Surface Strengthening of AISI4140 by Cavitation Peening

Author:  Alexander Klumpp, Franziska Lienert, Stefan Dietrich, Hitoshi Soyama, Volker Schulze
Source:  ICSP-13
Doc ID:  2017110
Year of Publication:  2017
Abstract:  
Introduction: Cavitation produces severe erosion in hydraulic machineries such as pumps and screw propellers,since severe impacts are generated when cavitation bubbles collapse. However, cavitation impacts can also be utilized to enhance materials properties in a similar way as shot peening. “Cavitation shotless peening”, or simply “cavitation peening” [1], is a recently developed mechanical surface treatment which makes use of such cavitation impacts instead of conventional shot impacts. The advantages of cavitation peening (CP) are as follows: Compared to conventional shot peening, the peened surfaces are smoother after CP. This is due to the absence of shots. Furthermore, the pump pressure is lower than that of water jet peening, as cavitation impacts are used. Moreover, regions difficult to reach such as narrow tubes or tooth roots of gears can be hit by cavitation impacts. During CP, cavitation is generated by injecting high speed water jets into water filled chambers. This type of submerged water jets with cavitation is called “cavitating jet”, or a “cavitating jet in water” [2]. Soyama [3, 4] realized a cavitating jet in air without a water filled chamber, by injecting a high speed water jet into a low speed water jet which was injected into air using a concentric nozzle. In this case, it was reported that cavitation peening introduced compressive residual stresses capable of relieving micro strains, which is further described in [5]. Regarding previous report [6],the Introduction of compressive residual stress by cavitating jets in air corresponds to shot peening using small shots at high velocity, whereas the introduction of residual stresses by cavitating jets in water corresponds to shot peening using large shots at low velocity. When the cavitating jet is injected in the water, secondary air bubbles follow the collapse of cavitation bubbles [7]. These secondary bubbles cause a “cushion” effect, reducing the cavitation impact energy. When the pressurized chamber is used for cavitating jets in water, the “cushion” effect is reduced by shrinking the secondary bubbles. Namely, the cavitation impact energy can be increased by using pressurized chambers. In the present paper, three types of cavitating jets were used for cavitation peening; a cavitating jet in water with an open chamber, a cavitating jet in water with a pressurized chamber and a cavitating jet in air.