PWHT

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INDEX

1.0 Definitions

2.0 Heat Treatment

3.0 Documentation

4.0 Codes and Standards & Tables

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1.0 Definition

1.1 Pre heating :

To reduce rates of cooling from fusion temperature

1.2 Post heating :

To give sufficient time to remove hydrogen & other impurity.

The application of heat to the weld material immediately before or

during a welding

1.3 Post weld heat treatment:

To reduce residual stresses

Temper the marten site which is a hard phase that may have resulted from welding. This action restores ductility. reduction of residual thermal stress, which is present in all welds. This action decreases the likelihood of pre-mature failure. PWHT may also increase corrosion resistance.

1.4 Methods of heat treatment :

furnace heat treatment Local heat treatment

. Flexible ceramic pads/ induction coil combustion burner – gas fired

Types of Heat treatment:

1. Annealing: Heating to and holding at a suitable temperature and then cooling at suitable rate for such purpose as reducing hardness, improving mach inability, facilitating cold working, producing a desired microstructure or obtaining desired mechanical, physical, or other properties.

2. Normalizing: a process in which the ferrous metal is heated to a suitable temperature above the transformation range and is subsequently cooled in still air in room temperature.

3. Quenching: Rapid cooling of a heated metal

4. Stress relief: Uniform heating of a structure or the major portion there of to a sufficient temperature to relive the major portion of the residual stresses , followed by uniform cooling slowly enough to minimize development of new residual stresses.

heat affected zone: that portion of the base metal which has not been melted but whose mechanical properties or microstructure has been altered by the heat of welding, brazing, soldering, forming or cutting.

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2. Post weld heat treatment or Stress relieving

Requirement of PWHT: PWHT shall be carried out for the cases as specified by the applicable engineering specification.

Heating/Cooling Rate:- For Heating & Cooling rate follow the requirement of Design Code/approved WPS/Specifications.

Soaking/Holding time & Temperature:- For PWHT soaking time & temperature requirements follow the requirement of applicable Design Code/approved WPS/Specifications.

Equipment for PWHT: All equipments are purpose built. Electric power sources are 440 V, 3 phase primary supply or 220V single phase with either single phase secondary output (distribution unit) or step down transformer secondary output (heat treatment unit) giving low voltage circuits. Temperature control equipment is either manually operated through energy regulators

Automatically operated using modular controls. Recording of temperature is by means of calibrated chart type temperature recorder.

Heating Elements: PWHT work under this procedure shall be performed by induction heating using ni-chrome wire covered with ceramic beads/heating pads/flexible ceramic pads as per parameters specified in applicable Design Code & approved WPS.

Thermocouples: Material of thermocouples to be used shall be the Nickel-Chromium (cromel) /Nickel, Aluminum (alumel) type. Compensating coil & thermocouples shall be calibrated. In addition, re-calibration shall be done at any point of time when there is justified reason to believe that they are in error. Thermocouple shall be attached to the work piece by Direct Wire Capacity Discharge method (Fig.1) or by nut and bolt connection (Fig.2). Where thermocouples are being joined by capacitance discharge method, these are to be attached with two shots (with attachment unit) as shown in Fig.1. The distance between two wires shall be min 6mm. Insulation to be removed from the end of the wire shall not be more than 15mm. Care shall be taken to connect the positive side of the thermocouple with positive side of compensating cable. Similarly negative side of the thermocouple shall be connected with the negative side of the compensating cable. Fig.1 6mm If the nut and bolt connection method is used (Fig.2), the materials shall be of compatible composition.

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Fig.-1

6 mm

Direct Wire Capacity Discharge method

Fig.-2

Two tack welds (opposite to each other)

Thermocouple insertion

Nut and Bolt connection method

Temperature recorders shall be of the potentiometer self compensating type having chart feed recording. Recorders shall be calibrated in every six months and calibration stickers shall be affixed to the recorders. In addition, recorders shall be re-calibrated after every major repair and also for the cases where there is reason to believe that they are in error.

PROCEDURE

Surface Preparation : Preparation of surface to the extent to remove coatings, grease, oil etc. shall be ensured within the area covered for preheat and post heat (wherever appl icable) and for PWHT to avoid damage due to short-circuiting of the heating elements.

Job knowledge evaluation of operating personnel: Before starting the job with induction coil heating (which covers preheating, post heating and PWHT), technician shall be interviewed by REAL QMD personnel to explain and/or demonstrate the necessary arrangement and control to the satisfaction of REAL QMD personnel.

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Location and Number of Thermocouple(s): The following guidelines shall be followed as minimum requirement for number and location of thermocouples:

i. One thermocouple for pipe diameters up to and inclusive of 6" NPS. For 2” and below, wherever joints are there within the range of 100mm distance, one thermocouple is permitted, provided single coil/pad is used.

ii. Two thermocouples at 180° for pipe diameter above 6" and up to 12" NPS inclusive.

iii. Three (3) thermocouples at 120° for pipe diameters of above 12" NPS .

Thermocouples shall be fixed within the area of heating band. Additional thermocouples may be fixed as per requirement to confirm hot zone or gradient limitations, if necessary. Thermocouple (s) and their position (s) shall be identified on the relevant temperature-recording chart by number or color/symbol of stamp.

Thermocouple requirement for branch connection and pad shall be in

line with the requirements as mentioned in above. In all cases, as a minimum there shall be one thermocouple per coil to control the heat treatment process.

Heating Element position

Heating band : Welds being locally PWHT treated shall have a circumferential band around the entire component with the weld located in the center of the band. The width of the heated band shall be as follows:

A. For girth welds, the width of the heated band shall be at least three times t he wall thickness at the weld of the thickest part being joined and shall be maintained on either side of the joint. (Fig-3.)

B. For nozzle and attachment welds, the width of the band heated to the PWHT temperature shall extend beyond the nozzle weld or attachment weld on each side at least two times the header thickness and shall extend completely around the header.

Fig,-3 3A Min. Insulation Band width

A A-min. heating band width A

3t 3t t-Wall Thickness

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Assembly Of Welded Valves

For PWHT of Welded Valves, The width of the band heated to the PWHT temperature shall extend up to 3t + weld width. Insulation width shall be 6t+ weld width both ( heating & insulation bands) on pipe side only as shown above in fig 4. Valve body shall be free from insulation, Valves shall be kept open during the full cycle of PWHT and No bonnet bolts to be loosened during PWHT cycle.

Heating element: Heating elements shall be fixed on weldments by means of black annealed iron wire.

Insulation: The insulation shall be extended to a minimum distance of 3 times the heated bandwidth. The number of layers and extent of insulated area depends on:

i. wall thickness (es)ii. pipe or fitting diameter

iii. gradient requirementsiv. site condition

But in all cases minimum two (2) layers of insulation with minimum thickness of 50mm shall be used. Open ends to pipe and/or fittings shall be blanked to avoid through draught cooling during cycle.

Fig –4

6t + weld

3t + weld T

3t + weld --- Heating Width 6t + weld --- Insulation Width

FIG. - ASSEMBLY OF WELDED VALVES

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Temperature Recording : The following are required to be verified :

1. chart speed is set at appropriate speed,2. Temperature recording system is recording all thermocouples and

providing information on temperature differences and trends during the heating/cooling cycle.

Verification : Preheat temperature shall be verified through the temperature indicating crayons. Welding shall not commence till the preheat temperature is attained. This shall be monitored by REAL QMD inspector from time to time. Post heating temperature (for burner heating) shall be verified through the temperature indicating crayons. This shall be monitored by REAL QMD inspector from time to time.

For post heating (electric heating) and PWHT, after fixing the heating coils and the thermocouples with proper connections with the recorder, the arrangement shall be certified by the Authorized representative of REAL QMD who shall sign on the graph to indicate that the arrangements are certified. Then the insulation shall be applied on the heating coils. Again after attaining 300°C during heating, the graph shall be re certified by REAL QMD representative. This requirement shall be treated as mandatory for the acceptance of Post heating (electric heating) as well as for PWHT cycle except for nitrogen purged/digital recorder.During PWHT cycle front door of recorder shall be kept in closed condition.

Interrupted PWHT: Due to power loss, equipment failure etc. due to weather condition/permit problem, there may be an interruption in heat treatment cycle.

1. If interruption occurs during heating period, the stress developed shall get moderated during soaking, so the cycle can be continued.

2. If interruption occurs during soaking period, then the soaking temperature shall be re -established at a rate no greater than that specified in code/established welding procedure and the soaking cycle shall be repeated.

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3.0 Documentation

3.1 PWHT Recorder, Thermocouple & Control unit Calibration Certificate

3.2 PWHT Procedure

3.3 PWHT Request

3.4 PWHT chart

3.5 PWHT completion record

3.5 Records

Prior to starting the electrical heating, the following information shall be hand written

on the recorder chart by the technician.1-Date, 2- ISO No.(s), 3- Agency’s Name, 4-

Weld Joint No.(s), 5- Chart No

The charts shall be identified with XXX XXX / XXXX XXXX

1 2 3 4First 3 digits indicate unit number in numeric,2nd 3 digits alphabetic for the name of

immediate contractor. 3rd four digits indicate name of the sub contractor and 4 th four digits

indicates sequential numbering of the chart i.e. 312 RCC / HRIS 0001

Record of PWHT shall be maintained in the format 2 QM 00 FMT ME 138.

Thermocouples tack welded with pipe shall be removed after PWHT

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4.0 Codes Specification and Standard

ASME SEC VIII: RULES FOR CONSTRUCTION OF PRESSURE VESSELS.

ASME B 31.1: POWER PIPING

ASME B 31.3: PROCESS PIPING

SPECIFICATION 22960 –3PS-GAW-004

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4.1 Code Requirement

PWHT requirement as per ASME Sec – VIII Div. -1

PWHT as per Sec. VIII Div. 1

Material Gr.No.Holding

Temp.deg.C

Min. Holding Time For Nominal Thikness at normal Temp.

Up to 2 IN. Over 2 IN.. to 5 IN. Over 5 IN.

P NO.-1

1

5931hrs .-15

minuts min.

2hrs+15 minuts min.for each

additional inch over 2inch

2hrs+15 minuts min.for each additional inch

over 2inch

2

34 N.A. None. None. None.

P NO -3

1

5931hrs .-15

minuts min.

2hrs+15 minuts min.for each



over 2inch

2

3

P NO -4 1,2 5931hrs .-15

minuts min.1hrs/in.


over 5inch

P NO -5A,5B,5C 1 6771hrs .-15

minuts min.1hrs/in.


over 5inch

P NO -5B 2 7051hrs .-15

minuts min.1hrs/in.


over 5inch

P NO -9A 1 593 $ 1hrs min.+ 15 minuts/in.for thk. Over 1 in.( 25mm)

P NO -9B 1 593 $ 1hrs min.+ 15 minuts/in.for thk. Over 1 in.( 25mm)

P NO -10A 1 593 $ 1hrs min.+ 15 minuts/in.for thk. Over 1 in.( 25mm)

P NO -10B 1 5381hrs min.+ 15 minuts/in.for thk. Over 1 in.( 25mm)

for all thickness.

P NO -10C 1 538 $ 1hrs min.+ 15 minuts/in.for thk. Over 1 in.( 25mm)

P NO -10F 1 5381hrs min.+ 15 minuts/in.for thk. Over 1 in.( 25mm)

for all thickness.

$ - Refer note of ASME -Sec -VIII Div.-1 UCS - 56 Page no.- 180 -186

PWHT requirement as per ASME b31.1

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PWHT as per ASME B 31.1

Material Material Gr.No.Min. wall thk.mm

Holding Temp.Rang

deg.C

Holding Time based on Nominal Thikness

Up to 2 IN.( 50 mm) Over 2 IN. ( 50 mm)

Carbon Steel P NO.-1

1,2,3 <=19 None. N.A. N.A.

1,2,3 >19 600 -6501hr/in(25mm) 15

minuts min.2hrs+15 minuts min.for each


Alloys Steels Cr<=0.5%

P NO -3

1,2 <=16 None. N.A. N.A.

1,2>16 600 -650

1hr/in(25mm) 15 minuts min.

2hrs+15 minuts min.for each additional inch over 2inch

Alloys Steels 0.5<Cr<=2%

P NO -4 1,2<=13 None. N.A. N.A.

>13 700 -7501hr/in(25mm) 15

minuts min.2hrs+15 minuts min.for each


Alloys Steels 2.25%<Cr<=10%

P NO -5A

1 <=13 None. N.A. N.A.

1>13 700-760




P NO -5B1,2

<=10 None. N.A. N.A.

1,2>10 700 -760



410 P NO.-6

1,2 <=10 None. N.A. N.A.

1,2,3>10 760-800



405 P NO.-71,2


1,2>10 730-775



Austenitic Stainless Steel

P NO.-81,2,3,4

all None. N.A. N.A.

Ni Alloys steel P NO.-9A1


1>13 600-650



Ni Alloys steel P NO.-9B1


1>16 600 -650



5 Ni steel P NO.-10HHT is neither required nor prohibited.temp range for Rapid cooling Alloys

1)S31803 -1022-1099 Deg.C,2)

PWHT requirement as per ASME B31.3

PWHT as per ASME B 31.3Material Material A.No. Holding Time

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Min. wall thk.mm

Holding Temp. Range. deg.C Brinnal

Hardness

min/mm

Hr/In

Min. Time Hr.

Carbon Steel P NO.-1 1<=19 None. N.A. N.A. N.A. N.A.

>19 593- 650 2.4 1 1 N.A.

Alloys Steels Cr<=0.5%

P NO -3 2,11<=19 None. N.A. N.A. N.A. N.A.>19 593-718 2.4 1 1 225All 593-719 2.4 1 1 225


P NO -4 3<=13 None. N.A. N.A. N.A. N.A.>13 704-746 2.4 1 2 225All 704-746 2.4 1 2 225


P NO -5A,5B,5C

4,5

<=13 None. N.A. N.A. N.A. N.A.>13 704-746 2.4 1 2 241

All (>3%cr)

704-746 2.4 1 2 241

High Alloys Steels Martensitic

P NO.-6 6All 732-788 2.4 1 2 241

A240 Gr429 All 621-663 2.4 1 2 241

High Alloys Steels Ferritic

P NO.-7 7 All None. None. None. None.

High Alloys Steels Austenitic

P NO.-8 8,9 All None. None. None. None.

Ni alloys steels P NO.-9A,9B 10<=19 None. N.A. N.A. N.A. >19 593-635 1.2 0.5 1

Cr-Cu Steel P NO.-10 nil All 760-816 1.2 0.5 0.5

Duplex Stainless Steel

P NO.-10H nil AllHT requid as per

material spec.1.2 0.5 0.5

27 Cr-Steel P NO.-10 I nil All 663-704 2.4 1 1

8 Ni ,9 Ni steel P NO.-11A SG 1 nil <=51 None. N.A. N.A. N.A. >51 552-585 2.4 1 1

5 Ni steel P NO.-11A SG 2 nil >51 552-585 2.4 1 1

Zr R60705 P NO.-62 nil All538-593 HT with in 14 days after

weldingnote-1 note-1 note-1

note -1 --Hold time shall be increased by 0.5hr for each 25 mmthk. Cool to 427 deg C at the rate <=278 deg C/Hr. per 25 mm nominal thk.278 deg C/Hr max. coll in still airfrom 427 deg C.

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Documents

PWHT