The inner box was partially filled with the trichloroethylene and then the broken pieces of dry ice were gradually added to the liquid. Violent effervescing would occur until the liquid cooled to its low temperature of approximately -60°C. Once the effervescing discontinued, it was not necessary to add more dry ice.
Of course, the use of trichloroethylene is now a prohibitive liquid because of health and safety issues, so what alternative can be used instead of trichloroethylene? A simple alternative is commercial kerosene, which will act as the liquid in place of trichloroethylene. Be sure that you place the equipment for the freezing treatment into a well-ventilated area.
The use of the kerosene as the freezing mixture is not quite as effective as liquid nitrogen, but it is effective as a freezing mixture and will serve the purpose of decomposing retained austenite. Be sure that you let the freezing mixture of dry ice and kerosene get down to its lowest temperature. This can be physically seen when the liquid stops effervescing. Place the parts to be treated very carefully into the freezing mixture. It is mandatory that you are wearing protective gloves and a face mask or goggles to prevent being splashed with the freezing mixture. (This has the ability to cause a bad freeze burn.) Cover the inner box with a loose-fitting lid to allow CO2 ventilation.
Once the procedure has been completed, remove the parts from the liquid and place them onto a table with a mesh face to allow shop air to naturally pass over the work surfaces. Do not force the rise up to temperature by blowing the parts with a warm-air blower. This can induce surface stress into the freshly formed martensite, which has decomposed from the retained austenite.
You will see the work frost over when it is removed from the liquid. When the frost has completely disappeared, you can consider a light temper at around 420°F. The selection of the tempering temperature will be dependent on the hardness after the cold treatment and the final hardness required of the tool. Do not, under any circumstances, put the tool straight into the tempering furnace after removal from the freezing mixture.
Part 4, coming next Friday, will address the use of liquid nitrogen.