Question 5 of 7: Three Stages of Tempering in a Water-Quenched Mid-Carbon Steel (SAE 1045)

Question V: Three Stages of Tempering in a Water-Quenched Mid-Carbon Steel (SAE 1045) (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The as-quenched structure is supersaturated, carbon-strained (tetragonal) martensite, with a small fraction of retained austenite trapped between the martensite laths. As the piece is progressively reheated, three overlapping stages of microstructural change occur:

V.1 — Stage 1 (roughly 100–250 °C): transition-carbide precipitation

The as-quenched BCT martensite is highly supersaturated in carbon and thermodynamically unstable at any temperature above room temperature. In this first stage, carbon begins to segregate and precipitate out of the martensite as an extremely fine transition carbide, $\varepsilon$-carbide (Fe$_{2.4}$C), coherent or semi-coherent with the matrix. As carbon leaves solid solution, the tetragonality of the surrounding martensite decreases (the $c/a$ ratio relaxes toward 1), producing what is called "tempered martensite." Hardness drops only modestly from the very brittle as-quenched state, while toughness improves noticeably because the worst of the internal quench strain is relieved.

V.2 — Stage 2 (roughly 200–300 °C): decomposition of retained austenite

Over this overlapping temperature range, the retained austenite trapped between martensite laths (present from the incomplete transformation on the initial water quench) decomposes into a mixture of ferrite and cementite, similar in character to lower bainite. This removes a mechanically unstable constituent (retained austenite can itself transform to hard, untempered martensite under later service stress or further cooling) and further stabilizes the microstructure.

V.3 — Stage 3 (roughly 250 °C and above): equilibrium cementite formation, coarsening, and ferrite recovery

At higher temperature the metastable transition carbide dissolves and is replaced by the equilibrium phase, cementite (Fe$_3$C), which nucleates as fine, discrete particles and then progressively coarsens (Ostwald ripening) into larger, more widely spaced, increasingly spheroidal particles as tempering temperature and time increase. Simultaneously, the surrounding ferrite matrix — now essentially free of dissolved carbon — recovers: dislocations introduced by the shear transformation annihilate and rearrange into lower-energy configurations, and at the highest tempering temperatures some sub-grain growth/recrystallization of the ferrite can occur. The end result of this final stage is the familiar "tempered martensite" microstructure: fine, uniformly dispersed spheroidal cementite particles in a soft, recovered ferrite matrix, which is the microstructure that gives quenched-and-tempered medium-carbon steels their favourable combination of strength, ductility and toughness.