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Green tint after forging and quenching doesn't automatically point to a hazardous coating. Since the bar was originally used as a machine shaft and appeared more corrosion-resistant than typical scrap, stainless steel is definitely a possibility. Stainless alloys can develop unusual oxide colors after heating, and the exact shade depends on alloy composition, temperature, atmosphere inside the forge, and the cooling process. Old water can also contribute to surface discoloration, especially if it contains minerals, algae residue, or dissolved metals from sitting outside for a long period. If the color is only on the surface, try lightly sanding or grinding a small area to see whether the green tint disappears. That can help determine whether you're looking at a surface oxide layer or something deeper. The magnet test mentioned above is a good next step. Another useful check would be comparing a freshly heated piece of known mild steel in the same quench water when you get the forge running again. Interestingly, surface coloration caused by heat treatment isn't limited to forged steel. [Commercial material removed.] In this case, though, the lack of smoke and the limited rusting history make stainless steel oxidation seem more likely than a dangerous coating.