Heat treatment is a critical step in the tool making process. We harden and temper our blades to give them the right blend or hardness and toughness. T0 harden the blades, we heat them up to 820 degree C, till the steel is glowing "Cherry Red" then quickly transfer them to a quenching bath, where the temperature is rapidly bought down to 20 degrees. This aggressive treatment captures the steel in a unique crystaline structure making it very hard, but brittle. To give the blades the necessary toughness we then temper them back in another furnace at 360 degrees for 2 hours. This slow gentle process, releases the stress and allows the formation of more granular of the steel structure. This also gives the steel its blue / grey appearance.
Most garden tools are cutting tools. They need to be strong to deal with significant bending and impact forces, but they must also be able to hold an edge so they can cut through soil, roots, plants and even rock. It is the quality of this working edge which often determines the quality of the tool. We use a .55% Carbon Spring steel with Boron which allows us to heat-treat the steel to 65 C Rockwell to give it extreme hardness, then temper it back to 50 C to give it toughness. It is this combination of hardness and toughness which allows the steel to be able to cut through roots and heavy soil, but also be able to absorb the impact of hitting rocks and not chip or break. Spring steel is ideal for this purpose. It holds it edge but also absorbs the impact force and like a spring, It can be bent slightly without losing its shape.
Many toolmakers use Stainless steel, but such steel doesn't heat treat to the same hardness. It therefore losses its edge quicker and can be more easily bent. A big advantage of stainless steel is that is doesn't easily rust and heavy soil is less likely to stick to it. Our spring steel is either powder coated, or finished with burnished oil to resist rust and sticking, however with time, wear and exposure the tools can rust if not cared for. In future blogs I will detail the correct care for your tools to ensure they last a lifetime.