Sorry this was a bit slow in coming. Spring is here and yard work is taking over my life. Spring also means that my companies customers are gearing up for summer construction season, and some of them are under the impression that the economy is still chugging along. I am not convinced, but that doesn't mean that I can dodge customer service calls. Here is my article on carburizing of steels. Hope that you can get some good information from it. I'll follow this in a month of so with a report on the re-carburization of a 1934 Mexican Mauser that I have been reworking into a 7mm Mauser sporter.


Carburizing:

The idea here is to take a steel that is relatively low in carbon and alloy, make a part, and then add carbon to it so that it is harder, stronger, and more wear resistant. This process is commonly used to batch process large quantities of parts because it is very economical. It also allows you to manufacture your part out of cheap and easy to machine steels.

Alloy steels, as described previously, are hard on tooling. The alloys in them form carbides (chrome, molybdenum, vanadium, titanium) that chew up tooling and dies. If you make parts out of 4140 or 4340, you typically order it fully annealed so that it is as soft and easily machined as possible, but it is still abrasive compared to plain carbon 1040 steel.

To save on tooling costs, designers will specify a non-heat treatable steel with less than 0.25% carbon in it with the intent of having it carburized. In modern firearms and many other applitcaitons, 8620 is a common alloy used. It is relatively cheap, easy to machine, and specifically designed for carburizing. More on this later.

Before I start to describe the carburizing process, it is pertinent to have a short discussion on the phenomenon of grain growth. This is something that the carburizing process affects that needs to be understood and compensated for.

A Word About Grain Growth:

Grain growth? I the last installment on Heat Treating, the concept that metals are made up of smaller units was introduced. That is only part of the structure of metals. The crystals all arrange, like a brick wall, into what are known as grains. So each grain is an ordered arrangement of crystals. The grains then are arranged to make up the metal. Crystals make up the grains, and the grains combine to form the whole piece.

When the metal is initially made, it is melted, and cooled into a solid. The crystals form into grains as the metal solidifies, and then the grains butt up against each other and form the metal solid. The brick wall idea fits here as well. Each grain is a brick, and all of the bricks laid in make up the wall.

Now, imagine, if you will, a brick wall made up of different size bricks. Some being twice as large as others. Some being only a little larger than others. Nature doesn't like this situation, because the bricks don't necessarily fit together well. There are voids and tight spots here and there in the wall. It isn't a neat and orderly wall. At room temperature, the bricks have to stay put. There isn't any way for them to change.

I'll switch terminology here and start to use the term grains instead of bricks. So the grains are of different sizes, and they aren't fitting well together. To get the perfect arrangement, energy must be added in the form of heat. So you heat up the metal to whatever temp works for that metal. For steel, you generally need to get above 1150F. When the steel gets above the critical temp for grain growth, the iron atoms gain enough energy to move around and re-arrange themselves. The hotter you go the more energy is available, and the faster the atoms can move. This is what happens during annealing and is a valuable tool to engineer metals.

In effect the larger grains eat the smaller ones, and all that you are left with are large grains of approximately the same size. Why is this important? Toughness. As the grain size increases, toughness decreases. So, an ideal situation is one where all of the grains are small and all about the same size. This will come into play when the process of carburization is laid out below.

So the take away here is that high temperatures can increase the grain size, and can reduce the toughness of the part. The person designing the carburization cycle needs to be mindful of this and put in place the necessary elements to correct it.

How to Carburize:

Carburizing relies on diffusion. Diffusion is a process where a high concentration of one element or material moves to enrich a low concentration area. Think rain soaking through your clothes. Outside your clothes there is a high concentration of water. The water passes through your shirt to you and you get a high concentration of water on you in the end.

So, you put a piece of low carbon steel in a high carbon environment at high temp and it will absorb carbon. Carbon is absorbed as a constant rate dictated by the concentration of the atmosphere, and the temperature. The very surface gains the most, and there is a gradual decrease the further you go into the steel, until the carbon is equal to the base level.

The mechanism of carburization isn't precisely known, but it is theoretically worked out. At high temp carbon monoxide gas created by the carburizing media is reduced by the iron in the steel to form iron carbide. Because the part is at high temperature, the iron carbide immediately dissolves and the elemental carbon is free to work its way into the steel.

To carburize you need a furnace, and a source of carbon. The carbon source can be a gas, it can be a solid high carbon material (charcoal, bone, leather), or it can be liquid. I'll skip the liquid carburizing because it isn't very applicable outside of industry, and it involves some very dangerous chemicals (sodium cyanide). Carburizing using charcoal, or solid carbon sources is referred to as Pack Carburizing, and is the technique used by manufacturers at the turn of the 20th century. The current state of the art is gas carburizing.

Gas Carburizing:

For gas carburizing the part is placed in a basket and suspended in the furnace, or if the part is large, the whole thing is placed on spacers on the hearth of the furnace. The furnace is purged of air so that only the carbon containing gas is present in the furnace. Common gases used are methane and carbon monoxide. Once the furnace is purged, the temperature is raised to a typical 1600F to 1650F and the part is held at this temperature for a predetermined time.

The time at temp is based on the depth that you want the carbon to penetrate into the steel and is figured with an equation. When done the part can be cooled to room temperature, or dumped immediately into quench media. Parts that are not quenched, would be reheated in an inert atmosphere to prevent scale formation (black oxide on the surface) to an appropriate austenitizing temperature for 30 minutes to an hour and then quenched. This allows more control of the finished grain size. The austenitizing temperature would be around 1550F for most steels. In some cases, the part will be austenitized a second time at a lower temperature, and quenched again. This refines and shrinks the grain structure of the core material to improve toughness. There will be more on this in a later chapter.

The quench media could be water, brine, or oil. This ensures that the carbon enriched surface transforms to martensite, and provides the hardness and wear resistance that you are after.

Pack Carburizing:

To pack carburize parts, you place the parts in a container and pack charcoal, bone charcoal, etc. around the part making sure that it is completely covered inside and out. The container is sealed from the atmosphere, and placed into the furnace. Again it is heated to 1600F-1650F and held for a predetermined time. As in gas carburizing, the parts can be directly quenched, or cooled and heat treated in a separate step.

The solid material used for carburizing can be as simple as powedered charcoal made from wood, bone, leather, or about any carbonaceous material. It can also be more complex and have additives that increase the carburization rate. A common mix is powdered wood charcoal, and 5% by weight barium carbonate, or sodium carbonate. The carbonates add a little oxygen to the system, which in turn creates carbon monoxide and increase the carburization rate.

There are several sources of pack carburizing media. Park Metallurgical is one, and Brownells sells media as well. There are also case hardening compounds such as Kasenit that do the same thing. Wood charcoal is the slowest media to use if it is used on its own. It contains very few impurities as they are burned off in the charcoal making process. Bone charcoal is probably the best single media choice. Bones contain many elements other than carbon and these other elements make the charcoal produce the needed CO gas more readily. So bone charcoal needs no additives to be effective, and wood charcoal is best if mixed with bone charcoal or mixed with an energizer (barium or sodium carbonate). Sodium carbonate would be the better choice since barium containing waste is generally considered hazardous.

Problems with this processing are part distortion or warping, and inconsistent case depth everywhere on the part. Distortion and warpage typically are the result of either the wrong quench media or improper part packing in the furnace. If the quench is too fast it can warp or crack the part. If the part isn’t all the same temperature, or the part is packed with other parts, it will not quench everywhere at the same rate, and it can warp. So, keep your parts separated, and don’t put too much in the furnace at once. Also use a quench media appropriate for the size of the part. To minimize warpage, it is best to air cool from the carburizing temperature, and heat treat in separate steps.

The time it takes to carburize a part to a depth of 0.020” is between 2 and 4 hours depending on the type of media the parts are packed in and how much material the furnace is heating up.

Charcoals are terrible heat transfer agents. Therefore it takes a long time to get the part up to temperature when it is being insulated by charcoal. This adds a good hour to the process just to get the part up to a sufficient temperature to start carburizing. Further information can be provided about carburization time and depth if needed. Please PM me.

The End Result:

The Carbon on the surface ends up at 0.7% to 1.2% depending on process conditions and the hardness achieved is 60-65 Rockwell C at the surface. After quenching, the part should be placed in a furnace again, heated to300 to 350F and held for an hour. This is called a safety draw, and it is intended to stress relieve the part to some extent and prevent cracking. For case hardened parts, temperatures above 400F are generally too aggressive and soften the case too much.

Keep in mind that the case hardened layer is not uniform throughout. The case at the very surface will have a higher carbon content and hardness than it will a few thousandths of an inch in. This is because the carbon diffuses slower the farther in it has to go. So a case with a depth of 0.030 inches will have a carbon content of 1% at the surface that gradually decrease to the base metal carbon content at 0.030 inches into the steel. This means that the hardness also decreases the deeper you go.

Polishing, and lapping of critical areas removes the hardest and most wear resistant part of the case. The case thickness is also reduced over time through normal wear of parts against each other.

To summarize, you place the part in a carbon rich environment at 1600-1650F for several hours, and then quench it into water, brine or oil. The surface now has a layer that is high in carbon, and after quenching is very hard martensite. Finally, the part should be tempered at 300 to 350F for an hour.

Double Heat Treating:

Earlier the concept of grain growth was discussed. In the carburization process, the part is held at very high temperature for in some cases many hours. All of this time, the carbon is enriching the surface, and the austenite grains are growing.

When steel is heated over 1330F it starts to transform from its low temperature phase called ferrite, to its high temperature phase called austenite. This was discussed in the heat treating article.

Ferrite grains can grow at temperatures of 1150 to 1330F. Above 1330, austenite grains form from the ferrite and the austenite grains grow as long as they are above 1330F. Carburization is carried out at 1600-1650F (typically) and there is a lot of drive for the austenite grains to grow.

When the part is subsequently quenched, the case transforms to martensite, but the core material transforms to either pearlite (a layered ferrite carbide structure), or a mix of pearlite and a small amount of martensite (thinner sections). The pearlite that forms from large austenite grains is very coarse and relatively brittle. This situation means that the whole part has been embrittled, and further work must be done to correct the issue.

Enter the concept of double heat treatment. I have no proof, but this is where I believe the heat treatment of low serial number 1903 Springfields went wrong. From my limited research on the topic, it appears that they chose too long of a cycle. This gave them a unnecessarily deep case, and a coarse pearlite core structure. The fix for it was as follows.

After the part is carburized, let it air cool. We know that the surface carbon content is between 0.70% and 1.00% carbon. And if we selected the material, we know that carbon content of the base metal. If you don't know the base metal carbon content, an assumption can be made that the carbon is around 0.25%. This is the lowest carbon content that would be used for a carburized part. Knowing the carbon content of the base metal is important in the next steps, but not critical. Close enough will produce acceptable results.

For a steel with a carbon content of 0.70%, the temperature where it is100% austenite is 1340F (roughly). For a 0.25% carbon steel, this temperature jumps to 1550F. These temperature are taken from the iron-carbon phase diagram. This diagram is a very useful tool for steel design and treatment, but it is difficult to describe its use.

Now, you can see that the austenite transformation temperatures for each region within the part are very different. This can be used to our advantage to refine the structures in each region, and make the part as tough as it can be. Add 50F to the temperatures above to ensure that the part is fully austenitic in the heat treatment cycles described next.

First heat the part to the austenitizing temperature for the lower carbon core: 1600F. Hold for 30 minutes to ensure that it is fully austenite, and quench into oil. This process transforms the pearlite into austenite, but doesn't allow the austenite to grow due to the short time. The small austenite transforms into small pearlite or pearlite/martensite forming a tough core.

1600F is too high a temperature for the case, and it will end up with a coarse martensite structure that isn't as tough as we need it to be. The next step will refine the case structure and restore its toughness.

Step two is to heat the part back up to the austenitizing temp for the high carbon region: 1400F. Hold for 30 minutes, and quench into oil. 1400F isn't high enough to fully transform the core, so it maintains the fine structure that we imparted to it in the previous step.

Perfrom a safety draw at 350F for an hour, and check the hardness using the C and D scales. The C scale checks the composite case plus core hardness. The D scale checks the case hardness alone. Temper the part again to get the hardness into the range that is desired.

Jeremy

Interesting, and a good read. Thanks!

"Hatcher's Notebook" has an excellent description of the low number 1903 HT problems.

My 1943 M98 was redone because of very spoty
carburizing as was common during WWII.However it was tempered back to eliminate all possibility of a too hard/too brittle case . I don't remember the hardness though ,it's been too many years though the procedure met with my approval. Present day use of 4140 is through hardened to about 40-42 HRc, again to eliminate any brittleness.

Mete,

Thanks for the info. I have seen many references to that work, and will have to find a copy.

Jeremy

Thanks, again, for sharing such learned and erudite knowledge.