Introduction to Cracks
Understanding Severity
Cracks in welding are serious imperfections that demand immediate attention. These fissures, often caused by rapid cooling or stress concentration, jeopardize structural integrity and longevity. Severity varies, from hairline surface cracks to deep-rooted fissures compromising the entire joint. To gauge severity, consider factors like crack size, depth, location, and environment. Superficial cracks might not impair functionality but could propagate under stress. Deep cracks compromise mechanical strength, leading to catastrophic failure.
Mitigating crack severity involves meticulous inspection, proper welding techniques, and material selection. Preheating, stress-relieving, and post-weld heat treatment can minimize crack formation. Implementing stringent quality control measures and adhering to welding standards mitigate risks.
Understanding crack severity is crucial for welders, engineers, and inspectors to ensure structural integrity and safety. It is a cornerstone of welding education, empowering professionals to recognize, assess, and address cracks effectively, safeguarding against potential disasters.
Shape of Cracks
Crack Shape Demystified
Longitudinal
Longitudinal cracks run parallel to the direction of the weld typically found along the length of the weld bead.
Transverse
Transverse cracks occur perpendicular to the direction of the weld often observed across the width of the weld joint.
Radiating
Radiating cracks extend outward from a central point commonly seen near the weld termination or in areas of high stress concentration.
Crater
Crater cracks form at the crater end of the weld typically found at the termination point of the weld bead.
Branching
Branching cracks have multiple offshoots from a main crack typically observed in areas of high stress or where discontinuities exist in the weld.
Types of Cracks
Addressing Cracks in Welding
1) Hot Cracks
Hot cracks, also known as solidification cracks or centerline cracks or crater cracks, form during the solidification process of the weld metal. They occur at elevated temperatures and are typically found in the weld metal or heat-affected zone.
When they occur along the centerline of the weld as a result of the solidification process then they are called as solidification cracks.
When they occur in the coarse grain HAZ, in the near vicinity of the fusion line as a result of heating the material to an elevated temperature, high enough to produce liquation of the low melting point constituents placed on grain boundaries then they are known as liquation cracks.
Solidification Cracking Mechanism
Key Factors affecting Solidification Cracking
Chemical Composition
The chemical composition of the base metal, filler metal, and any impurities present can significantly influence the likelihood of solidification cracking. High levels of certain elements like carbon, sulfur, phosphorus, copper, zinc, etc. can increase the susceptibility to cracking by altering solidification behavior.
Unfavorable Bead Shape
The shape of the weld bead can affect the distribution of heat and molten metal during welding. Unfavorable bead shapes, such as large depth-to-width ratio of the solidifying weld bead (deep & narrow), can lead to uneven cooling rates and segregation of impurities within the weld. This uneven distribution of heat and material can create stress concentrations and promote solidification cracking.
Heat Flow Disruption
Disruption of heat flow conditions, such as stop/start welding operations or intermittent welding, can contribute to solidification cracking. When welding is interrupted, the temperature gradients within the weld zone are altered, leading to uneven cooling rates and potential segregation of impurities. This disruption in heat flow can create regions of high stress and promote cracking during solidification.
Prevention of Solidification Cracking
Chemical Composition Considerations
Proper selection of base metal and filler metal with compatible compositions is essential to prevent solidification cracking. Choosing materials with low levels of impurities such as carbon, sulfur, phosphorus, copper, zinc, etc., can reduce susceptibility to cracking.
Optimizing Weld Bead Shape
Ensure proper weld bead geometry to promote uniform heat distribution and minimize stress concentrations by avoiding large depth-to-width ratio of the solidifying weld bead i.e. avoid deep & narrow weld bead, as they can lead to uneven cooling rates and segregation of impurities, increasing the likelihood of solidification cracking.
Controlling Heat Flow
Maintain consistent heat input and welding parameters to ensure uniform heat distribution and minimize disruption to the solidification process by minimizing stop/start welding operations or intermittent welding to prevent abrupt changes in temperature gradients within the weld zone, which can promote cracking. Also, implement preheating and post-weld heat treatment techniques as necessary to control residual stresses and promote uniform cooling, reducing the risk of solidification cracking.
Affected Materials
High Carbon Steels
High carbon steels are susceptible to solidification cracking due to the presence of high carbon content, which can increase brittleness and promote cracking during solidification.
Low Alloy Steels
Low alloy steels containing elements such as sulfur, phosphorus, and manganese are prone to solidification cracking, especially when welding thick sections or using high heat inputs.
Stainless Steels
Certain types of stainless steels, particularly those with high sulfur or phosphorus content, can experience solidification cracking during welding, especially in the heat-affected zone.
Nickel-Based Alloys
Nickel-based alloys, commonly used in high-temperature and corrosive environments, can be susceptible to solidification cracking due to their complex solidification behavior and sensitivity to weld parameters.
Aluminum Alloys
Certain aluminum alloys, such as those with high magnesium content, can experience solidification cracking, particularly in thick sections or when welding at high speeds.
Copper Alloys
Copper alloys, including brass and bronze, can be prone to solidification cracking due to the presence of impurities and the complex solidification behavior of copper-based materials.
Cast Iron
Cast iron, particularly gray cast iron, can experience solidification cracking during welding due to its high carbon content and brittle nature.
High Strength Steels
High strength steels, including quenched and tempered steels, can be susceptible to solidification cracking, especially when welding thick sections or using high heat inputs.
2) Cold Cracks
Cold crack, also known as Hydrogen induced crack or Hydrogen induced cold crack or HAZ crack or Delayed crack or Underbead crack or Toe crack, forms after 48 hours to 72 hours of the completion of welding process and typically occurs in the grain coarsened region of the HAZ.
It lies parallel to the fusion boundary and its path is usually a combination of inter and transgranular cracking. The direction of the principal residual tensile stress can in toe cracks cause the crack path to grow progressively away from the fusion boundary towards a region of lower sensitivity to hydrogen cracking. When this happens, the crack growth rate decreases and eventually arrests.
Sometimes, HIC cracks can form in weld metal under certain circumstances and recognized with welds in higher strength materials, thicker sections and using large weld beads. Hydrogen cracks in weld metal usually lie at 45° to the direction of principal tensile stress in the weld metal, usually the longitudinal axis of the weld.
HIC Cracking Mechanism
Key Factors affecting HIC Cracking
Hydrogen Level
High levels of hydrogen more than 15ml / 100g in the weld metal increase the likelihood of hydrogen-induced cracking. Hydrogen can be introduced into the weld metal from various sources such as moisture and contaminants present in the base metal or filler metal, or the decomposition of hydrocarbons present in the atmosphere.
Stress
High levels of tensile stresses (applied and residual stresses), particularly exceeding 50% of the material's yield stress, can promote hydrogen-induced cracking. Tensile stresses act as driving forces for hydrogen atom movement and embrittlement, increasing the susceptibility to cracking.
Temperature
Hydrogen-induced cracking is most prevalent at temperatures below 300°C, where hydrogen embrittlement mechanisms are most active. Low temperatures promote the accumulation of hydrogen atoms at grain boundaries and other microstructural defects, leading to embrittlement and crack initiation.
Microstructure
A microstructure with a hardness exceeding 400HV (Vickers hardness) or 380 HB (Brinell Hardness) indicates increased susceptibility to hydrogen-induced cracking. Hardened microstructures, such as those resulting from rapid cooling or high-strength materials, are more prone to hydrogen embrittlement and cracking.
Prevention of HIC Cracking
Controlling Hydrogen Level
The main source of Hydrogen is moisture and principal source is welding flux. Some fluxes contain cellulose and this can be a very active source of hydrogen. So, ensure proper handling and storage of fluxes (coated electrodes, flux-cored wires and SAW fluxes) to prevent moisture contamination, which can introduce hydrogen into the weld zone by either baking then storing low Hydrogen electrodes in a hot holding oven or supplying them in vacuum-sealed packages. Use low-hydrogen welding processes (that do not require flux), such as Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW), to minimize hydrogen pickup during welding. Implement preheating of the base metal to reduce hydrogen levels in the weld metal and promote hydrogen diffusion out of the weld zone. Employ shielding gases with low moisture content and high purity by checking the dew point (must be below -60°C) to minimize the introduction of hydrogen during welding. Other sources of hydrogen are moisture present in rust or scale and oils and greases (hydrocarbons). Ensuring the weld zone is dry and free from rust / scale and oil/ grease.
Reducing Stresses
The magnitude of the tensile stresses is mainly dependent on the thickness of the steel at the joint, heat input, joint type and the size and weight of the components being welded. Tensile stresses in highly restrained joints can be as high as the yield strength of the steel and this is usually the case in large components with thick joints and is not a factor that can easily be controlled. Ensure proper fit-up and alignment of the joint to minimize the introduction of stress concentrations that can promote HIC cracking. Implement stress-relieving techniques such as preheating, post-weld heat treatment (PWHT), or stress relieving annealing to reduce residual stresses in the welded joint. Use proper welding techniques and parameters such as increasing the travel speed as practicable to reduce the heat input to minimize distortion and avoid the introduction of excessive residual stresses. Keep the weld metal volume as low as possible by proper joint design.
Elevating Temperature
Increase the temperature of the base metal and / or weld zone through preheating to reduce the risk of hydrogen-induced cracking. Elevated temperatures promote hydrogen diffusion and reduce the susceptibility of the material to embrittlement and cracking. Maintain proper preheat temperature throughout the welding process to ensure uniform heating and minimize thermal gradients that can lead to cracking.
Controlling Hardness
A susceptible HAZ microstructure is one that contains a relatively high proportion of hard brittle phases of steel, particularly martensite. The HAZ hardness is a good indicator of susceptibility and when it exceeds a certain value that steel is considered susceptible. For C and C-Mn steels this value is ~350HV and susceptibility to H2 cracking increases as hardness increases above this value. Also, the higher the CEV the greater its susceptibility to HAZ hardening therefore the greater the susceptibility to H2 cracking. Procuring steel with a CEV at the low end of the range for the steel grade (limited scope of effectiveness). Using moderate welding heat input so that the weld does not cool quickly and give HAZ hardening. Applying preheat so that the HAZ cools more slowly and does not show significant HAZ hardening; in multi-run welds maintain a specific interpass temperature.
Affected Materials to HIC Cracking
High Strength Steels
High-strength steels, including quenched and tempered steels, are highly susceptible to hydrogen-induced cracking due to their high hardness and martensitic microstructure. These steels are commonly used in structural and pressure vessel applications and are prone to cracking if proper precautions are not taken during welding.
Low-Alloy Steels
Certain low-alloy steels, particularly those with high hardenability and alloying elements such as chromium, molybdenum, and nickel, can be susceptible to hydrogen-induced cracking. These steels are often used in high-stress applications where strength and toughness are critical.
Stainless Steels
Some stainless steels, especially those with high strength and hardness levels, can experience hydrogen-induced cracking during welding. These steels may contain elements such as chromium, nickel, and molybdenum, which enhance corrosion resistance but also increase susceptibility to hydrogen embrittlement.
Tool Steels
Tool steels, used in the manufacture of cutting tools, dies, and molds, are susceptible to hydrogen-induced cracking due to their high hardness and complex alloy compositions. Welding of tool steels requires careful control of hydrogen levels and stress-relieving techniques to prevent cracking.
High-Carbon Steels
High-carbon steels, commonly used in applications requiring high strength and wear resistance, are susceptible to hydrogen-induced cracking, especially if they are hardened or quenched. These steels require careful preheating and post-weld heat treatment to minimize the risk of cracking.
Cast Iron
Certain types of cast iron, such as gray cast iron, can be susceptible to hydrogen-induced cracking during welding. The high carbon content and brittle nature of cast iron make it prone to cracking if proper precautions are not taken to control hydrogen levels and stress.
3) Lamellar Cracks
Lamellar tearing is a form of cracking found beneath welds, typically in rolled steel plates with poor through-thickness ductility. This type of defect is characterized by cracks running parallel to the weld fusion boundary and plate surface, often originating at stress concentration points.
Identification of lamellar tearing involves observing the characteristic fibrous and woody fracture surface. Metallographic examination reveals transgranular tearing with a stepped appearance due to elongated inclusions.
Lamellar Tearing Cracking Mechanism
Key Factors affecting Lamellar Cracking
Susceptible Rolled Plate
A material susceptible to lamellar tearing has very low ductility in the through thickness (short transverse) direction and is only able to accommodate the residual stresses from welding by tearing rather than by plastic straining. Low through-thickness ductility in rolled products is caused by the presence of numerous non-metallic inclusions in the form of elongated stringers. The inclusions form in the ingot but are flattened and elongated during hot rolling of the material. Non-metallic inclusions associated with lamellar tearing are principally manganese sulphides and silicates.
High Through-Thickness Stress
Weld joints that are T, K and Y configurations end up with a tensile residual stress component in the through-thickness direction. The magnitude of the through-thickness stress increases as the restraint (rigidity) of the joint increases. Section thickness and size of weld are the main influencing factors and lamellar tearing is more likely to occur in thick section, full penetration T, K and Y joints.
Prevention of Lamellar Tearing
Susceptible Rolled Plate
Procurement of steel plates with good through-thickness ductility is crucial to prevent lamellar tearing. Ensure that the steel plate has low sulphur content (below ~0.015%) to minimize the presence of non-metallic inclusions, which can contribute to lamellar tearing. Through-thickness ductility of the plate can be evaluated using tensile test pieces taken perpendicular to the plate surface. Higher values of % reduction of area (%R of A) indicate better resistance to lamellar tearing.
Through-Thickness Stress
Reduce the magnitude of through-thickness stresses induced during welding by modifying weld joint designs or sizes. Consider changing the joint design, such as using forged or extruded intermediate pieces, to minimize through-thickness stresses experienced by the susceptible plate. Ensure proper fit-up and alignment of the joint to minimize stress concentrations that can lead to lamellar tearing.
Affected Materials to Lamellar Tearing
Carbon-Manganese (C-Mn) Steels
These steels are commonly used in construction, shipbuilding, and pressure vessel applications. They are particularly susceptible to lamellar tearing due to their relatively low through-thickness ductility and the presence of elongated non-metallic inclusions.
Low-Alloy Steels
Certain low-alloy steels, which contain small amounts of alloying elements such as chromium, nickel, and molybdenum, can also be prone to lamellar tearing. The susceptibility of these steels depends on their specific composition and microstructure.
High-Strength Steels
High-strength steels, including quenched and tempered steels, are often used in structural applications where strength and toughness are critical. However, their high hardness and martensitic microstructure can make them susceptible to lamellar tearing if proper precautions are not taken during welding.
Stainless Steels
Some stainless steels, particularly those with high strength and hardness levels, can be susceptible to lamellar tearing. These steels may contain alloying elements such as chromium, nickel, and molybdenum, which enhance corrosion resistance but can also increase susceptibility to lamellar tearing.
Cast Steels
Certain types of cast steels, such as carbon and low-alloy cast steels, can also be affected by lamellar tearing. The presence of non-metallic inclusions and the casting process can contribute to the susceptibility of these materials to lamellar tearing.
4) Sensitization
Sensitization in steels is a phenomenon where certain stainless steels become susceptible to intergranular corrosion and cracking due to the precipitation of chromium carbides at grain boundaries. This occurs when these steels are exposed to temperatures between 400°C and 850°C for an extended period during welding, heat treatment, or service. As chromium carbides form, chromium depletion at the grain boundaries renders them less resistant to corrosion, compromising the material’s integrity.
Sensitization can affect various stainless steel grades, particularly those with high carbon or low chromium content. Preventive measures include proper heat treatment, selection of low-carbon grades, and post-weld heat treatment to restore corrosion resistance. Understanding sensitization is crucial for maintaining the performance and longevity of stainless steel components in various applications.
Sensitization Cracking Mechanism
Key Factors affecting Sensitization
High Carbon Content
Carbon atoms in stainless steel have a strong affinity for chromium, forming chromium carbides at grain boundaries. Higher carbon content promotes the formation of chromium carbides, leading to chromium depletion in the adjacent matrix and sensitization. Therefore, stainless steels with higher carbon content are more susceptible to sensitization, particularly in welding applications where temperatures within the sensitization range are reached.
Elevated Temperature
Sensitization occurs within a specific temperature range typically between 450°C to 850°C. Exposure to temperatures within this range, such as during welding or post-weld heat treatment, promotes the diffusion of chromium and carbon atoms to the grain boundaries, where they form chromium carbides. Prolonged exposure to sensitizing temperatures increases the extent of sensitization, as more chromium carbides are allowed to precipitate and deplete chromium from the matrix.
Cooling Rate
Rapid cooling rates after welding can help minimize sensitization by reducing the time available for chromium carbides to precipitate. Proper post-weld heat treatment processes, such as solution annealing or sensitization annealing, can also help alleviate sensitization by restoring the chromium-depleted regions and promoting the dissolution of chromium carbides.
Prevention of Sensitization
Material Selection
Choosing the right stainless steel grade is crucial in preventing sensitization. Low carbon content grades such as SS304L and SS316L are preferred for applications where sensitization is a concern. These grades contain sufficient chromium to provide corrosion resistance while minimizing the formation of chromium carbides at grain boundaries.
Alloying Elements
Incorporating alloying elements like titanium or niobium with materials such as SS321 and SS347 can enhance the resistance of stainless steel to sensitization. These elements form stable carbides or nitrides, reducing the availability of chromium for carbide precipitation. This helps in maintaining the integrity of the material's grain boundaries and preventing sensitization.
Heat Treatment
Performing heat treatments like solution annealing helps in alleviating sensitization. Solution annealing involves heating the material above the sensitization temperature range (850°C) to dissolve chromium carbides, followed by rapid cooling to prevent their reformation. This restores the material's corrosion resistance and mitigates sensitization-related issues.
Sensitization Temperatures
Prolonged exposure to sensitization temperature range of 400°C to 850°C allows chromium carbides to form at grain boundaries, depleting the matrix of chromium and reducing corrosion resistance. To prevent sensitization, minimize time in this range during welding, heat treatment, or service. By reducing time in the sensitization range, the formation of chromium carbides is limited, preserving stainless steel integrity and corrosion resistance.
Affected Materials to Sensitization
Austenitic Stainless Steels
Austenitic stainless steels, particularly those with high carbon content, are highly susceptible to sensitization. Grades such as SS304, SS304H, SS316 and SS316H are commonly affected due to their composition, which includes an adequate amount of carbon content which can lead to the formation of chromium carbides. During welding or exposure to elevated temperatures, chromium carbides can precipitate at grain boundaries, leading to sensitization and reduced corrosion resistance.
Duplex Stainless Steels
Duplex stainless steels, characterized by a dual-phase microstructure consisting of austenite and ferrite phases, offer a combination of high strength and corrosion resistance. However, these alloys can also be prone to sensitization under certain conditions. While duplex stainless steels generally exhibit better resistance to sensitization compared to fully austenitic grades, prolonged exposure to sensitizing temperatures can still lead to carbide precipitation at grain boundaries, particularly in regions with higher ferrite content.
