• Lindroos, Matti

Abstract

The demand for more wear resistant materials originatesfrom modern applications of many industries, such asmining, automotive, aerospace and civil structures. Themotivation to develop more efficient engineeringstructures and components can be seen bene?cial in botheconomically and environmentally. Lighter, higherstrength and more wear resistant solutions can beattractive, for example because of savings in energyconsumption (e.g., petrol and running costs), higher loadbearing capability per material thickness/volume, andincreased component lifespan. Steels remain still todayvery competitive materials for various wear applicationsbecause of their relatively good wear resistance in manyconditions arising from their excellet mechanicalproperties, and because of the reasonable cost ofmanufacturing and processing of the components.The steels exposed to high stress abrasive and impactwear conditions, for example in the equipment used inmining, are required to withstand heavy static anddynamic loadings for long periods of time. The evaluationof the performance of different steels in these type ofconditions is often performed with experimental setupsimitating the real loading conditions and materialcharacterizations done afterwards, giving an insight intothe material's wear behavior in a particular tribosystem.This work concentrates on the characterization of themechanical behavior of wear resistant steels subjected toabrasive and impact loadings by hard particles. Themechanical behavior of the steels was ?rst characterizedat a wide range of strain rates from 10-3 to 4000 s-1 .Although the increase in the ?ow stress with theincreasing the strain rate is well established, limitedinformation is available of the behavior of these steelsin the dynamic range. For example, the localizationphenomena, such as adiabatic shear banding, have animportant role in the failure behavior of the martensiticsteels. On the other hand, the strain hardening behaviorof austenitic manganese steels that evolves with strainand strain rate is affected largely by the twinningphenomenon. Two in-service cases including samplematerials from a jaw crusher and from a cutting edge of abucket loader were also characterized and analyzed. Theobservations made on the deformed microstructures of thelaboratory and in-service samples formed the basis forthe simulation approaches developed in this work.High stress abrasion experiments were performed andfurther developed for the testing of wear resistantsteels to study their capabilities to surface harden andto withstand wear. The results show that the surfacehardening of the steels has a substantial effect on theirwear rates. The common single scratch experiments,however, were shown to be insufficient to reveal allimportant aspects related for example to the surfacehardening of the studied materials, and thereforedifferent types of multi-scratch experiments were alsoapplied. The characterization also showed that themartensitic steels generate two types of tribolayersdepending on the prevailing contact conditions.High velocity impact testing was conducted with a novelhigh velocity particle impactor device. The steels showeddependence on several external factors and conditions,such as impact energy, impact angle, and incidentimpulse. It was shown that the wear characteristicsdepended on the deformation mechanisms such as ploughingor cutting in addition to some more special mechanismssuch as shear banding, which becomes active only athigher impact energies and/or higher strain rates. Thestrain hardening had both positive and negative effectson the material's resistance against impacts depending onthe loading conditions.Two numerical crystal plasticity models were implementedto assist the development of the understanding of thedeformation behavior at micro-scale. First aphenomenological model including dislocation slip andtwinning was formulated to describe the micromechanicalphenomena occurring in austenitic manganese steels. Themodel was found capable of representing the materialbehavior with a satisfactory accuracy in the studieddeformation conditions, starting from the single crystalbehavior and extending to the polycrystal level. Amulti-scale method linking the application andmicrostructural scales was also demonstrated using a jawcrusher as an example. Implementation of a crystalplasticity method for BCC microstructure in the largedeformation framework was also carried out. The model wasextended to include a phenomenological description of theshear banding phenomenon in the microscale. The extensionwas demonstrated with simulations on single crystals withfour different initial orientations. The resultsindicated that shear banding is a heavily orientationdependent phenomenon, but its relevance for theperformance of polycryst...

Topics

• impedance spectroscopy
• surface
• single crystal
• experiment
• simulation
• wear resistance
• strength
• steel
• dislocation
• plasticity
• deformation mechanism
• Manganese
• crystal plasticity