The crankshaft may undergo extreme conditions of pressure, heat, torsional forces, and vibrational forces that necessitate careful planning when designing these parts and choosing the materials with which they will be constructed. The crankshaft has since the steam engine, which often used only a single piston and commonly appears in four-stroke and six-cylinder engines used in contemporary automotive design. Manufacturing of crankshafts has changed dramatically over the last century, including a bigger number of main bearings, more counterbalances, and more sophisticated heat-treating processes (Huppertz 2008).
The basic iron crankshaft is cast, and has the lowest strength, with a tensile strength of about 80,000 PSI. Nodular Iron is actually a slightly improved version of cast iron, with a composition by weight of C 3.4, P 0.1, Mn 0.4, Ni 1.0, Mg 0.06 and tensile strength around 95,000 (Walbot 2008). It has a yield strength of 63 KSI, elongation of 18% in two inches, and a hardness index of 170 according to the Brinell scale (Lyons et al. 2006). Nodular iron parts, such as crankshafts, are heat-treated after casting so that the graphite particles form distinctive spherical particles rather than flakes, which inhibit cracks and give nodular iron its name (Watson 2010).
The heat treatment process for nodular iron crankshafts is sophisticated, as they are complex parts that sometimes require additional stress relief. For such large parts, nodular iron castings should be mold-cooled to 600°F/315°C before shakeout to ease structural stress. Additional stress relief should be performed only when required at a temperature of 1150-1250°F/620-675°C. The Annealing process will aid in the decomposition and spheroidization of carbides in the metal, creating structural strength by preventing crack formation. This process should be conducted at 1750-1900°F/960-1035°C for at least one hour and up to five, depending on the size of the part and degree of spheroidization necessary.