A new metalworking technique referred to as positron and laser assisted magnetic energy intensive multifunction cavitation (PLMEI-MFC) using a narrow nozzle was developed and applied to SCM440 Cr-Mo steel. The effects of varying the laser wavelength and power were initially assessed without positron irradiation. In this system, water jet cavitation (WJC) bubbles were subjected to ultrasonication in a magnetic field and the resulting cavitation clouds were irradiated with laser beams having wavelengths of 450 or 405 nm and with ultraviolet light. Exposure to a broad range of ultraviolet light having a peak at 365 nm was found not to contribute to the generation of residual stress in the metal. Because the narrow nozzle produced small WJC bubbles, high compressive residual stress and significant hardness were imparted to an insulated thin specimen having a low heat capacity. The flow rate of the narrow nozzle was also shown to greatly affect the processing rate. In this system, electrons and positrons in the bubbles evidently underwent annihilation to generate γ-rays and ther 
forms of energy such that energy was concentrated in the bubbles. This PLMEI-MFC processing dramatically improved the surface strength of Cr-Mo steel specimens. Employing this peening apparatus with positron irradiation provided a stress increase of 1160 MPa based on converting tensile residual stress into compressive residual stress. This PLMEI-MFC system using a narrow nozzle allows compressive residual stress to be applied to the surface of steel without generating peening marks.