Welding Preheat Considerations


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.
  1. The use of Pre-heat, eliminates the surface moisture; consequently reducing the hydrogen induced cracking.
  2. It tweaks fusion and deposition of weld metal in a better way.
  3. It lowers the development of stresses by rendering uniform expansion and contraction between weld and base metal.
  4. 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.
  1. If the parent metal thickness is less than and equal to 50 mm, then A should not exceed 50 mm from joint.
  2. 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.

5 comments:

  1. This is a very informative piece. I had always been taught to pre-heat the weld area, but I hadn't really had a clear understanding of why. Thanks.

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  2. It's refreshing to see the endorsement of preheat (PWHT) when so many welders don't think that it is necessary.

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  3. The heat treatment of a weld metal plays a vital role to ensure the best suitable micro-structure and mechanical properties. The heat treatments are of different types in a welding and joining processes; Pre-heating, Post Weld Heating and localized heating after weld.

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  4. The pre-heating temperature depends of several factors, as: the chemical composition of base material (steel with more than 0,22% Carbon must be pre-heated), the carbon equivalent content [CE = %C + %Mn/6 + (%Cr + % Mo +%V)/5 + (%Ni + %Cu)/15] - steels with CE > 0,45 must be preheated), thickness of base material (steels with 0,25 % Carbon must be pre-heated at min. 100 Celsius degree from thickness > 50 mm, steels with 0,45% Carbon must be pre-heated at 150-300 Celsius degree from thickness 5 mm). The pre-heating is important for reducing the cooling speed, reducing the humidity and decrease the quantity of hydrogen that that can enter in the weld and as result reducing the risk of cold cracking.

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