Marquenching/Martempering: How Does it Work in Achieving Red Hardness of Steel?22 November 2019
Red Hardness, the terminology paints quite a picture. After a steel workpiece slides into a furnace, it soaks up so much thermal that it becomes “red hot.” Alloy microstructures alter when they reach this point. The steel hardens. More importantly, the metal retains this hardness quotient after it cools. That’s an important detail here. Marquenching, well, this low-distortion cooling technology blends with red hardness processing to raise a number of interesting heat treatment posers.
Red Hardness Heat Treatment
As usual, this is a term that can mislead the untrained eye. Sure, steel components shift in colour as they get hotter. Pushed towards a transformative state change, where the alloy will deform and melt, a furnace contained steel part will undergo a red-shift. But the term is also swapped out for another heat treatment phrase. Known as secondary hardness, this implies a phase state where a primary hardness quotient is infused. That’s true enough, then the hardness rating drops slightly as the alloy part is tempered. Peculiarly, though, the hardness climbs again when the tempering heat is maintained at around 600°C. This is the Red Hardness or Secondary Hardness effect, as produced by an intelligently regulated tempering phase.
MarquenchingAs An Interruptible Or Stepped Heat Treatment Method
That’s all very well, the secondary hardness phase creates tougher, more fatigue-resistant steel parts. But there’s a problem. If the component is also to pass through a distortion-reduction process, then the 500 to 600°C tempering phase requires a more controlled monitoring mechanism. You see, to produce distortion-reduced workpieces, the process can’t utilize an aggressive quenching medium. Out goes the water or oil quenching medium, then in comes the Martempering quench. With Marquenching, also known as Martempering, hot salts replace the water or oil pool. The problem being, red hot steel parts, especially those in hot salt baths, can undergo unpredictable phase changes when they’re being tempered. Of course, the very best heat treatment services know all about such issues. And, by using a red hardness oriented quench cycle, high secondary hardness quotients are still very much achievable.
By adjusting the temperature of the hot salt and the length of time a thermally soaked steel component is held at its upper transformative temperature, superior hardness ratings are realized during the Martempering phase. More importantly, they’re also retained as the Martinizing process slowly returns the component to room temperature. Hold times are key here, as are the salt bath temperatures that quench the metal. As a matter of fact, you could be looking at multiple salt baths, each held at a different distortion-reducing quenching temperature.
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