Quenched steel

[martin007]

The steel is subjected to a heating and subsequent cooling process to harden it.


> https://en.wikipedia.org/wiki/Tempering_(metallurgy)


Today I saw a video on YouTube in which it is said that a piece of steel (already tempered) and then machined, has to be tempered again.
Does it make any sense or maybe I misunderstood?

[Burgerman]

Yes. Before you temper steel you harden it. By heating and cooling fast. Then you remper it carefully to get the corect properties.
Machining changes this hardness or tempering by causeing stresses and localised heating where the cutting takes place.

Hardening, or tempering is an art form and very complex. A hardened screwdriver for e.g may break easily. So it is also tempered to give the corect amount of springyness, ductility or "toughness" so it doesent break etc. If you made aspring and just hardenned it, it would fail. Would work, buy be too brittle.

Its also extremely hard to machine carbon hardened and tempered steels. And the careful tempering may only affect a tiny part such as the very end 1mm of a screwdriver.

Think about how hard a hacksaw blade or screwdriver tip, or file is. Think about say the tools in my lathe. Those can all cut normal steels including stainless steel. But try machining a high carbon steel tempered lathe tool... Theres only really one sensible way and thats grinding. When tempering things the actual colour, and the hardened part can cover a small area only like the cutting edge on a chisel. The blade end of a scewdriver. So machine that away and the rest may be softer.

So to make say a cold chisel you would use a forge, a file to shape it etc. Then harden it by heating and queching. Then temper the very EDGE by cleaning so t shines, and heating to the correct colour, then cooling. To get the strength reqired at the cutting edge. And so it didnt break. You cant file or shape it once hardened. A file just slides off...

[martin007]

OK.
I understand.
The machining of steel was minimal and precision.
It sounded strange to me a tempered (hardened) second.

[Williamclark77]

It depends on the part. You typically "stress relieve" material after machining. It depends on the shape of the part and how it was machined. Long skinny parts are the most difficult and require the most stress relieving.

For example - a 16 inch long shaft that requires 10 inches of one side to be machined flat, leaving it in the shape of a D. You can almost guarantee the shaft will warp towards the area machined away because of the internal stress of the steel is no longer there to hold it straight.

Long hydraulic shafts with many features are typically turned and left oversized slightly larger than the heat treatment will penetrate. They're heat treated then "hard turned" to final size. Any common carbon steel can be turned when hardened. I've done this process many times. I recorded a video of some cutting does I did this way. I'll dig through my phone and find it and upload tomorrow.

[martin007]

Did you work in the metal industry?

[Williamclark77]

I've done some sort of designing, manufacturing, machining, CAD/CAM, etc for most of my life. I don't do it as a profession now. I still do a lot of CAD and CAM design, CNC machining, manual machining, and welding in my personal shop.

[Williamclark77]

Crap phone video recorded with my crap hands. These are dies for cutting sheet metal. They're machined slightly oversized then "hard turned" as seen here to final dimensions because they slightly warp during hardening. The width and diameter is not critical but they must be nearly perfectly flat and parallel. Hard turning also leaves them very sharp. They can also be resharpened this way several times until you break through the hard surface. These are about rc60 in hardness.

https://www.willsjunk.com/Other/BackupAll/Machining/n-9wQLhh/Images-from-folder-Machining/i-c4wkx4j/A

Finished

https://photos.smugmug.com/Other/Backup ... 326-X2.jpg

[Burgerman]

My old lathe wouldnt touch that! Quite amazing that you can still turn that hardened stuff to me!

[Williamclark77]

It's less about horsepower and more about rigidity. I did quite a few things like this on my old 1.5hp South Bend Heavy 10 lathe from the 1950s. It didn't like it though and didn't leave as good of a surface finish.

I bought this lathe five or so years ago. Precision Matthews 14x40 inch. It's only 3hp but much more rigid. Older small lathes just weren't really designed to use carbide insert tooling.

I have some video somewhere machining some 2 inch IHCP rod for a dozer track tensioner on my little South Bend lathe. IHCP = induction hardened chrome plated. It's hardened very deep and resist bending much better than regular hydraulic chrome rod. Epic pain to machine!

[martin007]

What would be better as a mechanical fuse (bolt)?

a) A bolt with a high carbon content.

b) A very tempered bolt.

[Burgerman]

It would depend on the properties this fuse was meant to prevent. Or what it was supposed to protect. Is vibration protection for e.g and consistency the important values? Such as the pins that are designed to fail on a airliner engine that lets an engine drop off in the event of failure to protect a wing structure. Or something that is ment to fail due to fatigue after a certain level of "wear" or something else like temperature changes? Like a fire protection fuse or electrical fuse?

Whatever th reasons are then the answer is complex and you need a metalergist! And physical design matters too. So I couldnt possible tell you the answer. Even if you could describe the required properties or parameters.

[martin007]

Once again everything is more complex than it seems...

Thanks.

[Burgerman]

Thats life though.

Most people do not understand enough about a subject to understand quite how much they dont know.

The "fuse pins" holding a jet engine on o an aircraft wing has to fail when vibration forces exceed a certain level. But not weaken normal daily stresses or flight vibration or loads, or fatigue over time or fail early at new lower figure. And must fail at minus 60C and +55C equally at the same load point. So who knows how they design and test those or what temper or alloys are choosen... Not me!

[martin007]

Most people lack sufficient IQ.

[Burgerman]

Failed ALMOST fuse pin.

2 engines fell off a 747 in a dutch accident years ago.