Decarburized Steel Critical for Induction Hardening Rotary Blades
For in-house heat treat operations, the number one goal is to produce a reliable product with consistent in-service performance. Yet supply chain and specialized processes can cause consistency stressors. In this article, Heat Treat Today underlines the importance of consistent feedstock for in-house induction heat treater, National Steel Rule, and how the essential mill process of controlled decarburization can be actualized.
This informative piece was first released in Heat Treat Today’s April 2026 Annual Induction Heating & Melting print edition.
If you have any comments or queries, on this article, let us know at editor@heattreattoday.com.
Para leer el artículo en español, haga clic aquí.
Introduction: Steel Rule that Bends
National Steel Rule manufactures rotary cutting rule for the corrugated box industry. Located in Linden, New Jersey, the company supplies products to the die making and die cutting industries globally. They have established a high standard of sourcing, researching, and testing material for their rule, in addition to a complete testing laboratory with both rotary and flat die cutting equipment.
Their steel rule is purchased from a mill that performs a controlled decarburization on the entire feedstock. When National receives the steel feedstock, they work the steel to create teeth, employing induction hardening as part of the process. The finished cutting rule is then sold to steel rule die makers who mount these blades and an ejection rubber on laser cut wooden boards. The manufacturer must ensure their rule blades are sound, as even microscopic cracks will open during the die cutting process.
National’s rotary blades and other products rely on purchasing decarburized steel. “Flexibility and formability are paramount,” states Ed Mucci, president of the company, and Alexander Heucke, chief engineer. Cutting rule must be bent to form a circular blade; in service, that blade rotates to cut into the corrugated material. The curve geometry can be extreme, often bending up to a 7-inch interior diameter. As such, the purchase of decarburized steel is critical for the manufacturer’s business. At present, National sources the material internationally. Mucci explains, “Manufacturers aren’t using large quantities of decarburized steel, making it challenging to source, at least domestically.”
Rotary rule feedstock typically involves C36 (SAE 1036) to C50 (SAE 1050) carbon steel with a hardness range of 32–34 HRC. Mucci and Heucke note that their steel of choice has a total decarburization layer to a depth of 0.0005” depth, with partial decarburization of at least another 0.0005–0.00075”. This ensures that when the rule is bent, the surface stretches versus cracks. Bending the rule is itself a test of whether it has been properly decarburized, with metallurgical testing serving as a quality control verification that suppliers are producing the appropriate decarburization levels.
Precise Induction Hardening Teeth
While bending is essential to forming the appropriate curve, the teeth must be resistant to wear and breakage. National’s rotary cutting rule has performance expectations of at least 750,000 impressions on paper, itself a highly abrasive material. To do this, their in-house heat treat operations induction harden the edge of the rule to ensure a long die life.
There are two methods used to harden the teeth. The primary method is to shave a profile into the strip steel and then induction harden this edge. Serrated teeth are then ground in. “This gives us better control of hardening depth,” according to Mucci and Heuke. The second method is to induction harden after the serrated teeth are ground in. “We have to make sure we don’t harden the teeth too deeply, or we can affect the bendability.”
Induction hardening involves short cycles, and as such requires careful process control to guarantee consistent results; temperature-indicating crayons that melt at a specific temperature are used as one of the process control methods. Hardness testing is performed as well.
Decarburization Revisited
“Usually, one tries to prevent decarburization or even add carbon,” states Mark Hemsath, executive consultant at WINGENS CONSULTANTS and longtime expert and innovator in the thermal processing industry. “Decarb often occurs by accident in poorly designed annealing systems, especially in continuous-type furnaces.”
Oxygen, in the form of air or water vapor, is key to the decarburizing process. Less carbon on the surface means a softer, more malleable steel, and while the art of a controlled decarburization process is well known, it can be challenging. Decarburization is a process usually performed below 1500°F. “The preferred method is to use water vapor or steam as a source of the oxygen,” notes Hemsath, pointing to the stability of hydrogen-to-water vapor (H₂/H₂O ratio) derived from the Ellingham diagram. These H₂/H₂O ratios indicate the non-oxidizing qualities of the gaseous mixture, which will allow it to be the carbon reducing agent in the atmosphere. Most furnace companies can provide the necessary equipment and customize size specifications to make it suitable for this special process, and these furnaces are typically retort-based bell or pit type.
Two Methods to Control the Decarb
There are two ways that a decarburization process can be intentionally completed. The first is decarburizing the entire product. In this method, even decarburization is applied to the entire coil sheet surface. “This cold rolled steel, typically with lower carbon, is used for appliances that need enamel adhesion,” Hemsath explained, noting U.S. Steel and AK Steel, now a part of Cleveland-Cliffs, have used this form of controlled decarburization.
Another form of decarburization is selective surface decarburization. Hemsath shared, “If selective decarburizing is required only on the edges, then you could keep the coils tightly wound and the decarburization would affect mainly the coil edges. There would be ingress of carbon loss, reducing towards the center of the wound coil surfaces.”
Conclusion
“Decarburized steel just isn’t in high demand,” according to Mucci, as “most industries are looking to harden and temper the steels they use.” In fact, preventative steel decarburization is more typical and often emphasized in trade shows, technical presentations, and in thermal processing publications. Yet there are products that rely on intentional decarburization to be successful.
Controlled decarburization at the mill brings challenges, in part because successful, consistent decarburization is not often cost effective for the North American thermal processing market. These challenges encompass regional access issues, niche market access, equipment selection needs, and technical process execution.
National’s experience underlines the challenges North American mills face in providing local, in-house heat treaters with reliably, well-controlled decarburized steel that will maintain service life.
Acknowledgements: Heat Treat Todayextends thanks to Dan Herring, The Heat Treat Doctor® at The HERRING GROUP, Inc., who was instrumental in the development of this article.
For more information: Contact Heat Treat Today’s Editorial Team at editor@heattreattoday.com.
Main image: RP8 rotary rule edge hardened | Image Credit: National Steel Rule
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