Heat Treating Steel Explained

cryst1

Understanding how a crystal formation occurs in volcanic activity is much like what happens in steel. Liquid magma dumped straight into the ocean instantly solidifies, locking in the natural state of the magma. If allowed to cool slowly over thousands of years the different elements begin to clump together forming crystals. Quartz, silver, gold. Cripple Creek Colorado is an excellent geological example of this. This same concept can be applied to heat treating. Instead of thousands of years as a time metric, you’re dealing with seconds.

dendrite

The normalizing process I find particularly interesting. If you do several normalization cycles each time lowering the temperature you take it to prior to air cooling you can produce a very fine micro grain structure.

Each time you normalize the crystalline structure has time to grow as it cools. The goal is to take your metal and make the entire piece uniform in its crystalline structure, with the smallest crystal growth possible.

For example, lets say you are working with CPM S110V (my favorite fillet knife material for my personal blades) stainless. This would be the correct heat treatment for the blade.

All steel manufacturers include data sheets for their products. It doesn’t have to be a special alloy steel, you can find these data sheets for many common metals both stainless and non-stainless.

Crucible Industries makes S110V, the data sheet can be found here:
http://www.crucible.com/PDFs/DataSheets2010/Datasheet CPM S110Vv12010.pdf

CPM S110V much be quenched at 2150F. That would be your minimum temperature for normalization cycles. This is the austenitization temperature. It should be non-magnetic, and the lowest temperature that loosens the bonds between the crystalline lattice in the material.

~~Normallization~~

Cycle 1:
Set your soak temp for 2250F and soak for 1hr. This will homogenize – think of it as almost liquidizing the crystalline structures in place, they are still there roughly but are in a state where they will reform during cooling. The bonds that hold them together are broken, but they are still there “thermal memory”. You will never completely get rid of them without melting down the steel. The higher your temp is the longer it takes to cool. You want to start above your quench temperature, but without excessive heat. Go with the closest margin of error your heat treat furnace can achieve. Given the same rate of cooling in air a higher starting temperature will result in larger crystals. This is important to remember. Once you start your normalization cycles you want to make sure you never cycle at a higher temperature than your last cycle. If you do it will continue to improve uniformity, but not so much the grain size. By reducing the temperature a bit on every cycle you both create uniformity of crystalline structure AND force smaller crystals by reducing the time in which the crystals have to grow.

Cycle 2:
Set soak temp for 2225F and soak for 30mins, air cool. This slightly lower temperature will reduce the time it takes for the steel to cool below the austenitization temperature. The crystals that formed in the first cycle will be unable to reform to their original size and the micro-stresses of the crystal formation will force the bigger crystals (thermal memory here) from the previous cycle to break up. This is very good.

Cycle 3:
Set soak temp for 2200F and soak for 20mins, air cool. Again lowering the temperature a bit forces the thermal memory of the previous cycle to break up even smaller.

Cycle 4:
Set soak temp for 2175F and soak for 20mins, air cool.

At this point you have a very uniform crystalline structure throughout your steel and a very small grain structure. By stepping down your temperature each cycle you have forced less of the crystals to homogenize each time, creating an effect that fills in the gaps so to speak between the larger left over crystalline structures that formed during the first 2 or 3 cycles.

grains

~~Quench~~
At this point you want to go for your quench at 2150F.
Set soak temp for 2150F and soak for 20 minutes. Quench appropriately (interrupted oil quench, I suggest ISO 32 hydraulic fluid)

Note this is below the normalization heats. If you go above your normalization heat temps you ruin the work you have done by the step down cycles. Make sure you never do this – never quench from a heat higher than your normalization heats. While the quenchant will cool much more rapidly than air, you don’t want the small grain structure you built to dissolve. This is an extremely important concept – the longer your steel takes to cool the larger your grain structure will be AND thermal memory (IE achieved crystalline structure size) is affected by temperature. Combine those two concepts and it will hopefully make sense as to why you don’t want to quench above your last normalization cycle’s temp. You want your steel conditioned perfectly by the normalization process so that when you go for a quench you can do it right at the austenitization temperature. If you skip the normalization structure you lock in whatever stresses and non-uniform crystal growth size is in the steel. This is why you see smiths on forged in fire quenching at cherry red end up with high performing blades and those quenching at bright yellow end up with chipping and rolling. Both are hard, but only one is strong. This is also why quenching in water is very dangerous for steels with high rates of crystalline formation. Where pure iron may survive, high carbon alloy steel with very complex crystalline and even carbide micro structures just shatter like glass under that amount of internal stress.

After the quench the uniform/tiny crystal formations are set.

~~Temper ~~
Set soak temp for 975F and soak for 2 hours. Make sure it completely cools to room temp each time. Repeat 2 more times.

6 thoughts on “Heat Treating Steel Explained

  1. Hello,

    I make knives as a hobby. I read your article and found it very interesting, however I’m not sure I understood everything clearly.
    I was wondering, if you could clear a few things up for me:
    *Do I air cool after the first cycle too?
    *To what temperatures do I cool it down to?
    *Do I put the steel in when the furnace is hot?
    * “You want to start above your quench temperature, but without excessive heat. Go with the closest margin of error your heat treat furnace can achieve. Given the same rate of cooling in air a higher starting temperature will result in larger crystals.” Above quenche temperature, but without excessive heat? The closest margin of error? Same rate of cooling? Could you please elaborate?
    Thank you!

    p.s.
    I have 2 furnaces.

    Best regards,
    Valents

    • So the furnace I’m talking about it a precision controlled electric heat treat oven. You can do the same thing with like a propane furnace or coal for plain carbon steels but for the higher end stainless steels that require precise temperatures you’ll want to build yourself a kanthal-A1 furnace. I have a writeup for mine here – https://davidproberts.com/diy/2300f-heat-treat-oven/

      Air cool to room temp on each normalization cycle.

      You can put the steel in while the furnace is hot or cold, doesn’t matter. All that matters is that it bakes at the proper temperature/time. Exact temp/times can be found on the manufacturers data sheet for the steel in question.

      The closest margin of error – this would be how much set point variance you have on your furnace. On mine it’s about 12 degrees F. For example, say I set my temp at 2000F and hit go. The coils activate and heat the furnace until it reaches 2000F on the ceramic thermometer. At that point the coils shut off but the temp may reach say 2005F before it starts to slowly drop again. Once the temp reaches 1999F the coils re-activate and begin producing heat. As this happens the temp continues to slowly drop until the coils reach cooling temp. During this time the temp may reach like 1990F before it stops dropping and heats back up to 2000F. On mine I use a 10,000watt voltage regulator to fine tune this “margin of error” delay depending on the temp. If you are soaking at say 500F for a 2 hour temper cycle you would want to use a much lower voltage than you would for say 2200F to give the coils time to heat the ambient air inside the chamber. Wouldn’t want your knife to hit 1500F before the thermometer was able to sense that it was 500F.

      Same rate of cooling – air cooling rates can be altered by forced air such as placing it in a pipe with a blower at the end of it. Use caution when setting this up, don’t just hold it in front of a fan. You MUST have even air flow on all sides of the blade or you will introduce internal stresses or even worse warps into your steel. It’s recommended for normalization cycles that you just hold it in the air, don’t lay it down on anything that can suck heat out of it unevenly. For rapid air quenching (final, not normalization) – this would be for air hardening steels – you definitely want to cool it with some forced air. Most higher end stainless steels (S110V/S90V/S30V for example) call for “positive pressure gas quench” this basically means ‘blow on it real hard evenly’. A leaf blower attached to a steel pipe with riser holders for the blade would be perfect. I usually opt for the interrupted oil quench.

      Example of a data sheet – S110V
      http://www.crucible.com/PDFs/DataSheets2010/Datasheet%20CPM%20S110Vv12010.pdf

      For this data sheet you would want 3 normalization cycles – 2200F for 20mins, 2175F for 20 mins, 2150F for 20 mins, then a final 2150F 20 minute bake and IMMEDIATELY into the oil/positive pressure quench. You’d then reset the temp for your oven to 975F, adjust your voltage regulator to minimize temp margin of error and cook it for three 2hr temper cycles, letting it room temp cool between each cycle.

  2. Thank you for the reply! It cleared up a lot of things!
    I found a mistake in my furnace as well and was able to fix it. Thank you!
    One more question: Where do you find the normalisation temperatures in the data sheet? I couldn’t find the temperatures given anywhere in the sheet. Also, why do the temperatures given in your article and in your reply differ for the same type of steel?

    • Q:Where do I find the temps for normalization on the data sheet?
      A: It’s on the data sheet but you need to look for the austentation point. For the example S110V that is “Austenitize: 2150°F (1175°C) Hold 20 minutes”
      http://www.crucible.com/PDFs/DataSheets2010/Datasheet%20CPM%20S110Vv12010.pdf

      Q:Why do the temperatures given in your article and in your reply differ for the same type of steel
      A: The only one that matters is the final temp which does match – 2150F – which is needed precisely for the quench. As long as you are not burning the metal starting at 2250F or 2200F and stepping down doesn’t matter. Both are examples of the process to illustrate the temp step down between normalization cycles. You don’t need to use any set temp it just needs to be higher than the austentation point. Let the variance of your heat treat furnace dictate what that step is. If you can hit a 10F variance on the head then you can do maybe 15F steps. If you’re more 20-30F variance you’ll want 40-45F increments.

      When you normalize you’re bringing the metal up past it’s “critical temperature” – the temperature in which the crystalline lattice bonded edges start to break loose and loose their grip on each other. You’ll know you’ve hit it when the steel becomes non-magnetic. Going a little bit higher is fine but don’t go crazy. Keep in mind the higher past critical temperature you go the faster/larger your crystals are going to be.

      Austenitizing Definition
      Austenitizing heat treatment is heating a steel above the critical temperature, holding for a period of time long enough for transformation to occur. The material will be hardened if austenitizing is followed by quenching at a rate that is fast enough to transform the austenite into martensite.

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