Hi William and Paul,
I just did a file bite test on a Seth Thomas pallet arbor. It bit in but It does appear to be harder than average. Your arbor may have snapped due to the flat that’s milled across it for the pallet strip not having a corner radius. You can check the hardness on the old arbor by drawing a file across it. If you’re using a high carbon steel then you can harden and then temper it down to a blue color. This will leave it fairly hard (tough) and will take the “brittleness” (is that a word?) out of it. I’ll try to find the video that covers this in a few minutes and will post it up here.