164598784 Post Weld Heat Treatment

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HEAT TREATMENT OFPRESSURE VESSELS

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HEAT TREATMENT

MECHANICAL PROPERTIES OF METALS &ALLOYS DEPEND ON

CHEMISTRY (ALLOYING ELEMENTS)

&

HEAT TREATMENT

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WHAT IS HEAT TREATMENT?

• MATERIALS TREATED BY APPLICATION OFHEAT - NORMALLY DONE IN SOLID STATE

• HEATING BY VARIOUS SOURCES– HEATING IN A FURNACE – Oil, Gas, Electrical– HEATING OUT SIDE WITH ELECTRIC HEATING

COILS ARROUND THE JOB – Local Heat treatment– PASSING HOT AIR INSIDE A CLOSSED VESSEL –

Internal firing

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HEAT TREATMENT

• HEAT TEATMENT PARAMETERS– RATE OF HEATING– SOAKING TEMP.– SOAKING TIME– COOLING RATE– COOLING MEDIA

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WHY HT REQUIRED ?• TO BRING THE PROPERTIES TO THE

DESIRED RANGE–Strength, UTS, YS & Elongation–Toughness–Hardness

• CARRIED OUT IN–FABRICATION INDUSTRIES- Fabricated Components–STEEL PLANTS – Plates, Pipes, Tubes, Sections– FOUNDRY SHOPS - Castings– FORGING SHOPS – Forged Components

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MATERIALS HEAT TREATED

• METALS & ALLOYS– SINGLE PHASE– MULTIPHASE

• ALLOYS– Single Phase : Copper Nickel– Multiphase : Steels

• METALS (Single phase )– Titanium

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HEAT TREATEMENTCRITERIA• COLD WORKED to NORMAL

– All materials• UNEQUILIBRIUM PHASES to

EQUILIBRIUM– Stainless Steels ,Maraging Steels

• STRESSED to UNSTRESED– All Materials

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MATERIALS HEAT TREATED

• CS

• C-Mn , C-Mo , Cr-Mo , Cr-Mo-V, Ni -Steels

• Stainless Steels

• Non Ferrous Materials

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TYPES OF HEAT TREATMENT

• NORMALIZING• ANNEALING• STRESS RELIEVING• SOLUTION ANNEALING• HARDENING• TEMPERING• AGEING

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IRON CARBON DIAGRAM

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NORMALIZING• The steel is heated to 40° C above the upper

critical temperature followed by cooling in thestill air.To achieve

• Uniform structure• Change in Mechanical properties,

– UTS, YS & Elongation– Hardness– Impact properties

• Refined grains

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ANNEALING

• Steel is heated 10 to 50°C above the uppercritical temperature and held for the desiredlength of time followed by very slow coolingwithin the furnaceTo achieve:

• Softness & better ductility• Stresses free material – Stress generated due to

mechanical working / previous HT• Uniform property through out the material

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SOLUTION ANNEALING• Austenitic Stainless steels is heated to above

1050°C and held for the desired time followed byfast cooling to room temperature within fewminutes by quenching / blowing the air.

Solution annealing is done on stainless steel andnon ferrous alloys

• To soften the material• To remove carbide precipitation formed at grain

boundaries during manufacturing process ( SS )• To improve Corrosion Resistance

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AGEINGThe Material is heated to a certain temperature,and held for the desired time; followed byquenching or cooling in air

Ageing is done on materials susceptible forageing characteristics : Maraging Steels

• Normally increases strength

• Improves Toughness

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AGEING

Maraging Steels

M250

• Temperature : 485° C

• Normally 3 Hrs 15 mts• Heating Rate : 200 C per hr per inch thick• Cooling Rate : Cool in Air / Quench in water

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STRESS RELIEVING

The steel is heated to a temperature below close to thelower critical temperature with a specific rate ofheating. It is held at the temperature for a desiredlength of time, followed by cooling with a specificrate up to certain temperature.There is no change in grain structure.

Stress relieving is done Fabricated Components ofCS & LAS:

• To reduce Internal Stresses• To soften the steel partially• To soften HAZ

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STRESS RELIEVINGC-Mn , C-Mo , Cr-Mo (< 2% Cr)

• C - Mn Steels , C - Mo Steels, Cr-Mo Steels– SA 515Gr 70 , SA204GrA, SA387GR11,CL1

• Temperature : 593° C Min– Normally 600 - 640° C, 650-690° C

• Time : 15 minutes min ( 1 hr / inch thick)• Heating Rate : 200° C per hr per inch thick• Cooling Rate : 260° C per hr per inch thick

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Cr-Mo SteelsCr - Mo Steels (Cr >2%)

– SA 335P22 ,SA335P5• Temperature : 676° C Min

– Normally 680 - 700° C 2.25Cr– 704 - 720° C 5 Cr

• Time : 15 mts min (1 hr / inch thick)• Heating Rate : 200° C per hr per inch thick• Cooling Rate : 260° C per hr per inch thick

STRESS RELIEVING

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Ni -Steels• Nickel Steels : 1,2,3% Ni

– SA 203 GrA ,D• Temperature : 593° C Min

– Normally 600 - 640° C,• Time : 60 mts min (1 hr / inch thick)• Heating Rate : 200° C per hr per inch thick• Cooling Rate : 260° C per hr per inch thick

STRESS RELIEVING

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Steels enhanced by Heat Treatments• Q&T Steels :

– 9.5% Ni Steels , SA 517 Gr E• Temperature : 538° C Typ

– Normally < 600° C• Time : Min 15 minutes to 2 Hr ( 1 hr / inch thick)• Heating Rate : 200° C per hr per inch thick• Cooling Rate : 260° C per hr per inch thick

STRESS RELIEVING

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LAYOUT OF A TYPICALFURNACE

(Electrical or Gas fired )

RECORDER P.I.D.

FURNACE

JOB

COMPENSATINGCABLE

THERMOCOUPLE

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THERMOCOUPLES

• PRINCIPLE OF A THERMOCOUPLE

• THERMOCOUPLE MATERIAL

• TYPES OF THERMOCOUPLE BEINGUSED IN HED POWAI / HZW

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PRINCIPLE OFTHERMOCOUPLE

The basic principle of thermoelectricthermometry is that a thermocouple developsan emf which is a function of the difference intemperature of its measuring junction &reference junction. If the temperature ofreference junction is known, the temperatureof the measuring junction can be determinedby measuring the emf generated in the circuit.

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THERMOCOUPLE MATERIALREQUIREMENT

1. High coefficient of thermal emf.

2. Continuously increasing relation of emf to temperature overa long range.

3. Freedom from phase changes or other phenomenon givingrise to discontinuity in temperature emf relationships.

4. Resistance to oxidation, corrosion and contamination.

5. Homogeneity and reproducibility to fit an establishtemperature & emf relationship.

SPEED OF RESPONSE MAY BE IMPROVED ANDRADIATION & CONDUCTION ERRORS MAY BE REDUCEDBY THE USE OF SMALL DIAMETER THERMOCOUPLES.

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TYPES OF THERMOCOUPLEBEING USED IN HZW

K type :

Material : Chromel + Alumel

Nickel based ( 10 %Cr ) + ( 2 % Al )

Properties : Non-Magnetic + Magnetic

In this type of thermocouple, the wires arejoined at one end only to form a point-typetemperature sensor. Instrumentation convertsthe millivolt signal to related temperature.

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TYPES OF THERMOCOUPLEBEING USED

K type :

Dia : 2.5 mm 0. 7 mm

Insulation Bare(ceramic) Refractorycoated

Attachment Mech Capacitor

Usability Reusable Disposable

Location PIT F/c except PIT F/c

Color - Red & Yellow

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recorder

pid

Compensatingcable

Thermocouplewire

Weldedjunction

CONSTRUCTION OF A

K TYPE THERMOCOUPLE

Accuracy : 0.75%

PROPORTIONALINTEGRAL

DERIVATIVE

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‘S’ TYPE THERMOCOUPLE

• ‘S’ TYPE THERMOCOUPLE ARE THE STANDARDTHERMOCOUPLES.

• IT IS USED FOR CALIBRATING “K” TypeTHERMOCOUPLES.

• MATERIAL OF CONSTRUCTION90% PLATINUM + 10% RHODIUMPLATINUM

• OXIDATION RESISTANCE , SO MORE LIFE .

Accuracy : 0.25 %

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COMPENSATING CABLE

COMPENSATING CABLE IS DEFINED AS A PAIR OF WIRESHAVING SUCH EMF TEMPERATURE CHARACTERISTICSRELATED TO THE THERMOCOUPLE WITH WHICH THE

WIRES ARE INTENDED TO BE USED, THAT WHENPROPERLY CONNECTED TO THERMOCOUPLE THEEFFECTIVE REFERENCE JUNCTION IS IN EFFECTTRANSFERRED TO THE OTHER END OF THE WIRES.

MATERIAL ==> +ve COPPER ( white )-ve COPPER NICKEL (blue ) for “ K “ TYPE .

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P.I.D.PID = PROPORTIONAL INTEGRAL DERIVATIVE

• PID FUNCTIONS BOTH AS PROGRAMMER ANDCONTROLLER

• PID CONTROLLER CAN BE ZONE WISE• PROGRAMME IS MADE IN SEGMENTS AS PER

DIFFERENT STAGES OF HEAT TREATMENT• DIGITAL DISPLAY IS AVAILABLE FOR PROGRAMME

TEMPERATURE AND FURNACE TEMEPERATURE• TYPICAL OR REPETITIVE HEAT TREATMENT CYCLE

CAN BE STORED IN PID(PROGRAMMER)

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RECORDERTYPES OF RECORDER

• PAPERLESS -- WITH COLOUR DISPLAY SCREEN ,HARD DISC AND FLOPPYDRIVE. NOT USED IN HED HZW.

• WITH PAPER -- CURRENTLY BEING USED IN HZW.• 24 CHANNEL -- CURRENTLY BEING USED IN PFS( CHINO MAKE-- model no.I003

/Graph ET 001).• 12 CHANNEL -- CURRENTLY BEING USED IN MFS1 AND HFS1

( CHINO MAKE -- model no. EH100 / Graph ET 201).COMPENSATING CABLES ARE CONNECTED BEHIND THE RECORDER SCREEN INCHANNELS.

• X-AXIS IS FOR TEMPERATURE (RANGE = 0 TO 1200’C)

• Y-AXIS IS FOR GRAPH SPEED.

• VARIOUS SPEED OF GRAPHS ARE 12.5, 25, 50, 100 MM / HOUR

• GENERALLY KEEP 25 MM / HOUR.

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GRAPH PAPER• GRAPH PAPERS ARE USED FOR PLOTTING THE

FURNACE /JOB TEMPERATURE VIA THERMOCOUPLE.THEY ARE FITTED ON THE RECORDER.

• GRAPH PAPER RECOMMENDED ON RECORDER ONLYTO BE USED

• GRAPH PAPER FOR MFS1 AND HFS1 FURNACE==> ET 201 CHINO MAKE, JAPAN

GRAPH PAPER FOR PFS FURNACE==> ET 001 CHINO MAKE, JAPAN

• THE LENGTH OF ONE BUNDLE OF GRAPH PAPER ISGENERALLY 2000 MM.

• DOTTING TYPE RECORDER INK (CHINO MAKE, JAPAN)IS USED IN RECORDER FOR PLOTTING OF GRAPH.USUALLY , 6 COLOURS ARE FILLED FOR PLOTTING.

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PROCEDURE FOR EMPTY FURNACECALIBRATION

Calibration of PIDS ( indicator & controller )

1. Connect the millivolt source to the temperatureindicator or controller by a compensating cable.Care should be taken to clean the wires andterminals thoroughly before connections aremade.

2. The millivolt output for various temperatureranging from 00C to 10000C in steps of 500C is fedto the indicator / controller.

3. After the millivolt value / temperature readingdisplayed is steady, the reading ofindicator/controller shall be noted.

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Calibration of recorder

1. Connect the millivolt source to the recorder by acompensating cable. Care should be taken toclean the wires and terminals thoroughly beforethe connections are made.

4. If the error in the indicated readings is more thanthe specified accuracy ( +/- 10C ), then correctionto be carried out for the indicator / controller andpoints 1 to 4 shall be repeated till the specifiedaccuracy is obtained is obtained.

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2. The millivolt output for various temperatureranging from 400 C to 10000C is fed to therecorder and is allowed to plot on a graph.

3. The graph thus obtained is reviewed for timeand temperature values. These values shouldmeet the accuracy requirements.

4. If there is error in the values plotted on thegraph, then correction to be carried out forthe recorder and points 1 to 4 shall berepeated till the specified accuracy isobtained.

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EQUIPMENT REQUIRED ACCURACY

1. 20 Nos. big K-type thermocouples +/- 0.25%

2. 10 Nos. small K-type thermocouples +/- 0.25%

3. Millivolt source (wahl unit )( 1 micro volt at 1000 micro volts )

4. Heat treatment fixture.

5. Temperature indicators (PID) +/- 10C

6. Recorder +/- on temperature scale.

+/- minutes on time scale.

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PIT FURNACE CALIBRATION PROCEDURE

1. Ensure that the PIDs are calibrated as mentioned above.

2. Ensure that the recorder is calibrated as mentionedabove.

3. Ensure that all the thermocouples used are calibrated.

4. Ensure that the thermocouples are attached to the heattreatment fixture as shown in sketch-I.

5. Place the heat treatment fixture inside the furnace withthermocouples in position.

6. Close the furnace lid. Start the furnace and the recorder.

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7. Set the temperature of controller to 4000C.

8. After reaching the set temperature, it is allowed tostabilize for half an hour.

9. Measure and record the temperature indicated byeach of the 20 thermocouples. The temperature is tobe read through WAHL UNIT.

10. Three sets of readings are to be taken for eachthermocouples at an interval of 10 minutes.

11. Also record the readings indicated by each of thethermocouples at an interval of 10 minutes.


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12. The temperature is then raised in steps of 50 C up to10000C. ( I. e. 4000C, 4500C, …….., 9500C, 10000C. ) Themeasured temperature is stabilized for 30 minutes.PID reading are also to be recorded along with this.

13. The allowed temperature variation with respect to theset temperature is +/- 50C up to 8000C and +/- 100Cabove 8000C.

14. This is allowed to plot on the graph and thusobtained for time and temperature values.

15. Calibration of furnace is valid for 1 year.

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STANDARD OPERATINGPRACTICES FOR LOCAL

STRESS RELIEVING

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LOCAL STRESS RELIEVINGWhen•Local SR to be done only when furnace SR not feasible

• Only when certain weld joints on components to be heattreated

HOW• Done by Electrical heating coil• DETAILS ON ENSURING PWHT TEMP. IN WELDMENT AREA• Soaking band(SB) = Widest weld width ‘x’+ ’t’ or 2 inches

whichever is less from edge of weld• Heating band width (HB)

• Induction stress level• Through thickness criteria• SB + 4 rt where r = Inside radius, t = thickness

• Insulation band width (IB)• Axial gradient• HB + 4 rt

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LSR -BAND WIDTH

‘X’ = Weld width X

T

Soak band

Insulation band

Heating band

Lesser of 1T or 2”

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LOCAL STRESS RELIEVING SET UP

1. Provide multitonne roller on one end of vessel duringLSR of circular seam when job is horizontal.

2. If both ends are open during LSR, provide insulationfrom inside. If not possible , prevent airflow so thattemperature on inside surface do not drop down.

3. Spider/prop shall be provided in such a way that upperportion of spider / prop is not welded with insidesurface to allow contraction/expansion of shell surface.

4. Spider/prop shall be between 200- 500mm from heatingzone.

5. Temporary attachments, provided for holding insulation,shall be within soak band only.

6. Minimum two thermocouples shall be provided frominside, when accessible.

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LOCAL STRESS RELIEVING

No Welding at top

Multitonne roller

200 to 500mm from heating band

LSR of C/SSB+HB+IB

Spider orprop

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GOOD ENGG. PRACTICESFOR FURNACE CHARGES

& L S R

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SUPPORTING ARRANGEMENTS

1. Minimum distance between wall of furnace and the

job shall be 600mm.

2. Minimum distance between floor of the furnace

and lower most part of the job shall be 300mm.

3. The distance between the flame of burner and

saddle support shall be 600mm.

4. Minimum 90 degree saddle to be used, however

120 degree saddle is desirable.

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600mm

900

600mm


450mm

5. Saddle shall be arranged in such away that openend of the vessel is maximum 450mm from saddlesupport.

6. Saddle shall be located as close to spiders(temporarily arranged to control deformation) aspossible.

burner

300mm(point no:2)

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7. Spiders shall be provided as per annexure-5

8 Saddle supports shall be selected as per annexure.-3

9. Spiders or vertical prop shall be provided at open

ends, center and below man way/nozzles above 24”

10. Avoid gap between saddle support and job surface

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Zero gap

Supportingarrangement


11. Locking/clamping of job, restricting the movement

(axial/lateral) during heat treatment shall be avoided.

12. All long nozzles projecting outside job surface shall

be supported.

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13. Checklist shall be prepared and attached with HT

request before furnace is fired as per Ann-1

14. Spot check report shall be filled by supervisor as per

Annexure-2 during job is being heat treated.

15. Moonplate support and welding inside surface

prior to release for Heat treatment as perAnnexure-4

16. General idea about thermocouple locations and its

attachments is as per Annexure-6


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GENERAL

1. Blocking the flame of the burner is not desirable

2. Burner shall have blue flame and not yellow

3. Flame shall not directly impinge on job

4. All burners shall be fired at a time

5. Keep all job nozzles open during heat treatment

6. Above “24” nozzles / manways shall be located

towards bottom

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GENERAL

Temp. support

Furnace floor

Gasket machined surface

7. Deoxidization agent shall be applied on allmachined and gasket faces

8. Gasket / machined face of loose assembliesshall not be touching any object.

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THERMOCOUPLES

1. All the thermocouples shall betagged with aluminum sheetand identification hard punchedon it.(For PIT furnace only)

2. Minimum two thermocouples tobe attached for any charge.

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3. Minimum 8 thermocouples to be used for a charge in

HFS- I furnace if the job occupies all 8 zones

4. Maximum distance between two thermocouples for a

sample job is as shown in annexure- 6

5. PTC shall have separate thermocouple

THERMOCOUPLES

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1. Use only TAU-90 capacitor Discharge Weldingmachine for thermocouple connection

2. Use specified WPS for attachment ofthermocouple for cs/alloy steel material

3. Only trained person by welding engineering shallattach thermocouple.

4. A list of qualified person shall be by WeldingEng.

THERMOCOUPLESATTACHMENTS

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5. Clean surface prior to attachment.

6. Two wire of thermocouple shall be attached oneafter another.

7. Gap between two wire of a thermocouple shall bemax. 3.0mm

8. Only calibrated thermocouple shall be used.Calibration shall be by QA.

9. After PWHT, thermocouple area shall be groundand DP shall be carried out.

THERMOCOUPLESATTACHMENTS

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THERMOCOUPLESATTACHMENT UNIT

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SPECIAL NOTE

IF THE TEMPERATURE OF HEAT TREATMENT

EXCEEEDS 650-DEGREE CENTIGRADE, THE

MATERIAL AND SIZE OF SPIDERS AND SUPPORTS

TO BE DECIDED BY PLANNING AND APPROVED BY

DESIGN.

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CODE EXTRACTS

•FURNACE PWHT• L S R

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REQUIREMENT OF HEAT TREATMENTAS PER ASME-SEC VIII Div.-1

• SERVICE CONDITION (UW-2)

• MATERIAL (UG-85, UW-40,UCS-56,UAT-80,UHA-32,UNF-79)

• THICKNESS (UG-85, UW-40,UCS-56,UAT-80,UHA-32,UNF-79)

• LOW TEMERATURE

OPERATION (UCS-68)

• COLD WORKING (UG-79)

• CUSTOMER SPEC.

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CODE EXTRACT FOR HEAT TREATMENT( 1 ) The soak band shall contain the weld, heataffected zone and a portion of base metal adjacent to theweld being heat treated. The minimum width of thisvolume is the widest width of weld plus 1T or 2 inches,whichever is less, on each side or end of the weld. Theterm ‘T’ is the nominal thickness.( 2 ) The operation of postweld heat treatment shall beperformed either by heating the vessel as a whole in anenclosed furnace or heating the vessel in more than oneheat in a furnace, provided the overlap of the heatedsections of the vessel is at least 5 feet ( 1.5m). When thisprocedure is used, the portion outside of the furnaceshall be shielded so that the temperature gradient is notharmful. The cross section where the vessel projectsfrom the furnace shall not intersect a nozzle or otherstructural discontinuity.

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CODE EXTRACT FOR HEAT TREATMENT( 3 ) When the vessel is required to be postweld heattreated, and it is not practicable to postweld heat treat thecompleted vessel as a whole or in two or more heats; anycircumferential joints not provisionally heat treated may bethereafter locally postweld heat treated by heating suchjoints by any appropriate means that will assure therequired uniformity.( 4 ) While carrying out local postweld heat treatment,the soak band shall extend around the full circumference.The portion outside the soak band shall be protected sothat the temperature gradient is not harmful.( 5 ) Heating a circumferential band containing nozzlesor other welded attachments in such a manner that theentire band shall be brought up uniformly to the requiredtemperature and held for the specified time.

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CODE EXTRACT FOR HEAT TREATMENT( 6 ) Where more than one pressure vessel or morepressure vessel part are postweld heat treated in onefurnace charge, thermocouples shall be placed onvessels at the bottom, center, and top of the charge or inother zones of possible temperature variation so that thetemperature indicated shall be true temperature for allvessels or parts in those zones.( 7 ) Postweld heat treatment, When required, shall bedone before the hydrostatic test and after any weldedrepairs. A preliminary hydrostatic test to reveal leaksprior to PWHT is permissible.( 8 ) For pressure vessels or parts of pressure vesselsbeing post weld heat treated in a furnace charge, it is thegreatest weld thickness in any vessel or vessel partwhich has not previously been postweld heat treated.

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CODE EXTRACT FOR HEAT TREATMENT( 8 contd...) The nominal thickness is the total depth of

the weld exclusive of any permitted weldreinforcement.

For groove weld, the nominal thickness is thedepth of the groove.

For fillet welds, the nominal thickness is thethroat dimension.

If a fillet weld is used in conjunction of grooveweld, the nominal thickness is the depth of thegroove or the throat dimension, Whichever isgreater.

For stud welds, the nominal thickness shall be thediameter of the stud.

( 9 ) For P–1 material ( carbon steel), minimum holdingtemperature during postweld heat treatment shallbe 1100 Deg. F ( 593 Deg.c).

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CODE EXTRACT FOR HEAT TREATMENT

P. NO. HOLDING TEMP. NOM.THICKNESS

SOAKING PERIOD

1 ( CARBONSTEEL) & 3(LOW ALLOYSTEEL)

1100 DEG. F(593’C)

UPTO 2” 1 HR. PER INCH. ,HOWEVER 15 MINUTESMINIMUM

OVER 2”TO 5”

2 HOURS , PLUS 15 MIN.FOR EACH ADDITIONALINCH ABOVE 2”

OVER 5 ” 2 HOURS , PLUS 15 MIN.FOR EACH ADDITIONALINCH ABOVE 2”

* POST WELD HEAT TREATMENT IS MANDATORY ON P-NO.3 GR. NO. 3MATERIAL IN ALL THICKNESSES.

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( 10 ) Postweld heat treatment is mandatory inFollowing conditions :

• For welded joints over 1. 5” nominal thickness.• For welded joints over 1.25” nom. Thickness

through 1.5” nom. Thickness, unless preheat isapplied at a min. Temperature of 200’F ( 94‘c )during welding.

• Vessels or parts of vessels constructed of basematerial with corrosion resistant integral or weldmetal overlay cladding or applied corrosionresistant lining material shall be postweld heattreated when the base material is required to bepostweld heat treated. In applying this rule, thedetermining thickness shall be the total thicknessof base material.

• When the PWHT is a service requirement.


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SERVICE CONDITION

• LETHAL SERVICE PWHT IS MANDATORY

• EXEMPTIONS ARE FEW


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CODE EXTRACT FOR HEAT TREATMENT( 11 ) Postweld heat treatment is not mandatory for carbon

steel jobs (P1 material ) in Following conditions (UG2):

If groove welds is not over ½” in size or fillet weldwith a throat thickness of ½” or less used for attachingnon pressure parts to pressure parts provided preheatto a minimum temperature of 200’F is applied when thethickness of pressure Part exceeds 1.25”. If studs are welded to pressure parts providedpreheat to a minimum temperature of 200’F is appliedwhen the thickness of the pressure parts exceeds1.25”. for corrosion resistant weld metal overlaycladding or for welds attaching corrosion resistantapplied lining provided preheat to a minimumtemperature of 200’f is maintained during applicationof the first layer when the thickness of the pressurepart exceeds 1.25”.

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• The temperature of furnace shall not exceed 800’F( 4270C) at the time when the vessel or part is placed in it.• Above 8000F( 4270C), the rate of heating shall not be morethan 4000F Per hour (2000C/Hour) divided by the maximummetal thickness of the shell or head plate in inches, but in nocase more than 4000F Per hour( 2220C Per hour ).• During the heating period, There shall not be a greatervariation in temperature throughout the portion of the vesselbeing heat treated than 2500F( 1390C) within any 15 feet (4.6m) interval of length.

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• During the holding period, there shall not be agreater difference than 1500f ( 830c) between the highest andthe lowest temperature the portion of the vessel being heated• During the heating & holding periods, the furnaceatmosphere shall be so controlled as to avoid excessiveoxidation of the surface of the vessel. The furnace shall be ofsuch design as to prevent direct heat impingement of theflame on the vessel.• Above 8000F ( 4270C), The rate of cooling shall not bemore than 5000F Per hour (2780C/Hour) divided by themaximum metal thickness of the shell or head plate in inches,but in no case more than 5000F Per hour ( 2780C Per hour).

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when it is impractical to postweld heat treat at thetemperature specified in table mentioned in Sr.. No. 9, It ispermissible to carry out the post weld heat treatments atlower temperatures for longer periods of time as shown intable below :


DECREASE IN TEMP. BELOWMIN. SPECIFIED

TEMPERATURE IN ‘F

MINIMUM HOLDINGTIME AT DECREASED

TEMPERATURE (NOTE 1)

NOTES

50 (10’C) 2 HOURS ----

100(38’C) 4 HOURS ----

150(68’C) 10 HOURS 2

200(94’C) 20 HOURS 2

NOTES :1. MINIMUM HOLDING TIME FOR 1” THICKNESS OR LESS ; ADD 15 MINUTES PER INCH OF

THICKNESS FOR THICKNESS GREATER THAN 1”.2. THESE LOWER POSTWELD HEAT TREATMENT TEMPERATURES PERMITTED ONLY FOR P-

NO.1 GROUP NO. 1 AND 2 MATERIALS.

Documents

164598784 Post Weld Heat Treatment