The development of abrasive-corrosive wear resistance of steels by microstructural control

Doctoral Thesis


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University of Cape Town

The performance of developmental alloyed steels with improved abrasive-corrosive wear resistant properties has been evaluated. The synergistic effect of abrasion and corrosion in the accelerated wear of steels is examined and the main parameters identified. A model of the process is proposed. The model is used to develop the optimum abrasive-corrosive wear resistance in steels for applications in the gold mines of South Africa. A wide range of engineering steels, both commercially available and experimental, has been evaluated in laboratory simulated abrasive and abrasive-corrosive wear tests. An appraisal of the wear tests and the applicability of the results to in-service conditions has led to the development of an additional abrasive-corrosive wear test. It has been established that both the microstructure and chemical composition determine the resistance of a material to wear. Control of the microstructure by alloying and heat treatment is attempted in order to optimise the abrasive-corrosive wear resistant properties for each class of microstructure whilst maintaining adequate formability and weldability. Abrasion of a metal surface has been shown to accelerate the rate of corrosion. Three categories of corrosion behavior are defined. A model of the abrasive-corrosive wear process is proposed to account for the behavior. The model adequately predicts the outcome to a change in system parameter, namely: an increase in the corrosivity of the water, an increase in the frequency of abrasive events, a change in the chemical composition and the degree of passivity inherent in the material. Recommendations are made to maximize the abrasive-corrosive wear resistant properties without resorting to expensive highly alloyed steels. To satisfy the needs of the mining industry, two microstructures of note are identified: a metastable austenitic (TRIP type) steel and a 0.25% carbon lath martensitic alloyed steel. A basic chemical composition is proposed with each microstructure. The austenitic steel is shown to achieve its abrasion resistance through the high degree of work hardening it undergoes during abrasion and the high ultimate strength of the strained material. The lath martensitic steel has the necessary strength to toughness ratio for good abrasion resistance. A 20% degree of work hardening in conjunction with a bulk hardness in excess of 500 HV is prescribed for superior abrasion resistant properties in the wear system of the mines. The life time cost of the martensitic alloyed steel recommends it for applications in the gold mines of South Africa.

Bibliography: pages 212-223.