Case Hardening

You don't need to temper anything that is case hardened as the core's carbon content is not altered. It just gives a wear surface

At first glance, like when I started model engineering, it’s easy to assume that case referred to the creation of a hard skin ‘encasing’ a component. Later reading revealed that the process used to be done by enclosing the part in an iron box or “case” ? with carbon bearing materials like bone or leather. So maybe the term case hardening referred to the box. I have heard it named box hardening. This is surmise on my part so I would be interested in comments.
On a somewhat more practical and up to date point it is possible to selectively harden parts of components by choosing when in the machining sequence the casing or carburisation is carried out. For example on a part with a fine screw thread or something that will distort when heat treated like a cam follower bucket if the area to be hardened is finished machined but the rest is left oversize the part can be carburised but not quenched. The surfaces to be left soft can be skimmed, thread cut or fully machined to remove the carburised surface. Heating and quenching will harden the target surface and leave the rest soft to ensure non brittle threads or in the case of the cam follower the ability to turn the outside cylindrical after hardening has distorted it.

regards Martin

Edited By Martin Kyte on 22/12/2022 10:00:11

Yes Martin you are quite right about box hardening, I use a it fir small-gears and other instrument parts. Not only do they come out a beautiful shade of grey but as some parts have thin star shaped teeth there is no chance of overheating as they are protected by the canister. Out of interest I put a brown paper tube in the canister to use up the oxygen which results in the nice clean finish

Dalboy, - by the dimensions you stated it sounds like you are talking about Eccentric Rods. If those are the parts you are working on, you only case harden the small end - not the entire part. The case hardening is only to provide a long-wearing surface on the 1/8" (?) dia hole on the one end. You only heat that end and dip it in the powder. I would have a scratch rod to make sure the powder gets into the small hole.

If that is correct, I'm not sure why one would want to do that? Much simpler, and better, to leave the rods as they are and case harden the pins.

Andrew

Andrew, I would have thought a lot less work to remake a worn pin some time down the line than a worn rod

Jason I seem to remember something about tempering to refine the core due to grain growth when quenching the case but cannot remember if that was just for cyanide hardening. Where I done my time we had a big cyanide hardening shop, lovely place to be on a freezing cold winters day, the fumes also cured blocked up noses very effectively!

When it comes to selective carburising hardening of surfaces another method is to coat the surfaces to be left soft with copper sulphate solution. The costing of copper sulphate prevents the absorption of the carburising compound & thus retains the properties of the original steel. You can even apply it to a small area of a surface if you will need to drill a hole in a particular place after hardening. Carburised hardened surfaces can also be tempered after hardening to reduce the tendency of the glass hard surface to chip or otherwise break up under load.

The tool and die company where I served my apprenticeship had an extensive heat treatment department that dealt with hardening & tempering steels of all grades. From low carbon up to & including exotic tool steels. All used in the manufacture of automotive body press tooling. Low carbon steel parts to be carburised would be packed in carburising compound in metal boxes & placed in an oven for specific times depending on the depth of case required. Those jobs were nearly always left overnight, often soaking for several hours if a deep case was required. Most, if not all parts were tempered afterwards. Examples were guide plates for the large draw dies used to form a car door or trunk lid from a flat sheet. They would have a case depth specified of I think 0.025”. That included an allowance for finish grinding after heat treatment. I think that the opposite side of the plate was left soft by the application of copper sulphate.

As an aside, copper sulphate can also be used instead of “marking blue” to highlight scribed lines on steel. There is a simple method to remove it after the wok is complete but exactly how has disappeared from my memory unfortunately. It is more durable than ”marking blue”.

Edited By Brytech on 22/12/2022 15:34:43

I remember my using Kasenite to harden gun parts, as already mentioned you had to get the part to red heat and then dip it in the powder, if the temperature was correct you would see it fuse and run like liquid all over the part and only then would you plunge it into water or oil.

I used to visit the gun quarter in Birmingham where they would harden parts buried in trays of old leather and bonemeal kept at red heat in an oven for several hours in order to produce lovely mottled colours on the lock parts

I think Ammonia is used to remove the copper film after marking out (or case hardening).

I have often wondered if heating silver steel to red heat and quenching may be case hardening. To get a piece of steel up to a uniform temperature takes one hour per inch of thickness (heating from both sides). Holding half inch diameter silver steel bar at red heat for a minute or two before quench should not give a hardened centre. Just a thought.

Frequently parts to be case hardened in a cyanide bath or by gas (usually propane or butane) in a furnace were held on mild steel wire. After hardening a length of the wire was kept with the parts as a quality check.

JA

After case hardening a piece of round bar does the actual diameter increase ?

The carbon will migrate deeper if the time at temperature is extended. When I was working, the foreman gave me a very warped piece of steel, about 12 x 2 x 1/2 inches and asked me to straighten it. The end result was two pieces, and you could see the grain structure of the hard parts had penetrated at least 3/16". I couldn't help sniggering as I handed him the bits, and he stormed off in a huff.

Just to add to some of the discussion from the earlier part of the thread : the version that you can dip a hot item into is the ferrocyanide as in Kasenit. The version that you put in a box and heat the item in for a long time is different - it is a mixture of charcoal (or organic material that decomposes in to charcoal like leather) and lime ie calcium carbonate (or old crushed dried bones - not modern garden bonemeal which is too fresh and organic hence smells). The finer powdered the better for max contact but the patterns made by leather added to the mystique in the old days. The purpose of the charcoal is well obvious and the lime is there to decompose into CO2 driving out the air before it can cause oxidation of the part and to discourage the carbon from burning away.
You can find people selling the mixture like it was made of gold dust either white with black specs or black with white specs 'cos the haven't the slightest idea how it works just had some old notes that said you needed both ingredients.

I don't think so. Some Carbon diffuses into the Iron, fitting into the gaps left between atoms and altering the crystalline structure at the surface. Case hardening doesn't add an external layer as does electroplating or paint, it's a solution effect.

Similarly, dissolving a spoonful of sugar in tea doesn't increase the total volume by a spoonful because the sugar dissolves in the liquid.

Dave

I'm told you can achieve some level of surface hardness using sugar. Hot plus carbon. If I were going to the trouble I'd get some kasenit. Tubal Cain book on hardening and heat treatment is highly recommended.

When I worked on narrow gauge locos we had case hardened pins in case hardened holes. I think they have since fitted thin bushes, but still hard pins.After 100 years the holes had gone oval, so bore out with carbide and fit bushes.

60 years ago I worked as a draftsman at an Axle manufacturing Plant in Spencer, Ohio. We made Axles for various Ford and GM Trucks, Jeeps, and large Axles for large Off-Road Equipment.

The typical Axle for the large Off-Road Equipment was around 3" diameter x 68" long. Both ends were upset forged. On one End was an SAE Spline. On the other End would be a Spur Gear about 5-1/2" OD with about 18 teeth (these are all rough numbers from memory). The splined End would enter the Differential Gearing. The Spur Gear would be the Sun Gear in a Planetary Gearing set-up in the Wheel Hub.

These shafts were made from SAE 8620 and 9310 Steels. The last two Digits on the SAE number lists the Steel's Carbon content (xx20 = 0.20% Carbon, xx10 = 0.10% Carbon). Note the normal Model Engineer's Cold Drawn Steel (or Bright Steel - a term not used over here) is 1018 (0.18% Carbon). A common leaded Steel is 12L14 (0.14% Carbon). Ball Bearing Races are made from 52100 which has 1.00% Carbon.

We would fully machine the Axle Shafts. We did not have Gear Grinding equipment but would use a Gear Shaver to improve the Gear Tooth finish, to bring the Gear to final size, and crown the Gear Tooth Profile. The end of the Shaft would be tapped 1/2", still preserving the Lathe Centers on both Ends. The Shafts were held vertically in the Carburizing Furnaces., say qty 80 at a time. It was always a sight to see them removed and quenched in the Oil Tank - lot of Flames. The Oil Tank had a large Screw (Propeller for the non-nautical Types) in the bottom which churned the Oil to eliminate hot spots.

As SOD says above the Carbon diffuses into the Steel (which is on the order of 97% Iron). The outside Surface would probably be in the Range of 0.90 to 1.00% Carbon. The Carbon would diffuse a good 3/16" into the material but the percentage would be less and less. After quenching, the Axles would be tempered. The 86xx and 93xx part of the SAE number indicates the Alloying composition - 10xx steels have no Alloy Constituents. The Issue arising during Quenching is the Heat Transfer Rate. Some Alloys reduce the required Heat Transfer Rate to obtain proper Grain Transformation. That is the reason many Alloy Steels can be Oil quenched. The advantage of Oil Quenching is slower Grain Transformation which leads to lower incidence of cracking. It also allows the Steel deeper in the part to harden where the Quench Rate is lower than near the Part Surface.

At this point a couple of Gear Teeth would be cut from a test Sample Shaft, about 1/4" thick. I would place the Sample in a Mold, pack it with some sort of thermosetting Plastic, heat it up and press a Plunger to encase the Gear Sample. I would then polish the exposed cross-section of the Gear Teeth on a Lapidary Wheel, Acid etch it and study the Microstructure under Microscope. I would them use a Diamond Point Rockwell Machine to find the location of the 50Rc point of the Case as it transitioned from 62 Rc on the Surface to 28 Rc in the Core. The distance from the Surface to the 50Rc point would typically be .06" (from memory). I made the measurement with the Microscope and recorded it with the Heat Treat Lot.

If the Heat Treat Properties were incorrect, it would be back into the furnace (which could create. another set of problems). Depending on the Manufacturer's Lot of the Steel, the Spur Gear would increase in size which led to annealing the Shaft and re-shaving the Gear Teeth - but that did not happen often. They would make that correction on all further Axles made from that Lot of Steel.

If an Axle required an area to be non-carburized, it would be copper-plated before going into the Carburizing furnaces. It was common for the Customer to specify the Outside Diameter of the Spur Gear to be non-carburized. This prevented chipping of the Gear Tooth Edges. In this Case the End of the Axle was copper-plated before cutting the Gear Teeth.