In certain welding applications, the surfaces to be welded together are required to be preheated before welding commences.
What is preheat temperature in welding?
The preheat temperature is defined as the minimum temperature at which the base material needs to be before any welding takes place. For small parts, preheating is usually done on the entire part. For larger weld structures, usually only the area surrounding the weld is preheated.
Preheat temperature requirements specify a minimum preheat temperature.
The temperature measurements are often done with Tempilstik® temperature-indicating crayons that melt at a specific temperature.
A minimum preheat temperature is required to be maintained to control several key features.
One important feature is hydrogen cracking, or cold cracking, which occurs when hydrogen gets entrapped in the welded metal.
Preheating the work piece reduces the cooling rate of the weld bead and increases the time spent at higher temperatures when hydrogen can diffuse out of the weld bead more easily.
Another parameter that can be specified in the weld procedure is interpass temperature, the temperature at which subsequent weld passes are deposited after the first pass in a multi-pass welding process.
Interpass temperatures are measured usually just in front of the location where the next weld pass will start. Once again, the slower the cooling rate of the weld bead reduces the chance for hydrogen cracking. It can often be treated similarly to what is done for preheat temperature, although the requirements for minimum interpass and preheat temperatures may differ.
Both preheat and interpass temperatures are crucial parameters in welding. They affect the weld bead cooling rates which, in turn, determine the metallurgical microstructures that are formed in the weld and the adjacent base metal.
Higher interpass temperatures tend to reduce the weld metal strength, but at the same time, improve the toughness of the weld. However, above a certain temperature the trend is reversed, and the toughness will decrease with increasing temperature.
For this reason, the maximum interpass temperature is often specified to not exceed a certain level, for example, 290° C (550° F). Interpass temperatures can also affect the yield and tensile strength of the weld bead: if the interpass temperature is too high, the bead strength may fall below the process requirements.
Minimum interpass temperatures are specified to control hydrogen cracking, and in most cases are similar to the preheat temperature requirements. A minimal interpass temperature is specified to reduce cold cracking by increasing the ductility of the weld joint – a minimal interpass or preheat temperature can minimize the probability of cold cracking by increasing time for hydrogen to diffuse out.
The requirements on the minimum interpass temperatures are in most cases similar to the preheat temperature.
The interpass temperature can change from welding pass to pass. It may either increase or decrease in the consequent passes depending on the thickness of the joining parts or the weld cross-sectional area.
If the cross-sectional area is large, it is likely that the interpass temperature will not increase significantly with each of the passes. In smaller parts, heat is easier to collect in the parts and the interpass temperature will increase over the passes.
A similar phenomenon occurs in Wire-arc additive manufacturing (WAAM) processes where the built up structure collects the heat and the temperature between the deposition passes keeps increasing.
Monitoring the interpass temperature for WAAM is crucial for the stability of the process and for the integrity and quality of the deposited layers.
Xiris’ XIR-1800 SWIR thermal camera can be used to monitor the cooling rate which is directly affected by preheat and interpass temperatures.
It can also monitor the maximum allowed interpass temperature directly, as the XIR-1800 sensitivity starts at around 250C and is sensitive enough to detect when the temperature of the previous pass is above a recommended value.
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