Needle peening uses a bundle of metallic needles projected at high velocity onto the surface by a pneumatic tool (needle scaler). Each needle creates a small imprint; the multiplicity of impacts generates a continuously cold-worked surface condition with moderate to high residual compressive stresses.
Needle peening stands out for its accessibility to tight geometries (tube-inside welds, fillet joints, sharp angles) where neither shot nor flapper peening can operate. Its high noise level and workshop/field environment make it primarily suited to naval, railway, energy and oil & gas applications.
References: DIN EN 1011-2 (structural steel welding), IIW recommendations and offshore OEM best practices (ABS, DNV-GL).