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Defects - lamellar tearing
BP Forties platform lamellar tears were produced when
attempting the repair of lack of root penetration in a brace weld
Lamellar tearing can occur beneath the weld especially in rolled steel plate
which has poor through-thickness ductility. The characteristic features,
principal causes and best practice in minimising the risk of lamellar tearing
Visual appearanceThe principal distinguishing feature of lamellar tearing is that it occurs in T-butt and fillet welds normally observed in the parent metal parallel to the weld fusion boundary and the plate surface , (Fig 1). The cracks can appear at the toe or root of the weld but are always associated with points of high stress concentration.
Fracture faceThe surface of the fracture is fibrous and 'woody' with long parallel sections which are indicative of low parent metal ductility in the through-thickness direction, (Fig 2).
MetallographyAs lamellar tearing is associated with a high concentration of elongated inclusions oriented parallel to the surface of the plate, tearing will be transgranular with a stepped appearance.
CausesIt is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur:
Factors to be considered to reduce the risk of tearingThe choice of material, joint design, welding process, consumables, preheating and buttering can all help reduce the risk of tearing.
Steels with a higher strength have a greater risk especially when the thickness is greater than 25mm. Aluminium treated steels with low sulphur contents (<0.005%) will have a low risk.
Steel suppliers can provide plate which has been through-thickness tested
with a guaranteed STRA value of over 20%.
Joint DesignLamellar tearing occurs in joints producing high through-thickness strain, eg T joints or corner joints. In T or cruciform joints, full penetration butt welds will be particularly susceptible. The cruciform structures in which the susceptible plate cannot bend during welding will also greatly increase the risk of tearing.
In butt joints, as the stresses on welding do not act through the thickness of the plate, there is little risk of lamellar tearing.
As angular distortion can increase the strain in the weld root and or toe, tearing may also occur in thick section joints where the bending restraint is high.
Several examples of good practice in the design of welded joints are illustrated in Fig. 4.
Weld sizeLamellar tearing is more likely to occur in large welds typically when the leg length in fillet and T butt joints is greater than 20mm. As restraint will contribute to the problem, thinner section plate which is less susceptible to tearing, may still be at risk in high restraint situations.
Welding processAs the material and joint design are the primary causes of tearing, the choice of welding process has only a relatively small influence on the risk. However, higher heat input processes which generate lower stresses through the larger HAZ and deeper weld penetration can be beneficial.
As weld metal hydrogen will increase the risk of tearing, a low hydrogen process should be used when welding susceptible steels.
ConsumableWhere possible, the choice of a lower strength consumable can often reduce the risk by accommodating more of the strain in the weld metal. A smaller diameter electrode which can be used to produce a smaller leg length, has been used to prevent tearing.
A low hydrogen consumable will reduce the risk by reducing the level of weld metal diffusible hydrogen. The consumables must be dried in accordance with the manufacturer's recommendations.
PreheatingPreheating will have a beneficial effect in reducing the level of weld metal diffusible hydrogen. However, it should be noted that in a restrained joint, excessive preheating could have a detrimental effect by increasing the level the level of restraint produced by the contraction across the weld on cooling.
Preheating should, therefore, be used to reduce the hydrogen level but it should be applied so that it will not increase the amount of contraction across the weld.
ButteringButtering the surface of the susceptible plate with a low strength weld metal has been widely employed. As shown for the example of a T butt weld (Fig. 5) the surface of the plate may be grooved so that the buttered layer will extend 15 to 25mm beyond each weld toe and be about 5 to 10mm thick.
In-situ buttering ie where the low strength weld metal is deposited first on the susceptible plate before filling the joint, has also been successfully applied. However, before adopting this technique, design calculations should be carried out to ensure that the overall weld strength will be acceptable.
Acceptance standardsAs lamellar tears are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).
Detection and remedial actionIf surface-breaking, lamellar tears can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic examination techniques but there may be problems in distinguishing lamellar tears from inclusion bands. The orientation of the tears normally makes them almost impossible to detect by radiography.
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