I studied metallurgy for years in college and after. I rarely see RU-vid presentations of heat treatment without misinformation. You did an accurate and clear job of presenting a complex subject and it was entertaining.
Mare, can you recommend any good reads for hobbyists? I have no background in metallurgy, though there are some instruments I would like to make for my work.
@@therealKJMD I'd really recommend Machinery's Handbook, there is tons of extremely good info in there on most metalworking & mechanical engineering topics. Current editions are kinda expensive (150 USD for the current edition), but older editions used off ebay are just fine for hobbyist use. Don't buy this book off of amazon, most of the listings there are counterfeits (at least in the US, don't know about AU) and can possibly have misprints. I paid ~20 USD for my copy of the 26th edition, I believe they are currently on the 31st edition.
"Traditionally you would use wrought iron, but we no longer live in the 18th century" 😂 I'm so glad you're able to take advice from others and admit some mistakes. That's a very hard thing to do. You weren't bad with your method though and in your case I think case hardening would be a good idea. You definitely do a ton of research to find different methods for doing a thing, their effectiveness, and problems with them, and weigh the cost and labor for each of them. It's only natural to miss the occasional detail but I honestly think you do a fantastic job on your research and it's very insightful to listen to! And of course you do test things for yourself so you're not just relying on book knowledge and know that slight adaptations you make will still produce the same result. Well done!
An old machinist showed me several methods of case hardening for plow points and the like, harking back to "olden days" before cars and such. My favorite was an iron box filled with small pieces of bone and antler, which was supplied by my breaking up dead critters. The box was about 3/8ths of an inch thick, which closed with a snail cam. He would then chuck it in the forge and I would get to work pumping the bellows to his satisfaction. I was pleased to stop when he figured it was time enough and he would let the fire die for the night. In the morning I would drag out the box and empty it. The hardening usually brought out some colors on the parts. For the tempering, he used rendered bear fat because it would not crack or check the metal. Just thought I might mention a different method. Good video, thanks.
I'm afraid bear fat would be tad difficult to get nowadays... ;-) Also, that "charred animal stuff" (charred leather in this case) method was shown by Chris from Clickspring channel - "Antikythera Fragment #4 - Ancient Tool Technology - The First Hardened Steel?" (ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-V_Mp1fNzIT8.html). That method was indeed "used before cars and such"...
@@MrKotBonifacy I do appreciate your letting me know about that. I've heard tell of that antikythera. It was supposed to be older than the Sumerians (I think that was who, could have been the Greeks), and I do believe I heard tell that it was brass or bronze. so that could help you date it, if an age was needful. I'll check out that video tomorrow, since it's gotten late.Nice to know that the old ways of doing things can still be counted on.
This is a compliment, this will save me a lot of time, I enjoy reading however work takes mostly all my time, and a lot of times I just need the answers to the test fast, this was a great thing you are narrating, and illustrating and will save me and others time and that money thing we need, and it's always a topic I am interested in too. You have from day one provided me with all like-minded topics and interests thank you for all you do hope to see more. Good day to you.
Thanks for the great overview and taking all the time to do the comparison testing. From personal experience I know how much time can be consumed by experimentation to validate the things people suggest. Not to mention the exhausting effort needed to comb both old literature and pinpointing the specific relevant information amongst the mind boggling plethora of search returns that only pertain to the general topic your searching. You may have called it a simple introduction but from my perspective even without covering some of the finer points on the subject, you did a fantastic introduction/Q & A that would help over 95% of the people wanting to case harden parts, 🍻.
I’ve heard about pack carburize but I have never done this. Back in the mid 90s I got a job at a local commercial/tooling heat treatment shop. I first started running the salt pots running tooling O-1, O-6, S5 and many more tooling materials. It was a salt to salt or salt to oil process. I moved on to run the vacuum furnaces, temper and endo furnaces. At least a few years in Each department. After 10 years of working in this tooling shop I decided I wanted to learn more and ended up getting a job at the words biggest heat treatment companies (BTP) this was one great experience that I will always be grateful for. I started as a vacuum operator, moved to a temper furnace operator, Nitride supervisor, shop foreman, production manager and finally plant manager. All these jumps was within a 12year time period not overnight. By this time I had 22years experience and made a decision to slow things down, I moved back to my first job as a facility manager. I am now in charge of the furnaces and future development of the plant. But I always love to hit the floor and reach the new guys a few tricks, especially the guys that show interest in the process. But getting back to carburize, I’m not to familiar with pack carburize since this is not allowed in aerospace parts. We process the parts in a furnace with temperature and carbon controlled instruments. But for a tooling part this seems to be good enough, if it fails you just try again. I might make some videos on the process using controlled instruments. I’ll see if I have time this year.
That is great for you to accomplish a career in a very important field. I'm a machinist 45yrs, worked tool & die 2 yrs and learned some heat treating tool steels. It is an enormous field to learn. Thanks for your inspiring story. You are proof that we can accomplish big things if we work hard and keep learning, move forward !!
I have a question for you! Heat treating knives, the data sheets demand a preheat to equalize. I just put the knives in at cold, and ramp as fast as it goes up to aus temp. Shits getting hard, so it’s working so to speak but I’m just wondering if I can maximize everything. I want to heat treat these knives as good as I possibly can. The data sheet for m2 suggests ramping 400 degrees f per hour up to normalize temp (like 1550f) then rapid ramp from there to aus. I’m assuming this is for large tooling parts not thinner cross sections right? And finally. I’m planning on making some knives from forged a11, the forging range says 1700f, do I have a tiny window? Or what? Can I reverse the or what? I’m assuming I’ll cycle the material after forging but I don’t want to do any permanent damage to the grain or carbide or anything. Oh yeah and sorry, I also want to forge weld simple stainless to these high carbide tool steels like a11, would I need to do that in a tool wrap, with argon? You have my dream job. If I didn’t go into psychosis I’d be graduated from Colorado school of mines by now. Getting my rocks off in the home shop now.
Without really knowing what I was doing, I managed to case harden some model locomotive parts using almost exactly the same process you've shown. I did use the sodium carbonate too. I've read that old fashioned blood and bone fertiliser is a good source of carbon but most modern fertiliser has other additives and I wouldn't trust it. Apparently, boot leather can also be used. Good video on a not widely documented topic.
Cheers Preso. The 1st edition machinery handbook that I have does reference crushed up bone as a pack hardening material, although it seems to be absent in later versions. Not sure about fertilizer but it could have some nitriding effects. I might have to look into that. Cheers
I watched with fascination and awe at you making the video on small home machinery and made comment to such . But boy you just took it to a whole new level in this video you are very informed and pass that knowledge over easily Thank you for such facinating content
Excellent video & discussion! From time to time, a case hardened part is far more efficient to make and use for my needs. Thanks for taking the time to better explain the process.
Old case hardening salts usually had sodium chloride and cyanide salts mixed in to aid in the dissolution of the carbon, but ofcourse those salts can sound a little terryfying, BUT by making the charcoal by carburizing something organic like leather in an oxygen free enviroment you can get carbon that already has very useful amounts of natural cyanide compounds in it. And the carbon doesnt all have to be carburized leather or some such, but something a third or a quarter of the whole carbon content. Additionally the rest doesnt have to be wood charcoal, it can be something else rich in organic carbon chains, such as sugar or flour. I don't know the exact chemistry behind it but the sodium chloride and cyanide apparently react together with the iron in such a way that the dissolvin of the carbon into the steel speeds up considerably
I greatly appreciate the info you've provided. In light of what you've presented, I was using too high of a temperature for far too long. Container was a lidded ceramic jar placed in a pottery kiln. Cycle time was on the order of 10 hours. 😯 Very haphazard.
Interesting, never thought about using a ceramic jar. There are charts online that will give you a good idea of case hardening times, for the vice, I case hardened it for around 10 hours and some times parts are case hardened for longer (in industry)
If you are stuck for flat bar, try old leaf spring steel .Though it can't be guaranteed as 0.7% C., (eutectoid) ,it's superior for hardness, tensile, etc.tjere are colour charts available if you've no thermocouple or welders crayon for the heat treat.your quench method is fantastic .evens out the chill crystal structure. * still will require normalizing. 250° /hr/cubic inch. Great vid man.
It certainly would not be >0.7% C given that the specification for the two common spring steels is not >0.60%. Also contain around 2.0% Si. One grade also contains an amount of Cr (approx 0.50%). Be aware of so called spring steels being made in sub continent factories from old rail steels. NO Si content there.
When quenching a part, you should move it rapidly in the quenching medium, regardless what it is. Move it in a figure of 8 or circles and up and down. This movement prevents the steam jacket. If only a section of an part should be hardened, move the part up an down, to achieve a smooth transition from hardened to not hardened material.
Nice explanation. For future projects I would suggest not using Linseed oil. The modern process for producing it leaves behind some impurities in the oil that in and of themselves cause upredictable results. Actually, canola oil makes a passable quench medium if you don't have a right proper quench oil like Parks. Maybe in the future you could repeat your experiment when you are on another project and use Canola and test the results you get.
I think all the seed oils have low cooling rates (too low) in the vicinity of pearlite formation. I know the blood Knife Steel Nerds has discussed that at length.
Canola doesn’t fully quench. Maybe something 1/8 and under but research has been done suggesting you cannot get a full quench. Parks is abundant as hell now. About the same price as canola. No excuse imo but time. Brine if nothing else.
I worked in the tool making industry many years ago and we would case harden low carbon steel in a non sealed box packed with charcoal. We would heat treat tool steels in the same un sealed boxes but packed with cast iron chips to prevent carbon being lost from the steel.
Great Video. As a chief engineer in the gas turbine industry I have designed many heat treated nickel, cobalt, steel, titanium and aluminum based super alloys parts, but I don't normally don't get to see the step-by-step details of how the process works. All beatings in the gas turbine world get case hardened mostly using M50-Nil as the base alloy. Even in ceramic bearings, the inner and outer races are still case hardened steel.
I always heard the term case hardened bolts , but never really knew what it meant other than it was harder , then a couple of years ago I saw some videos where they were case harding gun parts with the same method of heating it with charcoal Great video great explanation . Thanks
I have done my fair share of heat treatment case hardened or other you will never eliminate distortion. It is simply part of the game especially on intricate parts with variations of thickness such as your vice. I would say you were successful in the end. Quenching in water regardless of movement up or down or side to side the rate of change in temperature is extremely fast. Oil drops the rate of change more slowly but you would still experience distortion. I love the fact that you are not shy about trying new process and procedures keep up the good work.
Mike, I totally disagree with your statement. Countless times I used A2 tool steel and had it vacuumed heat treated. On critical pieces double or triple heat treatment would be in order. A2 would basically hold its size and hit the Rockwell required. D2 tool steel ditto.
@@gordhildreth4630 That's fantastic I haven't had that experience myself but I'm glad you are able to old tight tolerances on parts post heat treatment. Myself if I'm looking for a flat square or tight tolerances work I grind the finish dimensions. I must be doing something wrong of course Air hardening steel is going to be much more stable. I can't say I have ever ran any parts that needed to be heat treated multiple times an annealing process perhaps more than once but never heat treatment cycles repeatedly.
I worked as a heat treater for about 12 years in WA state. We used induction, molten salt baths, open flame and endothermic gas electric furnaces with internal quench chambers to case harden. Carbon flows like electricity as it tries to reach a neutral carbon level. In a high carbon environment a low carbon material will absorb carbon. A high carbon steel in a low carbon enviornment will lose carbon. The higher the temperature the faster a material will absorb or shed carbon. In a endothermic gas filled furnace we would generally carborize at 1750 deg. The depth of penetration was determined by the material 1040, 8620 etc. and time at temp. The way of introducing carbon was simple. We simply plumbed in natural gas which burns and the byproduct is carbon. A carbon sensor on the furnace controlled a barrel valve to keep the carbon at a specific level. A product called condersol can be painted on a part to prevent carbon penetration into a area of part. Most large cities have several heat treating shops and a tour can be very informative. Keep in mind some steel such as 1018 will not respond well to case hardening and to prevent scaling you need a protective atmosphere. Thanks for the video. MH
Tried the motor oil for trigger sets in airsoft guns, worked great. Just made sure I did any tuning/shaping of surfaces before I did the hardening. Made a huge difference in the life of the parts.
Thank you very much for the informative video. You have done an excellent job producing it. Your vice looks very nice as well. I have not done any case hardening since high school in the mid 60"s at which time we used kasenit and I understand it is no longer available. I did use 10% sodium carbonate as a catalyst and my final product turned out just as I wanted. I made a couple wear parts for a forming die and solved the problem- thanks. Thanks again and keep up the good work.
👍This channel just keeps getting better and better. Thanks for the excellent explanation! The short technical "lecture" was really well written and explained, and the pace that you narrate is very easy to listen to, without any rambling. Well done! One question - what state are you located?
Thanks for such great content. Living down under with the super high cost of specialty alloys (and pretty much everything else except sunshine) forces us to use our ingenuity which is to our benefit.
Nice work! A have home o traditional Oven, having a pretty high vent,,so I made a mix of backing soda and charcoal,all stuff in a sealed box, and kept it inside that oven for few hours under intense heat ,then from oven right into water.It Was amazing hard,I could scrach glass with...
@@artisanmakes I wasn't speaking to practical realities, only mentioning how the theory metallurgists use categorizes this is a "random walk" type behavior: you can get a linear fit over short times (ie. distances) but accepted mathematical models of it assert that there will always be diminishing returns as you keep going, due to the fact that carbon is diffusing outward as well as inward.
I've used brine quench in a running ultrasonic cleaner. The ultrasonic energy breaks the steam bubbles quite effectively but it also degasses the brine. The effect of salt in quench water is to reduce dissolved gases which nucleates boiling. Also effective on pure water quench which has slightly better specific heat capacity.
Great video! I was considering case hardening for some fixtures at my work, but I found a good spring steel, 5160, for a reasonable cost that worked great for our application. I have not tried heat treating it, because it has been doing great at the C25 it came with. It has not been too bad on tools. 5160 might be good for you or your viewers too. McMaster Carr description "Yield Strength: 97,000 psi Hardness: Rockwell C25 (Hard) Heat Treatable: Yes Max. Hardness After Heat Treatment: Rockwell C63 Specifications Met: ASTM A689 5160 alloy steel handles repeated cycles of stress without breaking just like spring steel. It’s often used to make flat springs"
I recently read a 1925 patent, it said to use sodium bicarbonate. Which could make sense. It breaks down at a low temp to CO2 and sodium carbonate This would fill the chamber with CO2 before it gets red hot. This would displace the air and increase the amount of CO the carbon could produce. It also said to use a large proportion of hard wood sawdust. I assume this would reduce the density of the packing to allow the CO to defuse to the part.
Very well done and packed with well thought out comments. Since you have done a lot of the trial and error to find some of the better home shop techniques, how about putting together a video of the single best home technique with out so many "ifs and maybes" so we can concentrate on one procedure. This comment is not meant to be critical in any way. This video was very informative and well presented. Just asking for a step by step procedure for dummies to follow. I will be watching for your next video no matter what it is. Thank you.
Just a thought on the used motor oil idea, I wonder if used diesel motor oil might be the key because of the amount of soot (carbon) that would be saturated in the oil compared to a much cleaner oil out of a gasoline engine.
Great info. I noticed that your reference material had info from the Pennsylvania Rail Road's Altoona shops which is near my neck of the woods. Very interesting. They did a LOT of their own R&D with metallurgy to try and stay at the forefront of technology, but they held on to steam tech too long while most other RR's were converting to diesel-electric tech. The reason for this was that they worked hand in hand with the coal industry, and they didn't think using oil would pay off. They were wrong and eventually went bankrupt. Of course Air travel and trucking had a lot to do with it as well.
11:43 The use of oil or water must be made considering the steel alloy (Cr, Ni, V, etc) and not the carbon content. Alloy steels can be quenched in oil and non-alloyed steels must be quenched in water. What changes, between one type and another, is the hardenability.
Yes, but this video is not about alloy steel and I purposely exclude talking about it because as you correctly point out we then have to start talking about hardenability and TTT diagrams which I did not want to talk about in this video. This is just for plain carbon steel
I worked in the industry for many years and quenching a material like 4140 or 4340 was done in heated oil or a low temp salt brine. If we were induction hardening we used a heated glycol. A water quench is too much of a shock and would frequently lead to cracking. 1040 and maybe 4130 you could get away with a water quench. Regardless of what the final desired hardness was by the customer each material has a temperature at which its quenched at. The final hardness is determined by the secondary heating/tempering this is where the higher the tempering temp the lower the hardness. 4340 would usually soak at 90 minutes at 1525 deg. Get a 140-180 deg oil quench and have a as quenched hardness of RC (150kg) Rockwell of 55-60. It must have a secondary temper of at a min. 300 deg for 90 minutes or risk cracking from internal stress. When dealing with complex very expensive machined parts you only get one try to get it right.
When your learning there's always teething problems. Back in the 80s I purchased a can of Hardite / barium chloride 20% and potassium ferric cyanide Great for case hardening. Maby there's something in the book of armaments to consider , after all the Holy hand grenade was a winner ch2 v9. Sodium carbonate is a good tip added to quench water .
My shop instructor in the 80's recommended the used motor oil trick - I can't say for certain that it does or does not work, but even if it's a myth, it's one that has been around for a while.
Long ago I would put the steel in the carbonizing pack two or three times before quenching it for a deeper case. For a precision vise we would cycle the parts between an oven and a freezer several times to relieve stress.
Very interesting vid, thanks for sharing! I tried a method of case hardening I found over on the ‘Clickspring’ channel, don’t know if you found that one? He was making historically accurate tools for a project to re-create the Antikythera mechanism. Basic recipe had leather charcoal, flour and salt in it, I’d have to look up the proportions though. He did his in a sealed clay packet / crucible affair, I just used a metal box like you did and got good results on a small hammer head I made after about an hour in an electric kiln at about 900°. I didn’t get all the gal off the metal box before it went in the kiln, so the zinc oxidised and turned into a nice floaty white powder that went everywhere!
Metallurgy and materials science was one of my favorite classes in college. (Aside from the lab reports of course) but we did so much cool stuff with different metals and plastics.
I use Stainless foil and make a pouch I fill with part and charcoal. Bring to the "critical temperature" ..rapid quench in a "brinded" oil. (Salt). Clean off the scale and draw back at half temp/time. 58/62 45c Rc. Every time !!! P.S. 1550 deg for low carb. 1850 high carb.
Back in the 40's and 50's you could buy special case hardening powder. I can't remember exactly how it worked but I believe you packed the part in the powder and hested it in a forge to a soecified temp and time. I think it was some kind of high carbon content powder that would infuse the steel when heated, hardening it to a specific depth depending on how long you keep it in the forge and the amount of powder used. Tubalcain or MrPete222 has some excellent and very informative videos on the case hardening process and how to do it correctly and how to achieve the depth of harening that you want. I would reccomend checking out his videos. He is an infinite source of knowledge concerning everything to do with metalworking , machining and forging.
Most useful content. +1 I will likely experiment with this to case harden some linear rails for a rather large format 3D printer. I am aware, of course, that I will need to pay special attention to warping.
try using a paste of charcoal with flour, let it dry and then use a normal thick clay jacket around everything, obviously just let everything air dry first and slowly rise the jacket's temp. i think clickspring did something like that
Another channel on youtube called Clikspring also made a video on case hardening and how it might have been done in ancient times. He cased his raw material in clay which id imagine might be much cheaper and more expendable to use overall than galvanized steel. He also added salt to his carbon source and used flour as a binder for it. You should look up his video on it.
To get a precise temperature inside the casehardening box, you could use a thermocouple. A good one will have a probe that is resistent to the high temps inside the furnace, and the display should provide an accurate temp reading of the probe.
A tip for making Charcoal powder more uniform would be to use a ball mill. But as those can be expensive you can make one cheaper by getting a rock tumbler/polisher and loading it with ball bearings.
Charcoal is probably soft enough to process in a coffee grinder. Bob Adkins' warning about the fine dust applies to the coffee grinder as well as a ball mill. Use a good dust mask or you'll have black snot for a few days.
Great video, really interesting and succinct. It's too bad you can't harden steel with the irritation you get from Steel suppliers at having to deal with hobbyists... Here in Adelaide, you'd only need 5 minutes of a salesman's eye-rolling and grumbling to turn Mild Steel into hardened 4140
This seems like it was outside your wheel house. Fascinating and an excellent video. I’ve been experimenting with a product called ‘Cherry Red’ out of the US and has worked great for case hardening clamps for my precision vice. I like the idea of using a forge for larger parts. My heat treat oven only goes to 6x6x4 inches.
The carburizing time is in the ballpark of minutes ( let's take half an hour ), as you know. Using oils (which are a hydrocarbon compound and therefore contain carbon) doesn't change physics and does only one thing, when quenching: Bringing you far away from the carburizing temperature in the fastest possible way. Also, with old motor oil: Possibly giving you cancer as birthday present and providing a nice fire hazard, hehe. But of course it is "true" that you can induce a carburization in this way. However, the effect is so small that it is unfortunately not measurable. Let's call it "steel homeopathy", or?:))) Anyway, thanks for the video and showing your working solution(!!!) off. Great work! Editaddendum: Incidentally, I also recommend that you read the German Wikipedia (under the term "Aufkohlen") and have it translated. The various procedures are explained in more detail there. Not going into depth, of course, but one thing is clear: carburizing in carburizing granules unfortunately always has poor reproducibility. I know this is difficult for reproducibility at home, but maybe a process in a vacuum furnace or with protective gas (CO2 + Nitrogen, which could carburize at the same time) would be better? Of course, this requires a much more complicated oven, with separate heating and combustion chambers. ... but as is well known, the bride always has to be inventive before the wedding. Please don't even think about cyanide-based liquid solutions (potassium cyanide and potassium cyanate): You're guaranteed to kill yourself and anyway, because of the lack of environmental compatibility and economic efficiency, this is not a good idea. P.S.: "A lot of people say" ... and that they say different things ... is because this method does NOT lead to reproducible results (In the sense that you have a considerable tolerance when it comes to "carbon inoculation").
While this is all true, the home workshop is very rarely concerned with getting an exact depth of case. The added complication of a more sophisticated process is very unlikely to have any noticeable payback. Pack hardening in a carbon rich medium has been done for centuries, and gives results that are good enough for all but the most exacting or mass industrial needs.
works for me thankyou. I want to have a go at making a sledgehammer in wrought or mild steel and case hardening it for knocking in fence posts and the sodium carbonate and charcoal method looks ideal
CHARCOAL: When you were crushing the charcoal, I thought of a suggestion. You can get a jar of powdered charcoal, sold as an art supply. It’s very fine, like cornstarch, and might perform better or faster, given that there would be less air. Of course, anyone can crush charcoal more finely, but the powder is still pretty cheap, homogeneous, and ready to use. I’m only speculating, but I also wonder if the charcoal source matters at all. It’s not likely, but some charcoals have different structures, and there might be a “best” charcoal. Bamboo has more silica in it, which might matter, for instance. If anyone has thoughts on this, I’d love to hear them.
I was just thinking that maybe the sodium carbonate only works in the enclosed environment. Perhaps in open air it gets neutralised by the oxygen. Still worth a try. None of it is expensive.
Quenching in water or oil is used to obtain different surface hardness. Also, for some steel grades, quenching is suitable only in a certain environment, quenching in an unsuitable environment leads to a shortage of hardness or internal defects of the workpiece. Blanks from low-carbon steel can be strengthened by the method of riveting.
Some years algo i try this of hardering stell soft;and i use wood charcoal and Bariun carbonate..60..plus..40..hardering some knifs whit good end..and quench whit motor oil.
That is why i am here. I often heat treat with oil. Keeps my drill steel sharp longer. But i have learned to quick dip my steel. I l8ke it almost white hot. Time between dipping 8s put out the fire than dip. Pull out when fire starts put out dip untill no more fire starts than let cool. Touch up edge if you want with grinder. Want to know more about carbon.
The space in the sealed box won't matter, as the oxygen is soon exhausted. We used large cast iron boxes, filled with short pieces of charcoal sticks, similar to what we used for drawing in art class at school, with the machined components completely surrounded by charcoal. They weren't filled to the brim, plus lots of air between the sticks. The cast iron lids were sealed with fire clay, and the muffle furnace heated (from memory, early 70's) to 900 C. The furnace ran for hours, then allowed to cool overnight. Subsequently reheated in electric furnaces and quenched in whale oil. I can't recall if the components were then tempered, but from your video comments, probably. Only the 'skin' would be hard. Then ground to size.
If you do any more experiments, I'd try putting a chunk of dry ice at the bottom of the container, below the charcoal, then seal with the morter, leaving a small gap to allow off-gassing, until the dry ice finishes sublimating, then seal the gap. Theoretically, it would displace any oxygen in the charcoal with carbon dioxide, which could help with the chemical reaction.
No, CO2 doesn't help there, you want CO, which comes from the oxygen reacting with the carbon. Steel in high temperature then robs the carbon and the oxygen recycles. CO2 is quite a stable molecule which doesn't break so easily.
Carburizing is also called "cementing", There are several different ways to get the hard surface, depending on the exact properties and costs that you want. The thick side armor on battleships was face-hardened, usually having a thin carburized and very hard surface layer and a somewhat softer, but very thick, hardened layer behind the surface, with a very thick soft layer making up the rest of the armor as a shock absorber and tough mounting layer.
You could probably get a result like that using a steel with a moderate carbon content and low hardenability like 1045 which you subsequently case harden and quench. The thick cross section would garuntee a soft core, and the moderate carbon content would give a tough layer under the hard skin.
Very interesting video. If you need in the future to show better the difference from the hardened and sof layer grind and polish the surface, the smoother is better and then put on a bit of acid, the martensite will react differently from the austenite
