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SOLUTIONS: Evaluation of welds

Heat cracks

Heat cracks may appear during or just after the cooling off period. There are two main causes: Impurities in the base material which have a tendency to segregate and may form a layer in the middle of the weld. This layer prevents fusion of the crystals. Segregated substances are first and foremost carbon and sulphur. In cases where heat cracks are caused by these substances, switch to basic electrodes. If heat cracks appear when welding with basic electrodes, the material is not weldable.

Tension across the weld can cause heat cracks even if the base material does not segregate in the weld. At a narrow, critical temperature range, just after coagulation of the bead, there is very little deformation property in a weld, Heat cracks and if shrinking in the metal is greater than the stretch in the weld, a heat crack will result. This type of crack can be avoided by clamping workpieces in special jigs which control shrinkage. A heat crack will occur in the middle of the bead and will appear as a straight crack on the surface.

Shrinkage cracks

Shrinkage cracks occur when the deformation property (toughness) of the weld is less than the actual shrinkage movement. Shrinkage

cracks will usually appear across the weld direction and be caused by considerable lengthwise shrinkage. Basic electrodes are the best safeguard against shrinkage cracks.

Shrinkage cracks

Hydrogen cracks (Cold cracks) Weld metal cracks are caused by hydrogen and may occur in any type of steel which has been hardened or which may become hardened during the welding process. Steel with a high yield point will contain a certain amount of hardened structure, normally martensite. The higher the melting point, the higher is the risk of hydrogen cracks. The most common reason for hydrogen cracks is that moist or damp electrodes is used during welding.

The water in the coating will change into hydrogen in the arc and end up as hydrogen porosity dissolved in the weld metal and the Hydrogen cracks heat affected zone (HAZ) immediately (cold cracks) adjacent to the molten zone. When combined with hard phases in the weld and sufficient stress it will form cracks.

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