Abstract
Advanced aircraft gearboxes operate under high mechanical loads. Currently, aircraft gears are manufactured from chromium–nickel–molybdenum steel grades such as AISI 9310 or Pyrowear 53. The major causes of gear failure are wear and fatigue cracking. As the crack initiation occurs predominantly on the component surface, the gears are routinely subjected to surface hardening processes such as low-pressure carburizing and case hardening. The gears are manufactured in a multiple operation process, in which teeth grinding is a crucial step. Selection of improper grinding conditions can lead to local heat concentration and creation of grinding burns, which are small areas where microstructure and properties changes are induced by high temperature generated during grinding. Their presence can lead to significant reduction of gear durability. Therefore destructive and non-destructive (NDT) quality-control methods such as chemical etching or magnetic Barkhausen noise (MBN) measurements are applied to detect the grinding burns. In the area of a grinding burn, effects related to the over-tempering or re-hardening of the carburized case may occur. In this paper, the results of the studies on the characterization of microstructure changes caused by local heating performed to simulate grinding burns are presented. The areas with the over-tempering and re-hardening effects typical for grinding burns were formed by laser surface heating of carburized AISI 9310 steel. Analyses of the microstructure, residual stresses, retained austenite content, and non-destructive testing by the MBN method were performed. The correlation between the MBN value and the properties of the modified surface layer was identified. It was also found that the re-hardened areas had similar characteristics of changes in the Barkhausen noise intensity, despite the significant differences in the width of the overheated zone, which depended on the laser-heating process conditions.
Highlights
The material under consideration was the AISI 9310 steel grade developed for lowpressure carburizing and used extensively for aircraft gearing
Visual analysis of the tracks generated by laser processing revealed the evidence of Discussion surface oxidation.3.The extent intensity of particular effects such as melting, over-temVisual analysis of the tracks generated laser processing revealed the evidence of pering, and re-hardening in the irradiated zone were by dependent on the laser-processing surface oxidation
The extent and intensity of particular effects such as melting, overconditions of power density and beam traverse speed (Figure 4). It was obtempering, and re-hardening in the irradiated zone were dependent on the laser-processing served that at a constant power density, width of thespeed track(Figure decreased with an increase conditions of power density the and beam traverse
Summary
High wear and fatigue resistance within the surface layer is critical to ensure a long operation time of the gears These properties are obtained by heat treatment or thermochemical treatment, mainly by induction hardening, carburizing, and nitriding. Several non-destructive testing (NDT) techniques are used in quality control of carburized case-hardened gears, such as Barkhausen noise analysis, micromagnetic multiparameter microstructure and stress analysis, eddy current signal, and recently digital image processing [4,15,16,20,25,26,27,28]. The thermal effects simulating grinding-burn damage were produced by means of laser-beam surface heating of carburized case-hardened AISI 9310 steel. Determination of this correlation distinguish over-tempered and re-hardened zones using non-destructive technique
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