Post Weld Heat Treatment (PWHT) Requirements for Carbon Steel Pipe

Many projects involving carbon steel piping may bypass PWHT by taking advantage of BS 2633’s and similar codes’ tolerance of thicknesses up to 19mm without PWHT for low alloy steels with lower carbon content; such requirements tend to meet or surpass those set forth by U.S. codes; however BS 2633 stipulates a higher preheat requirement.

Procedure di saldatura

PWHT (Post Weld Heat Treatment) is not just another procedural step: it is an integral engineering process that directly affects equipment integrity, worker safety and long-term performance. PWHT reduces residual stresses while refining microstructures to prevent stress corrosion cracking and to avoid stress corrosion cracking altogether.

PWHT involves heating the weldment to an elevated temperature for an extended period, allowing its metal matrix to absorb and redistribute internal stresses in order to reduce risk of brittle fracture, improve toughness and ductility, and prevent hydrogen damage.

To achieve optimal PWHT levels, welding procedures must include measures for both preheating and postweld heat treatment, including selecting appropriate preheating types and temperatures as well as monitoring heating/holding temperatures and implementing quality control measures.

PWHT alone may not be sufficient to reduce hardness and increase ductility of material used in welding; additional processes may need to be conducted by highly-qualified welding staff.

Preheating

Recent research developed an experimental and analytical method of preheating during multi-pass repair welding using thermovision cameras and heat transfer analysis; both methods proved accurate.

Since the 2014 edition of ASME B31.3, thick carbon steel piping has been exempt from PWHT as long as it is preheated to above critical austenitic temperature and welds are performed using multi-pass welding processes – an important difference from previous requirements.

While these new exemptions may help save money, they come with risks that should be carefully considered before choosing them. These welds could potentially suffer corrosion and service issues; additionally, lack of PWHT requirements could result in improper design that necessitates rewelding in some instances; preheating is recommended to prevent this.

Postweld Heat Treatment

Postweld heat treatment (PWHT) involves heating weldments to a specific temperature for a brief time and then slowly cooling them, helping relieve stress and increase toughness of weldments. However, PWHT must be performed carefully to avoid either over- or undertempering which weakens material properties.

PWHT involves heating the weldment to temperatures between 550-625 deg C for about an hour per inch of thickness, changing its microstructure from austenitic to equiaxed twinned martensite and altering its mechanical properties by increasing toughness while decreasing hardness.

PWHT may be necessary in certain instances of structural weldments; however, in others it isn’t. ASME B31.1 and B31.3 allow limiting wall thicknesses of C-Mn steel pipe used in pressure vessels, piping systems and storage tanks without postweld PWHT provided multipass welding is utilized and preheating measures are implemented.

Qualifiche

Table 331.1.3 provides details of significant modifications made in 2014 to PWHT requirements for carbon steel process piping that was subject to PWHT measurements, in response to concerns that previous PWHT thickness limit exemptions may have been too restrictive or excessively conservative.

EPRI-sponsored tests (Ref 1) demonstrated that P No 4 materials underwent dramatic fluctuations in hardness and toughness as the heat treatment temperature increased, particularly at dissimilar material joints between P No. 1 and P No 4 materials.

The 2014 modifications to B31.3 allow all carbon steel materials that fall into the P No 1 material group and all control thicknesses and weld types to be exempted from postweld heat treatment if a preheat of at least 95 degC is used prior to welding. This approach is similar to what BS 2633 in Britain recommends; however, an NDE inspector must still verify that all processes complied with Code requirements.