Heat Treating & Resisting Wear

To give a material the toughness and hardness needed to be an effective wear detail, you have two choices: altering the properties of the base material, or introducing another material to the base material. Heat treating falls into the first category and is (at least) a two step process: the first step, hardening, involves taking a high carbon steel and heating it to the point where the pearlite in the steel is transformed into austenite and then quenching it to lock in the new structure, which makes the resulting material brittle and hard. To reduce the brittleness and hardness, and to stabilize the material, a secondary process called tempering (or drawing back) is used, which is a controlled reheating and cooling of the hardened material. Depending on how much the steel is tempered, hardness (the ability to resist deformation and wear) is traded for toughness (the ability to resist fracture). This tradeoff is very important, because as a general rule, the harder something is, the more durable it is, but also, the more likely it is to fracture under use, whereas lower hardness values create more wear and deformation under load, but the detail will be less likely to break.

The second category, introducing another material, can be a totally additive process like plating, an infusion that allows the base material to behave differently, like caruburizing, or a combination of addition and infusion like hard anodizing. A brief description of these methods is given below.

First, a Few Words on the Rockwell Hardness Scale

The standard hardness scale in our industry is the Rockwell "C" scale (denoted as R/c). Hardness testing is done with a standardized indenter (that standard size is where the C comes from; there are also A and B indenters) under a controlled load and the resulting indentation depth corresponds to a hardness value. The R/C scale runs from 1 to 72, but practically speaking, we typically only see values from around 35 to 65.

Through Hardening

Any tool steel will always through harden and that needs to be kept in mind when designing your details. There is no such thing as selective hardening of a tool steel because the higher carbon content is present throughout the entire piece. Thin walls, sharp corners, thick to thin transitions, etc, all create stresses in through hardened details that could cause a fracture, and should be kept at a minimum, although tool steels such as A2 and S7 (see Tool Steel Selection post) are pretty forgiving even when machined into complex shapes as long as the application isn't too extreme (high impact, material held in tension, etc).

Because of "modern" (ok, 50 years or so) techniques such as vacuum heat treating, control of the final hardness can be achieved pretty well with tool steels, but it is typical to specify a range of acceptable hardness rather than a single value. For the majority of details, R/c 56-60 is a good choice. It's a pretty decent mix of hardness and toughness, and, it gets one tempering, so it's possible to send details out one day and get them back the next (depending on your local logistics, of course). Going to a lower hardness requires secondary tempering and pushes the delivery into an extra day or sometimes two. But, expediency shouldn't be the primary factor over function. Remember the tradeoff between hardness and toughness? Even a shock resistant material like S7 (see Tool Steel Selection post) loses a lot of its benefit if the steel is left at R/c 56-60. Any time that you anticipate shock loads or are forced to have thin walls with no radii, you should always specify an R/c of 50-55. That applies to all tool steels, but especially when you go to S7 because if you've chosen S7, you are looking for that capability anyway.

Surface or Case Hardening

There are several instances where you don't want to through harden something. Examples are male threads (see through hardened thread post), barrel and plate cams, certain shafts, and strike anvils. Even when you draw back the hardness on tool steel to R/c 50-55, the piece is still many times more brittle and prone to stress risers than soft steel. But soft steel would not be adequate for most part touch, impact, or wear situations. The solution to this is surface or case hardening a low carbon (non-tool) steel (see Low Carbon Steel Selection post). The upside is an excellent combination of wear resistance with none of the brittleness. The downside is it usually requires more time at heat treat and because the underlying material is not as stable when heated, it almost always requires more secondary ops than a tool steel, so it can be pricier overall and time consuming. But again, you can't implement a bad solution just to save time or money. The idea is to look for the lowest cost solution that performs as needed. Here are a few techniques for surface hardening.

Carburizing

For this process, a toolmaker will make a slightly oversized detail from your print and send it to the heat treat facility. They put the details (in bulk) into a unit that floods the details with a gaseous form of carbon under pressure and heat. This literally infuses the steel with the carbon. The time and pressure and the part geometry controls the depth of penetration. Then, the piece is heat treated just like a normal piece of tool steel, but the lack of carbon in the center keeps that soft. The detail is then brought back and finish ground to final dimensions, maintaining the case hardness. There are two things that need to be considered, one by you and one by the toolmaker: first, when designing details, case hardening is only really effective on details that are at least 1/4" thick, or even more if possible. While they have a decent control of the carburizing process, it isn't an exact science. You can get depths as low as .02-.03" [0.5-0.8mm] and as high as .300 [7.5mm], so if your material is too thin and the carburization is too deep, it can eliminate the benefit of a soft core. Regardless, your callout on your print should specify a max thickness for the carburizing (ie, 8620 Carb & Harden .06 Deep Max, R/c 56-60). You can get hardness pretty much all the way to +60R/c with this method. As far as the toolmaker, he or she has to make sure they don't put too much extra stock and there is still a case left when they do final operations.

If you want the core to also harden to maybe a R/c of 20 or so, a higher alloy material such as 8620 cold rolled should be used. It's a little more expensive, so if that tougher/harder isn't needed, a simple 1018 cold or hot rolled steel can be used.

Nitriding

Nitriding is another great option because the process doesn't occur at above the transformative temperature of steel. So you can use low carbon steels, high carbon tool steels, or even stainless, and the underlying metal won't change or move. You can achieve hardness above 60 R/c, so there are very few limitations there. It would be my go-to solution, as long as you recognize its main limitation: it can only penetrate about .02-.03" deep [0.5-0.8mm] into the metal. This means that you can "dent" it with a hard blow since the case is so thin and the other drawback is that the hard case could be machined off if you do any post treatment ops.  

Selective Surface Hardening

Sometimes, it is advantageous to only harden certain parts of a detail, and there are techniques that can be utilized to make that happen. Here are a couple:

Carburizing & Nitriding

There are two ways to utilize the carburizing and nitriding processes described above selectively. First is masking. This is easier with nitriding because a coating can literally be painted or dipped on that will not allow the nitriding process to affect that area of the detail and the relatively low temperature of the nitriding process won't affect the masking material. There is a similar method that can be employed with a carburized detail, but because the temperature is so high, it is actually a copper plating that is used, so, much more expensive and time consuming to have done, but it is possible.

Second, you can make areas of the detail that you want to remain soft a larger "blank" that can be machined post-treatment so that the hard shell is removed. This is a pretty good way to go, but does require carbide tools to break through the shell, so it's not the easiest machining to do. Follow this link to see an example of how to apply this method.

Flame Hardening

Flame hardening conjures up images of medieval blacksmiths making swords for knights, and it is a bit of a throwback technology, but it's very effective in a few situations. As the name implies, a torch is used to bring a very localized area of a carburized detail through the transformative stage to heat treat that very specific area. It's used mostly for large plate details that have wear surfaces such as cam paths in them. This is done this way for two reasons: first, it's cheaper than a giant plate of tool steel that's through hardened, and second, you will likely not believe it until you experience it yourself, but there are all kinds of stresses set up in through hardened, large details. I have seen a 15" x 30" A2 plate with a cam path cut into it crack literally in half after only being in service for a few months.

Plating & Coating

I included this because plating and coating can be used as a case hardening method, but as the names imply, they are additive processes instead of transformative, which means you almost always have to allow for the plating thickness in your design and do post-plating grinding (unless you have a wide open tolerance situation).

The most traditional high wear/high hardness (R/c 65-69) is hard chrome plating. You should design your details around .006-.010 [.15-.25mm] undersize in the area that needs the high tolerance, specify .010-.015 [.25-.40mm] plating thickness, and finish grind to nominal.

For aluminum parts, hardcoat anodize is the primary method for adding wear resistance to details. Using aluminum as a wear detail is almost always less desirable than treated steel, but for certain applications (plastic or bronze on aluminum, low duty cycle applications, and applications where weight is the most important factor), a hard anodized piece of aluminum may fit the bill. It adds .0005-.002 in overall part dimension and has good wear capability, although will still dent rather easily upon impact due to the thinness of the layer.

There are a ton of other coatings that I have limited or no experience with, but would be worth a google search if you need a lubricity or other characteristic beyond what hard chrome provides for your details. Diamond Electroless Nickel, NIBOR, and diamond chrome are just a few.