Bainite in Steels 2nd Edition PDF

We begin with a historical survey of the exciting early days of metallurgical research during which bainite was discovered, covering the period up to about 1960, with occasional excursions into more modern literature. The early research was usually well conceived and was carried out with enthusiasm. Many of the original concepts survive to this day and others have been confirmed using the advanced experimental techniques now available. The thirty years or so prior to the discovery of bainite were in many respects formative as far as the whole subject of metallurgy is concerned. The details of that period are documented in the several textbooks and articles covering the history of metallurgy. but a few facts deserve special mention, if only as an indication of the state-of-the-art for the period between 1920-1930.

The idea that martensite was an intermediate stage in the formation of pearlite was no longer accepted, although it continued to be taught until well after 1920. The β-iron controversy, in which the property changes caused by the paramagnetic to ferromagnetic transition in ferrite were attributed to the existence of another allotropic modification (β) of iron, was also in its dying days. The first evidence that a solid solution is an intimate mixture of solvent and solute atoms in a single phase was beginning to emerge (Bain, 1921) and it soon became clear that martensite consists of carbon dispersed atomically as an interstitial solid solution in a tetragonal ferrite crystal. Austenite was established to have a face-centred cubic crystal structure, which could sometimes be retained to ambient temperature by quenching. Bain had already proposed the homogeneous deformation which could relate the face-centred cubic and body-centred cubic or body-centred tetragonal lattices during martensitic transformation. It had been established using X-ray crystallography that the tempering of martensite led to the precipitation of cementite, or to alloy carbides if the tempering temperature was high enough. Although the surface relief associated with martensitic transformation had been observed, its importance to the mechanism of transformation was not fully appreciated. Widmanstatten ferrite had been identified and was believed to precipitate on the octahedral planes of the parent austenite; some notions of the orientation relationship between the ferrite and austenite were also being discussed.

It was an era of major discoveries and great enterprise in the metallurgy of steels. The time was therefore ripe for the discovery of bainite. The term 'discovery' implies something new. In fact, microstructures containing bainite must have been encountered prior to the now acknowledged discovery date, but the phase was never clearly identified because of the confused microstructures that followed from the continuous cooling heat treatment procedures common in those days. A number of coincidental circumstances inspired Bain and others to attempt isothermal transformation experiments. That austenite could be retained to ambient temperature was clear from studies of Hadfield's steel which had been used by Bain to show that austenite has a face-centred cubic structure. It was accepted that increasing the cooling rate could lead to a greater amount of austenite being retained. Indeed, it had been demonstrated using magnetic techniques that austenite in low-alloy steels could exist at low temperatures for minutes prior to completing transformation. The concept of isothermal transformation was already exploited in industry for the manufacture of patented steel wire, and Bain was aware of this through his contacts at the American Steel and Wire Company. He began to wonder 'whether exceedingly small heated specimens rendered wholly austenitic might successfully be brought unchanged to any intermediate temperature at which, then their transformation could be followed' and he 'enticed' E. C. Davenport to join him in putting this idea into action.

^tNotable historical works include: The Sorby Centennial Symposium on the History of Metallurgy, published by the A.I.M.E.in 1965(includes an article by Bain himself), the commentary by H. W. Paxton, Metallurgical Transactions 1(1970) 3479-3500, and by H. W. Paxton and J. B. Austin, Metallurgical Transactions 3 (1972) 1035-1042. Paxton's 1970 article is published along with a reproduction of the classic 1930 paper on the discovery of bainite by Davenport and Bain, and is based on first hand historical knowledge obtained directly from Davenport and Bain.

Edited by: H. K. D. H. Bhadeshia