All about how we make our knives

It’s not just the shape of the cutting edge that determines how sharp a knife is, but the quality of the steel plays its part too. In fact, it is crucial to determining how the steel can be worked and processed. What’s more, the cutting edge has to remain stable and resistant to corrosion over the long term. That’s what we, and most certainly you too, expect from a good knife. To meet this expectation, ZWILLING only works with select raw materials as part of highly-sophisticated production processes subject to strict quality controls.

ZWILLING has optimized the interplay between temperature and force. The result: upset-forged SIGMAFORGE® knives. We control temperature and use it precisely where it’s needed. In the pre-treatment phase, heat is only applied to the part of the blank which is to be shaped into the bolster during forging. The quality of the steel structure is preserved in its entirety, completely unaffected by the heat - a prerequisite for creating the particularly hard, flexible and edge-retaining SIGMAFORGE® knives. During the forging process, SIGMAFORGE® knives are precision-forged from a single piece of steel and the bolster is formed by applying temperature and force. To this end, it is of the utmost importance to intelligently control the interplay between process parameters in order to obtain a precise result.

Our knives are also made from a single piece of steel as part of the traditional forging process we use. In contrast to the SIGMAFORGE® knives made by ZWILLING, the traditional forging process involves the entire steel blank being heated and shaped at high pressure. This process is used to create the sophisticated full-tang design of the knives in the TWIN® Cuisine series. During the forging process, the steel is heated to above 1100 °C and struck into a die with a drop hammer to give it its shape. This takes place in three to four steps until the shape required to produce the knife blank has been achieved. Each knife type (bread knife, chef’s knife etc.) must have its own die. The blank created then continues to be worked on.

For knives without bolsters, usually known as stamped or household knives, a sheet of steel is taken as the starting point and used to stamp out the blade blank. This blank is the same shape as the finished product for full-tang knives. Depending on the shape and size of the blade, the blank’s position on the sheet may vary greatly to make the most out of the material. The pre-production process for knives in this category is simple and therefore economical, and it is even possible for several blades to be stamped out with a single press stroke. What’s more, the process of shaping these blades involves much fewer steps than shaping blades as part of the forging process as there is no bolster to form.

Knives have been made from multiple layers of steel for centuries now. Since day one, the production process has mainly involved drawing on the various properties possessed by the types of steel being processed in order to combine resistance to corrosion with a sharp cutting edge. To date, classic Damascus steel knives have been created in a highly-sophisticated production process. Over the years, however, other production processes have caught on and nowadays, the striking look and protective properties of Damascus steel are achieved through using multi-layer steel. To this end, the ultra-hard core of the blade is encased in at least two softer roughened layers of steel, which are then fused together. The hot steel is then rolled out, subjected to more heat and sandblasted. This process is repeated once again after the steel has cooled down. Blanks are then cut out of this multi-layer material which has been firmly welded together, before being misshapen so that the straight layers transform into the waved pattern characteristic of Damascus steel. The blank is then cut into the desired blade shape and further processed until a knife is created. This highly-sophisticated production process has its advantages in that the pattern which appears on the blade is individual and distinctive, and the steel layers which surround the core play a protective role. The grind is the only place where the ultra-hard core is on show, meaning that any premature corrosion or breakages are avoided, but the cutting edge is razor-sharp. There is a fine balance to be struck when it comes to the hardness of the material: if the steel is too soft, then the knife will quickly turn blunt, but if it’s too hard, the knife can break easily. ZWILLING has optimized the hardening process so optimum material hardness can be achieved.

FRIODUR® is an optimized hardening process developed by ZWILLING and denotes products made from steel which has been thermally treated and hardened in the following steps:

1. The steel is heated to above 1000 °C.
2. It is then very quickly cooled so its temperature falls below 800 °C, subsequently being left to slowly cool down to room temperature.
3. Ice-hardening causes the steel to transform even more, further improving its hardness, as well as its resistance to wear and tear and corrosion. This step also optimizes the steel’s structure.
4. Tempering harmonizes the grain structure and relieves any stress from the material. This creates flexibility, makes the knife safe to use and prevents the blade from breaking.

The result: high levels of hardness, elasticity and resistance to corrosion

ZWILLING has developed an optimized hardening process for multi-layer steel with a Rockwell hardness rating of 60 and above, which involves fine-tuning hardening temperatures in line with the properties of the steel. Steel is hardened in an ultra-modern vacuum furnace which enables temperatures to be precisely controlled and rules out negative oxidation states. Blades are quenched to room temperature in a matter of seconds and then cryogenically cooled and hardened. Different temperatures are used for tempering depending on the properties of the steel. The result: optimal, long-lasting hardness, supreme cutting performance (initial performance and edge retention) and top-class resistance to corrosion and blade flexibility