We continue our discussion of the heat treatment of fasteners.
Surface Hardening of Steel Fasteners
It is often desired that fasteners have a hard exterior while retaining a ductile interior. This is achieved through surface treatment, which affects only the outer layer of the steel. The end result is a fastener with improved exterior wear resistance and surface hardness while retaining fatigue strength.
Case hardening is a general term that refers to the formation of a hard outer surface ("case") on the fasteners by carburizing or nitrocarburizing, which diffuses the surface with carbon. Case hardening can be done on low-carbon steels, which would otherwise require a higher carbon content to provide the hardness.
Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures in the range of 850-950°C (1560-1740°F) then quenching to form a martensite exterior. Carburizing increases the grain sizes by dwelling for a long time in the austenitic region of the phase diagram, and requires a secondary heat treatment to refine the grains. Carburizing forms a hard layer approximately 0.5-1.5 mm (0.019-0.059 inch) thick.
Nitriding is a method of hardening the surface of the steel by saturating it with nitrogen, typically in the form of ammonia, which reacts with iron and other elements (particularly aluminum and chromium) added for this purpose. This results in hard nitrides, which cause a considerable increase in hardness. Nitriding is done at 500-600°C (902-1112°F). The depth of the surface hardness is determined by the alloying elements and the heating time (Fig. 2).
Nitrocarburizing is similar to nitriding except that carbon is also introduced. It is performed at temperatures of 750-880°C (1350-1616°F). After heating, the fasteners are tempered in oil, which reduces stress.
Heat Treatment of Aluminum Fasteners
Aluminum fasteners are heat treated using a two-part process of solution treatment followed by artificial aging. This is referred to as the T6 heat-treatment process.
Solution Heat Treatment
Solution treatment (Fig. 3) involves heating the aluminum to a temperature of 427-538°C (800-1000°F), at which point alloying constituents are taken into solution (i.e., brought near their melting point), prior to a rapid quench. This retains the grain structure but leaves the material soft, requiring a subsequent aging operation to increase the hardness. The heating time and temperature is dependent on the alloys contained in the aluminum and the cross-sectional thickness of the material.
The time required to extract the material from the furnace and quench it (quench delay) must be kept within a specified limit (typically 5-15 seconds), and it varies depending on the alloy being processed and the material thickness. Thinner, cold-worked aluminum alloys require a faster quench than cast alloys. The quench medium is most commonly heated water or glycol.
After solution treatment and quenching, aluminum is age hardened (aged) either at room temperature (natural aging) or more commonly at elevated temperature (artificial aging), which is referred to as precipitation heat treatment (Fig. 3). The aging process increases the strength and hardness of the material. Artificial aging requires temperatures of 115-200°C (240-392°F) and heating times of 5-48 hours. The time-temperature parameters for aging are carefully selected based on the alloy used and the desired mechanical properties.
Natural aging can be suppressed or delayed with some alloys until a controlled artificial age can be performed, typically for maintaining production flow and desired batch sizes. Conventional practice allows for refrigeration; e.g., of 2014-T4 alloy rivets at minus 18°C (0°F) for several days.