How To Control Hydrogen in Welding
From filler metal selection to proper preheat, learn some tips to help eliminate hydrogen-assisted cracking in welding.
Reduce hydrogen cracking
Hydrogen in outdoor work environments is inescapable. Nearly all organic compounds — everything from lubricants and oils to naturally occurring substances in the field and moisture in the atmosphere — contain hydrogen.
Hydrogen-assisted cracking, or heat-affected zone (HAZ) cracking, is one of the greatest threats to weld integrity in many applications, including transmission pipeline and process pipe welding. Mitigating hydrogen in the welding zone starts with understanding the many sources of hydrogen and how to eliminate or minimize them.
Learn more about some best practices that can help reduce the occurrence of hydrogen-assisted cracking, as well as some specific precautions for different grades of steel and filler metal.
Why do you need to control hydrogen in welding?
Hydrogen-assisted cracking, also referred to as delayed cracking or cold cracking, can be slow to take effect and may appear hours or days after the weld has been completed. This cracking can result in costly repairs and downtime.
Because hydrogen ions are extremely small and highly mobile, they can easily diffuse out of the weld zone and coalesce along discontinuities present in the microstructure. The hydrogen ions may recombine to form hydrogen gas, further stressing the microstructure. These pockets of hydrogen eventually build stresses that can lead to cracking. Several factors must be present for hydrogen cracking to occur, including:
- A crack-susceptible microstructure
- The presence of residual stresses
- The presence of hydrogen
In general, susceptibility to hydrogen-assisted cracking increases as the strength of the base metal increases. Reducing the amount of diffusible hydrogen and taking steps to reduce or eliminate residual stresses will lessen the chances of hydrogen-assisted cracking.
This can be accomplished by using low-hydrogen filler metals, improving pre- and post-weld heat treatment, maintaining interpass temperatures and, in some cases, changing the welding process. In addition, better attention to material handling and storage methods that help prevent moisture absorption will go a long way toward preventing hydrogen-assisted cracking.
Tip 1: Choose appropriate filler metals and use them properly
One source of hydrogen in the weld is the filler metal. The makeup of the filler metal, as well as the environment and manner of storage, can affect hydrogen levels in the filler metal and resulting weld.
Cellulosic shielded metal arc welding (SMAW, or stick) electrodes provide the highest hydrogen levels of filler metals commonly used in transmission pipeline applications, with levels far exceeding 16 ml per 100 g of weld metal. Low-hydrogen stick electrodes, with designations of H4 and H8, are also available and provide less than 4 and 8 ml of hydrogen per 100 g of weld metal respectively. Unfortunately, low-hydrogen stick electrodes do not offer the same penetration and performance characteristics as cellulosic electrodes and are generally not acceptable for root-pass pipe welding.
- Welding process change: Transitioning to the Regulated Metal Deposition (RMD®) welding process with a metal-cored or solid (GMAW) wire for the root pass can lower hydrogen levels to 4 ml per 100 g of weld metal or lower. Good alternatives for fill and cap passes are a gas-shielded flux-cored process where H4 and H8 options are available or a self-shielded flux-cored process that is typically less than 8 ml per 100 g. These wires are particularly suitable for high-strength steels, which tend to be more prone to hydrogen-assisted cracking. Bottom line: use a filler metal that contains the lowest level of diffusible hydrogen and is still capable of meeting the desired mechanical properties.
- Proper filler metal storage: All filler metals should be stored in a clean, dry area and remain in the original packaging until the time of use. Keeping the filler metal sealed helps prevent moisture from entering the package and degrading the filler metal. Minimizing the transfer of filler metals from cold to hot environments will also help minimize condensation, which adds hydrogen.
- Filler metal handling: Filler metals should be handled with clean, dry gloves whenever possible. Sweat, oils and dirt on the hands can easily transfer to the surface of the wire or electrode and introduce additional hydrogen and contaminants into the weld.
- Take care with cellulosic stick electrodes: Cellulosic stick electrodes (those of the EXX10 and EXX11 classifications, such as E6010) present a unique set of challenges related to hydrogen. While hydrogen is generally undesirable, a cellulosic stick electrode should never be dried to remove the moisture manufactured into the electrode. They should be stored at room temperature, protected from the environment. Welding high-strength pipe with these electrodes should be avoided when possible. If proper precautions are taken, these electrodes are generally acceptable for X60 and lower strength pipe. While cellulosic stick electrodes should not be stored in electrode ovens, low-hydrogen electrodes (those of the EXX18, EXX15 and EXX16 designations, such as E7018) should always be stored in hermetically sealed containers or in electrode ovens. Be sure to follow the manufacturer’s recommendations for storage and reconditioning of low-hydrogen stick electrodes.
Tip 2: Understand preheat and heat treatment
The rapid heating and cooling of the base metal that takes place during welding puts stresses into the part and can spur the creation of hard, strong grain structures that are susceptible to hydrogen embrittlement. Rapid cooling provides less opportunity for hydrogen to diffuse out of the weld and heat-affected zone and can lead to cracking.
Maintaining required preheat and interpass temperatures is critical, both for producing a softer, less crack-susceptible microstructure and for allowing hydrogen to diffuse out of the weld metal and HAZ. In some cases, it may be necessary to apply a post-weld soak (typically 24 to 48 hours at 200 to 400 degrees Fahrenheit) to further reduce the amount of hydrogen trapped in the weld. Stress relieving through post-weld heat treatment may be recommended for some types of steel.
Many pipe welding applications rely on oxy-fuel or propane torches to bring the weld joint to temperature. This equipment can pose a problem in that most fuel gases are hydrocarbons and the process of igniting the torch and applying the flame to the pipe actually introduces hydrogen into the weld joint. Heating with a torch also does not ensure uniform heating throughout the joint and HAZ, leading to cold areas that can heat and cool at uncontrolled rates.
Induction heating is recommended for optimal hydrogen diffusion and uniform heating throughout the part. Heat is induced in the part by placing it in an alternating magnetic field created by liquid- or air-cooled induction heating cables or blankets that are wrapped around or placed on the part, creating eddy currents inside the part to generate heat. Induction provides a fast time-to-temperature, offers safety benefits and delivers a strong return on investment compared to other heating methods. With induction, tools do not heat up and there is little noise and no toxic byproducts. In addition, operating costs are low compared to other methods. With flame heating, fuel costs can run up to $50 per hour and operations may have to pay fire-watch personnel*, while resistance heating requires installation of electrical infrastructure and often involves hiring third-party contractors who may charge up to $2,000 per joint.
Watch this video to learn more about how induction heating works and the many applications where it can be used:
The key factor with induction heating is control. The operator controls the ramp-up speed, interpass temperature and post-weld soaking or stress relieving to exact parameters. This controls cooling and ensures the HAZ and weld retain the desired mechanical properties — and at the same time aids in the removal of diffusible hydrogen.
Tip 3: Use welding techniques to control hydrogen
There are some welding technique changes and process changes that can help reduce the amount of hydrogen that makes its way into the weld. Focusing on low-hydrogen welding practices should be a priority.
- Adjust contact-tip-to-work distance: In the case of a solid, metal- or flux-cored wire, welding with a longer contact-tip-to-work distance (within the recommended range) can help remove hydrogen from the weld area. The longer electrical stick-out results in greater preheating of the wire and burns off greater amounts of hydrogen before it crosses the arc and transfers to the molten weld pool. It’s been shown that a 1/8-inch difference in electrical stick-out can have a substantial effect on diffusible hydrogen. However, using too long of a stick-out can increase the risk of losing shielding gas and can lead to other problems.
- Consider a welding process change: As mentioned above, a change to the welding process can also help control hydrogen. For operations using stick welding, a switch to RMD (a modified short-circuit MIG process) for the root pass and flux-cored welding for the fill and cap passes can allow for the use of filler metals formulated to remove hydrogen from the weld. These processes also deliver other benefits, including improved productivity and ease of use for operators.
- Pay attention to proper joint prep: Joint preparation is crucial, as residual cutting oils from pipe beveling, paint and certain coatings applied to the pipe — as well as organic materials such as dirt or rust — can all introduce hydrogen into the weld. Grind both the inside and outside surfaces of the pipe 1 inch from the joint to prevent contaminants from entering the weld pool. Also, always make sure the joint is completely dry prior to welding.
Addressing sources of hydrogen in welding
The effect of hydrogen on weld metal and the HAZ has been well established. A push for higher-strength steels in many industries has resulted in the need for lower-hydrogen welding consumables and processes. Careful attention to numerous factors, including material and filler metal handling, filler metal selection, and preheat and post-weld heat treatment, can minimize the risk of hydrogen-assisted cracking in many welding applications.
* As specified in ANSI Z49.1, Supervisors shall assure that fire watchers are assigned, and hot-work authorization procedures are followed where required.