Cracks That Appeared After the Truck Left
A 4140 steel shaft, 8 in. diameter, is being rebuilt at a journal. The repair procedure calls for E7018 low-hydrogen electrodes, a preheat of 250 F, interpass maximum 400 F, and no post-weld heat treatment. The welder applies a 1.25-in. build-up in 12 passes, then covers the repair with a fire blanket and lets it cool in still air. The shaft is machined 24 hours later. Magnetic particle inspection of the finished journal shows three transverse indications at the weld toe in the base-metal heat-affected zone, each about 3/8 in. long. A portable hardness check gives 42 HRC at the indication location. The shop wants to grind out the indications and reweld using the same procedure. The customer requires the shaft to be returned to service within 72 hours.
1. What cracking mechanism is most likely, and what evidence in the setup supports it?
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The most likely mechanism is hydrogen-assisted cold cracking in the heat-affected zone. AISI 4140 has high hardenability and a relatively high carbon equivalent, so the HAZ transforms to hard martensite at moderate cooling rates. A preheat of 250 F is low for this section and chemistry; it does not provide enough slow cooling or enough time for hydrogen to escape. The procedure specifies no post-weld heat treatment, so any diffusible hydrogen from the E7018 deposit remains mobile, and the hard HAZ is sensitive to delayed cracking. The cracks appeared 24 hours after welding, which is consistent with delayed hydrogen cracking rather than hot cracking or solidification cracking. They are transverse at the weld toe, where residual stress concentrates, and the 42 HRC reading supports a hard, crack-susceptible HAZ. The 1.25-in. build-up also adds restraint and residual stress.
2. What additional examination would you require before accepting any repair?
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Before any repair is accepted, I would require the indications to be removed by grinding or machining to sound metal, with magnetic particle inspection after each removal step to confirm the cracks are gone. I would require a hardness traverse across the weld, HAZ, and base metal to map the hard zone and to set a maximum acceptable hardness for the service. I would require ultrasonic testing or another volumetric method to check for subsurface cracks that do not reach the surface, because the surface indications may be only the visible part of a larger crack. I would also require the repair procedure to include a hydrogen bake-out or post-heat and a delayed inspection at least 48 hours after welding, because hydrogen cracking may not appear immediately. The original 24-hour inspection interval is not adequate for a delayed-cracking-sensitive repair.
3. Evaluate the shop's proposal to grind out the indications and reweld with the same procedure.
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The proposal is not acceptable as written. Grinding out the visible indications and rewielding with the same 250 F preheat and no post-weld heat treatment would reproduce the same conditions that caused the cracking: a hard HAZ, residual stress, and diffusible hydrogen. A repair procedure for 4140 should use a higher preheat, controlled interpass temperature, low-hydrogen electrodes that are properly stored, and a post-weld heat treatment or tempering treatment to reduce HAZ hardness before the shaft is returned to service. The procedure should be qualified or at least supported by hardness and crack-susceptibility data for the actual section and chemistry. The 72-hour turnaround may not be achievable if a proper post-weld heat treatment and delayed inspection are required; the customer should be told that schedule pressure cannot justify accepting a known cracking mechanism. The inspector should not approve the repair until the revised procedure and examination plan are accepted.