What is Preheating?
Preheating involves rising the temperature of the parent material locally, on both sides of the joint to a value above ambient. The need for preheat is usually determined by the pertinent fabrication code and verified by the weld procedure qualification test. Preheat may be required as an aid to welding for one of the four basic reasons:
- To control the rate of cooling. especially in the heat affected zone, to reduce hardness. High carbon and low alloy steels harden if they are quenched from high temperatures (above cherry red). Exactly the same process can happen in a welded joint at the fusion face with the parent material. By raising the temperature of the base metal to be welded, to reduce the temperature differential between ambient and the resultant heat input, hardening may be controlled as the weld cools. Reducing hardness reduces the risk of cracking.
- To control the diffusion rate of hydrogen in a welded joint. The intensity of the electric welding arc breaks down water, present as moisture, into its base elements of hydrogen and oxygen. Both of these gases are easily dissolved into the weld metal at high temperatures and hydrogen can play an important role in weld and heat affected zone cracking with a phenomenon known as hydrogen or cold cracking. Preheat can also help by ensuring that the weld preparation area is dry and remains dry throughout the welding operation. The presence of preheat, and associated benefits on cooling rate, helps to facilitate the diffusion of the hydrogen molecules out of the metallic structure.
- To reduce thermal stresses. Thermal strains are set up as the molten weld pool cools. Partially made welds can crack as the parent metal restrains the contraction of the weld metal and the cross sectional area of the joint is insufficient to with stand the resultant stress. Preheat can control the level of strain by reducing temperature differentials and reducing cooling rates.
- Compensation for heat loss. Thicker section steels with high thermal conductivity benefit from preheat during welding with improved fusion.
Where preheat is applied, every effort should be made to ensure that the correct levels for a particular application are attained, both uniformly over the length of the joint and for the duration of welding process. Post Heat is the terms given to the extension of preheat on completion of welding at the same or increased temperature. Its purpose is to effect diffusion of hydrogen from the joint and reduce susceptibility to the associated form of cracking. It is usually applied to the higher strength carbon-manganese steels and the low alloy steels where the risk of hydrogen cracking is higher. Post heat treatments are not reflected in standards or codes, but are often specified by the client who has incorporated their equivalent into the weld procedure qualification test.