What is Preheat in Welding?

What is Preheat?

In welding, Preheating involves heating the base metal surrounding the weld joint, to a specific desired temperature, called the preheat temperature, prior to welding. Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source. Normally a preheat maintenance i.e. preheat throughout the welding is required for more exotic materials such as Cr-Mo steels, HSLA steels, etc.

The interpass temperature, defined as the base metal temperature between the first and last welding passes, cannot fall below the preheat temperature. Interpass temperature will not be discussed further here. Preheating can produce many beneficial effects; however, without a working knowledge of the fundamentals involved, one risks wasting money, or even worse, degrading the integrity of the weldment.

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Welding Preheat- What, Why & How?

Why Preheat?

Preheat in welding offers:

  1. it lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking
  2. the slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking
  3. it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints and
  4. it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as notch toughness.

When Should Preheat be Used?

The preheat requirements are depends on following factors:

1.       code requirements,

2.      section thickness,

3.       base metal chemistry,

4.      restraint,

5.      ambient temperature,

6.      filler metal hydrogen content and

7.      previous cracking problems

   When there are no codes governing the welding, one must determine whether preheat is required, and if so, what preheat temperature will be appropriate. In general, preheat usually is not required on low carbon steels less than 1 in, (25 mm) thick. However, as the chemistry, diffusible hydrogen level of the weld metal, restraint or section thickness increases, the demand for preheating also increases. There are several methods to determine the required preheat temperature for a given base metal and section thickness that will be discussed in the next section.

How to select Preheat temperature:

The selection of a correct preheat temperature is a critical task and requires sound knowledge of codes, standards, metallurgy and engineering practice. Briefly, preheat temperature can be selected by:

1. Based on construction code and welding recommendation codes or standards.

2. Based on alloy chemistry, thickness, level or restraint & hydrogen level.

Preheat requirements according to various codes and standards:

The table below specifies the various code references for the preheat temperature. This guide helps to easily locate the applicable code preheat requirements such as ASME Section VIII, ASME B31.1, ASME B31.3, AWS D1.1 & EN 1011-2 latest respective edition.

Preheat temperature calculation based on alloy chemistry, thickness, restraint & hydrogen level:

When no welding codes are specified, and the need for preheat has been established, how does one determine an appropriate preheat temperature?

The preheating temperature depends on the following input data:

  • Carbon equivalent CET (see above): The CET can be explicitly filled in here or be calculated by the contents of the alloying elements in the menu carbon equivalent. The CET is inserted in weight-%
  • Plate thickness d: The plate thickness is inserted in mm. It should be considered that the influence of the plate thickness is of minor importance for plate thicknesses above 60 mm due to the three-dimensional heat flux.
  • Hydrogen content HD: The hydrogen content H2 is inserted in ml/100g. Here either a value between 1 and 20 ml/100g can be inserted directly or a typical value depending on the weld process used can be selected:

You can use our Preheat Calculator to easily find the preheat temperature based on alloy chemistry, thickness & hydrogen level.

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