Canvassing views on something I really ought to know more about...

A customer has informed me that his sword is starting to take a bend. This can be corrected, but it comes back. From what he described, it sounds to me like fatigue is setting in. It is a broadsword (blunt), forged from the equivalent of 5160 spring and hardened and tempered to 50RC. It has been subjected to 3 or 4 full speed bouts once or twice a week, plus general drilling, for nearly two years.

Oddly enough, one of my personal swords (made to the same specs) has just started to do the same thing.

I know there are any number of variables that can affect the outcome, but -generally speaking- how long is it reasonable to expect a blade like the one described to last with regular heavy use?

Thanks in advance


Tim
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Nonnumquam proctalgia

By reputation, it should be extremely tough (expected to withstand millions of cycles as a hard working spring.) Hardness sounds a little low. That one is normally not tempered fairly hot. If you describe your heat treat in more detail, someone with more experience with it might be able to tell you if the condition relates to the heat treat process.

Actually, about the only application were metal gets more present day punishment than in a vehicle's leaf spring is serving as tank armor! The 1970's 5160 material was tested in several test rigs (giant motorized hammers plus hydraulic twisting apparatus impacting leaf spring assemblies at different angles) at my college campus (Champaign IL.) They were still waiting to find the fatigue limits of several leaf spring assemblies in the mid 1980s (springs being struck a little faster than once every 2 seconds around the clock, over a decade....)

I think there are important differences though. Swords have that sharp edge that will concentrate stresses during edge to edge impact. In contrast, a spring is made as blunt as possible with a semi-circular radius edge. Surface hardness has also been considered an important part of spring life for some materials, some manufacturers going for shallow surface hardening. I don't know what the present day view and practice for 5160 is though. If the swords in question were failing at cracks emanating from the cutting edge, it would seem understandable. If they are bending without visible surface cracks, implying to me that the core material is yielding in the plane of the flat, stress concentration does not really explain it. The material is not supposed to be difficult to heat treat, so this is doubly puzzling. Pictures of what is occurring (good surface detail in bend regions) plus explanation of the heat treat is likely needed. There is such a thing as "tempering embritelment" that can occur if a drawing (stage of tempering) heat is too low. I think 5160 is a candidate (chromium related) for this, but I am not sure at what temperature region it would occur.

Can you describe "take a set?" I am wondering if there are very small cracks that might not be obvious without using a good quality magnifying glass, dye, and light?

Leaf springs are really subjected to something more analogous to buckling, plus twisting. It is imparted with such abruptness that it is analogous to a shock and best simulated with mechanical hammering. They do well in that situation and dissipate a lot more energy than my arm could withstand through sliding action (kind of like a sword deflecting a glancing strike rather than a parry with a hard stop against flat plus loads of deflection....

I would consider it unfair to complain about 5 good years of rigorous use, particularly if the blades are rather light and geared towards nimble cutting performance. (I would characterize some of your swords that way as an intended complement.)

I suspect that there is some heat treat which would be optimal for withstanding fatigue yielding. Usually this falls in a brinell range of 450, Rockwell upper 40's. But, that heat treat might not be considered "optimal" in terms of edge hardness and durability in cutting. A swordsman who had survived 5 years of rigor with a blade, had all around good performance and durability of the edge, would most likely be pretty grateful to its maker.

Gentlemen,

Thanks for your responses.

I have not seen the blade in question yet, but I do know that it is misbehaving in the most likely trouble spot - the transition between forte and foible, around the distal end of the fuller.
I have my blades heat-treated commercially, and know nothing of the technical details of the process. In some cases, there will be a degree of post-treat warping, and more often than not, that region is where it will occur.

I can understand how correcting that will leave a core weakness. I use the Hrisoulas method - heat as small as possible an area at the apex of the warp to temper colour with an oxy-acetylene torch, manually straighten and quench ( in oil). There can sometimes be be a bit of a lag between straightening and quenching. Initially, this makes no appreciable difference to blade resilience, but I see how it could prime an area for trouble later on.

The school where this blade has been used is not known for delicacy. They employ a vigorous, percussive style featuring plenty of Silver's "downright blows" and a lot of edge-to-edge contact. So, if two years is a reasonable life-span for a blade under those conditions, fair enough.

I operate on the principle that if a blade fails in the first six months, there is obviously something wrong, but I found myself with no idea of how long a blade could be expected to last. I certainly have more of a clue now - at least regarding what I would class as heavy use.

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Nonnumquam proctalgia