Repair welding: fixing a crack so it stays fixed
A repair weld is harder than the original one. The groove sits in a rigid part, the steel is often unknown, and whatever caused the crack is still there. Below is the order of work for a crack in steel, cast iron or aluminium; replacing rusted-through sections and building up worn surfaces are covered on the repairs and rebuilding page.
Short answer
Repair welding means removing the defect completely and replacing it with sound weld metal, under conditions no worse than those of the original joint. For a crack: find its real ends with penetrant (PT) or magnetic particle (MT) testing, drill 3–4 mm holes just beyond them, grind or gouge a V or U groove down to sound metal, test the groove again, preheat to the value from the preheat calculator (S355J2, 20 mm, low-hydrogen filler: 75 °C), weld in short sections from the ends towards the middle, keep the interpass temperature between preheat and 250 °C, and inspect the repair again — on hardenable steels no earlier than 24–48 h after welding.
Crack repair step by step
- Identify the material and the cause. The grade and its carbon equivalent come from the certificate or an analysis (CE and CET calculator); the cause from where and when the crack appeared — hot, cold or fatigue crack. A crack welded up with its cause still in place comes back in the same spot.
- Find the ends. Grind the surface to bright metal: the visible end is rarely the real one. PT (ISO 3452-1) works on any metal; MT (ISO 17638) works only on ferromagnetic steel but also shows cracks just below the surface. Mark the ends with a centre punch.
- Drill crack-stop holes of 3–4 mm just beyond each end, so the tip is removed rather than left in the metal. The holes become part of the groove and are welded with it.
- Excavate to sound metal. Grinding or air-arc gouging along the full length and depth of the crack. A V or U groove with a rounded bottom, open enough for the electrode to reach the root — about 60° for stick, 50–60° for MAG, as for a new butt joint — with the ends run out at a shallow slope. A crack through the full thickness gets a proper butt preparation with root face and gap (joint preparation). Grind gouged faces back to bright metal.
- Test the groove. PT or MT on the clean groove. Any indication: grind deeper and test again. Note the length and depth of the excavation — the record and the filler quantity both need them.
- Preheat. Take the value from the calculator, not from habit. The repair groove is held by the whole part and is more restrained than the original joint, so the calculated value is the minimum. Heat a wide band and check it with a temperature indicator stick on both sides of the groove. The cooling time can be checked with the t8/5 calculator.
- Weld in a planned sequence. Low-hydrogen filler; basic electrodes rebaked as in the rebaking chart. First the holes and the ends, then short sections towards the middle; stringer beads layer by layer, slag removed from each, every crater filled.
- Hold the interpass temperature. Not below preheat and not above about 250 °C for ordinary structural steel — the upper value the preheat calculator uses. Austenitic stainless stays below about 150 °C, 6xxx aluminium below about 120 °C.
- Inspect the finished repair. Visual first, then PT or MT over the weld and the metal around it, UT for internal defects in thick sections. On steels with a high carbon equivalent the final check comes no earlier than 24–48 h after welding: cold cracks need time. Acceptance is the same as for the original weld, and cracks are not allowed at any ISO 5817 quality level.
- Record it. Where, how long and how deep; NDT before and after; preheat and interpass as measured; consumable, welder and WPS.
Preheat for a repair: values from the calculator
A few grades and thicknesses, worked out with the calculator’s formula. Hydrogen matters as much as thickness: on S355J2 at 20 mm, going from HD 4 to HD 12 (rutile electrode) raises preheat from 75 to 120 °C.
| Thickness, mm | S235JR CET 0.25 · HD 12, °C | S355J2 CET 0.34 · HD 4, °C | S355J2 CET 0.34 · HD 12, °C | S460N CET 0.42 · HD 4, °C |
|---|---|---|---|---|
| 10 | — | 35 | 85 | 100 |
| 20 | 50 | 75 | 120 | 135 |
| 30 | 80 | 100 | 150 | 165 |
| 40 | 100 | 120 | 170 | 185 |
| 60 | 120 | 140 | 190 | 205 |
Heat input 1.5 kJ/mm, result rounded up to 5 °C — the same formula as the calculator (EN 1011-2, method B). "—" means no preheat by the formula. Grade CET values are the calculator’s starting points; for a repair use the CET from the certificate or an analysis.
Repair by material: decision table
The order of work stays the same; the details change with the metal. Cast iron is covered in full under welding cast iron, aluminium under welding aluminium and stainless under welding stainless steel.
| Material | Finding the crack and excavating | Preheat and interpass | Filler and technique | Watch out for |
|---|---|---|---|---|
| Unalloyed structural steel (S235, S355) | PT or MT; grind or air-arc gouge a V or U groove, grind gouged faces back to bright metal | From the calculator: S355J2, 20 mm — 75 °C with HD 4, 120 °C with HD 12; interpass from preheat up to 250 °C | Basic electrode or solid wire MAG of matching strength; stringer beads from the drilled ends to the middle | The cause: a fatigue crack comes back if the detail stays the same |
| High-strength or thick steel (S460 and up, over 30 mm) | MT preferred; excavate the full depth, the crack goes deeper than it looks | Never below the calculated value: S460N, 40 mm, HD 4 — 185 °C; the repair groove is more restrained than the original joint | Low-hydrogen consumables only (HD 5 or lower), rebaked basic electrodes; t8/5 within the steel maker’s window | Cold cracks appear hours later: final NDT after 24–48 h |
| Austenitic stainless steel (1.4301, 1.4404) | PT only — MT does not work on austenitic steel; grind with discs used on stainless and nothing else | No preheat; interpass below about 150 °C | Filler matching the grade, L grades (C under 0.03 %); low heat input, short beads | Sensitisation at 450–850 °C; remove heat tint after welding |
| Grey and ductile cast iron, cold method | PT, or warm the oily part gently — seeping oil marks the crack; 3–4 mm holes, U groove with a rounded bottom, burn the oil out | No preheat; the next bead only when the part is hand-warm | Ni-CI or NiFe-CI (EN ISO 1071), about 25 A per mm of diameter, beads of 20–40 mm, peen each one while hot | The HAZ turns to white iron: the repair holds but cannot be machined |
| Cast iron, hot method | As for the cold method | The whole casting at 500–600 °C, held through the work, cooled slowly in the furnace over many hours | Gas welding with a cast-iron rod and flux | Needs a furnace; the only route to a joint that machines like the casting |
| Aluminium alloys | PT; oxide off with a stainless brush kept for aluminium, excavate with a carbide burr | Moderate warming only; 6xxx no hotter than about 120 °C and for no longer than 15 min | TIG on AC or MIG; 5356 or 5183 for 5xxx, 4043 or 5356 for 6xxx, 4043 for castings | Heat removes the age hardening of 6xxx in the HAZ — the repaired zone is weaker |
| Cast steel | PT or MT; casting cavities below the surface are often larger than the indication — excavate until the groove is sound | As for rolled steel of the same composition: CE or CET from the certificate, then the calculator | Low-hydrogen filler matching the composition | If the casting was heat-treated, its specification may call for heat treatment after repair |
Documentation, WPS and procedure
On a part made to a standard, the repair is welded to a WPS like any production weld. If the process, material group, thickness and position of the repair groove fall within the range of an existing procedure qualification (WPQR to ISO 15614-1), the original WPS can be used; a different process or a groove outside that range needs its own qualified procedure. Under a quality system to ISO 3834 a repair is handled as a nonconformity: a written repair procedure, re-inspection to the original requirements and a record. Repairing without paperwork is fine for a bracket in your own yard, not for a part someone else has to sign off.
When not to repair-weld
- Pressure equipment and pressure-bearing parts — only by an organisation and to a procedure that the applicable rules accept, with inspection. Not a workshop job.
- Lifting gear, brake and steering parts, and anything whose failure can injure someone: replace them.
- Fatigue cracks with no change to the design. The weld restores the same notch and the crack returns. Find the cause first — load, detail, stiffness — with the designer.
- Hardened and heat-treated parts — springs, case-hardened gears, quenched and tempered shafts. Welding heat destroys the treatment and the HAZ hardens. Only with a procedure that includes heat treatment afterwards; otherwise replace.
- White cast iron and hard castings are practically unweldable; cast iron scaled by years in hot service cracks again beside every bead.
- A spot already repaired twice. The steel there has been overheated twice and is brittle.
- Steel that cannot be identified. No CE means no preheat value; first an analysis, or at least a hardness check.
Repair mistakes: cause, check, fix
Eight mistakes in the order they happen during a repair.
| # | Cause | How to check | Fix |
|---|---|---|---|
| 1 | Crack welded over without excavating | A line or a PT indication left in the groove bottom | Excavate to sound metal and confirm with PT or MT before the first bead |
| 2 | Crack ends not found or not drilled | Within days a new crack runs on beyond the end of the repair | Find the real ends with PT, drill 3–4 mm just beyond each one, include the holes in the groove |
| 3 | Preheat too low or from one torch spot | Temperature indicator stick or contact thermometer on both sides of the groove, not just on the surface next to it | Heat a wide band evenly to the calculator value and hold it through every pass |
| 4 | Damp basic electrodes | Spitting arc, fine surface pores, a transverse or HAZ crack the next day | Rebake at 300–350 °C for 2 h, then keep in a quiver at 100–150 °C |
| 5 | Final inspection straight after the last pass | "No indications" an hour after welding | On steels with a high carbon equivalent, final NDT not earlier than 24–48 h after welding |
| 6 | Fatigue crack rewelded with the same detail | Crack at a weld toe, hole or cut-out under cyclic load, beach marks on the fracture face | Find the load and change the detail with the designer; the weld alone restores the same notch |
| 7 | Third repair at the same spot | Signs of earlier repairs, a hard brittle zone | Replace the part or the section: twice-overheated steel is brittle |
| 8 | Steel electrode on cast iron | Crack along the fusion line after a few days, the drill skids on the seam | Nickel electrode, short beads, peening |
Where to go next
Why cracks form and how they are numbered on a report: cracks in welds. What each inspection method finds and misses: the table in the weld defects catalogue. Inserts, hardfacing and repairs of steel structures and machinery: repairs and rebuilding. The numbers for your own steel: preheat, carbon equivalent and t8/5 cooling time.
Frequently asked questions
Can a crack be welded over without grinding it out?
No. The crack tip stays under the new weld, and the crack keeps growing from it. The crack is excavated to sound metal, the groove is checked with penetrant or magnetic particle testing, and only then is it welded.
What size should crack-stop holes be?
3–4 mm, drilled just beyond each end once penetrant testing has shown where the crack really stops. The hole removes the sharp tip; it then becomes part of the repair groove and is welded up with it.
What preheat does a repair weld need?
At least the value for the original joint, worked out from CET, thickness, hydrogen and heat input. For S355J2 at 20 mm and 1.5 kJ/mm the calculator gives 75 °C with a low-hydrogen filler (HD 4) and 120 °C with a rutile electrode (HD 12). The repair groove is more restrained than the original joint, so the calculated value is the floor, not the target.
When can a repair weld be inspected?
Visually and with penetrant straight away, but on steels with a high carbon equivalent the final NDT is done no earlier than 24–48 h after welding, because cold cracks appear with a delay. The actual waiting time is set by the product standard or the inspection plan.
Does a repair need its own WPS?
Not always. If the process, material group, thickness and position of the repair fall within an existing procedure qualification (WPQR to ISO 15614-1), the repair is welded to that WPS. A different process, a groove outside the qualified thickness range or a different material needs its own qualification.
Cast iron: cold or hot repair?
Cold, with Ni or NiFe electrodes, short 20–40 mm beads and peening, for most brackets and housings. Hot, with the whole casting at 500–600 °C and slow cooling in the furnace, when the repaired area has to be machined afterwards.
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