Here you go gents, has nothing to do with corrosion resistance, it revolves around the hardening process. Or to be more precise, preventing the hardening process. It allows you to have hardend bearing surfaces, while preventing the hardening of the full rod body, so she will bend, rather than fracture. What did we ever do without the Google machine?? Grab a cup of coffee and enjoy the read...
...A connecting rod in an internal combustion engine provides linkage between the piston and the crankshaft. Particularly in two stroke cycle internal combustion engines, this linkage is done in a low lubricant environment. To accomplish this linkage, the connecting rod generally uses rolling element bearings between the crankshaft and the connecting rod at a first, large end of the connecting rod and also between the piston pin and the connecting rod at a second, small end of the connecting rod. The first, large end of the connecting rod includes a large end hole. Conventionally, after a rod is forged or cast, the rod is cracked in a brittle manner to form a two component connecting rod that is assembled on to the crankshaft through the large end hole. Thus, after cracking, the connecting rod may be reassembled with bolts in a manner that gives optimal registry. Cracking of connecting rods is known in the industry.
It is also known that the connecting rod may be forged from a low carbon steel alloy having, at the very least, less than 0.70% carbon by weight, and usually having less than 0.20% by weight. However, in these conventional connecting rods, the rolling element bearing cannot successfully run on the forged surface because the low carbon steel is not strong enough to resist mechanical deformation caused by the load transmitted from the piston through the roller bearings during the combustion process. To resolve this problem it is known to carburize low carbon steel connecting rods to increase the carbon level near the surface area. The higher carbon level material can then be heat treated to form a specific microstructural constituent known as martensite.
Carburizing is the addition of carbon to the surface of low carbon steels at temperatures generally between 850° and 950° C. (1560-1740° F.) at which austenite is the stable crystal structure. It is known that austenite has a high solubility for carbon and therefore it is ideal to carburize at the austenite temperature. Hardening of the carburized surface is accomplished when the high carbon surface layers are quenched to form martensite. Thus, the carburization process allows for a high carbon martensitic case with good wear and fatigue resistance to be superimposed on a tough, low-carbon steel core.
During the martensitic heat treatment process after carburization, the low-carbon steel connecting rod undergoes a solid state transformation. Initially, the part is in a body centered cubic (BCC) structure at room temperature. The BCC structure is a fairly soft metallic state and is only able to dissolve a limited amount of carbon. During the heat treatment, the part is heated until it reaches a temperature where the low energy condition of the material is preferable to transform into a face centered cubic (FCC) structure. In the FCC structure, many more carbon atoms are able to fit into the interstitial portions of structure as compared to the BCC structure. After the carbon molecules have diffused to the interstitial positions, the part is rapidly cooled or quenched. During the quenching process, the part is transformed at a temperature where the structure is generally of the BCC type. However, if the cooling is sufficiently fast enough, then the carbon atoms do not have enough time to diffuse from the interstitial positions of the FCC structure, and the carbon atoms remain packed in the interstitial positions. At room temperature, the diffusion coefficient of carbon is very low and carbon will essentially be trapped in the position it is in. Since the BCC structure cannot contain this much carbon at room temperature, a third structure, martensite, with a body centered tetragonal (BCT) structure is formed. This crystalline structure has a very high amount of internal stress. Due to this internal stress, the product is extremely hard but brittle, usually too brittle for practical purposes. This internal stress may also cause stress cracks on the surface of the product. From the quenched condition, the part is tempered to increase the toughness, but only slightly, as a surface hardness of 60-63 hardness Rockwell C is desired. The tempering process is well known to those in the art.
Forging is a manufacturing process where metal is shaped by plastic deformation under great pressure into high strength parts. There is no melting and consequent solidification involved. Forging's plastic deformation produces an increase in the number of dislocations resulting in a higher state of internal stress. This strain hardening is attributed to the interaction of dislocations with other dislocations and other barriers, such as grain boundaries. Simultaneously, the shape of primary crystals (dendrites) changes after this plastic working of the metal. Dendrites are stretched in the direction of metal flow and thus form fibers of increased strength along the direction of flow.
Conversely, the manufacturing process of casting consists of pouring or injecting molten metal into a mold containing a cavity with the desired shape of the casting. Metal casting processes can be classified either by the type of mold or by the pressure used to fill the mold with liquid metal. Since casting is a solidification process, the microstructure can be finely tuned, such as grain structure, phase transformations and precipitation. However, defects such as shrinkage porosity, cracks and segregation are also linked to casting's solidification process. These defects may lead to lower mechanical properties in some castings. Subsequent heat treatment is often required to reduce residual stresses and optimize mechanical properties in cast products.
The connecting rod is machined to a near-net shape before the heat treating process. This is done because the cost to machine large amounts of hard material is costly and it is also difficult to create the hardness profile that is required for the part. Unfortunately, the solid state phase transformation distorts the connecting rod. Particularly, this distortion occurs during the quenching operation of the current process when the crystal structure changes from FCC to BCT. Such connecting rods are measured for distortion and straightened by bending. Accordingly, it is desired to create as little distortion as possible of the connecting rod from heat treatment through the phase changes. Ideally, the distortion is limited to less than the final grinding allowance. Thus, in conventional connecting rod manufacturing, the connecting rod has to be straightened before it can be used. Such straightening is a non-value added operation during the manufacturing process of a connecting rod.
Problems also arise if the connecting rod is completely carburized. The section of the connecting rod between the ends usually has a configuration of a thin-walled I-beam. Carburization of the thin-walled I-beam results in an unfavorable through thickness hardness condition. While this condition has good strength properties, it has poor fracture toughness. Thus, the connecting rod is subject to brittle fracture from any impact type loading, and an engine misfire event could cause a fractured connecting rod.
To address the through thickness hardness condition, connecting rods are normally copper plated. Since the diffusion of carbon in copper is very low, the copper effectively masks the carbon and prevents diffusion. After the copper plating process, the connecting rod may be selectively machined to remove the copper in areas where the part is to be carburized. As aforementioned, these areas are typically where the bearing rollers contact the rod at the thrust faces of the first and second ends of the connecting rod. Since these areas have to ultimately be machined, the copper plating is not considered to be a value added operation.