It’s Not Just the Heat, it’s the Cool – Part 2.

Design Considerations

We ended the last blog talking about examples of quench severity applications to particular parts during the flame hardening process with flame hardening equipment. Examples like a gear with small teeth: it would require, at minimum, some percentage of polymer in the quench to make it less severe. Straight water, a pretty severe quench, would create cracks even if the gear is 1045. If it’s an alloy like 4140 you will need even more polymer in the quench solution. The cooling needs to be slowed down to avoid cracking, but again, it can’t be too slow or you lose your hardness levels. Conversely, if you’re hardening a 1045 steel shaft ten inches in diameter, the mass of the shaft would allow you to use straight water without cracking. The large mass, once heated, helps to slow the cooling rate of a faster quench solution like water. If you tried a very slow quench, such as air cooling, the shaft would likely lose the hardening entirely.

So since the last blog post went over the materials best suited to different severities of quench, this blog will now address the other three factors in good quench design: application, temperature, and agitation.

If you use the progressive method of flame hardening, the quench is applied in a spray straight from the flame head. It’s doing double duty: keeping the flame head itself cooled, and then cooling the gear tooth or whatever part it’s quenching. The quench solution circulates through the system so that it remains at as constant a temperature as possible (in our machines we provide a temperature sensor in the quench supply tank). The heating and cooling cycles work in tandem, heating up and cooling down at about the same rate and within a certain defined temperature range, with application methods that give the hardness pattern and depth required. All this is achieved in our engineering designs of flame hardening equipment: every variable is accounted for, and tested to make sure the results meet spec.

For most other flame hardening methods, we use a quench tank to dunk the heated parts into the tank. These tanks usually constitute the largest footprint of the whole flame hardening machine. They must be large enough so that they can maintain constant temperature at the required depth to cover all the heated sides of a part. In addition, agitation must be introduced to a quench tank because air bubbles can form otherwise, creating inconsistencies in hardness levels. We usually install a large pump to recirculate the water through the quench tank. If nozzles are installed in the quench bath that drive the quench directly at the part, you can sometimes use that to improve the hardness level and consistencies across parts.

No matter how you apply quench, and no matter which solution you use, temperature is the final variable that you must understand to successfully flame harden. A quench that is too cold create cracks. If the quench is too warm, you won’t get to the required hardness levels. For a quench tank, we install heaters that heat up the quench to above 70 degrees F (for most applications). Conversely, we also install heat exchangers with chillers to keep the quench below 110 degrees F (typically). Sometimes polymer quenches need to be above 110 or 120 F to work correctly with high alloy steels. The temperature, and the need for heaters or chillers, will be determined by the mass and size of the part.

Well, this is the end of my very brief, high-level overview of quenching during heat treating. I hope it gives you an idea of how important this part of the process is, so that when you have to flame harden any parts you take a look at how the part must go through the entire cycle – heating as well as cooling – and make sure you have considered at least all the variables mentioned here:

  1. Pace
  2. Application
  3. Temperature
  4. Agitation

Call me with any questions you have about heating and cooling. I appreciate your reading the blog and thinking about flame hardening and flame hardening equipment! 919-956-5208,



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