Purpose of Heat Treatment and Its Requirement
The purpose of proper heat treatment is to place pressure vessels, piping, tanks, weldments or other equipment or components into the best condition to resist failure by cracking and/or corrosion. Heat treatment is thus performed to reduce residual stresses, remove cold work, and improve the metallurgical structure.
 
When the reduction of residual stresses is the primary consideration, an improvement in the resistance to failure may be desired in service environments involving the following:
  1. Fatigue loading (mechanical or thermal)
  2. Brittle fracture conditions (usually at low temperatures)
  3. Stress corrosion cracking
  4. Localised corrosion
  5. Geometrical stability (minimised distortion)
   
Heat treatments to produce desired metallurgical changes may be made to provide:
  1. Homogenisation in the metallurgical structure
  2. Solution of specific metallurgical phases
  3. Stabilisation of specific metallurgical phases
  4. Age hardening
  5. Formation of specific structures
  6. Removal of hydrogen (which can cause delayed cracking, also known as cold cracking, underbead cracking, etc.)
 
The results may be hardening, strengthening, softening, improved formability, better weldability, or other properties.

Proper heat treatment is as essential to a soundly fabricated pressure vessel, tank, piping system or other component, as are good design, forming and welding procedures.

Service failures have been far more common than generally recognised when heat treatments either were not done at all, or when the heat treatments were not performed correctly. Catastrophic failures of pressure vessels, tanks, piping and other types of weidments have resulted from improper or insufficient heat treatments.

Improperly performed or controlled heat treatments, or the use of undesirable equipment has also resulted in cracking of welded equipment, pressure vessels or piping, and has necessitated costly repairs - often exceeding by a very substantial margin the cost of reliable heat treating equipment, or of proper procedures carefully supervised and controlled by specialised personnel highLy experienced and trained in all aspects of heat treatment engineering. The downtime required for the repair operation frequently results in a business interruption loss far more costly than the entire cost of the original equipment.
 

Preheat Treatment
On medium carbon (0.25% C) steels, and on many low-alloy steels, heating prior to welding is an effective method of avoiding weld or heat-affected zone cracking. This cracking tends to occur when the weld has cooled to near room, temperature conditions. Thus, it is also referred to as “delayed” and “cold” cracking. Where this cracking occurs in the heat-affected zone of the base metal, it is also referred to as “underbead” cracking.

The beneficial effects of preheat treatments in these steels involve primarily the reduction of stresses, and the increased ductility of the weld heat-affected zone; making it more resistant to cold cracking. When hydrogen is present in the weld or heat-affected zone, the tendency towards cold cracking increases. This tendency can be reduced significantly with increasingly higher preheat temperatures of up to 400 to 600°F.

The need for proper preheat treatment is particularly high when welding the first (or root) pass in butt welds. It is also more important with welding processes which are characterised by depositing essentially thin weld passes in the root, as in the case of inert-gas tungsten-arc welding.

Heavy wall sections of unfavourable weld joint contours may also necessitate preheat treatments, sometimes even at temperatures higher than those normally recommended.

The interpass temperature during welding should, of course, be maintained at the level of the preheat temperature. To be most effective, the entire weld area should be preheated as uniformly as possible.
 

Postheat Treatment
A number of different postheat treatments are applied to ferrous and to some non-ferrous metals. On welds, the most commonly applied heat treatment, also referenced “Postweld Heat Treatment”, involves stress relieving after welding. In carbon and low-alloy steels, this heat treatment is accomplished by heating the weld deposit and adjacent base metal slowly to below the “transformation range” (or critical temperature) characteristic of the particular steel involved. The section is held at the temperature for a suitable time, alld is normally slowly cooled.

Other heat treatments also applied involve annealing, normalizing, tempering, quenching and tempering, sub-critical annealing, and solution heat treatment. Pressure vessels, tanks, boilers, piping and other components are generally stress relieved in accordance with the appropriate section of codes such as ASME, ANSI, BS, JIS and etc.

Localized heating as caused by welding, hot forming, flame cutting, or other manufacturing operations may cause metallurgical changes of significance to lead to service failures. Some of these are related to stress corrosion cracking. Non-uniformity in heat treatments may also do more harm than good, and may, in effect, increase the level of residual stresses. Proper postheat treatment utilising adequate equipment, and providing uniformity over the heating, holding and cooling cycles may thus be extremely important in assuring that the weldment meets the applicable service requirements.