Steel C70 and its alternative K720.
The C70 is one of the steels widely used in cheap cutlery and can be a good steel to start making the first knives since it is cheap and if you want you can harden it with some homemade equipment if you do not want to take it to some friend maker who has the oven.
It is used on low-end knives despite having excellent qualities but not that of corrosion resistance so they are steels that when used for cutlery need more maintenance and the foresight to be dried well after their use and that the scabbard is not wet.
Surely it is not a steel to choose if you know that it will often remain in humid environments but there are also those who appreciate this vintage look that is created and in any case you can always easily create a passivation of the blade that protects it from oxidation.
The so-called carbonaceous steel is an alloy formed essentially by Fe (iron) and C (carbon) plus some other chemical element depending on the classification and the manufacturer.
It is a “simple” steel (at least on paper!) to be processed both by forging and by removal from a bar.
It is an inexpensive steel, so it is one of the first steels that is used by novice makers , but also very appreciated by those who have a lot of experience for some types of knives because of its physical and mechanical skills.
Personally I used this steel for my first attempts to build knives, precisely because of the low cost and ease of processing by removal as well as for its simplicity for the reclamation process (hardening and tempering).
Carbonaceous steels are the only ones where it is possible to carry out a hardening process in forging.
A steel that can also be recovered from some mechanical parts, such as the crossbows of old vehicles or simply by reforging some old tools of disused blades.
The steel that is used is normally the C70 (at least here in Italy) which derives from the Italian DIN classification, equivalent to 1.1520 for classification according to UNI EN 10027-2, in the classification of the American Iron and Steel Institut AISI 1070.
Chemical composition of steel C70:
Temper and Reach:
- Temperature of hardening 850 degrees C
- Cooling Oil, Air, Water
- Found at 150-200 degrees C in the oven for 60/90 minutes.
- Hardness 59 HRC
A clarification for the hardening of C70 steel
What I have indicated are general temperatures but for the correct temperatures it is essential to contact the steel manufacturer because the various foundries insert elements that can change the various CCT diagrams (cooling speed curve).
From the previous table we see that Manganese is an element in percentage equal to Carbon, this element improves the structure of crystalline grain and contributes to hardenability.
Other manufacturers include some other element, although in much smaller quantities, of Silicon Nickel and Chromium.
Each recipe therefore changes the characteristics of the alloy and its reclamation process (hardening and tempering).
Even if it is a “simple” steel in reality this simplicity will lead you to understand soon that it is not always so easy to harden to get a great result, if you search in the forums you will see how many discussions of makers who have mistaken the hardening of the C70.
An alternative can be the K720 but the C70 manages to have elasticity qualities well above the k720.
To remind you the C70 is a simple steel with 0.7% carbon (aisi – 1070) therefore less carbon than the K720 and contains few other binders such as silicon or manganese (in a small percentage).
Some time ago it was widely used as spring steel but also in cutlery it has made its history
The Bohler K720 is an alloy steel with 0.9% carbon (90MnCrV8) and contains some binders such as silixium, manganese, chromium and vanadium to give it superior characteristics.
Widely used by mold makers, especially in folding and drawing molds.
It is also known as “aisi – O2” and is known for having excellent cutting skills.
The K720 hardens at a temperature a little lower than the C70, so it can be advantageous if it does not have a hardening or forging oven, and if you only have a small forge made with jars you should be able to harden it.
- The c70 I would say that you can harden it to 850 degrees in oil and then send it back 1 hour to 220 to get about 58 HRC.
- For the k720 you temper it at 800 degrees always in oil and personally make a double tempering, at 240 degrees the first and 220 degrees the second, you get a hardness of about 58 HRC and a good elasticity.
Personally I prefer quality carbon steels such as U10A steel but they are tastes, experiment because both the C70 and K720 are cheap carbon steels but that can give great satisfaction.
In my opinion when you start the C70 and the MA5MV are the steels to be used even if I must say that the MA5MV has nothing to envy to super steels much more expensive and often fashionable, there is because anyway the makers create (the good ones) or follow the trends to give value to their creations.
Steels are to be known so have fun trying them to find what best responds in some way to the types of blades you want to create and to your personal goal, without forgetting the intended use of the blade where for example for a kitchen knife steel C70 is not particularly suitable, try cutting an apple with a opinel in the carbon version and feel the taste, pinch, maybe you like it but it is not ideal to contaminate the food.
It is a carbon steel that reaches a high hardness and if sharpened well has a very high cutting capacity, you re-sharpen it quickly enough and holds the sharpening well.
It tends to oxidize easily and therefore a good maintenance of the knife must be done.
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