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Busted slitter


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I was at the forge today trying to slit a piece of axleshaft, when my homemade slitter broke clean in half. The slitter was made from a piece of 5/8" coil spring. The flat taper was about 3" long, and broke 1" before the transition to round. I don't understand why it snapped. I wasn't wailing on it, and it was straight vertical to the workpiece.

Maybe my heat treat wasn't correct? I used veggie oil to quench, and tempered it to a straw-ish color. Upon closer examination, I think I see vertical cracks on the cylindrical portion

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Could be all sorts of reasons, do you have a picture showing the broken pieces fractured ends ?

Have you used this before on workpieces succesfully?

Most likely causes

Spring fatigued when in original use and material not in good condition to make tooling from

Worked at the wrong temperature when forging to shape

Incorrect heat treating/ depth of insertion into cooling medium giving a fracture line

If you are a novice at this, the tendency is to leave the slitter in contact with the workpiece too long resulting in negating your HT proceedure, then cooling it resulting in making it brittle and fracturing.

Just some stabs in the dark, others will also give opinions

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I've used it several times before, and use the 'two whack' method between dipping the tip in water to cool. The steel came from a 69 bronco, so it is possible that it was stressed. I am thinking the HT was not the greatest. IIRC I may have only treated the working end.

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Here's my 2 cents worth. Hope it helps. It's so far up the piece, away from the hardening and tempering, I'd guess it wasn't normalized to take the hardness and stress out of all of the tool before heat treating it. If it was normalized and you hardened it that far up with out swishing it around to wash the air bubbles off that can create a break line, too.Then again, how cold was it in your shop in Alaska? A frozen tool isn't good either. I almost always quench coil spring tools in water and never have had a problem. Hey, it's an opportunity to make another tool!

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Its great to hear from a fellow Alaskan. I think the more experienced members have given you good advice, I don't have anything to add. But, I will be makeing some of these tools soon. Its good to have this information. So, to review for my own benifit. First normalize the steel. Then forge to shape. Lastly heat treat. Now here is where I see alot of different ideas. What color to draw the colors to? Some say straw. Some say blue. And some even say use as forged and no heat treat is needed. For me at least it can be confusing. Thank you everyone for a very informative topic.

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Please read the "That does it, I don't like junkyard steel" thread from a couple days ago. Almost identical circumstances, and my comments there apply here as well. Your grain doesn't look ideal, and I would guess you undertempered. Straw is likely to be quite hard, especially if you got that color by letting the colors run from an unquenched part of the tool (which makes for a very short tempering cycle). I'd suggest you add at least one more normalizing cycle, and temper significantly hotter - particularly if you don't have the time or inclination to do multiple oven tempers.

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Normalize AFTER forging, not before. Another thought of mine was the possibility of leaving a cold shut in a square cross section, sometimes all it takes is tiny ding. Stresses will be conducted to any flaw in shape. For instance a moderately heavy person can carefully stand on an empty aluminum can without collapsing it but all you need to do is touch the side with a sharp pencil and it's pancaked.

In effect there could've been a ding in a corner or as appears a line across one face as you'd make to define a shoulder fefore forging it. Either can easily concentrate stresses and cause premature fatigue and failure.

Straw is also harder than I temper hot chisels, you're going to be cutting HOT steel, you can get away with mild steel on a temporary basis so why even try for perfection. The heat in the cut is going to run the temper out in short order anyway, the edge on your's is black.

I don't recall, were you at the last meeting at Kevin's? Are you going to make the next one at Pat's? Mark is an old hand at heat treating tools.

Frosty The Lucky.

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i

Matt, it does seem to be the same problem that I posted. However, the verdict there was that the water quench I used was the problem, and here it was quenched in oil, which shouldn't cause cracks. I would say the spring fatigue is a good guess. ~Mitch


Water quenching wasn't your only problem, and I'm not certain it was the cause of your break, although it might have been. But your grain was a bit large -- which causes brittleness -- and I still think you undertempered. Excessive hardness will indeed cause a struck tool to crack.

Fatigue is one possible problem here. But again the grain looks larger than ideal, and the fact that this piece was oil quenched doesn't mean it wasn't too hard. If it's an oil hardening steel -- which I'm guessing -- it probably through hardened. And again it was tempered to straw (although we don't know how it was tempered).

I guess what I'm saying is that I don't blame bad steel unless I'm positive I had the heat treat dialed in to start with. Every broken tool provides an opportunity to learn something about how well it was heat treated. If nothing else, I always look at the grain. If it isn't satiny to the point of individual grains being almost invisible, I know that I could have done better.
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Its great to hear from a fellow Alaskan. I think the more experienced members have given you good advice, I don't have anything to add. But, I will be makeing some of these tools soon. Its good to have this information. So, to review for my own benifit. First normalize the steel. Then forge to shape. Lastly heat treat. Now here is where I see alot of different ideas. What color to draw the colors to? Some say straw. Some say blue. And some even say use as forged and no heat treat is needed. For me at least it can be confusing. Thank you everyone for a very informative topic.


For a hot-work tool the temper will end up fully drawn. This is coil spring, believed to be 5160, fully hardened then drawn fully (about 1000F, dull red) the hardness when cold is similar to normalized. These tools need to be kept cool because the metal will start to soften at about 700F (grey). This is also true for 4140 and 1045.

Normalizing before working is not needed because a trip above 1450F causes recrystallization anyways, so heat and work it.

Phil
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I'm pretty new to grain structures and stuff. I have no idea what a good grain vs. a bad one would look like. This busted piece looks like a solid powder (kinda).

Frosty, the last meet-up I was at was a good while ago. Out at Pats place I think (near fairgrounds?)

Looking at the piece, it looks like there was a small (pinhead size) portion that blew out as well. Relevant? I don't know.

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One way to get an idea of what good vs bad grains look like is to make yourself some examples.

Take 2 pieces of the same steel. Stick one in the fire, overheat and let it sit in there for too long ( just don't let it melt away) quench and break it. Now take your other piece and give it a proper heat treat. Anneal, 3x normalize, quench and then again break it. Don't temper them and a solid tap should be sufficient to break them. You should notice that the abused piece has a very grainy looking appearance along the fracture while the heat treated piece should look and feel almost velvety.

You could probably get a good comparison without doing the full anneal on your heat treated piece and just doing a 3x normalization/quench to save some time. The normalization process is what refines the steel grain structure back to smaller size after heating to forge has caused it to grow. Hope this helps.

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Let's start from the beginning. Cut off your piece of coil spring. Straighten it out. Stop! Inspect for cracks. If none proceed. I find it helps if you slightly square up the piece, leaving rounded corners.. This way it won't roll off the anvil and makes it stronger, especially in coils that are 3/8" round and less. Stop! Inspect for cracks again. If none, proceed. Hammer a very slight taper on the hitting end. Helps to center your hammer blows and makes it easier to clean up when it does mushroom. Leave that end hot and turn piece around to work on business end. Shape end to desired use. Turn the air off the coal forge or the gas off a gas forge. Put the piece back into the fire and heat up at least 3/4's of the tool. Rotate so it's an even heat and bring the heat up very slowly. When solid red or slightly hotter lay the tool on a fire brick. Make sure not to lay it on any metal as it will work as a heat sink and that can be a break point. Leave until cool. About an hour or so will work. I usually do this near the forge so there's not cold air blowing on it. So that's the normalizing. Check again for cracks. If none, proceed. Now you're ready to harden. Slowly heat up about 2" to 3" of the working end, rotating in the fire for a solid heat. Don't guess by color as that is different in each shop. Use a magnet and keep checking the flat of the tool end. When the magnet has no grip any more, that's critical temperature. Quickly cool off about 1-1/4" or so of the end by moving the piece through the water in a figure 8 and slightly up and down. When the end is cool quickly polish up the end on a brick or with sand paper to shiney. Hold tool with working end up. Heat rises. Watch for the colors. Straw will come first. For hot cutting tools I take the end to purple. If there isn't enough residual heat to run the color that far put the tool backwards in the fire and slowly heat and then check colors again. Once purple cool off just the polished end. As long as there is water on the end you aren't running any more colors. Once it's all air cooled check again for cracks. If none, proceed to use your new tool. The first time I use a new coil spring I cut off a short sample piece and forge to a chisel end and check for cracks. If the piece starts splitting I just throw the spring away. Only happened once. Then I harden the sample piece in water. If no cracks that's what I use for my tools. In almost 40 years of tool making and 35 of teaching tool making I have done this process and always use water for coil spings. I hope this helps.

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After having a short slitter fracture that had been made from a coil spring I have started anneling after cutting to length and straighting. After forging and welding a handel {if necessary} I polish and reheat to gray and let cool to normalize. Then heat treat.
I have found that the extra steps of anneling and normalizing remove any stress. If nothing else it gives me piece of mind.
I try to make several tools at one time and normalize at the end of the day. This way I can shut the forge down and leave the tools in the forge to cool overnight. When using my coal forge I heat up a fire brick or a piece of one inch plate to dull red, put the tools on them and cover with kaowool and cool overnight.

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whoa jeez, Matt hit it on the nail..... thats some big grain growth you have there... i'd be brittle no matter whatcha do

normalize it several times.. ... heat just to non-mag and let cool down to room temp... ( grain grows the higher the temp/time )

then heat to non-mag and quench... crack it in two, and you should have a much finer grain... break a file in two... look how silky smooth the grain is, now that is what you want it to look like...

good luck

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If you want to know what really good grain looks like: go buy yourself a good Nicholson or Simonds file -- or better yet, take an old, dull one that is destined to be a knife blade -- but one that HAS NOT BEEN HEATED ABOVE CRITICAL since it left the factory. Lock it in the vise with an inch or so protruding from the top. Give it a good, solid whack with a heavy wooden mallet. The tip should snap off. Look at the grain along the fracture. That is what you are aiming for. Those guys know how to heat treat their steel.

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It's hard to tell from the pic but If I look at the fracture surface, right piece, lower left corner... The corner appears darker that the rest of the fracture surface. This may have been where a crack started. The upper right corner on the left piece looks similar. I am thinking you had cracks at these locations prior to your recent use of the tool.

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It's hard to tell from the pic but If I look at the fracture surface, right piece, lower left corner... The corner appears darker that the rest of the fracture surface. This may have been where a crack started. The upper right corner on the left piece looks similar. I am thinking you had cracks at these locations prior to your recent use of the tool.

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I made a new one a couple days ago out of the same spring. First I heated it up to yellow, then put it in some sand to cool. A few hours later I pulled it out and re-heated it and forged it straight and put the taper in. After forging, I let it get yellow again, and turned off the gas, closed the forge, and waited till the next day. That was the normalizing step right? After this step, I brought it up to non-mag, and quenched in used fryer oil using a swirling motion. Then I set it on top of my forge hood for a while until it showed some bluing.

Just to test it out, I chucked it up in the vise, and smacked it with a hammer. The vise would not hold it. I think it was because I had the taper in the jaws. Today I will put the round part in the jaws and try to break it off.

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