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How to Relieve Pressure Stress in Vessels When Portable Welding

Welding is a highly technical skill with a variety of industry-specific terms. When welders talk about vessels, they're referring to a structural container, such as a boiler or ship's hull, which are almost always fabricated with portable welders. When a seam in a boiler is welded, the steel is superheated. This effectively heat-treats the steel, creating a hardened band that is brittle relative to the remainder of the vessel. The weld-joint doesn't respond to stress or loads in the same way the surrounding metal does. Returning the welded area to a a consistent temper is usually done by heat treatment, and it restores ductility -- the ability for the metal to stretch slightly without breaking.

Things You'll Need

Heat-treating resistance wires
Metal clamps
Steel wire
Leather welder's gloves
Leather welder's bib and jacket
Leather work boots
Eye protection

Instructions

- 1
  Complete your welds prior to stress relieving, also called post weld heat treatment. You want to be able to conduct all the stress relieving or heat treatment at once, not a section at a time, as you weld.
- 2
  Apply heat-treating resistance wires specifically developed for steel heat treating along your weld seam. These heating wires are capacitor heated, much like an electric blanket. Cover the weld-seam and a band of 3 to 4 inches on either side of the weld seam with the wires. Secure them to the heat-effected areas with metal clamps or twisted steel wire as used to secure rebar in concrete work.
- 3
  Conduct a safety check. Make sure you can safely heat the exposed wires without risk of personal injury or fire. The wires may not provide a visual indication of their heat the way a hot burner might. So, cordon off the area, display signs or do whatever else is reasonable to ensure the safety of your job. Remove all flammable material from potential contact.
- 4
  Heat the wires, bringing the temperature up to approximately 300 degrees Celsius, maintaining this temperature for up to nine hours or more. It is important that you check engineering specifications for the specific type, thickness, configuration and intended use of the steel alloy you're treating. While this is a general guide, each of these factors will affect the specific temperature you apply and possibly how fast you apply it. Do not guess. You must have the heat specifications to achieve your objective.
- 5
  Cool the metal by cooling the wires gradually or incrementally. Do not just discontinue heat, as it is critical for the molecular structure of the metal to cool gradually. Depending on your equipment, you may have an automatic cool-down setting, or you may have to turn the heat down in increments of 50 degrees per hour, for example.

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