Conrod Materials. Alternate Materials

Depending on the particulars of the application and the resultant loads, any of a number of different materials may be used for connecting rods.

Cast materials. The casting materials used most widely for connecting rods are nodular cast iron (GGG-70) and black malleable cast iron (GTS-70). GGG-70 has both technical and economic advantages when compared with malleable cast iron. In particular, the specific oscillation resistance, which is important for conrods, is considerably greater for GGG-70.

GGG-70 is an iron-carbon casting material; graphite inclusions that are largely spherical are introduced into a basic structure that is primarily pearlitic. The compact shape of the graphite gives the material an optimum strength and ductility. At the same time, the graphite is also responsible for the good casting properties. The required structure is created during the casting process without additional heat treatment.

In the case of malleable iron, which is also an iron-carbon material, the structure is determined by applying heat subsequent to casting.

Forging steel. The large majority of all conrods are manufactured from steel in the drop forge process. In most cases, microalloyed steel such as 27MnVS6 BY or carbon manganese steels like C40 mod BY are used. Steel with high carbon content (C70 S6 BY) is used for forged and fracture-split conrods. These materials attain tensile strength of Rm = 1000 MPa.

Available for high-performance conrods is 34CrNi-Mo6 V (or 42CrMo4), a steel alloy that achieves tensile strength of 1200 MPa. In this case, additional heat treatment (hardening) is required.

New developments in steel have reached tensile strengths—even in materials used for cracking—of up to 1300 MPa at 0.2 offset limits in excess of 700 MPa. These steels are identified with the designation "C70+" in the table of materials.

Powdered metal. Materials such as Sint F30 and Sint F31 are available for manufacturing conrods from powdered metal. They achieve tensile strengths of up to 900 MPa.

Alternate materials. In addition to the materials used for conrods in mass production, explorations into using alternate materials pursue above all the objective of reducing conrod weight while maintaining load-handling capabilities. Carbon fiber reinforced aluminum or carbon fiber reinforced plastic is used for this purpose.

Widely used in racing are titanium conrods, with which a considerable weight reduction is achieved. The disadvantage of the titanium conrods is the strong tendency for bores to expand during operations, which has a deleterious effect on the tightness of the seat for the bearing shells. Another drawback is the fact that titanium is not a good "friction partner" for steel. Consequently, slip coatings on the mating surfaces are needed to protect against scuffing (friction-induced damage) and/or on the bearing's steel backing to prevent fretting.

Common to all conrods made of these alternate materials and fabricated for individual engines are the high manufacturing costs that hinder greater use in mass-production engines. The most important materials and their properties are summarized in Fig. 7-38.

Fig. 7-38. Properties of conrod manufacturing materials

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