Post weld heat treatment (PWHT) is a controlled process in which a material that has been

welded Welding is a fabrication process that joins materials, usually metals or thermoplastics, primarily by using high temperature to melt the parts together and allow them to cool, causing fusion. Common alternative methods include solvent w ...

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 Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable s ...

and

nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: *Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics * Nuclear space *Nuclear ...

industries, it has a specific meaning. Industry codes, such as the

ASME The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing edu ...

Pressure Vessel and Piping Codes, often require mandatory performance of PWHT on certain materials to ensure a safe design with optimal mechanical and

metallurgical Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

properties. The need for PWHT is mostly due to the residual stresses and micro-structural changes that occur after welding has been completed. During the welding process, a high temperature gradient is experienced between the weld metal and the parent material. As the weld cools,

residual stress In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening im ...

is formed. For thicker materials, these stresses can reach an unacceptable level and exceed design stresses. Therefore, the part is heated to a specified temperature for a given amount of time to reduce these stresses to an acceptable level. In addition to residual stresses, microstructural changes occur due to the high temperatures induced by the welding process. These changes can increase hardness of the material and reduce toughness and

ductility Ductility refers to the ability of a material to sustain significant plastic Deformation (engineering), deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic def ...

. The use of PWHT can help reduce any increased hardness levels and improve toughness and ductility to levels acceptable for design. The requirements specified within various pressure vessels and piping codes are mostly due to the chemical makeup and thickness of the material. Codes such as ASME Section VIII and ASME B31.3 will require that a specified material be post weld heat treated if it is over a given thickness. Codes also require PWHT based solely on the micro-structural make-up of the material. A final consideration in deciding the need for PWHT is based on the components' intended service, such as one with a susceptibility to

stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SC ...

. In such cases, PWHT is mandatory regardless of thickness.

Application

Rate of heating, hold times and temperatures, and rate of cooling are all important variables that need to be controlled and monitored precisely, or the desired effects may not be achieved. When PWHT is mandatory by a given industry code, requirements for these variables will be specified.

Heating

The rate of heating when PWHT is performed is typically based on the component's thickness and is specified by the governing codes. If the rate of heating is not performed properly, either by heating too quickly or unevenly,

temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature spatial gradient is a vector quantity with Dimensional analysis, ...

s within the component can become detrimental to the component. As a result, stress cracks may occur and residual stresses not previously created can form when the component is cooled to ambient temperatures.

Holding temperature and time

Holding temperature and time are governed by the material and thickness respectively. Regarding material thickness, longer holding times are needed for thicker materials. This is to allow the material to reach a stable condition where the distribution and levels of stresses become more uniform and decrease. The specified holding temperature is one that is at a high enough temperature to relieve high residual stress levels, yet is still below the lower transformation temperature. In addition to the reduction of stress, high hold temperatures below the transformation temperature allow for microstructural transformations, therein reducing hardness and improving ductility. Great care should be taken as to not heat the component above the lower transformation temperature, as detrimental metallurgical effects and impaired mechanical properties can result. In addition, the holding temperature should not be greater than the original tempering temperature unless later mechanical testing is performed. Holding above the original tempering temperature can reduce the strength of the material to below

required minimums.

Cooling

As with the heating rate, the cooling rate must be controlled, as to avoid any detrimental temperature gradients that could cause cracking or introduce new stresses during cooling. In addition to this, rapid cooling rates can increase hardness, which may increase the susceptibility of a brittle fracture.

Monitoring technique

Thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...

s are typically attached to the component undergoing PWHT to check and ensure that heating rates, hold temperatures, and cooling rates meet code specification. Computer software is typically used in conjunction with the thermocouples to monitor the fore-mentioned variables and provide documentation that the PWHT was performed properly.

References