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Weld-cracking:Origins and Avoidance.Solutions with Effective, Powerful AdviceWeld-cracking is obviously a most dangerous situation that compromises the ability of a fusion welded structure or machine to perform the design functions in a safe and consistent way. Cracks are discontinuities presenting fracture. They have two dimensions quite large relative to their opening, and a sharp tip that tends to propagate under stress. Therefore cracks must always be avoided or eliminated. In previous pages we briefly considered Weld Cracking as a danger and cracks as unacceptable defects (Click on Welding Defects) and discussed some welding conditions that may promote the appearance of cracks in Alloy Steels (Click here).
Here we wish to present the different types of Weld-cracking discussing the conditions for their appearance and the measures that can be employed to avoid them. Four main groups of Weld-cracking are described, known as hot cracks, microfissures of Heat Affected Zones, cold cracks and lamellar tearing. Hot cracks Of the four types of Weld-cracking discussed here, hot or solidification cracks appear near the end of the solidification process in the fusion zone. They result because of the incapacity of the molten and semisolid material to absorb without tearing the thermal shrinkage strains due to weld solidification and cooling. Weld-cracking, formed in the weakest places like grain boundaries, relieves the associated stresses. Some microstructures are more prone than others to the formation of solidification Weld-cracking. The presence in the metal composition of low melting elements like sulfur, lead, bismuth etc. is deleterious. The tendency of certain metals to exhibit hot Weld-cracking is called hot shortness, caused by low-melting constituents segregated at grain boundaries. For welding, these susceptible compositions should be avoided. (Example: free machining steels or stainless steels). Welding parameters should be selected with care, because they can influence the formation of hot Weld-cracking by the rate of strain application (lower rate = lower danger), which is higher for welding processes that cause rapid solidification and cooling. Joint design should strive to limit the molten mass, joint gap should be contained and good fit-up should be preferred, in order to reduce shrinkage strains and Weld-cracking. Weld speed, an important productivity factor, may have to be contained at such levels that do not cause solidification Weld- cracking, other data being constant. For any given alloy there may be a possible range of welding parameters that permits avoiding hot Weld-cracking. This range is narrow for alloys with a large melting interval, that are more prone to hot shortness. On the contrary, alloys with a narrower melt interval enjoy a larger range of possible welding parameters. It so happens because cumulative shrinkage strains are proportional to the magnitude of that melting temperature range, that depends essentially on chemical composition. Crater cracks are a common occurrence at the end of a weld if the welding current is interrupted abruptly. It should instead be reduced gradually, possibly directing the arc on top of an already welded beam. Solidification cracking susceptibility can be measured in different alloys by performing a series of tests (varestraint testing) and measuring the maximum length of the induced cracks. The results permit to rate the different compositions as to their hot Weld-cracking susceptibility. Heat Affected Zone Cracks Microfissures are cracks that develop in a place of partial melting occurring near the Heat Affected Zone, bordering on the fusion line. As partial melting occurs below the melting point of the alloy, especially if secondary elements segregation is present along grain boundaries, local thermal strains that manifest themselves can induce grain boundary separation similar to what explained above. HAZ cracks, may occur with or without the presence of liquid. In the presence of brittle structures associated to intermetallics, failures happen if the thermal stresses associated with the weld thermal cycle exceed the local tensile strength at the actual temperature. Some alloys undergo microstructural transformations in the Heat Affected Zone as a consequence of thermal cycles brought about by welding. Typical is the martensitic transformation, producing a hard and brittle crack-susceptible structure, which is accompanied by a volumetric change inducing noticeable stresses. Cold cracks Hydrogen induced delayed cold Weld-cracking occurs as a consequence of contamination with this gas being absorbed in the molten metal while welding. Hydrogen sources should be avoided and removed: not only water and humidity must be controlled by drying and preheating. Organic matter contamination has to be removed by cleaning and good housekeeping. Three conditions are necessary for the formation of cold cracks: the presence of a certain stress, of a suitable microstructure and at least a critical level of hydrogen. A long standing theory claimed that atomic hydrogen moving interstitially within the solid metal aggregated to molecular form with pressure increase sufficient to tear apart metallic bonds. This has been recently challenged and replaced by a competing model involving the presence of preexisting defect sites in the metal, where, under stress, hydrogen preferentially diffuses, reducing the local cohesive strength. Fracture occurs when the remaining strength falls below the intensified stress level. Hydrogen would then accumulate in the newly generated voids and the process would repeat itself. Hydrogen induced Weld-cracking is a serious cause for concern especially with high strength steels. Preheat and postheat procedures, depending on material type but also on thickness and joint constraints, are commonly employed to reduce the danger of hydrogen Weld-cracking. The purposes are to eliminate water, to reduce cooling rate in order to avoid dangerous structures (untempered martensite), to temper and soften this hard structure if formed, to reduce and relieve thermal stresses, and to allow for hydrogen gas to escape if entrapped. An Article on Preheating, a technique useful for reducing the risk of Weld-cracking, was published in Section 2 of Issue 37 of Practical Welding Letter for September 2006. To read the article click on PWL#037.
Lamellar Tearing Lamellar Tearing is a kind of Weld-cracking that forms beneath a weld. It is a dangerous condition occurring when certain plate materials presenting low ductility in the thickness (or through) direction are welded to a perpendicular element. Tearing always lies within the base metal, generally outside the HAZ and parallel to the weld fusion boundary. The problem is caused by welds that subject the base metal to high shrinkage stresses in the thickness direction. Tearing is not visible on the outside, generally, but it can be found by ultrasonic testing. The problem can be avoided by selecting base material having adequate ductility in the thickness direction, that has been screened by ultrasonic testing for absence of laminations or other internal defects. It can also be managed by paying due attention to joint details and avoiding massive welds that develop significant shrinkage stresses. Although appearing sometimes long after welding, a structure stress relieved and ultrasonically tested for lamellar tearing after welding should be considered safe. Special design details were developed for corner joints, to put the bulk of shrinkage stresses in the direction of rolling of one of the elements, and freeing the other element from stresses in the through direction. An Article on Lamellar Tearing was published (7) in Issue 46 of Practical Welding Letter for June 2007. Click on PWL#046 to read it.
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Checking the hardness of an unknown material before welding may help avoid Weld-cracking. To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. For anyone of the following Quality related subjects, click on the underlined
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