International Conference on Welding Technology

 

Consumables Corner: Preheat and interpass temperatures defined in welding, Part I

                  

 We certainly can, and we’ll take a little deeper look at how these welding variables affect weld and base material properties.

Let's start with the basic definitions of these terms:

• Preheat temperature is the base material’s minimum required temperature immediately before any arc can be struck or applied to the weld joint. This includes welding, tack welding, or carbon arc gouging.
• Interpass temperature is the minimum or maximum allowed temperature measured immediately before the next arc is struck in the weld joint.

Defining these terms is the simple part. Interpreting "how" and "when" is where it can get confusing.

Preheat temperature is straightforward and defined by many welding codes. The welding procedure specification (WPS) should list, if required, the minimum required preheat temperature for the applicable weld joints. The best way to confirm that the part has reached the minimum temperature is by using a temperature-indicating crayon, such as a Tempilstik, which has an accurate melting point. A crayon mark on a base material will remain solid below the stated temperature, or it will melt when the temperature is exceeded.

Welding codes often dictate preheat temperature measurement location. For example, AWS D1.1 states that preheat measurement shall be 3 in. in all directions from the edge of the weld joint, and through the thickness of the material if less than 3 in. This is a good rule of thumb for any application where preheat should be used to maintain the base material’s mechanical properties, primarily the heat-affected zone (HAZ), and to reduce the possibility of cracking.

In most cases, preheat and minimum interpass temperatures will be the same value, ensuring that the base material does not cool too quickly. This will help prevent the formation of an undesirable microstructure like martensite.

Interpass temperature can be a minimum value, maximum value, or both depending on the application and base material. Some carbon steels are less affected by high interpass temperatures that may occur during welding production and only require that a minimum temperature be maintained. However, many carbon steels have impact requirements and can be affected greatly by high interpass temperatures, requiring welders to adhere to a maximum temperature limit.

Minimum interpass temperatures should follow guidelines for preheat temperature location measurement. Conversely, maximum interpass temperature may exceed what’s acceptable if it is measured at the same location. Therefore, measure maximum interpass temperatures approximately 1 in. from the edge of the weld joint. This should ensure excessive temperatures are not reached during welding, thereby maintaining HAZ integrity.

For continuous welding applications like pipe or pressure vessels, where small-diameter sections reach high interpass temperatures during multiple weld passes, a good measurement location is at the weld start. As the part rotates and the weld overlaps the previous weld bead, the welder should verify that the minimum or maximum temperature meets WPS requirements.

It's important to understand that preheat is necessary to prevent rapid base material cooling. Rapid cooling can lead to high hardness due to martensite formation; preheat slows the cooling rate and reduces martensite formation, thus reducing hardness.

Minimum interpass temperatures, much like preheating, are a way to ensure slow cooling rates are maintained. Maximum interpass temperature limits prevent excessively slow cooling rates. Slow cooling rates result in grain growth, which can negatively impact weld and HAZ properties.

 

 

 

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