What happen to this blade causing it to fail?

SkagSig40

SkagSig40

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Feb 1, 2001
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Not a knife blade but I'm curious what might be going on with this bolt cutting blade. This set of bolt cutters is made by H.K. Porter and is used daily commercially to cut 25-100 bolts off of containers at a dock per day. They were several years old when they failed. I noticed the inside of the blades grain looks uneven. Looks very "tight and small" at the cutting side and seems to get larger towards the back. Just wondering if the steel experts might be able to tell what is going on here? Enjoy!


 
Forged blade like that is probably only edge quenched, or induction hardened or some other method that only partially hardened it. Likely over years of use a small stress crack started in the hardened portion and simply propagated unnoticed as a weak point until it was one bolt too many.
 
I've cut a few container bolts. Probably seen some twisting, some prying, and been dropped and banged into some of the cans.
 
I agree, that cutting edge hardened higher than the rest of the jaw, cracked, then propagated. Whatever, you got your moneys worth cutting thousands in its lifetime.
HKPs are about best there is. New jaws are reasonable priced too.
 
Thanks guys! So the larger looking grain behind the edge is softer? How exactly are blades like this forged? A big machine pound them out? I would think they heat treat and temper hundreds of blades at the same time. I would guess because of this the blade would be one hardness. I saw an episode of "How it's made" and they wee making saw blades and it looked like they were heating hundreds of them on a conveyor belt and then they dropped them into a quench before tempering in a oven.
Any idea what steel these bolt cutter blades are made of?
 
It is called metal fatigue. It starts when a stress is applied. The leverage of the cutters is enough to apply stress. Twisting adds more stress. As the cutters get dulled, the amount of stress greatly increases as there is more force applied to do the same cut. Little stress cracks start forming ( the lines up from the side) as the grain boundaries start to fail. Then one crack starts moving down the grain boundaries and a fatal crack forms. All the stress will now run down this path and break the blade from side to side.
 
So are the stress cracks those "feather" looking spots on the left-hand side of the bottom picture? Any guesses on what steel this might be made out of? I have a few of the old blades and was thinking it would make a great knife. The stuff obviously a super durable.
 
Yes, the "feathers" are where the stress cracks started. Because they are pulling away from one side only, I suspect the bolt cutters were being twisted or laterally pulled when they broke.

No telling what the steel is. It may not even be a steel that would forge into a blade well. You could try contacting the maker and ask them what steel they use. Unless there is some great sentimental attachment to the cutters, buying a new bar of 1095, W2, or whatever steel you like would be far wiser.
 
As far as hardening, I think the cutter edge is harder than the body. You can see it in the different texture where its broken.
In manufacturing that sort of differential hardening is common. Probably done by induction but maybe a fast torch passthru.
 
That may have started out as a crack on the leading edge , developing into a fatigue failure [smooth area] then final failure .A better photo might help with the answer.
Steel type could be various types depending on what's being cut.
Jaws of Life for autos had some specific specs but then , when Boron Steels started to be used in some car parts things changed. Boron steels became used to help meet crashability tests. That required redesign of the jaws -- bigger, stronger and a better steel for the cutting edge !!
 
Question for those of you with much more experience than I, Can you read what the metal fatigue you are reading from looking at the steel? Would you please read it to us dummies?
 
golfer1 said:
Question for those of you with much more experience than I, Can you read what the metal fatigue you are reading from looking at the steel? Would you please read it to us dummies?
Click to expand...
You sure can but you're better to inspect in person and often with some sort of magnifier.

This is a picture of an alloy part but the failure mechanisms are very much the same.

https://en.wikipedia.org/wiki/Fatigue_(material)#/media/File:Pedalarm_Bruch.jpg
Fatigue_(material)

Fatigue_(material)
 
I always carry a 10X magnifier and a flashlight with me ! Lots of techniques especially when you are trying to photograph !
Danke's photo , on the left side , shows a secondary fatigue failure !
Failures often involve more than one cause . Combined fatigue + corrosion etc. Often we wouldn't get all the details. One crane hook showed the cause under a microscope .A microstructure showing VERY high temperatures -- either hit by lightning or a high power line !! Failure analysis is a Sherlock Holmes thing .You have to think and often out of the box !
 
mete said:
That may have started out as a crack on the leading edge , developing into a fatigue failure [smooth area] then final failure .A better photo might help with the answer.
Steel type could be various types depending on what's being cut.
Jaws of Life for autos had some specific specs but then , when Boron Steels started to be used in some car parts things changed. Boron steels became used to help meet crashability tests. That required redesign of the jaws -- bigger, stronger and a better steel for the cutting edge !!
Click to expand...

Even with the newer technology and tools out, we still stay away from pinching the nader bolt with the tips of our cutters. I've seen cutter jaws fail and break from pinching the bolt with the tips.
 
mete said:
I always carry a 10X magnifier and a flashlight with me ! Lots of techniques especially when you are trying to photograph !
Danke's photo , on the left side , shows a secondary fatigue failure !
Failures often involve more than one cause . Combined fatigue + corrosion etc. Often we wouldn't get all the details. One crane hook showed the cause under a microscope .A microstructure showing VERY high temperatures -- either hit by lightning or a high power line !! Failure analysis is a Sherlock Holmes thing .You have to think and often out of the box !
Click to expand...

Do you use a magnifying glass or a loupe? I'm thinking of getting one of the jeweler sort of visor/magnifier/light combos for the workbench. Right now I have a magnifing glass and lighting I can direct. For really small stuff I take a macro photo and blow it way up on screen. Definitely a lot of thinking in a quiet room too.
 
I have the original one from my school days , one in my 'shop' one at my desk, one in my truck !! A little thing less than 1"x1"x1", 10X. Don't get the cheapest ones , they have poor lenses !
The truck one is mostly for ticks .Others are for splinters ,various parts gunsmithing stuff. I also have 'glasses' which are 2X. These are best limited to 2X , higher has too little depth of field.
There are some newer things that work from the USB port of your computer. Have any of you tried them , are they worth getting ?
Before I retired I always had good equipment in the lab especially for photography.
But I had to be prepared for anything out in the field !
 
I poached a set of my wife's reading glasses that are around about 2x that work well and then a 4x glass. I have an endoscope that I can plug into my laptop but 99.9% of the stuff I look at is in simple plain view.

If I had too look inside more stuff the endoscope would be a lifesaver.
 
" .... There are some newer things that work from the USB port of your computer. Have any of you tried them , are they worth getting ?....."

I have three different ones. Even though they are good, I use a 10X corrected loupe 99% of the time.

The higher power ones are hard to use and I wouldn't recommend them. They work OK at the lower powers, but are not very clear at 100X and up. They need to be rigidly mounted on a heavy stand to keep down any vibrations.

The lower power (20X max, IIRC) is pretty good. You can do macro photograph with it. It isn't high resolution like a 12M camera, but is more than clear enough for things like this discussion. If it doesn't come with one, get the stand and focusing attachment. These are also fairly low price.

The best one is the eyepiece camera lens for a microscope. Just switch it out with a lens and plug it in to the computer or any monitor device. You then focus the microscope as normal, play with the lighting, etc., and then you can examine the larger image on the screen with none of the problems of using a scope. This is especially good for those who wear glasses. The ability to save these images easily is also a great plus. From there you can crop, vignette, label, etc. the images with any photo editing program.
As in most good optical devices, you get what you pay for. The low price ones are much poorer than the medium price units. There are professional grade ones that rival a high grade Zeiss lens. Another plus is that the lens camera can be placed in most any optical device ( telescope, etc.) and used to view and store the image. These type cameras have replaced squinting through a telescope lens for many people.

Aside -
I remember when the only telescope lens cameras were CCD devices that needed to be cooled with cryogenic liquids and took a long time to form an image on the computer. Most of the time, you had to build them yourself. They were horribly expensive, on the scale of getting a new power hammer for your shop.




"Anyone else have a guess what steel these might be made of? Just curious...."
Carbon steel .... that's as close as I can guess.
 
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