Cracking After Welding a Repaired Casting
A repair shop is welding a cracked cast iron pump housing. The casting is thick and irregular, and the shop plans to use a nickel-based electrode with preheat. The welder proposes preheating to about 200 C, welding short beads, and peening each bead. A second housing previously repaired the same way developed a crack parallel to the weld after cooling. The shop has a hardness tester and a magnetic particle kit. The customer needs the pump back in service quickly and asks whether the repair can skip post-weld heat treatment to save time.
1. Explain why the earlier repair likely cracked and what the preheat and peening steps are meant to accomplish.
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Cast iron is brittle and sensitive to thermal stress and to the formation of hard, crack-prone zones in the heat-affected area. A 200 C preheat is on the low side for a thick, irregular casting; uneven heating and rapid cooling generate restraint stresses that can open a crack parallel to the weld. Peening is intended to relieve tensile stress by mechanically stretching the bead, but it must be done on clean, hot metal with controlled force; heavy peening can itself initiate cracks. Short beads and controlled interpass temperature limit heat input and distortion. The prior failure suggests insufficient or uneven preheat, rapid cooling, or hard zones that were never tempered. The hardness tester and magnetic particle kit are the right tools to check for hard zones and crack indications before returning the part to service.
2. The customer wants to skip post-weld heat treatment. What is your recommendation and what evidence would support it?
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Do not simply skip it. Post-weld heat treatment or a controlled slow cool is often used on cast iron repairs to reduce residual stress and temper hard zones, and omitting it raises the risk of delayed cracking in a pressure-retaining housing. Recommend a slow, uniform cool under insulation at minimum, and post-weld heat treatment if the procedure or customer specification requires it. Support the decision with evidence: hardness surveys across the weld and heat-affected zone showing values within an agreed limit, magnetic particle inspection showing no indications, and a documented procedure. If schedule pressure is the driver, propose a controlled cool in an insulated blanket overnight rather than a fast air cool, and explain the risk of a field failure in a pump housing.