The Requirement For PWHT

Post weld heat treatment (PWHT) is a controlled process used to eliminate residual stresses caused by welding as well as temper hard, potentially brittle microstructure regions. Requirements for PWHT vary based on code specifications for fracture toughness requirements.

PWHT regulations for pressure vessels and piping typically start at 32mm thickness, though exact requirements vary according to code. However, exemption from PWHT can often be found.

Omezující tloušťka

Postweld heat treatment (PWHT) involves heating weldments beyond their yield strength in order to alleviate tensile residual stresses caused by welding, thus decreasing the chances of environmentally assisted cracking which could otherwise lead to leakage and failure of equipment.

PWHT is currently required for weldments made of low alloy steels used in pipe fabrication standards; in contrast, most general structural steelwork connections in bridges and buildings do not need PWHT testing.

This study’s objective is to establish whether it is possible to resolve conflicting rules between piping and pressure vessel codes regarding thickness requirements above which PWHT must be conducted and Charpy test requirements, using similarities that do exist between them – such as using specially designed repair procedures and specifying minimum Charpy energy calculated using fracture mechanics analysis as indicators of success.

Charpy Test Requirements

The Charpy Impact Test is an assessment method of metals and alloys to measure their toughness. The process entails striking the specimen with a pendulum hammer, measuring how much energy was absorbed, and then identifying whether the material is ductile or brittle. It may be combined with other tests, such as J-Da ductile tearing resistance curves to ascertain whether an assembly can withstand impacts under stress.

Charpy test results are reported in joules, an SI unit for energy, and usually plotted as a load-time curve (Figure 3). An advanced version of this test known as an instrumented Charpy test allows engineers to accurately capture crack progression while measuring strains during fracture.

Even though pressure vessel and piping standards often call for PWHT in excess of certain thickness thresholds, as depicted in Table 1, rationalization of differing PWHT requirements in an as-welded condition could prove challenging due to varying steels, welding processes and quality levels that result in different Charpy impact toughness requirements.

Výjimky

Weldability (weldability) of structures and components is crucial to their safe operation. This can be ensured by four factors: (1) ease of weldability, (2) suitability of welding process, (3) condition of material after welding, and (4) environment conducive for welding.

Review of chromium-molybdenum materials used in nuclear service has revealed significant opportunities to relax thickness requirements and Charpy test requirements, thereby creating significant savings opportunities both financially and through reduced outages. This could provide utilities with significant cost-savings in both dollars and outage time.

Interviews with industry experts revealed that current PWHT requirements for nuclear-grade SS-316L and CS-430 weldments are driven more by successful practices found within petrochemical and power industries rather than technical data, design calculations or experimentation; this trend may have serious ramifications for nuclear piping projects in future.