Stratified tears may sometimes occur in rolled steel plates with poor thickness ductility. The main characteristic of lamellar tear is that it appears in the base metal parallel to the weld fusion boundary and the surface of the plate, usually in the T weld and fillet welds. Cracks can occur at the toe or root of the weld, but are always associated with the point of high stress concentration.
The fracture surface of the lamellar tear is fibrous and has a long parallel part, which indicates that the ductile ductility is low in the thickness direction.
Because lamellar tearing is associated with high concentration extended inclusions parallel to the surface of the plate, tearing will be transgranular with stepped appearance.
The occurrence of lamellar tearing must satisfy three conditions: the first is the transverse strain. The shrinkage strain in the welding must be used in the short direction of the plate, that is, through the thickness of the plate; the second is the welding direction, the fusion boundary will be roughly parallel to the plane of the inclusions; the third is the material sensitivity, and the plate must be in the direction of thickness. It is necessary to have poor ductility.
Therefore, if the stress generated during welding acts in the thickness direction, the risk of lamellar tear will be greater. The risk also increases. The factors that can effectively reduce the risk of welding tear are material, joint design, welding process, consumables, preheating and coating selection.
Lamellar tear occurs only in rolled steel plates instead of forgings and castings. Generally, the steel with low transverse shrinkage area (STRA) associated with high concentration of rolling sulphides or oxide inclusions is more susceptible. In general, more than 20% of STRA steel can basically be torn apart, while STRA less than 10%-15% steel plates can only be used for lightly constrained joints. Especially when the thickness is greater than 25mm, the steel with higher strength has greater risk. The risk of low sulphur content (<0.005%) aluminum treated steel is low.
Lamellar tear also occurs at joints where high pass thickness strain is generated, such as T joint or corner joint. In T or cross joints, full penetration butt welds are particularly susceptible. The cruciform structure that can not be bent in the welding process will also greatly increase the risk of tear.
In butt joints, there is almost no risk of lamellar tear due to the welding stress will not function through the thickness of the plate. Because the angle deformation will increase the strain at the root and / or toe of the weld, tear will also occur at the thick section joint with high bending constraint.
Because tearing is more likely to occur at fully permeable T joint, if possible, use two fillet welds. Double sided welding is more difficult than large-area single side welding, and balance welding to reduce stress, which will further reduce the risk of tear at the root. Large single sided fillet welding should be replaced by smaller double sided fillet welding. Redesigning the joint structure to make the fusion boundary more perpendicular to the sensitive plate surface is especially effective for reducing risk.
Typically, when the weld foot length of fillet weld and T type joint is longer than 20mm, the flake tear is more likely to occur in large weld. Because constraints can cause this problem, the thinner sections of the truncated panels which are less susceptible to tear may still be at risk under high constraints.
Because the design of material and joint is the main cause of welding tear, the selection of welding process has little influence on risk. However, it may be advantageous to generate low stress higher heat input processes through larger HAZ and deeper penetration.
Because welding metal hydrogen can increase the risk of tear, low hydrogen technology should be used when welding sensitive steel.
Where possible, the selection of lower strength consumables can usually reduce risk by accommodating more strain in the weld metal. A smaller diameter electrode has been used to prevent the tear.
Low hydrogen consumables will reduce risk by reducing the level of diffused hydrogen in welded metal. Consumables must be dried according to the manufacturer’s recommendations.
Preheating will have a beneficial effect on reducing the level of diffused hydrogen in welding metal. However, it should be noted that, at the restricted joint, excessive preheating may have adverse effects by increasing the constraint level produced by the shrinkage of the weld during the cooling process. Therefore, preheating should be used to reduce the hydrogen level, but preheating should be used so that it does not increase the shrinkage of the weld.
In addition, coating the surface of sensitive boards with low strength welding metal has been widely used. As shown in the T butt weld, the surface of the plate can have grooves, so that the butter coated layer will extend beyond 15 to 25mm of each weld toe, and the thickness is about 5 to 10mm.
In situ bonding, that is, low strength welding metal is first deposited on the induction board and then filled with joints, has also been successfully applied. However, the design calculation should be carried out before the docking technology is adopted to ensure that the overall welding strength is acceptable.
Because the lamellar tear is a linear defect with sharp edges, therefore, according to the requirements of BS EN ISO 5817:2007, the welding of quality grade B, C and D is not allowed. Using visual inspection, liquid osmotic or magnetic powder detection techniques can easily detect lamellar tearing on the surface of the stainless steel plate, but the internal cracks require ultrasonic inspection, but there may be some problems in distinguishing the lamellar tearing of the inclusion zone.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)