January 2014, Vol. 241 No. 1
Features
Simple, Reliable Melting Point Standards Useful In Temperature-Measuring Applications Necessary For Welding
The materials used in fabricating offshore and onshore jackets, rigs and platforms for the oil and gas industry vary from regular mild steel to high-alloy, high-strength steels. The pipelines carrying oil and gas also consist of high-strength, high-alloy steels.
The integrity of the welds for all of these must be of the highest standards since many lives depend on it. Because of the grade and thicknesses of these steels preheating, interpass temperature monitoring and post-weld heat treatment play a critical role.
To ensure that these critical operational parameters are adhered to, a temperature indicating stick plays the most significant role in ensuring that the correct temperatures have been measured, monitored and controlled. For measuring these surface temperatures, the simple method is often the best: use of temperature-indicating products based on the absolute predictable phase-change (solid to liquid) of a pure chemical compound.
Melting point – the temperature at which phase-change occurs – is a physical property; nothing affects it, and it occurs, simply and reliably, every time the exact temperature is reached. 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 industry. The melting point never needs recalibration.
The first phase-change temperature indicator was the stick mentioned earlier, used to mark the workpiece in welding applications. Now temperature-indicating sticks are available for measuring more than 100 exact temperatures, from 100 to 2000EF. The sticks 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.
Other phase-change temperature indicators include pellets (to be placed on or inside the piece – in an annealing furnace, for example), liquids (to be painted on the piece), and labels coated with temperature-indicating material (which are affixed to a component to measure temperature exposure). The pellets and liquids indicate their temperature rating by changing from a dry to a wet (melted) appearance.
Temperature-indicating liquids are composed of phase-change materials suspended in an inert, volatile, non-flammable vehicle. They may be used to mark polished metals, glass and other smooth or shiny surfaces difficult to mark.
Temperature-indicating monitors provide the means for rapid data acquisition. No wires or readout boxes are required. They provide the user with temperature measuring accuracies to ” 1%, and are particularly useful for measuring temperatures in isolated systems, on moving or rotating assemblies wherever other recording instruments are impractical.
Applications include monitoring operating conditions of electronic parts, transformers, resistors and circuit board components. These temperature-indicating labels enable the user to visually recognize an increase in temperatures by observing a white “bulls-eye” that turns black when a specific temperature is reached.
The accuracy of phase-change materials for surface temperature indication is usually within 1%: for example, within 3.5 degrees at 350EF. 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, of course, tell the exact temperature like a thermometer but, it does determine, very 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. This important step in quality control may begin with either the operator or inspection personnel.
The most common preheat method in today’s higher technology metallurgy involves automatic electric resistance preheating. But time to preheat is an inexact science, with thickness of the metal and other factors affecting the necessary duration before desired temperature is reached. The wise fabricator checks from time to time to see how preheat is proceeding.
A phase-change indicator of a desired calibrated temperature can be used to mark the workpiece before heating begins, or may be stroked on the piece as heating proceeds. When the temperature rating of the selected indicator is reached, the dry opaque mark undergoes a phase change to a distinct melted appearance.
Temperature indicating pellets (to be placed on the surface of the workpiece), are occasionally used for preheat indication, but are usually employed in higher-temperature and more prolonged heat-treating applications.
Since the temperature indicator is in intimate contact with the surface to be tested, the phase change is virtually instantaneous. No set-up time, calibration and recalibration are required, and no operator training or experience is necessary. It is only necessary for the operator to check that the dry mark has melted.
Obtaining Correct Annealing Temperature
Even procedures where welding is fully automatic, annealing is usually not automatic. Following machine welding the piece may need to be stress-relieved to an appropriate ductile condition by annealing – a process of controlled reheating which returns the over-hardened metal to the approximate hardness of the rest of the material. To do this, the piece is repositioned. The welding machine is energized and, in what is essentially a manually controlled operation; heat adjustment is varied until the correct annealing temperature is reached.
Correct temperature is extremely important. Too hot and weld strength will be substandard. Not hot enough and annealing does not occur. The weld remains over-hardened and brittle.
Phase-Changing Lacquers
Phase-changing lacquers may be used when a very smooth or soft surface is to be tested or in instances where the surface is not readily assessable for application of a crayon mark during the heating process. Within seconds after application the lacquer dries to a dull matte finish and responds rapidly when the temperature to be indicated is reached.
Temperature Sensitive Labels
Temperature sensitive labels consist of one or more heat-sensitive indicators sealed under transparent, heat-resistant windows. The centers of the indicator circles turn from white to black at the temperature rating shown on the label. The change from white to black is irreversible, caused by absorption of the temperature-sensitive substance into its backing material.
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