We are manufacturer of pressure vessels and one of the majorclient specifications explains about the simulated heat treatment of materialas under
“Materials with properties enhancedby heat treatment cycles such as tempering, intermediate stress relief (ISR) andthe final post weld heat treatment shall be tested to verify that theirmechanical properties have been retained after all heat treatment cycles. Thesetests shall also include two additional post weld heat treatment cycles toaccount for future repairs or alteration.”
Query: 1: My understandingis that the above paragraph is referring only to the materials for shells &heads of the equipment.
Query: 2: My understandingis that the above paragraph is referring only to the material undergoing PWHT inorder to achieve certain properties like HIC resistance etc. (or) some specificmetals which mechanical properties may enhance due to PWHT not due to thicknessfor PWHT (ie 38 mm above as per UCS-56).In this case, combined three cycles ofSPWHT will fully satisfy the above paragraph requirement.
Query: 3: If for the abovequeries (1 & 2), suppose my understanding is not correct, Please check thebelow condition for the applicability of Para.
Post-weld treatment acts as a tempering process by reducing the hardness of the heat-affected zone and the weld metal. Tempering is a heat treatment whereby the material is heated to a temperature below the lower critical temperature (often assumed to be approximately 1340ºF 725ºC for carbon steels).
Post weld heat treatment (pwht) In order to keep the strength of the material maintained after welding, a process called post-weld heat treatment (PWHT) is usually carried out. This process can be used to decrease residual stresses, as a means to control hardness, and enhance the strength of the material.
Susceptible materials should be given a post-weld heat treatment to a temperature greater than 950 °C to take the carbides into solution. Carbide precipitation problems are largely overcome in stabilised grades, for example types 321 and 347, by addition of elements (Ti and Nb) which form carbides preferentially to.
For example
Ng sfix rom download windows 7. ·Dish Head/Toricone of the pressurevessel requires Stress Relieving due to exceeding the allowable fiberelongation as specified in UCS-79(d)
·Instead of heat treating separatepetals of the dish head / toricone, the dish head/toricone petals are weldedand also shell portions are welded to it. The complete assembly is then heattreated to relieve residual stresses of Dish head / toricone. (This is done toavoid any distortion caused during Stress relieving of separate petals of thedish head / toricone)
·In this case, with reference toabove Para, whether mechanical propertiesafter one heat treatment cycle need to be tested only for dish head orfor all the materials included in the assembly?
Please note due to some reason, we are not clarifying theabove directly with client. I need opinion from experts.
Regards
M.M
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]
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[edit]
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[edit]
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[edit]
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[edit]
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[edit]
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] 2k17 upload.
References[edit]
Post Weld Heat Treatment Requirement
^ abcdefghi'Post Weld Heat Treatment of Welded Structures'(PDF). www.wtia.com.au. February 2003.
^ abcde'Heat Treatment of Welded Joints - Part 1'. www.twi-global.com.
^ abcWelding Inspection. Miami, FL: American Welding Society. 1980. pp. 38–39. ISBN978-0-87171-177-9.
^ abcdefg'Heat treatment of welded joints - Part 2'. www.twi-global.com.
^ ab'Heat Treatment Part 3'. www.twi-global.com.
^ abcdeCroft, D (1996). Heat Treatment of Welded Steel Structures. Cambridge England: Woodhead Publishing Ltd. pp. 16–18. ISBN1 85573 016 2.
^Thielsch, Helmut (1977). Defects and Failures in Pressure Vessels and Piping. Malabar, Florida: Krieger Publishing Company. p. 305. ISBN978-0-88275-308-9.
Post Weld Heat Treatment Chart
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