The commercial grade of AISI H-13 tool steel was selected for the present work. Table 1 shows chemical composition of this material in annealed condition

which was analyzed by vacuum emission spectrometer. Small samples of ap- propriate dimensions required for microstructural, hardness, stress rupture and wear analysis were prepared from this material for planned research. All pre- pared samples were then exposed to regular heat treatment including hardening and tempering. Both of these treatments were carried out in muffle furnace with controlled heating. During hardening samples were held at 1030˚C for 60 minutes for austenitizing followed by quenching in oil stirred bath. After hardening, samples were then tempered at 560˚C for 2 hours followed by air cooling to room temperature. This was the conventional heat treatment cycle used for all specimens under consideration in this study and samples named as harden & tempered (HT) samples.

Few samples from HT group were then deep cryogenically treated in a com- puter controlled cryo-processor (Sanmer make, −185˚C). The temperature inside the cryo-processor was controlled by supplying calculated gasified liquid nitrogen through solenoid valve. The samples were first cooled down to −185˚C at the rate of 3˚C/min to avoid thermal shock and then soaked for 32 hour as shown in Figure 1. After the completion of soaking time, samples were removed from the cryo-processor and stored in insulated thermocol until they attain room temperature. All samples were then processed through soft tempering at 110˚C with 2 hours soaking, followed by air cooling to room temperature.

Microstructural analysis was carried out by using scanning electron microscope (EV018 Carl Zeiss make). The samples were subjected to series of emery paper (1/0, 2/0, 3/0 and 4/0) polishing followed by velvet cloth polishing with intermittent application of fine suspension of diamond paste. Finally samples were cleaned with acetone and etched with 2% nital etchant. The impact toughness of already prepared notched samples (as per ASTM A370 standard) was determined by using Zwick Roell make impact tester. Average impact toughness (J) of three samples for each material condition was taken as measure of impact toughness. Hardness measurement was carried out on Matsuawa with Celmex make micro-hardness tester.

Samples were polished by using 300 and 400G polish papers before hardness testing. The averages of three measurements were recorded for each material condition.

Stress rupture tests were conducted with Applied Test System (ATS) model 2410 having three zone furnace controlled by WINCCS software. The specimens used for stress rupture testing were similar to that used for tensile test (ASTM standard E8-91). Test setup and specimen conditions are shown in Figure 2. The specimen was placed in the center zone of the furnace to maintain a constant

temperature along the gauge length of the specimen. Thermocouples were attached to top and bottom of specimen to monitor the specimen temperature during the test. The specimens were gripped in the furnace with the help of assembly.

The creep tests were conducted by using following procedure based on the standard practice ASTM E139-83. Load required to apply in pan is calculated as [17]

where, w―amount of load to be placed in pan, s―stress required, a―Cross section of specimen and r―Lever arm ratio.

The test was started with a preload of about 5% - 10% of the total load. Next, the test piece was heated. When the required temperature was reached, the sample was soaked for about 60 min. The load was applied without any jerk. The test load was applied along the test axis in such a manner that it minimized the bending and torsion of the test specimen. The WINCCS software provides computer automation of the entire creep test process. After completion of the test, the specimen dimensions were measured for calculation of percentage elongation and reduction of area. In stress rupture test, range of stresses applied was 120 to 600 MPa. Temperature was varied from 550˚C to 650˚C. For the present research work based on forging die working condition applied stress was 600 MPa and temperature was 650˚C.

High temperature wear testing was carried out on pin-on-disk DUCOM make tribometer. The samples shown in Figure 3 were manufactured and given planned treatments as already described. Pins with two different types of material conditions (HT & DCT32) were tested. This tribometer works on the Archimedes equation given below.

where, k―wear rate in mm³/Nm, ΔV―the worn out volume of material (mm³), F―normal force and L―sliding distance.

Disk and sample pins were cleaned with acetone to avoid any surface contamination. Fresh disk track was used for every new experiment. Table 2 shows testing parameters which were used during wear testing. The results of wear testing reported in terms of weight loss of pins and wear rate. Weight loss of pins was calculated by taking weight of pin before and after testing whereas wear rate was calculated by using Archimedes equation as mentioned above.

The microstructure of harden and tempered (HT) sample shows needle type

tempered martensitic structure (Figure 4(a)) along with carbides of chromium, molybdenum and vanadium. The evolution of microstructure by execution of deep cryogenic treatment is shown in Figure 4(b). It is observed that martensitic needles are finer in case of cryogenically treated samples.

The microstructure of conventional (HT) sample shows mainly primary and secondary carbides as shown in Figure 5(a). In case of cryogenically treated

sample (Figure 5(b)), very fine carbides (tertiary carbides) are also present along with primary and secondary carbides which increase carbide density. The carbide density was measured by using image analysis software and it was 4.3% for conventional (HT) sample. Whereas it is increased 5.7% for cryogenically treated sample Cryogenic treatment induces residual compressive stresses which then released during soft tempering creating nucleation sites for fine carbides [12]. Smaller carbide size and their more uniform distribution were also observed in case of deep cryogenically treated (DCT only) sample. The above findings are in good agreement with the published literature [13].

Deep cryogenic treatment also transforms retained austenite to martensite. The XRD analysis of conventional (HT) and deep cryogenically treated samples do not show presence of any retained austenite. Though conventional harden and tempered sample expected to have some amount of retained austenite (RA), but this amount is very low (below 4%) and it is difficult to measure. Almost complete transformation of austenite to martensite during hardening and tempering may be due to oil quenching used during hardening of this tool steel. Though small amount of RA was there after conventional treatment, DCT ensures complete transformation of retained austenite to martensite [1] [2] [3] [4].

The hardness and impact toughness of conventionally (HT) and cryogenically (DCT32) treated samples are presented in Table 3. Cryogenically treated sample resulted in slightly higher hardness as compared to conventional sample which may be due to increased carbide density and complete transformation of retained austenite to martensite. Moreover additional microstructural modifications, like finer martensitic structure, more uniform distribution of carbides and martensitic needles significantly increases impact toughness of cryogenically treated samples.

The equations Stress rupture performance of Conventional heat treatment and deep cryogenic treatment (DCT) is given in Figure 6. This shows significant impact of deep cryogenic heat treatment on rupture time. Rupture time for cryogenically treated sample is increased by 11.49% compared to conventionally heat treated sample. It may note that rise in rupture time could to attributed to increase in small sized tertiary carbides and their more homogeneous distribution.

The wear performance of tool steels is very complex phenomenon and it not only depends on the hardness but also on the microstructure, process variables and properties of sliding materials [17]. In the present research work, wear performance in terms of wear rate for heat treated and deep cryogenically treated at

32 hours (DCT 32) material conditions is given in Figure 7. The wear rate of cryogenically treated samples (DCT32) are significantly lower than conventional harden and tempered (HT) samples. It is clearly due to formation of tertiary carbides and their increased density during deep cryogenic treatment. Deep cryogenically treated sample reduces wear rate by 32% as compared to conventional harden and tempered (HT) sample.