Dou turbine slewing gear fracture analysis of the causes

Dou 2018 - turbine slewing gear fracture reason analysis 10 - 02 Zou Junwen ( Zhejiang electric power boiler pressure vessel inspection co. , LTD. ) Maintenance personnel in the process of on-site inspection found 3 # dou turbine slewing mechanism falls off a pinion gear on the west side has a, the bucket turbine gallons of a 3 # just ended. Carefully examined and found 3 # dou turbine drive pinion along the both sides of the tooth root axis direction integral fracture occurs, as shown in figure 1. Figure 1 broken teeth macro fracture module of gear according to Mn = 24, number of teeth z = 17, material is 42 crmo + QT forgings, tooth surface intermediate frequency quenching, hardness, 57± 6) HRC。 The physical and chemical test to find out the specific reason of the fault gear. 1. Macro check figure 2 is a fracture of the gear profile of macroscopic, the location of the fracture in external gear and internal tooth roots; Fracture macroscopic no obvious plastic deformation, no fatigue, a brittle fracture; Figure in the outer wall of fracture macroscopic has obvious radial streaks, its convergence to the outer wall, the fracture is outside the root of the tooth to the extension. Figure 2 fracture macroscopic according to 2. Composition analysis using SPECTROTESTCCD movable metal analyzer direct reading spectrometry was carried out on the fracture of the gear chemical composition test, the results are shown in table. Connecting rod joint chemical composition ( The mass fraction) （ %） According to GB/T3077 - 1999 standard for alloy structural steel in the chemical composition of 42 crmo steel quality requirements. 3. Metallographic and mechanical analysis diagram 3 is a photo of the gear microstructure, the microstructure of ferrite + pearlite + tempered martensite. 42 crmo under the tempering process should be tempered martensite or normal tempered sorbite of microstructure of the gear under the tempering process, the requirements of normal microstructure. Figure 3 metallograph sampling with mechanics performance test, the tensile strength of the gear material the Rm of 880 mpa, break elongation is 13%. 4. Electron microscopy (SEM) analysis diagram 4 is observed under SEM fracture morphology, microscopic fracture is flat, fracture macroscopic no obvious plastic deformation, not seen fatigue lines, and the fracture microstructure of quasi cleavage fracture characteristics, for brittle fracture. Figure 4 shape of 5. Analysis and discussion of turbine slewing gear due to improper heat treatment, the heating temperature is low, the original pearlite has transformed into austenite, the ferrite is not completely dissolved, there are a small amount of ferrite residue in the grain boundary, quenching, and because of slower cooling, has been along the grain boundary precipitates dissolved ferrite thin lamellar pearlite. Part of supercooled austenite martensite in the formation of temperature into a quenching martensite structure, formed the gear ferrite + pearlite + tempering martensite microstructure, this organization belongs to incomplete hardening organization. With reference to GB/T3077 - 1999 standard for alloy structural steel, 42 crmo after conditioning treatment in the normal course of its tensile strength can be up to 1080 mpa, and the fault gear quenching caused by tempering process does not reach the designated position of the workpiece hardening effect reach full play, the tensile strength of 880 mpa. In the running gear gear whole shows brittle fracture as a result of insufficient strength. Source: metal processing: hot working, 19, 2016