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't come in contact with the skin. The specimen and the dry ice were mixed using the spatula and the thermocol box was kept in the freezer of the refrigerant for duration of 12 hrs. The temperature of the specimen and dry ice in the freezer for a time period of 12hrs ethanol of required quantity was directly added into the thermocol box. The temperature of the specimen along with the ethanol and dry ice was noted.

The thermoses box containing specimen, dry ice and ethanol was again placed in the freezer of the refrigerator for a time interval of 48 hrs. After treating the specimen for 48hrs the thermocol box was taken out of the freezer and the temperature of the specimen along with the ethanol and dry ice is once again noted. The treated are then brought down to room temperature by opening the lid of the thermocol box and kept it opened till it attain room temperature then they are taken out of the thermocol box. The purpose of using dry ice and ethanol is to bring down the temperature below 0C. Dry ice has a temperature of -30C to -40 C and ethanol has freezing temperature of -110C. Thermocol box is used to retain the temperature during treatment. By the above procedure the temperature of the specimen is brought in a step-by-step manner so that the specimen retain that temperature of -80C.

3. Comparative Study:

A comparative study is made between cryogenically treated specimen and non-cryogenically treated specimen to know about the significant change of the mechanical properties of cryogenically treated materials like wear resistance, hardness and impact strength.

3.1 Wear Test and Frictional Force:

Nadig D.S, Jacob. S, Kasthurirengan. S, Karunanithi R and Geetha Sen had conducted an experiment to study the effect of cryogenically treated HSS tool (M 42) with following composition:

Carbon: 1.1% Molybdenum : 9.5%

Vanadium : 1% Cobalt : 8%

Chromium : 4%

Tungsten : 1.5% The square tool of dimension 3/16”x3/16”x4” length which was cryogenically treated at -80C was subjected to a standard sliding wear testing pin on disk apparatus.

For comparative study a non-cryogenically treated HSS tool (M42) was tested for wear resistance and frictional force.

Grinding Wheel: A100Q5v30 Load: 20 N Speed of the grinding disc: 400 RPM Tool track radius: 20mm Test duration: 10 minutes

3.1.1. Procedure:

The pin on the disc works on the principal of sliding wear. The tool was located in a fixture and is made to press against a rotating grinding wheel. To ensure proper forced contact between the tool-grinding wheel interface, selected dead weights were mounted. When the horizontal grinding wheel rotates at a selected speed the pressing tool face continuously wears with time. Due to the continuous wear of the tool, its height reduces which is sensed by an LVDT. The output of the LVDT in micron indicates the wear resistance property of the tool. Output of the LVDT is connected to the computer for data acquisition. The data of the continuous wear and frictional force between the tool and grinding wheel are recorded by computer.

3.1.2. Result:

a. Wear Resistance:

From the above experiment it was observed that the Cryo-treated tool has developed higher wear resistance property. The improvement in the wear resistance was around 33%.It can be said that this improvement in wear resistance is caused due to the conversion of soft retained Austenite to hard Martensite, formation of Eta-carbide and development of a uniform and refined micro structure of atom with high density.

b. Frictional force:

The frictional force between the tool and rotating grinding wheel reduced after Cryogenic treatment by around 30%, as shown in graph (2). The reduced frictional force reduces the friction between the tool and material and problem of formation of built of edges, gumming effect, frequent sharpening of the tools etc., can be avoided.

3.2. Hardness Test:

Shivkumar B. J, R. Venkatatram, B. S. Ajaykumar, Pushpa Iyengar and Gopal Prakash. (2003) conducted hardness and impact test on a Cryogenically treated HSS tool to know about its variation in hardness and impact strength. Brinell hardness testing machine was used to know about the change in Brinell hardness number in a cryogenically treated HSS (M2 grade) in comparison with a non-cryogenically treated HSS tool of same grade.

3.2.1. Procedure:

Load and indentor diameter were selected depending upon the specimen. The specimen was placed on the anvil and it was moved upward by turning the elevating screw. Load was applied by means of hand lever for 30 seconds. After this the specimen was removed from the anvil and the diameter of the impression on the specimen caused due to the ball indentor was measured using a micrometer microscope. Results for both cryogenically treated and non-treated specimen were tabulated.

3.2.2. Result:

It is observed that there is an improvement in the hardness of Cryogenically treated M2 HSS specimen by 18.6% in comparison with non-cryogenically treated specimen.

3.3. Impact Test:

Izod impact testing machine was used to know about the impact strength of a Cryogenically treated HSS (M2 grade) tool in comparison with non-cryogenically treated HSS tool of same grade.

3.3.1. Procedure:

Lateral dimension of the specimen at the notch was measured, the pendulum was lifted and latched to the latching tube. The friction pointer was adjusted, the specimen was kept on the vice as a cantilever beam with the notch facing towards the striker. Then the pendulum was released, as it swung down it broke the specimen and the reading was taken and tabulated.

3.3.2. Result:

It is observed that there was a decrease in impact strength by amount of 32.3% of the Cryogenically treated HSS M2 grade tool in comparison with non-cryogenically treated HSS tool of same grade.

4. Conclusion:

From the above study we can conclude that by treating a HSS tool Cryogenically which involves subjecting it to a sub zero temperature of about -80C , soaking the specimen for certain interval of time and then bringing it back to normal temperature. There is a significant improvement in its Mechanical properties.

From the above test we can conclude that:

i. With the increase of wear resistance in a Cryogenically treated HSS tool, tool wear decreases and hence increasing its life and efficiency.

ii. With decrease in frictional force in case of Cryogenically treated HSS tool, there is less heat generation at tool chip interface and hence there is an increase in cutting efficiency and reduction in machining time and machining cost.

iii. There is an improvement in hardness of a Cryogenically treated HSS tool at the cost of ductility to a small extent.

iv. With increase in hardness there is a decrease in decrease in impact strength and hence the tool loses its toughness to a small extent.

So treating a HSS tool Cryogenically improves its mechanical properties like wear resistance and hardness and hence increases its productivity at the cost of manufacturing cost.

5. Acknowledgement:

Assistance of Dr. Madhu, Mr. H.V. Panchakshari, and our colleagues are gratefully acknowledged in making this paper.

6. Reference:

1. CARY, R. F., and ROBERTS, G. A., 1980, Tool Steel, 4th ed. (Metals Park, Ohio: American Society for Metals), pp. 667-670

2. G. N. Haidemenopoulos, G. B. Olson. Morris Cohen, and K. Tsuzaki. 1989, (Dept. of Material Science and Engg, MIT, Cambridge.), pp. 207-212

3. "Nadig D.S, Jacob. S, Kasthurirengan. S, Karunanithi R and Geetha Sen (Center for Cryogenic Technology, Indian Institute of Science, Bangalore. And ISRO, Bangalore.)

4. "Effect of Cryogenic Treatment on Mechanical Properties of HSS”. Shivkumar B. J, R. Venkatatram, B. S. Ajaykumar, Pushpa Iyengar and Gopal Prakash. (2003) (BIT and East point college of Engg, Bangalore)

5. "Performance characteristics of cryogenically treated high speed steel drill” S. Chatterjee (1991)

6. Zhmud, E. S., 1980,”Improved tool life after shock cooling”. Metal Science and Heat Treatment, October 701-703.

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