[JEB]

I don't know what steel is used in K-80 receivers, perhaps someone can enlighten me, but it is probably similar to 4140/50 HT or 4340. That would put it in the DIN 1.6+++ to DIN 1.7+++ catagory. The yield of 4340 heat treated to a range of Rc 35 is 103,000 psi. Not heat treated, (annealed) Rc is 17, it is 62,000. 4140/50 would be less but not by much. These are not case hardening steels, but through hardening. They are also excellent for nitriding, which would give they an ultra high surface hardness, somewere around Rc 68, I do not believe that Krieghoff does this but others may. In the inside of my K-80 receiver there is a mark from the diamond point used in hardness testing, so obviously Krieghoff has a certain standard. Given the high yield point of this type of alloy, I believe that the only possible cause of a "blow up" must be some sort of an obstruction probably coupled with an ultra hot load. HMB points this out in another thread. The steel used in receivers is just too strong for anything else. These alloys are common in aerospace applications and I would very much doubt that there is a possibility of metal fatigue involved.

Incidentally about a year ago. I sold a TM-1 to a shooter who had blown up a TMX the previous week. He was a brand new reloader. It ain't just Krieghoffs.

J Bergman

 

[TOOLMAKER 251]

Jeb, You can case harden 4140 thru induction hardening.

 

[Hap MecTweaks]

I've replaced the purlin framework over the top of blast furnaces many times during my ironworking careerer. Even mild steel will shatter when struck with a sledge hammer. Heating and cooling changes the molecular structure of the normally softer metal which causes this. The heating and cooling processes in steel will act in the same manner regardless of which grade it's made from. Hap

 

[JEB]

I am not sure Hap if you are referring to temperature changes during shooting. They are not great enough to effect either the tensile or yield points. You are correct as to the relationship of temperature and strength, that is what annealing is all about. Toolmaker you are correct also, 4140 range steels will produce a hard case if certain types of induction hardening are performed, but I would doubt that it is done with a receiver. My guess is that they are machined from pre heat treated steel to eliminate any distortion in hardening. It is a very common machining practice, you probably have done it a million times.

JEB

 

[Hap MecTweaks]

JEB, I made no references to what happens during firing of firearms. I mention only what happens to steel when subjected to heat and cooling cycles.

How hot is the gasses spewing out of hulls even if only momentarily? One wouldn't think aircraft aluminum would stress crack like it does either since it's very high strength. It does crack and the only way heat is involved is through movement or flexing, mostly during take-offs and landings.

My thoughts on making the steels is sometimes suspect though. If any impurities are present or hidden inside those metals, machining may not expose them. The only way to know is by X-raying the piece to expose any internal flaws. I also question whether or not all gun makers rely totally on the steel manufacturer for a pure product or simply machine the product and proof test it once.

Hap

 

[TOOLMAKER 251]

Hap, I would say manufactures rely totally on the metal producers. Without lab certification on a steel purchase you could be getting a flawed batch. Dan Lilja got a run of 416SS a few years back and people got some not too good world class barrels. Dan makes some of the best barrels in the world with many records to prove it. The company I retired from built a Ford econoline chassis blanking die from steels made with A2 tool steel by a major steel supplier. When most of the work was done and after hardening the steels were cracking. The manufacturer of the steel after tests were performed determined their product had structural flaws. The bottom line is we ate the labor cost and was given new steel.

 

[KENENT1]

I have been machining bar stock for 33 years and have seen first hand the many quality problems from steel suppliers. Everything from old screws imbedded in the middle of a bar, to bars that crumble when you try to machine them, to bars that have cracks lengthwise down the middle.

I have recieved bar stock that has a part formed on the end more than once, this means that some other shop had a problem with the material and sent it back to the supplier, and they in turn just sent it to another shop, and usually when you start to cut these bars, you can see why it was sent back.

Most shops do not have the capability to check the chemical and phisical propertys of the steel they use, and have to depend on the steel suppliers.

One of the problems now is that quite a bit of the barstock is rolled off shore, especially the smaller diameters, there is not enough profit for the US steel companys to manufacture it.

tony