The phenomenon of heating the
joint in the weld zone before the commencement of welding procedure is known as
pre-heating. Normally it is applied by the use of ovens, flame producing torch,
resistive heating elements and high frequency heating elements. It helps to
eliminate welding interruption and several imperfections and set the joining
phenomenon on smooth track.
Why to use Pre-heat?
Following are the main reasons
for using the pre-heat in a welding procedure.
- The use of Pre-heat, eliminates the surface moisture; consequently reducing the hydrogen induced cracking.
- It tweaks fusion and deposition of weld metal in a better way.
- It lowers the development of stresses by rendering uniform expansion and contraction between weld and base metal.
- One of the main objectives, is to slow down the cooling rate in order to ensure uniform weld solidification. Doing so, the more refined micro-structure is produced with adequate mechanical properties.
Types
Based on the application
considerations, the Pre-heat phenomenon can be defined in different ways.
- Pre-Heat Temperature – The minimum temperature of the weld joint promptly before the commencement of welding. In WPS it can be specified in specific range.
- Inter-pass Temperature – The maximum temperature of the weld in multi-pass welding before the start of next pass. It should not be lowered than the minimum value of pre-heat temperature.
- Welding Maintenance Temperature – The minimum temperature of weld zone that should be maintained throughout the welding procedure. If any interruption occurs the weld zone temperature shall not be lowered than the maintenance temperature.
Where to apply Pre-heat
temperature?
Normally pre-heat is applied on
the base metal at the specific distance from joint. Let’s suppose A is that
specific portion from the joint edge then following are the two concerns while
calculating the joint distance for pre-heat application.
- If the parent metal thickness is less than and equal to 50 mm, then A should not exceed 50 mm from joint.
- If the parent metal thickness is greater than 50 mm, then A should not be less than 75 mm from joint. (Ref: CSWIP 23.4)
Equipment
Now a days modern equipment have
been developed which are being used in several small/commercial industries for
the purpose of pre-heat temperature indication, measurement and control. Listed
below are the common temperature sensing and measuring equipment;
Contact Thermometer – used to
measure the temperature accurately up to 350 degree Celsius. It consists of
temperature sensing device known as ‘thermistor’ whose resistance is
considerably reduced by heating i-e the resistance is inversely proportional to
temperature. However need of ‘Periodic Calibration’ is the down side of this
equipment in order to obtain accurate results.
Crayons and Paste – consist of
the material that melts or changes its color by heating at the specific
temperature. Available at low cost they are also easy to use, however they
don’t give accurate measurements.
Thermocouple – works on the
principle of measuring thermo-electrical potential difference between the hot
weld metal and the reference junction, in order to calculate the temperature.
It is applied during the post weld heat treatment for the purpose of continuous
monitoring and controlling of heating and cooling temperature. The equipment
gives accurate measurements over the wide range of temperatures but the need of
periodic calibration is the weighty concern.