The Gard P&I Club has issued an article explaining how crankshaft annealing can be a viable and attractive alternative resulting in substantial savings.
Annealing is a heat treatment process to remove hardness and stress within metal and to increase its ductility. Hardness is a measure of how resistant solid matter is to permanent shape change under application of force. Ductility is a material’s ability to deform under stress.
On a molecular level, when a metal such as steel cools down rapidly, its crystalline grains transform into a lenticular (lentil/lens) shape. This causes the metal to become harder and more brittle in the process. Heating the metal to the appropriate temperature causes a homogenous growth of new crystals. The more heat applied to the metal, the more its ductility increases and hardness decreases.
In its factory state, a crankshaft has a hardness of 250-350 HB. Its grain structure is homogenous and provides the desired strength, rigidity, resistance to wear, corrosion and impact.
During a bearing failure, a crankshaft absorbs a lot of heat. When the engine stops, the uncontrolled cooling can cause areas of excessive hardness of 600 to 700 HB or more. The grain structure of the steel becomes uneven and highly susceptible to cracking and potential shearing.
The repair process starts with machining the crankshaft’s journal surface free of cracks and drawing a hardness map to record hardness levels and determine the locations for annealing.
The main bearings and flywheel are removed to prevent any damage from the heating process and ceramic tiles, control thermocouples and monitoring equipment are mounted and the journal insulated.
The crankshaft is heated to annealing temperature, during which the steel molecules recrystallize to a state consistent with their original ductility and hardness. The crankshaft is then cooled to ambient temperature at a carefully monitored, computer controlled rate and the annealing equipment is removed.
This repair process takes a fraction of the time it would take to replace a crankshaft. This means considerably lower repair costs, less operational downtime and reduced loss of hire. There should always be an evaluation of the viability of repairing instead of replacing a damaged crankshaft.
Further details may be found by reading Gard’s Insight Crankshaft annealing – a repair alternative to replacement
Source & Image Credit: The Gard P&I Club
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