F3125 Manufacturing Process
Cutting
This is the process of cutting steel round bar to the desired length. This can be done by either shearing or saw cutting.
Shearing is typically done with smaller diameters and can be completed in a shorter amount of time. Saw-cutting is a longer process usually reserved for larger diameters or small quantities.
Headed bolts are measured from the underside of the head to the end of the bolt. To account for the steel required to forge the head extra steel is added onto the length that is cut and will then be compressed, or forged to create the head.
Threading
There are two options when it comes to threading. Threads can be either cut or rolled.
Roll threading is the faster of the two options but is typically reserved for smaller diameters and low to medium-strength steel. The process of roll threading is pressing the steel into the proper shape, displacing the steel, and forcing it between threading dies.
Cut threading typically takes longer but can be used for longer thread lengths, larger diameters, and high-strength steel. The process of cut threading is when steel is cut away from the round bar to form the threads.
Heading
The heads of the bolts are made by heating up one end of the rod and using a hydraulic press or upsetter to forge the heads. The process of forging is essentially forcing the heated steel into the proper shape. The F3125 specification requires the head of the bolt to have the corresponding grade symbol and manufacturer’s mark forged or engraved into the head.
Chamfering
Chamfering is the process of slightly rounding off the end of the threads and removing the first thread of the bolt. This is done in order to help nut assembly in the field.
Heat Treating
Heat-treated steel is commonly called quenched and tempered steel as that is the process they go through in order to obtain the proper ductility and mechanical properties for the specific grade that is required.
This process is essentially two steps. Bolts are heated to a very high temperature and then cooled rapidly, and then heated again at a lower temperature.
Bolts are sent off to heat treating after the heading process as the process of heating the steel to head it would unravel the proper properties obtained through heat treating.
Galvanizing
Galvanizing is the process of adding a corrosion-resistant protective zinc coating. This is typically needed for bolts that will be exposed to the weather and bolts being used near the coast.
The most common form of galvanizing is hot dip galvanizing. The process of hot dip galvanizing is when metallurgically bonded to the steel. This is done by dipping the bolts into an 840-degree tank of molten zinc. After a pre-determined amount of time, typically no longer than a few minutes, the bolts are removed from the tank and spun in a high-speed centrifuge to remove the excess zinc. The ASTM specification F2329 covers the hot-dip galvanizing of threaded fasteners
For the zinc to attach properly to the bolts they go through a process to prepare the steel. This includes wheelabrating the bolts with extra heat-treating scale. Then bolts are submerged in caustic soda which dissolves all excess first that may be on the surface of the bolt. Finally, the bolts are submerged in sulfuric acid to etch the surface of the bolts to allow the zinc to fully attach to the bolt.
Rotational Capacity Testing
According to the specification, galvanized F3125 A325 bolts that are being ordered with a nut and washer are required to be rotational capacity tested. The only way around the test is if the purchaser waves the test.
The purpose of the rotational capacity test, also known as the rocap test, is to test that the specific heat lot of nut, bolt, and washer as a unit, will function together with the required preloads. This testing needs to be done at least once per heat lot. The test is performed by using a torque wrench and Skidmore Wilhelm tension measuring device. To pass the test the assembly must reach a minimum rotation. This test typically adds around 3 days to the lead time.