Sep 18, 2020
What to Know About Stress Relieving After Welding
For critical applications, particularly those found in oil- and gas-related industries, stress relieving can be an essential part of safeguarding the integrity of finished welds.
Stress relieving involves heating the weld and base material for a specific length of time at a recommended temperature schedule. Heating can be accomplished using a furnace, resistive heating equipment or induction heating equipment. The temperature threshold must be below the lower transformation temperature of the weld — or the point at which changes to the microstructure occur — to prevent damage. After holding the temperature for the required time, the heat is slowly ramped down in increments to allow even cooling.
The process tends to be used for low alloy steels and applications with thick and highly restrained joints that contribute additional residual stresses based on their design.
The complexity and cost of stress relieving can vary depending on whether an entire part needs to be stress relieved (and the size of the part) or whether the process can be localized at the weldment.
What are the benefits and potential pitfalls?
The time/temperature schedule during stress relieving will vary according to the application, material and filler metal being used. But when completed properly, stress relieving offers several key benefits, including:
1. Reducing residual stress in the weld and heat-affected zone (HAZ)
2. Improving corrosion resistance
3. Minimizing the risk of caustic weld cracking
4. Improving dimensional stability
5. Improving the service life of a weldment
It is critical to follow the exact procedures for stress relieving. Bringing a part up to temperature too fast, holding it too long or at too high of temperature, or cooling it too quickly can all be detrimental to the material’s microstructure. If the microstructure is altered, it can lead to potential failures.
Filler metals for stress relieving
Some filler metals are specially formulated for use with stress relieving. Typically, these filler metals are required to have classification testing performed in the stress-relieved condition. Refer to the applicable American Welding Society (AWS) filler metal specifications for information on required stress-relieving schedules.
If stress relieving is deemed beneficial or required, never use a filler metal that isn’t designed or intended for stress relieving (for example, one formulated for the as-welded condition). The heat introduced by the process can lower the weld’s tensile strength, so a weld made with an as-welded filler metal could be weaker than required. It could also affect the weld’s toughness. The weld may fail unexpectedly, which can lead to dangerous consequences in an oil and gas application.
Note, certain manufacturers may test the use of stress relieving with filler metals that are not required to use the process and will report the results for informational purposes only.
It’s also important to always stress relieve a weld made with a filler metal formulated for this process. Doing so restores ductility and reduces weld tensile strength to within the intended range for service. A ductile weld that is not overmatched to strength of base metal helps to reduce the risk of residual stresses leading to weld or base metal cracking.
Filler metals for stress relieving are available in stick electrodes, solid wires, metal-cored wires and gas-shielded flux-cored wires. Submerged arc welding flux and electrodes are also an option.
AWS defines the stress-relieving hold time for testing of the following filler metal suffixes and temperatures at one hour unless otherwise specified. That means the filler metal has to maintain its mechanical properties at the given temperature for one hour to receive the specific designation. Some examples are:
Per A5.5/A5.5M:2014 Specification for Low-Alloy Electrodes for Shielded Metal Arc Welding
E8018-C1, E8018-C2: 1,125° F
E8018-D1, E10018-D2, E8018-D3: 1,150° F
Per A5.28/A5.28M:2005 Specification for Low-Alloy Steel Electrodes for Gas Shielded Arc Welding
ER80S-B2, E80C-B2: 1,150° F
ER90S-B3, E90C-B3: 1,275° F
Per A5.29/A5.29M:2010 Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding
E8XT1-A1C, -A1M: 1,150° F
E8XT5-Ni1C, -Ni1M: 1,150° F
Per A5.23/A5.23M:2011 Specification for Low-Alloy Steel Electrodes and Fluxes for Submerged Arc Welding
A1, A2, A3, A4, B1, B5, Ni1, Ni2, Ni3, F1, F2: 1,150° F
B6, B6H, B8: 1,375° F
These are just a few specific examples. For a comprehensive list of products tested for stress- relieved properties, review the appropriate AWS filler metal specification.
Also, many real-world applications may require different temperatures, mechanical properties or longer stress-relieving times. If that is the case, it is best to contact a trusted filler metal manufacturer to determine which product meets those needs. The filler metal manufacturer may be able to provide supporting documentation to assist with design or engineering approval.
Removing residual stresses from critical welding applications is key. With proper stress-relieving process, oil and gas industries that require components to maintain corrosion or cracking resistance can have peace of mind that elevated service temperatures or multiple repair cycles will not cause damage. As with any aspect of the welding operation, however, welding operators must follow and implement every step of the stress-relieving process carefully. Consulting filler metal spec sheets, as well as the welding procedure are also both important ways to ensure success.