In the process of coal extraction, a fractured zone is developed in the overburden above the goaf. If

the fractured zone is connected with an aquifer, then water inrush may occur. Hence, research and analysis

of the height of overburden fractured zone (HOFZ) are of considerable significance. This study focuses

on the HOFZ determination in deep coal mining. First, general deformation failure characteristics of

overburden were discussed. Second, a new method, numerical simulation by orthogonal design(NSOD),

have been proposed to determinate the HOFZ in deep coal mining. Third, the validity of NSOD is verified

in the practical application, compared with empiric al formula in Chinese Regulations and in-situ test.

These three methods were applied to determine the HOFZ of working face No. 111303 in No. 5 coal

mine. The pre dicted HOFZ of NSOD is found to be similar to the result of the in-situ test (8.9% relative

error), whereas the HOFZ calculated by the empirical formula has extremely large error (25.7% relative

error). Results show that the NSOD can reliably predict the HOFZ in deep coal mining and reduce time

and expenses required for in-situ test.

In this work, thermo-mechanically treated 42CrMo steel was subjected to cryogenic treatment conducted by means of orthogonal design method, followed by low-temperature tempering to investigate the effect of different parameters of cryogenic treatment on wear resistance of 42CrMo steel and to optimize parameters of cryogenic treatment for improving wear resistance. The results of hardness test and wear test show that cryogenic treatment significantly improves wear resistance with marginal changes in coefficient of friction and hardness. Specifically, cryogenic temperature has the largest impact on wear resistance of 42CrMo steel, holding time has medium impact, and the parameter of treatment cycles has the least impact. The optimum parameters of cryogenic treatment are −196°C for 12 hours with one cycle for improving wear resistance. The results of scanning electron microscopy (SEM) and X-ray diffractometry (XRD) analysis indicate that marginal changes in hardness and coefficient of friction may be owing to little amount of transformation of retained austenite, and the significant influence of cryogenic treatment on improving wear resistance of 42CrMo steel can be mainly attributed to segregation of carbon atoms promoted by cryogenic treatment resulting in more precipitation of carbides in subsequent tempering.