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Case Hardening :

Surface Treatment of Low Carbon steel

Can you direct me to sections of your website or to other referance sources for information re case hardening and color case hardening? If so, please advise.Please assume that I know nothing about the subject. Thank you.

John Dick - Saturday, 07/21/01 02:12:49 GMT

Case Hardening: John, I've posted on the subject in the past but our archives are a bit of a bear to wade through so here goes again. .

There are various case hardening salts and a process called "nitriding" we will cover these later.

CASE HARDENING, is the process of producing a hard surface on wrought iron and steel parts by increasing the carbon in the surface by absorption, then hardening by the usual method (heat and quench).

Finished low carbon steel parts are packed in a ceramic, graphite or steel box with powdered charcoal. Bone charcoal is supposedly the best. The box is then closed with a cover and sealed with fire clay. The box and parts are then heated to a red orange heat (1700°F to 1800°F) and held for a period of time depending on the depth of the case required. This can be from 15 minutes to 4 hours or more.

The cabon alows the steel to be hardened but also uses up the oxygen and prevents oxidation of the part. A crucible with a lid can be used. Special graphite boxes called "Diamond Boxes" have also been made for the purpose but I have never seen them for sale.

The parts can then be cooled and then later hardened and tempered OR they can be quenched directly from the furnace. This can be messy as the entire contents of the box are often dumped into the quench. However, this is the recommended practice for finely finnished parts such a gun parts or precision parts or screws.

COLOR CASE HARDENING, is a specialized process and an art. The parts are first finished to a high degree and cleaned so that there is no oil or fingerprints. The better the finish the brighter the colors. Then the procedure as above is followed. The difference is the quench tank. In order to produce the temper colors on the surface of the parts, air is bubbled through the water quench. As the parts cool to different temperatures and the air contacts the parts it produces different temper colors. After quenching the parts are removed, cleaned and dried. Then they must be oiled or lacquered to protect from rusting.

The process takes quite a lot of air and how it is bubbled through the tank can be varied to produce varying results. Much testing and experimentation is required to setup a color quench that produces the expected results.

- guru Saturday, 07/21/01 04:44:45 GMT


My son and I are not blacksmiths but are having a lot of fun building a Cap Lock Muzzleloader Rifle. We have done a good job on making and polishing all the brass parts and have a great bluing on the barrel. The only dilemma we have left is how to get the full color of the case hardening on the steel lock itself to come out. The Lock came in the kit and while you can see the color gradations they do not stand out like I am used to seeing. Moreover, the base color is not that of polished steel, but more of a brown/blue color.

Does this piece simply need application of a polishing process like used on other metals or is there another process that need be used to attain the maximum color?

Thank you,

Mike R - Thursday, 07/13/00 01:46:11 GMT

Color Case Hardening: Mike, this coloring is part of the process of "color case hardening". The colors are temper colors (same as heating any piece of clean steel) and are very thin (one atomic layer) and cannot be improved.

The part must be polished FIRST. The better the polish the brighter the colors. The part must be absolutely CLEAN. It is then packed and sealed in the case hardening case with powdered charcoal. It is then heated to a red heat and held there for a time determined by the weight of the part and the desired case thickness. Last, it is removed from the heat broken open and quenched in clean water. The TRICK is that the quench tank has air bubbling from a grate in the bottom. As the part cools the air oxidizes the surface at different rates creating various temper colors.

Afterwards the part can laquered but most gun parts are just oiled. There is no repairing or refinishing the part and keeping the temper blue. It IS possible to clean, finish and color case harden again. The difference is that the part is not held at temperature to absorb more carbon. Most parts are blued rather than trying to reproduce the color case hardening.

This question was what got me started in blacksmithing and took me 20 years to find.

- guru - Thursday, 07/13/00 08:07:26 GMT

Greetings from a warm and balmy Friday afternoon in Perth, Western Australia. I am a 69 year old retired electrican/engineer/Technical College Lecturer and I have restored several old cars, motor bikes and stationary engines. I have a modest backyard workshop with the usual hand tools plus a lathe, etc. I have a home-made forge.

Would appreciate any advice regarding case hardening. I have consulted my old friends Baumeister and Marks, Audel, Machinery's Handbook, as well as Tubal Cain and Ian Bradley, but none of them answer my specific questions in words of two syllables or less.

The tappet adjusting screws (1" x 5/16" BSF) on my 1956 LE Velocette motorcucle are a bit moth-eaten, and have a little 1/4" hex head. I also have a set out of a later model with a larger hex head which I'd prefer to use, but they came out of a bike which was burnt, and are soft. I would prefer these as their heads have more wear area (tappets rotate). I have ground and polished the heads of these, and I attempted to case them using an oxy torch, holding them at cherry red while sprinkling on "Hardite" powder for a while, then quenching in water, but I feel that this will have only given them a very superficial hardness.

I am going to try it again using total immersion in molten Hardite in an iron pot in my forge for a longer period, an hour or so, to try for a decent thickness.


(1) Can I re-case harden these parts, or should I anneal them first?

(2) Would I be better off starting again and making new screws?

(3) Should I protect the threads from case hardening? if so, what is the best method? Someone suggested Copper sulphate (sulfate).

This "Hardite" powder is labelled "Poison - contains 20% Barium Chloride". I recall the old fitters at the power houses using "Kasenite"(?) which I think contained a lot of cyanide, but I think this has been banished now. (Perhaps they weren't that old, maybe they just looked that way from using Kasenite).

Any suggestions appreciated. Thank you. Jack Watson

Jack Watson - Friday, 02/01/02 11:47:46 GMT

Case Hardening: Gary and Jack.

Not all steels are considered suitable for casehardening. Some alloy steels are not and only low to medium carbon steels benefit. See references such as MACHINERY'S HANDBOOK for applicable steels. See the ASM Heat Treater's Guide for specifics on methods and depth of case.

1215 is a resulphurized free machining steel with only 9 points carbon. It can benifit from light casehardening but is very soft and casehardening only improves surface wear.

4140 is a medium hardening steel and is not recommended to be case hardened.

The old Casenit had cyanide salt. The new Casenite does not.

Case hardening to a maximum depth of .032" (.8mm) takes four hours at 1600°F. The depth follows a curve but is fairly proportional accordiing to time. The lower the temperature the longer it takes. At 1450°F the case is only .005" (.13mm) after 1 hour but .016" (.4mm) at 1600°F after the same time. A few minutes with a torch at less than an orange heat and you have less than a tissue paper thick case.

Threaded parts are often case hardened to produce stronger more wear resistant threads. Parts that need higher mechanical strength (heads of nuts or bolts) do not get it from case hardening unless they are very small.

For the hardest case, parts are quenched directly from the bath. They are often untempered but if core steel needs tempering then that is a different situation. It is best to temper all hardened parts at least to the minimum of 350°F.

For more information and references see our Heat Treating FAQ.

- guru - Monday, 02/04/02 18:33:23 GMT

My questions,

Is this the correct way to increase the carbon content of steel? (If not could you tell me how itís done, regardless of how difficult or complicated it is):

Heat the steel between 1400 and 1500 degrees Fahrenheit and seal it in an airtight (and fireproof) container with some carbon and hold it at that temperature for a long while.

If this is correct, how long does it have to be held at that temperature? I know itís different depending on the size of the steel and how high you want the carbon content, but about how long would it take? (2 days, 2 weeks, 2 years? Etc.)

But that is only half my question. I have seen some material on increasing the carbon content of steel, but nothing on how you decrease it. I guess no one would really want to do that, but I do. Specifically how is it done? I would sorta assume that the steel has to be exposed to lots of oxygen to attract the carbon out of it, but how? I would like to try to reduce the carbon content of a piece of mild steel as much as I can. How low of a percent can I get it? Can I turn it into real wrought iron (commercial type) without the slag mixed in? Finally, how long would reduction take?

Sorry but I really would like you to be specific. Is it even possible to do this carbon content changing stuff at home or shop on a small scale?

I really appreciate you helping me out. Many thing about iron/steel continue to remain a mystery to me.

Robert - Saturday, 02/09/02 08:02:42 GMT

Casehardening You don't seal the steel in the box box first. It must be packed full with charcoal then sealed. The steel must be clean (scale free) to start. 1425°F is the minimum recommended case hardening temperature and it takes a lot less time at the maximum of 1625°F.

Carbonnitriding is done by using a liquid salt bath (cynaide was commonly used) and the time/temperature ratio is supposed to be more or less the same for case hardening by carbon packing and gas methods. The results are the same.

Carbonnitriding starting with low carbon steel (SAE 1008):

1 hour   @ 1425 to 1450°F results in a case .004" deep.
2 hours @ 1425 to 1450°F results in a case .006" deep.
3 hours @ 1425 to 1450°F results in a case .009" deep.
4 hours @ 1425 to 1450°F results in a case .011" deep.

1 hour   @ 1600 to 1625°F results in a case .015" deep.
2 hours @ 1600 to 1625°F results in a case .021" deep.
3 hours @ 1600 to 1625°F results in a case .026" deep.
4 hours @ 1600 to 1625°F results in a case .030" deep.

Heat Treaters Guide, 1982, ASM, p.25 chart (referenceing Metals Handbook 8th ed., Vol 2, ASM.
Before modern steel making methods were developed "blister steel" was created using this method and soaking at temperature for days. The result was very burnt and blistered. This product was then taken and folded and forge welded numerous times to homogenize the result since even after days of soaking the outer surface would have much higher carbon content than the core. This produced a suitable but variable quality steel. The crucible steel process was invented to make perfectly homogenous steel from blister steel.

Modern steels have carbon either left in when it is decarburized OR it is inoculated with a carbon bearing substance.

Generaly you do not remove carbon from steel. It happens accidentaly when forge or furnace atmospheres are too oxidizing. It removes carbon from the surface and ruins the steel. In steel manufacturing, air or pure oxygen is blown through liquid steel (resulting in GREAT fireworks) to reduce the carbon content from the smelting process.

- guru - Saturday, 02/09/02 16:44:28 GMT

Hello everyone,

I have made a small carving knife and I want to case harden it. I think there is a technique using clay to cover the upper part of the blade and then heating and quenching.

Can you please give me some instructions and details on this procedure?

Thank you very much.

George Chrysochoides - Saturday, 10/07/00 11:15:46 GMT

Case hardening: George, The clay covering technique is used on Japanese swords to create a differential hardness. The clay reducing the quench rate on the back of the blade I think. (I am NOT an expert on Japanese sword making). The exposed edge has a different crystal structure and the line between the edge and the body is called the "hammon" line. This is often turned into a decorative pattern by scraping the clay with a knife producing waves and swirls along the line. The hammon line is another of the high arts of the Japanese sword.

Case hardening is the absorption of carbon into the surface of low carbon steel making it high carbon steel and therefore hardenable. The hard surface on a soft body makes a strong wear resistant part using a low level of technology.

To case harden a part it is first cleaned. Then it is packed in a steel, clay or graphite container with ground charcoal. The container is closed and sealed with clay to keep out air. The box containing the part and carbon is then heated to a red heat (1700°F - 1800°F, 830°C - 980°C). The container is held at that temperature for 15 minutes to several hours. Then the box is opened and the entire contents (charcoal too) is dumped into water to quench.

Sealing in the box prevents air from oxidizing the part and bright finished parts should have the same finish when case hardened. Various types of charcoal are recommended. Bone charcoal and charcoal made from old shoe leather are common although any wood charcoal will also work.

Case hardening is not suitable for making good cutting tools that are to be sharpened on both edges.

- guru - Saturday, 10/07/00 15:13:28 GMT

More about Case hardening: George, case hardening often penetrates 1/32" (1mm) in a short time without damaging the part. On a thin blade this would be full penetration. The problem is that the case has more carbon on the outside and lessens as it goes into the metal. There would be no uniform area and a thin edge may have too much carbon.

Claying the part MAY prevent some carburization but what happens in the case hardening box is that the carbon dissassoiates and becomes an airless carbon vapor. The steel at this heat becomes metalurgicaly active and absorbs the carbon.

To do what you want to do, you are going to need to case harden and test several sample before doing the final part.

Trial and Error. - guru - Saturday, 10/07/00 15:35:45 GMT

Primitive case hardening: One of my gunsmithing references describes case hardening in a wood stove (heating stove), without a permanent case hardening box.

The part is prepared. It is then made into a "packet" using old dry leather with some charcoal packed around the part. Then the packet is surrounded with clay, the whole made into a clay ball.

The clay is then allowed to dry for several days. Then the ball is put into the coals of a HOT wood stove and left there for a half hour to several hours depening on the size of the part and ball. When removed it is broken open above the quench tank and the whole mess tossed into the tank.

- guru - Saturday, 10/07/00 15:46:51 GMT

Could anyone tell me if its a good idea to carburize or case harden with kasenit stainless steel...also if someone knows how to make their own carburizer/case hardener powder.

joe - Thursday, 08/09/01 23:51:47 GMT

Case Hardening Stainless: Joe, Most stainless will not case harden. Kasenit only puts a very thin almost superficial hard surface on steel.

- guru - Friday, 08/10/01 00:28:42 GMT

Case Hardening Compound: Well. . . I mixed some up for early medieval experiments: bone meal, cow horn shavings, leather scraps, antler dust, powdered charcoal and a pinch of bloodmeal . I filled a heavy walled pipe with this mixture and embeded stripts of wrought iron into it. Sealed up the "case" and put it in a large bonfire where it glowed red for 6 hours---the modern stuff probably works better. (also Theophilus in 1120 A.D. suggested greasing files then wrapping them in leather and covering the leather with clay and heating them and quenching as a surface hardening process.

Thomas Powers - Friday, 08/10/01 13:49:25 GMT

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2002 Jock Dempsey,

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