What is annealing?
Annealing is a heat treatment process commonly used to remove the stress and hardness within a metal and to increase its ductility. Hardness is a measure of how resistant solid matter is to permanent shape change under application of force, and ductility is a material’s ability to deform under stress.
On a molecular level, when a metal such as steel cools down rapidly, the crystalline grains in the metal transform into a lenticular shape. This causes the metal to become harder but also more brittle in the process. Heating the metal to an appropriate temperature causes a homogenous growth of new crystals. The more heat is applied to the metal, the more its ductility increases and its hardness decreases. During annealing, the steel is heated above its critical temperature and held at this temperature for a period of time before controlled cooling. The controlled cooling process allows the crystals to diffuse into a more flexible, cubic-shaped state.
SO HOW DOES THIS APPLY TO CRANKSHAFTS?
FACTORY TEMPERED STATE
Unhardened crankshafts generally have a hardness in the range of 250-350HB
Homogenous grain structure with a hardness that provides desired strength, rigidity, resistance to wear, corrosion and impact.
During a bearing failure, a crankshaft absorbs an incredible amount of heat. Emergency stopping of the engine introduces an uncontrolled cooling or quenching that often results in areas of excessive hardness in the range of 600 to 700+HB
Uneven grain structure with significant and visible differences in hardness create areas that are highly susceptible to cracking and potential shearing under force. Oftentimes, this hardness goes deep into the metal and without annealing would result in the condemnation of the crankshaft.
ANNEALING THE SHAFT
Heat is applied to the shaft in a tightly controlled manner above its critical temperature but well below the austenite range where the crankshaft could be permanently damaged. This temperature is held for a time period related to shaft diameter and extent of damage before being cooled in an equally controlled manner.
During annealing, once the metal reaches the critical temperature, the molecules are allowed to recrystallize (realign) to a state consistent with its original ductility and hardness when it was delivered from the factory. Hardness levels are reduced to within acceptable parameters and within maker tolerances.
Once the shaft is cooled, the surface of the journal is rough with scale from oxidation of the metal during the annealing process. The shaft must then be machined to finish diameter and machine polished to the required finish.
The journal now has an acceptable hardness across the entire surface of the pin and the surface has been polished to a finish generally of 0.03Rα or better.
Sampling Of Crankshaft Annealing Jobs Completed By Goltens Since Receiving GL Class Approval In 2012*
USA: MaK 8M20, MAN B&W8L27/38
HAITI: Wartsila 18V38
DOMINICAN REPUBLIC: MAN B&W12V28/32, Wartsila 18V46, Wartsila 18VW32B
MEXICO: Bergen BR M8, Bergen 33L8P, MaK 8M25
EL SALVADOR: MaK 16CM32
NICARAGUA: MaK 16CM32
TRINIDAD & TOBAGO: Wartsila 6L46C
ECUADOR: Wartsila 18V26
CHILE: Wartsila 18V32
AND MIDDLE EAST
TURKEY: MAN B&W 6L48/60
SPAIN: Bergen KVG18-G3
ITALY: Bergen B35:40 V12, Bergen KVG16, Bergen KVG 18, Bergen KVGS18-G4.2
VOYAGE IN MEDITERRANEAN: MAN B&W 5L28/32
UNITED ARAB EMIRATES: MaK 6M20, MAN B&W 6L28/32, MAN B&W 6L28/33, Ruston 6RKCM-H, Sulzer 8ZAL40
SEYCHELLES: Sulzer 8ZAL40 SENEGAL: Mitsubishi KU30B
ASIA AND OCEANIA
JAPAN: Colt Pielstick 10PC 4.2V, Mitsubishi KU30B
CHINA: MAN B&W 9L32/40
BANGLADESH: Bergen B35/40V12AG
INDIA: Bergen B35:40 V16AG, Bergen B35:40 V16AG, MAN B&W 7L40/45, MAN B&W 7L40/45
PHILIPPINES: Deutz 6SVM 640, Sulzer 14ZAV40S, Wartsila 18V46
BRUNEI: Mirrlees Blackstone K6, Major MK3
SINGAPORE: MAN B&W 6L28/32
INDONESIA: MaK 8M552, MAN B&W 18V40/45
SAMOA ISLAND: Deutz 12M640
AUSTRALIA: Bergen C25:33L9P