Question 7 of 7: Jominy Hardenability Test; Interpreting Four Heat Treatments on the TTT Diagram; Cryogenic Treatment

Question 7: Jominy Hardenability Test; Interpreting Four Heat Treatments on the TTT Diagram; Cryogenic Treatment (supplementary)

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.

Note on numbering. The printed paper numbers the four cases (i), (ii), (ii), (iii), repeating “(ii)”. They are answered below in printed order as cases (i)–(iv).

Given. Fig 3, 0.8% C eutectoid steel, austenitised at $800^{\circ}$C (above $A_1=727^{\circ}$C, so fully austenitic). Read off the printed diagram (digitised): nose of the C-curves at $\approx550^{\circ}$C, where transformation starts at $\approx0.75$ s and finishes at $\approx4$ s. At $650^{\circ}$C: start $\approx1.5$ s, finish $\approx15$ s. At $350^{\circ}$C: start $\approx24$ s, finish $\approx290$ s. At $300^{\circ}$C: start $\approx85$ s, finish $\approx800$ s. $M_s$ line $\approx260^{\circ}$C (where the start curve ends). $M_f\approx-50^{\circ}$C, i.e. below room temperature.

Find. (a) The Jominy test and what it tells you. (b) Microstructure and properties for each of the four cases. (c) Why a liquid-nitrogen quench plus a 100°C anneal improves properties.

Approach

For each isothermal hold, compare the hold time with the start and finish curves at that temperature. Past the finish curve means transformation is complete and the final quench changes nothing. Before the start curve means the steel is still austenite, and the quench turns it to martensite. Between the two means a mixture. Because $M_f$ lies below room temperature, any quench to room temperature leaves some retained austenite in the martensite.

1. (a) The Jominy end-quench hardenability test. A standard round bar ($25.4$ mm/1 in diameter, $100$ mm/4 in long) is austenitised, then held vertically and quenched from one end only by a controlled water jet. This gives a continuous, reproducible range of cooling rates along the bar: fastest at the quenched end, slower with distance from it (controlled by axial heat conduction). A flat is then ground along the bar and Rockwell C hardness is measured at fixed intervals from the quenched end, giving a hardenability curve (HRC against distance). Useful information: it measures hardenability, meaning how deep and at how slow a cooling rate martensite still forms, as a property of the steel, independent of part geometry or quench medium. Steels can be compared and specified by it (H-band steels). Using published charts that relate Jominy distance to the cooling rate at the surface, mid-radius and centre of round bars quenched in water or oil, it also predicts the as-quenched hardness profile of a real part.
2. (b)(i) 350°C, held 750 s, then quenched. At $350^{\circ}$C the transformation starts at $\approx24$ s and is finished by $\approx290$ s, so the 750 s hold goes well past the finish curve and the austenite transforms completely at $350^{\circ}$C. The final quench has no austenite left to act on. Microstructure: $100\%$ bainite (fine acicular ferrite + cementite, formed near the upper/lower-bainite boundary). Properties: high strength and hardness (roughly mid-40s HRC) with good toughness and ductility. It is harder than any pearlite and much tougher than untempered martensite, with no tempering needed (this is austempering).
3. (b)(ii) 650°C, held 500 s, then quenched. At $650^{\circ}$C the transformation starts at $\approx1.5$ s and finishes at $\approx15$ s, so it is complete long before 500 s and the final quench changes nothing. Microstructure: $100\%$ pearlite, fairly coarse because it formed with only $77^{\circ}$C of undercooling below $A_1$, where diffusion is fast and the lamellae are widely spaced. Properties: the softest and most ductile of the four cases (roughly 15–20 HRC), with modest strength.
4. (b)(iii) 300°C, held 10 s, then quenched to room temperature. At $300^{\circ}$C the start curve is not reached until $\approx85$ s, so after 10 s no transformation has begun. The steel is still austenite (it was quenched past the nose too quickly for pearlite to form). On quenching through $M_s\approx260^{\circ}$C to room temperature it transforms athermally to martensite. Because $M_f\approx-50^{\circ}$C is below room temperature, some retained austenite is left as well. Properties: the hardest and strongest condition (about 60+ HRC for 0.8% C), but brittle, with very low ductility and toughness and high residual stresses. Unusable without tempering. (This hold is effectively a marquench: the part equalises in temperature above $M_s$ before martensite forms, which reduces distortion and cracking.)
5. (b)(iv) As (iii), then reheated to 400°C for 3600 s and cooled slowly. The first stage gives the martensite + retained austenite of case (iii). The reheat is a tempering treatment, not a path on the isothermal diagram (that diagram applies only to austenite cooled from above $A_1$). During 1 h at $400^{\circ}$C the martensite decomposes into tempered martensite: fine cementite particles in a ferrite matrix. The quenching stresses are relieved, and the retained austenite also decomposes (to bainite/ferrite + carbide). Slow cooling afterwards causes no further change. Properties: hardness drops from the as-quenched level to roughly 45–50 HRC, while toughness and ductility improve greatly. This is the classic quench-and-temper combination of high strength with usable toughness, and far more serviceable than case (iii).
6. (c) Liquid-nitrogen quench plus 100°C anneal. On Fig 3, $M_f$ ($\approx-50^{\circ}$C) is below room temperature, so the martensite reaction stops before it is complete when the part reaches room temperature. The untransformed part remains as retained austenite, which is soft and metastable. It lowers hardness and wear resistance, and it can transform later in service (under stress or with time), causing dimensional instability and fresh brittle martensite exactly where it is not wanted. Quenching further in liquid nitrogen ($-196^{\circ}$C, well below $M_f$) drives the martensite reaction to completion and converts most of the retained austenite. The new martensite is highly stressed and brittle. The following low-temperature anneal at $100^{\circ}$C is a mild stress-relief/temper: it relaxes the transformation stresses and starts fine transition-carbide precipitation, without the softening a normal temper would cause. Net result: higher and more uniform hardness, better wear resistance and dimensional stability, and less brittleness. (This is standard practice for gauges, bearings and tool steels.)

[Figure not reproduced: Fig. Q7 — the printed Fig 3 isothermal diagram, redrawn from its digitised start (solid) and finish (dashed) curves, with heat-treatment paths (i)–(iii). Path (iv) is case (iii) followed by a tempering reheat and is not an isothermal-transformation path. See the official exam paper.]