Stress Relieving

There seems to be some confusion among shooters, and even gunsmiths, about stress relief in rifle barrels. Maybe the following will be helpful.

Commercial barrel makers, whether they are using the cut, or the button rifling process, stainless or chrome-moly steel, all get their barrel blanks in the same metallurgical condition. That is, in a heat-treated and tempered state, i.e. hardness R/C 28 to 32, with a tensile strength of around 135,000 to 150,000 PSI. This state of affairs is reached by the usual heat-treat methods that is, stated simply, by bringing the steel bars to their critical temperature (1550°F, in the case of 4140 (chrome-moly)), and quenching (usually) in a liquid medium such as oil.

This process leaves the steel in a full hard state, too hard to use for making rifle barrels, not to mention full of stresses. To relieve these stresses, and to reduce the degree of hardness to an appropriate level, the steel is then tempered, or "drawn," by reheating to 1000°F (again for 4140 or equal). This lowers the hardness to the 28-32 R/C range that we are looking for.

The secondary effect of this tempering operation is that it relieves residual stresses within the bar. Unless the bar has been subjected to some post heat-treat mechanical trauma such as bending, it is relatively stress free. This is why we can use it as received for making rifle barrels.

If you're a "cut rifle" barrel maker, you're home free. If you button rifle, you're not out of the woods yet. Button rifling, as you know, swages, or cold forms the grooves into the bored and reamed hole. In doing so, compression stresses called "radial" or "hoop" (tensile) stress are introduced into the barrel bar. These stresses radiate in diminishing intensity from the bore toward the outside of the barrel.

Any metal removal from the outside of the bar, such as turning, removes some of these compression stresses from the outer boundary, which, in turn, allows the remaining stresses, less encumbered with the containment of an outer shell, to relieve their force outward. This relaxation of stress outward manifests itself in a larger bore diameter.

If, for instance, a taper is turned from the muzzle to the breech, the resulting bore diameter will be inversely proportional to the turned taper. (In proportion only - not dimensionally). This would be a sad state of affairs but for one other procedure. Button barrel makers stress relieve their barrels a second time - at least the makers of match grade barrels do.

This second stress relief (the first one was by the steel maker) is done by soaking the finished but un-turned blank in an atmosphere controlled furnace at 1000°F or so, which removes most of the residual "hoop" stress. The barrel is then turned to final dimensions, and lapped to specs.

There's a bit of a down side to the second heat relief procedure. By raising the temperature of the barrel to 1000°F, a second "drawing" or further softening of the steel takes place, along with a corresponding loss of tensile strength. This is a result of the tempering process being cumulative.

This extra stress relief is the reason button rifled barrels will be about 8 to 10 points lower in hardness on the Rockwell C scale than cut rifle barrels. It also takes away the argument that button barrels are harder because of the cold working effect.

You might be thinking about a possible advantage in barrel life that you'might get with extra hardness, but it's doubtful that you'll gain much. Barrels stop shooting for reasons other than friction wear. I've checked numerous barrels that have lost accuracy (benchrest accuracy), and they all show one common characteristic: a constriction in groove diameter of 0.0003 to 0.0005" approximately 1" ahead of the throat. Although I'm not absolutely certain, I believe this to be a by-product of thermal reticulation (random directional cracks and fissures in the surface of the metal, caused by differential heating and cooling). Sections of a barrel have been examined and some of these fissures appear quite deep, causing the formation of small "platelets". It's quite possible that high pressure at high temperatures penetrates these fissures and forces the "platelets" into the bore, thereby constricting the diameter.

A reminder might be in order about buying barrel steel. It must ALWAYS be made clear to the steel warehouse that you want heat-treated and tempered steel. Ask for a certification that will spell out the analysis as well as the hardness. Never accept anything less than 28 R/C or less than 130,000 PSI tensile. Suitable steel usually has the suffix "H.T." attached to the alloy designation, indicating that it has been heat treated ... e.g. 4140 HT.

a few comments on

+6 -3

Responses

  • Ernesta
    Are sako barrels stress relieved?
    10 months ago

Post a comment