Post Weld Heat Treatment (PWHT) for Carbon Steel

Post-weld heat treatment, or PWHT, is an integral component of the welding process, serving to alleviate residual stresses caused by welding while tempering hard or potentially brittle microstructural regions.

PWHT can be completed in furnaces powered by electricity, natural gas or oil; however, in this article we’ll focus on what temperature must be reached to achieve optimal results with carbon steel PWHT.

Thermodynamics

Welding is one of the most widespread engineering manufacturing processes. However, after welding residual stresses may form that lead to operational difficulties. To alleviate these internal tensions Post Weld Heat Treatment (PWHT) can help relieve residual stresses by softening hardened zones, improving microstructure and chemistry characteristics, lowering hydrogen content content in weld area and relieving residual stresses; however excessive or inappropriate temperature or holding times adversely impact mechanical properties [1].

Carbon and low alloy steels require PWHT in order to reduce residual stresses, control material hardness and enhance mechanical strength. Each specific material’s requirements for PWHT temperatures depend upon its code of use; PWHT temperatures for certain chemical compositions or thickness ranges such as for piping and pressure vessel grade steel with less than 1.5% chromium content typically require temperatures higher than 1050degC as defined by code requirements. For instance, when used for pipes and pressure vessels with such requirements as for example with 1.5% chromium content steel requires PWHT temperatures above 1050degC to enhance mechanical strength as set forth by code requirements imposed by codes that regulate how the PWHT will function and meet these objectives effectively.

However, general structural grades such as BS 5400 for bridges, BS 5958 for buildings and EEMUA 158 for offshore structures allow significantly thicker as-welded thickness limits linked with greater Charpy energy requirements. These differences appear to be related to factors like preheat temperature, alloy content and martensite formation – this article’s goal is to help readers better comprehend these influences when determining a PWHT temperature for your steel.

Microstructure

Some materials require Post Weld Heat Treatment (PWHT) in order to achieve desired strength and ductility. PWHT involves heating welded fabrications between 580degC and 620degC for 1 hour per 25mm thickness in order to relax any residual stresses created during welding; this helps ensure sufficient tensile, Charpy impact strength as well as decreased risk of fracture.

STEM microscopy was employed to examine dislocation structure and alteration of MX carbonitrides at various PWHT-times. An as-welded specimen displayed a high density of severely tangled dislocations within its bainitic matrix; no ordered structures such as dislocation nets or subgrain boundaries could be discerned at this stage. By increasing PWHT time, dislocation density decreased substantially; new ordered structures such as dislocation nets or subgrain boundaries became evident, as did coarsened MX carbonitrides which reduced their pinning effect upon dislocations; ultimately leading to subgrain boundaries becoming visible again.

At PWHT times of 8 and 16 h, the weld metal morphology improves with lower dislocation densities; however, dislocation density remains much higher than quenched steel. Furthermore, precipitates at grain and subgrain boundaries have coarsened significantly due to Ostwald ripening of MX carbonitrides, leading to larger particles on grain/subgrain boundaries than within grains themselves.

Stress Relief

PWHT of carbon and carbon-manganese steels is an essential step after welding to minimize brittle fracture. This process entails heating the weld area to high temperatures for an extended period, then cooling. At higher temperatures, stress distribution becomes more uniform, thus decreasing residual stresses while tempering softens hard microstructural regions of base material that could otherwise lead to cracking.

Welding introduces high levels of residual stresses into metals, particularly thicker welds. When combined with service load stresses, these residual stresses can result in reduced fracture toughness and susceptibility to stress corrosion cracking (SCC). PWHT serves to alleviate these residual stresses and enhance mechanical properties of the joint being weld together.

PWHT requires subjecting structures to high temperatures for long hold times, which can be challenging and expensive for larger structural fabrications. To ensure proper insulation and heating in the weld area, with an accurate temperature reading from an digital recorder/controller.

An improper PWHT can reduce material strength. Furthermore, an inefficient cooling rate during PWHT can result in distortions and reduce its final tensile strength.

La force

As noted above, carbon steel’s tensile and yield strengths tend to decrease with increasing temperature; however, its ductility (KJc) increases due to tempering producing larger martensite-austenite constituents that enhance impact toughness.

As such, it is crucial that PWHT conditions be tailored not only to the properties of steel delivered as-delivered but also to anticipated service conditions of the component welded. Therefore, steel-makers must take this into consideration when determining a tempering temperature for their mill products.

Construction codes and standards (such as BS 1113 and ASME VIII ) often stipulate a lower limit for PWHT thickness. These limits are determined based on an assumption that both tensile strength and yield strength remain within acceptable parameters after PWHT has taken place.

Of course, this isn’t always possible and that’s why any pipe or vessel designed for hydrogen service must undergo post-weld heat treatment (PWHT). Both its temperature and duration must be carefully chosen to bring about desired metallurgical changes while meeting all relevant code/standard/contract specifications; expert advice must also be sought regarding times and temperatures specific to carbon steel types.