In my experience, PWHT of low carbon steel for structural purposes is very seldom used. The only exceptions would be if the steel is to be machined after welding, or in rare cases to inprove the fatigue life. I do not believe PWHT would be required per AWS D1.1 By - Date 04:44. The impact of Post-Weld Heat-Treatment (PWHT) on the hardness and microstructure of of 0.165% carbon steel was determined in this study after welding operation. Autodesk Inventor Simulation CFD 2015 Application Software for Visual Style –Wire Frame and Visual Style –Shaded Mesh was used to simulate the Heat Affected Zone (HAZ) and welded pool on three planes (YZ, XZ, XY). REVISION DATE 00 Initial Release TREATMENT PROCEDURE 1. SCOPE The procedure covers the general requirements of Post Weld Heat Treatment (PWHT) of weld joints between alloy steel to alloy steel, alloy steel to carbon steel and carbon steel to carbon steel material of piping fabricated in accordance with ASME B31Pressure Piping and ASME BPV Code.
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MIZAT ADVANCED NDT P.O. BOX 31712 AL-KHOBAR 31952 TEL. NO. +966 3 802 9708 FAX NO. +966 3 802 9709 E-mail: [email protected] Website: www.mizatalkhleej.com.saDocument Title: Document No.: Revision No.:Rev. No. 00POST WELD HEAT TREATMENT PROCEDURE MAN-QP-PWHT-001 Rev. 00Description of Change Initial ReleaseRev. DatePrepared by Approved by24.02.2010GCCISGDOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDURE1. SCOPE The procedure covers the general requirements of Post Weld Heat Treatment (PWHT) of weld joints between alloy steel to alloy steel, alloy steel to carbon steel and carbon steel to carbon steel material of piping fabricated in accordance with ASME B31Pressure Piping and ASME BPV Code. 2. REFERENCES ASME Sec. VIII Division IRules for Construction of Pressure VesselsASME B31.3, 2008Process PipingASME B31.4, 2006Pipeline Transportation Systems for Liquid Hydrocarbons and Other LiquidsASME B31.8, 2007Gas Transmission and Distribution Piping Systems3. RESPONSIBILITIES 3.1 Safety Personnel working in the heat treatment shall be trained in PWHT work. It shall be the responsibility o f the employed PWHT technician to ensure personnel safety and clearance within the facilities before performing the activity. 3.2 Qualification The PWHT technician shall be either a science/technical graduate or an electrical apprentice high school graduate with basic electrical knowledge to undertake the heat treatment at the site. 4. EQUIPMENT 4.1 Power source Power source shall be a 3 Phase 65 KVA double wound class “H” insulated dry type power transformer suitable for primary supply of 380/415/440 V and giv es 3 Phase output voltage of either 60 V or 80 V AC between phase and neutral. Output from the power source is taken through connectors and made available on the output sockets. The unit is fitted with circuit breaker and panel mounted cam-lock output terminals. 4.2 Programmable Temperature Controller It is a proportional controller used for co ntrolling the temperature during post weld heat treatment. The magnitude of control signal is proportional to the deviation between work piece t emperature and set point temperature. Power supplied in an ON-OFF mode but the set point is shifted electronically by a measure at how far the process temperature is above or below the set point. “K” type thermocouples attached to the work piece shall be connected to the temperature controller using compensating cables. Programmer is provided with automatic cold junction co mpensation.MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 1DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDUREThe six (6) channel controller energizes the co ntractorised heating circuits of the power source. Time proportional action is used to control heating/cooling rates and “Dwell” action to maintain soaking temperature. 4.3 Temperature Recorder Post weld heat treatment shall be progressively recorded using a multipoint, potentiometric type strip chart recorder with built in thermocouple reference junction compensation circuit. Chart speed shall be selected either on 2 5 mm/hr or 50 mm/hr and can be verified by using a stopwatch. Inputs shall be “K” type thermocouples connected using Copper-Constantan compensating cable of 30 mm length via programmable temperature controller. 12 Nos. Thermocouples can be connected to the recorder and each channel will be printed in different colours. Chart shall be fully identified with date, chart speed, joi nt identification, temperature-time cycle, name of the operator, heat treatment specification and chart receipt number. Temperature recorder shall be calibrated every three (3) months and a proper valid sticker shall be displayed. Calibration frequency shall be extended up to 12 months with the approval of Client Inspection Department if the documented calibration checks for that particular recorder demonstrate acceptable accuracy for a suitable period. 4.4 Thermocouple Wire Type “K” (Chromel-Alumel) thermocouple wire (22 SWG, pair) shall be used for measuring temperature of post weld heat treatment. Thermocouple wire shall be covered with h igh temperature glass fibre material which acts as an electrical and t hermal insulation as well as protect the wire from atmospheric contamination. Standard limits of error shall be ± 2 °C for 0 – 275 °C and ± 0.75% for 275 – 1260 °C range as per ANSI MC-96-1-1975. Copper-Constantan compensating cables shall be used to conn ect thermocouple to instrument terminals. Thermocouple wires shall be attached to the j ob using a thermocouple attachment unit. Thermocouple putty shall be used on hot junction to eliminate the chances of error due to radiant heat. Care shall be taken to ensure that the thermocouple wires do not touch, away from the hot junction. The thermocouple to extension cable connection shall be clear off heating zone. Thermocouple plug and sockets shall be used for connecting thermocouple wires to compensating cables and compensating cables to the instru ments. 4.5 Thermocouple Welding and Attachment “K” type thermocouple wire (Nickel-Chromium and Nickel-Aluminum pair) shall be fixed to the job using thermocouple attachment unit working on capacitance discharge principle. Proper cleaning of the area where the thermocouple to b e attached shall be done (with wire brush or file) to make sure that it is smooth and free from dirt, grease, oil, etc.MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 2DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDURE Nickel-Chromium and Nickel-Aluminum wires shall be fixed with a distance of separation 4 – 6 mm for accurate measurements. Preheating is not required for fixing the thermocouple wire by using this equipment. 4.6 Compensating Cables Copper-Constantan compensating cables shall be used to connect “K” type thermocouples to the programmable temperature controllers and temperature recorders. The Copper lead (positive, blue) shall be connected to the Nickel-Chromium conductor (non-magnetic) and Constantan lead (negative, white) to the Nickel-Aluminum conductor (magnetic) of “K” type thermocouple. For Type K thermocouples, Copper-Constantan (Type T) compensating cables may be used with th e following conditions. a) The junction between the Copper-Constantan compensating cable and the type K thermocouple lead shall be a minimum of 0.5 mm outside the insulated area. b) The acceptable temperature range of the junction between the compensating cable and the thermocouple lead is from 0 °C to 80 deg. °C. 4.7 Flexible Ceramic Mat Materials Heating elements used for post weld heat treatment shall be flexible ceramic mat heater of 60 V, 2.7 kW. Standard sizes are 205 x 250 mm, 255 x 205 mm, 305 x 165 mm, 180 x 145 mm and 1220 x 45 mm. Flexible ceramic mat heaters are flexible and can easily be wrapped around pipes. Continuity checks shall be done for each heating elements before connecting it in the ci rcuit. 4.8 Insulation Ceramic fiber mats of thickness 25 mm and size 1800 x 600 mm or 900 x 600 mm with stainless steel wire mesh cover shall be used as thermal insulation. Density of ceramic fiber shall be 120 kg/m3. Minimum two (2) layers of insulation shall be used. 5. PRE-START UP CHECK 5.1 Site Verification MIZAT heat treatment engineer/supervisor shall visit the site 15 days before starting the job to assess the location of joints, accessibility, arrangement for power connections, weather protection for equipment, schedule of PWHT etc. List of joints to be stress relieved shall be made with weld identification, drawing number, lo cation, diameter, thickness, material, heat treatment cycle etc. and t o be submitted to client inspector at site. 5.2 Surface Condition The welds to be heat treated shall be prepared free of greases, lubricants, dust and coatings to avoid damage and short-circuiting of accessories/equipment.MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 3DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDURE5.3 Surface Protection During heat treatment it shall be necessary to protect from oxidation the mechanical surface line flange faces, threaded holes, threads by the application of coating such as deoxyaluminate or a ny other suitable coating materials. 5.4 Protection from Extreme Weather Proper protection to the job a nd equipment shall be done in case of extreme weather conditions. Tarpaulin roof/side cover shall be used in case of heavy wind and rain. 5.5 Procedure to Avoid Chimney Effects The ends of open pipes shall be closed off using ceramic fibre insulation mats in order to eliminate drafts or air circulation that could lower the temperature on the inside of the joint, unless the internal surface also insulated. All the valves on the line shall be kept closed during PWHT to avoid chimney effect. 5.6 Equipment Checking All equipment used for PWHT shall be checked to ensure i ntegrity before starting the job. Control circuit of power sources shall be checked before connecting the heating load. Calibration of temperature recorder shall be cross-checked using calibrated thermocouple potentiometer. Thermocouple attachment unit shall be checked for battery condition. All flexible ceramic mat heaters shall be checked for continuity before connecting in heating circuit, all the electrical connections shall be checked before switching on. Connection materials required for the heat treatment, shall be of standard quality and it shall be selected free of defects. 6. POST WELD HEAT TREATMENT 6.1 The PWHT heating and cooling rates above 316°C shall not exceed 222°C/hr. divided by the weld thickness in inches, but no case shall be more than 222°C/hr. 6.2 Soaking period shall be min. one (1) hour for P1 material and minimum two (2) hours for P4 material and also per inch of nominal wall thickness of the pipe. Welding or heating after the final PWHT is not permitted if the PWHT is required for service. Post weld heat treatment shall follow all welding and repairs but shall be performed prior to hy drostatic test. 6.3 Below 315°C, natural cooling under insulation in still air. During heating and cooling, there shall not be a greater variation in temperature than 120°C between any two points in the heating zone. 6.4 Pipes with diameter of 305 mm (12 inches) or less shall have at least one thermocouple. At least two (2) equally spaced thermocouples shall be used for circumferential welds on pipe diameters above 305mm through 610 mm (24 inches). Pipes larger than 610 mm diameter shall have at least 4MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 4DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDUREthermocouples equally spaced around the circumference. (see Appendix I THERMOCOUPLE LOCATIONS) 6.5 Flexible mat heaters are arranged about the weld seam to accomplish a heating area extending at least 3 times the thickness of the material being welded, but not less than 25 mm on either side from the edge of the weld. Gaps between heating elements shall not exceed the wall thickness or 50mm, whichever is smaller. 6.6 Where the butt joint constitutes two (2) separate material thicknesses, the heating elements shall be arranged in two circumferential bands with independent control. 6.7 A minimum of 2 layers (50 mm) ceramic fibre insulations shall be applied, extending to a minimum distances of 300 mm on either side of the weld. 6.8 In case of premature termination of heat treatment (due to power failure etc.) it shall be recorded and repeated from the point of termination. 6.9 Any reduction in post weld heat treatments or alternative temperatures, below normal holding temperatures, listed in the codes shall not be permitted. 7. DISASSEMBLY Power source shall be electrically isolated before dismantling the secondary power cabling system, ceramic fiber insulation shall be removed only after the temperature has cooled below 150 °C. After completion of PWHT, all thermocouples shall be removed and the attachment areas shall be ground smooth in to clean and sound metal. MT/PT should be appl ied to those areas when requested by the Client. 8. PRECUATIONS Prior to the start of PWHT, components shall be checked to ensure that all restraints are removed and the component is free to expand and contract. For safety reasons, job shall be perfectly earthed during PWHT. The immediate area of the job shall be roped off. General safety aspects of “Hot Work” shall be strictly followed. Check all cable connections using proper plugs and sockets. Protect the power cables and thermocouple cable from damage. Care shall be taken when shifting the equipment from one place to another. Electronic equipment like recorders, programmers etc. shall be transported suitably packed in shock proof containers. Heat treatment equipment shall be placed at site in covered space, which protects it from extreme weather conditions. 9. DOCUMENTATION AND RECORDS 9.1 Procure the approval of the PWHT cycle and assignment for the weld joint to be heat treated. 9.2 On a daily basis, a list of joints heat treated in the previous day along with all the relevant details including Time – Temperature graphs (chart) shall be submitted to QC department for approval, All identification of joints must be noted in chart and signed, mentioned start time by the technician prior to start of PWHT.MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 5DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDURE9.3 Inform the client for any deviation in the heat treatment cycle. 9.4 The heat treatment cycle chart shall be treated as the proof and record for any heat treatment completion. 10. HARDNESS TESTING 10.1 Hardness checking shall be performed to determine the satisfactory performance of Heat treatment process. Performance to SAES-W-011 Section 14 and/or NACE RP 0472 shall be made for the purpose of hardness testing, the maximum hardness for P1 materials is 225 BHN for sweet service and 200 BHN for sour service. 10.2 The hardness testing shall be performed with portable Telebrineller field hardness tester only. 10.3 Hardness testing of deposited weld metal should be taken at the center of the weld bead. Hardness limits applies to both weld metal and HAZ (heat affected zone). Hardness testing of HAZ at a point as near as practicable to the edge of the weld. Before any hardness measurements, the surface shall be made smooth by filing. 10.4 If any readings exceed the specified limits by no more than10 BHN, then a minimum of three (3) indentations shall be made near the original high reading. If all three (3) retest are below the specified limits, then the joint is acceptable. If any of the retest readings are found to exceed the specified limits, then the weld shall be considered unacceptable.MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 6DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDUREAppendix I THERMOCOUPLE LOCATIONSDiameters; mm (inch) D ≤ 305 mm (12 inches)Thermocouple LocationsD > 305 mm (12 inches)D > 610 mm (24 inches)MIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 7DOCUMENT TITLE:POST WELD HEATDOCUMENT NO.MAN-QP-PWHT-001REVISION NO. REVISION DATE00 Initial ReleaseTREATMENT PROCEDUREAppendix II PWHT Form 1 0 0 T H W P F Q N A MMIZAT AL K HALEEJ HOLDING MIZAT Advanced NDTPage 8
Post Weld Heat Treatment Services ( PWHT ) is defined as one of heat treatments done after welding/machining to improve the Chemical mechanical properties of weldment / machined surfaces. In concept, PWHT covers many different potential treatments. However, in steel fabrication, most common procedure used is Stress Relieving.
When Pwht Is Required For Carbon Steel Welding
Post-Weld Heat Treatment/PWHT purpose
The primary reasons that components are required to be subjected to PWHT within the ASME Code rules are that PWHT reduces residual stresses and tempers hardened microstructures. PWHT will achieve both of these results but might not positively benefit the overall properties of the weldment if not properly controlled, particularly in terms of the toughness in the heat-affected zone. When toughness is a requirement, the Codes will impose additional controls on the PWHT, such as time at temperature controls. PWHT done to meet the Code requirements is typically performed at subcritical temperatures.
Residual stresses: can contribute to increases in the susceptibility to corrosion mechanisms and to fatigue. Because residual stresses cannot exceed the yield strength of the materials, an immediate benefit of increasing the temperature of the material during PWHT is a corresponding drop in the yield strength of the material and thus a reduction of the maximum residual stresses in the weldment. In order to further reduce the residual stresses, the weldment will need to be held for Longer periods at the elevated temperatures (this reduction occurs by relaxation-recrystallization or primary creep mechanisms).
Although the reduction of residual stresses is a benefit of PWHT, most of the rules for PWHT specified in the Codes are targeted at the hardened microstructures. This is because the applications that might require a reduction in residual stresses are not addressed specifically in the Codes. Flash mac chrome.
Phase diagram of an iron-carbon alloying system
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). The PWHT for carbon steels is generally done in the range of 1100-1200ºF (600-650ºC), although some Codes specify only the minimum temperature of 1100ºF (600ºC). A secondary effect of tempering is to allow some additional transformation of the martensitic grain structure into ferrite, but the main objective is tempering the martensite. The result can be increased ductility and toughness in addition to reduced hardness. If the tempering temperature is too high or held too long, some corresponding reduction in the toughness can result.
NITS has a talented crew that uses diesel fuel as source of heating for stress relieving for pressure vessels with the sole objective of reinforcing process and component integrity and quality. The experience heat treaters from NITS have the experience, equipment, and expertise to develop custom configuration for your particular process. Our heating processes include low-Range, Mid-Range & High-Range Temperature Heating.
- Permanent Low Thermal Mass Furnaces for heat treatment by Oil / Gas / Electrical mode of heating. Types of furnaces comes under the category are Bogie Hearth Furnace, Top Hat Furnace, Fixed Hearth Furnace, Pit type Furnace, Box Furnace, Roller Hearth Furnace etc.
- Temporary Low Thermal Mass Furnace for Stress Relieving, Normalizing, Solution Annealing etc.
When Pwht Is Required For Carbon Steel Grating
Heat treatment Equipment:
- High velocity oil & gas fired burner system provides capacity from 5,00,000 kcal/hr up to max. 8,000,000 kcal / hr.
- Heat treatment low voltage power source of 40,50,75,100,125 kva capacity.
- Heat Treatment Distribution Panel.
- Flexible Ceramic Pads operating at 45V, 60V, 80V, 230 V.
- Electric Channel Elements.
- Programmer Controller, 03 Point, 06 Point, 12 Point Inlet.
- Thermocouple attachment unit.
- Twin / Trinary Heat Module for Portable Operation.
- Thermocouple (Simplex / Duplex, Wire type)
When Pwht Is Required For Carbon Steel Pressure Vessel
Oil/Gas Fired High Velocity Burner Equipment:
When Pwht Is Required For Carbon Steel Conductivity
The Indotherm Oil / Gas high velocity burner systems provide from 5,00,000 kcal/hr up to 8,000,000 Kcal/hr ( 2000,000 Btu/hr to 32,000,000 Btu/hr ). For On Site Heat Treatment from a flame supervised, automatic / manually controlled, nozzle mix high velocity burner which will operate on all commercially available fuels.