Many of us probably think a bolt is just a bolt; some of us may not even know the difference between a bolt and a screw. Heat treatment is the key difference from one bolt to another, and understanding that difference is key to correct assembly design. If the wrong bolt is chosen for a given design, it becomes the weakest link and catastrophic failure will result.
A bolt can have a square or hexagonal head, and it is threaded only part of its length. If the entire length is threaded, it is a machine screw not a bolt. A bolt is a cylindrical (not conical) threaded fastener that is held in place by a nut or a threaded hole on the other side. Bolts are made from a variety of materials with steel being the most common. For the sake of simplicity, the rest of our discussion will pertain only to inch-grade bolts, not metric.
Now that we have identified what a bolt is, it is important to understand that not all bolts are created equal. All begin by cold forging a piece of bar stock to a near net shape, called “heading.” For commercial bolts, the threads are then put in by another cold deformation process known as thread-rolling. It has been found that thread-rolling after heat treatment (vs. before) enables the bolt to carry double the load. Under constant loading conditions, rolling threads after heat treatment extends bolt fatigue life 100 to 1,000 times. Fasteners for aircraft and special aftermarket fasteners are made this way.
High-strength bolts usually have a hexagonal head with markings indicating the heat-treated strength level. The material strength is indicated by the grade. The Society for Automotive Engineers (SAE) has established a sequence of grades from 0 to 8. Commercially available grades run from 2 to 8, with 8 being the strongest. The lowest grades are not heat treated. Beginning with grade 5, bolts are heat treated to attain the final mechanical properties. This heat treatment is typical of medium carbon and alloy steels requiring austenitization, quenching and tempering at the appropriate temper to attain required properties.
The American Society for Testing Materials (ASTM) has a similar grading system. The head markings are the same for the same strength levels, but ASTM adds numbers in some cases (e.g., A325 or A490). Metric bolts carry an entirely different numbering system that relates to the required mechanical properties.
Even with proper heat treatment, here are a few facts to consider when fastening with bolts:
- It is not always a good idea to replace a bolt with a stronger one. Some bolts are deliberately chosen so that they are weak enough to fail before the stress or strain damages a more critical part of the assembly.
- Regardless of the heat-treated strength, bolts stretch slightly under load and the distribution of stress on the threads is not uniform. In fact, regardless of the bolt length, the first thread takes a third of the load with the first six threads taking the entire load. That’s why most nuts are not much longer than six threads.
- Two threads should be exposed above a nut because the first two threads of a bolt are often poorly formed and may not engage the nut properly. This may result in stripping the nut.
- Do not reuse nuts. Properly tightened nuts yield to distribute load among the threads.
- Using a washer under the head helps to prevent any drill burrs from scratching the bolt fillet, causing a stress riser.
Now that we understand what goes into the manufacture of a bolt, we can see that we would not be able to keep it together in stressful situations without heat treatment.