What makes a steel easy or difficult to mirror polish?

[Walking Man]

From what I understand 440C is known for it's ability to take a shine, but why is it better than others? Also, what other factors play into polishing a blade? Is hardness a factor? Thanks in advance

 

[Cliff Stamp]

Machinability is likely the biggest factor, it just takes *massively* longer to remove the coarse scratch patterns with the high carbide steels. It isn't hardness as much as it is carbide content and distributions. Note there isn't uniform agreement on polishing as with most effects. People have complained about D2 in that regard for a long time but there are makers who can produce a very high quality finish. I tried it once on D2 just to see how difficult it was, forget about it without power tools. There are other issues like actual uniformity of the steel, some people have complained for example that ATS-34 was/is "dirty" in this regard which prevents a high finish. Really low end steels can often take a very high finish so it definately isn't a sign of quality.

-Cliff

 

[STR]

All the 440 grades of steel have been used for making mirrors for a long time. (well not the later 440V I don't think) I read somewhere that the mirrors of rest areas and even the ones in airlines and the space shuttle were 440 steels.

Generally speaking most any of the high Chromium steels can be mirror polished. I've seen some stunning mirror finishes on ATS34 as well as H1.

As for why it takes a mirror polish better or easier with no evidence of lines it is the fact that its a high chrome steel. D2 actually has just enough to allow it to be polished up to a pretty good shine but when the Chromium drops to below what is in it it is pretty hard to get a mirror finish if not impossible. I remember once years ago trying to mirror polish a knife I made and I could never get it anywhere near what I'd seen others do. It is a lot of work. More than making the knives sometimes. For me it was an effort in frustration though because mine was A2 steel. Needless to say it didn't cooperate.

STR

 

[ghost squire]

As Mr. Stamp said low end steels often polish up well. I don't know the specifics of it but I believe this is because they tend to have a high chromium content, which makes them "smushy" and "smeary". You may have noticed this when trying to sharpen a Pakistani or Chinese made knife.

Anywhoo according to my knifemaking book by David Boye, when you polish a knife its actually smearing and closing the gaps between grain and scratches, forming a near uniform surface.

 

[JohnR7]

It is a lot of work.

Usually what I do is use a 600 sand paper, then I will buff the knife with the red buffing compound. It is fast and easy and I get the best results for the least amount of work. If the knife is in really bad shape, then I may use a 400 sand paper first. But that is rare. If the knife is in really good shape, then I may use a 1000 or 1500 sand paper to keep from scratching it.

 

[JohnR7]

Anywhoo according to my knifemaking book by David Boye, when you polish a knife its actually smearing and closing the gaps between grain and scratches, forming a near uniform surface.

I noticed that with the new scotch bright buffing wheels for the dremel. They clean it up real good and it looks fine, but if you go over it with your polishing compound, you will clean your nice finish off of the blade or knife.