Improving fatigue performance — name the surface treatment Which process is specifically used to increase fatigue resistance by inducing beneficial compressive residual stresses at the metal surface?

Introduction / Context:
Fatigue failures initiate at or near the surface where tensile stresses and stress concentrations are highest. Processes that create a compressive residual stress layer can dramatically improve fatigue life by suppressing crack initiation and early growth.

Given Data / Assumptions:

• Objective: improve fatigue resistance without changing bulk geometry.
• Applicable to steels and many non-ferrous alloys.
• Process is mechanical and cold (no phase change required).

Concept / Approach:
Shot peening (a form of cold peening) bombards the surface with small spherical media, plastically deforming a shallow layer. Upon unloading, the underlying elastic substrate attempts to restore shape, leaving the surface in residual compression. This compressive layer offsets applied tensile cycles, raising fatigue strength. Other listed processes either shape the metal (extrusion, heading, piercing) or coat it (electroplating) and do not inherently create the same residual stress state.

Step-by-Step Solution:
Identify the mechanism needed: surface plasticity → residual compression.Match to process: shot peening produces controlled surface compression.Confirm alternatives: extrusion/heading change geometry; piercing forms holes; plating adds a coating but may not impart compression.Select cold peening / shot peening as correct.

Verification / Alternative check:
S-N curves for peened specimens show higher endurance limits; Almen arc height provides process control for peening intensity.

Why Other Options Are Wrong:
Hot piercing, extrusion, and cold heading are forming processes without the required residual stress effect as the primary outcome.

Electroplating may even decrease fatigue life due to hydrogen embrittlement if not controlled.

Common Pitfalls:
Over-peening causing surface damage; skipping coverage verification; ignoring stress-relief heat treatments after peening which can relax beneficial stresses if done improperly.

Final Answer:
Cold peening / shot peening (impacting surface with shots/hammer)