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course or fine grain?
January 5, 2013
6:32 am
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Jason brooks
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Howdy again!

The enclosed photo shows a piece of coil spring that I quenched in water. It cracked (I am not terribly surprised...). Look at the grain: is that considered "fine" or "coarse"? Can I tell by the grain how the hardening went? (I mean, aside from "it failed 'cuz it broke"?)

Just curious...

--jason

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As a beginning Blacksmith, I make scale.

January 5, 2013
3:37 pm
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Mike Blue
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Jason brooks;16965 wrote: ...is that considered "fine" or "coarse"? Can I tell by the grain how the hardening went?

That's considered a coarse grain. The larger the grains the more fracture planes you have available and the more likely a crack can propagate through the material leaving you with two pieces, or more.

At this point you can safely assume the steel got hard. Other tests like resistance to files or an indent test would be helpful.

Thermal cycling the steel before final heat treatment would correct the problem. It really helps reduce the scrap rate.

January 5, 2013
4:59 pm
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Steve H
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With respect to Blue, I would differ.
Tool and alloy steels by nature are fine grained, particularly with your rapid rate of quench. Also the alloy content is designed to limit grain growth to maximize strength or hardness. Ductile materials such as wrought iron are large grained.

I would oil quench that coil spring and temper to straw for punches or chisels. Leave the struck end annealed or normalized unless you like shrapnel.

They only remember you when you SCREW UP~!!!

January 5, 2013
5:35 pm
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Mike Blue
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Steve H;16976 wrote: With respect to Blue, I would differ.
Tool and alloy steels by nature are fine grained, particularly with your rapid rate of quench. Also the alloy content is designed to limit grain growth to maximize strength or hardness. Ductile materials such as wrought iron are large grained.

I would oil quench that coil spring and temper to straw for punches or chisels. Leave the struck end annealed or normalized unless you like shrapnel.

But we don't know what the alloy really is. There are a good many coil springs that I would readily bet are not the steel specifications found in some old book. It's too dangerous an assumption to make.

While an alloy may be intended to be fine grained, the blacksmith can screw up perfectly good materials even unintentionally with ordinary shop practices. Even vanadium or other alloying elements that are designed to pin the carbides will not work as designed if other factors are at work to defeat that intent.

We also don't know, yet, what type of fire Jason's using, specifically his temperature controls, nor how many heats he may have used at too high a temperature if the temperature was not controlled below the coarsening temperature.

If you care to look, all steels are subject to coarsening processes if given enough heat and enough time and no method is applied to reduce the grain structures after forging.

Without knowing all the variables, it's a safe enough coincidence that an oil quench and drawing back to straw will work. I do not bet on coincidence.

The picture is enough evidence that grain coarsening has occurred. I think jumping to another conclusion is wrong in this case without further information from the OP.

January 6, 2013
1:13 am
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Jason brooks
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Mike Blue;16979 wrote: But we don't know what the alloy really is.

Ding. I inherited the coil spring.

Mike Blue;16979 wrote: While an alloy may be intended to be fine grained, the blacksmith can screw up perfectly good materials even unintentionally with ordinary shop practices.

Is this what overheating, and over-working do?

Mike Blue;16979 wrote: We also don't know, yet, what type of fire Jason's using, specifically his temperature controls, nor how many heats he may have used at too high a temperature if the temperature was not controlled below the coarsening temperature.

Propane, via NCtools whisper-daddy. Temperature controls? Zip. I have a magnet on a stick... Ah: that answers my previous question...

Mike Blue;16979 wrote: ... all steels are subject to coarsening processes if given enough heat and enough time and no method is applied to reduce the grain structures after forging.

Is this what annealing does?

Mike Blue;16979 wrote: Without knowing all the variables, it's a safe enough coincidence that an oil quench and drawing back to straw will work. I do not bet on coincidence.

At this time, I have been lazy and not created an oil quench tub yet. I have no idea what I am waiting for here: but then I only really get out there about 2/14 days.

Mike Blue;16979 wrote: The picture is enough evidence that grain coarsening has occurred.

I think I might look around for a class on heat treating/tempering.

Thanks!

As a beginning Blacksmith, I make scale.

January 6, 2013
2:54 am
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Mike Blue
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Propane, via NCtools whisper-daddy. Temperature controls? Zip. I have a magnet on a stick... Ah: that answers my previous question...

It would be ideal to have some way to throttle that forge back, but if I remember the one I used, they are either on or off. They can also have hot spots so you could be just right on one end of the spring and overcooked on the other.

If you could hold your working steel just above non magnetic and forge close to that you could get by without complicated equipment, but you'll be watching and checking temperatures a lot to avoid overheating. A good venturi forge with a needle valve can be setup to hold pretty much whatever temperature the venturi will allow. That's a good way to avoid overshooting temperatures and still cook with gas.

If you're going to be serious about heat treating, a thermocouple and a good thermometer arrangement are needful tools.

Is this what annealing does?

Annealing will reduce the hardness of the steel but will not reduce the grain size enough. Thermal cycling, e.g. heat to just above non magnetic then a. quench in oil until the steel has just gone black, not cold, then back in the fire, or b. allow to cool to black in air, preferably a still air box = no draft blowing on the bar; and repeat times three. Some folks call this triple quenching or triple normalizing. There is no magic to three times either, it's merely the point of diminishing returns. Once you get the grain reduced as far as your tools will let you, it will grow and reduce around that minimum point. More cycles will not make it any smaller. It gets confusing unless you understand what's happening inside the bar of steel.

There is some allowable room for discussion of mechanical grain reduction, ala, packing the grains, because it does reduce grain size, but it is fraught with potential for not reducing the grain structure evenly or throughly. Before some of this grain size stuff was well understood, it's all there was.

Make some sample tiles and run them hot and for a long time. Then try all three methods of grain reduction, soft quench, normalize and hammer, then do a final heat and quench for hardness and break the pieces and observe the changes with each method and with each second and third repeat. Pay special attention to the amount of effort it takes to break 1, 2, 3. I'd run that experiment for a bunch of new guys in a heat treatment class. But it's something you can do in your own shop too.

January 6, 2013
3:16 am
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Lee Cordochorea
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Mike Blue is spot on.

Each time we heat up the steel to austenite, new grains form at the boundaries of the old ones. If we do this repeatedly, without allowing time for the newer grains to grow, we end up having replaced all the old big ones with lots of tiny ones.

By the way, you've read the Dave Smucker article, have you not? http://blacksmith.org/forums/t.....ve-Smucker

No matter where you go... there you are.

January 8, 2013
4:44 am
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Lynn Gledhill
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Have you ever tried annealling the metal before hardening??? Simple, get some vermiculite from the garden shop or big box store, heat your metal to just non-magnetic and stick it in the vermiculite til it's cold...

My experience with spring steel is this: You can quench the tip end of a tool in water, as long as you draw back the temper... Use the colors of the oxide that develops to tell you what temperature to temper.... Blue color would be good for a hot tool..

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