Why Does Work Hardening Occur??

R

rlinger

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Mar 29, 2002
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I was grinding a tang taper yesterday and thought boy this is going real slow. I wondered did I work harden this darn thing? Must have been me because I checked it against a sample piece from the same stock and it was the same. But it got me wondering again about what actually occurs in the steel composition to cause steel to become hardened by simple, for example, hand working it.

Thanks. RL
 
It's 6am so i'll give you a fast answer. Work hardening creates disruptions in the crystal structure. Some metals work harden extremely rapidly such as 301 stainless steel and monel metal. This has been used for sharpening blades for thousands of years. For example a scythe blade is sharpened either witha stone or they used a small hammer and anvil to work harden the edge.
 
Mete,
I have a question that is off topic, but I have been wondering what effect alloying of steel with carbide forming metals, eg. Cr, V, W and Mo, have on the .85% carbon eutectic? How is the Carbon partitioned out between non-Fe carbides and Martensite C sites?
I know that in some mineral systems phase stability is extended by impurities so that normally exclusive phases can coexist.
 
Shgeo, thats a complicated question which I would have to research to answer .Instead I'll just say that the iron-carbon phase diagram is just that Fe-C.The multiple alloying elements and the dynamics in heat treating have an effect on all the parameters. I normally say the eutectoid ( not eutectic !) is ABOUT .85% and for practical purposes that's all that's necessary. The second question is equally complex. Some elements are strong carbide formers and some are not so strong.Alloying elements also go into the matrix.The choice of alloying elements involves ; is it a strong or weak carbide former ,the grindability of the carbide,does it add toughness or other properties to the matrix. To confuse you further - carbide in the iron- carbon system is what we call metastable, the true stable or equilibrium form is graphite.
 
I was told by a silversmith that work hardening is the result of the hammer blows creating some degree of molecular or crystal order in the metal as it is pushed around. Whatever, the results are dramatic, a soft piece of annealed silver moves easily under the first couple of hammer blows and then begins to resist movement until no amount of hammering will cause any more movement until the metal is re-annealed.
 
My brother is working on his masters in materials science engineering, and from some of the stuff he's told me I kind of have an idea how some of it works....maybe (please correct me if you know for sure, I know I will butcher this ;) )

In the grain structure of the steel there are things called slip planes (IIRC) and grain boundaries around the slip planes. In any given plane the particles can move a certain amount. If you increase the number of boundaries you limit the amount the material can move, which would seem to make it harder . By working with the steel, you disrupt/change the boundaries, and that affects the planes which hardens the material.
This would seem to explain why hammering on the material can work harden it. When you forge you have to keep it hot so that the particles are already moving quickly on their own, and you can easily displace them.

As far as work hardening on the grinder/drill press/mill etc. I would assume that has more to do the with heating and cooling having a similar effect to heat treating, but to a much lesser extent.
 
So far as this ignorant brain can tell it seems all responces agree in foundation. The silver is very interesting to me. I presume the silver referred to contains no carbon to speak of.

I wonder about another ignorant question: Would work hardening show up on a common Rockwell tester or is it too shallow to read??

Thanks for the responces. I hope there will be more in further detail.

RL
 
Matt, you need more lessons from your brother. Work hardening when drilling has nothing to do with heating and cooling .In fact when the piece and drill get hot it's because the metal has already been hardened and it's no longer cutting. When cutting something like 301 SS you better have adrill sharpened exactly right and use exactly the right speed and feed. Roger, work hardening has nothing to do with carbon or heat .All the metals can work harden. The coppersmiths, silversmiths etc all go thorugh the same process of hammering and annealing .Depending on the details the metal can be worked right through. When you buy brass for example you specify full hard, half hard , quarter hard or annealed . It will be the same hardness all the way through. BTW we used to make springs by just cold rolling 301SS. As I mentioned a scythe can be sharpened with hammer and anvil( www.scythesupply.com) and of course in the bronze age the bronze swords were sharpened the same way since the bronze could not be hardened by heat treating.
 
Great. Now paint me a picture of what happens inside. I have been told about matrices and crystal structures. Are they compacting?? What??

Thanks and I know I must be getting hard on y'alls patience.

RL
 
The silver referred to is sterling silver with no carbon at all, same thing happens with pure copper,and brass.

The slip plane theory sounds valid because the hammer blows or vibration from drilling would disrupt slip planes and cause them to lock up thus the apparent hardening. There is no doubt that carbon bearing materials like blade materials can and do harden if they get hot during drilling but materials like 304 and monel do not contain carbon in more than trace amounts, not enough to harden them if you tried, but they do work harden, more so if the drill bit is chattering than cutting smoothly.
 
Crystal lattices are regular repetitive structures with regular bond lengths between atoms of elements. Sometimes there are vacancies or impurities in the structure that interrupt it. These are called defects in the structure. They are generally what we call point defects in the lattice. Bonds can normally from around these sites for instance, leaving the point defect in place or the lattice can be deflected to form what are called line or screw defects, both of which interrupt the regular lattice in greater ways than just the points. Bonds can also be broken when stress is applied. Annealing occurs when the lattice is heated and held until the bonds reform to optimum.

When non-receverable strain builds up due to bonds broken by stress applied to the lattice, work hardening occurs. Hence the scythe sharpening/hardening and silver/bronze hardening

Quench hardening occurs in materials like steel that change lattices during heating (Ferrite/Austenite/Martensite) are suddenly cooled, changing the lattice and trapping material in sites that strain the lattice. The Carbon sites in Austenite distort the lattice in the iron forming the more rigid Martensite.

I am no metallurgist, but all materials that are crystalline behave in predictable ways due to the nature of their structure and with enough information, we can understand what happens in a given instance. The more complex the system, the harder it may be to predict or describe exactly what will happen in a particular case.
 
I like this question, mostly because it is what I am boning up on right now for a lecture at the Ashokan seminar next week.

Work or strain hardening, as has already been said, is the result of plastic deformation due to slip. Slip is s shearing action that occurs across planes in the atomic lattices of metals. This shearing action can only occur along planes of the closest atomic packing.

Example: take a whole bunch of ping pong balls and glue them to 2 boards on a grid of 1" spacing. Now do the same with two boards at 2" spacing. Put the two 2" spaced, together ping-pong balls to ping pong balls and tilt the stack. You will have to go a very steep angle in order for the top board to slide. Repeat with the 1" spaced boards and you will not have to go near as far in your tilting to get "slip".

The tightest ping-pong ball atomic lattice was more susceptible to slip. Now we are going to get techy- when you heat steel to Ac1 (what SOME may call critical temp) the iron atoms will shift from a body centered cubic (bcc) configuration to a face centered cubic (fcc) configuration. bcc is less tightly packed than fcc which is close packed, this is why steel moves so much easier at proper forging temperature.

Now slip will most often occur at certain angles within these configurations, imagine this angle like 45 degrees, 0 degrees won't slip and 90 degrees won't slip.

Example: take a deck of cards, an object consisting of many planes upon which slip can occur and the deck will deform. Lay the cards flat and push straight down and nothing moves, set the deck on edge and push straight down and nothing moves, now set the deck at a 45 degree and push straight down and the deck easily changes shape.

Now the 45-degree thing is represented in tech talk by indices (Miller indices) relating to a 3 dimensional grid and then number that corresponds with the 45 in fcc is (1,1,1). In bcc metals, like iron at room temp, the (1,1,1) plane is not available due to the way the atoms are stacked so it is much harder to get things to move.

Let's complicate things even more, you may regret asking this question;). The theoretical force required for slip is much, much higher than what reality presents us with- does the math lie? No. Here is where the aforementioned "line" and "screw" defects come into play, mostly just the line one though. The actual term for these are "edge dislocations" in the atomic stacking and they are incomplete or half rows in the stack that creates distortions in the lattice. What this does is creates and easier avenue for slip. If you have a huge area rug that you wish to move across a rough floor trying to slide the whole thing will take a lot of force (the theoretical numbers), but if you create a wrinkle in the rug the whole thing can be moved just by pushing that wrinkle down the line, this is how edge dislocations make slip easier. Easier until....

Now we get to the heart of your question. When these edge dislocations meet an obstacle like a grain boundary, a serious lattice defect or a large substitional atom (like a big old chromium in the middle of all those irons) they start to pile up and you get a serious dislocation traffic jam. To add to this, the process of cold working continually creates more dislocations. Now as soon as you heat the steal above Ac1 all will be erased and the whole thing can start over on a clean slate, that is annealing)

Very simple explanation: Imagine cold working like a huge cube in your back yard that is made up of hundreds of smaller blocks. Pick a row of blocks, any row, in that cube and give it a push. They slide and misalign to the rest of the cube. Now keep pushing from different directions on different rows and eventually the half dislocations will start to multiply and interfere with each other. Soon it will get very difficult to find a row that is not tangled with another and if you keep pushing the block may come tumbling down!:eek: This would be fracture in a metal that was pushed too far. If you were to reheat the metal it would be the same as setting all of the blocks in your cube back to the same straight and ordered stack you started with.

With extreme cold working the grains, that are not aligned to a plane that can slip, will deform in a process of deformation known as twinning (which I won’t get into here) or until they rotate or align themselves in a direction that slip can occur. This will eventually result in most of the grains being elongated in the direction of working resulting in different properties in that direction than a direction perpendicular to it. That is, it will be less ductile and have higher strength along its length but if you were to cross section it and, check it the other way (perpendicular) you would find the opposite to be true. It cannot be stresses enough that any heating approaching Ac1 will completely erase all of these effects.
 
Kevin, you are strange, first you compare mozzarella to wrought iron now you talk about ping pong balls in steel. LOL LOL I'm holding back on responding here to try to find a book with a good description. But I would like to know about your seminar , is that Ashokan NY ?
 
Not that strange, how many times have ping pong balls been used in atomic models over the years? But the strange label has been put on me more thatn once in my life time, most get a chuckle out of my schools of fish analogy for carbon/iron systems;). Yes I will be at Ashokan NY.
 
Originally posted by mete
... I'm holding back on responding here to try to find a book with a good description. ..
Click to expand...

Try "introduction to Physical Metallurgy" by Avner it has some nice models of slip systems as well as an intersting diagram of a catapiller crawling to demostrate slip with edge dislocations.
"Plasticity of Metals" by Kurrein is all right but is geered more for engineering and heavy industrial applications.
 
This is getting complex...in silversmithing class the credo was "if in doubt anneal it." How much good does annealing do for 304 stainless?
 
Of the typical austenitic stainless steels the one most quickly work hardened is 301 and the least quickly work hardened is 305. But whatever the metal the rules are the same ; work harden , anneal, work harden, anneal. For Kevin, if you ever want to demonstrate twinning - take a 1/4" dia rod of pure zinc, hold it near your ear and bend it . You will hear the deformation twins forming since they form at the speed of sound.
 
I have also heard that tin sheet will cry when bent and twinnning occurs, but I have little personal experience becuase tin, these days, is right upt there with silver or gold. I wonder how many close packed hexagonal metals sing like this? And do they sing the same tones or note? Some artistic whack job should record a whole series of such metals while deforming and then mix it all together to a beat and create the "twinning sonata" or the "twinning overture" would probably be a hit with material science dept. all across the country ;)
 
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