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