Carpenter C-158 and 9130 are virtually the same steel. 9130 is the generic AISI designation and C-158 is Carpenter's name for their version of it. AISI specifies a range of content for all the significant alloys, and Carpenter may hold theirs to a little closer percentages in some cases, but it's essentially the same steel. It's the same with all the alloy steels, you can, for example, buy Carpenter's Vega, an air hardening tool steel, or you can buy generic A-6. We like to think we get a more consistent product with the name-brand steels...probably we do. Does it significantly affect machinability or final performance? Who knows?
Sorry, my dyslexia took over in the above post... I meant to type 9310, not 9130.
