It is a process where reheating the metal below its lower critical transformation temperature, following a welding process. The material is then treated in a higher temperature for a prescribed period of time to control residual stresses, increase the strength, increase or decrease the hardness, and decrease the risk of cracking by micro-structural changes. A chain or array of heating processes can be used to carry out post-weld heat treatment.
Post-weld heat treatment (PWHT) reduces the levels of tensile residual stresses in a joint. It does not reduce these levels to zero, however, and, even in a very well controlled thermal cycle, the levels of final residual stress are unlikely to be much below 30% of the material yield strength. If a tensile residual stress of, say, 100 MPa, remains after PWHT, almost all applied fatigue cycles (assuming these are typically less than 100 MPa in stress range) still result in entirely tensile stresses at the weld toe and are fully damaging. As the whole applied stress cycle is almost certainly still tensile, fatigue design codes do not recognise any benefit in terms of fatigue life and require the same design approach to be used for as-welded and PWHT joints. However, PWHT does not make the fatigue performance of the joint any worse, and it may be carried out for other reasons such as dimensional control, improving fracture performance, or prevention of hydrogen cracking. High metallurgical structure and improved ductility of the material are the major advantages of Post Weld Heat Treatment.
Features of post-weld heat treatment
Five aspects of post-weld heat treatment must be addressed:
1. The hot zone should be adequate to raise the weldment to the required temperature and provide a uniform temperature profile to avoid creating additional undue thermally induced stresses. This aspect has greater significance in localised heat treatments, but nevertheless must also be considered in furnace heat treatments.
2. The heating and cooling rates should be at least compliant with the necessary code requirements. These rates will indicate absolute maximum values. The rates are calculated from simple formulae related to component thickness and offer protection against thermally induced stresses. With thicker and more complex structures an experienced heat treatment engineer may wish to consider lower rates than required by the code to ensure acceptable temperatures profiles and gradients, with a view to keeping thermally induced stresses to an absolute minimum.
3. In localised heat treatment, the temperature gradients away from the hot zone should not be unduly severe, again the objective being minimisation of thermally induced stresses.
4. The soak temperatures should be held within the upper and lower limits of the soak range for the appropriate time.
5. The heat treatment system (including insulation), zonal division and number of thermocouples should be such that the energy input and level of control are capable of enabling these objectives to be met, ensuring that the integrity of the overall structure is not jeopardised.
Method and Equipment adopted
· Electric-resistance method can be adopted onthe local post-weld heat treatment of the welded joints.After the completion of all welding or repairing operation.
· Electrically and thermally self-insulated resistance heater are used and is built to size for each individual pipes.
· Depending on the power requirements the applied voltages across the coils are either 220 or 380 volts AC.
How the power controlling panel is composed?
A digital kind of temperature controller indicator and recorder are used in order to control the power inputs to the coil a potentiometer device is used.Input, and output terminals for power and thermocouple connection is required. Most importantly electrical power contactors of the proper rating that will supply a single heating station and therefore for each heating operation, one panel will be needed. The role of potentiometer here is to adjust the heating and cooling rates manually.
Essential requirements for PWHT
Everything should be in accordance with all the essential variables of ASME SECTION IX.
· To avoid any harmful temperature gradient the portion outside the heated band shall be suitably wrapped under insulation throughout the cycle of heat treatment.Like that of the exposed surface of the pipe. For this purpose, the temperature at the exposed surface of the pipe should be less than400°C.
·Uniform temperature attainment at all points of the portion being heat treated should be ensuredwhile carrying out local post-weld heat treatment. Care shall be taken to ensure that the width of the heated band on either side of the weld edge shall not be less than 4 times of pipe thickness or 2″ whichever is greater.
· Automatic temperature recorders that have been calibrated shall be adopted. Prior to starting the heat treatment operation the calibration chart of each recorder shall be submitted to the owner and his approval shall be obtained. Recording equipment shall be calibrated at least once in every 12 months.
Preparation and Attachment of Thermocouple for PWHT
· An adequate number of thermocouples (based on the diameter of pipes) shall be attached to the pipe directly after performing visual inspection and removing surface defect and temporary tack welds. The minimum number of thermocouples attached per joint shall be 1 for up to 3″ diameter, 2 for up to 6″ diameter and 3 for up to 10″ and 4 up to 12″ diameter and above.
· In order to avoid incorrect temperature reading due to direct radiation to thermocouples, it shall be protected by ceramic fibre blanked or any other suitable insulation material.
· Thermocouples should be directly tack welded to the joint or heating band of jointly provided that they have a tail of the same material and approved filler wire or electrode not larger than 2.5 mm in diameter is used for tack welding. The thermocouples shall be placed on the joint and in firm contact with the pipe as near as possible to the weld area.
· Insulating materials shall be mineral wool/glass wool which can overcome the temperature employed. The minimum insulation thickness shall be 50 mm. To hold the insulation material in position wire mesh shall be wrapped around and tied or tied by other suitable means.
In the case of PWHT temperature and time record, the post-weld heat treatment temperature and time and its heating and cooling rates shall be recorded automatically and present the actual temperature of the weld area. Each thermocouple shall be connected to the controlling and recording instrument for each treated joint.
Check out Heating, Holding and Cooling of PWHT
· In the related welding procedure specification the heat treatment socking temperature and holding time shall be specified.
· The difference between the temperatures measured by various thermocouples shall be within the range specified.The heating temperature above 300°C shall be recorded and heating and cooling rate shall not be more than that specified in related WPS and standards but in no case, more than 200°C/hr.
· The cooling down to 300°C shall be controlled cooling. Below that the cooling down to ambient temperature shall take place under insulation coating without controlling.
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