28 September 2022

Boiler repair practices | Heat treatment requirement for butt welding of tubes | Welding Process | Preheating | Inter-pass temperature control | Post heating or delayed cooling | Post weld heat treatment

Boiler repair practices

Heat treatment requirement for butt welding of tubes / pipes

Metal working results in distortion of grain structure. A weldment also results in different grain structure at different distance from weldment.

A WELDING MAY GO WITH ANY OF THE FOLLOWING HEAT TREATMENT PROCESSES

  1. Preheating
  2.  Inter-pass temperature control
  3. Post heating or delayed cooling
  4. Post weld heat treatment

Chemistry of material, thickness of material & type of joints decide the type of heat treatment

Why Preheat?

Preheat reduces the temperature differential between the weld region and the base metal

  1. Reduces the cooling rate, which reduces the chance of forming martensite in steels
  2. Reduces distortion and shrinkage stress
  3. Reduces the danger of weld cracking
  4. Allows hydrogen to escape

Using Preheat to Avoid Hydrogen Cracking

 

Interaction of Preheat and Composition

 

 

Why Post-Weld Heat Treat?

  1. The fast cooling rates associated with welding often produce martensite
  2.  During post-weld heat treatment, martensite is tempered (transforms to ferrite and carbides)
  3. Reduces hardness
  4.  Reduces strength
  5. Increases ductility
  6. Increases toughness
  7. Residual stress is also reduced by the post-weld heat treatment

 

Preheating methods

Gas rings / muffle furnace ( at factory)
Electrical resistance heaters ( at site)
Induction heating ( at factory)
Oxy acetylene or other gas torches (at site)

Post weld Heat Treatment and Hydrogen Cracking

Post weld heat treatment (~ 1200°F) tempers any martensite that may have formed
Increase in ductility and toughness
Reduction in strength and hardness
Residual stress is decreased by post weld heat treatment
Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutes

 

Post weld heating methods

Gas rings / muffle furnace ( at factory)
Electrical resistance heaters ( at site)
Induction heating ( at factory)

Weld should be free of grease, oil , chloride and sulfur bearing compounds.
Heat band should cover 3 times weld thk / material thk.
Post weld heat treatment must be monitored with temperature recorders.

 

Material and P No Grouping

 

Girth Butt Weld Tube and Pipe Diameter <=108 mm (dissimilar) applicable for thickness 19 mm for P1 ABC and thickness up to 13 mm for others Material

 

Heat treatment requirement for cold / hot forming of tube bends

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