Heat-treating under protective atmosphere : how, and why ? 03 October 2023

How and why do you use protective atmosphere for the heat treatment  ?

Why is it strongly recommended to harden stainless steel in a protective atmosphere? Knifemakers who are used to hardening stainless steel blades have experienced this at least once: heat-treating a "bare" stainless steel part in the hardening furnace means signing up for a few extra hours of work after hardening and tempering!

During the heat treatment of a stainless steel part, the presence of oxygen leads to oxidation and the formation of scale on the surface of the part - resulting in additional work steps after austenitisation and tempering. To save time and ensure optimum hardening, it is strongly recommended that stainless steel blades are heat treated in a protective atmosphere.

Let's go into a little more detail: first of all, this preserves the quality of the material's surface by avoiding surface decarburisation (disappearance of carbon when heated in contact with oxygen). In addition, it reduces the risk of deformation or cracking due to a reaction with oxygen (oxidation). Lastly, it helps to improve the mechanical properties of the object by ensuring even, controlled quenching. In short, quenching in a protective atmosphere is essential to guarantee optimum results during heat treatment and ensure the quality and hardness of your treated steels.

As well as saving time, you save :

• Money: less abrasive used!
• in quality: we avoid surface decarburisation (disappearance of carbon when heated in contact with oxygen), which is the source of a drop in hardness (less carbon = less hardness) and much poorer resistance to corrosion..

 

What do you use to form said protective atmosphere ?

Stainless steel foil

The first solution is to use a stainless steel foil to protect your blade from the ambient air and ensure successful heat treatment. This strip is designed to withstand temperatures of up to 1200°C at its peak, making it ideal for heat-treating stainless steel blades, even the most technical ones that require a high austenitisation temperature. As with cooking fish (but not in the same type of oven!), you will need to make a papillote, in which you will enclose your stainless steel knife blade.

Cut your stainless steel foil to the right size, place your blade inside and close the foil. It's essential to expel as much air as possible and make the folds watertight to guarantee a good result: don't hesitate to hammer the folds with a small hammer to close them properly. You can also enclose small pieces of tissue inside your stainless steel foil: when they burn, these small pieces of tissue will produce CO2, which will drive out the remaining oxygen in the foil.

Important note: once the austenitizing phase has been completed, the part must be cooled, still wrapped in its wrapping paper!

Condursal Z1100 protective paint

For knifemakers who don't want to waste any more time with stainless steel foil, we also offer another solution to give your blade the right protective atmosphere: Condursal Z1100 protective paint. This technical paint is designed to withstand temperatures of up to 1100°C, for heating times of up to 7 hours or more.

How is Condursal applied to a knife blade?

It couldn't be simpler! All you have to do is mix the paint in the can until you obtain a homogenous texture. You can then apply the Condursal to your blade: either with a brush, or by dipping your blade directly into the pot - in the latter case, leave the piece to drain well to avoid the "little drop" at the tip of the blade, which could ignite during austenitisation. In both cases, the layer applied should be very thin - too thick a layer could impair the hardening quality of your knife blade.

Is the Condursal easy to clean?

After heat treatment, the Condursal simply eliminates. You can remove the residues with a steel brush, for example. And to clean your brush, you can use Xylene - water won't work properly.

As you can see, these two methods of hardening in a protective atmosphere will enable you to get the best out of the stainless steel grades you choose to harden, while preventing oxidation and scale build-up.