Abstract
The three-dimensional morphologies of inclusions in gear steel 18CrNiMo7-6 forging were investigated by a non-destructive extraction method, and the cleanliness of radial positions was analyzed, mainly including the variation of total oxygen content and the distribution of size and quantity of inclusions. In addition, fatigue performance was tested using an ultrasonic fatigue machine to investigate the fatigue characteristics of the steel. The results show that the quantity density of inclusions per unit volume in gear steel 18CrNiMo7-6 decreases exponentially with increasing size, oxide inclusions with a size less than 8 μm account for more than 90%, while sulfide inclusions account for more than 85%. The average value of the oxygen content can reflect the level of inclusions that were evenly distributed in the molten steel, and the accumulative total oxygen content increases significantly with increasing inclusion size. The fatigue specimen failed after the stress exceeded the critical value, and fatigue failure hardly occurred when the stress was below the critical value. Meanwhile, large-sized nondeformable inclusions such as Al2O3-CaO in gear steel 18CrNiMo7-6 are closely related to fatigue failure. It is recommended that the area from the center to the 1/2 radius with low cleanliness should be avoided, while the area from the 3/4 radius to the edge with high cleanliness should be selected during the machining of the gear.
Highlights
As a necessary transmission part of mechanical equipment, different kinds of high-alloy gears have been widely applied and popularized in industrial production to meet the requirements of aerospace, metallurgical manufacturing, automobile production, mining, lifting transportation and building materials machinery, etc. [1,2]
The application of 18CrNiMo7-6 according to the standard of DIN (German Institute for Standardization) in Germany has become increasingly widespread with the expansion of high-alloy gear steel production, and the predecessor of this steel is
Carburizing gear steel 18CrNiMo7-6 is a surface-hardened steel with high strength, high toughness and high hardenability, which is widely used in mining, transportation, locomotive traction, lifting and internal power, and other industrial fields [3]
Summary
As a necessary transmission part of mechanical equipment, different kinds of high-alloy gears have been widely applied and popularized in industrial production to meet the requirements of aerospace, metallurgical manufacturing, automobile production, mining, lifting transportation and building materials machinery, etc. [1,2]. As a necessary transmission part of mechanical equipment, different kinds of high-alloy gears have been widely applied and popularized in industrial production to meet the requirements of aerospace, metallurgical manufacturing, automobile production, mining, lifting transportation and building materials machinery, etc. High surface quality was the essential condition to meet the requirement of harsh working conditions of the gear steel, so it is necessary to control the steel cleanliness and inclusions characterization, including morphologies, size, number and distribution, for guaranteeing the fatigue property of the steel [7,8,9,10]. Fatigue tests were carried out using an ultrasonic fatigue testing machine to investigate the fatigue characteristics of the steel This provides theoretical guidance for avoiding areas of poor cleanliness in follow-up processing, thereby laying the foundation for further improving the quality of gears [15]
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