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A connecting rod is a part which connects a piston to a crankshaft in an internal combustion engine. For motorcycle engines, a high strength connecting rod is required to bear the combustion forces from piston and transfer to rotational torques to crankshaft. Forged connecting rods, made of chromoly steel, are manufactured by a series of processes including hot forging, rough machining of planes and bores, heat treatment, and precise grinding of planes and bores.
Fig. 1 Connecting rod assembled in the crankshaft
A motorcycle connecting rod is especially important in motorcycle engine because the bore race has to be hard (HRC60) to resist abrasion from crank pin and piston pin during high RPM operation. However, a hard material is brittle which causes connecting rod failure at rod area. To avoid from connecting rod breaking, the hardness at rod area is necessary to lower down to HRC35. The material at rod area will become ductile and then the motorcycle engine can operate safely with a good connecting rod.
Another important spec. of a connecting rod is the roundness of connecting rod bore. Roundness is the measure how closely the bore approaches to a perfect circle. The bore diameter is measured by different angles, and roundness is the difference of maximum and minimum of all the diameter measures. Usually speaking, a qualified motorcycle connecting rod requires roundness of 0.003mm for both big-end bore and small-end bore. Therefore, the perfect bore will ensure the engine operate smoothly and the durability of connecting rod will be greatly enhanced.
The characteristic of motorcycle connecting rod is that the bore race has to be hard (HRC60) to resist abrasion from crank pin and piston pin during high RPM operation. However, a hard material is brittle and it is easily to break. The hardness of rod area is necessary to lower down to HRC35 to avoid from connecting rod breaking by the brittle steel. The steel at rod area becomes ductile and then the motorcycle engine can operate safely.
To produce the perfect connecting rod which is hard at bore area and is soft at rod area, the heat treatment of connecting rod is the key process.
The common material of motorcycle connecting rod is chromoly steel, SCM415 or SAE4118. SCM415 is a low-carbon steel. The carburizing heat treatment is applied to increase the connecting rod hardness. However, the carburizing heat treatment causes the whole connecting rod hardened to HRC60 including the rod area. The connecting rod is too brittle if we just apply carburizing heat treatment.
There are two methods to produce soft rod area after carburizing heat treatment.
1. To apply partial annealing heat treatment on a carburized connecting rod:
Annealing is a heat treatment to increase ductility and reduce hardness of material. Usually, the material is heated above its recrystallization temperature, maintaining a suitable temperature with a proper time, and then cooling with proper temperature gradient. The black carbon is precipitated on the steel surface. Therefore, the rod area becomes black since it is annealed to lower the hardness.
Question: Which heat treatment method is better for a connecting rod?
The biggest difference between annealing type connecting rod and copper type connecting rod is the core hardness at rod area. The carburizing heat treatment will harden the steel surface around 1.0mm, which is called the carburizing depth. The hardness below carburizing depth becomes uniform with hardness HRC35 which is called core hardness.
For copper type connecting rod, the core hardness at rod area is HRC35 which is the same as the core hardness of carburizing heat treatment. However, for annealing type connecting rod, the core hardness at rod area decreases down to HRC25 because of the annealing heat treatment. As a result, the copper type connecting rod has better stiffness or strength than annealing type connecting. The power loss of copper type connecting rod is less than annealing type connecting rod.
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| Fig. 1 Characteristic of a motorcycle connecting rod |
To produce the perfect connecting rod which is hard at bore area and is soft at rod area, the heat treatment of connecting rod is the key process.
The common material of motorcycle connecting rod is chromoly steel, SCM415 or SAE4118. SCM415 is a low-carbon steel. The carburizing heat treatment is applied to increase the connecting rod hardness. However, the carburizing heat treatment causes the whole connecting rod hardened to HRC60 including the rod area. The connecting rod is too brittle if we just apply carburizing heat treatment.
There are two methods to produce soft rod area after carburizing heat treatment.
1. To apply partial annealing heat treatment on a carburized connecting rod:
Annealing is a heat treatment to increase ductility and reduce hardness of material. Usually, the material is heated above its recrystallization temperature, maintaining a suitable temperature with a proper time, and then cooling with proper temperature gradient. The black carbon is precipitated on the steel surface. Therefore, the rod area becomes black since it is annealed to lower the hardness.
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| Fig. 2 Annealing type connecting rod |
2. To apply copper plating layer to avoid from carburization:
A high density copper is plated on the rod surface before caburizing heat treatment. The copper layer can isolate the steel from carbon gas, therefore, the steel with copper layer will not be hardened. The cooper on bore races are removed before heat treatment. At last, the bore race is hardened with hardness HRC60, and the rod area is protected by copper layer with hardness HRC35.
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| Fig. 3 Copper type connecting rod |
Question: Which heat treatment method is better for a connecting rod?
The biggest difference between annealing type connecting rod and copper type connecting rod is the core hardness at rod area. The carburizing heat treatment will harden the steel surface around 1.0mm, which is called the carburizing depth. The hardness below carburizing depth becomes uniform with hardness HRC35 which is called core hardness.
For copper type connecting rod, the core hardness at rod area is HRC35 which is the same as the core hardness of carburizing heat treatment. However, for annealing type connecting rod, the core hardness at rod area decreases down to HRC25 because of the annealing heat treatment. As a result, the copper type connecting rod has better stiffness or strength than annealing type connecting. The power loss of copper type connecting rod is less than annealing type connecting rod.
A motorcycle connecting rod is required to be hard (HRC60) at bore area, and it is required to be ductile at rod area. Therefore, the chromoly alloy steel is applied for the motorcycle connecting rod. For the traditional application, SCM415/SCM420 or 15CrMo4 is the most commonly used low-carbon alloy steel. After carburizing heat treatment, the surface hardness is increased to HRC60. The abrasion resistance is increased. Meanwhile, the strength is still good at core area.
SCM415 Material Composition:
|
(C)% |
(Si)% |
(Mn)% |
(P)% |
(S)% |
|
0.13-0.18 |
0.15-0.35 |
0.6-0.85 |
0≦0.03 |
0≦0.03 |
|
(Ni)% |
(Cr)% |
(Mo)% |
(Cu)% |
|
|
0≦0.25 |
0.9-1.2 |
0.15-0.30 |
0≦0.3 |
|
For higher power engine or modified engine, the strength of SCM415 may be not enough. A higher strength nickel-contained chromoly alloy steel is used to replace traditional SCM415. Generally speaking, the more nickel dissolved in the steel, the higher ductility or the higher yielding strength it becomes. A popular nickel-contained chromoly alloy steel is SNCM220. The yield strength of SNCM220 is 10% higher than SCM415, then providing more strength and safety of connecting rods.