Preheating Temperature Requirements

  We have seen the purpose for Preheating and why we should not do preheating by gas cutting torch in our earlier Lessons.

Click Here for purpose of Preheating

Click Here for Preheating by Gas Cutting Torch

   In this lesson we are going to learn the requirements for Preheating. The preheating requirements predominantly comes from the following:-

- Material Composition

- Material Thickness

- Carbon Equivalent

- Weather conditions

Material Composition

   When the material composition consists of Carbon and other alloying elements such as Chromium, Molybdenum, Vanadium may influence the weldability of steels depends upon their composition. These steels will require preheating irrespective of thickness. (Low alloy steels-LAS)

Material Thickness

   Material thickness also will influence the requirement for preheating since to reduce the thermal gradient in the weld joint. when the material thickness greater than 20 mm  then preheating may be necessary to bring down the thermal gradient.

Carbon Equivalent (CE)

   This is the most influential factor in deciding the preheating requirements for Carbon steel Materials. The higher the carbon equivalent, will have less weldability factor and thus require preheating. Generally CE of more than 0.4 may require preheating.
Carbon Equivalent is calculated based upon Carbon, Manganese, Chromium, Molybdenum, Nickel, Vanadium etc.

The formula for calculating CE is

Weather Conditions

   Weather conditions are very important to decide whether preheating to be applied or not. If moisture or water or ice present on the weld surface to be joined, then it must be removed before start welding. Or otherwise, if the humidity is high i.e above 90% preheating may be necessary in such situations.

Remember: Preheating is MINIMUM temperature to be applied on the weld joint.

Minimum the Preheating Temperature

Generally the following specifies the minimum preheating temperature:-

- Construction Codes and Standards

- Client Specifications

- WPS

- Weather Conditions

Quiz Question: 

A client Specification requires preheating to 100 deg C. The construction Code requirements for Preheating is 75 deg C. WPS Qualified with 80 Deg C. What Preheat temperature you will apply to the weld joint?

Answer Options:

a) Apply Preheat 75 deg C since it is the governing Code for Construction
b) Apply preheat 100 deg C, since it is client requirement otherwise Client will raise NCR
c) Apply 80 deg C, since the WPS is Qualified with 80 deg C only.
d) Revise WPS preheat temperature to 100 deg C and preheat to 100 deg C

These kind of situations will arise every often to all the Welding inspectors and the right decision is important. Get involved in the Quiz

Post Your replies or queries in the comments box below

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