Sand spots, attached to a copper ball surface by means of polyvinyl acetate adhesive and distributed over the surface with areal density that ranges between one spot per 1.18 cm(2) (for low-density spots) and one spot per 0.51 cm(2) (for high-density spots), serve as a temporary heat transfer enhancer during the quenching in liquid nitrogen. Highest heat flux densities, achieved during quenching, lie in the range 10.8 to 20.2 W/cm(2), depending on the sand layer structure. Application of the temporary enhancer increases an amount of heat, evacuated by highly effective nucleate and transition boiling, by factor of 4.5 as compared with the bare sample. The process of sand layer preparation, data acquisition peculiarities, relationship between heat exchange efficiency and the spots areal density, along with sand grit size are discussed in this paper.

The effect of the rapid cryogenic treatment on hardness and wear resistance of several kinds of tool steel was examined. Two ways of cryogenic cooling were evaluated: direct immersion of the metallic samples into liquid nitrogen and three-stage rapid cryogenic cooling (1 precooling in LN2 to-20 degrees C, 2 formation on the sample of a frost layer from air by natural humidity, 3 second cooling of the frost-covered sample in LN2 to-195.7 degrees C). Material in "as is" conditions and after a preliminary heat treatment (850 degrees C) were used as the reference points. The HV microhardness and the wear rate under dry abrasive friction were evaluated. Despite the very different types of the examined metals' nature, microstructure, and hardening mechanisms, the rapid cryogenic cooling improves both the hardness and the wear resistance values. For all investigated metals rapid cryogenic cooling assisted with the frost layer produces the best results.

In some industrial activities, the use of permanent heat transfer enhancements such as metal coating, pins, and grooves in cryogenic treatment is quite problematic. Such permanent heat transfer enhancers were replaced with temporary enhancers made of different crystalline materials, here focusing on NaCl, MgSO4·7H2O, CaCl2·2H2O, and C6H5Na3O7·2H2O, which were used during quenching of the sample in the cryogenic liquid, i.e., nitrogen. The time required for sample cooling until the liquid nitrogen saturation temperature is reached decreases significantly when a crystalline coating is applied. The dependence of the cooling time on temporary coating creation parameters such as solution concentration, initial temperature of the sample, and solution spreading time is discussed.

Enhanced boiling conditions during quenching by use of frost as a temporary heat transfer enhancer were determined experimentally. The frost coating was obtained by the application of air moisture on the previously cooled surface of a product.We found that heat transfer intensity can be adjusted by the change in coating structure characteristics. These characteristics, on the other hand, depend on frost formation parameters such as a temperature of the preliminarily cooled sample, an air temperature, a relative humidity, an air movement velocity and a frost formation time range. During the subsequent sample quenching in liquid nitrogen, a heat exchange enhancement was 10.2-12.4 when cooling down to the nitrogen's saturation temperature. By limiting the cooling temperature within the range of 90 K-110 K, the additional heat exchange enhancement increased 1.4-1.5 times. (C) 2012 Elsevier Ltd. All rights reserved.