Figure 1. Gold-tin phase diagram[2]

I want to subcritical anneal a part in hydrogen that is made from 304 stainless steel. The annealing temperature I want to use is approximately 1700ºF (925ºC). The part is gold plated, and I noticed what appears to be the onset of melting. Am I in trouble?

Gold coatings are often used for their corrosion-resistant properties and due to their excellent electrical conductivity. The melting point of (pure) gold is 1947ºF (1064ºC), while the melting range of a 304 stainless steel is 2550-2650ºF (1400-1455ºC), so at first glance you might think that you are OK. However, the fact that you are observing the onset of melting tells me that there is a low-melting eutectic involved.

Eutectic melting occurs at a lower temperature than that of the melting points of the individual chemical elements present.

Gold plate is not always pure gold, so the melting point can change depending on what elements are present. If, for example, a gold-compatible solder is present, it is likely to be 80% gold and 20% tin. A look at the alloy phase diagram (Figure 1) shows that for this alloy the melting point drops to around 536ºF (280ºC).