Why Shot Peening is Essential After Heat Treatment

What is Shot Peening?

Heat treatment is essential in transforming raw metal into high-performance components. It hardens, tempers, and alters the internal structure of the material, but it also leaves behind something unwanted: tensile stress. This invisible threat can shorten the lifespan of even the toughest parts, which is where shot peening comes in.

Shot peening is a cold working process in which the surface of a metal component is bombarded with small spherical media, known as shots, to create compressive residual stress. This treatment significantly improves mechanical properties and surface integrity.

How Shot Peening Works

Tiny metal, glass, or ceramic beads are propelled at high velocity.
Each shot acts like a mini hammer, plastically deforming the surface.
The surface is compressed, which counters tensile stresses.
This prevents the initiation and growth of cracks.

Key Takeaway

“Think of shot peening as giving your metal parts a protective armor after they've been heat-treated.”

Why Shot Peening Is Always Recommended After Heat Treatment

When metals are heat-treated, whether for hardening, annealing, or tempering, the cooling and structural shifts often induce residual tensile stresses near the surface. These microscopic tensions act like ticking time bombs, weakening the material’s resistance to fatigue and corrosion.

Shot peening solves this by:

Neutralizing tensile stress with compressive stress
Creating a work-hardened surface layer that resists microcracks
Improving fatigue strength

In many industries, skipping shot peening after heat treatment is not just sub-optimal, it’s a critical flaw!

Shot Peening in Action: Real-World Examples

Industry	Components Treated	Results Gained
Aerospace	Turbine blades, gears	Increased fatigue life, safety margin
Automotive	Springs, crankshafts	Reduced cracking, longer service cycles
Oil & Gas	Drill bits, valves	Improved corrosion resistance
Medical Devices	Implants, surgical tools	Enhanced durability, surface stability

Shot Peening vs Other Finishing Processes

*Process*	*Purpose*	*Does it induce compressive stress?*
*Shot Peening*	Fatigue resistance & hardening	Yes
Sandblasting	Cleaning/roughening	No
Polishing	Smooth surface finish	No
Case Hardening	Surface hardening	Not residual stress-focused

The Science of Stress: Why Compressive > Tensile

Tensile stress tries to pull a material apart, while compressive stress pushes the material in on itself. When cracks form, it's usually because tensile stress exceeds the material's limit. Shot peening turns the tables:

“By converting harmful tensile stress into protective compressive stress, shot peening creates a permanent barrier against fatigue failure.”

This shift in surface dynamics is the core reason peened components last dramatically longer under repeated or fluctuating loads.

Shot peening is most valuable immediately after heat treatment, especially when:

The component is exposed to cyclical loading
There's a risk of crack propagation due to surface tension
The part has undergone machining or grinding, which weakens the outer layers
Long-term corrosion resistance is essential

Skipping it means leaving your investment vulnerable to early failure.