As increased regulations drive the need for higher safety factors in the pipeline industry, more pipeline owners are requiring materials with greater strength. Because the potential for hydrogen-assisted cracking increases as the strength of the steel increases, this push for higher strength steels has resulted in the need for filler metals and welding processes that promote lower hydrogen levels.
Hydrogen in outdoor work environments is inescapable because nearly all organic compounds contain hydrogen – everything from lubricants and oils to naturally occurring substances in the field and moisture in the atmosphere.
Pipeline welding contractors must be proactive in mitigating hydrogen in the welding zone because hydrogen-assisted cracking is one of the greatest threats to the integrity of pipeline welding applications. A good first step is understanding the many sources of hydrogen and how to eliminate or minimize them.
The problem
Hydrogen atoms are extremely small, highly mobile, and can easily diffuse out of the weld zone and coalesce along discontinuities that are present in the microstructure. Those pockets of hydrogen eventually build stresses that can lead to cracking. Hydrogen-assisted cracking, also referred to as cold cracking, may appear hours or days after the weld has been completed, which can result in costly repairs and downtime.
The three main factors that must be present for hydrogen-assisted cracking to occur include a crack-susceptible microstructure, the presence of residual stresses and the presence of hydrogen.
Pipe welding contractors can take several steps to reduce the amount of diffusible hydrogen in the welding process and reduce or eliminate residual stresses, which in turn will lessen the chances of hydrogen-assisted cracking. Using low-hydrogen filler metals; improving pre-heat and post-weld heating; maintaining interpass temperatures; and in some cases, changing the welding process are among the recommended steps. Proper handling and storage can also help reduce the likelihood of hydrogen-assisted cracking by preventing moisture absorption.
While the filler metal is a main source of hydrogen in the weld, hydrogen can also come from other common sources, such as residual cutting oils, dirt or rust; condensation in and around the weld joint; and shielding gas contamination or hose leaks.
To help prevent the introduction of hydrogen from those sources, be sure to prepare the joint by grinding the inside and outside surfaces of the pipe 1 in. from the joint; always make sure the joint is completely dry prior to welding; and check all shielding gas connections and fittings regularly.
The impact of filler metal
The makeup of the filler metal as well as the environment and manner of its storage can affect hydrogen levels in the filler metal and the weld metal. Different welding processes use different types of filler metal, but the key is to use a filler metal that contains the lowest level of hydrogen that is still capable of providing the needed mechanical properties.
Cellulosic stick electrodes have hydrogen levels far exceeding 16 ml of hydrogen per 100 g of diffusible weld metal – the highest levels among the filler metals commonly used in pipeline welding applications. Low-hydrogen stick electrodes with designations of H4 can provide less than 4 ml of hydrogen per 100 g of diffusible weld metal. However, they do not offer the same penetration and performance characteristics as cellulosic electrodes, and are generally not acceptable for root-pass pipe welding.
Hydrogen levels of 4 ml of hydrogen per 100 g of diffusible weld metal or even lower can be produced by transitioning to a metal-cored wire with a modified short-circuit MIG process for the root-pass welding. Completion of the fill and cap passes can be done with a self-shielded or gas-shielded flux-cored wire process, which typically has a hydrogen content of less than 8 ml of hydrogen per 100 g of diffusible weld metal. Some flux-cored wires are particularly well-suited for high-strength steels because the wires contain a number of compounds that combine with hydrogen to remove it from the weld.
When proper precautions are taken, cellulosic stick electrodes can typically be used on lower strength pipelines. However, they are not recommended for high-strength steels due to an increase in residual stresses and a more crack-susceptible microstructure. When you partner those two factors with the extremely high hydrogen content of a cellulosic electrode, it dramatically increases the chance for hydrogen-assisted cracking. Therefore, some companies do not allow the use of cellulosic electrodes above X65 Grade Pipe, which have a more crack-susceptible microstructure due to higher tensile and yield strengths.
The storage issue
Storing all filler metal in a clean, dry area and in the original packaging until the time of use helps prevent moisture from settling on the outside of the filler metal or becoming absorbed. If a filler metal is moved from one set of environmental conditions to another (such as from cold to hot), it should be protected from the environment and allowed time to normalize at the ambient temperature of the work location.
For metal- or flux-cored wire applications, the wire spool should be covered and/or removed and placed in its original box at the end of the day. Fully encased suitcase-style feeders can also be used to seal the wire from the environment.
Cellulosic stick electrodes commonly used in pipeline welding applications (EXX10 classifications, such as E6010) should never be stored in an electrode oven. Instead, store them at room temperature, protected from the environment. These electrodes have moisture manufactured into them in order to create specific arc characteristics during welding. Drying out the cellulosic electrode coating shifts the composition and can lead to weld cracking and poor operability. If a cellulosic electrode used in pipeline applications becomes wet it should not be reconditioned by drying it in an oven and should be discarded.
Low-hydrogen stick electrodes (EXX18 and EXX16 designations, such as E7018) have different storage recommendations. They should always be stored in hermetically sealed containers or in electrode ovens to prevent moisture absorption. Reconditioning of these low-hydrogen electrodes is also allowed, but be sure to contact the filler metal manufacturer for specific instructions.
The importance of heat
The rapid heating and cooling that takes place during welding can put added stresses into the pipe joint that can make it more susceptible to hydrogen embrittlement. It also provides less of an opportunity for hydrogen to properly diffuse out of the weld, which leads to a greater risk of hydrogen-assisted cracking. These factors make it critical to maintain required/proper pre-heat and interpass temperatures that can produce a softer, less crack-susceptible microstructure and allow hydrogen to diffuse out of the weld. Stress relieving through post-weld heat treatment may also be recommended for some types of steel.
Using old-fashioned oxyfuel or propane torches to bring the weld joint to temperature is one option. However, this method can pose problems such as introducing more hydrogen into the joint from the flame; difficulties in ensuring uniform heating through the joint and heat-affected zone; and the need to stop during the welding process to reheat the joint if it falls below the minimum interpass temperature.
Another method, induction heating, is recommended for optimal hydrogen diffusion, uniform heating throughout the pipe and fast time to temperature. This safer, non-flame method of heating steel uses conductive coils to create an alternating magnetic field within the steel, causing it to heat up. The coils themselves do not become hot. The induction cables are wrapped around the pipe and create eddy currents inside the pipe to generate heat. Induction heating systems also can be integrated with automated recording devices that create a permanent record showing that proper heating/cooling sequences were accomplished.
Control is the key benefit with this method, because it offers the operator control to exact parameters of the ramp-up speed, interpass temperature and post-weld soaking or stress relieving. Control over these factors also controls cooling, which aids in the removal of hydrogen and helps ensure the heat-affected zone and the weld retain the desired mechanical properties. This is particularly important when welding with cellulosic stick electrodes that introduce higher levels of hydrogen into the weld or when welding thicker, high-strength steels, which tend to be less ductile with greater residual stresses, making the steel even more susceptible to hydrogen-assisted cracking.
The bottom line
Focusing on low-hydrogen welding practices should be a priority when welding pipe. Practices such as proper handling of the pipe; selecting low-hydrogen filler metals; and optimal pre-heat, interpass and post-weld heating are great ways to reduce the amount of hydrogen in the weld.
By paying careful attention to these factors and practicing proper procedures, the risk of hydrogen-assisted cracking in pipeline welding applications can be greatly minimized.
