Creep is the time-dependent, permanent deformation of a metal or alloy under a constant load or stress, typically at elevated temperatures that are still below the material's yield strength. In marine diesel engines, creep is a critical concern for parts like exhaust valves, pistons, and turbocharger blades, which operate for long periods under high temperatures and stresses.

Conditions under which creep occurs:

• High Temperature: Usually above 0.4 times the absolute melting temperature (in Kelvin) of the material.
• Constant Stress: Load is sustained for an extended period.
• Long Service Time: Prolonged operation, such as those experienced on ship main engines during continuous voyages.
• Examples on Ships: Creep is most notable in exhaust components, turbine blades, and other heat-exposed engine areas where temperatures and stresses combine over time.

Primary creep : starts at rapid & Unsteady rate and slows with time

Secondary creep : relatively uniform rate.

Tertiary creep : accelerated creep rate and terminates when material breaks or ruptures

Alloys are metallurgically engineered for higher creep resistance using the following processing techniques:

• Alloying: Introducing elements like nickel, chromium, molybdenum, and vanadium forms stable carbides/solid solutions that hinder dislocation movement, thus enhancing creep resistance. For example, nickel-base superalloys for exhaust valves are chemically optimized for this property.

• Heat Treatment: Processes such as solution treatment or precipitation hardening refine grain structures, promote uniform distribution of strengthening phases, and help retain fine, stable precipitates that block dislocation movement.

• Grain Size Control: Employing processes (like forging or controlled solidification) to ensure a coarse, stable grain structure, or in the case of some alloys, very fine grains. Large (coarse) grains in alloys reduce grain-boundary sliding, a key mechanism in high-temperature creep.