Weld Defects - Cracks

Causes and Types of Cracking that may occur in Welds
Cracking in WeldsWe know that several types of imperfections may occur in a weld or the heat affected zone. Welds may contain porosity (see last months HERA News) slag inclusions, lack of fusion or cracks. Cracks in a weld are almost certainly the most unwanted of all weld imperfections. Because of the various materials and applications used in welding today, cracking is a complex subject. The base material's crack sensitivity may be associated with its chemistry and its susceptibility to conditions that reduce its ductility.The welding operation itself can produce stresses in and around the weld, introducing extreme localized heating, expansion, and contraction that may also cause cracking.

Hot - Cold Cracks: Cracks can be classified hot or cold. Hot cracks develop at elevated temperatures, propagate between the grains of a material, and commonly form during solidification of weld metal. Cold cracks develop after solidification of the weld, as a result of stresses, and propagate both between grains and through grains. Cold cracks in steel sometimes are called delayed cracks and often are associated with hydrogen embrittlement. 

Base Material Cracks: Heat-affected zone (HAZ) cracking most often occurs with base material that has hardenability. High hardness and low ductility in a HAZ often are the result of a metallurgical response to welding thermal cycles. In ferritic steels, hardness increases and ductility decreases with an increase in carbon content and a faster cooling rate.The HAZ hardness depends on the base material's ability to be hardened, which in turn depends on the base material's chemical composition. Carbon has a predominant effect on steel's hardenability along with other elements. For instance, material with a carbon equivalent (CE) of over 0.4 may suffer from cracking unless precautions are taken during welding such as electrode choice, considering cooling rates and residual stress invariably may result in base material cracking.

Cracks types:
Heat Affected Zone (HAZ) and Underbead cracks (sometimes called toe cracks or delayed cracking) are generally cold cracks that form in the heat-affected zone of the parent material. The following needs to be considered: • hydrogen• a microstructure of relatively low ductility• high residual stress• temperature below 200 deg C  Heat-affected zone and underbead cracks are normally longitudinal. They may be found at in the weld toe area of the heat affected zone or under the weld bead where residual stresses are highest.

Transverse cracks are perpendicular to the direction of the weld. These are generally the result of longitudinal shrinkage stresses acting on weld metal of low ductility.  Crater cracks occur in the crater when the welding arc is terminated prematurely. Crater cracks are normally shallow, hot cracks usually forming single or star cracks. These cracks usually start at a crater pipe and extend longitudinal in the crater. However, they may propagate into longitudinal weld cracks in the rest of the weld.

Solidification cracks are longitudinal cracks in the weld face in the direction of the weld axis. They are generally hot cracks. These cracks are typically caused by excessive transverse stress, high depth to width ratio in excess of 2:1, high sulphur and phosphorus content.  Toe cracks are generally cold cracks. They initiate and propagate from the weld toe where shrinkage stresses are concentrated. Toe cracks initiate approximately normal to the base metal surface. These cracks are generally the result of thermal shrinkage stresses acting on a weld heat-affected zone. Some toe cracks occur because the transverse tensile properties of the base metal cannot accommodate the shrinkage stresses that are imposed by welding. Root cracks are longitudinal cracks at the weld root. They may be hot or cold forms of cracks.

Stress corrosion cracking in stainless steel is due to caustic or chloride contaminants. The cracking is predominantly inter-crystalline. Reheat cracking is almost exclusively restricted to creep resistant steels and must be considered a very serious form of cracking. Reheat cracking can be caused by the generation of excessive thermal stress during the post-weld heat treatment leading to the initiation of cracking from existing very small hot or cold cracks. This can be controlled by correct heating rates; temperature variations and avoiding where possible stress concentrations. Another form of reheat cracking occurs at high temperatures in the material’s creep range where inter-crystalline cracking in the "larger" grained heat affected zone results from insufficient creep ductility. This occurs during post weld heat treatment or during high temperature service.

Miscellaneous cracks include other forms of cracking such as chevron and lamellar tearing. Generally, the following guidelines can be applied:
(d)  Chevron cracking in high strength welds metals in ferritic steels only.
(e)  Lamellar tearing-in principle possible in any material but in practice restricted to structural and pressure vessel ferritic steels.

Cracks are unacceptable defects and are detrimental to weld performance. A crack, by its nature, is sharp at its extremities, so it acts as a stress concentration. The stress concentration effect of a crack is greater than that of most other discontinuities. Cracks have a tendency to propagate, contributing to weld failure under stress. Regardless of their size, cracks; except for crater cracks in class GP welds of AS/NZS 1554.1, are not permitted in welds governed by most fabrication standards. They must be removed by grinding or gouging, and the excavation filled with sound weld metal.

Successful welding procedures for the materials be joined include the controls that are necessary to overcome the tendency for crack formation. Such controls are preheating temperature, interpass temperature, consumable type and preparation, and post weld heat treatment.