Hey steel nerds... subcritical anneal vs. slow cool anneal?

[Jason Fry]

Some folks anneal by a slow step-down cooling in a kiln. This seems very precise, and very time consuming.
Some folks throw a hot blade in vermiculite or similar to anneal. This seems very imprecise, and less likely to result in full effect.
Then there's the "sub critical" anneal, say 1 hour at 1200-1300 in the kiln.

Can anyone educate me as to what's going on here? My understanding is that thermal cycling reduces the grain size. What does annealing specifically DO? What's the difference in metallurgical effect between a kiln metered slow cool anneal vs. a subcritical?

 

[Rick Marchand]

What's the difference in metallurgical effect between a kiln metered slow cool anneal vs. a subcritical?

It is essentially giving your internal matrix a carbon laxative. Redneck annealing is like eating prunes and full on spheroidizing is the equivalent of chugging Ex-Lax.

That might not be completely accurate... I'll admit.

Seriously... While we are on the topic... Other folks will austenitize & quench on their last thermal cycle, then follow up with a subcritical anneal. Is there any benefit/drawback to subcritical annealing from martensite rather than a predominantly pearlite structure?

 

[Karl B. Andersen]

Also significant is what your next steps are going to be - and what your needs and wants are.
Both of these are going to allow easy machining. If you are doing heavy machining a full anneal will be desirable. (I haven't done a full anneal on a knife in over a decade) In knife making I employ a simple sub-critical anneal right out of the forge at the end of my reducing heat thermal cycling following forging. Just a few dark red cycles cooling to black will do enough of a sub-critical anneal to allow the amount of machining I need before hardening.
Both will also require getting that carbon BACK into solution and the spheroids dissolved.
A full anneal is going to result in highly spherodized/coarse carbon and potentially enlarged grain - both of which need to be taken into account before any approach toward hardening.
Prior to a sub-critical anneal you will want to be sure your grain has been dialed in as you will basically just be able to bring that steel up to temp with a little soak time to dissolve your carbon and quench for hardness. Simple steels like 10XX and W1/2 work well with these.

 

[mete]

You benefit with sub critical anneal because it avoids scale and decarb problems .It also avoids grain growth and formation of unwanted carbides [in grain boundries ]
All without laxatives !

 

[Jason Fry]

Prior to a sub-critical anneal you will want to be sure your grain has been dialed in as you will basically just be able to bring that steel up to temp with a little soak time to dissolve your carbon and quench for hardness. Simple steels like 10XX and W1/2 work well with these.

This is my current understanding. On a forged piece, i thermal cycle in my last few heats, then sub critical anneal. Then grind, then austentize and quench.

 

[DevinT]

Annealing steel is meant to condition the steel in preparation for machining or hardening and tempering.

The two basic types of annealing are cycle annealing and sub-critical annealing. Cycle annealing involves heating a steel above A3 and slow cooling, sub-critical annealing is when you heat a steel to just under A1 and hold and then cool.

Cycle annealing produces coarse sheroidite and sub-critical annealing produces fine spheroidite.

Simple steels have finer grain when the condition of the steel prior to austenitizing is fine pearlite or fine spheroidite. Finer grain in hardened and tempered steel is tougher, more wear resistant and keener.

Very fine grain for a particular steel can be achieved by first prequenching, then sub-critical annealing, followed by austeitizing, quenching and tempering.

Thermo cycling produces fine pearlite.

Hoss

 

[Willie71]

To add additional questions, when using higher alloy steels that are air hardening, the only annealing listed in the datasheets is cycle annealing. If one wanted to refine the grain after forging, or heavy machining, is a cycle anneal the only option? If this produces course spherodite, then do we need longer soaks (temps are 1950-1970f) ? If I wanted the finest, even grain, is there anything else I can do?

 

[DevinT]

To add additional questions, when using higher alloy steels that are air hardening, the only annealing listed in the datasheets is cycle annealing. If one wanted to refine the grain after forging, or heavy machining, is a cycle anneal the only option? If this produces course spherodite, then do we need longer soaks (temps are 1950-1970f) ? If I wanted the finest, even grain, is there anything else I can do?

After forging high alloy steels, it’s best to cycle anneal, prequench, sub-critical anneal, austenitize, and temper. I don’t recommend going straight from forging to sub-critical anneal. Stress relieving after machining also uses sub-critical temperatures but is not a true anneal.

High alloy steels have multiple types of carbides that develope, some dissolve at lower temperature than others. Some types of carbide are stable at higher temps, they are primary carbides formed in solidification. Secondary precipitated carbides are smaller and some dissolve during heating. Annealed steels have twice the carbide volume as hardened and tempered steels. The carbides that dissolve during heating put carbon and alloy in solution.

Heating high alloy steels longer will dissolve more carbides but can grow the grain.

A prequench, sub-critical anneal, full austenitize, quench and temper will produce the finest grain. Cryo does not refine the grain.

Hoss

 

[Willie71]

After forging high alloy steels, it’s best to cycle anneal, prequench, sub-critical anneal, austenitize, and temper. I don’t recommend going straight from forging to sub-critical anneal. Stress relieving after machining also uses sub-critical temperatures but is not a true anneal.

High alloy steels have multiple types of carbides that develope, some dissolve at lower temperature than others. Some types of carbide are stable at higher temps, they are primary carbides formed in solidification. Secondary precipitated carbides are smaller and some dissolve during heating. Annealed steels have twice the carbide volume as hardened and tempered steels. The carbides that dissolve during heating put carbon and alloy in solution.

Heating high alloy steels longer will dissolve more carbides but can grow the grain.

A prequench, sub-critical anneal, full austenitize, quench and temper will produce the finest grain. Cryo does not refine the grain.

Hoss

Thank you. I started working on my press this weekend. I’ve had this question in my mind for a long time now, great answer.

 

[Heli00]

After forging high alloy steels, it’s best to cycle anneal, prequench, sub-critical anneal, austenitize, and temper. I don’t recommend going straight from forging to sub-critical anneal. Stress relieving after machining also uses sub-critical temperatures but is not a true anneal.

High alloy steels have multiple types of carbides that develope, some dissolve at lower temperature than others. Some types of carbide are stable at higher temps, they are primary carbides formed in solidification. Secondary precipitated carbides are smaller and some dissolve during heating. Annealed steels have twice the carbide volume as hardened and tempered steels. The carbides that dissolve during heating put carbon and alloy in solution.

Heating high alloy steels longer will dissolve more carbides but can grow the grain.

A prequench, sub-critical anneal, full austenitize, quench and temper will produce the finest grain. Cryo does not refine the grain.

Hoss

D DevinT - Could you please explain what the prequench is?
Thanks in advance,
Constantin

 

[kuraki]

I don't understand the relationship between spherodizing and grain size (other than grain grows with time and temperature).

IE: Is it possible to have a coarsely spherodized piece of steel that's fine grained? Finely spherodized that's coarse grained? Is there no relationship between the two at all other than the time and temperatures used to get to a particular state of spherodization?

Where does lamellar annealling fit in with sub-critical and cycle (spherodizing) annealling?

Is sub-critical annealing really just the highest form of tempering?

https://pmpaspeakingofprecision.com/tag/lamellar-pearlitic-anneal/

 

[jdm61]

I guess that I technically do my "subcritical anneal" as a stress relief operation because it is done after any pre-hardening rough grinding. Is that problematic in any way?

 

[mete]

You have two things to deal with : 1 carbides
2 grain size
work on the carbides first , then adjust the grain size.
find out what you have , 2 work out a plan , then adjust grain size.

 

[DevinT]

I don't understand the relationship between spherodizing and grain size (other than grain grows with time and temperature).

IE: Is it possible to have a coarsely spherodized piece of steel that's fine grained? Finely spherodized that's coarse grained? Is there no relationship between the two at all other than the time and temperatures used to get to a particular state of spherodization?

Where does lamellar annealling fit in with sub-critical and cycle (spherodizing) annealling?

Is sub-critical annealing really just the highest form of tempering?

https://pmpaspeakingofprecision.com/tag/lamellar-pearlitic-anneal/

Grains of steel look like soap bubbles in a jar. During grain growth, grains eat each other becoming larger. Carbides occur within the grain and at the grain boundaries.

Within similar carbide volumes, the smaller the carbides, the more of them you have and the more evenly distributed they are. The more carbides you have, grains will have a hard time growing because they are bumping up against the carbides, or pinning the grain boundaries.

Pearlite occurs mostly in simple or low alloy steels. Under a microscope, pearlite looks like very fine lamellar carbides in between ferrite, they are much finer than normal carbides. The finer the pearlite, the finer the grain when heat treated.

The finer the spheroidite or the finer the pearlite, the finer the grain in the final heat treated piece.

Hoss

 

[DevinT]

D DevinT - Could you please explain what the prequench is?
Thanks in advance,
Constantin

A prequench happens before the final austenitize and quench. They can be at, above, or below the normal austenitizing temperature, depending on what you are trying to achieve. They are most times followed by another thermal cycle.

Hoss

 

[Heli00]

A prequench happens before the final austenitize and quench. They can be at, above, or below the normal austenitizing temperature, depending on what you are trying to achieve. They are most times followed by another thermal cycle.

Hoss

Thanks again. So if the prequench is at or above the austenitizing temperature, what is the purpose of this cycle?

Thanks,
Constantin

 

[DevinT]

Thanks again. So if the prequench is at or above the austenitizing temperature, what is the purpose of this cycle?

Thanks,
Constantin

A prequench below the normal aus temp produces more carbide volume and will help refine the grain during quench and temper.

A prequench above the normal aus temp is used to dissolve larger carbides putting more carbon in solution and producing smaller secondary carbides. Must be careful with this one.

A prequench at the normal aus temp is somewhere in the middle of the other two.

Hoss

 

[Heli00]

A prequench below the normal aus temp produces more carbide volume and will help refine the grain during quench and temper.

A prequench above the normal aus temp is used to dissolve larger carbides putting more carbon in solution and producing smaller secondary carbides. Must be careful with this one.

A prequench at the normal aus temp is somewhere in the middle of the other two.

Hoss

Thanks again.

I assume even the below aus temp prequench is still way above the sub-critical anneal and will never overlap it, correct?

So once the target temperature is reached in a prequench, is the cooling done in the air or slower?

Is there a soaking time necessary once prequench target temperature has been reached?


Thanks,
Constantin

 

[DevinT]

Thanks again.

I assume even the below aus temp prequench is still way above the sub-critical anneal and will never overlap it, correct?

So once the target temperature is reached in a prequench, is the cooling done in the air or slower?

Is there a soaking time necessary once prequench target temperature has been reached?


Thanks,
Constantin

The more answers I give, the more questions you have.

Most prequenches are done above Ac3, there are some that are done in the inter critical range between Ac1 and Ac3.

These are an actual quench and depending on the steel type, will require some amount of soak time and need the same quench type as the steel requires, water, oil, or air.

Normalizing and thermal cycling are done without a quench.

Hoss

 

[Heli00]

The more answers I give, the more questions you have.

Most prequenches are done above Ac3, there are some that are done in the inter critical range between Ac1 and Ac3.

These are an actual quench and depending on the steel type, will require some amount of soak time and need the same quench type as the steel requires, water, oil, or air.

Normalizing and thermal cycling are done without a quench.

Hoss

Thanks again Hoss. My apologies if I asked too many questions. I really appreciate you took the time to answer.
All the best,
Constantin