Fatigue failure of a normalized 1045 pivot pin with a keyway
A packaging machine uses a 30 mm diameter pivot pin, 120 mm long, in a reversing linkage. The original pin is made from 1045 steel, induction hardened to 55–58 HRC to a case depth of 1.5 mm over the central 60 mm, and loaded by a 12 kN alternating force. After six weeks the replacement pin, machined from 1045 bar and supplied in the normalized condition, has worn 0.6 mm oval and shows a fatigue crack starting at the sharp root of an 8 mm keyway. The maintenance fitter proposes to make the next pin from the same bar, case harden it, and increase the keyway to 10 mm to “give it more strength”. The machine runs 16 h/day, 5 days/week, and downtime costs are high. Surface finish of the worn pin is Ra 3.2 µm; the drawing requires Ra 0.8 µm at the bearing areas.
1. Explain why the normalized 1045 replacement pin failed and why increasing the keyway from 8 to 10 mm is likely to make matters worse.
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Normalized 1045 has relatively low hardness and yield strength, so the 30 mm pin has little resistance to wear and the alternating 12 kN load produces a fatigue cycle at the surface. The sharp keyway root is a stress concentration, and the rough Ra 3.2 µm finish adds surface discontinuities from which a crack can start. The 0.6 mm oval wear increases clearance and impact loading, which further raises the local stress.
Enlarging the keyway from 8 to 10 mm removes more material from the pin section and requires a deeper keyway. That reduces the effective diameter and increases the stress concentration at the root. The cracked pin is a fatigue failure at the keyway, so adding section loss is the wrong direction. The keyway should stay at 8 mm, with a generous root radius and a filleted key if possible. The real fix is a tougher core, a harder and cleaner surface, and a better finish.
2. Specify a suitable material and heat treatment for the next pin, including core hardness, surface hardness, case depth and surface finish, and justify each choice.
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A suitable choice is 42CrMo4/4140 quenched and tempered to about 28–32 HRC, then nitrided. Nitriding gives a surface hardness of about 550–650 HV (roughly 52–56 HRC) with a case depth of 0.3–0.5 mm on a 30 mm pin, while the tough core supports the bending load. Nitriding is carried out at a relatively low temperature, so distortion is small and the keyway can be finished before treatment with less risk of cracking than with induction hardening at the keyway edges.
The bearing areas should be ground to Ra ≤ 0.4–0.8 µm, and the keyway root should have a radius of at least 0.5 mm with no sharp corners. If induction hardening is used instead, a case depth of about 0.8–1.2 mm and a surface hardness of 55–58 HRC are reasonable, but the keyway edges must be tempered and inspected for cracks. In either case, the aim is a hard wear surface, a tough core and compressive residual stress at the surface, not simply maximum hardness.
3. What inspection and process controls would you require after heat treatment and before the pin is put into service?
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Require the material certificate and heat-treatment record first. After treatment, check hardness at several points on a sample or sacrificial coupon, and verify case depth by micro-hardness or a metallographic section. Check straightness and diameter because heat treatment and grinding can distort the pin. Grind the bearing areas after heat treatment to the final tolerance and Ra ≤ 0.8 µm.
Carry out magnetic-particle or dye-penetrant inspection, concentrating on the keyway root and any section changes. Check the keyway dimensions and root radius, and trial-fit the key and mating bore. If the pin is nitrided, remove any brittle white layer from highly stressed edges if the specification requires it. Record all results and do not release the pin on hardness alone; the failure was at a stress raiser, so geometry and surface condition are equally important.