtained can be reused to level the salt bath.
II. Advantages and Disadvantages.
Benefits:
Staged hardening: bainitic, martensitic or bainito-martensitic
small deformations,
no risk of fire,
no aging since oxidation of the baths is almost impossible,
wide range of working temperatures.
Disadvantages:
Bath maintenance,
Additional rinses,
HSE constraints (projections, fumes, explosion in case of overheating, sludge treatment)
the positioning of the parts to avoid the entrainment of salts during the extraction of the charge from the molten salt bath.
III. Cooling mechanism.
The cooling mechanism is different from that of quenching liquids (oils, polymers). Indeed, the phases of calefaction / boiling are non-existent which has the advantage of offering a rapid and uniform cooling of the whole part, especially when passing the transformation temperature Ms. The cooling laws obtained with the help of a drasticimeter (Inconel) can be seen in figure 1.
Figure 1: Cooling curves with salt at 180°C.
We see that we can vary the drasticity of the medium, by setting up a device for the addition and automatic control of water (see Figure 2). We can see that the Vmax characteristic obtained after a drasticity test is higher than what is traditionally obtained with many quenching oils.
Figure 2: Salt bath installation with water addition for drastic variation.
IV. HSE Usage Constraint:
Solid state handling is carried out using PPE (gloves, dust masks).
In the molten state, protective equipment against the risk of burns is necessary. Projections of molten salts are possible (example: immersion of wet parts)
Care must also be taken with repeated overheating of the salts, which can cause the bath to explode.
Fume collection is ensured by means of suction systems located on the upper level of the furnaces. Fume treatment is recommended (especially with barium chloride salts).
Quenching salts should not be stored with oxidizing materials; they should be carefully packed in airtight containers to prevent moisture pickup.
The sludge, composed of sodium carbonate, once skimmed, must be stored and treated by a company specialized in this field.
V. Maintenance and aging :
The formation of carbonates in the quenching salt bath requires monitoring and maintenance.
The temperature at which the bath is used determines the amount of carbonate that can be present in the bath. The lower the temperature of the bath, the less soluble the carbonates will be in the bath and the less likely they will stick to the parts of the load to be treated. This may result in the creation of soft spots associated with poor heat transfer from the part to the bath.
Laboratory analyses of bath samples in service allow us to determine the carbonate content and their solubility temperature. Depending on the results obtained, a sludge removal operation (operation consisting in removing the sludge by skimming in the colder zones on the surface of the bath) may be necessary.
A reprocessing of salts is possible with companies specialized in waste reprocessing.
VI. Recommendations for starting up an installation :
The crucible is filled with solid salt (small ingots if possible) and then melted. During the melting process, the level must be compensated by adding fresh salt.
VII. Maintenance of the system : users and manufacturer action
Maintenance is localized at the bath and equipment level.
a) At the bath level:
Regular clearing of the bath.
Analysis twice a year recommended and corrective action according to the results.
It is advisable to check:
Sodium carbonate content
The H2O level, which should not exceed 2%.
The melting point
The amount of chlorides, which may come from another salt bath used for heating.
The drasticity curve in the laboratory (standardized DIN test with inconel sensor),
b) In terms of equipment:
Annual inspection of the crucible (mild steel type C15 or bi-material Inconel / mild steel)
Cleaning the oven environment
Control of the pyrometric chain.
Agitation control