Almost all wrenches are made of tool steel, a carbon steel alloy. The exact chemistry varies depending on the purpose of the finished item. Some can be brittle, so the alloy used for wrenches usually includes chrome and vanadium. These make for a very hard tool, but one that has ductility – a little bit of give. They’re less likely to fracture under pressure. Chrome also resists rust, although many wrenches have chrome plating for further protection.