Fundamentals and practical approaches of optimizing martensitic steels for use under severe operating conditions

Martensitic steel is a well-established material for the equipment and tooling used in mining and processing. Martensitic steel was originally chosen for its high strength and hardness. However, the overall performance of industrial machinery requires that a number of other properties be considered, including toughness, ductile-to-brittle transition temperature, fatigue strength, resistance to temper and hydrogen embrittlement, corrosion, creep, abrasion resistance, bendability (cold forming), and machinability. Often, a number of these properties should be optimized simultaneously. This optimization is achieved by adjusting alloy composition or thermomechanical processing, or ideally, by a well balanced combination of both. A clear understanding of the microstructure of martensitic steel and how it governs properties must precede any optimization exercise. Accordingly, we begin by outlining some fundamental considerations concerning the microstructure of martensite and its influence on strength, toughness and failure mechanisms. Next, we discuss the role of the various alloying elements and typical impurities on the microstructure and, hence the relevant properties. Typical alloying elements such as carbon, molybdenum, niobium, chromium, manganese and boron not only have primary effects, but also have cross-effects and synergies when added in combination. Thirdly, we describe the variety of thermomechanical processes that make it possible to tailor properties to a particular performance profile. These span the range from traditional quench-and-temper (Q&T) treatments to more recent processes, such as direct quenching, quench-and-partition and intercritical heat treatments. Each step of each treatment must be analyzed for its influence on the final microstructure and thus, the ultimate combination of properties. Finally, we present specific examples of technologically successful martensitic steels. We describe their composition and processing, particularly focusing on the effects and benefits of niobium and molybdenum as alloying elements. (AU) Copyright © 2018 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved