Crack Beside a Cast Iron Repair on a Gearbox Housing
A grey cast iron gearbox housing has a 70 mm crack running from a mounting bolt hole into the flange. The section thickness is 25 mm and the part is highly restrained by the adjacent bearing bore. The workshop proposes to V-groove the crack, weld it with 3.15 mm E6013 electrodes at 90 A, without preheat, using short 25 mm beads, and cool the part in still air at 15 °C. The welder says cast iron 'welds like mild steel if you go slowly'. After welding, a crack appears in the parent metal about 5 mm from the weld toe; the weld itself is sound. The housing is needed urgently and the supervisor suggests welding over the new crack with the same procedure.
1. Explain why the E6013/no-preheat procedure caused a crack beside the weld rather than in the weld metal.
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E6013 is a mild-steel rutile electrode. On grey cast iron, dilution of the weld by the high-carbon parent metal raises the carbon content of the deposit, and the fast-cooling heat-affected zone can form hard, brittle martensite or white iron. The flange and bearing bore restrain contraction, so the weld and heat-affected zone cannot relieve shrinkage stress by yielding. The result is a crack in the low-ductility parent metal just outside the weld toe, where the stress concentration and hard zone meet.
Short 25 mm beads reduce heat input but do not remove the main problem: no preheat means a high cooling rate and a large temperature gradient. The weld may look sound while the heat-affected zone has already cracked. The welder's idea that cast iron welds like mild steel is wrong because cast iron has much lower ductility and a much higher carbon content.
2. Select a suitable electrode and write a repair procedure covering preparation, preheat, welding technique and cooling.
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Use a nickel-based cast iron electrode such as ENi-Cl or ENiFe-Cl, or a bronze electrode for a lower-strength repair. Do not use E6013.
Procedure: drill stop holes at both crack ends; grind or gouge the crack to sound metal and open a 60–70° V-groove with a 2–3 mm root; clean with acetone and check by dye penetrant that the crack is gone. Preheat the whole housing uniformly to 300–400 °C, following the electrode maker's recommendation, and maintain the interpass temperature. Weld with low current, short beads, back-step or stagger, and peen each bead immediately with a round-nose hammer. Avoid excessive penetration. After welding, slow-cool in dry sand, lime or a thermal blanket; do not quench or cool in a draught. Stress relief is difficult on cast iron, so controlled cooling is the main protection.
3. The supervisor wants to weld over the new crack with the same procedure. Give your response and the risks.
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Do not weld over the new crack with the same procedure. The crack is in the parent metal, not just the weld; welding over it will not remove the crack tip, and the new heat will add stress and may extend it. The crack must be mapped by dye penetrant or magnetic particle inspection, then ground or gouged out completely to sound metal and re-assessed.
The same E6013/no-preheat procedure is unsuitable and would repeat the failure. Use the qualified nickel-electrode procedure, with preheat and slow cooling, and inspect again after repair. Risks of the supervisor's proposal are hidden crack growth, sudden fracture under load, loss of bearing bore alignment, and a repair that cannot be inspected reliably. If the crack reaches the bearing bore or the housing is highly stressed, replacement or a specialist cast iron repair may be the safer judgement.