Imperfections in welds
Commonly used steels are considered to be readily welded. However, these materials can be at risk from the following types of imperfection:
Other fabrication imperfections are lamellar tearing and liquation cracking but using modern steels and consumables, these types of defects are less likely to arise.
In discussing the main causes of imperfections, guidance is given on procedure and welder techniques for reducing the risk in arc welding.
Porosity
Porosity is formed by entrapment of discrete pockets of gas in the solidifying weld pool. The gas may originate from poor gas shielding, surface contaminants such as rust or grease, or insufficient deoxidants in the parent metal (autogenous weld), electrode or filler wire. A particularly severe form of porosity is 'wormholes', caused by gross surface contamination or welding with damp electrodes.
The presence of manganese and silicon in the parent metal, electrode and filler wire is beneficial as they act as deoxidants combining with entrapped air in the weld pool to form slag. Rimming steels with a high oxygen content, can only be welded satisfactorily with a consumable which adds aluminium to the weld pool.
To obtain sound porosity-free welds, the joint area should be cleaned and degreased before welding. Primer coatings should be removed unless considered suitable for welding by that particular process and procedure. When using gas shielded processes, the material surface demands more rigorous cleaning, such as by degreasing, grinding or machining, followed by final degreasing, and the arc must be protected from draughts.
Solidification cracking
Solidification cracks occur longitudinally as a result of the weld bead having insufficient strength to withstand the contraction stresses within the weld metal. Sulphur, phosphorus, and carbon pick up from the parent metal at high dilution increase the risk of weld metal (solidification) cracking especially in thick section and highly restrained joints. When welding high carbon and sulphur content steels, thin weld beads will be more susceptible to solidification cracking. However, a weld with a large depth to width ratio can also be susceptible. In this case, the centre of the weld, the last part to solidify, will have a high concentration of impurities increasing the risk of cracking.
Solidification cracking is best avoided by careful attention to the choice of consumable, welding parameters and welder technique. To minimise the risk, consumables with low carbon and impurity levels and relatively high manganese and silicon contents are preferred. High current density processes such as submerged-arc and CO 2 , are more likely to induce cracking. The welding parameters must produce an adequate depth to width ratio in butt welds, or throat thickness in fillet welds. High welding speeds also increase the risk as the amount of segregation and weld stresses will increase. The welder should ensure that there is a good joint fit-up so as to avoid bridging wide gaps. Surface contaminants, such as cutting oils, should be removed before welding.
Hydrogen cracking
A characteristic feature of high carbon and low alloy steels is that the HAZ immediately adjacent to the weld hardens on welding with an attendant risk of cold (hydrogen) cracking. Although the risk of cracking is determined by the level of hydrogen produced by the welding process, susceptibility will also depend upon several contributory factors:
- material composition (carbon equivalent);
- section thickness;
- arc energy (heat) input;
- degree of restraint.
The amount of hydrogen generated is determined by the electrode type and the process. Basic electrodes generate less hydrogen than rutile electrodes (MMA) and the gas shielded processes (MIG and TIG) produce only a small amount of hydrogen in the weld pool. Steel composition and cooling rate determines the HAZ hardness. Chemical composition determines material hardenability, and the higher the carbon and alloy content of the material, the greater the HAZ hardness. Section thickness and arc energy influences the cooling rate and hence, the hardness of the HAZ.
For a given situation therefore, material composition, thickness, joint type, electrode composition and arc energy input, HAZ cracking is prevented by heating the material. Using preheat which reduces the cooling rate, promotes escape of hydrogen and reduces HAZ hardness so preventing a crack-sensitive structure being formed; the recommended levels of preheat for various practical situations are detailed in the appropriate standards e.g. BS EN1011-2:2001. As cracking only occurs at temperatures slightly above ambient, maintaining the temperature of the weld area above the recommended level during fabrication is especially important. If the material is allowed to cool too quickly, cracking can occur up to several hours after welding, often termed 'delayed hydrogen cracking'. After welding, therefore, it is beneficial to maintain the heating for a given period (hold time), depending on the steel thickness, to enable the hydrogen to diffuse from the weld area.
When welding C-Mn structural and pressure vessel steels, the measures which are taken to prevent HAZ cracking will also be adequate to avoid hydrogen cracking in the weld metal. However, with increasing alloying of the weld metal e.g. when welding alloyed or quenched and tempered steels, more stringent precautions may be necessary.
The risk of HAZ cracking is reduced by using a low hydrogen process, low hydrogen electrodes and high arc energy, and by reducing the level of restraint. Practical precautions to avoid hydrogen cracking include drying the electrodes and cleaning the joint faces. When using a gas shielded process, a significant amount of hydrogen can be generated from contaminants on the surface of the components and filler wire so preheat and arc energy requirements should be maintained even for tack welds.
Reheat cracking
Reheat or stress relaxation cracking may occur in the HAZ of thick section components, usually of greater than 50mm thickness. The more likely cause of cracking is embrittlement of the HAZ during high temperature service or stress relief heat treatment.
As a coarse grained HAZ is more susceptible to cracking, low arc energy input welding procedures reduce the risk. Although reheat cracking occurs in sensitive materials, avoidance of high stresses during welding and elimination of local points of stress concentration, e.g. by dressing the weld toes, can reduce the risk.
Sumber : http://www.twi-global.com/technical-knowledge/job-knowledge/weldability-of-materials-carbon-manganese-and-low-alloy-steels-019/
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