Combination square blades are normally plain carbon steel, not stainless.
Actually, almost nothing in a wood shop is stainless because it sharpens poorly[0] compared to O1, A2, or any of the other tool steels you'll find in a wood shop.
[0] Yes, I realize that stainless and carbon steel are two whole classes of alloys, all with different characteristics. Based on my experiences with kitchen knives, I'll stand by my claim that at least the common ones sharpen poorly compared to the common carbon steel alloys.
Actually, almost nothing in a wood shop is stainless because it sharpens poorly[0] compared to O1, A2, or any of the other tool steels you'll find in a wood shop.
[0] Yes, I realize that stainless and carbon steel are two whole classes of alloys, all with different characteristics. Based on my experiences with kitchen knives, I'll stand by my claim that at least the common ones sharpen poorly compared to the common carbon steel alloys.