John s. Eckersley Buzz Ferrelli Metal Improvement Company Bloomfield, connecticut Fatigue life increases, by orders of magnitude, can be e~pected on compressor components treated by Shot Peening-a controlled process that involves the bombardment of the metal component by millions of spherical particles of steel, glass or ceramic. Shot Peening is being applied to crankshafts and con-rods of huge reciprocating compressors and to the small valve reeds, only a few thousands of an inch thick, that are the heart of refrigeration and air conditioning sealed units. In what is perhaps the ultimate in design of axial and centrifugal compressors, the modern Jet engine, Shot Peening is used on all rotating parts, as well as many of the stationary ones, to prevent premature fai.lures from metal fatigue, corrosion and fretting fatigue, and from stress corros1on cracking. The paper reviews these and other applications for compressor englneers so that they will be able to increase the life and/or the loading on both new and existing designs, without increasing size or adding weight to critical components. The controlling parameters of the Shot Peening process are also discussed. HISTORICAL BACKGROUND. Shot Peening was first used, in a Production application, to e~ tend the life of the valve springs for the Buick and Cadillac engines of the early 1930s (Ref. 1). The process was d1seovered accidentally and, although the benefits were soon recognized, it was several years before a mechanism was proposed and even longer before is was generally accepted. It was recognized, at the time, that fatigue cracks initiated under repeated tens1le loads. John Alman postulated that shot peening produced the 1ncreases in fat1gue life from the introduction, of a hlgh residual compressive stress, which remained just below the surface of the part (Fig. 1, Ref. 2). •IGURE 1. EXAMPLE o• RESIDUAL STRESS PRO.lLE CREATED BY SHOT PEENING. In most applications for shot peening, the benefit obtained is the direct result of the residual compressive stress produced. A typical profile of residual compressive stress as 1t cnanges with depth is shown. It has four important characteristics: 1) ssSurface stressThe stress measured at the surface. 2) CS max Maximum Compressive stress The maximum value of the compressive stress induced, which normally is highest just below the surface. 3) d Depth The depth of the compressive stress is the point at which the compressj've stress crosses over the neutra axis and beCQIII9S tensile. 4). TS maxMaximum Tensile Stress The maximum value of the tensile stress induced. The offsetting tensile stress in the core of the material balances the surface layer of compressive stress so that tne part remains in equilibrium. TS max must not be allowed to become large enough to create early 1nternal failures. 1+1 TENSION 919 % ULTIMATE TENSILE STRENGTH 1-1 100 COMPRESSION