Post weld heat treatment

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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. [1] 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. [2] [3]

Contents

The need for PWHT is mostly due to the residual stresses and micro-structural changes that occur after welding has been completed. [2] During the welding process, a high temperature gradient is experienced between the weld metal and the parent material. As the weld cools, residual stress is formed. [2] 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. [1] In addition to residual stresses, microstructural changes occur due to the high temperatures induced by the welding process. [1] These changes can increase hardness of the material and reduce toughness and ductility. The use of PWHT can help reduce any increased hardness levels and improve toughness and ductility to levels acceptable for design. [1]

The requirements specified within various pressure vessels and piping codes are mostly due to the chemical makeup and thickness of the material. [1] 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. [1] Codes also require PWHT based solely on the micro-structural make-up of the material. [1] 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. In such cases, PWHT is mandatory regardless of thickness. [4]

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. [3] When PWHT is mandatory by a given industry code, requirements for these variables will be specified. [3] [4] [5]

Heating

The rate of heating when PWHT is performed is typically based on the component’s thickness and is specified by the governing codes. [1] [6] If the rate of heating is not performed properly, either by heating too quickly or unevenly, temperature gradients 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. [4]

Holding temperature and time

Holding temperature and time are governed by the material and thickness respectively. [4] [6] Regarding material thickness, longer holding times are needed for thicker materials. [4] This is to allow the material to reach a stable condition where the distribution and levels of stresses become more uniform and decrease. [2] [6] 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. [1] [2] In addition to the reduction of stress, high hold temperatures below the transformation temperature allow for microstructural transformations, therein reducing hardness and improving ductility. [6] 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. [6] 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 ASME required minimums. [4]

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. [4] In addition to this, rapid cooling rates can increase hardness, which may increase the susceptibility of a brittle fracture. [7]

Monitoring technique

Thermocouples 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. [5]

Related Research Articles

Heat treating Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Austenite Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

Carbon steel Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05% up to 2.1% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Piping

Within industry, piping is a system of pipes used to convey fluids from one location to another. The engineering discipline of piping design studies the efficient transport of fluid.

Tempering (metallurgy) Process of heat treating used to increase toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

Residual stress

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 imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. However, unintended residual stress in a designed structure may cause it to fail prematurely.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

Pipe (fluid conveyance)

A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.

Induction hardening is a type of surface hardening in which a metal part is induction-heated and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the part. Induction hardening is used to selectively harden areas of a part or assembly without affecting the properties of the part as a whole.

Thermowells are cylindrical fittings used to protect temperature sensors installed in industrial processes. A thermowell consists of a tube closed at one end and mounted in the process stream. A temperature sensor such as a thermometer, thermocouple, or resistance temperature detector is inserted in the open end of the tube, which is usually in the open air outside the process piping or vessel and any thermal insulation. Thermodynamically, the process fluid transfers heat to the thermowell wall, which in turn transfers heat to the sensor. Since more mass is present with a sensor-well assembly than with a probe directly immersed into the process, the sensor's response to process temperature changes is slowed by the addition of the well. If the sensor fails, it can be easily replaced without draining the vessel or piping. Since the mass of the thermowell must be heated to the process temperature, and since the walls of the thermowell conduct heat out of the process, sensor accuracy and responsiveness is reduced by the addition of a thermowell.

Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.

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Friction stir processing

Friction stir processing (FSP) is a method of changing the properties of a metal through intense, localized plastic deformation. This deformation is produced by forcibly inserting a non-consumable tool into the workpiece, and revolving the tool in a stirring motion as it is pushed laterally through the workpiece. The precursor of this technique, friction stir welding, is used to join multiple pieces of metal without creating the heat affected zone typical of fusion welding.

The ASME Boiler & Pressure Vessel Code (BPVC) is an American Society of Mechanical Engineers (ASME) standard that regulates the design and construction of boilers and pressure vessels. The document is written and maintained by volunteers chosen for their technical expertise. The ASME works as an accreditation body and entitles independent third parties to inspect and ensure compliance to the BPVC.

Vibratory Stress Relief, often abbreviated VSR, is a non-thermal stress relief method used by the metal working industry to enhance the dimensional stability and mechanical integrity of castings, forgings, and welded components, chiefly for two categories of these metal workpieces:

HY-80

HY-80 is a high-tensile, high yield strength, low alloy steel. It was developed for use in naval applications, specifically the development of pressure hulls for the US nuclear submarine program and is still currently used in many naval applications. It is valued for its strength to weight ratio.

Peening Process of working a metals surface to improve material properties

Peening is the process of working a metal's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot or blasts of light beams with laser peening. Peening is normally a cold work process, with laser peening being a notable exception. It tends to expand the surface of the cold metal, thereby inducing compressive stresses or relieving tensile stresses already present. Peening can also encourage strain hardening of the surface metal.

Friction hydro-pillar processing (FHPP) is a solid-state joining technology which can be used for filling of surface and sub-surface cracks in thick metals. For example, FHPP was attempted for the first time to repair cracks in space shuttle external components of high-strength aluminum alloys. FHPP is also considered in repairing surface cracks in steam turbine rotors of a high-strength, high-temperature-resistant steel (Ref). Alternative methods such as fusion welding processes for in-service repairing of cracks in components of these high-strength steels remained difficult because of their high hardenability and mandatory need of pre-heating and post-weld heat treatment. In contrast, initial FHPP trials could achieve joint strengths up to 90% of the base materials in high-strength steel components, especially those used for petrochemical and thermal power plants. In particular, pressurized pipes and vessels of AISI 4140 steel are widely used in the power generation, oil and gas, and petrochemical industries. Initial studies on FHPP of this alloy have showed promising results.

Advanced thermoplastic composites (ACM) have a high strength fibres held together by a thermoplastic matrix. Advanced thermoplastic composites are becoming more widely used in the aerospace, marine, automotive and energy industry. This is due to the decreasing cost and superior strength to weight ratios, over metallic parts. Advance thermoplastic composite have excellent damage tolerance, corrosion resistant, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life. Thermoplastic composites also have the ability to be formed and reformed, repaired and fusion welded.

References

  1. 1 2 3 4 5 6 7 8 9 "Post Weld Heat Treatment of Welded Structures" (PDF). www.wtia.com.au. February 2003.
  2. 1 2 3 4 5 "Heat Treatment of Welded Joints - Part 1". www.twi-global.com.
  3. 1 2 3 Welding Inspection. Miami, FL: American Welding Society. 1980. pp. 38–39. ISBN   978-0-87171-177-9.
  4. 1 2 3 4 5 6 7 "Heat treatment of welded joints - Part 2". www.twi-global.com.
  5. 1 2 "Heat Treatment Part 3". www.twi-global.com.
  6. 1 2 3 4 5 Croft, D (1996). Heat Treatment of Welded Steel Structures. Cambridge England: Woodhead Publishing Ltd. pp. 16–18. ISBN   1 85573 016 2.
  7. Thielsch, Helmut (1977). Defects and Failures in Pressure Vessels and Piping. Malabar, Florida: Krieger Publishing Company. p. 305. ISBN   978-0-88275-308-9.