Trust me on this one....through hardened male threads pretty much always end up looking like the pic you see here. Not sure if designers just haven't been shown the results of their design when it does fail, or when happens during debug, maybe they think it was a fluke and it won't happen again, but I've seen this story repeated over and over again. And there really doesn't seem to be a certain size that makes it go away or any rule of thumb along those lines. The bigger the shaft, the harder people torque on it, and the stress risers from the threads and the undercut are still there, so at some point, it will likely pop. It may work while it's on your floor, it may even work for a year, but the odds are very high that it's going to break. So, it's best to avoid the situation altogether. How? There are a couple of ways that work well. First, if you absolutely need male threads on a hardened detail, you can use a heat treating technique called carburize and harden, or maybe nitriding on smaller pieces. You could even hard chrome plate only the wear resistant area, but the common theme to all of these techniques, (which I explain in more detail on my heat treating and wear resistance post) involve using a non-tool (mild) steel (see posts on tool steel and low carbon steel selection) and only creating a surface hardness such that it leaves the core of the threads soft, and therefore, less brittle. I have modeled up a detail that would be suitable for this technique and outlined the steps for making it work (see below). It's all about creating a larger blank so that whatever method you choose to case harden the detail, the hardness won't penetrate deeply enough to affect the threads that will eventually be machined into the part. That idea can be applied to any detail you can dream up that has the same functional requirement of a male thread coupled with the need for a wear resistant surface.

The second technique is to simply design your details to avoid the male thread altogether. For some reason that I'm sure a metallurgist or even a more well-read engineer could explain better than me, female threads don't seem to suffer from the same breakage issues as male threads until you get into some really thin walls around the thread itself, so if you can rethink your design to replace the male thread with a female thread (or a counterbored hole with the threads in the mating piece) and use a bolt to secure the detail to the machine, you can make the detail out of the most appropriate tool steel that will through harden and still be safe. Generally, I like this approach because it usually requires less processing of the part, is a simpler design, and the final product is more stable over time. Here is an example of how you might rethink the detail from above to take advantage of this idea.

I am sure you can take the ideas presented here and apply them to any number of details you'll have to design. I know it seems like extra work on your part, extra work in the machine shop, extra processing time at heat treating, etc, but it will save you a lot of headaches and angry customer calls if you take it to heart that all those broken threads are not just a statistical anomaly that you don't have to worry about.

Through Hardened Male Threads - No, You Won't Get Away With It Either