Three Factors That Cause Cold Cracking
Cold cracking, also called hydrogen induced cracking, appears in weathering steel welds hours or days after the joint has cooled, usually at the weld toe or in the heat affected zone. It occurs when three conditions are present at the same time: the steel and the weld metal have a hardening tendency, the joint is restrained so that tensile stress builds up across the weld, and hydrogen is present in the joint and able to diffuse to the region of highest stress. Remove any one of the three and cracking does not occur, which is why the control measures work on all three fronts.
Hardenability is assessed through the carbon equivalent value and, for the low carbon weathering grades, through the crack sensitivity parameter. The carbon equivalent formulation adds manganese, chromium, molybdenum, vanadium, nickel and copper contributions to the carbon content, while the crack sensitivity parameter treats carbon, silicon, manganese, copper, chromium, nickel, molybdenum, vanadium and boron in a weighted form. Both are used to compare a proposed heat against the welding procedure and to set preheat and heat input. Because phosphorus is not counted in the conventional carbon equivalent, the high weathering grades with 0.07 to 0.15 % phosphorus can look deceptively easy to weld on paper while behaving poorly in practice, and they should not be used for primary welded structures.
Consumables and Joint Preparation
The first line of defence is the consumable. Basic low-hydrogen electrodes and fluxes produce weld metal with a low hydrogen content and good desulphurisation and dephosphorisation behaviour, together with high impact toughness. They must be handled as a hygroscopic product: electrodes are dried before use, typically at 350 to 420 degrees Celsius for one to two hours, so that retained moisture is driven off before the electrode goes into the holder. Electrodes left out overnight, quivers without heating, or flux that has absorbed moisture from a damp workshop all put hydrogen back into the joint.
Joint preparation matters just as much. Components to be welded should be cleaned carefully, and debris such as moisture, rust, oil, grease, paint and cutting slag must be removed from the joint faces and the adjacent surface. The gap between mating parts should be kept under control, in practice within about twenty millimetres of clean, matching surface, so that the joint closes without force and the arc can reach the root properly.
Assembly, Restraint and Welding Sequence
Weathering steel should not be forced into position during assembly. Pulling a distorted component into place leaves high residual stress in the joint before the first bead is deposited, and that stress adds directly to the tensile stress that drives cold cracking. Fitters should check the parts before pairing, correct the fit up on the bench, and use sufficient tack welds made with the same consumable and the same preheat as the production run.
The welding sequence should be planned to keep the joint as free as possible. The principle is to arrange the order of passes so that most of the weld metal is deposited while the structure is still comparatively unrestrained, and to balance the passes around the neutral axis of the joint so that distortion and residual stress stay small. Back stepping, symmetrical welding from the centre outward, and completing long welds in short sections rather than one continuous run all help. Where the section is thick or the steel is at the upper end of its strength range, preheating is applied, commonly to between 100 and 150 degrees Celsius for heavy restrained joints, and interpass temperature is monitored so that it does not run away in thin gauge work.
Environmental Control and Post-Weld Measures
Weld under cover or with screens in wind and rain. A wet joint surface is a hydrogen source.
Keep the joint dry between passes and allow no condensation on the cooled weld face.
Where the risk assessment shows a high hardenability heat, hold the completed weld at a moderate temperature for a short period to allow hydrogen to diffuse out before the joint cools fully.
Record preheat, interpass temperature and consumable issue for every joint in a critical structure.
Inspect after a waiting period, not immediately. Hydrogen cracking develops over time, so ultrasonic or magnetic particle inspection early in the next shift finds defects that a check on the same day can miss.
Selecting the Grade and Procedure for Welded Work
Grade choice is the most effective single control. The weldable weathering grades of GB/T 4172-2008 hold phosphorus at 0.035 % or below and are designed for fusion welding, and they should be used for any structure that is welded throughout. The high weathering grades of GB/T 4171-2008, with their deliberately higher phosphorus content, are intended for bolted, riveted and lightly welded exposed structures. Where those grades must be welded, the welding procedure should be qualified on the actual steel, with preheat, heat input and consumable specified, and the procedure should follow a recognised method for determining preheat temperatures such as EN 1011-2. Hardness checks on the heat affected zone, ultrasonic testing to GB/T 2970 and a defined inspection interval complete the quality package.
Frequently Asked Questions
Q: Why does cold cracking appear only after a delay?
A: Hydrogen diffuses slowly through the weld and heat affected zone and collects in regions of high stress. Cracking starts only when enough hydrogen has accumulated, which can be hours or days after welding, especially in cold weather.
Q: Can high phosphorus weathering steel be welded?
A: It can be welded for lightly loaded or non-critical connections, but it is not intended for primary welded structures. For fully welded work, specify the weldable NH grades of GB/T 4172-2008 instead.
Q: Why dry electrodes at 350 to 420 degrees Celsius?
A: Drying drives out the moisture that hydrogen produces on welding. One to two hours at that temperature brings the coating back to a low hydrogen condition, provided the electrodes are then kept in a heated quiver.
Q: Is preheating always needed?
A: It depends on thickness, restraint and the carbon equivalent of the heat. Thin, lightly restrained joints often need none, while thick sections and high restraint need preheat, typically 100 to 150 degrees Celsius, with the value confirmed by the welding procedure.
Q: When should a completed weld be inspected?
A: After a waiting period, in the range of 24 to 48 hours for critical joints, so that any delayed hydrogen cracking has time to develop. Inspecting immediately can miss the defect entirely.
