Steel is a structural material widely used in numerous industries and applications. The properties of different steels vary from hard and brittle to ductile and tough, from easily rusting to resistant to corrosion in extreme environments.
To fulfill the requirements of various applications, numerous steel grades have been developed to provide a certain strength, ductility, hardness, toughness, or corrosion resistance.
Such a wide range of properties is possible because unlike some other metals, steel has not one solid state but several solid-state phases. These phases may have different atom distributions on crystal lattice, different levels of dislocation and the deformations of crystal structure, and, consequently, different mechanical properties.
The transformations from one phase to another occurs during the heating and cooling processes at certain temperatures below the melting point. Welding is an example of such a heating-cooling cycle.
At a given point, the temperature of the weldment rises rapidly during the weld arc passage and then cooling takes place once the arc passes. The cooling phase is usually much longer than the heating phase.
The cooling rate is one of the key factors that determines the result of such transformations in a weld. The toughness and hardness of the weld will depend on how fast it cools down post welding. To quantify the cooling rate, the reciprocal of the rate is often used, i.e., the time it takes to cool between two temperatures. The cooling time between 800 ˚C and 500 ˚C is the most widely used measure, referred to as t8/5.
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Depending on the steel grade, special requirements on the cooling rate may apply. The requirements are closely connected to the term weldability.
Weldability refers to how “easy” it is to weld a certain grade.
For example, if an alloy is considered low or even non-weldable, it means that the allowed range of welding parameters is very narrow. These parameters include travel speed, heat input, arc voltage and current, preheat, and cooling rate. Any variations of the parameters beyond the specified limits will lead to a higher defect probability.
For this reason, controlled cooling rates are required in welding of many alloys. Too-fast cooling may result in hard and brittle microstructural phases that reduce ductility.
Fast cooling rates also increase the amount of hydrogen trapped inside the weld metal that may lead to cracks at room temperature hours after the welding is finished.
Hydrogen-induced cracking, or cold cracking, is a significant defect in many steel applications, such as pipelines, structural members, etc. Slower cooling increases the probability of hydrogen diffusing out of the weld, reducing the risk of cold cracking.
Too slow cooling, however, may reduce the yield and tensile strength of the base metal (Figure 1). It is particularly relevant to high-strength low-alloy (HSLA) steels.
Another factor to consider is the peak temperature that a particular point reaches during the weld cycle. In case of the weld bead, the melting temperature can be considered as the peak temperature.
For the base metal adjacent to the weld, the peak temperature will vary depending on the distance it is away from the melting line. The areas near the weld bead, where the initial base metal microstructure transformed during welding, is called the Heat-affected zone (HAZ) (see Figure 2).
The effect of the peak temperature on the base metal adjacent to the weld. The regions of the heat-affected zone (HAZ) transform differently depending on the maximum temperature reached during welding. Reprinted from [2].
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Cooling rate and peak temperature are usually estimated by welding engineers as a function of welding parameters using formulae derived from a point heat source model dating back to the 1940s.
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These formulae may be an oversimplification in case of a more complex geometry than a flat plate. They also require the thermal conductivity of the alloy used in the application. A table conductivity value for iron for example, may differ from the actual conductivity and may introduce an extra error in the cooling rate.
An alternative would be a thermal welding camera. Real-time surface temperature monitoring provides significantly more control over the cooling rate and the peak temperature at every point of the joint regardless of the workpiece geometry.
A thermal camera such as the Xiris XIR-1800 camera provides accurate temperature measurements of the weld arc in the video frame across a wide range of temperatures - from above melting point down to around 300 C depending on the optics chosen.
Such a range allows process developers to monitor both solidification and the temperature dynamics between 800˚C and 500˚C in a single frame. Such a tool can provide an unprecedented control for your WAAM design or welding procedure for an “unweldable” alloy.
References:
[1] Mičian, Miloš, et al. "Effect of the t 8/5 Cooling Time on the Properties of S960MC Steel in the HAZ of Welded Joints Evaluated by Thermal Physical Simulation. Metals 10.2 (2020): 229.
[2] http://arcraftplasma.blogspot.com/2016/09/method-of-calculating-cooling-rate-in.html
To learn more about how you can implement Xiris Thermal Camera, reach out to our product experts.
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