- Joined
- Nov 6, 1999
- Messages
- 2,639
I wrote this post earlier today in response to a question about fillet knife stainless steels. But I think it might be an interesting topic for a more general discussion. The question in the other thread that I was trying to address with the following was: are stainless steels with the same absolute chromium content equivalent in stain-resistance? For example, is 440A more stain resistant compared to 440C? Are there other factors in blade steels or their production that influence the stain-resistance of the metal? Another question raised in that thread was the correlation of RC ratings (hardness) to edge holding. These are different questions, but I am hoping here to start what I hope will be an interesting discussion of blade metallurgy.
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The chromium content of a steel is the Most important single factor in stain resistance. The problem is that the percentage chromium in the alloy does Not tell you the amount of free chromium in a hardened and tempered blade. And no steel is truly 'stainless'. They will all corrode given enough time in the wrong conditions.
Iron just Loves to react with the oxygen in the air, or the acids and salts in our foods and water to form iron oxides. There are two primary iron oxide forms, ferrous and ferric. The difference between these is not important here.
My understanding is that 440A really is more corrosion resistant than 440C, even though they Appear to have the same chromium content and are identical in all other alloying elements except for carbon content.
The reason for this is that the anti-corrosion protection produced by chromium is a Surface phenomenom. Free chromium oxidizes to chromium oxide in a one-molecule thick, self-repairing, and invisibile surface layer. It is this protective coating which inhibits oxidation of iron deeper in the blade. Mirror finishes help by minimizing surface area for oxidation on the blade. It reduces the size of pits and scratches which may break the chromium oxide surface layer and allow oxidizing agent like chloride ions (from salt) to oxidize iron to ferrous oxide (red rust).
Chromium is also a strong Carbide former. It chemically combines with Iron and Carbon in making rigid crystal structures that make blades harder and hold edges longer. Low chromium steels like 52100 hold edges better than some other similar steels in part because of the small amount of chromium that is added (less than 1%). This is not nearly enough to add any corrosion prevention, however. It takes about 12% FREE chromium (that not bound up in carbides) in a finished blade to make a steel 'stain resistant'.
The additional carbon in 440C (1%) compared to 440A (0.6%) will make the blade harder and more wear resistant given the same heat treating conditions by producing more hard iron and carbon 'carbides'. But it will also soak up more of the available chromium, reducing the amount of Free chromium in the metal.
The bottom line is that it is an error to expect that the absolute amount of chromium in a steel correlates exactly with corrosion resistance. The amount of free chromium is a good predictor of oxidation inhibition. But the amount of free chromium depends on the concentration of other elements in the steel, and the ways these effect the final unbound (free) chromium in the metal.
In regards to RC scales and edge-holding: there is a rough correlation here. However, all steels at RC58 will NOT be equivalent in edge holding. Some will be better than others. This is because the RC scale only measures resistance to Penetration. Not wear resistance, toughness, malleablility, or ductility. These physical properties of steels are influenced by the alloying elements in the metal. The final combination of elements into crystals of different sizes is determined in the heat-treating steps of quenching (hardening) and tempering (softening). Every steel has its own unique characteristics. Too much is made recently of having very high (eg RC62) hardenesses. This will produce better wear resistance, but will often be associated with brittleness (low resistant to mechanical forces like bending).
Some steels make fine blades at RC60-62. Some do not. Most simple carbon steels have a nice balance of edge holding and toughness in an all hard blade tempered at RC56-58. Smaller blades for slicing can be left harder. Larger blades for chopping and prying need to be tempered a little softer to add toughness.
Some blade materials like Talonite (not really a steel at all since its principle ingredient is Cobalt, not iron) have RC values in the mid 40's. This is very very low by steel standards. On the other hand, talonite seems to outperform many iron containing blade steels in the edge holding departments. Its physical properties are quite different from steel.
It is not realistic to compare RC values of cobalt alloys and iron alloys to compare potential edge holding. Or even to compare RC values of regular knife steels to predict performance as a knife. And folks like the bladesmiths of the ABS practice differential heat-treatments, producing blades with hard edges, but soft backs and tangs, dramatically improving the overall performance of a steel.
Hope this rambling chemistry lesson helps. If I am wrong about any of this, please let me know
Paracelsus
[This message has been edited by Paracelsus (edited 10-23-2000).]
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The chromium content of a steel is the Most important single factor in stain resistance. The problem is that the percentage chromium in the alloy does Not tell you the amount of free chromium in a hardened and tempered blade. And no steel is truly 'stainless'. They will all corrode given enough time in the wrong conditions.
Iron just Loves to react with the oxygen in the air, or the acids and salts in our foods and water to form iron oxides. There are two primary iron oxide forms, ferrous and ferric. The difference between these is not important here.
My understanding is that 440A really is more corrosion resistant than 440C, even though they Appear to have the same chromium content and are identical in all other alloying elements except for carbon content.
The reason for this is that the anti-corrosion protection produced by chromium is a Surface phenomenom. Free chromium oxidizes to chromium oxide in a one-molecule thick, self-repairing, and invisibile surface layer. It is this protective coating which inhibits oxidation of iron deeper in the blade. Mirror finishes help by minimizing surface area for oxidation on the blade. It reduces the size of pits and scratches which may break the chromium oxide surface layer and allow oxidizing agent like chloride ions (from salt) to oxidize iron to ferrous oxide (red rust).
Chromium is also a strong Carbide former. It chemically combines with Iron and Carbon in making rigid crystal structures that make blades harder and hold edges longer. Low chromium steels like 52100 hold edges better than some other similar steels in part because of the small amount of chromium that is added (less than 1%). This is not nearly enough to add any corrosion prevention, however. It takes about 12% FREE chromium (that not bound up in carbides) in a finished blade to make a steel 'stain resistant'.
The additional carbon in 440C (1%) compared to 440A (0.6%) will make the blade harder and more wear resistant given the same heat treating conditions by producing more hard iron and carbon 'carbides'. But it will also soak up more of the available chromium, reducing the amount of Free chromium in the metal.
The bottom line is that it is an error to expect that the absolute amount of chromium in a steel correlates exactly with corrosion resistance. The amount of free chromium is a good predictor of oxidation inhibition. But the amount of free chromium depends on the concentration of other elements in the steel, and the ways these effect the final unbound (free) chromium in the metal.
In regards to RC scales and edge-holding: there is a rough correlation here. However, all steels at RC58 will NOT be equivalent in edge holding. Some will be better than others. This is because the RC scale only measures resistance to Penetration. Not wear resistance, toughness, malleablility, or ductility. These physical properties of steels are influenced by the alloying elements in the metal. The final combination of elements into crystals of different sizes is determined in the heat-treating steps of quenching (hardening) and tempering (softening). Every steel has its own unique characteristics. Too much is made recently of having very high (eg RC62) hardenesses. This will produce better wear resistance, but will often be associated with brittleness (low resistant to mechanical forces like bending).
Some steels make fine blades at RC60-62. Some do not. Most simple carbon steels have a nice balance of edge holding and toughness in an all hard blade tempered at RC56-58. Smaller blades for slicing can be left harder. Larger blades for chopping and prying need to be tempered a little softer to add toughness.
Some blade materials like Talonite (not really a steel at all since its principle ingredient is Cobalt, not iron) have RC values in the mid 40's. This is very very low by steel standards. On the other hand, talonite seems to outperform many iron containing blade steels in the edge holding departments. Its physical properties are quite different from steel.
It is not realistic to compare RC values of cobalt alloys and iron alloys to compare potential edge holding. Or even to compare RC values of regular knife steels to predict performance as a knife. And folks like the bladesmiths of the ABS practice differential heat-treatments, producing blades with hard edges, but soft backs and tangs, dramatically improving the overall performance of a steel.
Hope this rambling chemistry lesson helps. If I am wrong about any of this, please let me know
Paracelsus
[This message has been edited by Paracelsus (edited 10-23-2000).]
