I would wonder if the SS630 is the cheaper replacement for a far more expensive material and you have reached the bottom of the list. The alternative is someone who selected it to begin with rejected a cheaper material. They were apparently smart enough to make a design that is still in production, but you are hoping they made a mistake and left a huge price gap in the parts list?

All stainless is corrosion resistant, but the SS630 has a higher chromium content (I expect the main driver of cost) and will be more resistant than the SS420.

If this is a design your company did there should be design notes that include all the rejected materials and why they were rejected. I expect they didn't keep that information. A former company I worked with actively rejected proposals to keep the information about the rejects - not on liability grounds, it was a worry that "star" performers weren't.

Thanks 3DDave!
Actually the material was selected by my client few years ago and they only used it after getting life problem from using SS304. Due to the urgency, they released SS630 for production based on positive life test result. Now, customer is asking us to review any cheaper choice and that's why I raised out this question.
I am still curious any cheaper SS can replace SS630. But if no, I would abandon material change and find other way for cost reduction.

As listed, 17-4 PH meets your requirements.

Agreed 17-4 might be your best bid. another option you may want to try though is Ni free, N+Mn doped stainless. Ni lean or free could get you cheaper material cost; high N leads to a high work hardening rate. A cold worked grade will bring you enough strength/harness

630 is 17-4PH.
A 400 SS would work as long as it has >16% Cr.
An alloy like 440A would work.
But even if the alloy costs less it requires a quench and temper to reach properties.
Though at 40HRC (fairly soft for this alloy) you should be able to pre=harden the bar and machine in the hard state.
Make sure that they do not use a free machining grade.
These parts are being chemically passivated in acid right?

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P.E. Metallurgy, consulting work welcomed

Edstainless and all, thanks for your advice. It is helpful.
Yes, the part should passivated after machining.