When I made a replacement pin for my dad's Walnut Hill thirty-odd years ago, I took the advice of a toolmaker at work to use oil-hardening drill rod for its added toughness (as compared to water hardening), and to draw it back from the quench to about a Rockwell C scale hardness of 50. An 800°F temper, IIRC. That's about what is used for high quality hammer heads so they can strike nails without indenting significantly or cracking and shattering. I kept the original rectangular profile.
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