Perhaps a basic curve discussion?

I was thinking not only of the obvious cracking but micro-cracking which some time later cause the blade to break. When we tell you to immediately temper after quenching it's not just for the obvious cracks but the hidden micro-cracks. One of my books has a perfect photo of this, one grain has a crack going through it !
 
mete said:
Kevin , with these mixes where the HT is very different with the two steels do you know of any cracking because of it ??
Click to expand...

I will avoid mentioning any specific mixes since invariably some folks will take it as a personal attack if they happen to use it:rolleyes: , but it has been a problem in some mixes that involve very radical differentials in the response to quenching. Often shearing of weld zones, or around the weld zone, can occur in coarser layer counts. It only makes sense that if rapid martensitic expansion is occurring in one layer while nothing at all is happening in the other- somethings got to give.
 
Mete, Kevin

Since we can vary the HT times from the specs. when dealing with thinner sections, would these micro cracks occur to the same extent ( in thinner stock) as with thicker pieces?

And if the cooling rate is more uniform w/ thinner stock, would there be less crack causing stress, or do the cracks occurr because of temp. transition regardless of cross section ( assuming we don't followup immediately with the proper temper)?

Rod
 
Kevin.....I just re-read an article that Dr. Batson did in Knives Illustrated a while back about cryo-treatment. What are your opinions on that?
 
I have been cryo treating tool and stainless steels for the last five years, but have now stopped, at least for the tool steels after reading the article linked below. I have posted this here before, but as far as I know only Fitzo has ever read it. All the other in depth reports I have seen were written as advertising by the Cryo companies. This one has quantitative results showing that for A2 there is a loss of toughness/impact strength in all of the cases tested, including one test using liquid Helium (4°K) which is much colder than lN2.

Bob Dozier has written in this and other forums on more than one occasion that if you saw a difference with cryo on D2, you weren't heat treating right.


http://www.airproducts.com/NR/rdonl...019GLB.pdf#search="cryogenic quenching steel"

Take a chance, read the paper linked here. It might broaden some perspectives.
 
"2. A large number of finely dispersed, Cr-V-depleted, dark
carbides with the typical size of 100-250 nm dia. was found
in cryogen-treated samples which coexisted with a set of
more conventional, typically micron sized, alloy-rich, bright
carbides. Dark carbide number frequency, material wear
resistance, hardness, and the loss of impact resistance were
proportional to the length of cryogenic holding time
indicating that, apart from the residual austenite
transformation, cryogenic treatments effect early stages of
aging."

Interesting. I see that this is what they observed, but I didn't see an explanation as to why it occurred. Did I just miss it? Why would cryogenic treatment increase 'early stages of aging'?
 
Aging is precipitation. It may just be the lower temperatures effect on lowering solubility thereby initiating precipitation.Note that they are different carbides, some metals have stronger bonds with carbon than others.....Steve , I have read the article .It's just one more thing that confuses the picture .I have long felt that the cryo industry and the ASM have done themselves a great disservice by not vigorously doing basic research to find exactly what happens and find the effect of all the variables !! ...mtnsct, you ask a complicated question. Kevin mentions problems with mixes of steels that have significantly different austenitizing temperatures ,different Ms temperatures so the HT stresses can be high between the two steels.
 
mtnsct said:
Mete, Kevin

Since we can vary the HT times from the specs. when dealing with thinner sections, would these micro cracks occur to the same extent ( in thinner stock) as with thicker pieces?

And if the cooling rate is more uniform w/ thinner stock, would there be less crack causing stress, or do the cracks occurr because of temp. transition regardless of cross section ( assuming we don't followup immediately with the proper temper)?

Rod
Click to expand...

Grain size can have a profound effect on micro-cracking. Thicker sections do complicate matters if things are not cooled carefully. I have found O1 bars in ¾ “ thickness to very readily crack when cooled without compensation for expansion rates. In pattern welding, the finer the layers, the less pronounced will be their differences.
 
Steve Hayden said:
I have been cryo treating tool and stainless steels for the last five years, but have now stopped, at least for the tool steels after reading the article linked below. I have posted this here before, but as far as I know only Fitzo has ever read it. All the other in depth reports I have seen were written as advertising by the Cryo companies. This one has quantitative results showing that for A2 there is a loss of toughness/impact strength in all of the cases tested, including one test using liquid Helium (4°K) which is much colder than lN2.

Bob Dozier has written in this and other forums on more than one occasion that if you saw a difference with cryo on D2, you weren't heat treating right.


http://www.airproducts.com/NR/rdonl...019GLB.pdf#search="cryogenic quenching steel"

Take a chance, read the paper linked here. It might broaden some perspectives.
Click to expand...

Thank you Steve for posting that link, such a perspective is quite refreshing for this topic, and many more people need to read that study.
 
jdm61 said:
Kevin.....I just re-read an article that Dr. Batson did in Knives Illustrated a while back about cryo-treatment. What are your opinions on that?
Click to expand...
I was a bit disappointed that it had the same Achilles heal that most writings on the topic have- lack of objectivity due to much of the information coming from people who have a vested interest in selling the cryo process. I would like to thank Steve Hayden again for posting that link to a very rare and valuable thing, i.e. an article that doesn’t portray freezing as a philosophers stone panacea. Not that Jim offers a cryo service, but I did have a hard time determining when I was reading and article and when I was reading an advertisement for the services who so generously provided information. I think Jim can forgive my skepticism, he is one of my favorite smiths for the reason that we can, and have, disagree on minor things without it affecting our friendship at all. He is very cool in that way, it says a lot about ones position when it is difficult to make them defensive.

I have noticed a concerted effort by cooperation’s of folks offering cryo services to portray themselves as a purely scientific body and find this topic whenever it is mentioned in order to promote this fantastic process. Even if they don’t latch onto this thread I hope it doesn’t get side tracked into an area where there are no clear answers yet, as the mere mention of cryo is a quick way to send a thread into a tail spin.
 
For the record, guys... are there ANY steels that can benefit from cryo treatment, or is this, as Kevin describes it, a Philosopher's Stone? I've heard cryo treatment for S30V as being crucial, and this is why I pose the question.
 
I read the paper in the link above. The cryo samples were harder and more wear resistant than the non cryo'd samples, but not as tough. That was their conclusion.
Since hardness and toughness are inversely proportionate, this comes as no surprise to me.
That cryo dies indeed improve wear resistance is important to me, because a more wear resistant cutting tool is a better cutting tool, IMO. If toughness is more important than edge holding, skip the cryo.

I think the real question here is, is some level of retained Austenite beneficial to the overall performance of the knife? Or, stated another way, does the highest achieveable level of Martensite result in the best blade?

I don't think that can be answered in a "one size fits all" blanket statement.

One problem I have with the method used in the article is that all the specimens were not tested at the same hardness. So, it was an apples to oranges comparison. If the non-cryo'd piece was tested at Rc60, like the cryo'd piece, what would the difference in toughness have been? Certainly greater, but not as much greater.

I have been experimenting with non-cryo'd CPMS30V blades, at the recommendation of Scott Devanna from Crucible. I will try and report back after I finish some up (today) and sharpen them. As people aren't breaking my knives in use, my interest in this is more towards edge degradation and failure during cutting, rather than total blade failure.

I will say, it is easy to get frustrated over all this. Usually, the more education and infiormation you have, the more confident you are in choosing a course of action. Not so in this area.
 
rj martin said:
I read the paper in the link above. The cryo samples were harder and more wear resistant than the non cryo'd samples, but not as tough. That was their conclusion.
Since hardness and toughness are inversely proportionate, this comes as no surprise to me.
That cryo dies indeed improve wear resistance is important to me, because a more wear resistant cutting tool is a better cutting tool, IMO. If toughness is more important than edge holding, skip the cryo.

I think the real question here is, is some level of retained Austenite beneficial to the overall performance of the knife? Or, stated another way, does the highest achieveable level of Martensite result in the best blade?

I don't think that can be answered in a "one size fits all" blanket statement.

One problem I have with the method used in the article is that all the specimens were not tested at the same hardness. So, it was an apples to oranges comparison. If the non-cryo'd piece was tested at Rc60, like the cryo'd piece, what would the difference in toughness have been? Certainly greater, but not as much greater.

I have been experimenting with non-cryo'd CPMS30V blades, at the recommendation of Scott Devanna from Crucible. I will try and report back after I finish some up (today) and sharpen them. As people aren't breaking my knives in use, my interest in this is more towards edge degradation and failure during cutting, rather than total blade failure.

I will say, it is easy to get frustrated over all this. Usually, the more education and infiormation you have, the more confident you are in choosing a course of action. Not so in this area.
Click to expand...
You are so correct about the confusion and frustration part, sir. When i see Mr.Cashen at Blade this summer, I am going to buy him lots of drinks in the Pit to show my gratitude for all the advice he has given me on here.........as a matter of fact, I am going to be SO grateful that i am going to buy him enough to make him hurl and have a hangover at his table the next morning to get even with him for making me paranoid about showing any knife in public that isn't absolutely perfect both artistically and metallurgically and causing my cranium to collapse with all of this way-too-complicated tecnical jargon:D just kidding about the puking part, Kevin;)
 
I have an open mind about the Cryo question in general. I wiil be interested to see what happens with the S30V. I have quit cryo treating A2 and D2 because thorough tempering can get the retained Austenite converted and I like to have the tool steels at their optimum toughness. For both of these steels Hrc 60 is the point where they peak in impact strength. I have been doing the cold treatments for years due to having the impression that I was improving the toughness as well.
 
Always great to read your threads Kevin.

If I get that mag off the ground, please, please write for it! :D
 
There is more than one application for steels where a little RA is beneficial.But that's a little not a lot !! We still have to approach the question from an individual steel basis. Some are far more prone to have RA. Of course the biggest cause is excessive austenitizing temperature . No generalizations please !!
 
I just wanted to say hello to Kevin and to Mete. I haven't been on this forum in a long time. Just got bored on some of the other sites.
 
Yes, I moved to the Houston area almost 2 years ago. Changed jobs, changed companies. Got too old and crabby to work in the mills any longer so I accepted a position as Director of Applications Engineering for a Canadian Pipe Manufacturer. A year ago we were sold to a Swedish Steel company and then in June of this year they sold us to the largest pipe maker in Russia. I wake up wondering just who I work for sometimes. I enjoyed the foregoing discussion on heat treating. I am back to bladesmithing but on a small scale: I make woodcarving knives. These typically have blades only 1"-1.5" long with a flat grind. The challenge is to forge them with a proper taper then final grind them without getting the dreaded blue spot. I work with simple tool steel (O1) so I don't get too sophisticated in my heat treating. I agree with your approach: full quench and temper from the spine to the edge.
 
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