Cold Crack Tending of Ferritic Stainless Steel Welds and Fusions
Abstract
This paper discusses the tendency of cold cracking when welding or fusion welding ferritic stainless steel parts together. First, the types of cracking that may occur under these conditions are analyzed, followed by a detailed overview of the welding process that is affected by cold cracking. Several preventive measures, such as proper preheating and post-weld treatment, are also discussed in order to reduce the chances of cold cracking, as well as corrective actions that can be taken in order to repair any damage resulting from cold cracking.
Introduction
Cold cracking is a common problem that can occur when welding or fusion welding ferritic stainless steel materials together. Cold cracking, also known as solidification cracking, is caused by a limited amount of time for the weld to solidify prior to thermal stress taking effect. High solidification stresses, as well as a lack of filler material, may also contribute to the development of cold cracking in these joints.
The Types of Cracking That may Occur
Cold cracking can cause several types of cracking when welding or fusion welding ferritic stainless steel materials together. These include hot tearing, underbead cracking, seam cracking, and toe cracking.
Hot tearing: Hot tearing occurs when the weld shatters under the thermal stress that is created between the soft filler metal and the hard base metal. The discontinuity created by hot tearing can lead to a lack of complete fusion, with hot tearing typically occurring near the toe of the weld.
Underbead cracking: Underbead cracking is caused by a lack of filler material within the weld, creating a continuous crack underneath the weld bead. The crack is usually visible on the edges of the weld and can propagated along the length of the weld in some cases.
Seam cracking: Seam cracking occurs when fractures are created in the weld metal along the length of the weld due to thermal stresses. The fractures can propagate along the entire length of the weld, resulting in a significant reduction in the strength of the joint.
Toe cracking: Toe cracking is caused by the weld metal at the toe of the weld becoming weakened due to thermal stresses. This cracking can propagate along the entire length of the joint and significantly reduce the strength of the joint.
Welding Process Affected by Cold Cracking
When welding or fusion welding ferritic stainless steel materials together, several welding process parameters must be considered in order to prevent cold cracking. These parameters include pre-heating, welding speed, dwell time, post-welding heat treatment, and proper joint design.
Preheating: Preheating is used to reduce the cooling rate of the weld and reduce welding stresses due to thermal expansion and contraction. It is critical to appropriately preheat the joint in order to prevent cold cracking.
Welding speed: The welding speed should be matched to the desired cooling rate of the weld in order to control the thermal stress and reduce the likelihood of cold cracking.
Dwell time: Dwell time is the amount of time that should be allowed between welding passes in order to allow the weld metal to completely cool before the next pass is added. This should be set according to the size of the weld and the desired cooling rate.
Post-welding heat treatment: In order to reduce internal stress, a post-weld heat treatment can be used. This heat treatment should be carefully controlled in order to ensure that thermal stresses are kept to a minimum and cold cracking is eliminated.
Joint design: The joint design should be designed in order to minimize the risk of cold cracking. This includes ensuring that the joint geometry is correct and the welding parameters are correct for the application.
Preventive Measures to Reduce Cold Cracking
Preventive measures should be taken in order to reduce the chances of cold cracking when welding or fusion welding ferritic stainless steel materials. These measures include preheating the joint, maintaining appropriate welding and cooling speeds, using appropriate dwell times, using a post-weld heat treatment and performing proper joint design.
Preheating: Preheating the joint should be done in order to reduce the cooling rate of the weld and reduce the likelihood of cold cracking. The preheating temperature should be selected according to the thickness of the material and the size of the weld.
Welding and cooling speed: The welding speed and cooling rate should be carefully controlled in order to minimize the amount of thermal stress that is created. This includes ensuring that the welding current and voltage settings are set according to the size of the weld and the thickness of the material.
Dwell time: A dwell time should be used between welding passes in order to allow the weld metal to cool before the next pass is added. The dwell time should be set according to the size of the weld and the desired cooling speed.
Post-weld heat treatment: A post-weld heat treatment should be performed in order to reduce internal stresses within the weld. This heat treatment should be performed according to the size of the weld and the material.
Joint design: Proper joint design should be used in order to minimize the risk of cold cracking. This includes ensuring that the joints are correctly aligned and that the appropriate clearances are maintained.
Corrective Actions for Cold Cracking
Corrective actions should be taken in order to repair any damage resulting from cold cracking. These corrective actions include repairing the crack, replacing the joint, re-welding the joint, stress relieving the joint and using filler material to fill in any cracks.
Repairing the crack: A crack can be repaired by chipping out the cracked material and filling it in with weld material. This process should be performed carefully in order to avoid further damage to the joint.
Replacing the joint: If the joint is too damaged to be repaired, it may need to be replaced. The joint should be inspected for damage prior to replacement in order to ensure that the replacement is suitable for the application.
Re-welding the joint: If a joint is cracked, it may need to be re-welded in order to ensure a strong, stable joint. This process should be performed carefully in order to ensure that the joint is suitable for the application.
Stress relieving the joint: A stress relief heat treatment may be performed on the joint in order to reduce stresses within the joint. This heat treatment should be done according to the size of the joint and the material.
Using filler material: Filler material may be used to fill any crannies or cracks that are present in the joint. The filler material should be chosen according to the size of the joint and the material.
Conclusion
This paper has discussed the tendency of cold cracking when welding or fusion welding ferritic stainless steel parts. Several types of cold cracking have been analyzed, followed by a description of the welding process that is affected by cold cracking. Preventive measures, such as proper preheating and post-weld treatment, have been discussed in order to reduce the chances of cold cracking, as well as corrective actions that can be taken in order to repair any damage resulting from cold cracking.