Would S30V (and all crucible steels) tend to brittleness?

L

Lavan

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Nov 8, 2000
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Logic (and logic only....no technical expertise) says to me that any knife blade made of a poured, sintered, or pressed conglomeration of grains would be more brittle and inclined to snapping on flex than a homogenous rolled or forged steel.

Is this correct?

:confused:
 
No!

Not to speak of any steel or company in particular, but you are seriously mistaken when you think of sintered and HIPed steel as being made up of "powdery grains". If the material is sintered and HIPed to full density there is no power or grain structure left (the word grain is a bit misleading: in sintering one speakes of grain growth which is the fusion of the powdered particles, but one also speaks of grains of (mostly) single crystaline structure in the steel. Here I meant the former). Due to the "surface free energy" of small particles the particles will "melt" and fuse at temperatures below the actual melting point of the bulk, this is the basis of sintering. The temperature at which that occures depends on the pressure. Depending on the sintering process, the material can be left porous (the powder particles weld together but do not completely fuse), but the HIPping (hot isostatic pressing) will yield usually close to 100% density, meaning the particles are completely fused together....there is no powder or particle structure left.
 
CPM steels are not quite like sintered metals where they take the powdered metal particles and press them under high heat. The CPM steels are made with purified powders of all the alloying elements in them, then melted together in a closed vacuum evacuated container, making molten steel like all other steels, then taken out and hot rolled into ingots and/or sheets. Instead of just taking so much iron in a furnace and dumping measured amounts of molybdenum, carbon, vanadium, chromium, etc in, it's all made from stock that is purified by atomization into powder, removing everything but the desired stuff. Purified components in precision measured quantities. IF the finished products made from CPM's steels are brittle, it would have to be the heat treatment, etc, as aside from the purity of the components that goes into it, those steels are not really that much different from other near equivalent conventionally made steels. S30V is similiar to BG42, and not too far off from 440C, and also the formula looks like M2 except with large amounts of chromium added, but with a lot more vanadium. My first S30V knife is on the way here, I cannot wait to test that sucker out and see just how well it does hold the edge. (Buck Signature 805).
 
The edges being brittle intially are not a bad thing necessarily. Sometimes it is necessary to add a 'micro bevel' after buying the knife. A brittle edge in a new tool is often considered to be a sure sign that it is a superior metal or tool that you are using. This is the case with many of my Japanese carving knives that are Rc 65. As you sharpen the knife the surface metal that was exposed to heat treat and formed during the making of the blade will be removed and replaced with the more durable underlying metal that is exposed with sharpening. This in conjunction with a microbevel can make the brittleness a thing of the past.

The only real issues with brittle blades are the ones that crop up after the tool has been sharpened and used and no longer a new tool. I believe there were some problems with S60V AKA as 440V steel a while back but they were somewhat corrected by a different heat treatment to a lower Rc. The way I see it the CPM S60V and CPMS30V are certainly both better than AUS8 and no one complains much or too loudly when the edges of a AUS8 blade chips or rolls. It is the added cost and the "premium" price tag that bring out the cussing when it happens with the S60 or S30V steels I think.
 
STR said:
A brittle edge in a new tool is often considered to be a sure sign that it is a superior metal or tool that you are using. As you sharpen the knife the surface metal that was exposed to heat treat and formed during the making of the blade will be removed and replaced with the more durable underlying metal that is exposed with sharpening.
Click to expand...


But isn't sharpening and final grinding done after heat treatment?
 
Yes final grind is done after HT. Sometimes the surface is decarburized [carbon is lost] and sometimes the surface has been damaged by improper grinding. In either case resharpening will fix the problem.
 
HoB said:
No!

Not to speak of any steel or company in particular, but you are seriously mistaken when you think of sintered and HIPed steel as being made up of "powdery grains". If the material is sintered and HIPed to full density there is no power or grain structure left (the word grain is a bit misleading: in sintering one speakes of grain growth which is the fusion of the powdered particles, but one also speaks of grains of (mostly) single crystaline structure in the steel. Here I meant the former). Due to the "surface free energy" of small particles the particles will "melt" and fuse at temperatures below the actual melting point of the bulk, this is the basis of sintering. The temperature at which that occures depends on the pressure. Depending on the sintering process, the material can be left porous (the powder particles weld together but do not completely fuse), but the HIPping (hot isostatic pressing) will yield usually close to 100% density, meaning the particles are completely fused together....there is no powder or particle structure left.
Click to expand...

Did you read the link I posted about Crucible steels? I don't think so.

http://www.ajh-knives.com/metals.html
 
DGG said:
Did you read the link I posted about Crucible steels? I don't think so.

http://www.ajh-knives.com/metals.html
Click to expand...

I do think so! That link states exactly what I said, only that according to that link Crucible HIPs into large billets which are then forged and rolled further. This link also does not mention the final density of the sintered product which according to an article in the Jan 06 issue of Blade is around 99.7%. All it says is that the Crucible steels have an even HIGHER strength because of additional steps taken. Quite frankly, the information in that link is a bit thin to come to much of a conclusion. No offense to the link, but it seems intended as an overview over different bladesteels, not as a paragraph on PMs.

With all respect to Crucible and their products (which I really like), the difference they stress to set themselves apart from their competitors (such as the way they create the powder in a nitrogen jet) are, I am sure, very relevant from an engineering point of view but from a scientific point of view, trying to understand the basic principle of PM the differences are negiable. I am not even sure, if I am willing to distinguish between sintering an HIPping. There are SO MANY forms of sintering under such different conditions (liquid phase sintering, solid phase sintering, glass phase sintering, sintering to porous structure, sintering to full density etc.) that HIPping is essentially nothing more than sintering at very specific conditions i.e. very high pressures and moderate temperatures. But then again, I am a scientist, not an engineer and we tend to simplify things.

I just wanted to counter the impression that the final product of PM still has remnants of the powdered structure leading to the impression that it could fall apart or suffer errosion like a sand castle, which is not the case. The article that I spoke of in Blade on the Kershaw Offset contains a short paragraph on MIM which is another, very specific form of sintering and it talks about some of the steps of the process and shows micrographs of the final product and you can see that there is nothing of a powder structure left.
 
HIP'ing and Sintering are two different processes. HIP'ing involves high heat and high pressure which essentially forge welds the powder particles together. A HIP'ed ingot is 99.9 percent dense if not fully dense out of the unit. Forging and rolling afterwords are done to impart some hot working properties and to get to final shapes. Sintering does not put pressure on the powder during heating so the density of the "green" part is the density of the finished part. It will be porous to some degree. MIM's final outcome is identical to a sintered part, there will be porosity. Kershaw is HIP'ing the blades after MIM so the density is nearly 100%.
 
Does all that technical stuff mean that they will stand up to ...bending... as well as a forged same hardness blade?

"not snapping" is what I mean.

Unless...hmm...a knife that cuts around corners....hmmm

;)
 
Satrang said:
Sintering does not put pressure on the powder during heating so the density of the "green" part is the density of the finished part. It will be porous to some degree.
Click to expand...

Satrang, I am very well aware of the fact that you work actually in the field and I really don't want to come across like a smart ass. But even though I am not professionally involved in this field, I am a surface scientist and simply read in and through a couple of standart graduate level textbooks on sintering as a hobby and they directly contradict what you say in the quote above. They cite several sintering techniques at elevated pressures and list sintered systems with large increase in density. This is exactly why I really don't see a need to distinguish between sintering and HIPing other than that HIPing is a subcategory of sintering. After all, sintering is a very generic term and you can sinter more than just metals. I am more than happy to let you educate me (I saved most of your posts for my personal reference and I have great respect for the knowledge that you bring to this forum), but I would like to know a bit more about this than just a brush off. But you are right, that in the case of the Offset the article states that the blade is far from fully dense after the "sintering" step, but that, according to the article, is in part due to an added component that aids the injection molding and which is removed in the sintering process, which, again, seems to me like a very specialized sintering process.
 
Metalurgy not spoken here, but pragmatically I expect any blade of HRC 60 or better to be subject to edge-chipping - regardless of alloy or manufacturing process.

I've experienced edge-chipping with blades of 440V(Military), S30V(Strider SMF) and ATS34(Buck Strider 882) - all advertised as HRC 60 or better.

An application such as digging out tree roots in your garden or even pointing dirty stakes will almost certainly result in edge-chipping with these blades.

Sorry if this is off-topic.
 
HoB. I'm with you on the basics, and you are right, we are talking about getting powder to hold together in a shape, but the true difference is the pressure. This alone puts Hip'ing by itself and actually above sintering on the food chain. I would put the true difference at the end product. Hip'ed products will almost always be 100% dense. Sintered and even sintered and pressed prdoducts will have a % of porosity. The major effect of porosity is the massive decrease in toughness of the mateirial. You can buy pressed and sintered cutting tool inserts and they work very well until you get to intermittent cuts or any chatter. They chip out under that load. There is even an in-between where powder is pressed mechanically then forged. A good number of automotive parts such as connecting rods are made this way.
 
Ah, thank you very much! That makes sense now. Actually, I wanted to appologize. When I wrote my last post, I came straight out of a meeting and I guess I carried my argumentative mood with me. Again, great information you bring to the forum!
 
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