Question 6 of 7: The Jominy Test; Isothermal Heat Treatments of Eutectoid Steel

Question 6: The Jominy Test; Isothermal Heat Treatments of Eutectoid Steel (20 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.

Given. Isothermal (TTT) diagram for a 0.8% C eutectoid steel: eutectoid temperature 727°C, nose of the C-curve near 250°C, $M_s\approx220$°C, $M_f$ below room temperature; four heat-treatment schedules quenched from an 800°C, 1 hr austenitizing hold.

Find. (a) Description and purpose of the Jominy end-quench test. (b) Microstructure and mechanical properties for cases (i)–(iv). (c) Why a liquid-nitrogen sub-zero quench plus a 100°C anneal often improves the properties of case (iii)'s product.

Approach

For each isothermal hold, the deciding question is where the hold time falls relative to the diagram's "start" and "end of transformation" curves at that hold temperature: past the end curve means the isothermal transformation runs to completion at that temperature (giving a single, uniform product); between start and end means only partial transformation, so the remaining untransformed austenite converts to martensite on the final quench to room temperature.

1. (a) The Jominy end-quench test. A standard round steel bar (1 inch diameter, 4 inches long) is austenitized, then mounted vertically and end-quenched by a jet of water directed at just one end face, giving a continuous range of cooling rates along the bar's length — fastest at the quenched end, slowest toward the far end. After quenching, a flat is ground along the bar's length and Rockwell C hardness is measured at fixed distance intervals from the quenched end, producing a hardenability curve (hardness vs. distance from the quenched end). Useful information: the test isolates hardenability (how deep a steel can be hardened, i.e. how far martensite/bainite formation persists as cooling rate falls) from hardness itself (which depends mainly on carbon content); it lets engineers compare alloys' ability to through-harden thick sections and predict, via standard Jominy-distance-to-equivalent- cooling-rate correlations, the as-quenched hardness profile of a real part of a given size and quench severity.
2. (b)(i) 350°C, 750 s. At 350°C the isothermal "end of transformation" curve is reached in roughly 10–20 s (well inside the 750 s hold), so the austenite fully transforms isothermally at 350°C, in the temperature band between the pearlite nose (≈250°C) and the pearlite region above ≈550°C. Microstructure: 100% bainite (fine, feathery ferrite/cementite aggregate, upper-to-lower-bainite boundary region). The final quench to room temperature causes no further change, since no austenite remains. Properties: bainite at this temperature gives a strong combination of hardness and toughness — harder and stronger than coarse pearlite, and tougher than martensite, with no further tempering needed.
3. (b)(ii) 650°C, 500 s. At 650°C, only ≈77°C below the eutectoid temperature, the end-of-transformation curve is reached very quickly (a few seconds), so the 500 s hold is far more than enough for complete transformation. Microstructure: 100% coarse pearlite (widely spaced ferrite/cementite lamellae, since diffusion is fast this close to 727°C). Properties: the softest, most ductile product of the four cases — low strength and hardness, high ductility, since coarse lamellar spacing gives relatively little Hall–Petch-type strengthening.
4. (b)(iii) 300°C, 10 s. At 300°C the start-of-transformation curve is crossed at roughly 1–2 s, but the end curve is not reached until roughly 20–50 s — so the 10 s hold stops during the transformation, with only part of the austenite converted to bainite. The final quench to room temperature (below $M_f$) transforms the remaining, still-untransformed austenite to martensite. Microstructure: a mixture of bainite and untempered (fresh) martensite. Properties: hard but brittle, dominated by the untempered martensite fraction; not a desirable end condition without further tempering, since fresh martensite is highly stressed and crack-prone.
5. (b)(iv) Same as (iii), then reheated to 400°C for 3600 s, slow cooled. This second stage is a tempering treatment applied to case (iii)'s bainite + fresh-martensite product. No further austenite-to-product transformation occurs (none remains after the room-temperature quench); instead, the hour-long 400°C hold allows the martensite to decompose into tempered martensite (fine, dispersed carbide particles in a ferrite matrix), relieving the internal quenching stresses, while the pre-existing bainite is comparatively stable and changes only modestly (some carbide coarsening). Properties: substantially improved toughness and ductility relative to case (iii), at some (deliberate, controlled) sacrifice of the peak hardness — the classic quench-and-temper trade-off, producing a much more usable engineering material than the as-quenched mixture of (iii).
6. (c) Liquid-nitrogen sub-zero quench + 100°C anneal on case (iii)'s product. The diagram's $M_f$ (martensite-finish) temperature lies below room temperature, so quenching only to room temperature (as in (iii)) leaves the martensitic transformation incomplete — some austenite remains untransformed as soft, dimensionally-unstable retained austenite. A further quench in liquid nitrogen (−196°C, well below $M_f$) completes the martensitic transformation of that retained austenite, increasing hardness and eliminating the risk of it transforming later in service (dimensional instability). The subsequent low-temperature (100°C) anneal is a mild stress relief / temper: it relieves the severe internal stresses created by the cryogenic quench and slightly reduces the brittleness of the freshly-formed martensite, without significantly softening it (100°C is far too low a temper temperature to cause meaningful carbide precipitation/softening). Net effect: higher, more complete hardness (from converting retained austenite) combined with reduced brittleness and better dimensional stability — the "improvement in properties" the question describes.