PREHEATING
The application of supplementary
heat to a joint before or after welding can be a very important part of the welding
procedure in order to get desired mechanical properties.
Elevating the temperature of the
work pieces immediately before a welding operation is called preheating.
INTER PASS TEMPERATURE
The inter pass temperature is to maintain the
uniform Temperature of joint during entire welding.
This temperature shall not be less than Preheat
temperature and maximum temperature should be as specified in WPS.
POST
HEATING
The application of supplementary heat to a weldment
during any interruption or completion welding is called post heating.
POST
WELD HEATTREATMENT
STRESS RELIEVING
NORMALIZING
ANNEALING
QUENCHED & TEMPERING
PURPOSE
OF PREHEATING
To avoid cold cracking in the heat-affected zones of
hard enable steels
To increase the toughness of the weld joint and
improve its ability to withstand adverse service conditions involving impact
loading.
To give controlled cooling rate in weld and heat
affected zones to obtain desired micro Structures.
To reduce residual stresses to a desired levels.
To minimize shrinkage and distortion.
To produce desired mechanical properties.
Post heating are adopted to remove diffused hydrogen
in weld and heat affected zones.
METHOD
OF HEATING
Gas burners
Heating elements
Applied on both side of joint
Measured 3” from joint preferably from back side
Check by Tempilstik or Digital Thermometer
GENERAL
RECOMMENDATION OF PREHEATING TEMPERATURE
< 19mm: 66 Deg. C
> 19mm - < 38mm: 80 Deg. C
> 38mm - < 65mm: 95 Deg. C
Above 65mm: 100 Deg. C
Max. Inter Pass: 250 Deg. C
TEMPERATURE
CHECKING
The preheat temperature and inter pass temperature
shall be checked immediately before stat the each welding pass.
The post heat temperature shall be maintained
whenever recommendation in the WPS and checked at any interruption or completion
welding.
The instrument to check temperature may be
temperature indicating Tempilstiks.
The temperature shall be checked on either side of
the joint at a distance not less than three times the thickness of thicker
plate but not less than 75mm from all directions of joint.
PREHEAT
CALCULATION - CARBON STEEL
A simple formula for calculating preheating
temperature based on research carried out on various low alloy steels with
experimental verification of the under bead hardness attained.
The preheating temperature ‘T’ should be found from
the formula.
T Deg. C = 350 SQUARE ROOT OF (C ) – 0.25 in which
(C ) is the total carbon equivalent.
PREHEAT
CALCULATION – LOW ALLOY STEEL
The total carbon equivalent is the sum of the carbon
equivalent or the alloying elements present in the steel i.e. the chemical carbon
equivalent (C) e, and the carbon equivalent for the plate thickness, (C) t a
function of the component dimension.
(C) = (C) e + (C) t
The value of 0.25 in above formula corresponds to
the top limit of carbon for carbon steels of ordinary weldability.
The chemical carbon equivalent has been defined as
360 x (C) e = SQUARE ROOT OF 360 C + 40 (Mn + Cr) +
20Ni + 28 Mo
The carbon equivalent for thickness (C) t must be
function of the plate thickness and of the hardanability of the steel i.e. the
chemical carbon equivalent.
(C) t = 0.005 x (C) e
Where e is the thickness in millimeters, the
constant in this formula was determined by experiment, hence
(C) = (C) e (1+0.005 e)
EXERCISE
PREHEATING CALCULATION
For example, the steel specification 25 CD 4 in plate
thickness 12mm and chemical carbon equivalent from the above formula is
360 (C) e = SQUARE ROOT OF (360 x 0.25) + 40 (0.8 +
1) + (28 x 0.25)
(C) e = 0.46
Therefore, (C) = 0.46 (1 + 0.005 x 12) = 0.48
T Deg. C = 350 SQUARE ROOT OF 0.48 – 0.25 = 170 Deg.
C
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