Krot, P. V.; Bobir, S. V.; Zharkov, I. P.; Nefedieva, E. E.; Timofeev, G. V.
December 2013
Metallurgical Processes & Equipment;Dec2013, Vol. 34 Issue 4, p1
Academic Journal
Background: Cryogenic quenching or deep cryogenic treatment (DCT) is a known method of microstructure changing of alloyed and carbon steels, which greatly increases the wear resistance and hardness of cutting tools, but process parameters and heat treatment (HT) schedules are not definitely known for certain steel grades. Various issues are considered of using cryogenic temperatures (down to -196 °C) in the HT process of tool steels. The most appropriate approach is when DCT is included into the standard schedules of HT. Materials and/or methods: Deep cryogenic treatment was carried out of samples of several alloyed steels: P6M5 (cutters), 9XC (drills), IIIX15 (bearings) been passed standard heat treatment schedules. The whole time of samples holding in LN2 was 18 hours. A simplified testing method is used for comparative analysis of abrasive wear of tools based on drilling and sharpening machines. Micro hardness of tools was checked with standard device PMT-3. Steel microstructures were investigated with the optical microscope "Axiovert 200 M". Samples wear was estimated by analytical weighing machine VLA-200 with accuracy of 1 mg. Results: It was developed a cryogenic apparatus with digital control providing the programming of different modes of cooling and heating for deep cryogenic processing of tools. The main technical parameters of cryogenic apparatus are as following: LN2 consumption - 1L/1kg of tools; cooling/heating rate - 1-5 °C/min; amount of treated tools - 20 kg. Microstructure analysis showed a homogenization of grains and fine carbides precipitation. Comparative tests on abrasive wear and measurements of micro hardness were performed. Experiments showed the absence of correlation between wear resistance and micro hardness changes of different steels after DCT. Small changes in micro hardness (3.3 % of drills (9XC) and 24.5 % of bearings (IIIX15)) corresponded to significant improvement in wear resistance (65 and 59 % respectively). While greatly increased micro hardness (170 %) of cutters (R6M5) resulted in relatively small wear resistance (26 %). That is due to different ways of DCT influence: retained austenite transformation in 9XC, IIIX15 and fine carbides precipitation in R6M5 which has higher temperature of initial quenching and was subjected to high-temperature tempering, hence, contained small amounts of austenite before DCT. Conclusion. Directions are formulated for further research and development in order to choose appropriate HT schedules for different steels to increase the efficiency of DCT. Namely, interval between stages of quenching and DCT and time of holding in LN2 need to be determined.


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