Page 144 - CITS - Welder - Trade Theory

WELDER - CITS

Maintenance of Inter pass temperature:
Introduction: During welding, the parent metal is heated to melting point and after that it is allowed to cool
rapidly. The adjacent portion to the welded zone is also heated by to a lower temperature. This causes certain
phase transformations and on rapid cooling, due to heat transfer through the colder portion of parent metal and
atmosphere, the materials hardness and hence mechanical properties are also affected. The width of parent metal
that is affected due to the above cycle is called ‘Heat Affected Zone’. It is quite clear that the hardness depends
on rate of cooling. Higher the cooling higher will be the hardness. In order to control the cooling rate pre-heating
and interpass temperature controls are adopted. In order to relieve the welding induced stresses and to achieve
better metallurgical structure to meet service conditions, post - weld heat treatment is followed. Heat input: The
energy supplied by the welding arc in a fusion welding process is called arc energy and is calculated from current
voltage and welding speed. However all the arc energy is not utilized for welding; some of it is invariably lost as
shown in Fig 6.

Fig 6

WLC22T0146

The extent of energy loss varies with the welding process, welding parameters, type of material, preheat
temperature etc. To account for the energy loss and estimate the actual energy given to the workpiece, a term
known as heat input is employed.
The heat input of a single pass weld is calculated by multiplying the efficiency of the welding process and arc
energy. Therefore heat input at best can serve as a rough guide to the amount of heat supplied to the workpiece.
Temperature changes in welding: Heat moves from one area to another whenever there is a difference in
temperature. Just as water flows downhill, so that flows down the temperature hill, warning cold objects at the
expense of warmer ones. When the source is moved away, the heat in the weld is conducted outward into the
plate. The temperature of the weld has fallen, while the plate temperature near the weld is rising. The weld has
cooled still further and the plate temperature is still rising. The metal reaches a maximum temperature less than
the melting point of the weld metal, and cooling sets in.
Use of temperature indicating CRAYONS
Although there are many other uses for measuring high temperatures, it’s critical to get accurate temperature
readings during the preheat stage of welding.
Whether you are using TIG (GTAW) welding, automatic welding or gas metal arc welding (MIG), many base
materials, like certain Quench and Tempered Steels, require you to raise the temperature in order to prevent
hardness in the heat affected zone and prevent hydrogen assisted cold cracking in joints of larger thicknesses.
With Q&T materials the maximum inter pass temperature must be monitored to ensure it is not over-tempered
(softened) this is result in loss of tensile strength. Otherwise, your welding joint may not have the required
mechanical properties.
There are other critical factors to achieving mechanical properties, such as the consumable electrode type.
Thermo crayons, however, are designed to help with temperatures indication and can be purchased for a specific
temperature. Normally you need a series of temperatures 50C, 75C and 100C. The temperature you need
depends on the base material type, thickness, welding process, and consumables.

They mark almost any base material, from steel, brass and aluminium. This temperature indication technology is
clearly useful when welding.

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CITS : C G & M - Welder - Lesson 33 - 43