Cryogenic treatment of drill bits: tested 2X lifetime and microstructure analysis

today on Applied Science we're gonna take a look at the process of improving drill bits by dipping them in liquid nitrogen a process known as cryogenic treatment so I'm going to show you how I did this in the shop here and then also how I tested it and then we'll also use the scanning electron microscope to take a look at the microstructure to try to figure out what's actually happening during this cryogenic treatment process but the video actually starts in an unusual location actually in the driveway I've owned this DeLorean for quite a few years and one of its many weird quirks and features is the doors are opened by a torsion bar there's a gas strut to hold the door open but the thing that actually starts the door opening is this torsion bar which is said to be cryogenically treated and the reason that's needed is because this bar has to go through a 90 degree twist every time you open the door and it has to withstand you know many thousands of these cycles so I started researching why you would ever want to cryogenically treat a piece of steel and what you actually get out of it does it actually make it stronger does it actually make it tougher like what are you actually getting out of it so instead of experimenting on my car I figured a good way to test this out would be to use drill bits so the idea is buy a whole bunch of drill bits all the same these are a quarter inch diameter stub length from McMaster and then cryo treat half of them and leave half of them untreated and just start drilling a bunch of holes and count up how many holes we get for each bit type before they break and the results are truly staggering so the untreated bits with this machining setup I averaged 32 holes per bit before it failed and then with the cryo treated bits I got 60 holes before the drill that failed so a huge difference I thought there might have been a systemic problem in my testing or maybe I was doing something else but as it turns out I'll link to some research papers that have very similar performance gains listed to what I got almost a doubling and drill bit life just with this cryo treatment process I should add that this is a pretty intense machine instead of so we're running at a hundred and thirty one feet per minute which is like forty meters per minute and we're drilling into mild steel and I mentioned the diameter is about a quarter inch or six millimeters and the depth of the hole is three diameters but initially I started off with set up 30 meters per minute than being the only change 0.1 millimeters per Rev or point 0:04 of an inch per Rev but I actually got all of these holes out of this single bit and it was still cutting at the end so the trick is that the surface speed makes a big difference and I just can't test a sort of more nor more normal surface speed because I don't have this much steel to go through so if I were to test this one with cryo and regular I would need you know yeah a four by eight foot of Steel the sheet of steel to test all this out so anyway so I went to a much more aggressive machining setup so that the bits would in fact fail faster the failure criteria that I used for this was done by sound and I realized this is somewhat subjective but when I started testing I found out that measuring whether a drill bit is really at its end of life or not is actually pretty subjective itself right I mean you can always keep pushing it until the thing actually just physically breaks into two pieces but that's kind of a little too aggressive for what I'm doing so based on my experience machining things I would listen to the sound of the operation and any sort of cracking or exterior spindle you know the lowering of rpm because of the load was considered a failure and I'll play some examples for it so you can hear what a failure sounds like [Music] I realized halfway through that I should have blinded myself so what I was set out doing was just going back and forth so untreated cryo untreated cryo and so on and then halfway through I realized why that was dumb since it's a subjective failure criteria I can't be aware of what it is or I'm contaminating it so then halfway through I marked the bottom of each bit and then put them into the bucket and then opens the top of the bucket so I couldn't see the bottoms of it took a bit out and then tested it and then continued on with the results so I did not know in fact which bit was which after this point it didn't make any difference I mean the difference is they're so huge for example between this untreated bit getting 23 holes in this cryo bit getting 84 holes it definitely was not a close call let's just say pretty dramatic difference let's talk about the cryo process itself it's actually pretty easy you just take the bits cool them down slowly to liquid nitrogen temperatures hold them there for about 20 or 30 hours and then warm them up slowly and that's it couldn't be much easier so lucky me I actually have a cryocooler that generates liquid nitrogen on demand so I took the bits and put them into a little foil pack with a temperature probe and put them into a vacuum flask and then put this cryocooler on top and used a very act to power up the cryocooler slowly so eventually this cryocooler uses about a hundred and fifty watts and will drip liquid nitrogen off the end of it and down into the flask I have a video on my liquid nitrogen generator if you want more details on this if you don't have access to a cryocooler you know heaven forbid and you're starting with a basically a tank of liquid nitrogen what you want to do is drip it slowly into the thermos so that you don't thermally shock the steel I get the feeling that this is actually not a huge concern you could probably just pour it in kind of slowly so that the you know the liquid nitrogen cools the thermos down a bit and then pour in a little bit more later or whatever I didn't experiment with trying to break this the steel due to thermal shock but all of the information on the internet claims you have to cool it down and warm it up slow and I'm using this packs instruments thermal logger which works really well except at liquid nitrogen temperatures it reads a little low so you can see on the graph here that my cooldown period was about an hour and that's correct but the ultimate temperature couldn't possibly have been negative 240 because that's much colder than liquid nitrogen it should have been about negative 200 but anyway it shows kind of the ramp down on the ramp up that that's realistic also one of the research papers that I linked to in the description claims that you have to temper the steel after the cryo treatment process so after it comes out of the cold and comes up to room temperature you're supposed to bring it up to 3 or 400 C and then back to room temperature and then back up to 3 or 400 see a few times to temper it another paper that I found didn't mention this at all and I didn't do it either I don't really see the point of that because tempering steel is meant to convert some of this very hard substance to a less hard substance within the steel and the whole point of the cryo treatment is to convert everything to this completely hard state right so I don't really understand even why that would work but since we're talking about steel structures let's talk about what actually is happening in this cryo process recall that when you want to harden a piece of steel provided it has enough carbon content about it over point 40.5% the way you do it is heat up the piece of steel to red heat and then quickly crunch it and rapidly cooling it is what causes this crystalline structure to form that's really hard it's not very tough it's actually very brittle but it is very hard and I have a whole video on steel hardening if you're interested in in that so this chart here shows the process happening you basically start off up here with a red-hot piece of steel and if you cool it down really quickly by dunking it in water or oil for example you end up with this really hard crystalline structure called martensite that's what the M means and the graph shows martensite starting to form at about 220 degrees C and they claim ninety percent will be converted to martensite at you know a hundred and twenty degrees C about it but the graph actually keeps going and so the first sort of you know obvious attempt at understanding this cryo treatment is that well it's colder and so for only getting up to 90% converted at 110 degrees C or 120 C if we're down at negative 200 degrees C we've got to be closer to a hundred percent because it's you know it's going up as we go down and so you know at first it seems weird cuz this drill bit was made you know months or years ago and it's been sitting on the shelf and then it was shipped to me and then I put it in the liquid nitrogen it seems like that length of time would sort of erase any ability to affect its heat treatment but actually as it turns out no it's still changing that the steel is still able to be modified by temperature and you can see on the graph the time is actually logarithmic so we could be out at you know 10 to the 8th seconds or whatever and you can still get it colder and still convert more of the structure to martensite so basically what's happening is we're just finishing off the heat treatment even after the steel has been tempered which is interesting so remote so tempering is the process of converting some of this martensite to the slightly softer form called pearlite and the reason you'd want to do that is so your drill bits don't shatter the first time you use them you want this thing to be really hard but also it needs to give enough so that if this thing hits a little you know an uneven spot on the surface it doesn't just chip the tip off and deciding sort of how tough and how hard the material needs to be is a good trade-off that tool makers have been doing for ages but the trick is that if you really do want to convert everything possible to martensite and then you give it a certain amount of temper you can then later go back and still convert more to martensite and that's what this cryo process is doing so you know theory is great at all but what's even better is seeing the change with our own eyes so what I wanted to do is take one of the treated drill bits and one of the untreated drill bits and make one of those really cool crystallographic images that you've seen maybe from metallurgical textbooks or whatever and I've always wondered how to make these things I you know it's not obvious right you can't just put a piece of metal under the microscope and see this beautiful crystal structure there has to be a couple things that happen one you have to polish the surface of the metal to an absolutely great degree because the structures that we're looking at are only tens or hundreds of microns big so if the surface is scratched even with you know tens of microns level of scratches you won't really see any of the structure so that's one step then you have to etch the metal with a special agent so that only the certain regions are etched that you want to see right like you need contrast if the etching just applied everything to the same you know applied equally to the whole surface the whole surface would just move down you wouldn't see anything so we need an etchant that preferentially attacks a certain structures over others and then we have to put it into a microscope where the light comes from the top so most biological microscopes like this one work by shooting the light up from the bottom or occasionally you can kind of aim it in from the side like this but it doesn't work for getting these really nice crystallographic images because the light has to be coming exactly on axis with the viewing angle or otherwise this this effect doesn't work where you're looking at the very tiny differences in surface quality so I hacked together this really cheesy setup it's basically just a piece of plastic that I actually used in another experiment my Raman spectrometer and it's just a it's just a piece of plastic with a beam splitter here and I put this on and shine some light in here and then put the camera on top and was actually able to get a not bad result however what we're looking at here is a piece of low-carbon steel that has been normalized or heated up and then cooled down at a relatively slow rate so the crystals are big the magnification here is you know on the order of one or two hundred X and these crystals are relatively huge for the crystals in the drill bits the size is down in the tens of microns and I just can't image it with this microscope the light microscope with my setup is just not powerful enough to see it but lucky for us have a scanning electron microscope that can go too much higher magnifications the Special Agent that I'm using is called night owl for nitrogen alcohol and it's made by starting off with methanol or ethanol and adding to it about five percent by mass nitric acid and this one is actually fairly dangerous nitric acid is a strong oxidizer and you're basically adding it right to a bunch of fuel the alcohol so it's very important that we don't exceed 5% by weight nitric acid there are some confirmed stories on the internet of really bad explosions that happened I guess because someone capped the bottle and it out gassed or maybe the thing just exploded outright but anyway if you're mixing this up be very aware that this is in fact a potentially explosive mixture the etching process is very quick I used about two minutes to etch this tool steel and that seemed to work just fine for softer Steel's like low-carbon steel even five or ten seconds in this 5% night-owl is enough so I made my samples by putting them into a little tub with some epoxy sanding it down and then going up through the grits all the way to these really fine polishing papers it's actually a cloth coated with really fine abrasive like 1/2 micron abrasive or something and you end up with this mirror finish crazy polished kind of thing it doesn't have to be super flat in fact mine or not because the epoxy is so much softer than the metal itself it kind of mounds up a little bit so the surface isn't crazy flat but it is locally flat it's very smooth and I loaded these up into the scanning electron microscope and lo and behold there is a difference again I thought the difference was so shocking that it something might have gone wrong and so if you look carefully you can see there's these grain structures pretty well defined in the untreated bit and then in the treated bit it's completely flat like there's no surface features that look like grain boundaries the white dots are very likely carbide precipitates so I mentioned that you need at least 1/2 percent carbon in the steel to harden it and a lot of that carbon forms these little hard particles that kind of hold the matrix together right it's kind of like concrete and cement the strength of the material comes from the little rocks in it and the cement itself is okay and strength but it's really the rocks that give it the ability to be abrasion resistant same thing with this steel these carbide precipitates are in for keeping the steel very strong additionally these drill bits are made from high speed steel which has molybdenum and chromium in there which also forms carbides like chromium carbide molybdenum carbide and those are especially good those are like really hard rocks to really hold the thing together so anyway the carbide seem like they're about the same treated versus untreated but certainly this grain boundary missing thing is weird and again I went online to see if this result is known and actually did find a couple papers that looked kind of like my results cryo treating it basically you know the martensite basically forms into itself like the the boundary is almost eliminated because the conversion to martensite is more complete at least that's my understanding of this so this seems like a no-brainer process like why wouldn't you just cryo treat all pieces of steel well remember that there is no free lunch so if we're making the steel harder or more wear resistant it's very likely that we're making it more brittle as well the change is not huge I mean we're not making it twice as hard or anything so we wouldn't expect a huge change in fracture toughness or brittleness but but it will be there and depending what kind of a tool you're making you may not really want the absolute hardest tool like with the drill bit for example this is actually a good one you know that people are gonna put this in a hand drill and you know lean on it back and forth like this and they might snap the drill off so you want it to have a little bit of give you don't want it to be as hard as you can make it and it's possible that the manufacturers have sort of optimized this drill bit to be flexible enough so that people don't break them when they're using them but hard enough so that it wears well and you know gives overall good performance if you kind of know what you're doing and you have a CNC machine or a machine shop and you know you're gonna use the drill bit in a very controlled way over and over yeah go for the hardest one in fact they make solid carbide drills that are basically that also if you search around you'll find plenty of manufacturers are selling cryo treated drill bits as like their selling point but generally these are coming from companies that don't make tools for professionals they're generally selling to no the home user market and the banner of cryo treated is kind of an exciting thing but you know the performance gained by it is maybe not as important to the home user so anyway a cryogenic treatment of Steel is something that I've wondered about for years and so I actually have a great time making these videos because I get to answer questions that I've had for a long time and hopefully it answers questions that you've had for a long time too so I hope you found that interesting and I will see you next time bye

Channel: Applied Science

Views: 1,048,506

Rating: 4.9185991 out of 5

Keywords: cryo, cryo treatment, treatment, hardening, steel, cryogenic, cryogenic treatment, cold, liquid nitrogen, applied science, ben krasnow, drills, hss, m2 steel, cold hardening, deep cryogenic, strength, wear resistance, machining, drilling, high speed steel

Id: hAxi5YXTjEk

Length: 17min 43sec (1063 seconds)

Published: Tue Sep 03 2019

Reddit Comments

[u/COMPUTER-MAN]

This channel is great. I love that he made an EL display for the Apollo DSKY

[u/tinman82]

Soooooo I need to dip my knives in liquid nitrogen? An oldschool vacuum thermos works a dewer right?

[u/Dark_Alchemist]

LOL, I was just watching that.