Adventures in steel
Ed Pickering from the University of Cambridge reflects on current research into applications of steel.
There are very few things today that are not made from, or using, steel. Far more steel is produced every year than all other metallic alloys combined, and it is second only to concrete in terms of weight manufactured. But while the uses of concrete remain relatively limited, steel continues to expand its applications, which are often highly specialised and subject to the challenging ambitions of today’s engineers. It is no surprise, then, that the physical metallurgy of steels remains a dynamic and ever-relevant area of research, which is explored internationally.
One particularly active area of research is bearing steels. Whether they are used in aero engines or the ever-increasing number of wind turbines, bearing steels must withstand huge cyclic loads for thousands of hours without failure (which can often be catastrophic). Hence there is a strong desire to understand the physical phenomena associated with damage accumulation and propagation during rolling-contact fatigue. Bearings are connected to drive shafts, and shaft steels are also the subject of current investigations in which issues such as high-temperature stability and fatigue resistance are being addressed.
Aside from gas and wind turbines, steels that are used in other methods of power generation remain hot topics of research. The ever-present desire to run engines hotter or run nuclear power stations for longer continues to drive materials capability forward. To these ends, the creep resistance of steels at elevated temperatures is a key issue for fossil fuel plants, while the effects of irradiation damage in ferritic steels remain the principal concern in the nuclear sector. The higher neutron fluencies expected in fusion and Generation IV fission reactors have necessitated investigations into novel steels, including dispersion-strengthened and reduced-activation varieties.
An important requirement of the oil and gas energy sector is the need to transport or extract resources, producing considerable interest in improving pipeline steels. Here, the main concern is embrittlement due to hydrogen being introduced into the material during service. Indeed, the hydrogen embrittlement of steels in general is currently a dynamic area of study.
Of course, hydrogen and other defects, such as inclusions (a principal source of cracks in bearing steels), often derive from production and fabrication processes. Although there is some overlap here with process metallurgy, physical metallurgists continue to be concerned with the impact of production routes (solidification, forging, heat treatment and finishing) on the microstructural and mechanical properties of steels. Some people believe that improvements in some areas of process metallurgy, such as enhanced cleanliness, have led to a shift of emphasis away from process understanding to a better knowledge of physical metallurgy.
Nonetheless, there are still areas of research, such as the welding of steels, where understanding of process and physical metallurgy is likely to be interlinked indefinitely. Welding continues to be a very active subject, as would be expected following the development of new steels and new applications. Electron-beam welding of large forged assemblies and friction stir welding are examples of particular interest at present. Friction stir welding, a solid-state process in which joints are formed by severe plastic deformation induced by a rotating tool, was originally developed for aluminium alloys but has since been applied to dissimilar metal and steel-to-steel welding. It has found applications in steel joining in the aerospace, shipbuilding and railway industries.
Another use of friction-stir welding (in a spot-welding capacity) is for assembly in the automotive industry. Automotive applications have instigated a number of exciting recent developments in our understanding of steels. Stronger and tougher steels for chassis and bodywork not only allow for greater energy absorption during car crashes, but also for lighter and more efficient vehicles through reduced material use. Dual-phase (martensite–ferrite) steels are an example, as are newly developed twinning-induced plasticity (TWIP) steels – austenitic alloys that deform through mechanical twinning.
Elsewhere in the transportation sector, rail steels, which remained essentially unchanged for around 150 years, are being modernised. The introduction of more wear-resistant bainitic steels (on the Eurostar route, for example) has facilitated faster and smoother journeys. There is also an ambition to introduce rails produced from superbainite, which comprises bainitic laths that are nanometres in size and that deliver exceptional mechanical properties, including strengths of up to 2.5GPa, with good ductility. However, weldability, impact toughness and high-temperature stability remain issues.
The theoretical basis for the prediction of superbainite was already in place at the time of its discovery, reflecting the advances that have been made in predictive methods. It is now possible to calculate the transformation kinetics, stabilities, morphologies and mechanical properties of bainite and other steel microstructures based on composition and processing, and this has opened up countless new possibilities for alloy design. There remain problems where computational capability falls short, for instance in the prediction of fracture toughness from microstructural data, but steels modelling is becoming more and more powerful as time passes.
The production of superbainite, friction-stir welding and TWIP steels were all topics of presentations given at the international conference Adventures in the Physical Metallurgy of Steels, held in Cambridge, in 2013. Other novel subjects included flash bainite, pulse-processed steels and non-cubic ferrite. A recent special issue of Materials Science and Technology (Vol 30, No 9) presents selected papers from the conference.
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