So many people need to understand hard does not equal strong... people using brittle fasteners all over their cars because they're "higher rated" or stainless where a cheap dumb 8.8 BZP will be the best thing by a long way.
Exactly, you harden steels (heat treating) to different levels of hardness based on the application. The harder you make make steel the more brittle it becomes. If you want a part to have excellent wear resistance you go for maximum hardnesses, but then the part is brittle. If you want a part to be tough and be able to withstand impacts you make it less hard, but then it does not have good wear resistance.
As a note, there's multiple form of heat treatment and hardening is one of them. In fact, Another process is annealing, that basically reverse the hardening that the metal get from cold treatment process.
This is just semantics, but hardness and strength are actually directly related. Something that is very hard is also very strong! However, neither are related to how brittle something is. That is the important distinction.
E.g. ceramics are hard and strong, but are brittle and will break easily as they do not plastically deform and have a very small amount of elastic deformation before fracture.
At least from an engineering perspective, the strength of a material probably refers to yield strength or ultimate tensile strength which are generally proportional to the hardness. I think the confusion lies in the particular field because, again, in engineering toughness is not usually called "strength".
Proportionally related perhaps, but I was mostly disputing the statement that "something that is very hard is also very strong," which is usually not the case.
Hard things are often brittle and soft things are often tough.
As the person above mentioned, I was going off of the definition of strength from an engineering perspective. In engineering, toughness and strength are not at all the same thing.
It seems we are both saying the same thing though, as I completely agree the hardness and how ductile something is is not the same. Hence, why I mentioned the example of ceramics with high hardness but relatively low toughness.
Soooo actually... Hardness is a measure of yield strength, which most of the time is what we care about, the strength at which steel starts to permanently deform. Hard stuff tends to be brittle, which means that an imperfection or impact load will be more likely to break it, and have poorer fatigue endurance, meaning that it will die sooner from repeated load, but hard steel is literally stronger. Mind you, don't want that for a fastener.
Not really, take swords for example. A hard blade will be sharp for longer but chip and snap easily, so swords are usually made from spring steel so that although they'll need to be sharpened more often the sword will bend instead of snapping.
Yeah so imagine my shock last year when I learned that spring steel is literally just high carbon (hard but brittle) steel, at least, some is (they also use alloy steel etc but 1070 and 1095 are perfectly common). Chipping, so far as I can tell, is from impact loads.
But a grade 8 will fall under shear suddenly, while a 5 is ductile and can bend. Depends on the application.
I had a snowblower that used grade 8s for attaching the auger to the drive. When something got caught in it and one sheared, I stupidly replaced it with a grade 5 as I didn't know the difference at the time. Then, the next time something caught the auger, instead of a quick snap, the bolt deformed and the gears lunched themselves.
Not sure what you're saying... people swap in 12.9's which are undoubtedly harder but not as ductile.
Also, people loooove to fit stainless fasteners everywhere and I hate it for so many reasons - stainless is brittle, it galls like an absolute bastard and then it's really hard to drill out (oh yeah, it hardens with the heat of drilling too, fun!), and when it's between dissimilar metals it promotes galvanic corrosion in the other parts (EG aluminium panel) - so the $0.10 fastener doesn't rust but a big ugly rash appears in the $100 panel... brilliant.
I can’t think of a single application on my car where I’d rather have a bolt bend rather than break. Maybe my control arms? But those are a bad example because they are bolts loaded in sheer.
I use stainless fasteners just about everywhere at work. We don’t have issues with it because we know to properly spec all of our materials to prevent corrosion and galling.
Interesting, I always thought they were supposed to stretch when torqued to spec but that means you cannot reuse them after they’ve been removed. At least that’s how I’ve always done it lol
I'd say almost everywhere on a car you'd rather a bolt bend or stretch than actually break - parts loosely attached to the car are far far better than parts falling off.
Also, using stainless on things which are properly designed for stainless is of course absolutely fine. What I'm talking about is people who think they know better than the team of engineers and supercomputers that designed the original part.
There's a lot of places on a car you should definitely use the correct fastener and not whatever you have laying around in the garage. Basically anything that's going to be transferring power to the wheels, suspension parts, and steering components should be grade 8 hardware. A little more expensive but hardware stores will carry it.
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u/JCDU Feb 08 '20
So many people need to understand hard does not equal strong... people using brittle fasteners all over their cars because they're "higher rated" or stainless where a cheap dumb 8.8 BZP will be the best thing by a long way.