Post Weld Heat Treatment

Post weld heat treatment (PWHT) is a process designed to relieve and redistribute any residual stresses generated during welding, while producing metallurgical changes that increase strength, ductility and toughness.

PWHT may be mandated by industry codes for some metals used in pressure equipment; however, its necessity must be evaluated carefully against its cost and potential adverse effects on a material’s properties.

Residual Stresses

Residual stresses can arise during welding due to temperature gradients between weld material and parent material, and can reach critical levels, leading to cracking or stress corrosion cracking of components with thick walls. Most pressure vessel and piping codes stipulate postweld heat treatment (PWHT) should weldments exceed certain design yield strength thresholds so as to limit potential brittle fracture.

PWHT is a process designed to eliminate or redistribute residual stresses and promote tempering, precipitation and ageing effects in weld metal, ultimately increasing toughness, ductility and resistance against stress corrosion cracking. For an effective PWHT process, however, an increased understanding of its mechanisms must be acquired.

To achieve this, finite element analysis (FEA) is used to simulate stress fields in both elastic and elastic-visco-plastic analysis models for weldments with various wall thickness. A comprehensive creep modelling approach is also employed in order to understand how plastic strain and back stress contribute to residual stress relief.

Results indicate that PWHT can produce satisfactory residual stress reduction and material property enhancement across a wide range of weldment materials, with final tensile strength reaching close to design value following treatment. Further investigations should focus on creating an appropriate Time-Temperature-Thickness relationship for thicker weldments undergoing PWHT to minimise risks related to brittle fracture and enhance overall reliability of their weldments.

Hardness

Hardness of metals is an indicator of their microstructure, and can have a dramatic impact on whether welding processes or filler metals are suitable for certain applications. While excessively hard microstructures tend to be less ductile and tough than softer metals, heat treatment may sometimes be required in order to enhance mechanical properties or decrease susceptibility to cracking.

PWHT (Process Warm Heat Treatment) is a form of heat treatment used to eliminate residual stresses and enhance weld strength, ductility and toughness. PWHTs may also help prevent cracking due to thermal cycling or stress corrosion cracking; their need will depend on metal type/thickness/dimension as well as welding parameters/service requirements.

Codes often stipulate that welding materials or weldments over a certain thickness require postweld heat treatment for protection. This is due to certain metals being more vulnerable to stress corrosion cracking or having higher risk of weld failure than others, while different welding processes produce harder deposits than others.

Toughness

Welding can produce residual stresses that, combined with load stresses, compromise material toughness. A controlled heating and cooling process known as PWHT can help improve mechanical properties, relieve residual stresses and increase strength and hardness of weldments; its necessity depends on several factors including welding parameters and service requirements; fracture mechanics analysis using assumed stress levels and flaw sizes can help identify whether PWHT is required.

An easy model was devised to calculate the minimum required toughness for specific materials and section thickness. Fracture toughness requirements (K mat) were then estimated for two different probabilities of failure, P f = 0.05 and P f = 0.5 using Annex J of BS 7910’s Master Curve correlation between fracture toughness and Charpy energy.

As shown in the figures below, results from our analysis reveal that even a relatively minor decrease in PWHT temperature can significantly lower estimated minimum requirements for K mat. This is likely due to PWHT’s effect of reducing crack-tip constraint and improving crack extension by ductile tearing; additionally, results demonstrate PWHT can also dramatically improve laser welded samples’ impact toughness by recovering their original brittle fracture mode with river patterns observed with untreated samples.

Ductility

Ductility refers to a material’s capacity for plastic deformation before cracking under stress, as opposed to elastic deformation which reverses upon stress release. Ductility is an integral factor when designing components as it shows how much strain a component can withstand before it fails; greater ductility indicates greater chances of its deforming under extreme loads.

Ductility of materials depends heavily upon their chemical composition, crystal structure and testing temperature. Metals tend to exhibit greater ductility as their metallic bonds enable valence shell electrons to be shared among multiple atoms and thus allow metal atoms to move past each other easily while still absorbing forces that would cause other materials to shatter.

Although closely associated with malleability, ductility and malleability remain distinct properties. Ductility refers to a metal’s ability to withstand tension stretching forces while malleability refers to how well it handles compressive stresses caused by hammering or pressing. Both properties should be taken into consideration when choosing materials for specific applications; high ductility materials like gold can only be stretched until breaking occurs while lower ductility materials such as copper may only stretch to very small thicknesses before snapping under strain.