I have heat treated Black Walnut many times.It works great.One of my favorite bow woods. I make them mostly out of saplings.Ax them down green , put them on a 5 inch back set form, and let them dry in my basement. Before I take them off the form I heat treat them.Once tillered I heat treat them again.Belly heat treat with Walnut makes a big difference in performance and set.Build it wide, and at 27 inch draw I make them 62 inches long Pyramid style.Two inches coming off of the fades to five eighths at the tips.Low density wood makes them very light ,and pretty snappy when built properly. It is an underrated bow wood.It can ding easy , but just adds character.Bob.