.png)
Tweet
All the photos can be clicked to view bigger. CC BY-SA version 3.0 unported license applies to them all.
It has taken me some time, but finally I was able to experiment case hardening with my home made heat treatment oven. Now that I have established some basics it is less of a mystery or black magic and I hope that others can benefit from this also.
All this was done at home in my little 'shop room' and produced no odors, smoke or anything so wife is happy and so am I.
For the record, I'm after actual case hardening and NOT color case hardening, as this is going to be used on tools and not decorative items, so depth of case, hardness and time are of interest. Tools will be turned, milled, ground or polished after hardening, so the oxidation colors will be gone if there is any.
When I tried to find information on this 'lost art', all I got was for color case hardening, which frankly is black magic to 'get correct'. All their recipies for the mixture included anything from plain charcoal to one-winged bats eyelashes thrown in to the mix. And color case hardening isn't so much after the deep case as it is for the colors (which by the way are awesome).
What I finally found was actual books on the subject, dating back to the early 1900's when this pack carburization or pack case hardening was done. These detailed many aspects and the process itself, so I read a whole week this new-old-information and armed with this knowledge I fired up my oven and got to work.
What is case hardening?
In case hardening the idea is to have a low carbon steel (like structural steel) workpiece that won't harden on its own. The workpiece is cooked in a carbonaceous material (like charcoal) at high temperature for X long period of time to diffuse more carbon to the surface of the steel, thus making a layer of high carbon steel that can be hardened by heating and quenching.
Why would you do it?
To get a hard, wear resistant surface on steel that won't harden on its own. Or to get a workpiece that has a hard skin but tough core, thus combining the benefits of both the hardness and toughness. Ordinary low carbon steel is soft to work with, doesn't cost a fortune like highly alloyed tool steels and hardening only parts of the workpiece is very easy.
What happens?
The carbonaceous material burns to produce carbon monoxide as there is more carbon than oxygen inside the container. When the temperature is high enough, this carbon monoxide breaks to produce carbon that diffuses in to the steel and the oxygen recycles again to carbon monoxide. The longer the steel is at the high temperature, the deeper the carbon can diffuse. The higher the temperature, the faster this happens.
If one includes carbonates in to the mix, they will speed up or enchance the formation of carbon monoxide, thus providing a better atmoshpere inside the container. Usual carbonates ae those of sodium, calcium, potassium and barium, though I would not recommend the barium carbonate due to its toxicality. Sodium carbonate is the same as washing soda and calcium carbonate is the same as chalk, both easily obtained and very cheap.
Container
First you need a box or container that is large enough. Large enough meaning that there should be about 25-50 mm of space around the workpiece when inserted. Bigger works too, but requires more heating, so a suitable sized container is the best. The container can be made from your basic steel by welding together, like from round or rectangular tubes and plates.
In the following photos you can see that my containers were made from 102 x 3.5 mm steel pipe with caps welded on as the bottoms. The bottoms and lids were sawn off from 100 mm diameter bar and cleaned up on the lathe (not necessary, but bugged my eye). To the lids I turned a small should, 5 mm long and 0.5 mm smaller than the tube inside diameter so it fits on easily but isn't excessively loose either. The open end of the tube I turned on the lathe to get the lid to sit flat on it.
Source of carbon
Next one needs a source of carbon. Charcoal is quite cheap, easily obtainable source for carbon, but the usual form that it comes in is briquettes. These are formed from whatever organic material was at hand and pressed together with some binder. Do not use them, but instead find a charcoal bag that says "100 % something wood", something being preferably a hard wood.
This charcoal has to be crushed somehow and it produces fine powder that will mess up everything. I crushed a 2 kg bag of it with a chunk of steel, but I would advice something else, especially if a machine can be used. After this I used an ordinary kitchen sieve to get the fine dust out of it (did it near a vacuum cleaner) and collected the rest in a plastic container. The 2 kg bag got me about 6 liters of crushed charcoal that doesn't contain dust.
Edit: The charcoal is best if it is 100 % hardwood (usually the packaging label says this) to ensure that it isn't made of whatever organic material was laying around. The pieces in my sack were 100 % birch and one could find bigger pieces that had the wood grain patterns still showing.
The other good source would be just buying activated charcoal, as it usually comes in a pill form and is thus ready to use. I haven't tested it yet, but it probably will be my source in the future.
If one wants to add carbonates, a 10-20 % should be enough. My own containers pulled 175 grams of charcoal, so I added 25 grams of carbonate to it and mixed well to get a 15 % mix.
Packing the container
Packing the parts in to the container needs some prep work, like cleaning them from oils and dirt. Then the container is first filled with the carbonaceous material, about 20-50 mm bed is good. The container is tapped on the table or with a hammer on the side to settle everything. Then the part is inserted in the middle of the container and more material poured around it to support it. It is advisable to tap the container once in a while while adding to make sure the part sits tight with inside the pack. Then on top of the part is again poured a thick layer of the carbonaceous material, 30-50 mm is enough. This extra is allowed for shrinkage, as the carbon slowly sinks in the container a little.
Once the container is full, put the lid on and make sure it closes down and doesn't bear on the carbon mix inside or that the seal area doesn't have the mix on it.
Carburising
Put the container(s) in to the oven, preferably to the middle to provide even heating. It would be best that the container have some legs under it or that it is located on two pieces of scrap iron so that the hot air inside the oven can heat the container from the below also. Close the oven, heat up your oven to 900 degrees Celsius and let it soak for 4 hours. After the 4 hours is up, cut off power to the heating elements and let the oven cool down on its own.
The parts have been now carburised. They are still soft, but the surface layer is of high carbon content steel. The parts can be machined further now to make threaded holes or other features to so that they will not harden later on.
In this picture the containers have been removed from the oven after they have cooled down. The surface on the lid looks like it has been just come off from the mill, scaled badly and the scaling comes off just by touching it. Quick cleanup on one of the lids and check with a vernier caliper shows no appreciable change in thickness.
Once the containers are opened, the drop in carbon level can be seen. Next to the lids is my control piece that is the same size as the parts in the containers but it didn't receive any heating. The carbonates can be seen in a few containers as non-black substance.
Some fishing and cleaning later I had my test specimen in a row. In this picture from left to right is control piece (K), plain charcoal (C), calcium carbonate (CA), sodium carbonate (NA) and barium carbonate (BA). Nothing special off them, a little darkness on the surface, the sodium one being black for a reason unknown. The coloring on the control piece is due to me playing with copper coating and sulfuric acid. All the pieces were cut off from a 25 mm diameter cold drawn S355 steel bar.
Hardening
I put the carburised pieces to the oven and set my oven to heat up to 800 degrees Celsius and while waiting, went to the local hardware store to buy the cheapest pipe tongs I could find. Back home removed the plastic handles from them, straightened the handles in a vise and cleaned them up a little. Lastly I welded pieces of 16x1 mm stainless tube over the handles with a MIG. Cheap and easy way to get long tongs. Also seen is my welding gloves to protect my hands. What is missing in the picture is a face shield.
Once the oven reached 800 degrees Celsius, I waited for 30 minutes for the heat to soak through the parts. Rule of thumb is that heat goes in 25 mm per hour, so a 25 mm piece heats up in 30 minutes.
It has taken me some time, but finally I was able to experiment case hardening with my home made heat treatment oven. Now that I have established some basics it is less of a mystery or black magic and I hope that others can benefit from this also.
All this was done at home in my little 'shop room' and produced no odors, smoke or anything so wife is happy and so am I.
For the record, I'm after actual case hardening and NOT color case hardening, as this is going to be used on tools and not decorative items, so depth of case, hardness and time are of interest. Tools will be turned, milled, ground or polished after hardening, so the oxidation colors will be gone if there is any.
When I tried to find information on this 'lost art', all I got was for color case hardening, which frankly is black magic to 'get correct'. All their recipies for the mixture included anything from plain charcoal to one-winged bats eyelashes thrown in to the mix. And color case hardening isn't so much after the deep case as it is for the colors (which by the way are awesome).
What I finally found was actual books on the subject, dating back to the early 1900's when this pack carburization or pack case hardening was done. These detailed many aspects and the process itself, so I read a whole week this new-old-information and armed with this knowledge I fired up my oven and got to work.
What is case hardening?
In case hardening the idea is to have a low carbon steel (like structural steel) workpiece that won't harden on its own. The workpiece is cooked in a carbonaceous material (like charcoal) at high temperature for X long period of time to diffuse more carbon to the surface of the steel, thus making a layer of high carbon steel that can be hardened by heating and quenching.
Why would you do it?
To get a hard, wear resistant surface on steel that won't harden on its own. Or to get a workpiece that has a hard skin but tough core, thus combining the benefits of both the hardness and toughness. Ordinary low carbon steel is soft to work with, doesn't cost a fortune like highly alloyed tool steels and hardening only parts of the workpiece is very easy.
What happens?
The carbonaceous material burns to produce carbon monoxide as there is more carbon than oxygen inside the container. When the temperature is high enough, this carbon monoxide breaks to produce carbon that diffuses in to the steel and the oxygen recycles again to carbon monoxide. The longer the steel is at the high temperature, the deeper the carbon can diffuse. The higher the temperature, the faster this happens.
If one includes carbonates in to the mix, they will speed up or enchance the formation of carbon monoxide, thus providing a better atmoshpere inside the container. Usual carbonates ae those of sodium, calcium, potassium and barium, though I would not recommend the barium carbonate due to its toxicality. Sodium carbonate is the same as washing soda and calcium carbonate is the same as chalk, both easily obtained and very cheap.
Container
First you need a box or container that is large enough. Large enough meaning that there should be about 25-50 mm of space around the workpiece when inserted. Bigger works too, but requires more heating, so a suitable sized container is the best. The container can be made from your basic steel by welding together, like from round or rectangular tubes and plates.
In the following photos you can see that my containers were made from 102 x 3.5 mm steel pipe with caps welded on as the bottoms. The bottoms and lids were sawn off from 100 mm diameter bar and cleaned up on the lathe (not necessary, but bugged my eye). To the lids I turned a small should, 5 mm long and 0.5 mm smaller than the tube inside diameter so it fits on easily but isn't excessively loose either. The open end of the tube I turned on the lathe to get the lid to sit flat on it.
Source of carbon
Next one needs a source of carbon. Charcoal is quite cheap, easily obtainable source for carbon, but the usual form that it comes in is briquettes. These are formed from whatever organic material was at hand and pressed together with some binder. Do not use them, but instead find a charcoal bag that says "100 % something wood", something being preferably a hard wood.
This charcoal has to be crushed somehow and it produces fine powder that will mess up everything. I crushed a 2 kg bag of it with a chunk of steel, but I would advice something else, especially if a machine can be used. After this I used an ordinary kitchen sieve to get the fine dust out of it (did it near a vacuum cleaner) and collected the rest in a plastic container. The 2 kg bag got me about 6 liters of crushed charcoal that doesn't contain dust.
Edit: The charcoal is best if it is 100 % hardwood (usually the packaging label says this) to ensure that it isn't made of whatever organic material was laying around. The pieces in my sack were 100 % birch and one could find bigger pieces that had the wood grain patterns still showing.
The other good source would be just buying activated charcoal, as it usually comes in a pill form and is thus ready to use. I haven't tested it yet, but it probably will be my source in the future.
If one wants to add carbonates, a 10-20 % should be enough. My own containers pulled 175 grams of charcoal, so I added 25 grams of carbonate to it and mixed well to get a 15 % mix.
Packing the container
Packing the parts in to the container needs some prep work, like cleaning them from oils and dirt. Then the container is first filled with the carbonaceous material, about 20-50 mm bed is good. The container is tapped on the table or with a hammer on the side to settle everything. Then the part is inserted in the middle of the container and more material poured around it to support it. It is advisable to tap the container once in a while while adding to make sure the part sits tight with inside the pack. Then on top of the part is again poured a thick layer of the carbonaceous material, 30-50 mm is enough. This extra is allowed for shrinkage, as the carbon slowly sinks in the container a little.
Once the container is full, put the lid on and make sure it closes down and doesn't bear on the carbon mix inside or that the seal area doesn't have the mix on it.
Carburising
Put the container(s) in to the oven, preferably to the middle to provide even heating. It would be best that the container have some legs under it or that it is located on two pieces of scrap iron so that the hot air inside the oven can heat the container from the below also. Close the oven, heat up your oven to 900 degrees Celsius and let it soak for 4 hours. After the 4 hours is up, cut off power to the heating elements and let the oven cool down on its own.
The parts have been now carburised. They are still soft, but the surface layer is of high carbon content steel. The parts can be machined further now to make threaded holes or other features to so that they will not harden later on.
In this picture the containers have been removed from the oven after they have cooled down. The surface on the lid looks like it has been just come off from the mill, scaled badly and the scaling comes off just by touching it. Quick cleanup on one of the lids and check with a vernier caliper shows no appreciable change in thickness.
Once the containers are opened, the drop in carbon level can be seen. Next to the lids is my control piece that is the same size as the parts in the containers but it didn't receive any heating. The carbonates can be seen in a few containers as non-black substance.
Some fishing and cleaning later I had my test specimen in a row. In this picture from left to right is control piece (K), plain charcoal (C), calcium carbonate (CA), sodium carbonate (NA) and barium carbonate (BA). Nothing special off them, a little darkness on the surface, the sodium one being black for a reason unknown. The coloring on the control piece is due to me playing with copper coating and sulfuric acid. All the pieces were cut off from a 25 mm diameter cold drawn S355 steel bar.
Hardening
I put the carburised pieces to the oven and set my oven to heat up to 800 degrees Celsius and while waiting, went to the local hardware store to buy the cheapest pipe tongs I could find. Back home removed the plastic handles from them, straightened the handles in a vise and cleaned them up a little. Lastly I welded pieces of 16x1 mm stainless tube over the handles with a MIG. Cheap and easy way to get long tongs. Also seen is my welding gloves to protect my hands. What is missing in the picture is a face shield.
Once the oven reached 800 degrees Celsius, I waited for 30 minutes for the heat to soak through the parts. Rule of thumb is that heat goes in 25 mm per hour, so a 25 mm piece heats up in 30 minutes.
Comment