26153887 post weld heat treatment

7/29/2019 26153887 Post Weld Heat Treatment

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PWHT

BY

T BALAMURUGANSENIOR WELDING INSPECTOR


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WHAT IS HEAT TREATMENT?

Heat Treatment is thecontrolled heating and cooling

of metals to alter their physical

and mechanical properties,

without changing the product

shape

PWHT is the one of Heat

treatment done after the

welding/ machining to improvethe mechanical property and

material structure. In steel

fabrication PWHT is commonly

known as stress relieving.


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PWHTPOST WELD HEAT TREATMENT


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WHAT IS PWHT?

REFERS TO THE PROCESS OFREHEATING A WELD TO BELOWTHE LOWER TRANSFORMATION

TEMPERATURE AT ACONTROLLED RATE, HOLDING

FOR A SPECIFIC TIME ANDCOOLING AT A CONTROLLEDRATE.


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Applications


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WHY THE NEED FOR PWHT?

Residual Stress

Effects of MechanicalProperties

Effect on Creep Properties

Other benefits


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WHY THE NEED FOR PWHT?

Residual Stress The development of residual stresses approaching or

even exceeding the yield stress is possible when weldingthick sections.

Petrochemical, Chemical, Oil and Gas, etc. the existence

of residual stress of this magnitude is completely

unacceptable

the stress developed due to thermal expansion of

pipework can take on a far greater significance than

residual stress due to welding.


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WHY THE NEED FOR PWHT

Effect on MechanicalProperties

As a series of very general statements, the following arethe consequences of post weld heat treatment comparedwith the as-welded condition:

Yield strength is decreased slightly, the effect falling offfairly rapidly with time.

The tensile strength is decreased.

The ductility is increased.

Hardness levels are reduced.

Toughness is slightly reduced at short times but theeffect can be significant over longer times.


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Effect on Creep Properties

For creep resisting material, post weldheat treatment is required in order to fully

develop the creep strength. This is

specially true for thicker components such

as headers.


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Other benefits

Improving the diffusion of hydrogen out of

weld metalSoftening the heat affected zone and thus

improving toughness (although not weldmetal toughness)

Improving dimensional stability duringmachining.


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Other benefitscontd

Improving ductility.

Improving the resistance to stresscorrosion cracking.

Reducing the effects of cold work.


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WHEN TO POST WELD HEATTREAT

SERVICE CONDITION

(Eg. Caustic) MATERIAL GRADE AND

THICKNESS

WHERE HARDNESS LIMITS ARESPECIFIED.


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HOW TO PERFORM POST WELDHEAT TREATMENT

FIXED FURNACE

TEMPORARY FURNACE

INTERNAL FIRING

LOCAL HEATING

PARTIAL HEAT TREATMENT


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FIXED FURNACE

Fixed furnaces tend to be large and

expensive to operate.

Gas Fired.

Oil Fired.

Electrical resistance heating elements.


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FIXED FURNACE


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FIXED FURNACE


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FIXED FURNACE


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TEMPORARY FURNACE

These are custom-built around a

vessel, rather than transport a vessel toa fixed furnace. The idea is to minimisethe air space between the vessel andfurnace walls, and they allow for fasterheating and cooling


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TEMPORARY FURNACE

..contd

Heating can be through resistant heating

mats placed on a concrete floor or via gas

burners placed at each end.

In the case of gas burners, care must be

taken to avoid direct flame impingement

on the vessel.


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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TEMPORARY FURNACE


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INTERNAL FIRING

Vessels of suitable dimensions and arrangement of

openings can be post weld heat treated by gas firing

through nozzles or manways.

Manways are large enough to accommodate the gas

burners, but care needs to be exercised with the

diameter and position of nozzles and expert opinion

should be sought.


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INTERNAL FIRING

.. Contd Care must also be taken to place deflector plates inside

the vessel and opposite the burner entry points to avoid

direct flame impingement on the shell. It is not advisable to post weld heat treat vessels that

contain internals in this manner.

The outside of the vessel must be completely encased in

insulating material, and again, at least a 12 pointtemperature recorder is advisable.


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INTERNAL FIRING


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INTERNAL FIRING


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INTERNAL FIRING


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INTERNAL FIRING


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LOCAL HEATING

Circumferential weld seams can be post weld heat

treated by heating a band around the weld.

Although not specifically stated, such heating is

essentially limited to resistance or induction heating,

mainly because of the controls required on heated band

width, width of insulation and temperature measurement

requirements.


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LOCAL HEATING


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LOCAL HEATING


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LOCAL HEATING

PARTIAL HEAT


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PARTIAL HEATTREATMENT

There are occasions, for example with very long vessels,

when the entire vessel will not fit into a fixed furnace.

This has been catered for in most Standards, and it is

permissible to post weld heat treat section of the vessel

first, then turn the vessel around and heat treat the

remaining section.

As with local heating, there are restrictions in this case

as well over the degree of overlap and the longitudinaltemperature gradient.


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ITEMS TO CONCERN

AREA TO BE HEATED

HEATING AND COOLING RATE

HOLDING TEMPERATURE ANDDURATION

TEMPERATURE MEASUREMENT AND

DISTRIBUTION

EQUIPMENT CALIBRATION AND

CONDITION

HEATING / COOLING


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HEATING / COOLINGRATE

The PWHT heating and cooling rates above 316C shallnot exceed 222C/hr divided by the weld thickness ininches, but in no case shall it be more than 222C/hr.

At least compliant with the necessary code

requirements. Related to component thickness to offer protection

against thermally induced stresses (thermal shocks).

With thicker and more complex structures, considerlower rates than required by the code to ensureacceptable temperature profiles and gradients with aview to keeping these thermally induced stresses to anabsolute minimum.


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HEATING RATE


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SOAKING PERIOD

COOLING RATE


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COOLING RATE


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THERMO COUPLES

To transmit the surface temperature to the recorder.

If localized PWHT is used, the following minimumnumber of equally spaced recording thermocouples(T/C) shall be used:

a) Pipe diameter of 305 mm or less: 1 T/C.b) Pipe diameter above 305 mm up to and including 610mm: 2 T/C.c) Pipe diameter above 610 mm: 4 T/C.d) Additional thermocouples are required if multiple heat

control zones are used, in which a control zone is notmonitored by any of the primary T/Cs.


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THERMO COUPLES

Type K (Chromel-Alumel) or Type J (iron-

Constantan) thermocouples are permitted.


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THERMO COUPLES

Thermocouples shall be attached to the

component by capacitive discharge welding

only. Other methods of attachment are not

permitted. The thermocouples shall not be in direct contact

with electrical heating elements or subjected to

flame impingement by gas or oil burners.


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THERMO COUPLES


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THERMO COUPLES

N i ti i lt


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