POST WELD HEAT TREATMENT (PWHT)
PWHT is heating treatment in which the
weld is heated at sufficiently high temperature held for specific time so that
stresses in the weld almost completely eliminated. The annealing and normalizing
are two different things other the PWHT.
Temperature of annealing
and normalising is around 750 to 950 degree but in PWHT the temperature below
the cortical range. The main difference is that in case of normalising or annealing
the the main grain structure, tensile properties also slightly dimensions also
changes but in case of PWHT the temperature below the critical range.
Advantages of
PWHT as follows,
1) It soften HAZ (Hear Affected Zone)
2) Brittle fracture failure chances reduces
3) Removes hydrogen like gasses from
weld and HAZ area
4) Lowers the yield and tensile strength.
5) Reduces the stresses involve in it.
Types of Heat Treatments.
1) Electric resistance method.
2) Internal firing
3) Furnace heating
4) Electric induction method.
Furnace
heating : in this type of heating the temperature
is controlled in order of (plus/minus)10 degrees, also rate of heating is very
easily controlled.
Pic01
Above image is picture of Furnace Heating and job (pressure vessel loaded inside the
furnace)
Furnace is totally sealed from atmosphere. This type of heating is expensive. The vessel is totally loaded in enclosed finance and heated by LPG. Temperature is maintained by thermo couples. The disadvantages of this type of heating is that furnace consume large size/area, if large length and diameter of vessel is required to heating then heating done by segments of given pressure vessel or job.
Pic01
Above image is picture of Furnace Heating and job (pressure vessel loaded inside the
furnace)
Furnace is totally sealed from atmosphere. This type of heating is expensive. The vessel is totally loaded in enclosed finance and heated by LPG. Temperature is maintained by thermo couples. The disadvantages of this type of heating is that furnace consume large size/area, if large length and diameter of vessel is required to heating then heating done by segments of given pressure vessel or job.
Electric Resistance heating
This heating is done by wires and beads.
The heating element wrapped around the power source the current of transformer is
control. Beads using in this heating is 98% made up of ceramics they do not brakes
during handling.
Gas Heating :
This heat treatments is easy also low
initial cost and cheap but some limitations due to flame Impingement on metals.
PWHT only done as per approved procedure in accordance with codes and drawing requirements.steps involve in
PWHT is,
1)LOADING
2) RATE OF
HEATING
3) SOAKING
4) COOLING.
THERMO COULES REQUIREMENTS,
For vessel or
tank thermo couple shall be placed every 4.5mm distance.
For pipes 4 inch - 1 thermo couple,
4-10
inch - 2 thermo couples
Above
10 inch - 4 thermo couples.
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