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21-Mat-B6 Ceramic Materials · May 2013

Question 4 of 7: Tempering of Quenched SAE 1045 Steel — Three Stages

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Reference texts: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 4th ed.; Callister & Rethwisch, Materials Science and Engineering: An Introduction, 10th ed.; ASM Handbook, Vol. 4, Heat Treating; Porter, Easterling & Sherif, Phase Transformations in Metals and Alloys, 3rd ed.

Question IV: Tempering of Quenched SAE 1045 Steel — Three Stages (15 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.

As-quenched, the 860 °C-austenitized, water-quenched SAE 1045 (∼0.45 wt% C) consists of highly strained, carbon-supersaturated BCT martensite (needle/lath morphology) plus a small fraction of retained austenite trapped between the laths (its $M_f$ is not far below room temperature at this carbon level, so retained austenite is present but modest compared with a higher-carbon steel). This structure is hard, brittle and carries large internal (quenching) stresses. On reheating, three overlapping but temperature-separated stages of change occur:

Tempering temperature (°C) Hardness 20 (as-Q) 250 300 700 I II III
Fig. 4.1 — Schematic hardness vs. tempering temperature for a quenched mid-carbon steel: Stage I (~100–250 °C, ε-carbide precipitation), Stage II (~200–300 °C, minor retained-austenite decomposition), Stage III (>250 °C up to 700 °C, cementite precipitation/coarsening and recovery of the ferrite matrix) — hardness falls fastest early (loss of martensite tetragonality) and levels off toward a soft, tough, spheroidized-carbide structure near 700 °C.

4.1 — Stage I (∼100–250 °C): transition-carbide precipitation

Fine, coherent transition carbide (ε-carbide, Fe2.4C, hexagonal) precipitates directly from the supersaturated martensite. The martensite's own carbon content drops toward ∼0.2–0.3 wt% C, so its tetragonal distortion ($c/a$) relaxes toward 1 and internal strain is partly relieved. Hardness falls only modestly in this stage, since the fine carbide dispersion itself contributes some precipitation strengthening even as the matrix softens.

4.2 — Stage II (∼200–300 °C): retained-austenite decomposition

Any retained austenite present decomposes to a bainite-like mixture of ferrite and cementite. In a mid-carbon steel like 1045 the retained-austenite fraction is small, so this stage is less pronounced than in a higher-carbon (e.g. eutectoid or tool-steel) quench, but it still contributes a further, small hardness change and removes the (otherwise metastable) retained austenite from the structure.

4.3 — Stage III (>250 °C, continuing to ∼700 °C): cementite formation, coarsening and recovery

The transition ε-carbide re-dissolves and is replaced by the stable phase, cementite (Fe3C), which now precipitates directly from the (nearly carbon-free) ferrite matrix. As temperature rises further, the cementite particles coarsen (Ostwald ripening — fewer, larger particles at lower total interfacial energy) and the ferrite laths themselves recover and progressively recrystallize into more equiaxed grains. By the top of this range the structure has become a coarse, globular dispersion of cementite in a soft, recovered ferrite matrix (classically termed tempered martensite grading into spheroidite as the temperature approaches 700 °C). Hardness and strength fall essentially monotonically through this stage while ductility and toughness rise; internal (quenching) stresses are progressively relieved throughout all three stages, most completely by the end of Stage III.