1. Introduction:

Mechanical properties like hardness, wear resistant, tensile strength are very important for cutting tool. The Cryogenic treatment on metal has been extensively employed since many years for various application like stabilizing dimension of precision machined part and gauges with change in temperature, removal of internal stresses, improving wear resistant and hardness.

Cryogenic treatment of metal parts means cooling these parts at a predetermined rate, up to a given Cryogenic temperature (-80C), maintaining these parts at that lowest temperature for a given duration of time and then allowing these parts to warm-up at a given warming-up rate to room temperature.

So the main variable of the Cryogenic treatment are:

a. Rate of cooling.

b. The lowest maintained temperature for a given duration.

c. The duration for which the specimen are maintained at the lowest temperature.

d. The rate of warming up

(Chillar, Agarwal. 1995)

Cryogenic treatment has been successfully applied on steel to improve its mechanical properties. This occurs because of transformation of almost all the retained Austenite in steel to Martensite thereby making the steel more Wear resistant.

2. Literature Survey:

Evidence of Cryogenic treatment was found during 1937 in Soviet Union (Carry and Robert 1980). Metals are gradually cooled to cryogenic temperature (-80C), soaked for a prolonged period and warmed to room temperature at a predetermined rate, the lattice structure of the atoms change due to stress being relived during Cryogenic treatment. In case of ferrous metal, the soft ductile FCC structure Austenite gets converted into strong and harder BCC structured Martensite. Apart from this, a wide precipitate of newly formed Eta-Carbides (responsible for increased wear resistance) into the hard Martenside structure induces a dense lattice structure.

Some theories by Zhmud (1980) are:

a. Cryogenic treatment was effective for intemperate and adequate tempered tool.

b. The soaked time (5min to 10 hrs) had no effect on the tool life.

c. A decrease in tool life within 5-7 days after treatment and increases again with repeated treatment.

Cryogenic treatment can be performed by different methods.One of such methods consists of a Cryo-treatment chamber and an auxiliary liquid nitrogen supply system. In this set-up Cryogenic treatment of the tool was done by indirect cooling and with no direct contact with liquid nitrogen. The Cryo-treatment is a doubled walled stainless steel container with inter-spaced filled with Polyurethane foam. The top cover made of stainless steel has a double end shaft fan motor assembly mounted centrally. The liquid nitrogen, connecting valve, pressure gauzes, feed through and outlet connections are all mounted on the top flange.

Below the top cover a cylindrical buffer tank is mounted where the liquid nitrogen get collected and evaporated vapour are vented through a vent pipe. The shaft of the fan motor assembly passes through this buffer tank and has fan blades mounted on both sides. A copper disk has been fixed to the bottom of the buffer tank to ensure better heat transfer. Aluminium fans are fixed at the bottom to ensure forced convection cooling of the space inside the Cryo-treatment chamber. The specimen kept in the stainless steel tray can be cooled by two ways:

i. A circulating fan housed below the buffer tank, induce forced convection currents picking up cold from stored liquid nitrogen in the buffer tank downward over the specimen. The cold gas moving down to the bottom of the chamber gets cooled and moves radially sideways. These gases re-enter on the fan region through the opening of the slots in the aluminium fins. Thus forced convection current loop is established within the chamber.

ii. Part of the liquid nitrogen stored in the buffer tank is made to circulate over the shroud by thermo-syphon effect. To facilitate this a copper tube is connected to the buffer tank and is brazed on the outer wall of the shroud over its entire length. A solenoid valve is operated by a PID controller with predetermined set points, is used to regulate the liquid nitrogen supply to the chamber. The temperature of the specimen is measured using temperature sensors.

Another method can be by using refrigerator with ethyl alcohol and dry ice as refrigerant for Cryogenic treatment. The specimen to be treated was placed in a clean and dry thermocol box. The specimen with the thermocol box were then kept in a refrigerator for duration of 12hrs to bring down the temperature of the specimen to 0C. After a period of 12 hrs, dry ice was added into the thermocol box using a spatula. While handling, care was taken such that cry ice doesn