Substitute molten salt bath with temperature range from 400–500 °C I'm looking for a suitable substitute for a $\ce{NaNO2/KNO3}$ salt bath (for quite obvious reasons due to the other applications of the latter chemical). The salt mixture needs to have around 400 to 550 °C of molten working temperature (the lower ranges are not interesting), and preferably be reasonably safe to work with (as safe as the aforementioned salt bath, so totally harmless isn't the goal). The purpose is precise annealing of brass alloys in a home shop environment (so no fume hoods and blast cabinets available).
I wasn't sure what other conditions you needed for your salt bath, yet I listed few for your convenience together with the salt bath of $\ce{KNO3/NaNO2}$ (bath 5) for comparison:
$$
\begin{array}{ccc}
\text{Bath#} &amp;\text{Composition} &amp; \text{Approx. melting poing} &amp; \text{Working temp. range} \\\hline
1 &amp; \ce{NaCl}, \ce{KCl}, \ce{BaCl2}, \ce{CaCl2}  &amp; \pu{400 ^\circ C} &amp; \pu{500-800 ^\circ C} \\
 &amp; (10\!-\!15 \!:\!20\!-\!30 \!:\!40\!-\!50 \!:\!15\!-\!20) &amp;  &amp;  \\
2 &amp; \ce{NaCO3}\, (45\!-\!55), \ce{KCl}\, (55\!-\! 45)  &amp; \pu{450 ^\circ C} &amp; \pu{550-900 ^\circ C} \\
3 &amp; \ce{NaCl}\, (20), \ce{KCl}\, (30), \ce{BaCl2}\, (50)  &amp; \pu{540 ^\circ C} &amp; \pu{570-900 ^\circ C} \\
4 &amp; \ce{NaNO3}\, (100)  &amp; \pu{370 ^\circ C} &amp; \pu{400-600 ^\circ C} \\
5 &amp; \ce{KNO3}\, (50\!-\!60), \ce{NaNO2}\, (50\!-\!40) &amp; \pu{135 ^\circ C} &amp; \pu{160-550 ^\circ C} \\
\end{array}
$$

I also listed $\ce{NaNO3}$ bath in case you won't mind using it since it is not potassium salt and working range is ideal for your requirement. As indicated, you can change composition as much as you wanted to to get right melting point and the working temperature range you wanted to.