There are often little notches, lines or numbers on the head of a bolt. Do you know what those are for? Those are indicators of the heat treating process that the bolt underwent before reaching the shelf at the hardware store.

Some of the physical properties of fasteners can be altered through thermal treatment after initial formation. In particular, steel alloys are heat treated routinely to increase their strength, hardness and other such properties.

There are multiple standards governing the heat treatment of bolts. The two most common standards are from the SAE International (formerly Society of Automotive Engineers) and ASTM international (formerly American Society for Testing and Materials). SAE International J249 covers bolts used in automotive applications, while ASTM International F3125 and A325 cover structural steel bolts. There are more standards from each organization covering corrosion resistance, high and low temperature exposure, and other physical parameters that require specialized fasteners

What is heat treating?

Heat treating is a thermal process where parts, such as nuts and bolts, are reheated and left to “soak” for a specified time. The temperature is well below the melting temperature of the alloy, but varies with its composition. The soak time is often determined experimentally, as the size of the part and its heat transfer capabilities will dictate how long the heating is required.

In addition to time and temperature, metallurgists can also alter the atmosphere in the furnace, as some alloys benefit from heating in carbon dioxide, carbon monoxide or nitrogen atmospheres. They can also alter the “quench”, meaning the cooling rate of the parts after heat treatment.

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Heat treating procedure

Standard heat treating requires either electrical or combustion to heat parts to the appropriate temperature. Electric furnaces are more common, due to the tighter temperature control possible with heating elements.

Different furnaces are available, some of which are batch furnaces and others are continuous. Batch furnaces are charged with parts, heated, and then allowed to soak. With continuous furnaces, parts are placed on a metal or ceramic process line and fed through different chambers and heating zones, such that new parts are constantly loaded in one end and heat-treated parts removed from the other. While continuous furnaces are often more time efficient, there needs to be sufficient parts processed to justify the additional capital and operating costs.

The heat-treating process also removes residual stress. The violent decrease in temperature during quenching or cooling leads to thermal stresses that are stored in the bolt. To relieve these stresses, a heat-treating process called annealing is implemented. In annealing, the temperature is increased to speed up diffusion, meaning atoms are allowed to reposition to locations that lower the internal stresses.

Another common heat-treating process is carburization or case-hardening. In this process, bolts are exposed to a carbon-rich environment, such as carbon monoxide or carbon dioxide gas. Within reason, the carbon content in steel increases the hardness. The carbon from the gas diffuses into the surface of the steel, making the surface harder. The time and temperature for this process are determined through the use of experimental data and diffusion constants, such that the “case hardened” layer thickness can be determined to thousandths of an inch.

Microstructure

Metallurgically speaking, steel can undergo several microstructural changes during heating and cooling. The microstructure, or how the metal looks under a microscope, ultimately determines the physical properties of the alloy. While composition plays an important role, two bolts with the same chemical composition can have different physical properties. Instead, the composition holds constant, but the individual atoms of iron, carbon and alloying elements move to different locations.

For example, an SAE Grade 1 bolt may have a tensile strength of at least 60 ksi (414 MPa). The same bolt that has undergone a quench and temper heat treatment becomes an SAE Grade 5.2, with double the tensile strength 120 ksi (828 MPa), with no change in chemical composition.

To determine what microstructure is likely, a Continuous Cooling Transformation (CCT) curve is used. Many bolts are quenched rapidly, forming a microstructure called martensite. As-quenched martensite is hard but extremely brittle, and is of very little use. If the bolts are cooled more slowly, they form bainite, or a combination of ferrite and pearlite.

Figure 1. A CCT Curve for a low-carbon steel alloy. Source: S zillayali/CC BY 3.0

One of the more common bolt microstructures is tempered martensite. This requires the steel to be quenched quickly, transforming all of the austenite (above 800° C in the figure) into martensite, the brittle but hard material. However, an additional heat treatment will retain some of the hardness, but also increase the strength and fracture toughness of the bolt.

Bainite is strong and tough, but it is not as hard as tempered martensite.

Pearlite is the weakest of the microstructures listed. It consists of thin layers (lamella) of ferrite and cementite (Fe₃C) ceramic, and cracks can propagate through the ceramic lamella. It is more ductile than bainite, as-quenched martensite or tempered martensite, meaning pearlite is used for other industrial applications.

Figure 2. A sample of AISI 1045 steel under a microscope. The dark areas represent Fe3C (cementite) and the light areas represent ferrite. The striped lamella of cementite and ferrite are called pearlite.

The tempering process only sacrifices a little of the hardness, but increases the strength; bolts are often made from tempered martensite. However, because the as-quenched martensite is so brittle, tempering must be performed properly.

Markings for heat treating

Rather than having to examine the physical properties of every bolt (which requires destructive testing), bolts that have been processed to standard are marked with indicators. The indicators are raised from the bolt head so that after years of service, they can be identified and replaced with the proper bolt.

Figure 3. A sample of AISI 1045 steel under a microscope. The dark areas represent Fe3C (cementite) and the light areas represent ferrite. The striped lamella of cementite and ferrite are called pearlite. Source: Wiki.Robotz.com

Care and feeding of heat-treated parts

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First, all heat-treated bolts should be purchased from a well-known and trustworthy vendor. Some “copied” bolts have been imported in the past that have the markings, but have not undergone the proper heat treatment. These bolts will fail prematurely. For critical applications, it might be worth sending the bolts to be tested at a metallographic testing center or university laboratory.

Heat treated parts must not be exposed to additional high heat. Heat-treatable parts can continue to harden and can become brittle if exposed to high temperatures. For example, welding near heat-treatable fasteners will transfer heat through the steel and can cause the fasteners to become brittle.

Case-hardened parts are harder and more wear resistant. Because of this, they should not be machined, as the case-hardened layer will be removed. Filing, sanding or grinding a case-hardened bolt removes the hardened layer.