PWHT Acero inoxidable

Stainless steels are widely utilized in industrial equipment due to their corrosion resistance and weldability, yet can only be used in high temperature operating environments if post weld heat treatment (PWHT) is undertaken to prevent embrittlement of sigma phase embrittlement and subsequent cracking.

PWHTs are often conducted in an atmosphere such as argon or nitrogen to minimize formation of harmful phases that cause corrosion resistance to decrease and mechanical properties to degrade over time.

Corrosion Resistance

Chromium is the main element in stainless steel, creating a protective oxide layer against further oxidation and corrosion. Addition of other alloying elements such as nickel (Ni) or molybdenum (Mo) may further improve corrosion resistance depending on environmental conditions and application needs; grades typically depend on industry needs or application needs to determine their composition.

Stainless steel does not melt at high temperatures like many other metals; however, welding processes may produce intense heat and rapid cooling cycles that cause microstructural changes to occur within it. Such as coarsening of weld metal (HAZ) and chromium carbide precipitation. Such modifications may compromise its strength, ductility and corrosion resistance capabilities.

Welding can contribute to galvanic corrosion, which occurs when two metals come into contact. For instance, stainless steel welds may corrode due to galvanic attack caused by copper (Cu) present in the weld metal reacting with oxygen from weld gas.

Duplex stainless steels such as 17-4 and PH13-8Mo provide greater corrosion resistance than austenitic grades such as 304 and 316; however, they still can be susceptible to sensitization and intergranular corrosion. To overcome this problem, duplex stainless steel welds should be placed in an environment free from corrosion; alternatively post weld heat treatment (PWHT) may reduce sensitization sensitivity as well as promote good tensile strength and hardness after welding.

Mechanical Properties

Stainless steels are generally strong and resilient materials; however, they are susceptible to changes during welding processes. When exposed to extreme temperatures during welding processes, weld metal and HAZ may become brittle due to rapid thermal cycling processes causing microstructural changes within its fabric and weld/HAZ materials.

PWHTs should always be applied after welding to help protect and preserve the integrity of components welded together, although their necessity depends largely on the type of weldment and its intended service conditions.

Austenitic chromium-nickel grades that are designed for use in severe corrosion environments may require PWHT in order to minimize sensitization upon elevated temperature exposure, while grades intended for less aggressive applications or previously heat treated may not require this additional process.

Recent studies have explored the effects of PWHT on plasma arc welded 316L austenitic stainless steel weld microstructure and mechanical properties during plasma wire heat treatment (PWHT). They discovered that as PWHT temperatures increase, so does ferrite content; however tensile strength, fracture toughness, uniform and total elongation all decrease with each increase in time spent PWHTing.

Welding Process

Welding can have a dramatic effect on the mechanical properties and corrosion resistance of stainless steel, but by choosing suitable welding parameters and applying postweld heat treatments engineers and welders can minimize these side effects and maximize its potential.

Welded stainless steel may undergo post-weld heat treatment (PWHT) to alleviate residual stresses and improve weldability, particularly for larger or thicker sections of material. Unfortunately, PWHT may also cause distortion to the weld section; to limit this possibility it should be left loose enough that its expansion and contraction doesn’t cause distortion at key connections.

PWHT not only reduces stress but can also lower susceptibility of sigma phase embrittlement and elevated temperature creep damage by minimizing temperature gradient between stress relief and solution anneal temperatures.

Under different welding heat inputs, optical microscopy was employed to assess the effects of welding on a weld overlay’s microstructure. Ferrite content increased with increasing welding heat inputs; this indicated that delta ferrite had been preferentially transformed to sigma ferrite during fusion, leading to weight loss measured through pitting corrosion resistance testing in an artificial corrosive environment.

Requirements

Post-weld heat treatment of stainless steels should maintain their mechanical properties. This is particularly true of austenitic grades where an austenitized surface layer formed by the formation of a passive layer containing chromium oxide remains stable after welding to minimize corrosion issues; PWHT treatment however could dismantle this barrier and therefore diminish corrosion resistance of these materials.

Therefore, it is crucial that individuals know when stainless steel requires postweld heat treatment or not following welding. The need for PWHT depends heavily on its grade, expected service conditions and welding procedure.

Many industrial applications that employ stainless steel do not necessitate PWHT, such as welds in pressurized plants. This can be attributable to advanced welding techniques, smaller welds, and an environment without intergranular corrosion issues.

There are certain weldments which require PWHT, such as those located in hostile environments or high-stress conditions. Grain boundary liquation cracking can lead to significant ductility loss and increased susceptibility for chloride stress corrosion cracking; PWHT works by heating the joint at temperatures that encourage fine niobium or molybdenum carbide formation while simultaneously suppressing growth of the ferrite part of an alloy called the “sigma phase,” otherwise known as thermomechanical stabilization.