In a word, stress! The degree to which special bar quality (SBQ) steels are truly “special” largely depends on how much stress it can handle. Not all SBQ steel is created equally, and only a select few manufacturers can produce it for the most demanding applications.
So let’s back up from the specifics of the most challenging portion of the SBQ world, and frame the definition starting with the fact that steel is, arguably, the most important engineering material in the world. It is adaptable for so many critical uses. Steel is produced and sold in several product forms, including “flat” and “long” products. Flat products include rolled sheet and plate. Long products include bars and seamless tubing.
The top of the long-product market is referred to as SBQ – in some parts of the world the term “engineering steels” is used. Because SBQ is often vaguely defined and spans a wide range of non-structural uses, it could be aptly renamed “long steel products engineered for tough applications” – way too long, but highly descriptive. The better way to describe the “special” in SBQ is by considering the stress the steel material handles in application. Greater stress increases the risk of failure and elevates the importance of consistency and integrity of the steel – in other words, the higher the stress tolerance, the more “special” the SBQ is.
While structural commodity steels may be loaded at 10-40 kilopound per square inch (KSI), critical SBQ applications like bearings, crankshafts, gears and drill-string components withstand high-cycle fatigue loads of 100-500 KSI. To create steel that can withstand that significant loading, the mechanical properties are often achieved through thermal treatment in near-final product form.
Two attributes are critical to high performance in products made from steel – chemical composition, which, in conjunction with thermal treatment, enables the attainment of required mechanical properties – and steel purity that allows consistent achievement of those properties. Chemical composition, once specified, is relatively easy to achieve. Purity, however, is the distinguishing attribute of SBQ and difficult to deliver consistently. In the highest SBQ application stresses, for components including bearings and gears, the actual fatigue and service life can be significantly reduced by the presence of the impurities inherent in the steel.
TimkenSteel, a leading producer of clean SBQ, manufactures these pure, high-performance steels in bar, seamless mechanical tube and component product forms. Our manufacturing process controls minimize creation of impurities called “non-metallic inclusions” in the steel while it’s still liquid. Our precise chemistry control during the refining process provides consistency of properties such as hardness and ductility and ensures that those properties are delivered during heat treatment.
Impurities are detrimental to fatigue life under high cycle and heavy application loads. Clean steels do not “stress out” in applications, while “dirty” steels shorten the products’ fatigue life. This can be as great as a twofold to tenfold difference! Not all SBQ is created equally, which is why data on cleanness and quality is critical when purchasing SBQ.
Ray Fryan is vice president of technology and quality for TimkenSteel. A career metallurgist recognized across the industry, Ray has spent his career advancing steel production and processes, quality systems and materials technology. Ray holds a bachelor's degree in metallurgical engineering from Grove City College and a master's degree and doctorate in material science and engineering from Case Western Reserve University. He is a graduate of the Executive Development for Global Excellence program at the University of Virginia Darden School of Business.