Why is Preheating Performed before Welding?

Preheating can be defined as the process of raising the temperature of the parent material before welding begins. Preheating is applied to ferritic steels principally to reduce the probability of Hydrogen Induced Cold Cracking (HICC) as per the following:

Item 1. To slow down the cooling, defined as cooling rate, which allows more hydrogen to diffuse from the weld area by extending the time period over which the weld is at elevated temperature. The reduction in hydrogen reduces the risk of Hydrogen Induced Cold Cracking (HICC)

Item 2. To slow the cooling rate of the weld and local base material, potentially resulting in weld metal and heat affected zone microstructures that are less hard and, as such, give a greater resistance to HICC.

With regard to hydrogen diffusion, Item 1 above, hydrogen is a gas that is often present in the atmosphere surrounding the arc. The hydrogen comes from the breakdown of water or organic material such as grease, rust or paint present on the metals surface. It is also sourced from humidity and moisture in the electrode coating. and can dissolve in the molten pool as shown in Figure 1.

High (fast) cooling rates improve the chances of trapping some of the hydrogen in the weld area (weld metal and HAZ) where it will do its damage and promote cracking. Therefore, preheat will slow down the cooling rate and allow more time for the hydrogen to leave the weld zone.  

Figure 1 Illustrating Hydrogen Diffusion from the Weld Zone which is Assisted by Preheat.

Regarding Item 2, the possible softening of the microstructure, will depend very much on a steel’s hardenability. Steels that produce hard microstructures have an increased probability of being subject to HICC and so the possible softening by preheat is another weapon that welding personnel have to control the onset of this type of cracking.

Preheating is very, very important as, even now well over 50 years since we understood the hydrogen cracking phenomena, we still see it. Amongst other things, this is due to insufficient care being taken in applying the prescribed preheats and the associated interpass temperatures.

Most fabrication codes, CSA W 59. Table 5.3 for example, contain guidelines on preheats that generally equate thickness and material type as per the generic list given in Table 1.

These tables are a guideline and usually give the minimum necessary preheat and interpass temperature (the temperature of the workpiece between welding the initial and subsequent passes). They should be included in any operational Welding Procedure Data Sheet (WPDS) or Welding Procedure Specification (WPS).

Table 1. Generic Preheat Table similar to those prescribed in Fabrication Standards

Preheat can be applied by various means which includes single or multiple heating torches, radiation heaters, induction heaters and electrical strip heaters,. For small components, furnace heating will be an option if the facility is so equipped.

It is important that preheat is applied correctly and accurately. 100 degrees C is just that, it is not 90 deg C. and it needs to applied with the correct monitoring and controls. Temperature indicating crayons, infrared digital thermometers etc. are available for these measurements.