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