Post Weld Heat Treatment and Corrosion Resistance

Industry-specific welding codes often mandate PWHT for certain materials. PWHT helps ensure the integrity of welded components under extreme conditions in environments such as fossil fuel power plants, nuclear reactor vessels and other types of pressure equipment.

Process can be time consuming and require special equipment. Distortions or warping of welded equipment could occur as a result of this procedure, necessitating corrective actions to rectify it.

Reduction of Residual Stresses

Post Weld Heat Treatment (PWHT) reduces residual stresses, controls material hardness and enhances mechanical strength after weld completion. PWHT should be performed after welding pressure vessels, pipes and nuclear power plants to mitigate weld failure risks such as higher cracking potential or susceptibility to brittle fractures; failure to do so can result in weld failure and increased susceptibility to fracture. PWHT offers an efficient and cost-effective method of meeting strict industry standards while meeting them more safely.

Dependent upon the application, residual stress levels in weld metal, heat affected zone and adjacent base metal can reach yield strength – in such instances PWHT can relieve and redistribute these residual stresses to improve actuation strain of welds while mimicking structural behaviour similar to base material.

This paper investigates the effects of uniform PWHT on X joint welded HSS plates with slightly overmatching filler metal. Sections were mechanically separated and subjected to various PWHT cycles and tensile testing before macroscopic and microscopical analysis was conducted on them. Results show that PWHT-induced changes such as formation of twinned martensite in HAZ and FZ can significantly lower residual stress levels to levels below that found in BM, while also significantly reducing stress raisers in weld area as well as improving actuation strain to levels that approach that value found in BM.

Hardness Control

Post weld heat treatment (PWHT) can reduce residual stresses caused by welding processes that leave behind residual stresses that could otherwise lead to hydrogen induced cracking or stress corrosion cracking issues. PWHT involves heating material then slowly cooling it again – helping redistribute and relieve stresses while improving ductility which increases resistance against stress cracking.

PWHT can also be used to induce phase transformations or precipitation that will improve strength and toughness of materials, but in order for this process to work successfully it requires many variables be controlled such as the temperature and duration of exposure of material.

Conducting a hardness test to assess whether a workpiece requires PWHT is relatively straightforward. Fill a sample cell with water, add buffer and indicator reagents, stir, then count how many drops were necessary to reach an endpoint (usually red and blue), which some photometers provide the option to test calcium hardness separately from total hardness; this gives an accurate picture of your material’s quality. You may occasionally hit a purple endpoint instead of blue; this is typically caused by interference from copper (from algaecides or pipes) but can be corrected by adding extra drops of titrant at the start.

Strength Enhancement

Post weld heat treatment not only reduces and redistributes residual stresses, but it can also produce metallurgical changes that increase strength. At higher temperatures used during this process tempering or precipitation may occur which lowers hardness while improving ductility and decreasing fracture risk.

PWHT equipment provides significant advantages to its users. It can help ensure welds can withstand high stress environments without failing or leaking, which is especially relevant to oil and gas refineries, petrochemical plants and nuclear power plants where equipment must withstand severe conditions.

PWHT processes can be costly due to their dependence on specialized facilities and equipment as well as time-consuming heating/cooling cycles, increasing project costs and delaying timelines. Repeated heating/cooling cycles may cause thermal fatigue in some alloys reducing their lifespan over time.

Intertek provides PWHT services to enhance weld microstructure and reduce residual stresses in welded components. We have extensive experience performing PWHT for applications spanning oil & gas, petrochemical, power generation, chemical processing and steel/metal fabrication industries – as well as both carbon and stainless/high alloy materials, meeting ASME Section VIII specifications or specific thickness limits.

Korrózióállóság

Corrosion resistance is one of the key characteristics that enable metals to be utilized in numerous practical applications. It stems from both its intrinsic corrosion resistant properties as well as any protective measures taken against corrosion damage.

Chrome in stainless steel helps form a passive layer of chromium oxide that deters oxygen from penetrating into its core metal and thus prevents corrosion and rusting. Other elements like nickel, manganese and molybdenum also increase corrosion resistance in stainless steels.

Lack of corrosion resistance can be fatal to biomedical equipment. Metal stents used in transcatheter heart valves require excellent corrosion resistance; failing to meet this standard could result in mechanical failures of the stent and negative biological responses that threaten both patient and healthcare provider alike.

PWHT improves corrosion resistance of materials by tempering or precipitating them to lower hardness levels while simultaneously improving toughness and ductility, helping reduce weld failures as well as risk of overload fracture. PWHT is frequently required in various industries – work-hardened steels, carbon and low alloy steels, plus oil & gas and nuclear industries often mandate PWHT when repairing pressure equipment.