Very cold temperatures and pinned bolsters

Robber58

Robber58

Joined
Oct 9, 2008
Messages
478
A few days ago, a gentleman I made a knife for 20 years ago stopped by to ask me to sharpen for him. The knife was a 154CM full tapered tang with pinned 416 bolsters and ironwood scales. When he handed it to me the first thing I noticed was that the three bolster pins were clearly showing and I could actually feel a slight sharp edge around them. I was pretty sure that was not the case when I delivered it so I pulled out a picture I had taken of it and sure enough it looked fine - no visible pins at all. So I asked him about his history in carrying and using it. He has used it hunting almost every year since I made it, with all the hunts in Arizona. It had never been dropped and I could see it had not been abused. However, late last year he went on a hunt in the high mountains in Colorado. He said it was as cold as he had been in his life with temperatures reaching as low as -25 degrees F. He was there for several days and when he came home he noticed the bolsters. I started thinking about what could be going on and did a bit of study on the properties of the 154 and 416. What I found out was that the coefficient of expansion for the two steels is actually quite different at low temperatures, with the 154 being about 3.9 millionths of an in/in/degree and the 416 at 5.5. Based on that, my current theory is that the pins and bolsters were shrinking more than the underlying blade, causing the pins to pull down very slightly. Frankly I can't come up with any other ideas. Anybody have any similar experience or ideas about what else could have happened?
Randy
 
That's a clever hypothesis, but if I've kept up with my zeros the delta only works out to a fraction of a thou over a 100 deg temperature swing across a typical blade width.

Are you sure that both the bolster and the pins are 416? The reason I ask is a bit of a stretch, but 416 is a martensitic stainless, where something like 304 is austenitic. I wonder if it is possible that some RA in the 416 converted, increasing the thickness of the bolsters relative to the length of the pins?

I don't know, that's a tough one. Pinging Mete!
 
Nathan,
Your right about it being a very small change but when your are dealing with bolster pins, it does not take much for one to show. I really don't have absolute proof that the pins and bolsters are the same material. I did buy them from the same source and based on an experiment I tried years ago, if you put 300 series pins in a 416 bolster, they will show. The color is slightly different and they will just not go away. Maybe some other alloy might be different though.
Randy
 
Nathan,
You did trigger a thought. Back then, I only used heat treated 416 and both the pins and the bar stock had been done. I wonder if some difference in the process for the pins and bar could have caused this cold temperature reaction?
Randy
 
Robber58 said:
Nathan,
You did trigger a thought. Back then, I only used heat treated 416 and both the pins and the bar stock had been done. I wonder if some difference in the process for the pins and bar could have caused this cold temperature reaction?
Randy
Click to expand...

Man I have no idea, but that may well be plausible.

Steels can shrink in one axis and expand in another after heat treat. I'd wager the grain direction on the pins is 90 degrees to the bolsters. Is it possible that -25 was enough to convert some old RA? That sort of thing happens some times in industry causing ejector pins to grow or warp and gauge standards to grow out of calibration etc.
 
A tiny bit of moisture under the bolsters at -25 degrees couldice and force the bolsters out.
 
Bill,
I had not thought of something like that. I have always put a small amout of epoxy under the bolsters of my knives to act as a seal to keep moisture out but I can't say it works perfectly. This really concerns me because I have an opportunity to sell some knives in Colorado and I certainly don't want them doing this. I may have to re-design a bit and get rid of the bolsters unless I can come up with some tests that allow me to isolate the problem. Thanks for the ideas guys.
Randy
 
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