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Weld Discontinuities - Part 3 Cracking
Cracks in a weldment are probably the most dreaded of all the weld discontinuities. Because of the wide range of applications and the many types of materials welded, cracking is an extremely complex subject. We will examine some basic theory and characteristics of different types of cracking in welded connections.
Cracks will occur in the weld metal when localized stresses exceed the ultimate strength of the metal. For this reason, we need to consider some important variables when designing a welding procedure to best resist cracking. The crack sensitivity of the base material may be associated with its chemistry and/or its susceptibility to the formation of elements which will reduce its ductility. The introduction of excessive stresses to the weld joint, particularly in conjunction with a material in a crack-sensitive condition, can cause cracking to occur. Stresses in and around the weld are characteristic of the welding operation, which often introduces extreme localized heating, together with expansion and contraction during the welding process. Cracking is often associated with stress concentration near discontinuities in welds and base metal, and near mechanical notches associated with the weldment design. Hydrogen embrittlement, which is a condition that causes a loss of ductility and exists in weld metal due to hydrogen absorption, can contribute to crack formation in some materials
Cracks are usually classified into one of two types: Hot Cracks and Cold Cracks.
Hot cracks develop at elevated temperatures, propagate between the grains of the material, and commonly form during solidification of the 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 are sometimes called delayed cracks and are often associated with hydrogen embrittlement.
We can divide cracks into an additional two types: Cracks in the base material and cracks in the weld metal.
Cracks in the Base Material
Heat-Affected-Zone (HAZ) cracking is most often associated with hardenable base material. High hardness and low ductility in the heat-affected zone is often a result of the metallurgical response to the welding thermal cycles. In ferritic steels, hardness increases and ductility decreases with an increase in carbon content and an increase in the cooling rate from the welding temperature. The heat-affected zone hardness is related to the hardenability of the base material, which in turn is dependent on the base material chemical composition. Carbon has a predominant effect on the hardenability of steel. Perhaps an extreme example of this hardenability and its effect on base metal cracking is when we consider the welding of cast iron. This material contains between 2% and 4.5% carbon, which gives the alloy great hardness and low ductility. If we attempt to weld this material without serious consideration to cooling rates and residual stress, we will invariably encounter base material cracking.
Cracks in the Weld Metal
We can divide weld metal cracks into three types: Transverse, longitudinal and crater cracks.
Transverse weld metal cracks are perpendicular to the direction of the weld. This type of crack is more common in welds that have a high degree of restraint.
Longitudinal weld cracks travel in the same direction as the weld and are often confined to the center of the weld. This type of crack may be an extension of a crack that originally initiated at the end of a weld.
Crater cracks can be formed by an abrupt weld termination if a crater is left unfilled with weld metal. These cracks are usually star shaped and initially only extend to the edge of the crater. However, these cracks can propagate into longitudinal weld cracks.
The Effect of Cracks on the Weld Integrity
Cracks in any form are usually unacceptable discontinuities and are considered most detrimental to the performance of the weld. A crack, by its nature, is sharp at its extremities and consequently 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 and can contribute to weld failure if subjected to stress in service. Cracks, regardless of size, are not normally permitted in weldments governed by most fabrication codes. They are required to be removed, usually by grinding or gouging, and the excavation filled with sound weld metal.
The successful welding procedure will incorporate in its requirements the controls necessary to overcome the tendency for crack formation. Such controls, dependent on material type, may be preheating temperature, interpass temperature, preparation of and type of welding consumables, and post-weld heat treatment. It is the responsibility of the welding inspector to evaluate these welding procedural controls during their inspections, thereby ensuring that welding is performed in accordance with welding procedures that have been designed to minimize the probability of weld cracking.