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Intro to heat treatment of steel (hardening and tempering)
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00:00:00.030 hey everyone I wanted to talk about heat 00:00:02.78000:00:02.790 treating steel a little bit in this 00:00:04.34000:00:04.350 video so if you search for this topic on 00:00:06.92000:00:06.930 the internet you'll find there's a lot 00:00:08.48000:00:08.490 of theoretical and background 00:00:10.00900:00:10.019 information and then also a lot of 00:00:12.22000:00:12.230 practical information without much 00:00:14.36000:00:14.370 explanation of why the things are done 00:00:16.35900:00:16.369 so I'm hoping to bridge the gap a little 00:00:18.65000:00:18.660 bit in this video when we talk about 00:00:21.71000:00:21.720 heat treating steel we're mostly 00:00:23.99000:00:24.000 interested in increasing the materials 00:00:26.33000:00:26.340 strength by exposing it to different 00:00:28.40000:00:28.410 temperatures in sort of a prescribed way 00:00:30.82000:00:30.830 but first a note about what strength 00:00:33.31900:00:33.329 actually is there's a few different 00:00:35.42000:00:35.430 quantities to keep in mind here the main 00:00:37.94000:00:37.950 ones being stiffness and strength so 00:00:41.15000:00:41.160 here's a graph here and it says strain 00:00:43.58000:00:43.590 on the bottom and stress on the vertical 00:00:46.27900:00:46.289 axis and what we're showing here is how 00:00:49.52000:00:49.530 hard we're pulling on the material here 00:00:51.61900:00:51.629 the stress is the force divided by the 00:00:53.56900:00:53.579 area and the strain is how far the 00:00:56.09000:00:56.100 material is moved so this is the the 00:00:58.22000:00:58.230 Delta how much the material has actually 00:01:01.09000:01:01.100 elongated or contracted divided by its 00:01:04.49000:01:04.500 total length so this graph takes into 00:01:06.77000:01:06.780 account whatever size material you might 00:01:08.66000:01:08.670 have so for steels and most ductile 00:01:12.67900:01:12.689 materials when we start yanking on the 00:01:15.32000:01:15.330 material that will deform a little bit 00:01:17.12000:01:17.130 and if we don't yank too hard we can let 00:01:19.64000:01:19.650 go and the material wills elastically 00:01:22.67000:01:22.680 returned back to its original shape so 00:01:25.92900:01:25.939 if we take this steel bar here and I 00:01:28.78900:01:28.799 flex it a little bit it snaps back to 00:01:31.99900:01:32.009 its straight shape with no problem and 00:01:34.16000:01:34.170 so what's happening here on the graph as 00:01:35.89900:01:35.909 I'm taking it up to about this point and 00:01:37.85000:01:37.860 letting it back down instead if I take 00:01:42.44000:01:42.450 this coat hanger and stretch it a little 00:01:44.60000:01:44.610 bit now when I let go the curve actually 00:01:46.85000:01:46.860 stays in the coat hanger so on the graph 00:01:49.96900:01:49.979 what's happened is I've stressed it so 00:01:52.39900:01:52.409 the stress has gone up and eventually 00:01:54.26000:01:54.270 we've gone so far that it's gone past 00:01:56.53900:01:56.549 its yield point and now we're in a 00:01:59.14900:01:59.159 region called the plastic region so we 00:02:01.45900:02:01.469 put a plastic deformation in this coat 00:02:03.38000:02:03.390 hanger by bending it like this what's 00:02:07.31000:02:07.320 important to note is that we can have a 00:02:08.83900:02:08.849 very stiff material that's not very 00:02:10.88000:02:10.890 strong or we could have a very 00:02:13.13000:02:13.140 material that's not very stiff the two 00:02:15.41000:02:15.420 quantities are actually not dependent on 00:02:18.32000:02:18.330 each other so let's pretend this line 00:02:21.55900:02:21.569 represented steel aluminum on this graph 00:02:24.41000:02:24.420 might look something more like this and 00:02:29.65000:02:29.660 there's a lot to talk about with this 00:02:32.75000:02:32.760 graph but I'm just going to do sort of a 00:02:34.40000:02:34.410 general kind of overview but 00:02:36.14000:02:36.150 hypothetically let's say we had an 00:02:38.03000:02:38.040 aluminum alloy that looked like this its 00:02:40.91000:02:40.920 strength could actually be equal to the 00:02:42.80000:02:42.810 steel now it's not going to be as stiff 00:02:44.84000:02:44.850 because the stiffness is actually a 00:02:46.49000:02:46.500 material property and we can't easily 00:02:48.83000:02:48.840 change the stiffness through heat 00:02:50.47900:02:50.489 treatment we would probably have to use 00:02:52.31000:02:52.320 a different material if we needed a 00:02:53.87000:02:53.880 different stiffness but if we had this 00:02:57.50000:02:57.510 this great aluminum alloy we could say 00:02:59.75000:02:59.760 well it's just as strong as steel 00:03:01.49000:03:01.500 because the stress-strain point where 00:03:04.52000:03:04.530 the material gives out and doesn't 00:03:06.19900:03:06.209 spring back anymore is actually at the 00:03:07.88000:03:07.890 same stress however the material has 00:03:11.72000:03:11.730 moved more in that in that loading 00:03:14.99000:03:15.000 because it's not as stiff the x over 00:03:18.80000:03:18.810 here indicates the point of breakage and 00:03:20.96000:03:20.970 so if we keep pulling this material 00:03:23.42000:03:23.430 farther and farther eventually we're 00:03:24.94900:03:24.959 into this plastic region where we can 00:03:27.31900:03:27.329 change the shape of the material without 00:03:29.68000:03:29.690 putting in all that much additional 00:03:31.67000:03:31.680 stress and then eventually the material 00:03:34.00900:03:34.019 gives out and it breaks so when we heat 00:03:37.52000:03:37.530 treat steel we're actually staying on 00:03:40.25000:03:40.260 this same slope here because we can't 00:03:42.53000:03:42.540 change the stiffness of it but what we 00:03:44.50900:03:44.519 can do is move the yield point up 00:03:46.61000:03:46.620 through heat treatment so let's just say 00:03:49.61000:03:49.620 we had a material that looked like this 00:03:53.97900:03:53.989 now this material is just as stiff as 00:03:57.83000:03:57.840 the original one it's still steel but it 00:04:00.80000:04:00.810 has a much higher yield point so we say 00:04:03.35000:04:03.360 that it's stronger one of the downsides 00:04:05.93000:04:05.940 though is that this line doesn't go over 00:04:08.63000:04:08.640 into the plastic region as far so what 00:04:10.61000:04:10.620 happens with this material is we load it 00:04:12.47000:04:12.480 and it's still elastic it's still 00:04:14.33000:04:14.340 elastic and then there's a tiny amount 00:04:15.89000:04:15.900 of plastic deformation but it breaks 00:04:18.17000:04:18.180 right away this is characteristic of 00:04:21.40900:04:21.419 extremely hard steels and it's also 00:04:24.23000:04:24.240 characteristic of 00:04:25.87900:04:25.889 brutal materials like glass so if we 00:04:28.67000:04:28.680 took a piece of glass just like a window 00:04:31.48900:04:31.499 and flexed it we could let go and it 00:04:34.07000:04:34.080 would snap back to its you know flat 00:04:35.95900:04:35.969 shape but eventually if we flexed it too 00:04:38.14900:04:38.159 far it would just break all of a sudden 00:04:39.89000:04:39.900 without really much warning and we can't 00:04:42.43900:04:42.449 take a piece of glass and bend it and 00:04:44.51000:04:44.520 then let go and expect it to retain that 00:04:46.87900:04:46.889 shape so glass is a brittle material 00:04:48.89000:04:48.900 because it doesn't have this plastic 00:04:50.89900:04:50.909 region you'll also note that it says 00:04:53.98900:04:53.999 maximum tensile strength here instead of 00:04:56.92900:04:56.939 here so what's the deal with that like 00:04:58.99900:04:59.009 why why do we actually count this as the 00:05:00.74000:05:00.750 material strength the reason is that 00:05:03.73000:05:03.740 let's say we were building like an 00:05:05.60000:05:05.610 airplane part or something that you 00:05:07.07000:05:07.080 wanted to support a load in an important 00:05:10.15900:05:10.169 way if the material is in this plastic 00:05:12.49900:05:12.509 region the part has deformed enough 00:05:14.51000:05:14.520 where it might be causing problems so 00:05:16.61000:05:16.620 let's say this or an airplane landing 00:05:18.05000:05:18.060 gear if you're up in this region over 00:05:19.93900:05:19.949 here the landing gear is not going to be 00:05:21.37900:05:21.389 the same shape anymore so it's true that 00:05:23.80900:05:23.819 you might get a little bit of additional 00:05:25.57900:05:25.589 strength out of the material but you 00:05:28.15900:05:28.169 really can't count on that part being 00:05:29.77900:05:29.789 sound anymore so for engineering this is 00:05:32.74900:05:32.759 the point where we say the material has 00:05:34.67000:05:34.680 failed it's yielded so the weight a 00:05:38.51000:05:38.520 hardened steel is to heat it up until 00:05:40.36900:05:40.379 it's glowing red and then very quickly 00:05:42.67900:05:42.689 reduce the temperature by plunging it in 00:05:45.52900:05:45.539 water or oil typically and what happens 00:05:48.55900:05:48.569 this is the crystalline structure inside 00:05:51.01900:05:51.029 the steel changes so it's very different 00:05:53.83900:05:53.849 from letting the steel cool down slowly 00:05:55.63900:05:55.649 and that rapid cooling is actually what 00:05:58.01000:05:58.020 causes us to make the graph that looks 00:05:59.95900:05:59.969 like this instead of like this but we 00:06:04.45900:06:04.469 have a problem 00:06:05.14900:06:05.159 I just said that this is behavior is 00:06:07.55000:06:07.560 like glass where you load the material 00:06:09.49900:06:09.509 and then suddenly it breaks really 00:06:11.32900:06:11.339 without much warning and we don't really 00:06:13.51900:06:13.529 like that behavior in very many 00:06:15.76900:06:15.779 materials and another problem is that 00:06:18.26000:06:18.270 the really freshly hardened steel like 00:06:21.20000:06:21.210 if you heat the steel up dump it in 00:06:23.45000:06:23.460 water take it out it's so incredibly 00:06:25.82000:06:25.830 hard and brittle that you can break it 00:06:29.05900:06:29.069 very easily even with your hands at the 00:06:30.74000:06:30.750 steel if the piece is small enough so 00:06:33.07000:06:33.080 typically all hardening operations are 00:06:35.54000:06:35.550 followed by a tempering operation and 00:06:37.67000:06:37.680 the tempering operation actually 00:06:39.74000:06:39.750 lowers the strength of the material but 00:06:42.98000:06:42.990 it increases the toughness so there's a 00:06:44.99000:06:45.000 very distinct trade-off there and the 00:06:48.32000:06:48.330 tempering process can be tailored to 00:06:49.91000:06:49.920 give us any sort of a a strength versus 00:06:53.24000:06:53.250 toughness trade-off so for example let's 00:06:56.18000:06:56.190 say we tempered it so that we had a 00:06:57.68000:06:57.690 material that looked like this instead 00:06:59.57000:06:59.580 of going all the way to full hardness we 00:07:01.97000:07:01.980 could temper the material and maybe we'd 00:07:06.20000:07:06.210 end up with something like that so now 00:07:08.12000:07:08.130 we've got all this extra room here in 00:07:09.80000:07:09.810 the plastic region and it's not quite as 00:07:12.86000:07:12.870 strong as the as the full hard but the 00:07:15.80000:07:15.810 tempered steel is much much more easy to 00:07:18.08000:07:18.090 use in an engineering application 00:07:19.97000:07:19.980 because it's not like glass it's more 00:07:21.77000:07:21.780 like a normal metal so to test this out 00:07:26.06000:07:26.070 I bought some w1 steel this is an eighth 00:07:30.35000:07:30.360 of an inch in diameter and w1 means 00:07:33.20000:07:33.210 water hardening so this steel is meant 00:07:35.24000:07:35.250 to be heated up and then tossed in water 00:07:37.61000:07:37.620 to quench it to cool it down and harden 00:07:40.04000:07:40.050 it and then you can temper it to give 00:07:42.86000:07:42.870 you any sort of a curve a desired 00:07:45.56000:07:45.570 toughness and strength and to test it I 00:07:48.85000:07:48.860 came up with this little test jig here 00:07:51.40900:07:51.419 so that I could load the samples in 00:07:53.54000:07:53.550 bending and carefully apply more load by 00:07:56.65900:07:56.669 hanging a bucket from it and I filled 00:07:59.18000:07:59.190 the bucket up with sand and bits of 00:08:01.07000:08:01.080 metal to see how much load I could hold 00:08:03.20000:08:03.210 with each with each piece of steel with 00:08:06.38000:08:06.390 each sample and what I did as I started 00:08:09.08000:08:09.090 off these samples are untreated so this 00:08:12.32000:08:12.330 is probably not fully annealed when I 00:08:16.43000:08:16.440 talked about cooling the steel down and 00:08:18.11000:08:18.120 you have a couple options you could heat 00:08:19.76000:08:19.770 it up to red-hot and then cool it down 00:08:21.53000:08:21.540 really really slowly by like putting it 00:08:24.02000:08:24.030 in an oven or in an insulator and that 00:08:27.86000:08:27.870 will give you full anneal that's the 00:08:29.60000:08:29.610 softest you can get if you heat the 00:08:31.85000:08:31.860 steel up and just let it cool down in 00:08:33.77000:08:33.780 air that's called normalized and so even 00:08:36.64900:08:36.659 that will give you some amount of 00:08:37.90900:08:37.919 hardening over the full and yield state 00:08:40.79000:08:40.800 and I don't know how this is sold for 00:08:43.19000:08:43.200 mcmaster this is probably normalized so 00:08:45.74000:08:45.750 they heated this up and then let it cool 00:08:47.42000:08:47.430 down at ambient temperature I'm guessing 00:08:49.67000:08:49.680 but it's it's relatively soft and so I 00:08:52.40000:08:52.410 was able to bend it by 00:08:53.51000:08:53.520 this just by applying 16 kilograms so 00:09:00.11000:09:00.120 note that we actually didn't get to 00:09:01.70000:09:01.710 breakage on this piece what would happen 00:09:03.35000:09:03.360 to us since we went up the graph and 00:09:04.73000:09:04.740 then stopped somewhere around here so it 00:09:07.13000:09:07.140 was plastic and eventually just slipped 00:09:10.79000:09:10.800 out at the fixture if we kept bending it 00:09:12.92000:09:12.930 eventually we get to fracture and it 00:09:14.33000:09:14.340 would break so next I tested one of 00:09:18.08000:09:18.090 these full hard pieces and this one I 00:09:21.52000:09:21.530 heated up to you know cherry red and 00:09:24.29000:09:24.300 then dropped it in water and took it out 00:09:27.26000:09:27.270 and put it in the loading jig and this 00:09:29.57000:09:29.580 one only held six kilograms and also as 00:09:32.33000:09:32.340 you can see there's no bending at the 00:09:34.04000:09:34.050 fracture so that we have this sort of a 00:09:36.95000:09:36.960 situation where it elastically deformed 00:09:39.71000:09:39.720 you can see it bending a little bit when 00:09:41.15000:09:41.160 we load it and then suddenly it 00:09:42.95000:09:42.960 fractures and snaps back there's very 00:09:45.59000:09:45.600 little if any plastic deformation at the 00:09:49.04000:09:49.050 breakage point now you might be saying 00:09:52.37000:09:52.380 well this only held six kilograms and 00:09:54.83000:09:54.840 the soft one held you know sixteen point 00:09:58.31000:09:58.320 two kilograms you know what's the deal 00:10:00.59000:10:00.600 with that I thought we were supposed to 00:10:01.70000:10:01.710 be getting a lot more out of this and 00:10:03.17000:10:03.180 the answer is that point loading is a 00:10:05.93000:10:05.940 very complex thing and so if we have a 00:10:08.00000:10:08.010 bar like this with a steel cable loading 00:10:11.78000:10:11.790 it like this right at the point where 00:10:13.52000:10:13.530 the steel cable is touching it there 00:10:15.47000:10:15.480 could be an additional stress caused by 00:10:17.39000:10:17.400 this loading scheme this is also the 00:10:20.57000:10:20.580 reason that glass is not considered a 00:10:22.25000:10:22.260 structural material because you can't 00:10:24.38000:10:24.390 really clamp a piece of glass without 00:10:26.18000:10:26.190 introducing a lot of local stresses that 00:10:28.64000:10:28.650 would break it so you can really think 00:10:31.79000:10:31.800 of super hard steel like this as a piece 00:10:33.77000:10:33.780 of glass where it's very um it's very 00:10:36.17000:10:36.180 touchy and so small small amounts of of 00:10:38.75000:10:38.760 local stress will cause it to fat to 00:10:40.94000:10:40.950 fracture which is why it's basically 00:10:43.10000:10:43.110 never used so now we've covered the 00:10:46.49000:10:46.500 extreme ends of this spectrum we've gone 00:10:48.35000:10:48.360 from normalized or very soft to full 00:10:52.34000:10:52.350 hard which is almost unusable because 00:10:54.44000:10:54.450 it's just so brittle so to temper the 00:10:57.86000:10:57.870 steel what we do is we heat it up a 00:10:59.81000:10:59.820 little bit and then let it cool down 00:11:01.67000:11:01.680 slowly and what happens here is we give 00:11:04.34000:11:04.350 up some of this hardness because we're 00:11:05.93000:11:05.940 letting 00:11:06.45000:11:06.460 that crystalline structure changed by 00:11:08.55000:11:08.560 heating it up a little bit and if we 00:11:10.53000:11:10.540 heat it up to a very specific 00:11:11.67000:11:11.680 temperature we can control how much 00:11:13.50000:11:13.510 strength were actually trading for 00:11:15.69000:11:15.700 toughness very conveniently steel will 00:11:19.17000:11:19.180 change color in air based on how how 00:11:22.83000:11:22.840 high we heat it up and the color change 00:11:25.53000:11:25.540 comes from an oxide layer that's forming 00:11:27.33000:11:27.340 on the steel and it's interfering with 00:11:29.76000:11:29.770 light and we can see what color or what 00:11:32.40000:11:32.410 temperature the steel is based on what 00:11:34.05000:11:34.060 color we see off that because the oxide 00:11:36.27000:11:36.280 layer is forming an optical interference 00:11:38.25000:11:38.260 pattern there so as we heat it up we'll 00:11:41.55000:11:41.560 see a straw yellow color and then kind 00:11:44.37000:11:44.380 of an orange color and then brown and 00:11:46.62000:11:46.630 purple and then blue and then light blue 00:11:49.50000:11:49.510 and the hotter we heated up the more 00:11:52.67000:11:52.680 strength we give up in return for 00:11:55.32000:11:55.330 getting more toughness and so there's 00:11:57.21000:11:57.220 quite a bit of research and fine-tuning 00:11:59.70000:11:59.710 to be done here but for home shop 00:12:01.80000:12:01.810 hardening and tempering it's actually 00:12:03.69000:12:03.700 quite ineffective and decent means of 00:12:06.69000:12:06.700 setting up tooling of course if you have 00:12:09.63000:12:09.640 access to a kiln it also makes a lot 00:12:11.85000:12:11.860 more sense to just set the temperature 00:12:13.14000:12:13.150 that you want to temper your steel to 00:12:15.06000:12:15.070 and put it in the kiln and leave it for 00:12:17.73000:12:17.740 the prescribed time which is actually 00:12:19.59000:12:19.600 like an hour - usually and then take it 00:12:22.05000:12:22.060 out of the kiln and let it cool down so 00:12:25.14000:12:25.150 interestingly enough I started with the 00:12:27.27000:12:27.280 the 300 degree Celsius piece that I kiln 00:12:31.32000:12:31.330 tempered and this piece held about 55 00:12:36.18000:12:36.190 kilograms in fact my bucket became 00:12:38.90000:12:38.910 overloaded I put all of the sand in 00:12:41.52000:12:41.530 there and that it was holding fine and 00:12:43.47000:12:43.480 then I put all kinds of random scrap 00:12:45.39000:12:45.400 bits of metal in there and it was still 00:12:46.98000:12:46.990 holding and I had to push down on it 00:12:48.45000:12:48.460 with my arms so I I completely didn't 00:12:51.39000:12:51.400 expect how strong I could actually make 00:12:53.28000:12:53.290 this steel compared to the full hard and 00:12:55.98000:12:55.990 the normalized state the results for the 00:13:00.48000:13:00.490 other tempered pieces were pretty 00:13:01.83000:13:01.840 similar except for this one this one i 00:13:04.65000:13:04.660 tempered only two straw yellow which is 00:13:08.24000:13:08.250 less tempering which means more brittle 00:13:11.97000:13:11.980 and stronger so I stopped recording how 00:13:15.90000:13:15.910 much weight these things held because my 00:13:17.31000:13:17.320 system was woefully in 00:13:20.02000:13:20.030 but what was interesting is that this 00:13:21.85000:13:21.860 one broke in a brittle sort of a 00:13:23.95000:13:23.960 fracture whereas these other tempered 00:13:26.41000:13:26.420 pieces that were tempered to higher 00:13:27.70000:13:27.710 temperatures did not break like that 00:13:29.38000:13:29.390 these yielded another really handy trick 00:13:33.31000:13:33.320 is to use the file to determine how hard 00:13:35.41000:13:35.420 the material is that we're working with 00:13:36.97000:13:36.980 so these normalized pieces if you just 00:13:39.46000:13:39.470 lightly run a file along and I'm hardly 00:13:41.86000:13:41.870 pressing down on the file I'm just 00:13:43.45000:13:43.460 pushing it along very gently you can see 00:13:45.79000:13:45.800 that it sort of grabs and after you do 00:13:48.76000:13:48.770 this a few times so we get a very good 00:13:50.05000:13:50.060 feel for what different steals at behave 00:13:53.02000:13:53.030 like but this is very grabby and if we 00:13:55.57000:13:55.580 take one of the full hard pieces the the 00:13:58.60000:13:58.610 file just absolutely glides along like 00:14:00.55000:14:00.560 it's on glass it's not even biting into 00:14:02.32000:14:02.330 the material at all and that's because 00:14:04.39000:14:04.400 this is actually harder than the file so 00:14:06.76000:14:06.770 when we drag the file teeth across there 00:14:08.68000:14:08.690 and the teeth don't dig into the metal 00:14:10.72000:14:10.730 at all whereas with a softer one the 00:14:14.59000:14:14.600 file teeth actually bite in and that's 00:14:16.21000:14:16.220 what's causing the drag also I should 00:14:23.14000:14:23.150 point out that hardness is related to 00:14:26.92000:14:26.930 strength so when we say a material is 00:14:29.83000:14:29.840 really hard what we mean is it's 00:14:31.27000:14:31.280 actually very strong and files are quite 00:14:34.54000:14:34.550 hard it's actually one of the hardest 00:14:35.86000:14:35.870 tools that you'll find in a common 00:14:37.84000:14:37.850 machine shop and the fact that we can 00:14:41.47000:14:41.480 run it across this and this is actually 00:14:43.09000:14:43.100 even harder than the file seemed would 00:14:44.80000:14:44.810 indicate that this is something that 00:14:46.00000:14:46.010 this is a hardness that you generally 00:14:47.47000:14:47.480 not encounter here's a graph that shows 00:14:51.40000:14:51.410 what's actually happening when we cool 00:14:53.17000:14:53.180 down a piece of steel so this is the 00:14:55.45000:14:55.460 first part of the process the hardening 00:14:57.28000:14:57.290 part of the process and we've got 00:14:59.05000:14:59.060 temperature on the y-axis and time and 00:15:01.30000:15:01.310 the x-axis and we're starting off at 00:15:04.18000:15:04.190 about 800 degrees C which is the cherry 00:15:07.54000:15:07.550 red color and what we want to do is get 00:15:10.78000:15:10.790 down into this phase down here we want 00:15:13.57000:15:13.580 to get below this line without going 00:15:15.82000:15:15.830 through this part of the graph so this 00:15:18.40000:15:18.410 whole deal with cooling it down quickly 00:15:20.23000:15:20.240 is because we need to get down to this 00:15:22.60000:15:22.610 part of the graph without interfering 00:15:24.67000:15:24.680 with this area this graph is called the 00:15:27.16000:15:27.170 time temperature transformation graph 00:15:29.62000:15:29.630 and we talked about going past the 00:15:32.69000:15:32.700 those of the TTT graph like this and so 00:15:35.63000:15:35.640 there's this critical cooling rate where 00:15:37.40000:15:37.410 we have to get down into here around so 00:15:40.34000:15:40.350 we don't get this hardening effect so if 00:15:42.26000:15:42.270 we take too long if we if we spend ten 00:15:44.42000:15:44.430 seconds cooling down from 800 we're 00:15:46.91000:15:46.920 gonna end up in this region and that 00:15:48.98000:15:48.990 means that we'll get some hardness so 00:15:51.26000:15:51.270 there'll be some hardening effect but it 00:15:53.27000:15:53.280 won't be anywhere near getting down to 00:15:55.49000:15:55.500 here and if you're curious the M is 00:15:58.19000:15:58.200 martensite which is the crystalline 00:16:00.17000:16:00.180 structure that gives us that really high 00:16:01.97000:16:01.980 hardness in steel when we temper the 00:16:05.63000:16:05.640 steel we're actually starting out down 00:16:07.73000:16:07.740 here and we take it up into this region 00:16:10.22000:16:10.230 so we're basically giving up some of 00:16:12.29000:16:12.300 this really hard crystalline structure 00:16:14.30000:16:14.310 and gaining some of this less hard but 00:16:17.21000:16:17.220 tougher structure and there's a lot of 00:16:19.70000:16:19.710 terminology involved here that probably 00:16:21.23000:16:21.240 won't help you understand it but if you 00:16:23.18000:16:23.190 go searching for this stuff you'll find 00:16:24.80000:16:24.810 quite a depth of information so you 00:16:27.62000:16:27.630 might be wondering what can I do this 00:16:28.94000:16:28.950 trick with a coat hanger if I heat it up 00:16:31.01000:16:31.020 and then cool it down and do this sort 00:16:32.66000:16:32.670 of transformation no the answer is nope 00:16:35.84000:16:35.850 you need to have steel that is hardened 00:16:37.88000:16:37.890 abaut and not all steels are hardened 00:16:39.77000:16:39.780 Abul and the thing that determines 00:16:41.15000:16:41.160 whether they're hardened Abul or not is 00:16:43.10000:16:43.110 the carbon content and to a lesser 00:16:45.74000:16:45.750 extent the other alloying ingredients so 00:16:48.38000:16:48.390 this w 1 water hardening steel that I've 00:16:51.11000:16:51.120 been using today has a carbon content 00:16:53.48000:16:53.490 fairly close to 1 1 % so this graph 00:16:57.14000:16:57.150 shows us temperature on the y axis and 00:16:59.24000:16:59.250 carbon content as a percent on the x 00:17:01.70000:17:01.710 axis and most tool steels are pretty 00:17:05.00000:17:05.010 close to about 1% and the reason for 00:17:07.91000:17:07.920 that is that it makes this crystalline 00:17:09.92000:17:09.930 structure that's very beneficial for 00:17:11.87000:17:11.880 having a very hard structure if we have 00:17:14.78000:17:14.790 tons and tons of carbon what we actually 00:17:16.73000:17:16.740 have is cast iron and if we have very 00:17:19.28000:17:19.290 little carbon we have cheap steel 00:17:21.56000:17:21.570 basically ok well I hope that was 00:17:25.52000:17:25.530 helpful 00:17:26.09000:17:26.100 see you next time bye
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