Steel is the heart of the knife January 27 2015, 0 Comments
Heat Treatment is the birth of the knife and its soul
In our article ‘Knifesteeleology’ and ‘Custom Knife – Materials – Steel’ we touched the basics of the most commonly used steels for knives, Carbon Steel – Stainless Steel – Damascus Steel.
Steel is actually a big, a very big subject. There are metallurgists which do nothing else in their whole professional career than analyze, study and design different steels for different applications. We are looking only for the steel in regards to cutting, especially knife application.
Choosing the right steel for the knife depends mostly on its use; you will certainly use a different steel for a straight razor (superior cutting) as you would use for a machete (chopping and shock absorbing).
All this comes together when the knife is born, when it gets its soul! The Heat Treatment.
We all know about ‘Quenching’ and Tempering’, we heard about oil-quench, differential quench, triple temper, differential temper etc. some disrespectfully label the Heat Treating as following the ‘cooking procedure’ which in our opinion relates to their knowledge about steel.
In order to do the Heat Treatment right one has to understand steel. Here some the basics:
- Steel is a combination of Iron (Fe) and Carbon (C)
- Knife steel has additional components like Chromium (Cr), Vanadium (V), Molybdenum (Mo), Nickel (Ni), Niob ( Nb) and others; see composition of the different steels example RWL 34
- Depending on the composition the steel has different characteristics; i.e. without min 14% Cr it is not a stainless steel; i.e. common CS heat treats @ ~ 1400 – 1500 F, SS ~ 1800 – 2000 F generally speaking but one should refer to the mills datasheet for the individual steel.
- The ‘making’ of the steel has an influence on its characteristics; i.e. modern PM (powder-metallurgical) steel has better properties than conventionally molten steel and is therefore more sophisticated
- The Fe-C (Iron/Carbon) diagram gives the most basic knowledge about steel and one can estimate how complicates it gets when more elements (like Cr, Mo, V etc.) are included.
- What face of the steel do I want to achieve with the heat treatment like Austenite, Martensite, Banite, Cementite, Perlite, and Ferrite etc? Did you find the Martensite in the Fe-C Diagram as this is the phase we need in the blade after the heat treatment is successfully done? What is Martensite?
- What about corrosion resistance, toughness, wear resistance? They are heavily influenced by the heat treatment, especially the tempering. Without tempering the blade would shatter very easily as it is very brittle when just quenched. Remember hardness is not everything in a blade, 60 HRC will not guarantee that there is enough toughness and wear resistance.
All these factors have to be understood and play a role in the heat treatment.
For the specific steel, the steel mill will give out Material Data Sheets. In these data sheets the mill gives, despite other data like Charpy-C testing, thermal coefficient, elasticity etc, the recommendation for the best heat treatment like
- Austenization temperature and holding time
- Quenching medium
- Expected hardness
- Tempering instruction
- Cryogenic information (what does Cryogenic treatment achieve in the steel?)
The diagrams shows clearly that a different temperature (and this can be only 50 F) will give different hardness values which again is influenced by the tempering and cryogenic. This diagram does not include recommendation for water quenching, air quenching, salt-bath, material thickness……. So no easy ‘cooking procedure’ here.
It is very crucial to have an accurate temperature control, estimating it by the color of the steel is Victorian and does not produce the results modern steels are designed for.
Simply increasing the temperature does not mean you are achieving increased hardness values too.
When the knife maker now puts the pre shaped blade into the fiery maw, the kiln, he should be very well aware about what he is doing as with a proper heat treatment he will destine the knife, he will birth the knife and give it its soul – the special characteristic the knife is designed for.