Is it true that Forged irons rust?
Not completely – it still depends on the material used. But yes, the 1025 mild carbon steel Mizuno uses will rust if it comes into contact with moisture. The chrome plating offers protection in most cases, but moisture can still penetrate surface abrasions made by sand and grit. As long as you keep the clubs dry, with headcovers off between rounds, rusting won’t be a problem. Adding a touch of lubricant like standard WD40 occasionally will help too. The rust issue is a trade off for the performance of a Grain Flow Forged 1025 mild carbon steel clubhead. You have to decide if the extra care it takes to keep the irons in good condition is worth the trade off.
Why do cast clubs tend to be made from stainless steel while forged clubs are made from mild carbon steel?
Mild carbon steel could be used for casting, but when it is melted its viscosity is high; it doesn’t flow into the mould as well, and so does not fill up the detailed part of the mould as efficiently. Molten stainless steel is more liquid, so you can avoid bumps or not filling a recess. But also 17-4 stainless steel, the famous one, is very strong, twice as strong as mild carbon steel that is used for forged heads. The strength is good for design flexibility – you can make thinner walls in cavity backs for example – but its hardness means it cannot be forged so well; the material cracks or breaks when it is hammered.