Blog post by Mark Sirrine with metallurgical help from Jim Senne of MetalPro Resources
Part of what makes our work so interesting – and sometimes, so frustrating – is that heat treating and flame hardening involve quite nuanced processes to achieve the exact transformation of steel the customer needs. Sometimes it’s frustrating because the old school thinking is still out there: Put a torch to it until it’s red and then drop it in a bucket of water and you’re good to go. But that’s why people end up calling us. In today’s exacting manufacturing climate, the old school thinking sometimes leads to cracking – instability in the resulting steel structure – in the field. But we don’t lose our temper when we run into these misconceptions about flame hardening. In fact, we do just the opposite: we temper, temper!
Heat treating isn’t just about the heat, OR the cool, as it turns out. It is about an entire process designed to make a steel or cast iron piece of equipment harder and more stable so it can handle abuse in the field. Whether it’s steel cables grinding down the grooves of wheels and pulleys and drums, or a pinion gear slamming against ring gears in locomotives, or landing gear on a 70,000 lb jet, the art of heat treating these components involves a carefully engineered process that often starts with localized flame hardening or induction, but ends with tempering. Tempering stabilizes the martensite structure of the steel even more, getting us the best of both worlds: durability and stability.
We’ve not talked much about tempering on our blog but we’ve just seen an increasing need for it as components come to us with more demanding manufacturing requirements. Flame hardening focuses on a specific hardness pattern the customer needs the part to have, and in the process of heat and quench we can achieve that hardness. But to ensure that hardness performs at the highest level, we often take the flame hardened part and put it in a tempering oven, followed by air cooling. Why? Tempering adds an extra level of stability to the hardened steel’s structure.
That’s because immediately after quenching, while the steel may be at its highest level of strength, it can also be brittle and subject to cracking if impacted. Tempering improves the fracture toughness and notch sensitivity, making the workpiece more suitable for use without failing. It’s added insurance that the toughness performs at the highest level in the field.
Untempered martensite (the predominant phase of steel after quenching) is subject to further phase transformation when the steel part is in service. We dare you to geek out on A H. K. D. H. Bhadeshia’s page from the University of Cambridge, with all the chemistry and metallurgy details you could want: “Tempering at temperatures around 650o promotes the segregation of impurity elements such as phosphorous to the prior austenite grain boundaries, leading to intergranular failure along these boundaries. The reversibility arises because the impurity atmospheres at the grain boundaries can be evaporated by increasing the tempering temperature. Quenching from the higher temperature avoids the resegregation of impurities during cooling, thus eliminating embrittlement.”
For the rest of us, it means tempering helps to stabilize the microstructure. Any act of heating and quenching steel can leave the workpiece in a state of high residual stress. Some steels, such as tool steels should be tempered two or three times after hardening. That’s why successful localized hardening, whether it’s through flame or induction, requires knowledgeable engineering that shepherds the piece through the entire transformation process of repeated heating and cooling to achieve the desired structure in the end. And that structure depends on how the piece will be used in the field!
So while we at Flame Treating Systems Inc. focus on the flame hardening part of the heat treating process, as manufacturers and service providers, we know that the flame hardening is one step in a process that often needs to include a trip to the tempering oven. Most of our customers who buy flame hardening machines already have tempering ovens in place to serve a variety of heat treating needs. For customers who send us parts to flame harden, we always make sure to understand the hardness requirements and performance expectations for the piece in the field. If we think the part will perform better being tempered after flame hardening, we will recommend they do that. Yes, it costs more money but the added stability is worth it. They may not know the difference between flame hardening and tempering or martensite and ferrite, they only know that the piece works in the field. And that’s fine with us. In the end, all flame hardening and heat treating should add value to the part by extending its useful life and level of performance under stress.
So although we are known as “the flame hardening experts,” being an expert means knowing how flame hardening best fits into the heat treatment process that returns the highest level of quality at the highest level of production for the lowest cost. When that process needs to include tempering, we temper, temper!
Don’t hesitate to email me at firstname.lastname@example.org for answers to any of your heat treating questions. Or call me at 919-956-5208. It might take me some time but I will get back to you! Thanks for visiting our website.