PWHT 및 탄소강 용접

Carbon and carbon-manganese steels and low alloy steels meeting hardness requirements standards generally do not need PWHT treatment, although differences between codes for pressure vessels and piping (as detailed in Table 1) often complicate rationalisation processes.

This article’s purpose is to describe and outline PWHT requirements and exemptions applicable to carbon and low alloy materials.

Stress-Relieving

As welding causes residual stresses to remain within metals, particularly low alloy carbon steels, these may lead to cracking and weakening. To alleviate these internal tensions, the material undergoes PWHT heat treatment, where temperatures lower than its transformation temperature are used and it soaks for an extended period of time before cooling uniformly across its cross section and surface area.

PWHT requires temperatures that can relieve welding-induced stresses while simultaneously avoiding metallurgical phase changes and temper embrittlement, so heating and holding times must be closely managed in order to maximize benefits and ensure their realization.

Pressure Wave Heat Treating, commonly used on pressure equipment, but also performed on other structures like bridges and buildings, should be understood so as to make informed decisions regarding any structures you build or renovate. It’s vital that when this process is necessary and its benefits, so as to make decisions with maximum knowledge regarding their construction.

Mechanical stress relief (MSR) may offer one method for relieving residual stresses, but it doesn’t offer the same metallurgical advantages that PWHT does; thus it shouldn’t be considered an alternative solution. MSR may still prove useful when moving parts directly into an oven for PWHT is impractical or not possible, but shouldn’t be seen as a replacement treatment option.

Temperature Change

Dependent upon the welding procedures employed, residual stresses may exceed material yield strength and lead to brittle failure in the weld area. PWHT reduces these stresses by redistribution thereby decreasing risk of failure in carbon steel structures welded using PWHT welding procedures.

PWHT welding treatments not only reduce stress relief, but they can also be used to soften and soften hard welded structures – increasing ductility while decreasing environmental assisted cracking risk – which is especially useful when welding welds for sour service piping applications.

PWHT changes can help reduce hydrogen-induced corrosion in carbon steels and increase their fatigue performance, as well as lower risk. It should be noted, however, that PWHT is distinct from tempering, solution treatment or ageing processes (although some of their effects may be achieved via PWHT).

PWHT requirements are defined in various codes, with thickness limits typically being set at 32mm for pressure vessels and piping applications requiring PWHT. There may also be variance between codes due to differing Charpy energies or inspection standards as well as chemical composition variations of carbon or C-Mn steels they cover, thus making rationalization unlikely.

Weld Defects

Residual stresses can cause weld defects that are both visible and invisible, including discontinuities, porosity and spatter; visible defects include weld discontinuities, porosity and spatter; invisibly detected defects include incomplete fusion, low ductility and poor mechanical properties. Residual stresses also compromise welds’ resistance to stress corrosion cracking while increasing their susceptibility to fatigue failure – especially with complex structures or long-term service applications.

As a rule of thumb, the higher the carbon and alloy content in weld materials and cross-sectional thickness of structures is, the higher is their potential need for post-weld heat treatment (PWHT). This is because welding residual stresses reduce fracture toughness in their tempered martensite state and thus require PWHT.

However, PWHT requirements do have some exceptions. According to current fabric standard rules, certain structures may be exempt from PWHT requirements if welded using specifically designed repair procedures and specified with an energy factor calculated using fracture mechanics approaches.

Welding is an active process and the weldments it produces can experience significant strain during its cooling phase, creating stresses which must be managed in order for these weldments to be used in critical applications. This can be accomplished by decreasing electrode travel speed, restricting current usage during welding operations and using shielding gases with appropriate composition for material type and thickness.

Safety

Welding is an integral component of building and maintaining oil, gas, and chemical processing assets. However, improper weld performance can inadvertently weaken equipment by inducing residual stresses into materials and weakening strength. To mitigate this effect, post weld heat treatment (PWHT) should be regularly performed following welding in order to minimize residual stresses in weld material, control hardness levels after welding processes, and in some instances increase mechanical strength.

PWHT is an insulate procedure which uses high temperature resistance heaters to raise weld temperatures up to around 300degF-1,125degF depending on the type of steel and its carbon content. Heat is applied using an electric resistance heater designed for pipe size that fits atop weld to be treated. All electricians involved with an installation must understand its safety implications during PWHT operations; all connections should be cordoned off properly while the area should be cordoned off as a danger zone to protect unknown individuals coming in contact with high voltage electrical cables.

PWHT requirements differ between fabrication codes. For instance, the thickness threshold at which PWHT becomes necessary varies significantly; for instance BS 1113 [22] and 2633 [23] limit it for carbon-manganese steels with up to 0.25% carbon content while PD 5500 and Pr EN 13445 allow its use on weldments up to 140mm thick, provided they meet an established fracture mechanics toughness requirement.