Analysis of destruction, micro-structure and properties hard loaded gears of different manufacturers

Abstract

Purpose. To study the causes, nature of destruction, structural, mechanical and operational characteristics of heavily loaded cemented gears from steel 20Х2Н4А with chemical composition (%): 0,17 С; 0,25 Si; 0,39 Mn; 1,33 Cr; 3,12 Ni; 0,022 W; 0,07 Мо; 0,01 Ti; 0,25 Cu; 0,016 S; 0,01 P of various manufacturers to improve the technology of their production and improve performance.To investigate the causes of destruction, worn-out gears manufactured by Joy Global Inc. were selected. (“Joy”) (USA), Machine-Building Enterprise “MAGMA” and Yasinovatsky Machine-Building Plant (“YaMZ”) (Ukraine) of heavy-duty gearboxes of mining tunnel combines, completely out of order, made of 20Х2Н4А steel after cementation and a full heat treatment cycle.Research methods. Macroscopic (GOST 10243-75) and microscopic analyzes, spectrographic and quantometric analysis of the chemical composition of the cemented layer depth, measurement of Rockwell hardness (HRC), micro-hardness of the layer depth (GOST 9450-76), tensile testing of mechanical properties GOST 1497- 84, metal-to-metal friction wear resistance tests.The nature of destruction, the macrostructure, the microstructure of gears, the distribution of carbon, and the microhardness over the depth of the cemented layer was investigated. Comparative wear tests under conditions of dry sliding friction were carried out.Obtained results. It was established that the causes of chipping of the active surfaces of the teeth of the heavily loaded gears of mining machines were the insufficient strength of the cemented layer at a certain depth, as well as extreme overloading of the elements of the engagement during operation. The carbon distribution in the depth of the cemented layer of gears produced by “MAGMA” is 1.5 times higher than that for parts produced by “Joy”, which does not exclude the latter being saturated with carbon and nitrogen by carbonitriding. The microstructure of the hardened layers in the gears of the manufacturers “Joy” and “MAGMA” consists of finely dispersed martensite, carbides and residual austenite, the ratio of these phases varies with the depth of the layer according to the decrease in carbon content. The mechanical properties of steel samples 20Х2Н4А, cut from the gears of the specified manufacturers, comply with the requirements of GOST 4353-71. The microstructure of gears produced by “YaMZ” is troosto-martensitic with larger grains (number 4–5), which resulted in a somewhat lower hardness and relative wear resistance. Samples cut from the gears of the production “Joy” and “MAGMA” have the highest wear resistance, a little less – “YaMZ”.Scientific novelty. Based on the analysis of damage, the causes of failure of heavily loaded gears, operating in difficult operating conditions of different manufacturers, are determined, their comparative studies and testing properties are carried out. The nature of changes in the microstructure, the distribution of microhardness depending on the concentration of carbon in the thickness of the cemented layer of gears.Practical value. The research results served as scientifically based recommendations for the improvement of the technological process of production and the hardening of heavily loaded gears in “MAGMA” to improve their performance.