Tool Steel Hardening Furnaces and Salt Bath Working Temperatures
A tool steel is only as good as the furnace cycle that hardens it, and the choice between a salt bath, a vacuum furnace and a controlled atmosphere furnace decides how much distortion, decarburisation and cleaning a die shop has to live with afterwards. The tables below are the salt bath line of that choice. They give six salt mixtures by composition, melting point and recommended working temperature range, grouped by the job the bath does in the cycle, so a shop can match a mixture to the preheat, austenitising or quench stage it is buying it for. This is a reference summary of published practice and not a specification of ours.
What a salt bath does that a furnace atmosphere cannot
In a salt bath the work is immersed in molten salt and heat reaches it by conduction, with the salt acting as a ready reservoir rather than as a radiating wall. The core of a tool then rises in temperature at roughly the same rate as its surface, which is the property the other furnace types cannot match, because convection and radiation heating cannot supply heat as fast as the whole tool absorbs it. The source gives the practical size of that difference, a 25 mm (1 in.) bar reaching temperature equilibrium in 4 min in a salt bath against 20 to 30 min in a convection or radiation furnace.
Three consequences follow and they are the reasons a salt bath is still the standard route for high speed steel tools. Distortion and residual stress are lower because the temperature is uniform and there is no thermal shock from a cold atmosphere. The surface comes out clean, free of carburisation, decarburisation and scale, which matters most for tools that cannot be ground after hardening or that need sharp edges. And the bath puts almost all of the power it draws into the work, about 93 to 97 per cent of the electricity consumed going directly into heating.
Salt mixtures by the job they do in the cycle
Austenitising salts, the high heat bath
The high heat bath is the austenitising step itself and these are the two barium chloride mixtures that run it. Mixture 1 is the straight 98 to 100 per cent barium chloride bath, the hottest of the six and the one used where the full 1300 °C top of the range is needed. Mixture 2 adds 10 to 20 per cent sodium chloride, which pulls the melting point down by 80 °C and widens the useful band downwards to 930 °C, so it covers the hardening temperatures of the hot work and high speed grades without going to the top of the range.
| Salt mixture No. | BaCl2, % | NaCl, % | KCl, % | CaCl2, % | NaNO3, % | KNO3, % | Melting point, °C | Melting point, °F | Working range, °C | Working range, °F |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 98-100 | … | … | … | … | … | 950 | 1742 | 1035-1300 | 1895-2370 |
| 2 | 80-90 | 10-20 | … | … | … | … | 870 | 1598 | 930-1300 | 1705-2370 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 6-1, Typical compositions and recommended working temperature ranges of salt mixtures used in heat treating of tool steels, printed page 106. The three dots are printed by the source where a mixture contains none of that constituent. This is a reference summary of published practice and not an Aobo Steel specification.
Preheat salts
A preheat bath does two jobs at once. It takes the thermal shock out of the step up to the austenitising temperature, and it shortens the time the tool has to sit at the high heat, which is what limits grain growth and decarburisation on the way in. Mixture 3 is the cheaper 70 per cent barium chloride and 30 per cent sodium chloride bath and mixture 4 brings in 25 per cent potassium chloride for a lower melting point and a lower working band.
| Salt mixture No. | BaCl2, % | NaCl, % | KCl, % | CaCl2, % | NaNO3, % | KNO3, % | Melting point, °C | Melting point, °F | Working range, °C | Working range, °F |
|---|---|---|---|---|---|---|---|---|---|---|
| 3 | 70 | 30 | … | … | … | … | 335 | 635 | 700-1035 | 1290-1895 |
| 4 | 55 | 20 | 25 | … | … | … | 550 | 1022 | 590-925 | 1095-1700 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 6-1, printed page 106. Mixture 3 is printed with a melting point of 335 °C, below the melting point of either of its two constituents; the figure is reproduced as the source prints it rather than corrected. This is a reference summary of published practice and not an Aobo Steel specification.
Quench and temper salts
The quench bath equalises the temperature on the way down and leaves the clean surface the earlier stages have produced, and a nitrate bath of this kind also serves the tempering step, where a salt at 500 to 675 °C does the same work as a tempering furnace with a tighter hold on the part. Mixture 5 is the chloride type at the top of that band and mixture 6 is the nitrate type, the lowest melting of the six and the only one built entirely from sodium and potassium nitrate.
| Salt mixture No. | BaCl2, % | NaCl, % | KCl, % | CaCl2, % | NaNO3, % | KNO3, % | Melting point, °C | Melting point, °F | Working range, °C | Working range, °F |
|---|---|---|---|---|---|---|---|---|---|---|
| 5 | 30 | 20 | … | 50 | … | … | 450 | 842 | 500-675 | 930-1250 |
| 6 | … | … | … | … | 55-80 | 20-45 | 250 | 482 | 285-575 | 545-1065 |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 6-1, printed page 106. The three dots are printed by the source where a mixture contains none of that constituent. This is a reference summary of published practice and not an Aobo Steel specification.
How to read the melting point against the working range
The two right hand pairs of columns are the practical part of the table, and they have to be read as a pair. The melting point is the floor of the bath, the temperature at which the salt is liquid at all; the working range is the band the source recommends for heat treating in it. The gap between them is the safety margin against freezing the bath on a cold load or a cold furnace wall, which is why the preheat and quench baths carry a working range well above their melting point and the austenitising baths, run hot and continuously, start closer to theirs.
Read the chloride group and the nitrate group separately. All four chloride mixtures are built on barium chloride and run to high temperature, and the sodium chloride and potassium chloride additions are there to move the melting point, which is the only thing a shop can tune without changing the hardware. The two nitrate mixtures sit at the other end of the scale and are used where the working temperature is under 700 °C and the bath has to stay fluid and clean around a finished part rather than harden it.
Related reference data
The grades that most often go through these baths are described with their forms and delivery conditions on M2 tool steel, H13 tool steel and D2 tool steel. The hardening and tempering parameters a salt bath has to deliver for the whole AISI range are tabulated on tool steel hardening and tempering chart, the temperatures each grade is held at are collected on tool steel heat treatment temperature ranges, and the surface treatments applied after hardening are described on tool steel surface treatment.
Compiled from Tool Steels (G. A. Roberts and G. Krauss), Table 6-1, Typical compositions and recommended working temperature ranges of salt mixtures used in heat treating of tool steels, on printed page 106, the chapter on practical aspects of tool steel heat treatment. The table is reproduced as published and is a reference summary rather than an Aobo Steel specification, so treat it as the practice of one metallurgical textbook rather than as a recipe for a particular furnace.