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Wear mechanisms of digger teeth

2 Literature review

2.2 Excavator digger teeth: introduction

2.2.2 Wear mechanisms of digger teeth

The excavation process of the particulates takes place by a combination of steps; penetration, breaking of material, scooping and lifting for loading into bucket. During these operations, the digger teeth undergo extensive abrasive wear and impact loads. This is very difficult to monitor and calculate the impingement

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P a g e | 66 velocity, impingement angle and contact stresses experienced by the tooth surface during ground engagement without systematic field trials. Tupkar and Zaveri [260] calculated maximum stresses which may generate at the tooth tip due to the regular and maximum contact with the soil. They used analytical method and maximum shear stress theory and verified the results with finite element modelling. The maximum shear stress reported at the tooth tip using the former method was 96.39 MPa whereas the FEM modelling showed a shear stress value of 112.98 MPa. Similarly the FEM study by Dagwar and Telrandhe [261] shows a maximum shear stress acting on the tooth of 43.45 MPa. The data presented above may give some indication about the contact stresses acting on the tooth, however, the field operating conditions may very case to case resulting in a significant variation in the contact conditions and wear of tooth material. Unfortunately, in the present study neither any field operating conditions were monitored nor any such data had been provided by the industry other than the tooth position and weight loss data.

Limited literature is available on the metallurgical failure analysis of a worn digger tooth. Based on the specific mining environment, Mashloosh, Eyre and Bulpett [6, 76] investigated a digger tooth of martensitic steel and reported presence of surface white layers. The white layer was found to be a structurally and chemically homogeneous phase. This consisted of fine equiaxed grained martensite with homogenous distribution of very fine M3C and M7C3 carbide

particles. The white layer grain size was reported to be 200 nm with a hardness of 1200 ± 30 HV. No traces of Oxides or Nitrides were found that could be responsible for higher hardness of the white layer [76]. The subsurface was reported to be localized tempered martensite caused by the heat evolved during the deformation process. The white layer was etching resistant but the subsurface layer showed increased etching response. The results were in good agreement with the impact abrasive wear mechanisms, as discussed in Section

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2.1.1.1. Figure 2.30a represents the worn tooth microstructure delineating three

distinct regions, 1) white layer, 2) tempered martensite and 3) undeformed matrix. A microhardness profile measurement is shown in Figure 2.30b, displays

the high hardness of white layer followed by a significant drop in hardness in the tempered region. The lower hardness value was attributed to the tempering occurred in that region by the heating effect during operation of the digger tooth. A similar thermal softening during impact wear is discussed in Section 2.1.1.1.3.3. The tempered region is followed by the undeformed matrix.

Martensitic steels of hardness ~700 HV with good work hardening capacity was suggested for the digger teeth steels by Mashloosh [6].

Figure 2.30. a) Cross-section of worn digger tooth: Region 1 - white layer, Region 2 - tempered martensite and Region 3 - undeformed matrix, b) microhardness through white layer towards undeformed matrix [6].

Bryggman et al. [235, 236] investigated a martensitic digger tooth which was worn during loading of wet blast stone. They compared the abrasion of digger teeth with gouging abrasive wear. The cross-sectional microstructure of the worn tooth material was similar to the findings of Mashloosh [6], comprising of white layer at the surface followed by the soft tempered martensitic layer and

Ha rd ne ss , HV Distance in mm

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P a g e | 68 unaffected matrix. The material removal was attributed to the surface fatigue, chipping and fragmentation by the grooving action of the hard abrasives. The heavily deformed subsurface favoured the material removal, on-setting cracks at the boundary between hard and brittle martensite (white layer) and tempered martensite (soft layer). Quenched tempered steels (510 – 545 HV), tool steels (430 – 560 HV) and HSLA steel (505 HV) were used for field tests. The field test results showed a good correlation with the hardness of the materials. High-Cr tool steel (560 HV) displayed higher wear resistance. They [235] also conducted pin-on-disc, single pass pendulum grooving and multi pass pendulum grooving laboratory wear tests with the same steels. The pin -on-disc and single pass pendulum grooving provided a rational correlation with the field results and the High-Cr tool steel exhibited higher resistance to wear. Interestingly the multi pass pendulum grooving laboratory wear test showed a poor correlation with the field results and a higher wear rate was reported for the High-Cr tool steel in this test. However, the research does not provide detail about the discrepancy found in case of multi pass pendulum grooving wear test [235]. Expectedly, the High-Cr tool steel was recommended for digger teeth application in contrast to Mashloosh [6] where a material of high hardness with a considerable amount of workhardening capability was proposed.

Changming et al. [262] proposed the digger teeth wear mechanism of impact abrasion nature. According to them, under external force the tooth surface starts deforming elastically. With further load by the blunt edges of rock particles the material was extruded forward. Once the tooth material reached the yield point, plastic deformation occurred. The rock grain pushed the material ahead to form a trench and plough the material to both sides of the trench. The plastic deformation resulted work-hardening and stress concentration. Thus, the local tensile stress generated microcracks which on further deformation resulted in

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P a g e | 69 wear loss by microchip formation. In contrast to Mashloosh [6] they recommended Hadfield grades for digger teeth applications due to the high workhardening capacity.