| 1. Introduction:Mechanical properties like | | | | Cryogenic treatment of the tool was done by |
| hardness, wear resistant, tensile strength | | | | indirect cooling and with no direct contact |
| are very important for cutting tool. The | | | | with liquid nitrogen. The Cryo-treatment is a |
| Cryogenic treatment on metal has been | | | | doubled walled stainless steel container with |
| extensively employed since many years for | | | | inter-spaced filled with Polyurethane foam. |
| various application like stabilizing | | | | The top cover made of stainless steel has a |
| dimension of precision machined part and | | | | double end shaft fan motor assembly mounted |
| gauges with change in temperature, removal of | | | | centrally. The liquid nitrogen, connecting |
| internal stresses, improving wear resistant | | | | valve, pressure gauzes, feed through and |
| and hardness.Cryogenic treatment of metal | | | | outlet connections are all mounted on the top |
| parts means cooling these parts at a | | | | flange.Below the top cover a cylindrical |
| predetermined rate, up to a given Cryogenic | | | | buffer tank is mounted where the liquid |
| temperature (-80C), maintaining these | | | | nitrogen get collected and evaporated vapour |
| parts at that lowest temperature for a given | | | | are vented through a vent pipe. The shaft of |
| duration of time and then allowing these | | | | the fan motor assembly passes through this |
| parts to warm-up at a given warming-up rate | | | | buffer tank and has fan blades mounted on |
| to room temperature.So the main variable of | | | | both sides. A copper disk has been fixed to |
| the Cryogenic treatment are:a. Rate of | | | | the bottom of the buffer tank to ensure |
| cooling.b. The lowest maintained temperature | | | | better heat transfer. Aluminium fans are |
| for a given duration.c. The duration for | | | | fixed at the bottom to ensure forced |
| which the specimen are maintained at the | | | | convection cooling of the space inside the |
| lowest temperature.d. The rate of warming | | | | Cryo-treatment chamber. The specimen kept in |
| up(Chillar, Agarwal. 1995)Cryogenic treatment | | | | the stainless steel tray can be cooled by two |
| has been successfully applied on steel to | | | | ways:i. A circulating fan housed below the |
| improve its mechanical properties. This | | | | buffer tank, induce forced convection |
| occurs because of transformation of almost | | | | currents picking up cold from stored liquid |
| all the retained Austenite in steel to | | | | nitrogen in the buffer tank downward over the |
| Martensite thereby making the steel more Wear | | | | specimen. The cold gas moving down to the |
| resistant.2. Literature Survey:Evidence of | | | | bottom of the chamber gets cooled and moves |
| Cryogenic treatment was found during 1937 in | | | | radially sideways. These gases re-enter on |
| Soviet Union (Carry and Robert 1980). Metals | | | | the fan region through the opening of the |
| are gradually cooled to cryogenic temperature | | | | slots in the aluminium fins. Thus forced |
| (-80C), soaked for a prolonged period | | | | convection current loop is established within |
| and warmed to room temperature at a | | | | the chamber.ii. Part of the liquid nitrogen |
| predetermined rate, the lattice structure of | | | | stored in the buffer tank is made to |
| the atoms change due to stress being relived | | | | circulate over the shroud by thermo-syphon |
| during Cryogenic treatment. In case of | | | | effect. To facilitate this a copper tube is |
| ferrous metal, the soft ductile FCC structure | | | | connected to the buffer tank and is brazed on |
| Austenite gets converted into strong and | | | | the outer wall of the shroud over its entire |
| harder BCC structured Martensite. Apart from | | | | length. A solenoid valve is operated by a PID |
| this, a wide precipitate of newly formed | | | | controller with predetermined set points, is |
| Eta-Carbides (responsible for increased wear | | | | used to regulate the liquid nitrogen supply |
| resistance) into the hard Martenside | | | | to the chamber. The temperature of the |
| structure induces a dense lattice | | | | specimen is measured using temperature |
| structure.Some theories by Zhmud (1980) | | | | sensors.Another method can be by using |
| are:a. Cryogenic treatment was effective for | | | | refrigerator with ethyl alcohol and dry ice |
| intemperate and adequate tempered tool.b. The | | | | as refrigerant for Cryogenic treatment. The |
| soaked time (5min to 10 hrs) had no effect on | | | | specimen to be treated was placed in a clean |
| the tool life.c. A decrease in tool life | | | | and dry thermocol box. The specimen with the |
| within 5-7 days after treatment and increases | | | | thermocol box were then kept in a |
| again with repeated treatment.Cryogenic | | | | refrigerator for duration of 12hrs to bring |
| treatment can be performed by different | | | | down the temperature of the specimen to |
| methods.One of such methods consists of a | | | | 0C. After a period of 12 hrs, dry ice |
| Cryo-treatment chamber and an auxiliary | | | | was added into the thermocol box using a |
| liquid nitrogen supply system. In this set-up | | | | spatula. |