Grains, Carbides, and You

Me2 has started a cool thread, but it's obvious that we are not all on the same page. As a suggestion, maybe one of the moderators could send an email -- or this thread -- to one of the major steel companies to see if they would be willing to have one of their engineers address this issue and maybe stick around long enough to answer a few questions. I know Crucible has done that in the past for other groups.
 
davek14 said:
So I'm assuming that when Chromium or Vanadium is in the mix that the excess carbon forms carbides with that instead. I'm still trying to understand where and how the carbides are distributed and held. I know the carbon is trapped "in solution" by the quench between the bonds of the iron atoms. Where are the undissolved carbides? Are they only at grain boundaries or in the grains themselves as well. You say "distributed based on prior heat treatment procedures". I'm trying to absorb basic concepts from this thread. Kind of a general idea of how things usually go to get a better working understanding.

Does small carbides (only) = smaller grain structure?

Interesting subject.
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Yes you are on the right track. Chromium, vanadium, niobium, titanium, etc. all form carbides easier than iron, and will bond with available carbon and replace iron carbide with (insert element) carbide. Nickel and manganese generally don't form carbides, nor does silicon. These will mostly be found dissolved in the iron matrix, be it martensite, ferrite, or austenite.

As to where you find the carbides, it depends on what kind. Some form at the grain boundaries, some form at the grain interior. Grain boundaries are very disorderly, and it's easier form carbides to form there in many cases. Particularly the large versions of Chromium Carbide, Cr23C6. Vanadium carbide tends to be a grain boundary carbide, which is nice as it basically acts as a speed bump for grain growth. Chromium carbide does too, but it dissolves at a lower temperature than vanadium carbide, so once it's gone, it doesn't help anymore.

As an example, let's use the 1095 above. Now this particular piece is not a good piece for knives, but it will illustrate the point. The gray/brown parts in the 1095 grains are areas of pearlite. Pearlite is made of alternating layers of ferrite and cementite, like layers in Micarta. So here we have cementite in the grain interior. The white parts around the grains are grain boundary cementite.

When we heat the 1095 to austenizing range, the ferrite will change to austenite and the carbides will dissolve, both at the grain boundaries and in the layers of pearlite. All of it won't dissolve until we get above the normal austenizing range for 1095, so whatever is left is undissolved carbide. For 1095, there will be some in the interior and some in the grain boundaries. For steel in the condition of the picture, we need to make sure the grain boundary carbide is not continuous, for reasons mete pointed out above.
 
One of the coolest threads on BF. Loads of info. I had no idea youtube had so many videos related to metallurgy.
Me2, thanks for posting/starting it.

P.S. I know it depends on HT method, but any ballpark number/method on calculating the amount of cementite for simple 10xx steels?
 
me2 said:
Yes you are on the right track. Chromium, vanadium, niobium, titanium, etc. all form carbides easier than iron, and will bond with available carbon and replace iron carbide with (insert element) carbide. Nickel and manganese generally don't form carbides, nor does silicon. These will mostly be found dissolved in the iron matrix, be it martensite, ferrite, or austenite.

As to where you find the carbides, it depends on what kind. Some form at the grain boundaries, some form at the grain interior. Grain boundaries are very disorderly, and it's easier form carbides to form there in many cases. Particularly the large versions of Chromium Carbide, Cr23C6. Vanadium carbide tends to be a grain boundary carbide, which is nice as it basically acts as a speed bump for grain growth. Chromium carbide does too, but it dissolves at a lower temperature than vanadium carbide, so once it's gone, it doesn't help anymore.

As an example, let's use the 1095 above. Now this particular piece is not a good piece for knives, but it will illustrate the point. The gray/brown parts in the 1095 grains are areas of pearlite. Pearlite is made of alternating layers of ferrite and cementite, like layers in Micarta. So here we have cementite in the grain interior. The white parts around the grains are grain boundary cementite.

When we heat the 1095 to austenizing range, the ferrite will change to austenite and the carbides will dissolve, both at the grain boundaries and in the layers of pearlite. All of it won't dissolve until we get above the normal austenizing range for 1095, so whatever is left is undissolved carbide. For 1095, there will be some in the interior and some in the grain boundaries. For steel in the condition of the picture, we need to make sure the grain boundary carbide is not continuous, for reasons mete pointed out above.
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Great stuff simply put. Thank you.
 
Gator97 said:
One of the coolest threads on BF. Loads of info. I had no idea youtube had so many videos related to metallurgy.
Me2, thanks for posting/starting it.

P.S. I know it depends on HT method, but any ballpark number/method on calculating the amount of cementite for simple 10xx steels?
Click to expand...

There is a way to do this. It's called the lever law or lever rule. It does require an iron-carbon phase diagram. I'll explain more later after work. This is where one has to be careful, as any alloying will change the diagram, even the small amount of Mn in 1095. This is one reason for the variation in the eutectoid point, anywhere from 0.77 to 0.84 percent carbon. If you look it up and try it, don't be surprised when the room temperature calculations show a lot more carbide than the heat treated condition. Also, keep in mind the diagram is an equilibrium diagram. With such rapid changes in temperature as quenching, we are not dealing with equilibrium in knife making.
 
I would also like to say how much better this time has gone than the last. I posted this on another forum and the thread was locked in less than 2 pages. I know why, and agree it was a good idea, but why have to in the first place?
 
That's better than posting a new thread and having no one respond !!
 
I just want to thank Me2 for the excellent jello with fruit analogy. Well stated, makes a nice visual because of jello being transparent.
 
me2 said:
I would also like to say how much better this time has gone than the last. I posted this on another forum and the thread was locked in less than 2 pages. I know why, and agree it was a good idea, but why have to in the first place?
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Some "spirited discussions" can get going it is sure. It was stated in an earlier post that this is not a perfect science as well. There are exceptions and there are certainly opinions. :) My only goal in this one is to get some of the concepts down. That's what gives a working knowledge you can extrapolate from. Hey, I had to look up and re-learn from high school the word "eutectic".:rolleyes:

At the risk of a specific question, are the PM steels small grained and do small carbides automatically equal small grain?
 
davek14 said:
Some "spirited discussions" can get going it is sure. It was stated in an earlier post that this is not a perfect science as well. There are exceptions and there are certainly opinions. :) My only goal in this one is to get some of the concepts down. That's what gives a working knowledge you can extrapolate from. Hey, I had to look up and re-learn from high school the word "eutectic".:rolleyes:

At the risk of a specific question, are the PM steels small grained and do small carbides automatically equal small grain?
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Small carbides do not mean small grains, as it turns out. In fact, the 2 are somewhat at odds woth each other. Small carbdes come from higher heat. Higher heat means bigger grains. However, some carbides do a good job of keeping grains from growing, until the temperature gets too high and they dissolve.

The pm steels are not neccesarily fine grained. That is the basis for the discussion twindog and Ihave been having. The only micrographs Ihave seen show grains between 20 and 40 microns in a pm steel. There is very little out there about it. The high temps used to austenize such pmalloys leads me to believe thegrains will be somewhat large, 20 to 40 um, but that is not a hard rule. I also take the lack of information about it as a small indicator of where the emphasis is in developing these steels.
 
As I have said - when dealing with these higher carbon steels , HT first to deal with the carbides , then HT to deal with the grain size. Remember that each steel is different and must be treated according to it's own properties. Carbides are different concerning their M to C bond strengths .Harder carbides require longer time and higher temperatures do dissolve !
 
me2 said:
Small carbides do not mean small grains, as it turns out. In fact, the 2 are somewhat at odds woth each other. Small carbdes come from higher heat. Higher heat means bigger grains. However, some carbides do a good job of keeping grains from growing, until the temperature gets too high and they dissolve.

.
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That's right, that's right, higher heat, more disolved carbides.

Vanadium? for keeping grain from growing?

I'm not even going to try to "get" multiple heat treats, yet.
 
me2 said:
Small carbides do not mean small grains, as it turns out. In fact, the 2 are somewhat at odds woth each other. Small carbdes come from higher heat. Higher heat means bigger grains. However, some carbides do a good job of keeping grains from growing, until the temperature gets too high and they dissolve.

The pm steels are not neccesarily fine grained. That is the basis for the discussion twindog and Ihave been having. The only micrographs Ihave seen show grains between 20 and 40 microns in a pm steel. There is very little out there about it. The high temps used to austenize such pmalloys leads me to believe thegrains will be somewhat large, 20 to 40 um, but that is not a hard rule. I also take the lack of information about it as a small indicator of where the emphasis is in developing these steels.
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I get that finer carbides do not necessarily mean that the steel is also finer grained. And I stand corrected that powder steels are not necessarily finer grained than simple carbon steels. I've learned a lot from Me2 and I appreciate that. However, steel processing has a huge effect on carbide size, independent of the subsequent heat treat. The whole point of powder steels is to reduce the carbide size and improve their distribution so that steels with a high volume of carbides can be manufactured without the loss of toughness that comes with ingot steels with high carbide volume.

Yes, there may be fewer carbides in powder steels than their ingot counterpart, but that doesn't change the carbide volume. When you have the same carbide volume with fewer carbides, then those carbides are going to be larger. Powder steel processing reduces carbide size without reducing carbide volume, so there are more and smaller carbides. It also makes the carbides more uniformly distributed. When you reduce the size of carbides and make their distribution more uniform, you get a finer grained steel than the ingot counterpart of that same alloy, although not necessarily more fine grained than simple carbon steels.

High alloy steels give steel a lot of valuable characteristics, such is much, much, much better wear resistance, but ingot steel is limited in its ability to hold high carbide volumes without losing a lot of toughness. In other words, ingot steel with high carbide volumes break easily for a number of reasons. By employing the powder steel processing techniques, you get the advantages of high carbide volumes without the loss of toughness that is expressed in ingot steels. CPM D2 is tougher than ingot D2. PSF27 (spray formed D2) is much better than ingot D2 in both toughness and wear resistance -- thanks to the smaller, better disbursed carbides.

Certainly the heat treat can affect grain size and carbide size, but so can steel processing techniques. Crucible, and other makers, say powder steel and spray formed steel will have finer grain structure, although that difference is not stated numerically, as well as much finer carbide size. No one has been able to make high carbide volume ingot steel without a huge loss of toughness. Powder and spray forming processes overcome that limitation. The only reason we can make a decent knife out of M4 or or S90V or 10V is because steel manufacturing processes reduce carbide size and improve carbide distribution. You can muck up those advantages with a poor heat treat, but you cannot overcome the limitations of large carbide volumes in ingot steels with a better heat treat. Those small, well disbursed carbides help reduce grain size -- as do specific elements such as vanadium -- but the big advantage is in the smaller carbide size and the better distribution of those carbides.

Here's a link where Crucible explains the very real advantages of powder steel:
http://www.crucibleservice.com/eselector/general/generalpart1.html
 
Your reasoning here is sound. I have seen the micrographs, but it was for only one sample. There has to be more out there. I'll see what I can dig up.
 
With a little bit of calculation & determination, one can make exception to the norm:
D3 Carbide Refinement Thermal Cycle

D2, CPM-M4, etc.. Thermal temp/time/cct tweaks needed for dealing with V & W carbides. Proper technique will induce nucleation of carbide & grain per subsequence step. As you can see in D3 case, CrC is consistently size around 2um with low variance especially on the over size. PM steels conveniently let knife makers start at step 5/6 w/o worrying about thermal dynamic calculation for particular composition.

Cool thread :thumbup:

Twindog said:
Certainly the heat treat can affect grain size and carbide size, but so can steel processing techniques. Crucible, and other makers, say powder steel and spray formed steel will have finer grain structure, although that difference is not stated numerically, as well as much finer carbide size. No one has been able to make high carbide volume ingot steel without a huge loss of toughness. Powder and spray forming processes overcome that limitation. The only reason we can make a decent knife out of M4 or or S90V or 10V is because steel manufacturing processes reduce carbide size and improve carbide distribution. You can muck up those advantages with a poor heat treat, but you cannot overcome the limitations of large carbide volumes in ingot steels with a better heat treat. Those small, well disbursed carbides help reduce grain size -- as do specific elements such as vanadium -- but the big advantage is in the smaller carbide size and the better distribution of those carbides.

Here's a link where Crucible explains the very real advantages of powder steel:
http://www.crucibleservice.com/eselector/general/generalpart1.html
Click to expand...
 
Twindog said:
CPM D2 is tougher than ingot D2. PSF27 (spray formed D2) is much better than ingot D2 in both toughness and wear resistance -- thanks to the smaller, better disbursed carbides.
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AFAIK, spray form(SF) is more economical form of CPM/PM technology. It won't produce better results than PM process.
 
bluntcut said:
With a little bit of calculation & determination, one can make exception to the norm:
D3 Carbide Refinement Thermal Cycle

D2, CPM-M4, etc.. Thermal temp/time/cct tweaks needed for dealing with V & W carbides. Proper technique will induce nucleation of carbide & grain per subsequence step. As you can see in D3 case, CrC is consistently size around 2um with low variance especially on the over size. PM steels conveniently let knife makers start at step 5/6 w/o worrying about thermal dynamic calculation for particular composition.

Cool thread :thumbup:
Click to expand...

me2 said:
That's a good one there. I have to save that.
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Downloaded in case the link stops working at a later date :thumbup: Thank you for the post!
 
Gator97 said:
AFAIK, spray form(SF) is more economical form of CPM/PM technology. It won't produce better results than PM process.
Click to expand...

CPM D2's benefits over ingot D2 are real, but not dramatic. Crucible ran off two heat treats of it for the knife industry, but it didn't sell well, largely because other PM steels offered higher wear resistance at a similar or greater toughness. Spray formed D2 (PSF27) is touted more highly. Jim Ankerson has agreed to test the new PSF27 mule, so we should have some good numbers soon for wear resistance.

Here is what the manufacturer says, although I know it's marketing and yet another comparograph:

psf27-chart.gif


PSF27 is a chromium, molybdenum, and vanadium alloyed cold work tool steel(AISI D2 analysis +) produced using the Spray Forming Process. The Spray Forming Process allows for rapid solidification resulting in materials with a very fine grain and homogeneous structure. This structure results in improved toughness, wear resistance, crack resistance, and higher hardness. It also yields more predictable heat treatment results-dimensional stability.

http://sb-specialty-metals.com/grades/psf27
 
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