Heat treat and corrosion resitance?

[culter]

I've read somewhere around here that the quality of the heat treat affects the corrosion resistance of a blade. Can anybody please explain (for dummies) how and why?

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Seek simplicity, and distrust it.
Whitehead, Alfred North (1861 - 1947)

 

[The General]

Does it have anything to do with the fine grain effect? The finer the grain and the more even the Chrome is in the steel, the better the protection?

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Wayne.
"To strive to seek to find and not to yield"
Tennyson
Ranger motto

A few useful details on UK laws and some nice reviews!
http://members.aol.com/knivesuk/
Certified steel snob!

 

[Paracelsus]

Culter, the quick answer for dummies is that Yes, heat-treatment can effect corrosion resistance of steel. But also No, it is not something that should be of any real concern for buyers of properly heat-treated modern high chromium (stainless) steels. It is sort of a nit picking point for steel nerds to talk about.

Stainlessness has to due with the amount of free (as opposed to bound) chromium in the hardened steel matrix. To be considered 'stain resistant' (there is no such thing as stainless steel), a steel (iron/carbon alloy) must have greater than 12% free chromium in it.

Surprisingly, 440C with at least 16% total chromium is Not stainless in its annealed state. Enough of the free chromium has been bound up in the making of carbon/iron/chromium crstal structures that annealed 440C has less than 10% free chromium.

This is a extreme example of course, nobody would actually use a knife of annealed 440C, it would be too soft for use as a knife. But the point is that the chemical rearrangements that occur during heat treatment (annealing, hardening, normalizing, and tempering) Can effect the 'stainlessness' of a steel. For most steels considered to be stain resistant, the free chromium is high enough after the usual knifemaking heat treatments that it is not really a concern.

I was inspired last fall to write a post which became an interesting thread: Why is Stainless Steel Stainless?

You may also want to better understand what is meant by 'heat treating'. A good thread for this is: Heat Treatment, What Is It?

Happy reading!

Paracelsus

 

[Paracelsus]

General, stain resistance is not really related to grain size Directly. But there is a correlation to be made. Steel in its annealed state is colled from the critical (non-magnetic) temperature very slowly. This allows the growth of carbide crystals (grain). Grain size will be largest in annealed steel compared to fully hardened and tempered (softened) steel.

Chromium is a strong carbide (grain) former. A little bit can be a great thing. For example 52100 is a relatively simple iron carbon steel alloy, but is has a very small amount of chromium in it. Not enough to make it anywhere close to stainless, but enough to improve the formation of martensitic crystal structure at very small scales. Chromium can be a good thing for other reasons than just providing stainlessness.

On the other hand, in stainless steels with very high concentrations of chromium, the grain size will tend to be much larger than in simple carbon/iron alloys (the so called high carbon or what I prefer to call low chromium steels).

Lots of chromium makes a steel resist oxidation, but it can also adversely effect the working properties of the steel. That is why some of us are carbon steel knuts, thinking that the overall working properties of low chromium steels are preferable to most 'stainless' steels.

So yes, grain size is effected by chromium concentration, but it is the chromium, not the grain size, that confers stain resistance. Q new technology like Crucibles particle metallurgy techniques are changing all the old rules by radically changing the way steels are made. But that is another story

 

[culter]

Paracelsus, thank you for sharing your knowledge.
If you can spare the time it would be great if you could turn your posts on this subject in a short article for the knowledge base. I think it would be an excellent addition to the steel faq.

------------------
Seek simplicity, and distrust it.
Whitehead, Alfred North (1861 - 1947)

 

[The General]

Yes, thank you! Very interesting and it aught to be in the FAQ!

------------------
Wayne.
"To strive to seek to find and not to yield"
Tennyson
Ranger motto

A few useful details on UK laws and some nice reviews!
http://members.aol.com/knivesuk/
Certified steel snob!

 

[Cliff Stamp]

Paracelsus :

carbide crystals (grain)

Note there is a difference the "grain" of a steel and the carbides, grain is a more general term. During the heat treat process the steel is first heated to the soak temperature at which point it is austenite and has a grain structure but no carbides. The grains are just localized crytal orientations in the austenite.

When the steel is hardened the carbides will form first along the grain boundries as the carbon and alloy elements are thrown out of the steel as it transforms into a crystal structue than has a lower solubility than the austenite (alpha ferrite). The carbides then spread throughout the body of the steel. Tempering can refine the carbide size of the steel, as well as change its crystal form to vastly increase wear resistance and durability.

A deep cryo treatment can do the same thing. Not only can it enhance the austenite -> martensite transformation, but it can actually decompose the martensite similar to tempering. As well it can cause the formation of eta carbides during the later tempering which have been shown to increase the wear life of cutting tools by a significant amount (100%).

There is an excellent picture in "heat treatment, selection and application of tool steels', which shows a piece of hardned D2 where you can clearly see the retained austenite grain boundries and the carbides along them. Note however, at times to confuse the issue, carbides are called grains, the latter is just a more general term.

Corrosion resistance can be vastly effected by the heat treatment, Tom Mayo has commented about this before according to Paul Bos. And as with anything there is a trade off. You can for example get a very aggressive edge with low RC heat treatments, but the corrosion resistance is low (as is the strength etc.) .

-Cliff