Premature spline pitting on induction-hardened EN19 shafts
A batch of 24 splined drive shafts, 40 mm diameter, is machined from EN19 (AISI 4140) bar, then induction hardened and tempered after spline milling. Specification: surface hardness 55–60 HRC, case depth 1.0–1.5 mm to 50 HRC, core 28–32 HRC, retained austenite below 10 %. After 600 h in a reversing load, three shafts show pitting and spalling on the spline flanks and one has fractured at a spline root. Sectioning shows surface hardness 46–49 HRC, case depth 0.6 mm, a 0.15 mm decarburised layer at the root, coarse needle martensite with 18 % retained austenite, and core hardness 36 HRC. The heat-treatment record shows tempering at 320 °C for 1 h instead of the specified 180 °C, and the induction coil was changed without re-validating power and frequency. The material certificate is missing; a storeman's spark test suggested 'chrome steel'.
1. Link each metallurgical finding to its most probable process cause, and decide which single deviation did the most damage.
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- Surface 46–49 HRC instead of 55–60 HRC: tempering at 320 °C rather than 180 °C over-tempers the martensite and drops hardness by roughly 8–10 HRC. Soft flanks cannot carry the reversing contact stress, so pitting and spalling follow.
- Case depth 0.6 mm instead of 1.0–1.5 mm: the new coil was not validated, so power, frequency, scan or dwell time gave insufficient austenitised depth. A thin case on a soft core flexes and spalls.
- Coarse needle martensite with 18 % retained austenite: austenitising temperature was too high for the actual material, coarsening the grain; high retained austenite is unstable under load and transforms, adding distortion and cracking risk at the root.
- 0.15 mm decarburised root layer: heating without adequate atmosphere protection, or a pre-existing decarburised surface, removes carbon and lowers hardness exactly where bending stress peaks. This explains the root fracture.
- Core 36 HRC instead of 28–32 HRC: over-tempering would normally soften, so a higher-carbon or higher-hardenability steel than EN19 is likely - consistent with the missing certificate.
Most damage: the tempering error, because it degrades the whole batch uniformly at the surface where the load acts; the shallow case and decarburisation then compound it.
2. What verification would you demand on the failed shaft and on the remaining batch, and what corrective actions would you write into the process?
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Verification of material and process:
- Optical emission spectrometry on the failed shaft and a retained sample: EN19 should show about 0.38–0.45 % C, 0.9–1.2 % Cr, 0.15–0.25 % Mo. A spark test is not evidence.
- Hardness traverse on a polished section: surface, 0.2 mm steps to case depth, and core, plotted against the drawing.
- Microstructure at the root and flank, with retained austenite measured; check for decarburisation by microhardness and etching.
- Magnetic particle inspection of all 24 shafts for root cracks; any crack means rejection, not rework.
- Review furnace and induction records: temperature chart, atmosphere, coil recipe, quench flow.
Corrective actions:
- Re-validate the induction recipe on test coupons with the new coil: power, frequency, scan speed, quench; prove case depth 1.0–1.5 mm and grain size before production.
- Temper at 180 °C and control it with a calibrated recorder and a thermocouple on the load; 320 °C must be impossible without an engineering change.
- Protect the surface during heating or machine off the decarburised layer after hardening; specify the atmosphere.
- Reinstate incoming material certification and a positive material identification check at goods-in.
- Update the control plan: hardness and case depth on a first-off and hourly sample, with the acceptance limits and reaction plan written down.