1045 Shaft Build-Up: Heat Input Within Limits but Preheat Ignored
A repair shop is rebuilding a worn bearing seat on a 1045 steel shaft, 60 mm diameter. The procedure calls for manual SMAW build-up with 3.2 mm E7018 electrodes at 130 A, 26 V, and 120 mm/min travel, using stringer beads. The shaft arrives at 08:00 in an unheated bay at 8 C. The welder cleans the area, tacks the shaft, and begins welding without preheat. Interpass temperature, measured with a contact thermometer after the third bead, is 90 C. The shop's written procedure specifies a maximum heat input of 1.8 kJ/mm and a preheat of 150-200 C for this steel. Twenty-four hours later, magnetic particle inspection reveals transverse cracks at the weld toe on the first side welded. The shop wants to grind out the cracks and reweld with the same parameters, arguing the heat input was within spec because the calculation was never checked.
1. Calculate the heat input and assess the welder's argument that the parameters were within specification.
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HI = (V x A x 60) / (1000 x travel) = (26 x 130 x 60) / (1000 x 120) = 1.69 kJ/mm. That is below the 1.8 kJ/mm maximum, so the welder's arithmetic is correct but irrelevant: the procedure also requires 150-200 C preheat, and the shaft was welded at 8 C with 90 C interpass. Heat input is only one variable; preheat and interpass control the cooling rate and allow hydrogen to escape. The procedure was violated, so the crack cannot be blamed on heat input alone. A high heat input can even coarsen the HAZ, so increasing current to compensate for low preheat would be the wrong correction.
2. Explain the likely cracking mechanism and why 1045 steel is susceptible.
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1045 is about 0.45% C, with a carbon equivalent near 0.45 + 0.75/6 = 0.58, so it is highly hardenable. Rapid cooling from 8 C forms untempered martensite in the HAZ; diffusible hydrogen from moisture or contamination plus restraint stresses then causes delayed transverse toe cracks. E7018 is low-hydrogen only if kept dry; a damp electrode, dirty joint, or humid shop raises hydrogen. The 24-hour delay is typical of hydrogen-assisted cold cracking. Grinding out visible cracks is not enough because cracks may extend under the surface, so the excavation must be verified before rewelding.
3. Recommend a revised procedure and verification plan for the repair.
Read the worked answer
Preheat the entire section to 180-200 C, measure it 75 mm from the weld, and maintain interpass within the qualified range, not at 90 C. Use E7018 from a holding oven at 120-150 C, clean the groove to bright metal, and weld stringer beads at the qualified heat input, about 1.5-1.7 kJ/mm, without raising current to compensate for poor preheat. Slow-cool under an insulating blanket. After repair, inspect by MT or PT after at least 48 hours, not immediately, because hydrogen cracking is delayed. Record preheat and interpass on the traveler. If cracks recur, consider a qualified temper-bead or post-weld heat-treatment procedure rather than a third identical repair.