whats so special about s30v??

[Satrang]

Cliff,
You have it right. The air solubility limit of nitrogen is roughly 0.1-0.2 percent. S30V sits right around 0.1% according to them. It is added during the melting process in the form of high nitrogen chromium. In this case it acts like carbon and stays "dissolved". When nitrogen sits in the interstitial sites it retards the formation of chromium carbide if it is exposed to carbide precipitation tempering ranges. No martensitic stainless should be tempered between roughly 800-1050 F. You get a secondary hardness bump but trash the other properties. Under 800 F, no precipitation of anything. The really high nitrogen grades will form primary nitrides and some on high temperature tempering. The nitrogen precipitation when it occurs is less damaging than carbide since nitrides tie up less than half the chromium.
Grades with higher nitrogen contents than 0.20% have to be produced outside of air-melting.

 

[Cliff Stamp]

No martensitic stainless should be tempered between roughly 800-1050 F.

This depends on the steel and what you are trying to achieve, in some steels this is the recommended range, BG-42 for example (950-1050F). There is often an embrittlement concern with high tempering for various reasons, usually precipitation along grain boundries which reduces toughness, but you can also can gain hardness, wear resistance and of course hot hardness. With stainless the main other concern is loss of corrosion resistance due to Cr carbide precipitation so again you have to examine the performance and see where the improvments needs to be made. On an interesting note some are running S30V quite soft, and some factory blades have been rehardened and checked to be 55-56 HRC, in general this isn't a good idea for this grade because it is usually achieved by either under soaking or over tempering, and if it it by overtempering it could explain the very low performance seen in those blades in regards to hardness.

-Cliff

 

[Satrang]

For BG 42 the high temperature temper is for bearing applications, which is what the grade was designed for. That range does drop the corrosion resistance and toughness but does offer maximum hardness (which is good for a bearing). It's safe to say that since nearly all the materials used for cutlery were never really intended for cutlery, the properties will never quite be optimal. Even the S30V mentioned was not "entirely" based on cutlery. Based on conversations, cutlery was a big part but not the only target for that material.

 

[Carl64]

What sort of losses in toughness would we have if a BG42 blade were hardened to ball-bearing-levels? Too bad to make it a useable knife for people wanting maximum cutting ability and endurance, or only beyond the safety margin for mass market products that may be abused?

Edit:

Exactly how hard would a ball bearing usually be?

 

[Cliff Stamp]

Optimal hardness is also useful for knives, blunting is often dominated by deformation, both direct compression and lateral folding and rippling. What is optimal for the knife depends on what it is being used for, often very low toughness is acceptable because it isn't needed, it depends on the primary method of failure.

M2 at 65/66 HRC makes a very nice knife for a lot of applications, the impact toughness and plastic deformation region are low, but these are not relevant for some uses. Wilson noted awhile ago that he had one of his S90V blades compared to a very hard BG-42 blade cutting hemp rope and the BG-42 blade did very well, which it would not have done if it was much softer.

The main problem with CPM steels, some of the newer ones, is that there is little materials data available compared to other tool steels for which you can easily turn up impact, strength and wear resistance info, with air/oil and room/cryo, treatments from various soak temps. Often in the CPM spec sheets this spectrum is very narrow.


-Cliff

 

[Satrang]

Bearings fail by compressive fatigue. Hardness and cleanliness are the two main things you try to maximize. BG42 will take a loss in impact toughness and corrosion resistance when tempered in the peak range. These losses are fairly insignificant for a bearing and the gains outweigh the losses. Estimate 20-50% loss in impact toughness and nearly the same in corrosion resistance when a martensitic stainless is tempered between 800-1050F. All this is due to chromium carbide precipitation at the grain boundaries of the steel. The reason for the wide range of loss is the wide range of precipiation you can get.

 

[Cliff Stamp]

Yes, but this isn't specific to stainless, the same general issues hold for tool steels in general, and can be for other issues besides carbide precipitation as you can get impurity segregates as well.

M2 takes a huge loss in toughness when ran full hard at 65/66 HRC, almost 70% to maximize the secondary hardening, this however doesn't mean you should not do it. It just depends on what the knife is being used for.

If you are getting primary corrosion resistance failure in the BG-42 at 64/63 HRC then you might want to consider moving out of the secondary hardening range, however if your knives are failure by wear/deformation it isn't going to help and will just make the performance worse.

Consider that even with 50% of the corrosion resistance of BG-42 at 64 HRC, how does it compared to the corrosion resistance of L6, much higher still, thus it could easily still be high enough to not be the method of failure.

-Cliff

 

[Satrang]

Even with the high tempering the stainlesses are higher in corrosion reisistance than non stainless steels, just a lot closer and a lot more sensitive to environments (salt water being the worst). A good point mentioned above is that precipitation is not specific to stainless. Any high carbon high chromium grades will be hurt by this temperature range. A2 and D2 being the most common. High speeds precipitate molybdenum/tungsten carbides between 1000 and 1075 F. This stresses the material to its maximum usable hardness but is too brittle for general purpose blades or anything requiring high impact strength. Pure push cutting or slicing it does work well, for example the paper cutting industry.

 

[Cliff Stamp]

Any high carbon high chromium grades will be hurt by this temperature range.

I have knives that are so hardened and they are not simply paper cutters. The higher hardness allows them to do things that you can't do with softer blades because they are too weak and it will be the method of failure. As noted, not every knife fails due to lack of impact toughness, not every knife is meant to be a chopping tool. So you can't say uniformly that choice of tempering is less than optimal.

This stresses the material to its maximum usable hardness but is too brittle for general purpose blades or anything requiring high impact strength.

Not every knife needs high impact strength, and if you are looking for high high impact strength, but why are you using those steels for those knives anyway. Even when tempered low to avoid secondary hardening, D2 and M2 are still *very* brittle tool steels. Using them for high shock applications makes no sense, they are optimized for different tasks, if you want high shock resistance, use other steels.

Concerning martensitic stainless and tempering, Paul Bos ran ATS-34 hot primarily for a long time and a lot of makers used him and raved about the performance. There was a big flare up on Knifeforums because a number of other makers were promoting the low tempering cycle and Hitachi also released comparions with wear resistance, toughness and corrosion resistance data between the two and people like Jim March tried to get him to try the low cycles but he would not (you can check the archives on rec.knives).

Similar for D2, some makers use the high temper, some use the low. It D2 the situation is somewhat reversed as the secondary hardening temper leaves the blade softer as the carbide precipitation can't compensate for the temper draw, however the wear resistance is still significantly higher.

-Cliff

 

[Satrang]

If Paul was doing the ATS 34 at high temperature tempers it falls into the bearing category. Sacrificing optimum corrosion resistance and toughness for higher hardness. Good for a bearing not good for cutlery.

D2 doesn't get a major kick in wear resistance when high temperature tempered. If you get a couple of HRC points this is the reason for the better performance. What the high temperature temper is applied to D2 the secondary hardening comes from the conversion of retained austenite. The carbide precipitation in the grain boundaries is too small to really kick up the bulk wear resistance and lowers the ductility. Not recommended for optimum properties. You would get a better result by cryo treating and staying in the lower tempering temperatures.

 

[Cliff Stamp]

If Paul was doing the ATS 34 at high temperature tempers it falls into the bearing category. Sacrificing optimum corrosion resistance and toughness for higher hardness.

No same hardness either way, he was tempering 90% of his ATS-34 blades at 950F, he did do the lower temper on request, this was in 2000. There was a big clash about this earlier with Jim March being a focal point due to him getting some custom large blades in ATS-34 done and trying to figure out which way to do it, the common net perspective was Bos based due to the massive amount of makers who used him at the same (many still do of course) but there were a few makers who argued for the the low temper. Hitachi released thier test data on both specs which promoted higher wear resistance for the high temper and lower corrosion resistance, with the low one being tougher which is ironic because toughness was always one of the promotional points of the Bos heat treatment of ATS-34.

[D2]

If you get a couple of HRC points this is the reason for the better performance.

No, it drops, as noted, it doesn't have enough secondary carbide formers to compensate for the temper softening. Bryson says the grain structure is finer but isn't clear what he is talking about exactly, the austenite grain boundries are already set by this point and the marteniste coarsens, so it may be the formation of alloy carbides vs cementite at the low temperature. He notes a gain of wear resistance and even toughness but gives no specific reference. I meant to check this out in ASM "Tool Steels" as he says this is where his data comes from.

What the high temperature temper is applied to D2 the secondary hardening comes from the conversion of retained austenite. .

Austenite will convert in second stage tempering from 200-300C, you don't need to go up to the secondary hardening range for that, plus at the lower tempers you can get bainite vs pearlite for the higher tempers assuming you are converting austenite which is not necessary with cold treatements anyway which will directly add 1-2 points to hardness on top of the existing specs which points to them being beyond simple austenite coversion.

-Cliff

 

[Larrin]

The main problem with CPM steels, some of the newer ones, is that there is little materials data available compared to other tool steels for which you can easily turn up impact, strength and wear resistance info, with air/oil and room/cryo, treatments from various soak temps. Often in the CPM spec sheets this spectrum is very narrow.

That annoys me too. Sorry, had to throw in my two cents..

 

[Satrang]

Austenite will convert in second stage tempering from 200-300C, you don't need to go up to the secondary hardening range for that, plus at the lower tempers you can get bainite vs pearlite for the higher tempers assuming you are converting austenite which is not necessary with cold treatements anyway which will directly add 1-2 points to hardness on top of the existing specs which points to them being beyond simple austenite coversion.

-Cliff

Whoa, slow down there. You get some retained austentite conversion at the lower tempers but D2 was designed many years ago to have some stable austenite after tempering. This was to compensate for growth. The high tempers will convert the remaining austenite over, overtemper the matrix and precipitate grain boundary carbide. A total mess. The mistake many people make when heat treating is not to temper twice in the higher range or temper again after Cryo. You will get no bainite or pearlite when low temperature tempering, just martensite. Cryo converts retained austenite and is the reason for the 1-2 point jump. Nothing else comes into play there.

 

[Cliff Stamp]

The high tempers will convert the remaining austenite over, overtemper the matrix and precipitate grain boundary carbide. A total mess.

What is this based on, see for example materials testing by Timkin which show that D2 when tempered high is significantly tougher than when tempered low which agrees with Bryson. The problem is though they don't say exactly what they are using to measure it, though they promote its use at that temperature.

As for grain boundry carbides, this isn't simply an alloy condition, precipitates tend to form at heterogenous cites, cementite will precipitate out at low temperatures along the grain boundries as well and in fact induces a more severe loss of toughness because of the way it can sheet along the boundries.

Yes this reduces impact toughness, and yes it can reduce corrosion resistance if it ties up Cr, however this doesn't mean it "hurts" the steel, it depends on how the steel is failing in use.

You will get no bainite or pearlite when low temperature tempering, just martensite.

I looked this up, the formation to bainite does happen at low temperatures, it just won't happen during normal tempering times. For example, if you quench W1 to 350 F and hold it, 50% will be martensite and the remaining austenite will transform to bainite very quickly. However if you do the same with D2, even after 15 hours only half of the austenite will decompose. The TTT curve for D2 (see Krauss) shows why, the transformation is shifted way to the right at low temperatures, at higher temperatures it is much further left.

Cryo converts retained austenite and is the reason for the 1-2 point jump. Nothing else comes into play there.

Cryo will convert the austenite into martensite, tempering can convernt it into bainite or pearlite, then there is the issue of the increase in wear resistance. It isn't simply a hardness issue.

In regards to in general wear resistance being primary carbides and not effected significantly by secondary hardening carbides, this is based on wear resistance studies which have little in common with knife edges. Primary carbides are of the scale of 10 microns (they get larger and smaller, D2 is 20-30, M2 is usually 1-10), I have knives which are 25 microns thick back from the edge, this isn't the thickness of the very edge, but the primary grind say a mm or so back.

Now a carbide of 10-30 microns can't even fit in the edge itself. If you look at how knives blunt, even under low mag, you see large regions just tore/broke away from the edge, and this happens more with steels with large primary carbides, these are in fact degrading the performance. However the secondary carbides are much finer. D2 with its coarse carbides can't even take a low angled edge let alone keep it, thus an arguement could be made to put more of the alloy in soak, reduce the size of the primary carbides and induce more secondary hardening.

This would force the use of cryo because it would vastly reduce the Ms temperature and without it the as quenched hardness would be really low.


-Cliff

 

[Satrang]

Posted by Cliff Stamp
Yes this reduces impact toughness, and yes it can reduce corrosion resistance if it ties up Cr, however this doesn't mean it "hurts" the steel, it depends on how the steel is failing in use.

If dropping the impact toughness and dropping the corrosion resistance doesn't "hurt" a stainless steel, what does?

Posted by Cliff Stamp

The TTT curve for D2 (see Krauss) shows why, the transformation is shifted way to the right at low temperatures, at higher temperatures it is much further left.

Thanks for clarifying my point. D2 will not form bainite and pearlite during tempering. It forms martensite due to conditioning of the austenite and when the steel goes through the Mf again it transforms.

Posted by Cliff Stamp

In regards to in general wear resistance being primary carbides and not effected significantly by secondary hardening carbides, this is based on wear resistance studies which have little in common with knife edges.

Just what wear studies are specific to knife edges?

Cliff, every time I post you get argumentative about the information I present. I base my information on my metallurgical engineering degree and 15 plus years of alloy design and "Practical" work with heat treating, application engineering, manufacturing, etc. I work with this stuff every day. You base your knowledge on the internet and random bits from various sources. Believe me, the internet is fluff. Don't quote a reference to W1 when D2 is the topic. Don't talk about tempring D2 for 40 hours to try to justify your mistake on bainite and pearlite in D2 tempering. It's ok to let the experts have their say without contstant bickering.

I'm going to leave this thread because if I have to come back to this again, I'll lose my mind.

 

[Cliff Stamp]

If dropping the impact toughness and dropping the corrosion resistance doesn't "hurt" a stainless steel, what does?

Properties relevant to how the knife is failing in use, which are often lack of strength and wear resistance or grain structure, and possibly hot hardmess, and thus these would be critical. M2 is a very brittle steel, S5 is a very tough steel, does this mean all knives should be made of S5. Obviously not.

AISI 420 is much more corrosion resistant than BG-42, thus should it always be used in knives. How about if you could harden BG-42 so that while it had less than optimal corrosion resistance, it was still a lot more corrosion resistant than various non-stainless tool steels and had a very high hardness and wear resistance. Isn't that attractive for a knife steel?

All that matters in regards to a property is if it is low enough to be a failure point. If the steel is too soft so the edge is rippling and cracking and you hardened it so that this doesn't happen, why does it matter if the impact toughness when down if the knife is primary used to cut and not chop things.

Phil Wilson is an obvious example of this specific to S30V, does his choice of heat treating give optimal impact toughness? Is this even relevant to his knives? (no and no)

It forms martensite due to conditioning of the austenite and when the steel goes through the Mf again it transforms.

I have read the conditioning comment before, but many texts specifically note the decomposition to bainite or pearlite directly, it could be steel dependent, alloy obviously, but often they refer to the same steels. I meant to check this some time ago either in Leslie, Honeycombe and Cary.

[update]

When tempered hot enough for the alloy carbides to precipitate, which requires high temperatures for them to diffuse because they are so heavy, the austenite is reduced in alloy content, it then will act as if it was a lower alloy steel in the "quench" after the temper as the Ms point is dropped so martensite will again form in the cooling after the temper.

The transformation in steels during tempering is indeed steel dependent and is basically told by the TTT curves. Steels like D2 are very resistant to isothermal transformations which is why they can be air cooled and the same mechanics limits formations during tempering.

Just what wear studies are specific to knife edges?

Roman Landes has done specific research (published), large primary carbides lower performance due to lack of edge stability so they don't add to wear resistance. Alvin Johnson noted the same thing 20 years ago in the knives he made, it was more readily apparent to him because his angles were lower and he came to realize a fine grain structure was of critical importance as well as hardness, and then tested this by making knives and having them used. Verhoeven takes about it as well in his work. Recently a number of users have report this behavior directly with various knives comparing steels like D2 to much finer grades.

You base your knowledge on the internet and random bits from various sources.

Specifically mainly text books, ASM works or books used in specific materials courses here at Mun or books/notes from courses that friends have done such as Meyrick's Physical Metallurgy of Steel. Plus I have a background in solid state physics so the relevant background material is familiar enough, outside of some of the really heavy statistical thermodynamics which I would need to review as it has been years since I did any of that.

Don't quote a reference to W1 when D2 is the topic.

The topic was transformation during tempering, these are are two limits, one isothermals rapidly the other slowly, most steels fall inbetween them. I was describing the general pattern of behavior, what happen in general in steels, and then noted the specific behavior of D2. Krauss covers this with W1, O1, S5 and D2, and they cover most of the range from one side to the other in regards to isothermal resistance.

Don't talk about tempring D2 for 40 hours to try to justify your mistake on bainite and pearlite in D2 tempering.

Nice exaggeration, the transformation to bainite in D2 is from 50-90% of retained austenite at 15 hours depending on the tempering temperature, it is faster with other steels and slower still with others. I should have caught this when Alvin posted a bunch of TTT curves a few weeks ago and I corrected the origional post. However the transformation is taking place to bainite not tempered martensite, it is just slow at the low temps and high at the higher ones which it switches to pearlite according to Krauss.

-Cliff

 

[Satrang]

Unbelievable.:jerkit:

 

[rhino]

This makes me glad I have forgotten almost everything I ever learned about this stuff in materials science and physical chem classes!

 

[blaster4]

just wanted you guys to know i am finding this thread fascinating. i couldn't tell you what metal was used to make this fork i'm eating my apple pie with now but if this thread keeps going i should in no time. LOL. i hope it keeps going. in a civil way.

--KJ

 

[Carl64]

just wanted you guys to know i am finding this thread fascinating. i couldn't tell you what metal was used to make this fork i'm eating my apple pie with now but if this thread keeps going i should in no time. LOL. i hope it keeps going. in a civil way.

--KJ

Well, since you brought it up, here it is from memory...

If it's anything decent, it's an austenitic steel of some sort.

Our pocketknives all (well, almost all) use martensitic steels (AUS6, ats34, etc), in which roughly 0.5-1.0+% Carbon mixes with the Iron to make it hardenable but also very rustable (even "stainless" steel). Austenitic steel uses about 8-10% Nickel to help make it hard, and not much Carbon, so it is almost completely stain proof, but isn't hard enough for a nice sharp pocketknife. Forks don't need to be extremely hard or sharp, so it's a good steel to use.

Austenitic steel also has a lot of Chromium (the thing that makes our pocket knives resist rust), often 18% in tableware, so your good forks may have been labeled either 18/8 or 18/10 for the Chromium/Nickel content, although the word on the street is the 10 (when used in place of 8) is total bull$#!t the they are just approximate rounded numbers for the real amount which is somewhere around there.

If you have cheap forks, they might be a martensitic steel with very low Carbon content, which would help make them much more resistant to rust than good knife steel but might still tend to get stains.

And on a last interesting note, austenitic steels won't stick to a magnet.