Unit 212/255 – Principles of Fabrication and Welding Technology Distortion

Using Distortion Bi-metallic strip Shrink fit

What Causes Distortion Distortion is caused by uneven expansion and contraction

Weld Contraction

Weld Distortion

Weld Preparations Weld preps can effect distortion rates Preps should be as close to parallel as possible Tapered preps will have more expansion at the top to the weld than the root causing uneven contraction

Weld Preparations

Pre-setting Allowing for distortion can help reduce the effect.

Heat Input Reducing heat input will limit distortion Least heat MIG MMA TIG Oxy-acetylene Most heat

Welding Procedures Skip welding Distributes the heat evenly through the component

Welding Procedures Step-back welding Produces heating and cooling zones that counteract each other. Cooling – contraction zone Heating– expansion zone Welding direction

Stress Relieving However little distortion that may be apparent, there will always be a certain level of stress within a welded structure. Controlling heat input and employing correct weld geometry will help to reduce stress, but will never eliminate it completely. In many cases this residual stress will not cause a problem with the operation of the component.

Stress Relieving However, in some situations, such as pressure vessels and high pressure pipe work the relief of theses stresses are important. The most common method employed is a controlled heating and cooling cycle. In the case of a pressure vessel it is important that the whole fabrication reaches an even temperature so as not to create stresses elsewhere. Therefore vessels are normally stress relieved in a furnace, and soaked for up to 24 hours at the required temperature. The furnace also allows controlled cooling of the whole fabrication.

Stress Relieving Pipe Stress reliving of pipe joints can cause problems as it is impractical to heat the complete pipe run. This is overcome by ensuring that the heat band created by the stress reliving heating elements spreads wide enough either side of the weld joints. Heating elements are normally covered by a insulating blanket to ensure controlled cooling times, especially for site welding, where draughts could chill the heated zone.

Stress Relieving