Spotlight: How too... employ quenching and partitioning
Professor David V Edmonds, University of Leeds, UK, explores a novel heat treatment concept for quenching and partitioning.
During the last decade a new term has entered the steel heat treatment lexicon – quenching and partitioning (Q&P). It originated from basic research at the Advanced Steel Processing and Products Research Centre (ASPPRC), Colorado School of Mines, USA, by a team led by Professor John G Speer.
This new heat treatment is similar to the well-known conventional quenching and tempering operation, but with stricter control of microstructural change through choice of quench temperature coupled with appropriate steel alloying. This difference is reflected by use of the term partitioning, which refers to the microstructural change during tempering, thus differentiating it from the usual procedure that is designed to produce 100% tempered martensite. In contrast, Q&P is designed to produce only a partial martensite structure, the balance consisting of untransformed austenite.
This is achieved by an interrupted quench to a carefully controlled temperature between the martensite start temperature (Ms), and the martensite finish temperature (Mf), which, in the absence of extensive carbide precipitation, allows carbon transport from the supersaturated martensite phase fraction to the untransformed austenite phase – the partitioning stage of the reaction. The customary tempering reaction of the martensite by formation of the iron carbide and cementite, is suppressed or delayed by careful alloying, generally with silicon, and in consequence the untransformed austenite fraction can be thermally stabilised by its increased carbon concentration to temperatures lower than Mf, generally to or below room temperature.
The result is a duplex microstructure of martensitic ferrite interwoven with retained austenite, which can deliver potentially attractive combinations of mechanical properties for a wide range of applications. This new Q&P concept follows the similar interest in silicon-containing bainitic steels with a duplex microstructure of bainitic ferrite and interlath retained austenite, the enhanced silicon concentration having suppressed carbide precipitation during the austenite to bainite transformation. Characteristically known as carbide-free bainite, this microstructure offered promise of better combinations of strength and toughness. The carbide-free bainitic microstructure has also been refined by appropriate alloying and low temperature processing to produce what has been termed nanobain bainitic steel.
A schematic representation of the heat treatment temperature profile and nominal evolution of the microstructure during the Q&P heat treatment cycle is shown above. Often the treatment is described as a one-step or two-step procedure conventionally. One-step means an isothermal partitioning anneal at the quench temperature (QT), generally possible because of the higher quench temperature, while two-step means isothermal partitioning at a higher temperature than the quench temperature (PT), which may allow a closer to ambient, or even sub-ambient, depending on steel composition.
Often shortened to Q&P the original basic process has been broadened to include variations: e.g. quenching-partitioning-tempering (Q-P-T), quenching-partitioning-austempering (Q-P-A), with the additional terms indicating added processing steps.
The Q&P heat treatment concept is very versatile, but can be managed to produce controlled fractions, chemistries and stabilities of retained austenite. Thus it can be applied to various steel grades, including, for example, gear and bearing steels, linepipe steels, stainless steels and cast irons, such as austempered ductile iron, ADI, containing silicon but conventionally with a bainitic microstructure. It is especially noteworthy that Q&P heat treatment can be made to fit continuous high-volume steel production, an immediate advantage being that the speed of the microstructural change is limited only by the very rapid kinetics of the austenite-to-martensite reaction and the carbon diffusivity.
Initial interest has immediately arisen for third generation advanced high strength (AHS) steels for automotive applications, where improvements are driven by the continuing requirements for lightweighting to provide greater fuel economy without compromising safety, necessitating the exploration of increasingly complex microstructures, including, in the present context, those containing significant fractions of retained parent austenite phase. Martensitic transformation of this retained austenite during subsequent forming can lead to transformation-induced plasticity, resulting in combinations of enhanced formability and higher strength. Developments of medium-manganese AHS automotive steels combined with Q&P heat treatment have also been explored.
In consequence, following the first ASPPRC research publications in 2003, Baosteel Corporation, China, launched a special 200,000 tonne capacity high strength steel line in 2009 and Baosteel Automotive Q&P pressings in 2012. Since then, widespread global interest and investment from steelmakers and automotive manufacturers in Europe, the USA, Japan and South Korea to exploit Q&P, among other technologies, for AHS automotive steel options has accelerated with the installation of new 500,000 tonne capacity continuous processing lines. These third generation AHS steels are claimed to possess improved formability and strength, thus providing further opportunity for vehicle safety and lightweighting. It would therefore not be unrealistic to expect Q&P steels to be in our cars, vans and trucks anytime soon, if not, in some countries, already. Such developments become proprietary so it is difficult to know, but there is sufficient information in the public domain to suggest that it is not a question of if, but when.