Imperfections in Welds

Whenever imperfections happen to be formed, they are going to be located in either the weld metal or the parent material immediately adjacent to the weld, referred to as the heat affected zone (HAZ). Considering that the chemical composition of the weld metal is what determines the risk of imperfections, the selection of filler metal is typically crucial not only in achieving acceptable mechanical properties and corrosion resistance but additionally in producing a sound weld. Nevertheless, HAZ imperfections are generally as a result of the adverse effect of the heat generated in the course of welding and can primarily be prevented by way of strict adherence to the welding procedure.

In arc welding, as the weld metal requires mechanical properties to match the parent metal, the welder will need to avoid forming defects within the weld. Imperfections are primarily the result of:

• poor welder technique;
• inadequate measures to accommodate the material and / or welding technique;
• high stress in the component.

Techniques to prevent imperfections that include insufficient fusion and slag inclusions, which often are a consequence of inadequate welder techniques, are comparatively widely recognized. On the other hand, the welder must be aware of the fact that the material itself could possibly be susceptible to the formation of imperfections resulting from the welding process.

Frequently used steels are considered to generally be readily welded. Nevertheless, these kind of materials may be at an increased risk from the following different types of imperfections:

• porosity;
• solidification cracking;
• hydrogen cracking;
• reheat cracking.

Additional fabrication imperfections are lamellar tearing and liquation cracking although by using modern steels and consumables, these types of defects are generally unlikely to develop. In referring to an important factors behind imperfections, guidance is provided on procedure and welder techniques for minimizing the risk in arc welding.

Porosity

Porosity is a consequence of the absorption of nitrogen, oxygen and hydrogen within the molten weld pool that is subsequently released on solidification to become trapped in the weld metal.

Porosity is produced as a result of the entrapment of discrete pockets of gas within the solidifying weld pool. The gas might originate from poor gas shielding, surface contaminants including rust or grease, or inadequate deoxidants within the parent metal (autogenous weld), electrode or filler wire. A particularly severe form of porosity is 'wormholes', resulting from gross surface contamination or welding by using damp electrodes.

The existence of manganese and silicon within the parent metal, electrode and filler wire is beneficial because they act as deoxidants combining along with entrapped air within the weld pool to form slag. Rimming steels, which has a high oxygen content, can only be welded satisfactorily using a consumable that provides aluminium to the weld pool.

To generate sound porosity-free welds, the joint area must be cleaned and degreased prior to welding. Primer coatings needs to be removed except in cases where it's considered suitable for welding by that particular process and procedure. When working with gas shielded processes, the material surface necessitates additional rigorous cleaning, including through degreasing, grinding or machining, accompanied by final degreasing, and also the arc is required to be protected from draughts.

Prevention

The gas source must be identified and removed as follows:

• Air entrainment
• seal any sort of air leak
• prevent weld pool turbulence
• make use of filler with suitable level of deoxidants
• minimize excessively high shielding gas flow
• prevent draughts
• Hydrogen
• dry the electrode and flux in accordance with the manufacturer's recommendations
• thoroughly clean and degrease the workpiece surface
• Surface coatings
• thoroughly clean the join edges immediately prior to welding
• ensure that the weldable primer is below the recommended maximum thickness

Solidification cracking

Both solidification cracking and hot cracking make reference to the formation of shrinkage cracks during the solidification of weld metal, on the other hand hot cracking may also make reference to liquation cracking. Hot cracking develops in the event the available supply of liquid weld metal is not sufficient to fill the spaces between solidifying weld metal, which has been exposed as a result of shrinkage strains. Consequently, the primary factors behind cracking are:

• Strain on the weld pool is too high
• Liquid is unable to reach the regions where its required resulting from insufficient supply or blockage/ narrow channels between solidifying grains

Solidification cracks manifest longitudinally as a consequence of the weld bead possessing insufficient strength to withstand the contraction stresses inside the weld metal. Sulphur, phosphorus, and carbon pick-up from the parent metal at high dilution increase the probability of weld metal (solidification) cracking specifically in thick section and highly restrained joints. Whenever welding high carbon and sulphur content steels, thin weld beads are often more vulnerable to solidification cracking. Nevertheless, a weld which includes a large depth to width ratio could also be especially prone. In such a case, the centre of the weld, the final section to solidify, are going to have a high concentration of impurities escalating the risk of cracking.

Solidification cracking is best prevented by way of careful attention towards the selection of consumable, welding parameters and welder technique. In order to minimise the risk, consumables having low carbon and impurity levels and relatively high manganese and silicon contents are generally preferred. High current density processes which include submerged-arc and CO₂ , have a propensity to induce cracking. The welding parameters must produce an suitable depth to width ratio in butt welds, or throat thickness in fillet welds. High welding speeds could also increase the possibility given that the amount of segregation and weld stresses will increase. The welder will need to be certain that there exists a good joint fit-up in an effort to avoid bridging wide gaps. Surface contaminants, including cutting oils, must be eliminated in advance of welding.

Hydrogen cracking

A characteristic characteristic of high carbon and low alloy steels is the fact that the HAZ immediately adjacent to the weld hardens upon welding with an attendant risk of cold (hydrogen) cracking. Although the associated risk of cracking is dependent upon the level of hydrogen that is generated by the welding process, susceptibility will likewise depend upon numerous contributory factors:

• material composition (carbon equivalent);
• section thickness;
• arc energy (heat) input;
• degree of restraint.

The magnitude of hydrogen generated is dependent upon the electrode type and the process. Basic electrodes generate reduced hydrogen as compared to rutile electrodes (MMA) and the gas shielded processes (MIG and TIG) generate only a small amount of hydrogen in the weld pool. Steel composition and cooling rate establishes the HAZ hardness. Chemical composition is what determines material hardenability, additionally, the higher the carbon and alloy content of the material, the greater the HAZ hardness. Section thickness and arc energy has impact on the cooling rate and therefore, the hardness of the HAZ.

For a specified condition accordingly, material composition, thickness, joint type, electrode composition and arc energy input, HAZ cracking can be avoided as a result of heating the material. By using preheat that reduces the cooling rate, promotes escape of hydrogen and decreases HAZ hardness consequently preventing a crack-sensitive structure getting formed; the recommended levels of preheat with regard to a variety of practical situations are detailed in the applicable standards including BS EN 1011-2:2001. As cracking only develops at temperatures slightly higher than ambient, maintaining the temperature of the weld area higher than the recommended level during fabrication is particularly vital. In the event the material is permitted to cool too rapidly, cracking can occur as many as several hours subsequent to welding, typically termed 'delayed hydrogen cracking'. Subsequent to welding, hence, it's beneficial to maintain the heating for a specified period of time (hold time), depending upon the steel thickness, to enable the hydrogen to diffuse from the weld area.

Whenever welding C-Mn structural and pressure vessel steels, the measures which are usually taken to avert HAZ cracking are likewise be suitable in order to avoid hydrogen cracking in the weld metal. On the other hand, with increasing alloying of the weld metal, for instance, whenever welding alloyed or quenched and tempered steels, much more stringent precautions can be necessary.

The risk of HAZ cracking is minimized by using a low hydrogen process, low hydrogen electrodes and high arc energy, and additionally by reducing the level of restraint. Effective precautions in avoiding hydrogen cracking can include drying the electrodes and cleaning the joint faces. When working with a gas shielded process, a substantial amount of hydrogen may be produced from contaminants on the surface of the components and filler wire consequently preheat and arc energy requirements must be maintained even with regard to tack welds.

Precautions recommended to avert hydrogen cracking:

• Prevent the ingress of moisture or hydrogen. This can be accomplished by way of ensuring the cleanliness and dryness of workpieces, and that also consumables are clean and dry before welding. With regard to critical applications, basic, low hydrogen, manual metal arc electrodes are typically made use of. Low weld metal diffusible hydrogen levels are obtained by utilizing electrodes from sealed packs or as a result of baking them in advance of use. Welding in high humidity environments is in addition likely to increase weld metal hydrogen levels, and additional measures could be essential to avert cracking.
• Characterise the chemical composition of the steel being welded. The carbon equivalent (IIW CE) is utilized to characterise the consequences of alloying elements on weldability, and represents the contribution of the composition to the hydrogen cracking susceptibility of steel. With increasing IIW CE value (more highly alloyed) and material thickness, provision has to be made to mitigate the adverse effects. Preheat may be required, and consideration must be provided for working with low hydrogen welding consumables and processes. Typically, steels having an IIW CE value of <0.4 aren't susceptible to hydrogen cracking, provided that low hydrogen processes are utilized. Instruction for the prevention of hydrogen cracking is specified in BS:EN 1011:2001 Part 2.
• Select an appropriate electrode and welding process. One of several major differences between the arc welding processes is the manner in which the molten weld pool is protected during welding. This is accomplished by employing either a flux which forms a protective molten slag or a shielding gas. With regard to thin sections and low IIW CE materials, rutile flux coated electrodes are utilized for general fabrication in manual metal arc (MMA) and flux cored arc welding (FCAW), and fused fluxes are utilized in submerged arc welding (SAW). With regards to larger sections and / or higher IIW CE steels, lower hydrogen consumables have to be considered. For MMA welding the electrodes are baked, and basic flux electrodes provide the lowest weld metal hydrogen characteristics. For SAW, basic agglomerated fluxes are often used. Metal inert/active gas (MIG/MAG) and tungsten inert gas (TIG) welding are lower hydrogen processes which make use of shielding gases ınstead of fluxes. Girth welds in pipeline materials are generally produced by using cellulosic MMA consumables or automated TIG set-ups for increased welding speeds, however with cellulosic flux coatings hydrogen contents could be high, and therefore proper care is required to be taken.
• Postweld treatment if required. Higher strength structural steels utilised in highly stressed conditions typically have to satisfy additional requirements on standard structural grades that include minimum toughness requirements and maximum hardness requirements, specifically in the HAZ. Postweld heat treatment should also be considered in many cases in the event the hardness of the weld is a concern or in case it is a requirement of the fabrication code. Quenched and tempered steels, employed for instance in oil and gas wells for drilling, demand specialized care over the electrode drying procedures, preheat temperatures, heat input levels, postweld heat treatment and postweld non-destructive testing.

Reheat cracking

Reheat or stress relaxation cracking might manifest in the HAZ of thick section components, typically in excess of 50mm thickness. The much more likely source of cracking is embrittlement of the HAZ during high temperature service or stress relief heat treatment.

As a coarse grained HAZ is more at risk of cracking, low arc energy input welding procedures reduce the risk. Despite the fact that reheat cracking develops in sensitive materials, prevention of high stresses during welding and elimination of local points of stress concentration, for instance, by dressing the weld toes, will be able to reduce the risk.