**2. Shot peening**

required. As a raw material, ADI is cheaper than steel. It also has a lower manufacturing cost due to the possibility of casting the components to near-net shape. The cost and weight of ADI per unit of yield strength can compete with cast and forged aluminium, and forged steel. ADI exhibits higher damping characteristics than steel, leading to lower noise emission and less vibrations. The presence of graphite in ADI dampens vibrations 40% faster than in steel gears,

ADI is a type of cast iron, more commonly referred to as ductile iron, having an austempering heat treatment process applied to it. It is made up of graphite nodules in a matrix of acicular ferrite and retained austenite frequently referred to as ausferrite (**Figure 1**). Optimum properties are obtained when the chemical alloy composition, solidification micro-structure and heat treatment parameters are carefully controlled. The austempering heat treatment cycle (**Figure 2**) consists of first austenitising the ductile iron to temperatures between 850 and 1000°C, followed by quenching in a salt or oil bath. The bath is maintained between 230 and

time to transform the austenite to ausferrite. This is followed by cooling to room temperature. The mechanical properties of ADI depend on the parameters of the austempering process, which determine the morphology of the ferrite, the volume fraction of retained austenite, the carbon content in the retained austenite, and the presence or absence of martensite and iron carbides in the austenite or ferrite. In general, the tensile strength of ADI varies from around 1500 MPa with a corresponding 1% elongation, to lower tensile strengths (900–1200 MPa) and higher corresponding elongations of up to 12%. The former group of ADIs is produced at lower austempering temperatures of 230–330°C and exhibits high hardness (~50–54 HRC), but limited ductility. These are used for applications requiring high resistance to contact stress. ADIs having lower tensile strength, which are produced at higher austempering temperatures of 350–400°C, have lower hardness ranging from around 23 to 34 HRC, but have high toughness and ductility [3]. This range of ADIs consist of structures with greater amounts of

**Figure 1.** Typical micro-structure of ADI (austenitised at 900°C for 2 hours, austempered at 360°C for 1 hour) [1].

and left there for sufficient

and it also results in a 10% reduction in density compared to steel.

26 Advanced Surface Engineering Research

450°C, a temperature above the martensite start temperature *Ms*

Shot peening (SP) is a conventional mechanical surface treatment during which the surface of a material is bombarded by spherical media (called *shot*) arriving at high velocity and under controlled conditions. During SP, shots are accelerated towards the surface using air pressure and a nozzle or a centrifugal wheel. Shots impart forces that form a dimple by plastic deformation and radial stretching of the material. The numerous dimples on the surface create several of such plastically deformed hemispheres, while the elastically stressed region tries to recover to the fully unloaded state. These inhomogeneous elastic-plastic deformations induce high residual compressive stress and high dislocation densities in the surface and to a depth of circa 120–500 μm [9]. Their magnitude is a function of the mechanical properties of the target material and is at least equal to half the yield strength of the material being peened [10].

The essential parameters of the SP process can be classified into three groups: shot (shape, hardness and size), workpiece (hardness, chemical composition, crystal structure, geometry) and flow parameters (shot velocity, impact angle, mass flow rate, peening time, coverage). These parameters need to be carefully controlled in order to achieve a uniform distribution of compressive stresses on the surface of a component.

The induced compressive layer and work hardening increase the resistance to crack initiation and propagation, which in turn prolongs components' lifetime. In fact, SP has been used for years to extend the bending fatigue life of various engineering components in the transport industry, mainly for automobile and aircraft parts. Such components include gears, axles, springs, connecting rods, crankshafts, I-beams in heavy duty applications, compressors, turbine rotors and shafts. SP can be applied to the whole component, or just confined to parts of the component that are expected to be highly stressed. For example, in gears, one can SP the entire gear tooth or alternatively focus the SP only at the tooth root fillets, which are exposed to the highest stress.

propagate in surfaces upon which a compressive force is acting. The induced compressive stresses shift crack nucleation to the sub-surface and hinders fatigue crack propagation at the surface. Apart from hardening the surface, SP also eliminates microscopic defects, machining marks and grinding defects. This also increases the bending fatigue strength. However, Uematsu et al. [22] showed that SP cast iron containing spheroidal vanadium carbides (VCs), dispersed in a martensitic matrix, did not eliminate large casting voids. These voids and clusters of VCs served as sources of crack initiation, and hence SP did not improve the bending

Shot Peening of Austempered Ductile Iron http://dx.doi.org/10.5772/intechopen.79316 29

Wear of materials is a complex phenomenon, and depends on the running conditions and the properties of the tribopair materials. SP should be beneficial in reducing wear rates because of the high hardness due to work hardening, stress-induced austenite to martensite transformation, and residual compressive stresses at the surface [10, 23, 24]. Compressive stresses prevent micro-cracks from forming, thus inhibiting pitting or spalling. Kobayashi and Hasegawa [25] showed this to be true for carburised steel gears. This was attributed to the compressive stress suppressing cracking and delaying crack growth. Champaigne [10] and Townsend and Zaretsky [26] reported an improvement in the contact fatigue life of SP steel gears of around 1.5 times. In another study, Townsend [27] reported that a higher peening intensity (0.38–0.43 mmA) resulted in higher compressive stresses, and hence lead to a 10% rolling contact fatigue life (*L10*) of 2.15 times that of the carburised gears peened with an intensity of 0.18–0.23 mmA. Adamović et al. [28] investigated the sliding wear characteristics of ground and SP steel under boundary lubrication, and reported a slight decrease in coefficient of friction and

The effect of the rough SP surfaces on the tribological behaviour has been reported in a number of articles. The dimpled surface is sometimes considered to be favourable in lubricated contact, which acts as reservoirs that aid in the retention of the lubricant and maintaining a full film thickness between meshing teeth. Better lubrication reduces fretting, noise, spalling, scuffing and the operating temperature by reducing friction. That said, Vaxevanidis et al. [29] still reported improved sliding wear resistance of SP tool steel tested under dry conditions. A higher coefficient of friction was reported at the beginning of the test, but as the test progressed, this decreased to a lower value than that of specimens, which were not SP. This can

In contrast, studies by other researchers show no improvement in the tribological characteristics of surfaces after SP [30–32]. The reason given is that surface roughening counterbalances

Very few works have been conducted to study the tribological behaviour of SP ADI [30, 31]. Work by Sharma [31] showed that at a given load, the contact fatigue life of SP Mo-Ni ADI austempered at 230°C is 35–45% lower than that of carburised steel. The author attributed this to the rougher surface and a lower hardness of the SP ADI when compared to carburised steel. Lubricated rolling contact fatigue tests carried out by Ohba et al. [30] showed that SP Cu-alloyed ADI exhibited similar wear rates to corresponding as-austempered specimens.

fatigue strength in this particular case.

a 30% increase in the wear resistance after SP.

**3.2. Influence of shot peening on tribological characteristics of ADI**

be attributed to flattening of the rough surfaces during the wear test.

the positive effects of compressive stresses and hardening caused by SP.

Although SP is not a new process, it is still very popular and has evolved considerably in the late twentieth and early twenty-first centuries. A great deal of research has been carried out to study the effects of SP on the materials being treated, including metallurgical, mechanical, and geometrical effects and also the effects of SP on the mechanical properties, tribological characteristics and corrosion resistance of a wide range of metals and alloys [11]. Recently, advanced surface modification technologies show the use of variants of the traditional SP process, four of which are shown in this issue, namely ultrasonic SP, severe SP (SSP), surface mechanical attrition treatment (SMAT) and duplex SP. Other variants of SP can be found in literature, including laser shock peening (LSP), micro-peening, cavitation shotless peening and ball or roller burnishing.
