B31.3 PWHT Table

The B31.3 Code defines rules for piping systems used in oil refineries, chemical, paper & pulp, semiconductor manufacturing facilities, power generation stations and similar processing plants. This document specifies requirements for designing, fabricating, assembly erection examination inspection & testing of process piping.

An EPRI report (Ref 1) has recommended that the PWHT temperature for 1-1/4 Cr materials should be reduced, and this report will support that change within B31.1 and B31.3 codes.

1. PWHT Temperature Range

Different codes governing pressure vessels and piping vary considerably on various issues, including required PWHT temperatures. Most differences arise due to industry traditions rather than specific metallurgical or structural considerations; as a result, attaining uniform PWHT requirements across codes has proven challenging.

PWHT requirements depend on multiple factors, including temperature, duration and grain refinement requirements of material being treated; phase transformations; weldability considerations must also be made when considering high temperature PWHT (high-temperature treatments may result in cracking and spalling issues); additionally weldability should always be carefully taken into account as overheated materials could result in poor weldability when welding is involved.

EPRI-sponsored tests (Reference 1) on P No 4 materials have revealed that their lower critical temperatures are quite close to those specified in B31.1 and B31.3; suggesting that minimum PWHT temperatures could potentially be reduced without negatively affecting weldments they are used in.

Other elements that can influence PWHT requirements are chemical composition of broadly comparable steels, different Charpy test requirement limits and inspection standards; hence achieving compliance for exemption from PWHT in carbon steels depending on their chemical makeup isn’t easy.

2. Preheat Temperature

Preheat temperature is the minimum temperature that must be achieved before welding begins on certain materials, particularly thick ones like Cr-Mo Steels & HSLA High Speed Low Alloy Steels. Preheating also helps prevent cold spring and other stresses from developing in weldments and prevent cold springing during welding operations.

Preheat requirements can differ based on control thickness and nominal thickness. Control thickness serves as the benchmark thickness that requires PWHT; nominal thickness determines whether postweld heat treatment (PWHT) is needed at all; for instance carbon steel adhering to P-No 3 material group in its nominal thickness exceeding 25mm is exempted from such treatment.

3. Preheat Time

PWHT time is an integral component of fabricating/ welding process piping, used to determine whether postweld heat treatment will be required or not. As well as preheat temperature and range specifications, holding time specifications, and cooling range specifications; all must be considered when setting PWHT times.

ASME B31.3 (2014) introduces a table which allows carbon steel pipe with all control thickness and weld type welds exempted from mandatory postweld heat treatment when certain amounts of preheat are used, depending on its base weld metal group requirements.

4. PWHT Time

PWHT time is an integral component of its efficiency. As with any process, longer heat treatment duration increases energy costs and therefore costs associated with PWHT; so ensuring its duration as short as possible is of key importance in its successful execution.

Pipe, pressure vessel and structure weldments can be exposed to harsh environments ranging from high temperatures and corrosive chemicals, which require special protection to maintain structural integrity. In such an environment, PWHT treatment is often essential in mitigating issues like thermal fatigue and stress corrosion cracking – high strength steels and low alloy materials being particularly susceptible. PWHT therapy offers one solution.

PWHT can be an expensive process due to its lengthy hold times and slow heating and cooling rates, not to mention that high temperatures used during PWHT could potentially damage weldments if handled incorrectly.

Because of these concerns, several steel users have advocated for changes to the PWHT thickness limits set forth in B31.1 and B31.3 codes. This article investigates similarities and variations in exemptions from PWHT in various fabrication standards used by industries like EEMUA (petrochemical industry) and power generation (power generation industry), along with potential rationalisation solutions.