Making Santoku - Part 2

A minor household accident slowed work, but progress continued, and this week I managed to finish the most sensitive and important step of the manufacture – hardening.

As a first step, I ground the blades roughly with a 60-grit belt to remove all manufacturing scale. From experience, I know that it is better to do so now because, during the quench, the scale thickness would increase and would be harder to remove. In addition to that, this allows me to spot whether the steel has any manufacturing defects.

To prevent loss of carbon during the quench, I am using stainless steel foil. As an additional safeguard, I also add a piece of tissue paper inside the capsule.

In the past, I experimented with a protective coating based on borax because it is cheaper than stainless steel foil. It worked well, but it was subsequently very difficult to remove, so the money saved on foil was lost on abrasive belts.

Stainless foil is disposable, but with a bit of care it can be used two to three times before it must be discarded. That it has run its course is easy to spot – it becomes full of holes.

Unfortunately, I forgot to take pictures of my new forge in action. Well, next time. It reached the required 1050 °C with ease, and I quenched the blades between aluminium plates using specialized locking tongs. According to the manufacturer, N690 should be quenched in oil, but for blades this thin, it would be enough to let them cool off just in air. However, in air or in oil, blades this thin have a tendency to deform. The aluminium plates serve a dual purpose here – they remove heat fast enough for a good quench, and prevent deformation.

The greenish surface tint of hardened blades is caused by a thin layer of chromium (III) oxide. Without the stainless foil, this layer would be much thicker and practically black.

Quenched blades were laid out on a baking tray and subjected to two two-hour tempering cycles at 180°C in my kitchen oven. This slightly reduces the hardness but significantly increases strength and toughness.

Now the most time-consuming part of the process will follow – polishing. These blades won't get a mirror polish, but a satin finish takes more time, despite being less labor-intensive. About that, next time.