ASNT Level III- Visual & Optical Testing

The most common use for temperature indicators is the
measurement of preheat, postheat and interpass temperatures
for welding. In a typical application, the welder marks
the test surface with an indicating stick of a specific temperature
rating (see Fig. 67). When the mark changes phase
(melts), the material has reached the correct temperature
and is ready for welding. It is important for the user to
understand that change of color has no significance; only the
actual melting of the mark should be considered.
Oxyacetylene equipment cannot be used for welding or
cutting of high strength steels used in automotive components
because too much heat can reduce their structural
strength. However, in some instances an oxyacetylene torch
may be used if the critical temperature of 760 °C (1,400 °F)
for high strength steel is not exceeded.
When preheat temperatures are 370 °C (700 °F) or when
heating is prolonged, an indicating mark could evaporate or
could be absorbed by the test material, Under these conditions,
marks should be added periodically during heating.
Charlie Chong/ Fion Zhang