溶接後の熱処理

Many piping codes use fracture toughness of weldments as the basis for waiving postweld heat treatment (PWHT) requirements locally; however, this method may not always apply across materials or applications.

PWHT requirements for power piping material generally rely on the lower critical transformation temperature of its base metal.

Welding Procedures

Weldability for welded joints depends on various factors, including geometry, re-straint, welding preheat temperatures and interpass temperatures as well as current density. Furthermore, low carbon materials like AISI P-4 or P-5A also influence weldability; hardenability may also have an effect as may hydrogen delayed cracking potential.

Post weld heat treatment (PWHT) serves to improve notch toughness by relieving residual stresses and relaxing preexisting workpiece stresses caused by fabrication, welding and cutting processes. PWHT processes typically last one hour per 25mm of thickness at 600degC temperature.

PWHT requirements are typically driven by weldability criteria and fabrication codes provide detailed requirements with respect to local PWHT. A significant portion of these regulations focuses on thickness limit requirements – an assumption made based on brittle fracture being more likely without PWHT than with it – however this assumption has been challenged through fracture mechanics analysis; more specifically comparing master curves for girth butt welds in steel pipes has demonstrated this conclusion and found current PWHT thickness limits too restrictive in some fabrication codes.

Preheat

Preheating steels before welding is essential in minimizing welding stress and residual stresses, depending on thickness of weldment and alloy content. Temperature indicating crayons or thermocouple pyrometers should be used to monitor these temperatures accurately.

Preheating reduces the critical velocity gradient for any given equivalence ratio and increases laminar flame speed, creating more stable arcs with reduced spatter. (Fig. 20).

Preheating is used to increase weld metal and heat-affected zone (HAZ) hardness for carbon and low-alloy steels by slowing their cooling rate, thus decreasing defects such as porosity and voids in the HAZ, improving mechanical properties, and decreasing hydrogen embrittlement cracking (also referred to as delayed or underbead cracking) risk in carbon/low alloy steels as well as high nickel austenitic stainless steels such as Type 347/Incoloy 825.

Preheat and inter-pass temperature reductions may help increase weld thickness without postweld heat treatment (PWHT), especially for high pressure retaining weldments such as butt, socket, and fillet welds on threaded joints and nozzle connections. Based on E2G’s studies of brittle fracture, eliminating mandatory PWHT for low-alloy welded-to-pressure parts under ASME B31.3 would decrease weld toughness while increasing risk of brittle failures.

溶接後の熱処理

Post Weld Heat Treatment (PWHT) is an effective means of eliminating detrimental residual stresses in welded components such as pressure vessels and pipes, while attenuating their susceptibility to environmental conditions that cause crack initiation or brittle fracture. Compliance with code requirements should always be observed so as to maintain integrity of fabricated weldments.

PWHT processes are generally required for carbon steel weldments with thickness exceeding a predefined threshold to protect them from hydrogen delayed fracture and other stress corrosion phenomena, including hydrogen delayed fracture. PWHT involves heating, cooling and soak conditions which affect weld material properties resulting from this process – excessive or prolonged soak times can result in decarburization, debrittlement or embrittlement which will negatively affect its tensile strength, creep strength or notch toughness properties.

PWHT requirements for carbon steels vary across fabrication and repair codes depending on thickness and diameter limits, with variations possibly attributable to different interpretations of technical database or evolving fabrication/welding practices over time. Current ASME BP&V codes permit exemptions from PWHT testing up to 5/8 inch thick material used in nuclear service based on assumptions such as increased weldability from large diameter, thin walled materials; their inherent fracture toughness characteristics; being sufficient enough to support low energy fracture.

Cooling

PWHT procedures must be tailored specifically for each component, with temperatures depending on its thickness; ASME VII applies above 19mm while BS EN 13445 and BSPD 5500 cover 35mm or greater thicknesses. Cooling rates also depend on thickness; one hour for every 25mm of thickness and still air cooling is permissible below 800degF. PWHT will usually use either permanent fixed furnaces or portable bogie-loaded top hat type furnaces for large unwieldy parts.