EFFICIENT HEAT TREATMENTS:
“Plating tool steels to reduce the carbon footprint of heat treatment”
Decarbonization is a challenge for all energy-intensive industries. In heat treatment technology, production methods that use electric heating can impact their carbon footprint quickly and effectively by purchasing certified renewable energy. But does the potential for improvement end with green energy purchase?
Rationalization of energy consumption in heat treatment can be achieved by intervening on different factors like: working temperatures, holding time, carbon footprint of treated alloys, elimination of operations, rejection rate, thermal insulation, strain hardening, modification of pre- and post-heat treatment operations, energy recovery and reuse, use of heat from other manufacturing processes, etc.
In the following lines a practical case based on the application a the self-tempering process in cladded tooling developed by AZTERLAN is displayed.
Self-hardening of coated tool steels
Tool steels, especially high-speed steels, are characterised by high alloy levels and by sacrifying toughness in favour of wear resistance. They are typically austenised at very high temperatures, close to 1200°C, and require multiple tempering processes. The environmental impact is therefore twofold: on the one hand, due to the contribution of alloying elements to the carbon footprint and, on the other one, because of the need for very high temperature treatments. It is worth to mention that, in contrast, many quenching and tempering steels can be tempered at around 900°C.
With the advent of additive manufacturing technologies, and in particular of L-DED, it is increasingly common to find solutions that use tool steels as a coating on a lower-alloyed, tougher steel, separating functions: the coated surface is wear-resistant while the core material is tougher. Although the re-enhanced steel has a different microstructure to its solid counterpart, its performance in service is comparable.
The following images illustrate the concept of separation of functions (wear resistance and toughness) and the microstructural appearance of a high-speed steel in the hardened and tempered condition versus a high-speed steel in the quenched and tempered condition.
Representative micrographs of the concept of hardfacing with high-speed steels. Left: Functionalization of hardfacing by L-DED (tough substrate + wear resistant coating). Center: Hardfacing. Right: Solid high-speed steel reference.
The collateral impact of cladding is the replacement of the high-speed steel in the substrate with an alternative one with a lower carbon footprint. But does it offer a higher decarbonization potential? A traditional approach would involve performing a quenching and tempering treatment on the cladded assembly. However, at AZTERLAN we have demonstrated that, depending on the application, it is possible to take advantage of the tempering that occurs in the tool steel during welding to avoid having to reaustenitise the material. In this case, only the tempering is necessary, saving a significant amount of energy. A hardnesses of 66 HRC in service can be achieved.
Furthermore, in certain applications, a correct design of the alloy added on the substrate can be put into service in raw as-weld condition. In this scenario, heat treatment and, therefore, its associated carbon footprint are completely eliminated. In these cases, it is essential to regulate the resulting hardness so that the surface is machinable and a range around 60 HRC hardness is usually pursued.
The following diagram shows the three manufacturing routes mentioned, ordered from highest (#1, with four steps) to lowest (#3, with a single step) carbon footprint.
Alternatives for energy consumption savings by taking advantage of self-hardening of tool steels cladded by L-DED.