Cryo tempering

I'm not sure. Most stuff lately concentrates on using cryo as an extension of the quench with little or no delay. It also depends on the previous treatment. If there is some retained austenite then some might change, but not as much as if it were done soon after quenching. There would also need to be a temper cycle of any did change. If there weren't any, maybe nothing would happen.
 
dirtyphil said:
Alrightey... I need an overview of what happens to steel during HT...what's martensite, austentite, grain,etc
Click to expand...

OK, but just remember, you asked for it.

First lets start with iron. Pure iron melts around 3200 deg F. But, below that, it has different phases as well. All metals have an orderly structure with the atoms stacked in patterns. Iron atoms at room temperature stack like a box (cube with an atom at 8 corners) and have a 9th atom in the center. If one increases the temperature to around 1600 deg F, the atoms rearrange themselves to stack like a cube (8 corners) with one atom in the center of each face (6 more). There is more but this is what we are generally concerned with in steel.

Now add carbon. This is steel, iron with some carbon in it. In reality, it's difficult to get iron without carbon, but that's beside the point. Carbon atoms are much smaller and can fit between the iron atoms to some degree. At room temperature, with atoms arranged in the body centered cube (8 with one in the center of the box), the amount of carbon that can fit between the iron atoms is very small. So, any extra carbon that can't fit has to go somewhere. Where does it go? It forms iron carbides, which are little bits of basically ceramic mixed in with the iron atoms with carbon between them. It's the same as water with salt dissolved in it, but with too much salt, some will not dissolve. The difference is salt water is a liquid/solid solution, while steel is a solid/solid solution.

So, when we want to harden steel, we heat it hot enough that the iron changes from the body centered cube to the face centered cube (8 corners with an atom in the center of each face). The body centered cube arrangement is called ferrite, and the face centered cube arrangement is called austenite. Austenite has a lot more room for carbon between the iron atoms, and as the temperature goes up, more carbon will fit into the austenite arrangement. For plain carbon steels, like 1095 or any 10xx steel, you can heat it enough that all the carbon dissolves in between the iron atoms when they are arranged in the face centered cubes.

Now comes the quench. If we cool the steel slowly, the atoms will shift back to ferrite, and all the carbon that it won't dissolve will form iron carbides again. However, if we quench it, then the carbon doesn't have time to form carbides and gets stuck in the iron. The iron will try to change back to ferrite, but all the extra carbon stretches the cubes into rectangles, still with an iron atom in the center. This is called a body centered tetragonal (rectangle). This is quenched martensite.

Quenched martensite isn't terribly useful, as it is very brittle and hard. However, if one then tempers it, some of the trapped carbon can escape, the hardness goes down a little, and the brittleness is removed. This is the reason we heat, quench, and temper steel for blades.

To take it a step further to the cryo discussion, quenching to room temperature doesn't always finish changing the austenite to martensite. Sometimes the change doesn't finish until the temperature is well below freezing (of water). There is austenite left at room temperature and even below. This is called retained austenite. This is where the cold treatment and cryo come into play. These procedures keep cooling until the process is finished. Then one can temper as usual.

There is a fair amount of detail involved with all this, but that is the basic process. Wall of text over.
 
Glad you liked it. I keep reading it wanting to fix stuff, but that just makes it longer. Its long enough as it is.
 
Crto is part of the HT not an add on ! Cryo reduces RA but also tweeks the lattice .We don't have a lattice that is perfect like graph paper. We have lots of dislocations .To simplify, the tweeking moves the lattice ,creating some larger spaces. the larger spaces are where carbides can form more easily. On tempering carbides form in those spaces. The formation of carbides is a diffusion process ,much slower than the shear type of martensite. Info on how long it takes depends on the alloy. I have been saying 4-6 hours for lack of better info though I've seen much longer timesforsome. Then it becomes a question of practical time limits therefore picking the best alloys for cryo.If you come across info please let me know.I gather that a "snap temper " is not normally needed but if used stick to 300-350 F , no higher ! You must temper after cryo but it seems to me that 400 F should be a limit so as not to lose cohesion which is part of the strenghtening .
 
to be honest I don't see many benefits. the crystal structure of a well hardened blade should already be well enough to make a wonderful tool (depending on steel). I'm no expert, but it seems as if this would only make minor changes and if anything, adds another more risky process where one can shatter their blade due to the brittleness that steel takes on in cryogenic temperatures.
 
It looks dramatic but I don't really understand the units "wear resistance ratio" How is that arrived at? Can you give us an example?
Thanks


Natlek said:
Well ........... what about this ?

23jh8k9.jpg
Click to expand...
 
That is beautiful. Thank you. Now I will read it 50 more times to really plant it in my simple little brain.

me2 said:
OK, but just remember, you asked for it.

First lets start with iron. Pure iron melts around 3200 deg F. But, below that, it has different phases as well. All metals have an orderly structure with the atoms stacked in patterns. Iron atoms at room temperature stack like a box (cube with an atom at 8 corners) and have a 9th atom in the center. If one increases the temperature to around 1600 deg F, the atoms rearrange themselves to stack like a cube (8 corners) with one atom in the center of each face (6 more). There is more but this is what we are generally concerned with in steel.

Now add carbon. This is steel, iron with some carbon in it. In reality, it's difficult to get iron without carbon, but that's beside the point. Carbon atoms are much smaller and can fit between the iron atoms to some degree. At room temperature, with atoms arranged in the body centered cube (8 with one in the center of the box), the amount of carbon that can fit between the iron atoms is very small. So, any extra carbon that can't fit has to go somewhere. Where does it go? It forms iron carbides, which are little bits of basically ceramic mixed in with the iron atoms with carbon between them. It's the same as water with salt dissolved in it, but with too much salt, some will not dissolve. The difference is salt water is a liquid/solid solution, while steel is a solid/solid solution.

So, when we want to harden steel, we heat it hot enough that the iron changes from the body centered cube to the face centered cube (8 corners with an atom in the center of each face). The body centered cube arrangement is called ferrite, and the face centered cube arrangement is called austenite. Austenite has a lot more room for carbon between the iron atoms, and as the temperature goes up, more carbon will fit into the austenite arrangement. For plain carbon steels, like 1095 or any 10xx steel, you can heat it enough that all the carbon dissolves in between the iron atoms when they are arranged in the face centered cubes.

Now comes the quench. If we cool the steel slowly, the atoms will shift back to ferrite, and all the carbon that it won't dissolve will form iron carbides again. However, if we quench it, then the carbon doesn't have time to form carbides and gets stuck in the iron. The iron will try to change back to ferrite, but all the extra carbon stretches the cubes into rectangles, still with an iron atom in the center. This is called a body centered tetragonal (rectangle). This is quenched martensite.

Quenched martensite isn't terribly useful, as it is very brittle and hard. However, if one then tempers it, some of the trapped carbon can escape, the hardness goes down a little, and the brittleness is removed. This is the reason we heat, quench, and temper steel for blades.

To take it a step further to the cryo discussion, quenching to room temperature doesn't always finish changing the austenite to martensite. Sometimes the change doesn't finish until the temperature is well below freezing (of water). There is austenite left at room temperature and even below. This is called retained austenite. This is where the cold treatment and cryo come into play. These procedures keep cooling until the process is finished. Then one can temper as usual.

There is a fair amount of detail involved with all this, but that is the basic process. Wall of text over.
Click to expand...
 
Me2, can you send us to someplace to see illustration of the "face centered" vs "body centered" cube?
 
Sorry. I had removed this thread from my subscriptions and just stumbled across it. Yes that is a fine diagram. It also shows good examples of the various defects present in most metals. Thanks for the kind words.
 
blades-and-hammers said:
to be honest I don't see many benefits. the crystal structure of a well hardened blade should already be well enough to make a wonderful tool (depending on steel). I'm no expert, but it seems as if this would only make minor changes and if anything, adds another more risky process where one can shatter their blade due to the brittleness that steel takes on in cryogenic temperatures.
Click to expand...

Are you referring to specific steels or steels in general? For some steels is not terribly beneficial, some it helps and in some cases it makes things worse. It really depends on what the knife is intended to do. Some argue that a good knife should be cold treated or cryo treated to make it the best it can be, but there is considerable evidence that is not the case always.
 
RedFury said:
That is beautiful. Thank you. Now I will read it 50 more times to really plant it in my simple little brain.
Click to expand...

As you can see, I does not afraid of the text wall. If you have questions, there are numerous members here that can provide more information. I'll answer if I can, but don't just take my word for it.
 
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