Delayed HAZ cracking in a restrained 25 mm box-column weld
A fabricator is welding a 25 mm thick C-Mn steel box column with full-penetration single-V butt welds, 3.2 m long, heavily restrained because all four sides are welded in sequence. Plate chemistry (wt%): C 0.18, Mn 1.30, Si 0.30, Cr 0.08, Mo 0.02, V 0.01, Ni 0.05, Cu 0.10. The approved WPS uses rutile-coated E6013 electrodes taken straight from an open tray, no preheat, 200 A, 25 V, 150 mm/min travel, giving about 2.0 kJ/mm heat input. The shop floor is at 8 °C in January. Radiography immediately after welding was clean. Two days later, magnetic particle inspection found transverse cracks in the coarse-grained heat-affected zone about 2 mm from the fusion line on three of the four sides. Rework of one joint costs roughly Rs 45 000 and two days of the bay. The production manager wants to keep the same electrode and instead 'weld hotter and faster' so that preheat time can be avoided.
1. Compute the carbon equivalent and use it, with the other given conditions, to explain the type of cracking, its location and why it appeared two days late.
Read the worked answer
CE (IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 = 0.18 + 1.30/6 + (0.08 + 0.02 + 0.01)/5 + (0.05 + 0.10)/15 = 0.18 + 0.217 + 0.022 + 0.010 = 0.429 ≈ 0.43.
At CE 0.43, 25 mm thickness, 2.0 kJ/mm and no preheat, the HAZ cools quickly through 800–500 °C and transforms to hard martensite/bainite, typically well above 350 HV. Three ingredients then coincide: a hard susceptible microstructure, a high diffusible hydrogen level (rutile E6013 taken from an open tray is unbaked and moisture pickup is certain), and high restraint from the four-sided box sequence producing triaxial tensile stress at the weld toe.
That combination gives hydrogen-assisted cold cracking, not hot cracking, which is why radiography immediately after welding was clean: hydrogen needs time to diffuse to the process zone, and the crack propagates only after the joint has cooled and the local stress has built up. A 24–48 h incubation is the classic signature. The position, transverse to the weld, in the coarse-grained HAZ 2 mm from the fusion line, is exactly where hardness and hydrogen concentration peak. The 8 °C ambient makes the cooling rate worse and is the reason three of four sides failed.
2. Write the corrective welding procedure. For each change, state what it does to one of the three cracking ingredients.
Read the worked answer
Preheat to 100–150 °C, measured by contact thermometer or thermal chalk for at least 75 mm each side of the joint, applied before tacking and maintained through the root. This slows cooling through the transformation range (removes the hard microstructure) and lowers the thermal gradient, which reduces restraint stress; it also gives hydrogen time to escape. Preheat is the single most effective control here.
Change to a low-hydrogen electrode, E7018 (H4/H5), baked at 350–400 °C for 1 h and held in a quench oven at 100–150 °C, with a maximum of about 4 h out of the oven and a maximum 2 h in the portable quiver. This attacks the hydrogen term directly; the electrode class alone, without baking and handling discipline, changes nothing.
Cleanliness and consumables: remove mill scale, rust, paint and moisture from the joint faces; reject damp electrodes; never re-dry rutile electrodes as a substitute for low-hydrogen consumables.
Heat input 1.5–2.5 kJ/mm, stringer beads, weave limited to about three electrode diameters, interpass temperature capped at 250 °C. Too low gives hard fast-cooled HAZ, too high gives grain growth and distortion.
Root quality: TIG root or E7018 with a small land and a clean back gouge so no crack-like root defect remains to act as a starter.
Hydrogen soak: hold the finished joint at 250 °C for 2 h, or delay inspection for 48 h, so any cracking happens before final NDT and sign-off rather than in service.
3. Production proposes to drop preheat and raise heat input from 2.0 to 3.5 kJ/mm instead. Evaluate this proposal technically and commercially.
Read the worked answer
The proposal confuses two different levers.
What raising heat input does: at 3.5 kJ/mm the t8/5 time lengthens, so the HAZ is softer and less crack-susceptible in principle, and it does reduce the cooling rate.
What it does not do:
- It does not remove hydrogen. Only a low-hydrogen consumable, correct baking and handling, and a slow cool that lets the residual hydrogen diffuse out can do that.
- On a 25 mm single-V with four restrained sides, 3.5 kJ/mm means bigger, hotter passes, deeper penetration per pass and much more angular distortion and shrinkage stress; you would trade a cracking risk for a distortion and fit-up problem on 3.2 m welds.
- Grain growth in the coarse-grained HAZ at high heat input lowers toughness, which matters for a structural column.
The two controls are complementary: preheat acts before the arc strikes and across the whole joint, heat input acts only locally. A single short pass cannot compensate for a joint whose bulk is at 8 °C.
Commercially: preheat for this joint is perhaps 2–3 h of a bay and a heater, a few thousand rupees per joint. Rework is Rs 45 000 plus two days of bay time, plus the risk that a crack survives inspection and fails in service. The proposal is therefore rejected on a straightforward cost comparison, even before the inspection liability is counted.