Frank Turley Posted December 5, 2009 Share Posted December 5, 2009 A 1964 book, "Metals for Engineering Craftsmen," states for following regarding tempering colors. "The temperature is estimated by the surface colours of the steel, formed by oxidation, but these will only show it the steel is free from scale. It should, therefore, be cleaned with emery cloth or on the grindstone, but must not be polished, glazed, or burnished as this results in different colour changes" I wonder about this. Comments?http://www.turleyforge.com Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 5, 2009 Share Posted December 5, 2009 Interesting Frank. Seen people grind, polish or rub with a stone. Might be a nuance that has just a small effect for blacksmiths I'l bet. I've also read that the color (being related to the thickness of the oxide layer) will change with time even though the temperature stays the same. Meaning that if you leave a part in the oven for four hours, you can't judge the temper by the color. Quote Link to comment Share on other sites More sharing options...
HWooldridge Posted December 5, 2009 Share Posted December 5, 2009 Perhaps they were trying to convey that extra steps can skew the visual results, since 'polishing, glazing and burnishing' are essentially secondary or non-conforming processes to basic grinding. Polishing can contain rouge or other contaminants, glazing is usually considered a defect from worn belts or stones and burnishing is a mechanical process that usually closes grain structure. I know that oil contamination will change color so I typically wash in solvent after quench then hit the piece with a quick grind to expose the steel. I also agree with Grant on time and temperature but atmosphere can also play a role. If you temper in a vacuum or inert atmosphere, the colors will not be the same due to the O2 content in ambient air. I work in a machining jobshop and we have many parts heat treated in a variety of processes. One particular design which is vacuum treated and tempered in inert atmosphere always comes back as a bright gold color - almost looks like the parts have been plated. Quote Link to comment Share on other sites More sharing options...
Fe-Wood Posted December 5, 2009 Share Posted December 5, 2009 (edited) I think another important aspect is consistency. I used to make carving tools for professional wood carvers. Finding a good source for the steel is important but tempering was paramount. I found that grinding to the same polish (120 grit) everytime I was able to deliver good consistent results. If you think about it from surface texture, the heavier the grit the more surface available for oxidation. Would this make it oxidize faster or turn color faster because there are more microscopic peaks and valleys? Edited December 5, 2009 by Fe-Wood clarification Quote Link to comment Share on other sites More sharing options...
Doug C Posted December 9, 2009 Share Posted December 9, 2009 If the parts tempered in a vacuum turn gold what was the chemical process at play seeing as how there is no oxygen to create an oxide? Quote Link to comment Share on other sites More sharing options...
HWooldridge Posted December 9, 2009 Share Posted December 9, 2009 If the parts tempered in a vacuum turn gold what was the chemical process at play seeing as how there is no oxygen to create an oxide? On those parts, the heat treater adds an inert atmosphere in addition to the vacuum. I'm not sure what the gas is but I think it's either nitrogen or argon. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 9, 2009 Share Posted December 9, 2009 Well, you can heat in a vacuum, but you can't very well quench in one. For that they use inert gas, but few things are totally inert (non-reactive) at 2000F. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted December 9, 2009 Share Posted December 9, 2009 Sure you can! induction heating and then used cold quench blocks in the vacuum to pull heat out of the piece. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 9, 2009 Share Posted December 9, 2009 That'd work for flat things. Don't think my heat treater is set up to do that kind of thing though. Just cause you "can" doesn't mean anyone does. And why, when inert gas works so well? I think you're just messin' with my mind. Quote Link to comment Share on other sites More sharing options...
Brian D Posted December 10, 2009 Share Posted December 10, 2009 If you temper in a vacuum or inert atmosphere, the colors will not be the same due to the O2 content in ambient air. I work in a machining jobshop and we have many parts heat treated in a variety of processes. One particular design which is vacuum treated and tempered in inert atmosphere always comes back as a bright gold color - almost looks like the parts have been plated. Yes, it makes for a very consistent and nice looking part when heat treat with vacuum is called out. I call out "Heat treat with Vacuum" on a lot of my machine tooling. As said, it looks straw yellow to bronze depending on the heat treat process, material, and surface finish of the part.I think another important aspect is consistency. I used to make carving tools for professional wood carvers. Finding a good source for the steel is important but tempering was paramount. I found that grinding to the same polish (120 grit) everytime I was able to deliver good consistent results. If you think about it from surface texture, the heavier the grit the more surface available for oxidation. Would this make it oxidize faster or turn color faster because there are more microscopic peaks and valleys? Surface finish does make it look different, I don't know if it oxidizes faster, but same material with inconsistent surface finishes look very different than the same part with a consistent finish.If the parts tempered in a vacuum turn gold what was the chemical process at play seeing as how there is no oxygen to create an oxide? I do not think it is an oxygen free atmosphere, I think it just makes it much more consistent throughout.On those parts, the heat treater adds an inert atmosphere in addition to the vacuum. I'm not sure what the gas is but I think it's either nitrogen or argon. I don't think the shop I use purges the chamber with an inert gas, I think they just pull a vacuum on their chamber in the oven.Well, you can heat in a vacuum, but you can't very well quench in one. For that they use inert gas, but few things are totally inert (non-reactive) at 2000F. Most of the material that I call out a vacuum heat treat on is an air hardening material / heat treat process. Often makes for a more consistent, stable part minimizing the amount of operations that need to happen after heat treat. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 10, 2009 Share Posted December 10, 2009 (edited) I realize you have "air hardening" steel. So, how do they "air harden" in a vacuum? They don't (can't?). In "vacuum" heat treating the heating is done in a vacuum, but not the quenching. Most furnaces are equipped for either gas or oil quench. Edited December 10, 2009 by nakedanvil Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 10, 2009 Share Posted December 10, 2009 (edited) It should be noted that even "vacuum quenched" parts are often drawn in conventional draw ovens. Part of the problem is the industry uses terms like "vacuum heat treating" and "vacuum quenching" when usually it's just the heating for austenizing that is done in a vacuum. So they should just call it "vacuum austenizing". Edited December 10, 2009 by nakedanvil Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 10, 2009 Share Posted December 10, 2009 Brian, I don't want to sound like I'm beating up on you. I've formed certain perceptions about some things that I've later learned were totally wrong. I think our minds need an explanation and we form one that satisfies us with the information we have. A vacuum furnace look like an "iron lung" or a decompression chamber. The ""oven" is actually inside the vacuum chamber. Even in a vacuum, steel still requires the same heat-quench-draw that it requires in coventional practice. Stop by your heat treater sometime and ask them to show you the vacuum furnace and explain the whole process. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 10, 2009 Share Posted December 10, 2009 Actually I think (don't know for 100%) that the gold color is because they are drawn in a conventional controlled atmosphere furnace. Specifying "bright" meant they are drawn in vacuum too. Just like vacuum austenizing, vacuum drawing costs more too. Quote Link to comment Share on other sites More sharing options...
Brian D Posted December 10, 2009 Share Posted December 10, 2009 I realize you have "air hardening" steel. So, how do they "air harden" in a vacuum? They don't (can't?). In "vacuum" heat treating the heating is done in a vacuum, but not the quenching. Most furnaces are equipped for either gas or oil quench. I think the heating is done in a vacuum, and the quenching is done in a different "controlled" chamber where there is some kind of hand off of the part. It should be noted that even "vacuum quenched" parts are often drawn in conventional draw ovens. Part of the problem is the industry uses terms like "vacuum heat treating" and "vacuum quenching" when usually it's just the heating for austenizing that is done in a vacuum. So they should just call it "vacuum austenizing". I agree. I don't think I would use the term "vacuum quenching". It would probably make sense to use the terms "vacuum carburizing" and "vacuum tempering" or vacuum drawing.Brian, I don't want to sound like I'm beating up on you. I've formed certain perceptions about some things that I've later learned were totally wrong. I think our minds need an explanation and we form one that satisfies us with the information we have. A vacuum furnace look like an "iron lung" or a decompression chamber. The ""oven" is actually inside the vacuum chamber. Even in a vacuum, steel still requires the same heat-quench-draw that it requires in coventional practice. Stop by your heat treater sometime and ask them to show you the vacuum furnace and explain the whole process. Grant, don't worry about the way it sounds. I have thick skin. I have not physically "walked" the parts through the process. What I do know is that there are certain advantages that I have seen in the end result from a heat treat process with vacuum. Part stability (tighter tolerances) with less distortion and less finish grinding or other operation after heat treat is a big advantage. I understand that the steel still requires a heat-quench-draw, however, from what I understand, these all happen in different chambers and I understand the atmosphere is "controlled" in each. Just how, I don't know.Actually I think (don't know for 100%) that the gold color is because they are drawn in a conventional controlled atmosphere furnace. Specifying "bright" meant they are drawn in vacuum too. Just like vacuum austenizing, vacuum drawing costs more too. Quite right, Grant. It is where the color and consistency of it come from, and it is more expensive. It does not always make sense to have "bright" finish parts. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted December 10, 2009 Share Posted December 10, 2009 (edited) I know it's just the results you are after, but these threads fan out and people want to know the "how" and "why". The question just came up about how do you quench in a vacuum and it sounded like you thought it was not quenched with inert gas, but was in a vacuum during the whole process. "Vacuum quenching" is just an ambiguous term used in the industry. The parts are not transfered, they are quenched in the furnace with nitrogen or argon. "Controlled atmosphere" just refers to the normal gas fired heat treating furnaces. Edited December 10, 2009 by nakedanvil Quote Link to comment Share on other sites More sharing options...
northbayforge Posted December 11, 2009 Share Posted December 11, 2009 back a few thoughts in the thread... I temper in a salt bath. The scale from hardening is not removed from each blade, but at least one is, so I can check the color. A good (Fluke) dig. thermometer tells me the temp and has shown that there are not significant hot spots or temp drops or raises. OK, the interesting thing is that after 15 minutes in the bath at 410 F, there are no signs of tempering colors. But after an hour there are, and they get darker (light straw to dark straw) after a few hours. There is most probably limitted oxygen in the bath and as Grant said, the oxidation (tempering colors) is a gradual event. I do this long tempering time (at quite low temperature) because I think it yields greater toughness without sacrificing as much hardness. Quote Link to comment Share on other sites More sharing options...
pkrankow Posted December 11, 2009 Share Posted December 11, 2009 I looked up some stuff when this thread originally posted and the energy (temperature) to form the red oxide is much lower than the energy to form the blue oxide of iron. I suggest that if you are below 500F-510F and above 410F-420F then the blue oxides won't form, but a thicker coating of the red oxide will, which looks yellow in very thin layers, and will build to reddish brown as it thickens. I have noticed that things that are regularly exposed to high temperatures, but below 500F, frequently and for long duration (like oven racks) tend to turn a yellow brown or dark straw, but not go into the blue colors. Above 510 I am unsure about. I just looked this up quickly. Red iron oxide is iron(III), Fe2O3 Blue/black iron oxide is iron(II) FeO Phil Quote Link to comment Share on other sites More sharing options...
northbayforge Posted December 11, 2009 Share Posted December 11, 2009 pkrankow, I find that very interesting, as I was a chemical engineer in an earlier life (that was befor I got serious and realized I should live the dream of being a blacksmith with my one life). Anyway, I still appreciate understanding more of the chemistry of those magnificent colors! Thanks for researching it. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted December 11, 2009 Share Posted December 11, 2009 Yup just messing with you---we gave this some thought when we were brainstorming blacksmithing in space back in the mid '80's... Quote Link to comment Share on other sites More sharing options...
RustyLaidlaw Posted November 11, 2010 Share Posted November 11, 2010 I don't know about vacuums or salt baths, but I do know that when tempering a polish is a serious no-no. Try it some time. I temper everything by hand with either a torch or a hot piece of steel, or sometimes even over the coals in my forge. I will grind the work to almost a polish but not use anything other than a belt to get it there. I aim for 600 grit typically, but on some tools like hammers and pickaxes I'll go less, an old 80 grit belt or whatever is around. The finer the grit the better to see the color change, and that was the logic that says we should polish our work before tempering. Also it's easier to clean up the temper colors afterwards - less surface area to have to scrub color out of. The emery and carborundum stone method is good for quick jobs like chisels where you want to keep the heat and use it to temper the edge without re-heating the work. As we all know when we heat our work to tempering range colors start to develop and move from the area of applied heat down towards the cooler area as heat bleeds into the work. What causes the colors is an oxide layer forming on the steel. The thickness of the oxide layer is controlled by the temperature of the steel, and causes light to bounce off the layer and causes interference that we see as colors (same as looking at anything, I guess). Anything on the work, be it fingerprints, oil, grease (as most polishing compounds are based on) or scale will interfere with the formation of the oxide and give you a bad reading or at best a blotchy coloring. The best way to finish work for tempering is to grind it down, wipe it off with a clean cloth and begin! Unless of course you have a vacuum or salt baths ;) Quote Link to comment Share on other sites More sharing options...
dimenickel Posted November 11, 2010 Share Posted November 11, 2010 just my 2 cents ...burnishing or polishing sort of smears the top surface.. it protects it from oxidizing ... compare the two - buff a piece of iron... and polish a piece of iron with a water stone to same grit - then leave it in your humid shop... the waterstone/iron with the more open grain surface will easily rust also... temper colors change alot between alloys... i do enough heat treat in my oven.. and with the same alloys... sometimes the come out with a nice even color and other times they're darker or swirly... myself i think the temper colors are just one method of doing it, but with very wide window of accuracy Greg Quote Link to comment Share on other sites More sharing options...
maddog Posted November 11, 2010 Share Posted November 11, 2010 My guess is it has to do with sensitivity to the viewing angle. As was stated above, he oxide colors are a result of thin film interference. Light is reflected off the surface of the oxide which is transparent and also off the surface of the steel. When these surfaces are are about a wavelength apart the light waves from the two surfaces interfere and and combine to generate wavelengths of a particular color. The effect is analogous to the beat frequency that is produced by two tones that are close but not quite in tune. Since the effect depends on geometry, the color you see at a particular location depends on the angle with which you view the surface. Look at an oil slick on water, as you move, the rainbow shifts. If the surface is highly polished and smooth then this effect is pronounced. But if the surface is rough, then you are never viewing at any single angle instead you see a blended effect from all the different facets which is less sensitive to changes in viewing angle. Quote Link to comment Share on other sites More sharing options...
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