Define the term "work hardening"

Sea Farmer · Jan 28, 2015

I've always understood "work hardening" to mean a material that hardens through either heat or pressure. But a couple of recent threads have used the term in connection with O1 tool steel and stated that it does not work harden.

My experience with O1 in the annealed state is that a bit of friction heat generated by filing hardens it slightly. Same with the heat generated by a bend taken too fast, or grinding. Not the optimal hardness of a proper heat treat, but hard enough to cause some cracking in subsequent operations on the bit that was heated.

Is this "work hardening" or something else?

So is there a formal definition of the phrase?

johnoder · Jan 28, 2015

Easily demonstrated to oneself

Get a piece of soft wire, steel or copper would do

Bend it repeatedly in one place - back and forth

When it breaks, you have passed the point where it work hardened

The bending WORKED the metal

Want to bend less? Start with harder wire - a coat hanger for example, which has already been WORKED some

L Vanice · Jan 28, 2015

Work hardening is sometimes called cold working just to emphasize that heat is not a factor. The grain structure visibly changes, and can be seen on prepared specimens under microscopes. They do that sort of demonstration in engineering school, so I have seen it.

Some metals do not harden when cold worked. Pure lead and pure gold are examples. Gold leaf is an example of a product that depends upon the fact that gold does not get hard when rolled or beaten while cold. A sheet of pure gold leaf will crumple and blow away if you breath on it; it is still soft and incredibly fragile.

Other metals get harder when cold worked. Think about why some steel products are called cold rolled and others are called hot rolled. Brass can be purchased in states called soft annealed, half hard and hard. The harder states of brass are produced by rolling or extruding annealed brass. The amount of dimensional change in the last rolling operation after annealing controls the hardness. The hard or full hard state has been cold worked almost to the point that further cold work will cause it to fracture. Brass cannot be hardened by heat treating: it can only be softened (annealed) by heating and the speed of cooling does not affect final hardness.

Larry

DMF_TomB · Jan 28, 2015

work hardening
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metal stretched, compressed, pounded, pushed to its yield point (bent or deformed permanently) at a lower temperature generally below a red heat or 1400 degree F......... it in general increases tensile strength and hardness of metal and also lowers ductility (ability to further deform without cracking).
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for example pounding on soft copper it gets harder and eventually will crack if you continue to pound on it. by annealing or heating to a red heat or 1400 degree F it will soften, relax the metal to its lower tensile strength , softer condition and higher ductility state
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304 stainless steel that is cold rolled or work hardened can be 2x higher tensile strength and 2x harder to machine. Stainless require a more precise temperature and time heat treat to restore or anneal it without damaging its corrosion resistance
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some metal alloys work harden far more than others. some that are considered do not work harden might work harden a slight amount and so some might say it does not work harden compared to other metal alloys which work harden considerably more.

gwilson · Jan 28, 2015

I've had O1 harden while filing. The file suddenly screeches across it,grooving the file. Only hardens a few thou deep,but it can mess up your file when it happens.

I find it hard to believe that hammering O1 would not work harden it. But,I have not actually tried that.

magneticanomaly · Jan 28, 2015

Metal cutting is actually extruding on the microscopic scale, so always leaves a more-or-less-thin work-hardened layer. That is why climb milling and thick chips (high feed-per-tooth) are recommended for metals more prone to work-hardening, like the 300-series stainlesses.

Can't easily control feed-per-tooth when filing, so as the bevel gets wider, more teeth are in the cut, and the feed pressure-per-tooth declines, you would get into having to cut that work-hardened layer.

TDegenhart · Jan 28, 2015

Read these references. You will find more than you really want to know about work hardening.

Work hardening - Wikipedia, the free encyclopedia

Tom

Onepass · Jan 28, 2015

The older I get, the harder it is to work

rds6709 · Jan 28, 2015

When machinists talk about work hardening its usually the material hardening ahead of the cutter

from heat. It makes it difficult to keep tools sharp or shorten tool life when this happens. 316

stainless is very bad to do this and your best option is to keep feed rate up and rpms down to avoid

rubbing the material creating a hardness where you're trying to cut. The materials that work harden I try to avoid

being a pain to run and go through a lot of tooling if you're not careful. RD

DMF_TomB · Jan 28, 2015

gwilson said:
I've had O1 harden while filing. The file suddenly screeches across it,grooving the file. Only hardens a few thou deep,but it can mess up your file when it happens.

I find it hard to believe that hammering O1 would not work harden it. But,I have not actually tried that.

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i have machined quite a bit of different tool steels. they are often not especially hard but........ many can be very abrasive. hard spots in metal when grain of sand size are bad enough.
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bigger slag inclusions the size of a grain of rice i have seen destroy 40 carbide inserts. the effect is like trying to machine a grinding wheel.
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end mill life machining some tool steels will often average 20 minutes compared to 60 to 120 minute for 1018 steel.
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i would guess O1 problems it might not be strictly work hardening but more abrasive hard spots in the metal

jdj · Jan 28, 2015

rds6709 said:
When machinists talk about work hardening its usually the material hardening ahead of the cutter

from heat. It makes it difficult to keep tools sharp or shorten tool life when this happens. 316

stainless is very bad to do this and your best option is to keep feed rate up and rpms down to avoid

rubbing the material creating a hardness where you're trying to cut. RD

Nothing to do with heat. 316 ss cannot be hardened (any more than a point or two) by heat treatment. Friction and pressure (rubbing, that you mention! YES!) are what causes work hardening. High nickel SS and super alloys are the fucking kings of work hardening. O1? Could someone enlighten us as to WHAT element in O1 causes the alleged work hardening? It doesn't have THAT much nickel in it, does it?!?!?

Limy Sami · Jan 28, 2015

Define the term "work hardening"

Shit! another tip gone!

adama · Jan 28, 2015

Stainless, try drilling a hole through the head of a forged bolt down the central axis, if you hold the head and drill from the thread end you will know when you hit - get into the forged head area, trust me!!!

Rickyb · Jan 28, 2015

thermite said:
Generally applied to any forming operation rather than a heating operation.

Impact hardening by hammering was essential with copper tools before bronze was discovered, remained essential to hardening bronze at the edges of, for example, a sword, whilst the rest of the blade was held less brittle.

Also occurs from rolling - Gold springs are made by cold-working - or from rubbing pressure. High Manganese "Hadfield" alloy long used to self-harden a wear-resistant skin on railway switches, digging machinery blade tips.

Also has to be dealt with when turning, drilling, tapping, milling any 'work hardening' alloy of any race or tribe as the cutter creates problems for itself at any time it rubs or fails to shear cleanly. Have to be a seriously BAD hand-file to do that on bronze, not likely atall on O1. You may not HAVE O1?

When spinning, goods often require several stops for annealing, lest they become so brittle they crack.

Ditto bending of even initially very soft Aluminium or Copper. Brass can be far worse than either. Can surface-crack, then through-crack at the point(s) of bend. We rely on that to get the last grasp of lids off tin cans, even shorten fence wire when we have no suitable tools.

You've actually seen plenty of 'work hardening' already before that O1 walked in the door. Heat, if detectable, is a byproduct, not the cause. Most of it classes as 'cold' working.

A "formal" explanation and what CAUSES it at the molecular level will be in metallurgy texts. Those you can find online with newer dates than mine - most from the 1920's.

Bill

I'd like to take all of you a bit deeper in just one area of work hardening. A number of steels, especially those used in digging machinery tips have a high percentage of retained austenite in the metallic crystalline structure. As these steels with retained austenite are worked the austenite in the surface layer (which is naturally unstable) instantly converts to untempered martensite. This makes the surface layer file hard but does not affect the softer core material which was not rubbed. Now you have an extremely hard wear resistant surface layer able to withstand the scraping effect of rock. As the super hard layer wears away, a new one forms. Had the same tooth been through hardened to file hard at the start, it would be very brittle and break off almost immediately.

rds6709 · Jan 28, 2015

jdj said:
Nothing to do with heat. 316 ss cannot be hardened (any more than a point or two) by heat treatment. Friction and pressure (rubbing, that you mention! YES!) are what causes work hardening. High nickel SS and super alloys are the fucking kings of work hardening. O1? Could someone enlighten us as to WHAT element in O1 causes the alleged work hardening? It doesn't have THAT much nickel in it, does it?!?!?

I'm very interested in this subject so if heat doesn't work harden the material why does my high pressure help. I gave up

on a 316 ss job when I first starting running swiss that was turned and bored leaving only a thin shell out of .750 because

It was costing more in tools than the job payed. Later I added high pressure and I tolerate some of those jobs but

not what i enjoy doing. Running some of the tool steels ,316ss and other can be a real pain. So is friction the only culprit. RD

jdj · Jan 28, 2015

rds6709 said:
I'm very interested in this subject so if heat doesn't work harden the material why does my high pressure help. I gave up

on a 316 ss job when I first starting running swiss that was turned and bored leaving only a thin shell out of .750 because

It was costing more in tools than the job payed. Later I added high pressure and I tolerate some of those jobs but

not what i enjoy doing. Running some of the tool steels ,316ss and other can be a real pain. So is friction the only culprit. RD

Too much nickel, not enough carbon and not enough iron in 300 series SS to harden by heat. Friction AND PRESSURE, are the culprits. I machine a lot of 316 ss. The wear that I get on tools, really appears of the abrasive nature. Positive rake tools help to avoid work hardening. Less pressure and more clean shearing action, lessen the stress on the metal. I can get finishers to last well when turning 316, but roughers never last very long...and that is after trying almost every grade and brand of carbide on the planet. I can put a fire hose of coolant on 316 SS, but if the tools aren't sharp and the feed is too slow, it will still harden it.

alonzo83 · Jan 28, 2015

rds6709 said:
I'm very interested in this subject so if heat doesn't work harden the material why does my high pressure help. I gave up

. RD

What is high pressure? hammering?

You may observe heat in the action of pounding, bending twisting, pressing, rubbing all of them are pushing the molecules around which produces heat which is where all the energy goes to when applying a force.

Molecular friction a biproduct of using energy isn't what is getting the job done. .

tdmidget · Jan 28, 2015

Heat is more the result of work hardening than the cause.

gbent · Jan 28, 2015

Rickyb said:
I'd like to take all of you a bit deeper in just one area of work hardening. A number of steels, especially those used in digging machinery tips have a high percentage of retained austenite in the metallic crystalline structure. As these steels with retained austenite are worked the austenite in the surface layer (which is naturally unstable) instantly converts to untempered martensite. This makes the surface layer file hard but does not affect the softer core material which was not rubbed. Now you have an extremely hard wear resistant surface layer able to withstand the scraping effect of rock. As the super hard layer wears away, a new one forms. Had the same tooth been through hardened to file hard at the start, it would be very brittle and break off almost immediately.

Ricky, what steels are used? What heat treatment process is used to keep the retained austenite?

rds6709 · Jan 29, 2015

alonzo83 said:
What is high pressure? hammering?

You may observe heat in the action of pounding, bending twisting, pressing, rubbing all of them are pushing the molecules around which produces heat which is where all the energy goes to when applying a force.

Molecular friction a biproduct of using energy isn't what is getting the job done. .

high pressure cutting oil right at the cutter gets rid of a lot of heat helps keeps shaving cleared. RD

Define the term "work hardening"

Diamond

Diamond

Diamond

Diamond

Diamond

Titanium

Diamond

Hot Rolled

Aluminum

Diamond

Diamond

Diamond

Diamond

Cast Iron

Aluminum

Diamond

Stainless

Diamond

Diamond

Aluminum

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