Through-Hardening Shortfall in a Large 4140 Forged Shaft
A 250 mm diameter, 2.8 m long shaft in AISI 4140 (42CrMo4) is forged, normalised, rough machined, austenitised at 850 °C for 3 h, oil quenched with moderate agitation, and tempered at 560 °C for 4 h. The drawing requires 45–50 HRC at 12.5 mm below the surface and a 0.2 % proof strength of at least 1,000 MPa. Verification by a portable Leeb hardness tester converted to HRC gives 52, 51, 52 HRC at the surface and 37, 38, 36 HRC at 25 mm depth on the same section. Ultrasonic inspection is clean. Charpy V-notch impact tests at −20 °C on the specified test bar return 18 J, against a purchase-order minimum of 27 J. The customer wants a concession to accept the part because the project schedule has 5 days of float. You are the materials engineer signing the heat-treatment certificate.
1. Explain the most likely reason for the hardness profile. Does the part meet the drawing? What would you check before concluding?
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Diagnosis. The profile of 52 HRC at the surface and 37 HRC at 25 mm is the classic signature of inadequate through-hardening, not a tempering error. In a 250 mm section the cooling rate at 25 mm is only a few °C/s even in agitated oil, which is too slow for 4140 to miss the bainite knee at that depth; the surface cools fast enough to form martensite. Before concluding, I would:
- Check the mill certificate for C (0.38–0.43 %), Mn, Cr, Mo and grain size, because hardenability is composition-sensitive.
- Review furnace charts for austenitising time/temperature and quench tank agitation and oil temperature.
- Re-test on a cut section with a calibrated bench hardness tester; a Leeb conversion on a curved, possibly decarburised surface is unreliable.
- Confirm whether the 12.5 mm value was measured or merely inferred from surface readings.
Conclusion. Compliance with 45–50 HRC at 12.5 mm is not demonstrated, and the 18 J impact result is a separate non-conformance. I would not sign the certificate on this evidence.
2. The customer asks you to support a concession. Assess the fitness-for-service of the shaft for a torque-transmitting application with a keyway.
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Mechanical assessment. A shaft in torsion carries maximum shear stress at the surface, so surface hardness is favourable. However, the keyway has a stress concentration factor of roughly 2–3, and the low core hardness and 18 J at −20 °C give little reserve against brittle fracture under shock, misalignment or torsional oscillation. At 37 HRC the 0.2 % proof strength is likely to be borderline against the 1,000 MPa requirement.
Concession position. A concession is an engineering decision, not a waiver of judgement. Before supporting it I would require: (i) a torsion and bending stress analysis including the keyway Kt; (ii) a fracture-mechanics assessment using measured toughness and the 18 J result; (iii) a reduced torque or load limit with enhanced NDT; and (iv) the client's written acceptance. For a safety-critical shaft I would reject the part rather than accept a concession.
3. Recommend a revised heat treatment and verification plan, including how you would demonstrate compliance in production.
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Revised route. Re-austenitise at 850 °C with a soak adequate for the section, then quench in a polymer solution or oil with strong agitation to raise the quench severity; temper at 560–600 °C to the required strength. If the section cannot be through-hardened in 4140, change the steel to a higher-hardenability grade such as 4340/34CrNiMo6.
Verification.
- Heat-treat a representative test bar of the same heat and section, and cut it to measure hardness at 12.5 mm and 25 mm.
- Obtain a Jominy end-quench curve and compare with the mill certificate.
- Check temper embrittlement by step-cooling and repeat Charpy at −20 °C.
- Stress-relieve after rough machining and perform MPI/UT after final machining.
Only a measured section hardness and impact result, not a surface conversion, should release the part.