The preheating itself is often considered quite common. […]
The preheating itself is often considered quite common. It refers to heating the workpiece to be welded to above room temperature before or during welding. Current specifications usually require several preheating temperature levels based on the application standard of the material. The necessity and benefits of preheating and the consequences of improper preheating will be illustrated by examples.
Preheating refers to heating the workpiece to be welded to above room temperature before or during welding. Both pre-welding and post-welding specifications require preheating. However, under certain conditions, other methods of preheating can also be used. Regardless of whether preheating is required, preheating has the following advantages:
Reduce the shrinkage stress between the weld and the adjacent base metal, which is especially suitable for welds with high stress values.
Slow down the cooling rate of the weld in the critical temperature range during the cooling process, prevent excessive hardening and reduce the ductility of the weld and heat affected zone (HAZ).
In the temperature range of 400°F, slow down the cooling rate, so that hydrogen has more time to escape from the weld and adjacent base metal, and avoid hydrogen-induced cracking.
The amount of preheating is not determined by the minimum standard of the specification, but determined by one or more of the following methods:
Carbon equivalent estimation
Crack parameter estimation
Spark test estimation
Rules of thumb
The preheating temperature range is usually compatible with various weld groove dimensions and constraints. Although the minimum preheating temperature is specified in many specifications, in some cases a lower preheating temperature will be used, and in other cases a higher preheating temperature will be used.
There have always been various "preheating calculation tables" available. Many "preheating calculation tables" use linear or circular slide rules to predict the preheating temperature by identifying the material of the base metal and the thickness of the base metal.
Carbon equivalent (CE) is a way to determine whether preheating is required and to what degree.
If CE ≤ 0.45%, preheating can be arbitrarily selected
If 0.45 =< CE <= 0.60%, the preheating temperature range is 200°F to 400°F (100°-200°C)
If CE> 0.60%, the preheating temperature range is 400° to 700°F (200°--350°C)
When CE> 0.5, at least consider delaying the final non-destructive testing (NDE) for at least 24 hours to determine whether there is a delayed crack.