Pressure Vessel Design part-4 Post Weld Heat Treatment(PWHT) as per ASME Div-1
by Admin
Posted on 12-09-2023 05:14 PM
Post weld heat treatment (
pwht
) is a controlled process in which a material that has been welded is reheated to a temperature below its lower critical transformation temperature, and then it is held at that temperature for a specified amount of time.
It is often referred to as being any heat treatment performed after welding; however, within the oil, gas, petrochemical and nuclear industries, it has a specific meaning. Industry codes, such as the asme pressure vessel and piping codes, often require mandatory performance of pwht on certain materials to ensure a safe design with optimal mechanical and metallurgical properties.
Post Weld Heat Treatment (PWHT)
There are three fundamental reasons post weld heat treatment is carried out:
to achieve dimensional stability in order to maintain tolerances, during machining operations or during shake-down service. To produce specific metallurgical structures in order to achieve the required mechanical properties. To reduce the risk of in-service problems such as stress corrosion or brittle fracture, by reducing the residual stress in the welded component. Stress relieving is one of the main heat treatments for the above objectives.
Otherwise known as hydrogen-induced cracking (hic), hydrogen bake out removes any atomic hydrogen from the material before it can seep into the metal structure as can sometimes occur as the material cools down after welding. Hydrogen can cause cracks to the material and render it unsafe and therefore the hydrogen bake out will force any hydrogen out of the metal before it’s used and by definition is a post weld heat treatment. Industry regulations state that hydrogen bake out’s should be administered immediately post weld before the metal has had a chance to cool to avoid all risk of hydrogen damaging the material.
Several factors contribute to pipe damage in pipeline systems, including corrosion, erosion, welding defects, material defects, vibration stress, and mechanical damage. In addition, damage occurring in the weld area is often related to weld quality in the heat-affected zone (haz) and residual stress. To enhance weld quality, one effective method is the implementation of post weld heat treatment (pwht). Pwht is a critical component of the heat treatment process to alleviate residual stress generated during welding. The pwht process can be carried out in two ways: full pwht by subjecting the entire object to heat treatment in a furnace (typically applied to complete equipment such as pressure vessels or boilers) or localized heating near the welding area only (commonly used for pipes or high-pressure vessels).
In case of alloy steel materials such as cr-mo stels, if the post weld heat treatment is not performed immediately after welding, then the weld joint and adjacent portion of pipe, at least 50 mm on either side of pipe, shall be uniformly heated to 300°c. This temperature shall be maintained for half an hour minimum, and then wrapped with mineral wool before allowing it to cool to room temperature. If the post heating temperature specified in the welding specification chart exceeds 300°c, the same shall be followed. Similarly, if the welding specification chart specifies post-heat time, the same shall be followed.
Localized pwht with ceramic mat heaters ceramic mat heaters are used for localized post weld heat treatment to provide a controlled and consistent heat source for specific areas of a welded joint. These heaters are typically made from high-temperature ceramic materials that can withstand extreme temperatures and provide a uniform heat distribution. During post weld heat treatment, ceramic mat heaters are placed on the area of the weld that needs to be treated. The heaters are then turned on and the temperature is gradually increased until it reaches the desired level. The heat is maintained for a specific period of time, depending on the type of material and the desired outcome of the heat treatment.
Some people suggest that extra-low carbon grades do not need any heat treatment at all, that they can be used in the as-cast condition. We strongly disagree with this claim since the initial heat treatment performs important functions other than dissolving the chromium carbides. Some people have pointed out that stabilized grades may not be immune to intergranular corrosion since the welding operation can dissolve the columbium carbides thus providing carbon for chromium carbide formation. We agree that this is possible but have not seen any work to prove or disprove it. Many tons of both cast and wrought stainless steels have been placed in service without pwht and have performed satisfactorily.