Sergey Gricay continues to experiment with Substance Designer, building some fantastic stuff. Most recently, he uploaded a free Bolt Generator and made it available for free on Gumroad. The pack includes the filter (.sbsar) that generates the texture of the bolts (can be used with the free program), brush for Zbrush (.zbp), and an archive with alphas that can be used as a brush (.7z).
Sergey promises to update this section occasionally. Enjoy your bolts!