Altering high tensile bolt steel composition

[markx]

I'm planning a carburizing/case hardening experiment to alter high tensile (12.9 grade) bolt steel composition in the hopes of making it more hardenable and wear resistant. I need a cold working/die type of steel that could be hardened to extremes. For sure I could order a specific grade, but where's the fun in that

I noticed that 12.9 grade bolt steel supposedly has somewhat similar composition to O6 or some of the A group alloys used for cold working tools....that is if one could raise the carbon content in the bolt steel. In it's original form the 12.9 grade has quite low carbon content (about 0,3% assessable by spark test) and although it is tough as hell one can not really harden it any further with heat treatment techniques. Although it is possible to make it much more brittle by quenching.

I'm planning on running an accelerated carburizing treatment on a 9mm calibrating button that I machined out of the said bolt and see if I can get a really hard skin to form on the thing.
The process shall take about 7-8 hours out of which 5 hours are spent at 870C and the part is submersed in molten sodium carbonate/borax and carbon mixture.
I've used the molten carbonate approach previously for treating of mild steel and it really seems to accelerate the diffusion rate of carbon as opposed to just heating in the presence of a carbon source. Never tried it on alloy steel though....so it shall be interesting to see what happens. If it works at the same rate for alloy steel then I may expect the plug to carburize more than halfway through at the end of the cycle.

[lbender]

Have you considered nitriding instead of carburizing? Nitrogen has higher diffusivity in iron compared to carbon, so a through hardened part is more of a possibility.

[markx]

Have you considered nitriding instead of carburizing? Nitrogen has higher diffusivity in iron compared to carbon, so a through hardened part is more of a possibility.

Have not tried it yet.....but a salt mix and urea should be somewhat of a starting point. I would not like to deal with cyanide bearing compositions.

[rickyb]

I've seen something similar a long time ago on 52100 bearing steel. Carburize a through hardening bearing steel? Yes and long term service data did show improved wear characteristics.

Any concerns with fatigue or corrosive embrittlement? If so, your plan is not the way to go.

[markx]

She is heating up to temperature. The plug is contained in the vessel made from rectangular tube. I added a piece of wire to the tang so I can lift the piece out from the molten pack afterwards and quench it.

[markx]

Arrrgh....that was a bust!! The Great Architect was not supportive of my cause today. First of all the lid on the pack got stuck and by the time I managed to pry it open the salt mix had solidified, so I could not get the parts out to quench in oil. Had to soak the lot in hot water before they could be retrieved. Reheated in furnace and performed the oil quench, but surface layer had lost carbon by that time and did not harden. I repeated the quench into water by heating on a butane burner, but no luck. Then I machined the soft surface off and beneath it was a mediocre hardened layer, but it could be somewhat touched with a file, so no good.
Still the carburized sample was a bit harder than the original bolt steel when quenched into water.....also spark test showed an increase in carbon content, but it was a failed attempt in general. The carbon had not diffused and what little was there had obviously not traveled far in the material. Absolute minimal effect compared to the case that I have achieved with mild steel by that method. I do think that perhaps the cromium content in the bolt steel hindered the carbon diffusion. Chromium has that effect in stainless, creating a carbon diffusion barrier that blocks a carbon case from developing. Perhaps it also played a part in this failure.
I'll probably repeat the test and climb up to 950C for temperature....see if that improves things.

[754]

Or take to a heat treater that knows what to do and what material to use. Its pretty hard to discover what was learned over centuries, in a few tries at home with limited equipment.

[markx]

I'm doing this solely for fun and personal learning curve If I needed a specific part with guaranteed properties and within a tight schedule then I would certainly use someone's services. I find it very captivating to experiment and learn new stuff from both failures and successes. It is quite amazing how much can be accomplished with limited resources and tools if one has the motivation and interest...

[dian]

what are you doing exactly? mix natron and borax, heat it and add carbon to it? whats the melting temperature and what kind of carbon? what results did you achieve with mild steel? interestingly i had the impression recently that 12.9 screw harden quite well, although i didnt measure it (i made some tranfer screws).

btw, my understanding is that you cant just nitride any steel very well, thats what nitriding steels are for. if theres really enough cr and mo in 12.9 screws is hard to say.

[markx]

what are you doing exactly? mix natron and borax, heat it and add carbon to it? whats the melting temperature and what kind of carbon? what results did you achieve with mild steel? interestingly i had the impression recently that 12.9 screw harden quite well, although i didnt measure it (i made some tranfer screws).

btw, my understanding is that you cant just nitride any steel very well, thats what nitriding steels are for. if theres really enough cr and mo in 12.9 screws is hard to say.

https://bbs.homeshopmachinist.net/fo...nal-tap-making

I guess different alloys are used for producing the bolts, hence they may also show different reaction to further heat treatment. My general experience has so far shown that 10.9 are harder, can be further heat treated to harden even more, but become very brittle as a result. Also the 10.9 variety is very nasty to machine.....workhardens in a horrible way. 12.9 machines just beautifully, is tougher and does not substantially increase hardness wise upon further heat treatment, but also becomes more brittle.

[rickyb]

Chrome content of a 12.9 is about 0.4%, 400 stainless is~10% so you are not comparing apples to apples on carbon diffusion. How about trying something simple like wrapping the rod in paper then stainless foil? Carbon infusion and no decarb. Also something like Kasenit.

[nickel-city-fab]

Maybe better luck with sodium bicarbonate (baking soda) mixed with charcoal carbon? I have heard of others using this mixture successfully. And yes, do not allow air. Cover it in clay or use stainless foil.

[markx]

Chrome content of a 12.9 is about 0.4%, 400 stainless is~10% so you are not comparing apples to apples on carbon diffusion. How about trying something simple like wrapping the rod in paper then stainless foil? Carbon infusion and no decarb. Also something like Kasenit.

For sure, but I have no confirmation at which chromium content the diffusion barrier effect treshold might lie....could be at 0,1% but might be at 5% or at some other point. So it was purely a speculative statement that the alloy's chrome content might have been hindering the carbon diffusion.

It really does not matter if one uses sodium bicarbonate or sodium carbonate for the pack composition filler. Bicarbonate decomposes at around 150C with the release of water vapour and carbon dioxide, leaving sodium carbonate behind as a result.

The container was sealed with a machined lid....too tight fitting as it turned out. That caused it to become stuck and I could not get the parts out quickly. Also the point of the molten salt is to cover the part and hinder contact with excessive oxygen.

[markx]

Sure as heck the mild steel container that I performed the process in has carburized....almost straight through to the outside surface (that was a 2+ mm wall tickness tube) I cut it open and the bottom part of the inside is clearly bright which usually signals a rich case formation with the molten carbonate approach. I sliced some strips from the bottom part of the rectangular tube and bent one of them in the annealed state. The other strip was heat treated and quenched into water. Upon bending it snapped clean in two without deforming. The spark test also clearly indicates a carbon enriched steel. File would not touch the inside surface of the heat treated strip and only slightly scored the outside wall surface. So the process does work, but it seems that alloy steels (or at least the 12.9 bolt alloy that I used) do not carburize as successfully as simple mild construction steel....

[dian]

why dont you try some steel with known composition?