1084 Heat treat and grain structure trouble

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kyle840

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Jul 14, 2009
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I am having trouble with my heat treat being consistent from tang to cutting edge. When I heat treat a blade and then break it in half to check grain structure it looks spot on along the outer edge but the center is not a uniform grain, instead it looks textured and somewhat jagged.
A little about my setup and procedure
My Forge is eight inches in diameter and a little under a foot long. It's lined with kao wool and refractory cement and has a fire brick bottom. I am using a homemade Venturi burner hooked to my grill propane tank with a 30 PSI regulator. It is capable of getting to welding temperatures. Inside it I have a 3 inch muffle to keep the direct flame off of my work. I am quenching in a four-gallon bucket of canola oil heated to 125 degrees.
I put my blade in the forge, bring it to nonmagnetic checking it frequently and let it sit in the Forge for another 20 seconds and then pull it out and check it with a 1500 degree temperature melt pen. When I believe it to have reached 1500 degrees I quickly move it from the forge into the oil and move it back and forth quickly in a slicing motion for about 40 seconds. I am not sure why it does not have a uniform grain in the center of the piece after I Harden it. My only guess would be either the steel is not getting fully heated throughout or it is not cooling fast enough in the oil. Any insight into what the problem is?
 
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If you're just letting it sit in your muffle and not actively moving it in and out of the forge I'm guessing it's not getting to a uniform temperature.
 
A close up photo would help. It may be fine, and what you see is tearing. Examining grain by eye in a break isn't always accurate.

1084 is a shallow hardening steel and there could be some differences between the center and the skin. This can make the outside harder than the core, and when broken the outer skin will snap clean but the core will pull out and tear ( thing of WI in a break test).
 
I am not moving it inside of the muffle as it *appears* to be heating evenly. I will get a pic of the break on here in a few minutes. Stacy, what is WI in a break test?
 
I grind my bevels after I heat treat the blade. If the steel is shallow hardening, does this mean the center will be slightly softer therefore the bevels should be ground before HT so the cutting edge is completely hardened?
Here are a couple pics both with and without flash.

https://drive.google.com/file/d/0BxhWsAXsbssSMHpRYzV1WXFkZTg/view?usp=drivesdk
https://drive.google.com/file/d/0BxhWsAXsbssSaUZkeDhubGZ3OHM/view?usp=drivesdk
https://drive.google.com/file/d/0BxhWsAXsbssSbWswMzN1OHh4aDg/view?usp=drivesdk
https://drive.google.com/file/d/0BxhWsAXsbssSdzVpRHFpV0RuMHM/view?usp=drivesdk
 
if you are hardening in the muffle, like kuraki said you need to move the blade in and out of the forge..

With this kind of steel i would grind before HT, you will reach the temperature easier but you need to be carefull to the colors of the blade!

In the pictures you can see that one end of the stock had the correct grained size(in my opinion) and the rest was dull.

Hope this can help you!

regards
 
Have you been doing any grain refining/normalizing. When I heat treat 1084 I run it to 1600 and then 1500 and then 1400 with air cooling between each heat. I heat to 1475 and quench in 75° parks 50. But I use a oven for heat treating. What oil are you using?
 
I will try moving it in and out of the forge. I have not done any normalizing with the 1084. My steel comes from Aldo and when I spoke with them on the phone they said there is no need to normalize his steel unless forging. I would definitely give normalizing it a try if you think it will help.
 
I don't see any problem with the grain in the photos. You aren't seeing large grain in the wiggly area. It is the lines of shearing along the grain boundaries when the blade broke. I call this "tearing".
 
JT, I am using canola oil.
Stacy, I cant get any pics from the microscope with a camera but when I look at it up close the size of the grain in the middle section of the break looks a lot coarser than the edges. Kind of looks like comparing very fine sand to salt crystals. Ill see if I can get a better picture.
 
I looked at the photos again, and I still don't see any issue.

Think of the steel as a beach of sand. The grains of sand may be very fine, or very coarse. However, looking at the beach after the tourists leave for the day will show lots of ups and downs, and lines and ridges. These very large features may look like a rough beach, but it is the size of the individual sand grains that makes it coarse or fine ... not the shapes formed on the surface of the sand.

For an experiment, take a piece of the same steel and heat it to 1800-1900F for five or ten minutes. Quench in the same oil as before. Break that piece once it is cool. You will see a very different look than the blade you have. These are large grains.
 
I did not temper before the break. I broke it about a half hour after the HT. I break it by putting it in a vice and bending it with a pipe. I have broken a couple in the past that had uniform grain and when I broke them it took considerably less effort to break than the ones that have larger grain in the center. the uniform grained blades seemed a lot more brittle. after overheating a piece and breaking it I see what this looks like. my blades that have large grain in the center are definitely not overheated.

if a blade is overheated, say to 1700 degrees, and quenched, does it still reach its peak hardness? I know it will be brittle and have undesirable properties, but does it get softer as it is overheated past 1500 degrees?
 
During the quench the steel transforms to martensite which stresses the steel. That must be tempered to reduce quenching stresses .
Overheating will produce larger grains and that will make the steel more brittle ! Overheating can also change the grain boundaries chemistry which is one of the resaons for more brittle steel.
 
kyle840 said:
I will try moving it in and out of the forge. I have not done any normalizing with the 1084. My steel comes from Aldo and when I spoke with them on the phone they said there is no need to normalize his steel unless forging. I would definitely give normalizing it a try if you think it will help.
Click to expand...


It is my humble opinion that you need to normalize aldo steel. Non magnetic, just at or under non magnetic then 1400 very thorough with a bit of a soak. you will need to pump it in and out of your muffle, taking care to not overheat the tip. if you really want it to be special take it straight from the 3rd soak and let it slow cool overnight buried in vermiculite.
 
I found out a few things after talking to someone who is very good with 1084. 1084 actually doesn't get softer when it is heated past 1525 and then quenched. it will still reach peak hardness at higher temps, it just obviously wont make a good knife.
this is very helpful to know since some of my hardness readings were pretty low I definitely know that I was not overheating those particular pieces.
I also found out that since my steel is 3/16", it may not be fully hardening all the way through in canola oil. I was told that a faster oil would better suit 1084 that is of this thickness or greater.
 
I always normalize. The break looks like it got to hot in the center(middle) som crystal looking stuff there.Is your blade laying so the top and bottom touch the pipe? Try standing it up straight and try again.
Take Care
TJ Smith
 
I have never normalized but will give it a shot to the best of my ability with what I have at hand.
I put my blade inside the muffle while it is balancing on the spine or the belly so the least amount of surface area touches.
I am also going to try what Kevin and I spoke about and gives parks oil a try in the future.
I may also try overheating some steel and quenching just to see what kind of hardness numbers I get from it.
 
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