Carburized machine components have high endurance limit because:

Carburization is a heat treatment process that involves introducing carbon into the surface layer of a metal component. This process significantly improves the performance and durability of machine components. One of the key benefits of carburization is the increase in the endurance limit of the material. The endurance limit, also known as the fatigue strength, is the maximum stress level at which a material can withstand an infinite number of load cycles without fatigue failure. Carburization achieves this by introducing a compressive layer on the surface of the component, which helps to suppress stress concentrations and improve its fatigue resistance.

Introduction of a Compressive Layer:

During the carburization process, the component is heated in a carbon-rich environment. The carbon atoms diffuse into the surface layer of the material, creating a high-carbon concentration region. This region forms a hardened case on the surface of the component. The carbon atoms combine with the base material's iron atoms to form iron carbides, which are hard and brittle. This hardened case acts as a protective layer, enhancing the component's wear resistance and reducing the risk of surface damage.

The formation of the hardened case also introduces compressive residual stresses into the surface layer. These compressive stresses are beneficial for improving the fatigue strength of the component. When the component is subjected to cyclic loading, the compressive stresses counteract the tensile stresses that develop during each load cycle. This helps to distribute the applied stress more evenly throughout the material, reducing the likelihood of crack initiation and propagation.

Suppression of Stress Concentrations:

Additionally, carburization helps to suppress stress concentrations in machine components. Stress concentrations occur at areas of geometric irregularities, such as sharp corners, notches, or surface defects. These areas can act as stress concentrators, leading to localized high stress levels and potential failure points. The hardened case formed during carburization can effectively mask these irregularities and distribute the stress more uniformly across the surface, reducing the likelihood of stress concentration-induced failures.

Conclusion:

In summary, carburized machine components have a high endurance limit due to the introduction of a compressive layer on the surface. This compressive layer helps to suppress stress concentrations and distribute the applied stress more evenly throughout the material, resulting in improved fatigue resistance. Carburization also provides a better surface finish and enhances the wear resistance of the components, further contributing to their longevity and performance.