what grind before heat treat?

S

sunnybob

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Jun 25, 2020
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Just about to harden my first home made knife, and want to know at what grit stage am I wasting my time?
If I polish to mirror finish, how much of that will be lost after hardening?
If I only grind to 80 grit, how hard will it be to get back to mirror polish on hard steel?
the steel is 86/70, its a large bowie style with a full tang.
I will be using a converted BBQ with charcoal and forced air blower.
Thanks
Bob
 
You have to re-sand/grind after HT to remove the surface layer of decarb. Most folks go to 220 grit or 400 grit before HT. Beyond that you are just wasting time and materials. Leave the edge about .030" thick prior to HT. Make sure all deep scratches are gone ... because they are a lot harder to remove after hardening.

Good luck with the HT. A BBQ and blower may be tricky.
 
Stacy E. Apelt - Bladesmith said:
You have to re-sand/grind after HT to remove the surface layer of decarb. Most folks go to 220 grit or 400 grit before HT. Beyond that you are just wasting time and materials. Leave the edge about .030" thick prior to HT. Make sure all deep scratches are gone ... because they are a lot harder to remove after hardening.

Good luck with the HT. A BBQ and blower may be tricky.
Click to expand...


Good to know, thanks. That gives me some more work to do because at the moment its at 80 grit.
I have no option as far as a forge goes. Anything else would be wildly expensive and beyond my budget.
I dont even have a BBQ yet, I'm going to have to scrounge one from somewhere.
 
In Cyprus you have fewer opportunities than we have in the USA and Europe, but see if there is a maker near you who could help you do te HT. If not, post some photos of the BBQ and your air source. We might be able to help you.

One way to make a BBQ into a forge is to basically convert it into a charcoal forge. You will need some fireclay and about six firebricks. You can use all fireclay if no firebricks are available. In a tight budget, adobe will work.
1) Take the BBQ and coat the bottom of the inside with a 2" layer of the clay.
2) Place a 45 to 60mm (1.5" to 2") pipe down the bottom. Run the pipe through holes on both ends of the BBQ so one end sticks out 50mm/2" and the other sticks out 250mm/10". Add some more clay to bed the pipe about half way into the clay. Put a pipe cap on the short end ( just hand tight is fine) and a gate valve on the long end. Put a pipe npple on the valve that you will attach the pipe or hose from you blower, or the blower directly. Drill 6mm/1/4" holes in the top of the pipe every 25mm/1". The holes will deliver the air and should start about 100mm/4" from each end.
3) Put firebricks down the sides and ends to make the fire pit. Use more clay to flare the bottom up along the fire brick and cover the sides.
4) Use bituminous coal or wood lump charcoal if possible. Charcoal briquettes won't work. If nothing else is available, make a good wood fire with 50X100mm/2X4" chunks of wood and let it burn down to coals. (BTW - You can build a charcoal maker from an old 55 gallon drum.)
5) Start the fire in the middle and then start the air flow. Once it gets going spread it out as much as needed for the forging or HT task. Adjust the air to get the coals right. Too much and you will melt/burn the blade. Too little and it will not get hot enough. You will also have to keep the blade moving while it heats up to the target temperature.Play with some scrap steel until you get it right.
6) Use a magnet stuck to the side of ten forge to check when the steel becomes non-magnetic. This is about 50° to 75° F/10° to 25°C lower than the target temp.
 
A lot of info there, thanks for taking the time.

I'm doing this on a tiny or zero cost budget. I have a 30" square sheet of thin flat black iron that is surplus so I shall cut that and bend the ends and sides up to make a trough. Dont have any firebricks, but lots of sand and some acrylic tile cement to bond it to shape. Wood carbon is something that is easily available here. I can use the wifes hair dryer (what she doesnt know wont hurt her). I shall have to dig through the scrap pile for the air supply tube.

This is just an experiment to see if I want to get more involved in knife making, but even good steel is not on island. I had to get a visitor to bring this 86/70 in her suitcase from England. If I decide to make more I shall have to use car leaf spring from the scrapyards.
 
Improvise as needed to get roughly what I described. A hair dryer is OK for a small forge, but not really powerful enough for doing a larger HT forge. Make an old shop vacuum cleaner is somewhere around your place or in a yard sale.

Acrylic tile cement won't work. Use sand, any field or river clay you can get (red clay is best), and a small bag of portland cement or lime. I have seen people use plaster of Paris in a pinch. You can even make it with just the clay and sand, but it won't last long. If there are any caves near you, the mud/clay from the floor is usually excellent for this task, as it is high in lime..

About 25% clay, 70% sand, and 5% lime/portland works. Mix it to a thick goop like cookie dough (not wet or runny) and pack it in place firmly by hand or using a block of wood. WEAR GLOVES. If you have no lime or portland, use 1/3 clay and 2/3 sand.
 
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I have lots of sand, and red soil that can become very clay like when wet. Shame the acrylic wont work as i have 30 lbs of that and not a drop of cement or plaster. But thats not too much money. No caves, Cyprus is mostly stone and volcanic left overs.
I might be able to sneak the vacuum cleaner out for an hour. The metal pipe I can scrounge from a local iron works.
I'll make all this tomorrow, and hope to get it done on weds.
 
Don't rush things. Take your time and do it right!

The vacuum needs to have a place to connect the hose so it blows out air. You might be able to scrounge an old blower from a home oil heater. Check with any heating installation or repair places.

Make some balls of wet dirt/clay and see if they stay solid after a few days. If they crumble, it isn't clay.
 
You can always just dig a small trench and run a steel pipe into it And fill with charcoal. All you need is air and fuel and a way to keep your fuel (charcoal) in a pile. My very first heat treat I did with a steel box with a steel pipe along the bottom. I cut a slit in the pipe lengthwise facing up. I had about 4-5” of charcoal above the pipe. Fired it up and gave it some air from a small blower and promptly melted my first blade. You don’t need tons of heat as your only looking for an even 1500° for a heat treat.
 
Thanks for the reply.
I've made the steel trough and got a piece of 50 mm steel pipe, but then had visitors so the rest of the day is gone.
 
Well, I got the heat treat done, its in the oven now on its first cycle.
i used some old clay roof tiles to slope from the air tube up and out to the sides to reduce volume on the square metal box. mixed up some earth, sand and cement to pack the slopes and hold the tiles in place. Put wood charcoal on top and set it alight.
I used the wifes hairdryer on the end of 6 ft of hose so it wouldnt get overheated. I was very surprised how well that worked, I only had it on half speed as full speed was blowing sparks everywhere. I slit the air tube, rather than drill lots of holes.
The only possible problem is that I didnt get the blade evenly heated, but most it went to dull red and I kept it there for 3 minutes. I'm not fussed about perfect hardness on this knife, its a conversation piece and wall hanger.
From there i put it into a 30" long piece of 4" steel ducting tube mounted vertically, filled with old engine oil.
I know, its not the correct stuff, but this whole project is a dirt cheap run through to see if I want to spend more time making knives. Then into some angle iron clamps to stop warpage until it was cool enough to hold.
Then to the oven at 150c for 2 hours.
Total cost of the forge and treating was 8 euro (US$9) for the charcoal, which would do at least three more sessions.

The oil dip was interesting, I didnt expect it to flame when I pulled the knife out as the metal was only dull red, not even bright.
Now I have to wait the rest of the day to get it through the second oven tempering. I shall be cleaning it up and testing it tomorrow.
 
Color varies by the observer and the amount of ambient light, but dull red sounds too low for the blade to harden. The color wanted is a bright cherry red - around 815°C (1500°F) is a good target. It sounds like you needed more air than you were using.

Did you test the edge with a file to see if it hardened?

Increasing the temperature of the blade does not increase the hardness. It has to be over 780°C (1440°F) to harden. Otherwise it stays soft. Overheating a blade won't increase the hardness, either. It may damage the blade is n=many bad ways. You have to learn the right temperature and try to achieve it every time. It take some practice.

Steel-Color-Temperature-Chart.gif
 
I'm certain that I failed to harden the whole blade, but the tip became much hotter than the rest and I was afraid to lose it.
Even at that temp, and despite clamping it between 2 pieces of angle iron it still warped slightly, making cleaning the scale off awkward.
I've come to the conclusion that this hobby is not for me. I shall clean it up enough to become a wall hanger.
Thanks very much for taking the time to try to help me.
 
Don't give up yet. Start with a simple steel and work on the process of forging and HT. It is really fun one you get the hang of it. Few people start with knives that succeed.

Here is my first knife back in 1961:

View attachment 578985
my-first-knife-jpg.578985
 
I'm retired, and on a fixed income, so spending has to be considered for my hobby. Having tried this, I can see that I would not be happy carrying on like this. I would need a temperature controlled forge and a proper grinding wheel and thousands of dollars of sandpaper :( . So i'm going to quit now while my costs are very low.
If had more disposable income, i could see myself enjoying this.
Thanks again for your help and encouragement.
 
It was a piece of mild steel I found in my grandfathers garage. Probably simple welding steel similar to 1018. The handle was a deer antler my dad brought home from a hunting trip. It looked pretty plain, so I charred it with a propane torch. The rivets were 16-penny nails. I heated the metal with the propane torch and hammered it on the small anvil on the back of a vise with a 12 ounce ball peen hammer. I cut a slot in the anter and inserted the short stub tang. There was no HT. What I didn't know was ... well ... everything!
Grandfather gave me a book from his library on "Machine Shop Techniques and Formulae". It had a section on blacksmithing and the tools needed. I built a simple forge and the rest is history.
 
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