The Need for Pre and Post Weld Heat Treatment


The Benefits of Weld Preheating and Reducing Thermal Stress

PROBLEM:

  • Thermal Stresses Occur as Molten Weld Pools Cool
  • Cracking Can Occur Both During and After Welding
  • Colder Parent Metal Resists the Inevitable Contraction of the Weld Metal

SOLUTION:

  • Preheating reduces the temperature difference between the Weld Metal and the Parent Metal and Minimizes Potential Cracking

Pre Weld Heat Treating and High heat Loss

PROBLEM:

  • Heavy Wall Materials and Aluminum Alloys with higher thermal conductivity can act as a heat sink, drawing heat away from a weld and cooling at an accelerated rate.

SOLUTION:

  • Preheat Heat treatment will reduce the cooling rate of the deposited weld material and allow a proper fusion with the parent metal

Pre Weld Heat Treatment will Reduce Weld Porosity

PROBLEM:

  • If moisture is present at a welded joint, the high heat of the welding arc will break the water down into its elements, hydrogen and oxygen. The hydrogen by-product easily dissolves into the weld metal and causes weld porosity during weld solidification.

SOLUTION:

  • Performing a pre weld heat treat drives away moisture from the welded joint.

Pre Weld Heat Treatment will Reduce Weld Hardening

PROBLEM:

  • During and after completing of the welding process, both the weld pool and the Heat Affected Zone (HAZ) can harden and crack when cooling at a rapid pace

SOLUTION:

  • By preheating the weld and the Heat Affected Zone (HAZ) the rate of cooling can be controlled and reduced thereby eliminating problems associated with weld hardening

Pre Weld Heat Treating Prevent Hydrogen Cracking

PROBLEM:

  • Electrode Coatings and Fluxes can often introduce moisture directly to the arc and weld pool. The increase in moistures increases the possibility of weld or Heat Affected Zone (HAZ) cracking.

SOLUTION:

  • Preheating slows down the weld cooling rate which allows hydrogen to escape

Pre Weld Heating Improves Material Microstructure

PROBLEM:

  • Low Alloy Steels containing elements such as chromium, nickel, molybdenum, and vanadium are susceptible to cracking in the Heat Affected Zone (HAZ)

SOLUTION:

  • Preheating improves the microstructure inside the HAZ by reducing the post weld cooling rate. The reduced cooling rate produces:
    • A more desirable and ductile microstructure to be formed
    • Minimizes Under Surface Cracking

The Need for Pre-Weld Heat Treatment

Fumanite’s state of the art technology controls the heat up and in so doing;

  • Reduces the levels of thermal stress.
  • Provides compensation for high heat losses.
  • Minimizes the rate of weld hardening.
  • Reduced porosity in the weld.
  • Reduced hydrogen cracking.
  • Improved micro structure of heat affected zone (HAZ).

 

 

 

 

 

 

 

 

 

 

The need for controlled post weld heat treatment (PWHT)

  • Relaxes residual stress
  • Relaxes thermal stress
  • Tempers the metal
  • Removes diffusible hydrogen
  • Improves metallurgical structure
  • Enhances strength and life of the material

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