case color hardening again

[David Rase]

I case color hardened another set of parts this week and wanted to share some information.  First off let me say that I am not real happy with the results.  In the past I have packed my parts in the crucible with a 2 to 1 ratio of wood to bone charcoal mixture, placed the packed crucible into a cold oven and then brought the temperature up to 1500 degrees and soaked them for an hour.  I then quench the parts and then temper them or another hour in a 400 degree oven.  I really liked the results I get with this method.  After talking with different people and reading postings on this board it looks like I was the only one using 1500 degrees.  I think most others were using 1350 to 1400 degrees.  Because many of the parts on this latest build are pretty thin and or long, I decided to lower my temperature this go round to maybe avoid some warping, though I have not had problems in the past.  I soaked my parts at 1360 degrees for an hour once temperature was reached, just like in the past.  Upon removing the parts from the quench I was pretty disappointed with how grey everything was.  Where did all the darker blues and purples go that I had obtained in the past?  I have checked my oven with a type K thermal couple temperature probe and my oven and the thermometer track well.  The only thing I can think of is that even though the temperature is 1360 degrees, maybe the parts in the pack are not that hot.  That could also mean that at 1500 degrees oven temperature my actual crucible  temperature is less than the oven temperature.  Anyway, here are a couple of pictures to see what I am talking about. 
David
PS:  Unless someone can convince me otherwise, I am going back to using what I know works for me.

[smart dog]

Hi Dave,
I usually have my best results for colors and quality of the hardening at 1450-1475 degrees.  I use 50% wood and 50% bone charcoal. I experimented once with a temperature of 1375 (the lowest I've ever tested) and heat soak of 90 minutes. The colors were OK but I was disappointed with the depth of the hardening.

dave

[snapper]

Have you tried to do it at around 1325 F?

I have used Classic Guns in IL for several years and their colors are really vivid.  Small pieces of coarse do not get much color.

2 weeks ago I was able to stop in Crete IL and pick up my parts that they had for me. The parts can be seen in the Alex Henry post.

While I was there they gave me a tour of their facility and we discussed their process.  He told me that they use 1325 F for temp, and they do not temper the parts after the quench.  The fact that they did not temper was a concern for me.  So I ended up calling Turnbull to see what their process was.

Turnbull told me that they go over 1300 F but was not specific and they also do not temper after CC.

I was still a little concerned about the parts not being tempered.  So I call my buddy at Iowa State University and is a metallurgist.  He told me that the 1325 temps, tempering was not needed.  The metal was not hot enough to get into the phases that would require the stress relief.   As my buddy described it to me is that you are simply trying to get the carbon to diffuse in the out layer of the steel, and you do not have to get it hotter than 1325 to have that happen.

At Classic Guns, I did not ask them what they packed the parts in, however I noticed several bags of charcoal sitting around, and not any other materials that could be used.

Fleener

[smart dog]

Hi Snapper,
Thanks for that information.  I will try that for parts that strategy.  I've not gone below 1375.

dave

[oldtravler61]

Dave if your old process worked. I wouldn't change. But that's just me.

[ericxvi]

   Dave , I agree with your latest conclusion. I had an experience where after heat treating at 1500, I reduced the oven temp (programmable oven with digital readout) to 375 to temper the frizzen. After waiting till the readout indicated 375, I soaked the part like I usually do. The first time the lock was tripped the frizzen broke and no sparks! The frizzen was soft.
  I think when you start from a cold oven and allow only 1 hour, the crucible does not reach the intended temperature. In my case the oven was much hotter than the thermocouple was.

[smallpatch]

David,
We want to see more of that pistol!!

[Acer Saccharum]

Beware general statements for color case.

It depends on what kind of steel. 8620 is a medium carbon steel, and can be color cased with some surface hardness and a tough core. Receivers can be made from this, lockplates, standing breeches, etc. (definitely temper the threads on a standing breech so they are not brittle)

Pack hardening parts, such as set trigger parts, frizzens, lockplates, etc, where surface hardness is more important than the colors, you'd want to bring the pack to a higher temp for a longer time to deepen the case, and temper after quenching.

[David Rase]

Beware general statements for color case.

It depends on what kind of steel. 8620 is a medium carbon steel, and can be color cased with some surface hardness and a tough core. Receivers can be made from this, lockplates, standing breeches, etc. (definitely temper the threads on a standing breech so they are not brittle)

Pack hardening parts, such as set trigger parts, frizzens, lockplates, etc, where surface hardness is more important than the colors, you'd want to bring the pack to a higher temp for a longer time to deepen the case, and temper after quenching.

The components that make up the underhammer action are from 1018 and the tang was made from 12L14.  I have had success reducing the temperature from 1500 down to 1400 degrees in my oven.  Dropping the temp the additional 40 degrees to 1360 obviously was to low a temperature.  My next set of parts I will try to color harden at 1450 degrees.  If I decide to use lower than 1450 degree temperatures in a future hardening I will increase the soak time to maybe 1 1/2 hours to try and obtain a crucible temperature commensurate with the oven temperature. 
David

[David Rase]

David,
We want to see more of that pistol!!

Dane,
I need to finish it first.  Need to engrave the butt cap, finish fire bluing and pinning the barrel keys and rub in a few more coats of oil and .  Stay tuned.
David

[T*O*F]

We want to see more of that pistol!!

He will incur the wrath of the moderators.  That style of gun is banned here....not a sidelock.

[JPK]

I don't mean to offend any one with this side lock but it is for illustration of the color I get with my process. I pack in a 1 bone to 4 wood charcoal and place in the 1400 F oven. Once the temperature returns to 1400 F  I work it back down to 1375 F while the time is running for a total of 1 hour. The only tricky thing I do is wire the hammer and or other small parts into the tangs and loosely plug the receiver ring. Water quench and draw at 375 F for an hour.

[Bob Roller]

I don't mean to offend any one with this side lock but it is for illustration of the color I get with my process. I pack in a 1 bone to 4 wood charcoal and place in the 1400 F oven. Once the temperature returns to 1400 F  I work it back down to 1375 F while the time is running for a total of 1 hour. The only tricky thing I do is wire the hammer and or other small parts into the tangs and loosely plug the receiver ring. Water quench and draw at 375 F for an hour.

Fine job. Looks like a Freund to me.

Bob Roller

[snapper]

http://www.thefabricator.com/article/metalsmaterials/phases-structures-and-the-influences-of-temperature


phase diagrams are helpful in understanding the different phases the metal moves through when you are heating it.

diagram #2 is the one that is for up to 5% carbon.

My oldest son is studying to be a materials science engineer with a metals emphasis at Iowa State University.  It is nice to be able to talk to him about some of this stuff and pick his brain a little.

Last weekend at supper we talked about color case and he was excited to call me on Monday as they had a lecture on it.

Fleener

[Mark Elliott]

I have only been doing pack hardening for a short time,  and I have been using 100% hardwood charcoal from Lowes.   I have gotten  good colors between 1475 and 1625.    It was a little bluer and grayer at the higher temp meant to harden and not done for colors.     Still,  I would have been delighted with the color if I was doing it for that reason.   That leads me to believe that the type of charcoal doesn't really matter as long as it is free of contaminants.   

I use an electric oven that takes a long time to get to temp and I have held at least a hour at temp before quenching.   Usually it doesn't take much more that 10-15 seconds between the time I take the crucible out of the oven and dump the contents.   So,  I doubt that I loose much more than 50 degrees.    I have been using clay/graphite crucibles and covering them with stainless steel foil which is a pretty tight fit.     The crucibles and contents hold their heat pretty well.   

I have noticed in tempering operations that objects placed on the floor of the oven get hotter than the pyrometer reads; it being at the top of the oven.   I now make sure that I place objects on crucibles in the middle of the oven.   I don't think that would have a bearing on color case hardening though.    The temperature differential in my oven is supposed to be only 25 degrees, but I am not sure about that.    I am not sure how much you can trust just one thermocouple in an oven.   I sometimes put another thermocouble on my part to make sure I know the temp.    Usually, the thermocouples agree within a couple of degrees, but I still don't fully trust them.  Relying on heat transfer through air seems awful unreliable.