Phase-change materials for the mining industry
Melting point standards find a variety of uses necessary for welding in the mining industry
by Claude Bedingfield, welding instructor
Gillette College, Northern Wyoming Community District, Sheridan, WY
Instructors at Gillette College’s welding laboratory in Wyoming have found measuring surface temperatures of fabricated parts sold to the mining industry may be easily tested by temperature indicating products.
Phase-change temperature indicators are not influenced by external factors like static electricity, ionized air, time, humidity - or being dropped on the floor - that may cause electrical and electronic instrumentation to function erratically. This durability and reliability make phase-change indicators well suited for the service demands of the mining industry. The melting point never needs recalibration.
Temperature indicating sticks serve by marking the fabricated parts and are available for measuring more than 100 exact temperatures, from 37 – 1,100 °C (100 - 2,000° F). They come in a range of identifying colors, but color itself has nothing to do with the temperature indicating process. It is only when the dry, opaque mark changes to a distinct melted appearance, that the temperature rating of the stick has been reached. The liquid mark is distinct and well defined with no gray area or margin for interpretation.
The accuracy of phase-change materials for surface temperature indication is usually within 1 percent: for example, within 3.5°F at 176° C (350 °F). The remarkable accuracy of phase change materials is available because the stick, label or liquid is in intimate contact having excellent thermal equilibrium with the surface to be tested.
A phase-change indicator does not tell the exact temperature like a thermometer but, it does determine, precisely, when a pre-set minimum or maximum temperature has been reached. For checking one or a few limit points the phase-change is not only much less expensive, but actually more accurate than the digital probe or thermocouple.
Accurate determination of preheat temperature
Preheat for welding is almost always required or recommended, particularly in applications of surfacing welds and joining the heavier thicknesses or hardenable, higher strength allow commonly used in mining environments. The temperature indicators are used to ensure the recommended level of preheat has been achieved and maintained. Bringing the base metal up to heat before welding reduces the danger of crack formation. Maintenance of minimum and maximum interpass temperatures promotes ductility, reduces brittleness, increases toughness of the metal, improves controllability of the weld, reduces hard zones next to the weld area, and reduces distortion of surrounding surfaces. It also increases diffusion of hydrogen from the metal to limit weld contamination, and tends to vaporize moisture which may be present.