Nothing affects the properties of a material quite like temperature. Think of your own body and the constant need to regulate the temperature it is exposed to in order to function well. Although we’re not going to put a parka or Bermuda shorts on them, temperatures also affect fasteners in a big way. And in the design and construction phase we need to take those effects into account. What works for fasteners like Hex Bolts may not work for Hex Head Cap Screws. Not all Nuts and Bolts are created equal.
The key is to understand which materials are best in which situations.
Potential Problems
Obviously the thermostat swings both ways. There may be instances or prolonged exposure to extremely high heat (such as in an engine) or your design may call for resistance and strength in very low temperatures (such as exterior applications in a northern environment). Each circumstance could potentially cause the fastener to weaken or seize, as well as affect the plating and coating.
Heat can bring on oxidation and both extremes present the danger of corrosion. Basically, your fasteners need to able to handle the temperature or they will break down and fail.
High Heat
Most nonferrous (or those that do not contain iron) fasteners should be avoided in high temperatures. Plastics are out. Choose medium carbon (Grade 5 fasteners, for example) or low alloy steel for hot temperatures up to 450 degrees. But if your application is pushing above that range you will need to opt for a stronger material and possibly even move into heat treatments.
For conditions ranging between 450 and 900 degrees stainless steel is a great option. You’ll also be handling corrosion resistance with this option, although steel containing chromium-molybdenum can also be used.
From 900 degrees on there are super alloys that are specially developed for resistance and stamina. Inco 718 and A-286 are recommended up to 1200 degrees and fasteners made from nickel-cobalt alloys can withstand heat to 1600 degrees.
Heat Treatments
Ferritic steel (which covers steel that is mainly made up of ferrite and contains some chromium) are heat treated for strength in extreme temperatures. The process is somewhat different for each grade of steel, but generally the material is cooled to a low temperature, reheated to refine the grain and then quenched in liquid or normalized in air before being reheated for tempering.
There is a stated minimum temperature for the tempering process that ranges from 1100 degrees to 1200 degrees Fahrenheit, depending on the grade. Also, certain grades stamped with B6 and B6X are held at the tempering temperature (not that’s a tongue twister) for at least one hour. If the material is stress-relieved after tempering tests are necessary.
Stainless steel is treated in a slightly different way. The material is cooled then reheated, held there until the chromium carbide forms a solution and then cooled (in air or liquid). This process will result in a Class 1, 1B or 1C fastener. When a 1A class is needed the material will also be treated with a solution once the heating and cooling processes are done.
Extreme Cool
In certain circumstances your fastener may be exposed to prolonged cold. In consistent temperatures below minus 30 degrees there will be problems with toughness and ductility and the metal can become brittle. This means that materials that are notch sensitive shouldn’t be used in extreme cold, materials like low carbon steel (Grade 2 fasteners, for example).
Nickel-rich grades like AISI 4340 and 8740 can be used if the temperature stays above – 100 degrees Fahrenheit. For frigid conditions between -100 and -300 degrees Fahrenheit stainless steel is your best bet. Non-metallics have a chance to shine in cold temperatures, especially Teflon. Also alloys like Unitemp 212, Inco 718, Waspalloy and A-286 stand up to extreme cold. Interestingly enough, these same alloys also stand up to extremely high heat (up to 1200 degrees), creating a high demand in these unique circumstances.
Temperatures affect different fasteners in different ways. With some basic knowledge you can make a good choice, something that will stand up to whatever the surrounding climate throws at it. Research steams ahead at full power thanks to space exploration, where extreme temperatures take on a whole new level. But those of us still down here on earth are grateful for the information and data that helps us to make strong, dependable choices.