surface patterns in D2

[wnease]

Is anyone else seeing crazy patterns in hardened D2 during finishing? I see some in Bohler 678, high vanadium, too. Is it the vanadium? Is there a way to emphasize the patterns?

 

[mete]

A photo would help ! It may be segregation which we metallurgists try to avoid but some knife nuts think is cute !!

 

[Nathan the Machinist]

D2 does this, to some degree or another.

D2 is a very high carbon steel that is something like 15% carbide by volume. I believe this contributes to the swirly pattern you see. I have a blade by Spiderco with a VG10 blade that shows the same patterns, which I can not explain.

D2 needs a looong soak at austenitizing temps. If you're not soaking it enough to dissolve some of those carbides you can end up with a carbon lean martensite. Those patterns can be a sign you didn't soak long enough.

A dip into ferric chloride or vinegar might increase the contrast. The blade needs to be clean or you can get splotches. A weak solution with multiple dips, with cleaning between dips, works best.

 

[wnease]

thanks... i'd post a photo but it's tricky to make it show in pics... these are industrial paper cutting blades i get from a local plant and had analyzed... i would have assumed they got pretty good heat treatment as every second those blades are not functioning the plant loses $$$

 

[SteelSlaver]

I noticed the same thing on polished D2 and I was told what Nathan is saying. I took a Dulled punch from the punch press at work (very likely D2) and polished up the flat and there it was also. I know I soaked mine for over 30 minutes and I am sure the punch got a good HT. The amount of the patterning may indicate a poor HT though. I have to wonder if you could get better dispersion with some kind of normalizing, but the Heat Treater's guide says do not normalize.. I do like the idea of making it more visible. It is also the the reason D2 never going to pass a bend test, but, it will keep an edge forever.

 

[Nathan the Machinist]

these are industrial paper cutting blades i get from a local plant and had analyzed... i would have assumed they got pretty good heat treatment as every second those blades are not functioning the plant loses $$$

That is probably a good assumption, but I will point out that many industry standard heat treat for D2 is not optimized for knives. Retained austenite, which is probably a good thing in a shear or forming die, can be disaster for edge stability in a knife. There are ways of preventing it, and ways of addressing it, but these things can fall outside of standard D2 HT. Once stabilized, it is a pill.

I have developed a love-hate relationship with D2 for this reason. When it is working, it works. When it isn't, it doesn't. Aim for a little more hardness, and RA pops up again.

 

[wnease]

interesting...they are over RC60, and edge stability in fine edges (fine being 15 degrees per side, with very very thin edges) has been a problem, though as kitchen knives they are gangbusters, and with a beefy edge they are pretty good if you aren't hacking metal. D2 takes a soft back draw (for those who are into that sort of thing) pretty easily.

 

[Dawkind]

I have heard it described as 'Techno Wootz'.....looks sorta like Boyes 'Dendretic Damascus' with the 'waterfalls' through the steel.

 

[Nathan the Machinist]

interesting...they are over RC60, and edge stability in fine edges (fine being 15 degrees per side, with very very thin edges) has been a problem, though as kitchen knives they are gangbusters, and with a beefy edge they are pretty good if you aren't hacking metal.

Yup.

I have seen D2 at HRC 62 that suffered excessive edge roll (note: not chipping). Depending on the HT, it is possible to have as much as 15% retained austenite, which would be pretty hopeless as a knife. HRC numbers don't tell the whole story. And good edge stability at 15 deg per side is not always easy to achieve in D2, depending on how hard you're aiming for.

 

[James_Terrio]

D2 takes a soft back draw (for those who are into that sort of thing) pretty easily.

Really? I'd be interested in that for certain.

 

[Nathan the Machinist]

However, the impact strength peaks around 500 deg and goes down from there. Torsional toughness peaks a little higher. I don't think of D2 as being a good candidate for differential tempering, but I could be wrong.