Relative strength of different steels

Nathan the Machinist

Nathan the Machinist

KnifeMaker / Machinist / Evil Genius
Moderator
Knifemaker / Craftsman / Service Provider
Joined
Feb 13, 2007
Messages
19,125
I was looking through some steel data sheets today and found some interesting figures for different steels we use.

Of course the strength of a steel is directly related to it's harness. The harder the stronger, as measured by ultimate tensile strength, compressive yield strength and other measures (excluding impact toughness). I found out that:

D2 is very strong, with a yield strength of 319,000 PSI at 62 HRC.
That is 2.5 times stronger than 6-4 AL-V titanium, at less than twice the weight. Yup, better strength to weight than Ti

440A AKA "Surgical steel" peaks out at 229,000 PSI at 54 HRC

D2 @ 55 HRC is 270,000 PSI
A2 and O1 @ 55 HRC are both 261,000 PSI

At least according to Bohler-Uddeholm.

Just thought I'd share...
 
isnt that a measure on how much it can be pulled before breaking? how would this compare to a prying test? thanks!
 
Yeah, that is the tensile strength(how much to pull apart). And yeah that is super high, especially if you consider most structural steels for buildings and what not is like 70,000 Ultimate. As hardness goes up so does tensile. But the impact or ductility goes down. In the use of a knife I think its more about the hardness for edge retention not so much strength. A prying test, or maybe an Impact test would for sure be alot more meaningful because the harder they are the more brittle, thats why we temper. I think Blade mag just had something on impact testing a knife.
 
Thanks for posting that Nathan , you have definitely been a asset to this forum . It
is a pleasure to have you here . :thumbup:
 
Impact strength for both A2 and D2 peaks at HRC 60. Both lose impact strength at higher and lower hardness.
 
Keith_H said:
isnt that a measure on how much it can be pulled before breaking? how would this compare to a prying test? thanks!
Click to expand...

Well, tensile means in tension (pulled), compressive is in compression (squashed). Bending is a combination of the two. When bending, the outside of the bend is in tension, there is a "neutral plane" around the middle of the section, and the inside is in compression. So at that level it is still tensile and compressive forces, but it varies the further you are from the neutral plane. Which is why, for a given amount of material, a tube shape is stronger than a solid. I think in reality imperfections in the material and notch sensitivity prevent a sample from achieving its theoretical bending strength. And the harder and less ductile the steel, the greater the deviation between theoretical and actual strengths. So I expect there is a point where a tempered part is stronger in bending than a harder untempered part (Mete?) But there is going to be a correlation between high tensile and compressive strengths and high bending strength.

Ultimate strength mean the maximum load held before failure. Yield strength means maximum load before permanent deformation, which is where you cross from elastic deformation into plastic deformation. Or put another way, the point at which it doesn't spring back all the way, like a bent tip or rolled edge. Thus the yield strength of a material is always lower than the ultimate strength. In my opinion it is a more useful number to know for a knife steel. The numbers I quoted were yield strength numbers, not ultimate strength, which is higher.
 
Help a newb wrap his mind around this.

From what Steve and Nathan have posted the "strongest" -D-2 blade would be HRC 62 at the edge and HRC 60 along the spine? Or does impact and prying have drastically different effects on steel? I sure thought there was more variance than two units of Rockwell scale on a blade.
 
Soilarch said:
Help a newb wrap his mind around this.

From what Steve and Nathan have posted the "strongest" -D-2 blade would be HRC 62 at the edge and HRC 60 along the spine? Or does impact and prying have drastically different effects on steel? I sure thought there was more variance than two units of Rockwell scale on a blade.
Click to expand...


Yes, impact strength and "prying strength" are very different.

Most steel becomes weaker, tougher and more impact resistant at softer tempers. D2 is unusual in that its impact strength goes down at softer tempers. I expect it is because the large volume of carbides are not ductile and don't get softer at higher tempers, the martensite gets weaker, and you may get some grain boundary carbides develop as you temper, weakening the steel? In short, less strength without increased ductility = lower impact strength. But I really don't know, pure speculation.

I'm pretty sure A2 gets tougher as you go up the temper scale. I've broken an A2 machete at HRC 60 ish that would have been fine at a more reasonable temper. Silly in retrospect. It did cut like the devil though, for a little while...

I guess there may be some advantage to an A2 blade, properly tempered, with a hard edge and a somewhat softer spine. There would be no advantage to someone who uses a knife like I do, but other people might benefit from it. The trick would be maintaining the different temperature across the blade over the course of an hour or more as you temper the blade. It could be done.

I don't understand the appeal of an edge quenched blade. You get pearlite in the spine. That = weak...

I tend to think that a blade that is a good candidate for D2 would be best at a solid 61-62. IMHO...
 
thanks Nathan, it would almost seem that a knife would be best with a more tempered spine than edge, I wonder how one would go about this accurately?
 
You must log in or register to reply here.
Back
Top