doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Decalscence/Recalescence temperatures for Actions?

Has anyone documented the temps at which the phase change of the various maker's actions takes place. You have to know this to anneal, and also if you are recoloring, you daren't go above the decalescence temp of the core of the action otherwise you have a piece of glass. The common wisdom is to stay under 1325 but Brownell's actually recommends 1400 in their Case Coloring instructions. Brownells also says to temper to 350F but that's way too low to make any difference to the hardness.

Good question.

Found this info:

HARDENING TEMPERATURES FOR CARBON TOOL STEEL.- The best hardening temperatures for any given tool steel are dependent upon the type of tool and the intended class of service. Wherever possible, the specific recommendations of the tool steel manufacture should be followed. General recommendations for hardening temperatures of carbon tool steel based on carbon contend are as follows: for steel of .65 to .8 per cent content, 1450 to 1550 degrees F; for steel of .8 to .9 percent carbon content, 1410 to 1460 degrees F; for steel to .95 to 1.1 percent carbon content, 1390 to 1430 degrees F and for steels of 1.1 percent and over carbon content, 1380 to 1420 degrees F. For a given hardening temperature range, the higher temperature tend to produce deeper hardness penetration and increased compressional strength while the lower temperatures tend to result in shallower hardness penetration but increased resistance to splitting or bursting stresses.

It seems that one needs to find the sweet spot, ie go just over the hardening temp of the high carbon layer formed on the surface through carburization, and stay below the temp required to harden the low carbon core. That way you will end up with soft pearlite in the core layer, and hard martensite in the case layer.

So to ask the question another way, does anyone know the carbon content of the various steels used to make actions? and what elevated level is typically achieved in the case layer?

It sounds like Brownells may not be far off the mark with their 1400 degree temp.

Found some more interesting info: most shotgun actions are made from low carbon steel that CANNOT be through-hardened by quenching; only the case layer has enough carbon in it to respond to quenching; and 1450F seems to be the proper temp to cause that layer to harden, although 1650F is the right temp to cause it to absorb carbon with any degree of depth and in a reasonable time (hour or two). Carbon absorption is VERY slow at lower temps.

So....it seems that for initial case-hardening, you need to go to 1650F for a couple of hours to get the carbon to absorb. If the action is then annealed (eg for engraving), you only need to go back to 1450F for about 30 mins to reharden the case layer; and for recoloring, you don't need to go much above 1250F to 1300F and you don't need to hold it there, that temp won't affect metallurgy of the core or the case but it's enough to get fresh colors.

But here's the rub: the hinge pin may be made of different alloy than the rest of the action, ie higher carbon that WILL through-harden. So if the pin is still in the action, you need to make sure you don't screw up its properties by making it too hard thus risking brittle failure. That means staying below 1325F, or if you go above it, know exactly what alloy the pin is made of and heat treat accordingly (ie tempering of the pin may be necessary after quenching).

This is all conjecture based on limited reading so if there are any real metallurgists out there, feel free to correct what I have said.

Russ,
I'm no metallurgist, but you're on the right track now. My understanding is that all the vintage American actions are mild steel, of carbon content below which martensitic hardening is possible. This might be a .10 to .20 carbon content.

The exact temps and times for these processes seem to be somewhat protected by those that have learned the hard way by damaging/warping recievers.

I recall asking Oscar about casehardening a modern reciever that is a martensitic alloy. He didn't know, or wouldn't take on, coloring a modern medium carbon alloy reciever. He emphatically recommended Doug Turnbull for such work. As I understand it, the trick is to ensure proper hardening processing for such steels and still get colors. He warned not to trust just anyone that does casehardening to this job. A thorough understanding of martensitic hardening and tempering processing is needed.

Note that 1250-1300°F will affect the case. It will draw (temper) out virtually all the hardness put there by the quench. I had started a discussion privatly with Oscar, but unfortunately didn't have opportunity to complete. Oscar's technique was I think to heat part high enough for absorption of a little more carbon, then drop the temp to below the critical for the low carbon core, but still above that of the high carbon case for quench. This gave max hardness for the case but preserved full softness to the core.
The L C Smith "Parts & Specifications" book lists their frame for at least some part of it's life as being from 1020 steel. I think this is probably fairly typical for the period.
From all I am able to gather from the past, this procedure "Was" done for the "Hardness", color was a by-product, though they soon did work to develope the colors to their best, while retaining the hard case.

look at this thread.
http://doublegunshop.com/forums/ubbthrea...h=true#Post9138

Geno,

You and I are looking for the same holy grail, the exact science behind the art...unfortunately the diagram is gone from your posting but I know the one you are referring to. 727C is 1341F. So if you are around .8% carbon in the case, 1350F should create a hard martensitic surface when quenched.

Miller's point regarding the tempering effect on the case of stopping at 1250F, ie below the decalescence point is a good one. That's probably why Brownells says to recolor at 1400F. I got the 1250F recipe from a metallurgist who is also a gunsmith so I will ask him about tempering.

I think what Oscar was doing was going to around 1650 to get max carbon absorption, then coming back to maybe 1400 (or even lower as the recalescence point is much lower than the decalescence point when you come down in temp) before quenching to still get the case hard but to retain the colors (which disappear if you quench from too high temp...and you also risk cracking and/or warping if you quench from too high temp). In industry, this is normally done in two steps: first step is the caburizing at 1650F, then the part is allowed to cool. I suppose any working/polishing is then done. Second step is hardening of the case through quenching in water or brine from 1450F.

Anyone know if Oscar used a layer of oil on top of his quench tank?

I do remember a discussion here when Brownell's first marketed their case hardening kit. As I recall the main point Oscar made was that Brownell had the temperature schedule wrong. I do not know if they have changed it since then. Hopefully this will jog some one's memory.

As for the oil layer. I have read where several people claimed to use it. Never saw any of their work. I know of 1 commercial operation that does not use it.

Pete

Lot of apples and oranges in this discussion. Carbon tool steels are not what actions are made of. If we are talking about casehardened receivers (I assume we are) the steel involved is low carbon, about 20 points carbon, and is itself not hardenable by heating above the critical temperature and quenching, regardless of process. That is why it is often casehardened or carburized by one of several processes to give it a hard wear resistant THIN outer layer of high carbon steel to accompany a tough ductile core from which the strength of the structure arises.
That high carbon case is then tempered for several reasons, and this is indeed done at temps of 350-400f. That tempering (or the hardening quench for that matter) has only very minor effects on the low carbon core, and none that need influence this discussion.
It would be well to remember that carburizing of simple low carbon steels is an essentially very low-tech process that is well understood and has been carried on by primitive means for hundreds of years. The matter of whether or not colors are desired as a correlary of this process is another matter: You can have oxide colors without hardening or with it, or none, depending entirely on how the process is managed.
In point of fact, many of the finest London guns of the late nineteenth century, and probably the early twentieth as well, were heat treated and colored in boxes of bone charcoal over hand tended coke fires in the alley behind the workshop - literally.
It would come as a considerable surprise to these skilled artisans that they needed to know or care about decalescence or recalescense.