doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Lump hardness of chopper lump

I think most folks would agree, hinge pins are made of hardened steel, and yet the lumps, at least on chopper lump (Winchester Model 21 may be an exception because their barrels were of their harder Win proof steel) are not. Why do they not harden the lumps.....it would seem the mating of 2 equally hard substances would be best in terms of wear. I think this dichotomy in terms of hardness is at least one reason most guns will eventually shoot 'off face' if used enough and certainly if not lubricated properly. Does this make any sense?

Steels don't necessarily need a lot of carbon to work harden. Stainless steels are an example of this. Stainless steels that are of low carbon can work harden readily. You do need to exceed the elastic limit to work harden. That is you need permanent (plastic) deformation to occur to work harden.

I'm not aware of any hardening from age in the alloys common to barrels. There are age hardening aluminums that harden over time, usually at elevated temps.

Agree as far as SS ---------try to drill down through a chunk of Stainless you've just burned a bit up on from not enough lubrication. Stainless is evil

Buzz,
On most of the old doubles that are casehardened, the hinge pin is installed soft and casehardened with the frame.

Two equally hard sliding surfaces are usually a formula for galling. Most common arrangement of sliding(plain bearing) surfaces is one soft, one hard. This is why the main bearings in your engine are soft babbit and the crankshaft journal is nitrided (casehardened) iron or steel.

The first chopper lump barrels were damascus . They were not to sucssesful as the lumps being relativly soft did not stand up to the stresses and strains and also wore more rapidly than was required. Later steel barrels are an alloy steel that will harden to some degree if not treated with care duing there manufacture . As would the lumps on dovetaled barrels . I remember an instance when some barrel that had been brazed together were put outside in the winter to cool down quicker . They were then sent to have the lumps machined off beforer the ribs were put on. The barrels were set up as normal on a hoirizontal milling machine to have the sides and flats cut using a gang cuttter set up . Two sets a cutter swere blunted before it was realised what was going on .
The point is that yes the lumps will harden if only to some degree but the alloy steel is such that it is left in its anneled state . As must be the rest of the barrel,so care is taken when brazing and ribbing to allow them to cool naturaly to pevent this.

gunman,
That lump that cooled outside got a quench from the cold, which apparently was enough to harden the lump. That says the lump had enough carbon to produce martinsite. That would generally be of .30% or more. "Mild steels" would not be affected by that quench.

So Chuck and Gunman, what you guys are saying is it is better and more appropriate to have the lumps and esp the forward lump with the hook to be made of a softer metal than the hinge pin due to galling??? Ok....this paradox is making some sense to me now.

Thats about it .

It might be possible for a nonhardened steel to be difficult to machine, drill etc. I think if ribs were traditionally attached, the lug would not be likely to have been hardened, but the grain in it could have been mistreated, if it has enough carbon in it, to be hard on tooling and maybe more prone to failure.

Machineability is not all about hardness. For example, 4130, 4140, 4340 in it's normalized/annealed state is really crappy machining steel. It is impossible to make a fine smooth surface by cutting tools. If you harden 4130 to about 30-35 RC it will machine better with a nicer finish. Certain alloying agents aid in machineability in proper percentages. Lead is added in low percentages to make free machining steels for screw machine products, but it reduces weldability. Sulfur in appropriate quantities will make steel machine better too.