Abstract
The surface of 40CrNiMo steel, which is commonly used for the sprag clutch wedge, is prone to wear. In this study, hardening of the matrix material was conducted by broadband-laser scanning at various scanning speeds. The hardness distribution and structure evolution were analyzed along the vertical direction. Characteristics of the hardened layer were explored using scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. The friction coefficient, wear amount, and wear morphology of sliding friction against GCr15 steel were investigated under various conditions. The results show that the depth of the hardened zone decreases with increasing scanning speed. Under the experimental power and defocus, a laser scanning speed between 700–1020 mm/min can meet the general surface requirements of the sprag clutch wedge. After laser hardening, the main components of the hardened layer included lath-shaped and needle-shaped martensite and retained austenite. In terms of friction and wear, when the relative movement speed was within 300–500 mm/min, the relative movement speed decreased and the normal force increased, which led to an increase in the friction coefficient and its fluctuation, as well as an increase in wear volume of the hardened layer. The wear mechanism of the hardened layer included abrasive wear, adhesive wear, and oxidative wear. Excessive normal force resulted in obvious delamination of the sample. Within the scope of the experiment, the best laser hardening results were obtained with a scanning speed of 800 mm/min.
Highlights
Wear of the sprag clutch wedge often occurs during service, and failure due to surface wear is a prominent problem that can seriously affect the service life of the sprag clutch
In accordance with the conventional method of increasing the scanning speed by equal value, starting from 600 mm/min, laser hardening was performed at a scanning speed increment of 100 mm/min
When the scanning speed reached 900 mm/min, in order to explore the special phenomena that may be caused by the increase in scanning speed increment, laser hardening was performed at a scanning speed increment of 120 mm/min
Summary
Wear of the sprag clutch wedge often occurs during service, and failure due to surface wear is a prominent problem that can seriously affect the service life of the sprag clutch. As a material for manufacturing the wedge, improving the surface quality of 40CrNiMo steel by ordinary heat treatments is time-consuming. In order to improve the surface properties of 40CrNiMo steel, Li et al [1] used low-temperature laser shot peening (CLP) to surface treatment of 40CrNiMo steel. The crystal grains significantly reduced the wear quality loss and improved the high temperature wear performance. The results indicated that the main phases are NbC and Nb6 C5 in the niobiumized layer, and the loss of weight of carburized and subsequently niobiumizing strengthening specimens was 58.4% of conventional strengthening ones. Laser hardening is a relatively new surface treatment technology that uses a laser to heat the surface of a material to above its phase transition point. The approach offers several advantages such as short heating times, low surface roughness, and convenient operation [3]
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