Cryogenic Treatment of Tool Steel
Cryogenic treatment cools tool steel down to between -109°F (-79°C) and -320°F (-196°C) after quenching, to finish the transformation of retained austenite into martensite. The tables on this page give the temperatures, the hold times and the temper that follows, taken from the cryogenics chapter of the source. This is a reference summary of published practice and not an Aobo Steel specification.
What cold treatment does to tool steel
Quenching rarely finishes the job. The source puts a good heat treatment at 95% to 96% transformation of austenite into martensite when the recipe is followed exactly, the timing is right at every stage and the furnace is calibrated. On mass treated parts the number is far lower, and the source found 50% to 60% on commercially treated drill bits and saw blades.
The austenite that is left keeps transforming over a long period, but very slowly, so the part goes on changing in structure and in size while it is in service. That is why the source directs quality control gages through a cryogenic process. A gage that is still moving cannot hold a tolerance.
Cold treatment is the only method the source knows of that comes close to full transformation, at 99.8% to 100% martensite. Dry ice reaches up to 97% when the soak is long enough. The gain between the two is small in percentage terms and the source still calls it significant for wear resistance on many tools.
The three routes and the numbers for each
There are three practical routes, and the source states the numbers for each of them. Commercial firms treat tools as a service, in a batch large enough to fill the freezer, and typical charges run from $1 to $9 per pound with a $50 to $100 minimum. A processor that works only with dry gas will reach -260 to -300°F (-162 to -184°C), which the source calls the minimum temperature for the change to happen at all.
The route that reaches the coldest temperature introduces liquid nitrogen only after the load has come down slowly on gas, so the parts never take a thermal shock. Liquid nitrogen can also be bought in a dewar for in house work, but the source is direct that this needs training and equipment built for the process, not a chest freezer with a container inside.
Dry ice needs no capital equipment. The source describes a Styrofoam chest lined with plastic sheet, the tools in the bottom, K1 kerosene poured over them to carry the cold evenly, and a vented lid. The kerosene does not freeze at that temperature. The chest must be able to breathe, because dry ice expands to three or four times its volume as it thaws.
Cryogenic treatment routes and their temperatures and hold times
| Route | Temperature reached | Cool down | Hold at temperature | Warm up |
|---|---|---|---|---|
| Liquid nitrogen, dry gas first and then liquid | -320°F (-196°C) | 4 to 8 hours | 20 hours minimum | 20 to 30 hours, with no added heat |
| Liquid nitrogen, liquid let in at the end | -250°F (-157°C) on gas, then liquid | a few hours on gas alone | 18 to 20 hours covered | on its own, lid closed |
| Dry ice in a Styrofoam chest | -109°F (-79°C) | not stated | 8 to 10 hours minimum, longer is better | 6 to 7 hours to 2 days |
Source, Cryogenics chapter, printed pages 108 to 113. A processor limited to dry gas reaches only -260 to -300°F (-162 to -184°C), which the source calls the minimum needed to produce the required change.
How much of the austenite is transformed
The two tables above sit next to each other for a reason. What the part ends up with depends on what the heat treatment left behind, and the source is careful to say so. Tools that were poorly heat treated when they were made have the most to gain from a cold soak, because they start with the most retained austenite.
Austenite to martensite transformation by condition
| Condition | Martensite after treatment | What the source says |
|---|---|---|
| Good practice, recipe followed, equipment calibrated | 95% to 96% | the most a straight heat treatment normally delivers |
| Mass heat treated items, drill bits and saw blades as the example | 50% to 60% | what the source found on commercially treated parts |
| Dry ice, soaked long enough | up to 97% | set by how much retained austenite is left after tempering |
| Liquid nitrogen, dry or wet, load packed correctly | 99.8% to 100% | the only route that comes close to full transformation |
Source, Cryogenics chapter, printed pages 107 and 113.
(a) The source adds that the 97% and the 99.8% to 100% figures both assume a sound heat treatment that itself yielded 75% to 80% martensite.
Tempering after the cold treatment
Tempering after the cold treatment is not optional. The parts must be back at room temperature first, and then held at 300°F (150°C) for 2 hours per inch of thickness. If the tempering temperature used earlier in the process is known, going 25°F (14°C) below it gives a better result, and it still cannot soften the part below the intended hardness.
The source reports that not every commercial processor carries this temper out, and it suggests a simple check. If there is any doubt, the tools can be put in a workshop oven at 300°F, which is below any prescribed tempering temperature for tool steel and therefore cannot change the hardness that was already set.
Post cryogenic tempering requirements
| Step | Requirement | Why it matters |
|---|---|---|
| Tempering temperature | 300°F (150°C), or 25°F (14°C) below the tempering temperature originally used | 300°F sits below any prescribed tempering temperature, so it cannot change the temper hardness |
| Tempering time | 2 hours per inch of thickness | the source repeats the 2 hour per inch rate at every stage of the process |
| When it starts | after the parts are back at room temperature | it stabilises the martensite formed during the cold soak |
| What to confirm with the processor | that the temper is actually carried out | the source reports that some commercial processors skip it |
Source, Cryogenics chapter, printed pages 109, 111 and 112.
The thin film effect on freshly treated tools
A freshly treated tool usually wears a little better and nothing like as well as it should. The reason is a surface layer of less than 0.001 in (0.0254 mm) that keeps close to the wear of untreated steel. Grinding removes that layer and the improved wear appears behind it.
The practical sequence that follows is the one the source recommends for a first trial. Send tools that are already dull for treatment, then sharpen them when they come back. Sharpening has no effect on the process, because the treatment goes through the whole section rather than sitting on the surface as a coating does. It also does not make the tool more brittle.
Safety in handling liquid nitrogen and dry ice
Liquid nitrogen and dry ice both need care. A sealed container of either will rupture as the gas expands, so the chamber has to be vented. Liquid nitrogen displaces air, so the area has to be well ventilated to avoid asphyxiation. Frozen parts are brittle enough to fail catastrophically from thermal shock and are not to be taken out of the chamber early, and letting air into a cold chamber brings condensation and oxides onto the parts.
Before you heat treat
Tool steel is supplied in the annealed condition, which is described on the annealed condition of tool steel page, and the treatment that comes before a cold soak is on the tool steel heat treatment guide, the tool steel hardening and tempering chart and the tool steel tempering chart. Size change at the hardening stage is covered on the tool steel size change in heat treatment page, and gage work on the tool steel for gages and precision tools page. The grades most often sent for cold treatment are on the D2 tool steel, H13 tool steel, M2 tool steel and O1 tool steel pages.
Reference data compiled from the Cryogenics chapter of Heat Treatment, Selection, and Application of Tool Steels (William E. Bryson, 2nd edition, Hanser), printed pages 107 to 113, with the retained austenite figures also used in the CPM 10V chapter, printed page 105.