HEAT TREATMENT
Heat treatment is a process used to thermally change the metallurgic (micro)structure of steel in order to obtain the required properties. This process benefits the working life of a fastener as it may increase its strength, surface hardness and temperature resistance.
ANNEALING
The steel is maintained at a temperature just below 721°C for several hours and then allowed to cool slowly. During this process, the structure of the steel changes from a hard lamellar pearlite into soft globular pearlite, which is the optimal condition for cold heading.
NORMALIZING (RECRYSTALLIZATION)
By heating the steel to between 800°C and 920°C for a short period and then letting it slowly cool down, the steel’s coarse brittle grain structure, which resulted from e.g., hot rolling or hot forging of especially thicker areas, is returned to its original fine grain structure. By reducing the grain size in this way, the yield strength and impact strength are improved without the tensile strength being significantly reduced.
STRESS RELIEVING
During cold forming, internal stresses are induced in the material, which increases the tensile strength but reduces the yield strength. Heating the steel fasteners to a temperature of 550°C to 650°C for about two hours’ time removes approximately 90% of these internal stresses. The fasteners must be cooled down slowly in a furnace or ambient air. A slow cooling speed is important in order to avoid internal temperature differences within the steel, which creates internal tensions. This type of heat treatment is used for cold-headed fasteners of property classes 4.6 and 5.6.
HARDENING
When steel with a minimum carbon content of 0.3% is heated to a temperature above 800°C and then quenched in water, oil or a salt bath, a very hard but brittle martensite structure forms.
The achieved hardness depends on both the carbon content – the more carbon, the harder the steel – and the percentage of martensite, which is formed in the core of the material at a certain critical cooling speed. With thinner bolts made of unalloyed steel, the critical cooling speed is affected all the way to the core. However, with thicker sizes, the heat from the core cannot be transmitted to the outside quickly enough, and it is then necessary to add alloying elements like boron, magnesium, chromium, nickel and molybdenum, which promote through hardening by decreasing the critical cooling speed.
In general, when a type of steel with such through hardening is chosen, approximately 90% martensite is present in the core of the material after cooling. The cooling medium also influences the cooling speed. Fasteners are mainly quenched in oil. While water is more effective than oil, its use produces too much risk of resultant hardening cracks and warping.
Martensite structure
TEMPERING
With increasing hardness, however, the hardening stresses rise, and therefore the brittleness of the material also increases. Normally, a second heat treatment, called tempering, must follow as quickly as possible. In temperatures of up to 200°C, the brittleness only slightly decreases, and the hardness is hardly affected. Above 200°C (high tempering), there is a clear reduction in the stresses; the hardness also diminishes, and the toughness is improved.
QUENCHING AND TEMPERING
Quenching and tempering is a combined heat treatment that comprises quenching immediately followed by high tempering at temperatures of 340° C to 650°C. This is the most important and commonly practiced heat treatment for fasteners. The process reaches an optimum compromise between, on the one hand, a reasonable level of high tensile strength and a high yield strength/tensile strength ratio, and on the other hand, sufficient toughness. Sufficient toughness is necessary for the correct operation of a fastener that must be able to handle all sorts of external loads. The higher property classes 8.8, 10.9 and 12.9 must therefore always be quenched and tempered.
CASE HARDENING
This heat treatment is a carburizing process that is carried out in a carbon emitting gas. A thin, carbon-enriched layer is formed on the outside of the heated metal, which becomes hard and resistant to wear, while the core of the material remains tough. Case hardening is applied to fasteners like tapping screws, thread rolling, thread cutting and chipboard screws. Similar treatments are carbonitriding, which uses carbon and nitrogen gas, salt bath nitriding and nitriding, which uses nitrogen gas.
INDUCTION HARDENING
For special applications, a wear-resistant layer is formed, without gas being supplied, in a high frequency coil without contact with the work piece. After heating, the steel is rapidly cooled down in oil or water. This treatment is often used to locally provide better hardness and wear properties on a part or to heat treat long parts, like threaded bars.
The relationship between iron and carbon, heat treatments, types of steel and mechanical properties are summarized in the figure below.
