Question

The endurance limit for the carburized machine components is high because

• A. introduces compressive layer on the surface
• B. produces the better surface finish
• C. suppresses the stress concentration produced in the component
• D. raises the yield strength of the material

Hint:

Surface treatments like carburization, nitriding, and shot peening are often used to improve fatigue resistance. The common mechanism for this improvement is the induction of residual compressive stresses on the surface, which effectively reduces the net tensile stress experienced during cyclic loading, thus delaying or preventing fatigue crack initiation.

Answer 1

The Correct Option is A

Step 1: Understand the terms endurance limit and carburization.
Endurance Limit (or Fatigue Limit): This is the maximum stress that a material can withstand for an infinite number of load cycles without failing. It is a critical property for components subjected to cyclic loading (fatigue).
Carburization: This is a surface hardening heat treatment process in which carbon is diffused into the surface of low-carbon steel components. This process increases the carbon content in the outer layer (case) of the material, making it harder and more wear-resistant.
Step 2: Analyze the effect of carburization on material properties, especially fatigue life.
Carburization significantly enhances the fatigue life and, consequently, the endurance limit of machine components. The primary reason for this improvement is the introduction of residual compressive stresses in the surface layer of the component.
When carbon is diffused into the surface and then quenched, the outer layer experiences a phase transformation and often a volume expansion that is constrained by the core. This constraint induces residual compressive stresses at the surface.
Step 3: Explain how residual compressive stresses improve endurance limit.
Fatigue failures typically initiate at the surface of a component, especially at locations where tensile stresses are highest or where stress concentrations exist.
The residual compressive stresses at the surface effectively counteract any applied tensile stresses during cyclic loading. This means that for a given applied tensile load, the net tensile stress experienced by the material at the surface is reduced. Since fatigue crack initiation is primarily driven by tensile stresses, reducing these stresses at the surface delays or prevents crack formation.
This phenomenon leads to a higher endurance limit for carburized components compared to their uncarburized counterparts.
Step 4: Evaluate the given options.
(1) introduces compressive layer on the surface: This is the primary reason why carburization enhances the endurance limit. The compressive residual stresses suppress fatigue crack initiation.
(2) produces the better surface finish: While surface finish can affect fatigue life, carburization's main contribution to endurance limit is not primarily through improved surface finish.
(3) suppresses the stress concentration produced in the component: Carburization does not inherently suppress stress concentrations caused by geometric features (like notches or holes). It primarily deals with the stress state at the surface.
(4) raises the yield strength of the material: Carburization does increase the hardness and strength (including yield strength) of the surface layer. While a higher yield strength is beneficial, the specific reason for the high endurance limit is more directly related to the residual compressive stresses, which prevent the initiation of fatigue cracks under cyclic loading. The increase in surface yield strength contributes to the overall hardness but the fatigue resistance comes from the compressive stresses. The most direct and significant reason for the high endurance limit in carburized components is the introduction of a residual compressive layer on the surface. The final answer is \( \boxed{\text{introduces compressive layer on the surface}} \).