Shot peening is a separate cold working process designed to increase the exhaust life cycle by producing a compressive residual stress layer on the outer surface, which modifies the mechanical properties of the metals.
WHY ARE STRESSES PUT IN THE MATERIAL?
The residual tensile stress from welding metals such as inconel, stainless steel and aluminium is created because the weld consumable is often applied in a liquid state. The welding process heats the metal and the weld is applied in its hottest, most expanded state. When the much cooler consumable material is bonded to the base material, the weld tends to cool rapidly and will attempt to shrink from the ‘drop in’ temperature. Because the base material is usually much stronger and not in a molten state, this cannot shrink, leaving the material remaining in a highly stressed “tensioned” state.
This zone is usually just next to the weld joint, which explains why a crack may appear close to the weld.
HOW DOES SHOT PEENING WORK?
The shot peening process impacts the metal surface with small pellets (round metallic, glass or ceramic particles) with a high enough force to deform the surface plastically. A compressive layer is generated when the impact of all the particle shots produce small indentations in the top surface layer. The layer beneath the top surface is then compressed, generating a compressively stressed layer underneath the shot peened area. This layer helps to prevent the stressed area from cracking as a crack cannot propagate in a compressive environment.
The shot peening process has been proven to be very beneficial in many components operating in a highly-stressed environment such as motor racing.
TYPICAL SHOT PEEN BEHAVIOUR
The graph below [D1] shows a typical situation of a welded metal component before and after shot peening. As can be seen from the graph, the residual stresses in the welded metal are positive, which puts the surface layer in a tensional state.
When shot peened the material is compressed and the residual stresses are negative, putting the material in the compression state where the cracks cannot propagate. This graph illustrates how the residual stresses after shot peening are the most negative in the layer underneath the shot peened top layer and gently increase as we go deeper in the material.
This graph does not refer to any specific material and is purely for illustration purposes.[G2]
SHOT PEENING QUALITY STANDARDS
All shot peening processes are rigidly quality controlled and regulated by a series of SAE standards that control the process, media used, etc..
Shot Peening Media SAE-AMS-2431.
Automatic processing SAE-AMS-2430
Computer controlled processing SAE-AMS-2432
Almen Strips & Gauges SAE-J442
Almen Strip usage SAE-J443
GoodFabs manages both the shot peening and heat treatment processes as part of an exhaust programme for a race team or engine builder. Inspection is normally required both before and after heat treatment and shot peening, so GoodFabs coordinates both procedures prior to final delivery of the finished and inspected part to the customer.