**1. Introduction**

Compared to other ferrous materials, austempered ductile iron (ADI) has marked economic advantages such as low melting temperature, low shrinkage, excellent castability, good machinability and high damping capacity. Its versatility and wide range of properties make it widely used in the transportation industries, defence, heavy machinery, agricultural machinery and for general engineering applications. ADI can compete with steel on considerations of strength, for a given level of ductility. However, some alloyed and hardened steels exhibit better properties than ADI, and the use of ADI is limited when extreme tensile strength is

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

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 450°C, a temperature above the martensite start temperature *Ms* and left there for sufficient time to transform the austenite to ausferrite. This is followed by cooling to room temperature.

austenite (*Vγ*), having high carbon content (*Cγ*) and provide a bending fatigue strength in the region between 200 and 500 MPa as reported in various studies carried out on both alloyed

**Figure 2.** (a) Temperature-time plot for a typical austempering treatment and (b) austempering treatment superimposed

Shot Peening of Austempered Ductile Iron http://dx.doi.org/10.5772/intechopen.79316 27

When ADI engineering components require high toughness and ductility at the core of the component coupled with high bending fatigue strength and good tribological characteristics at the surface, the ADI can be first austempered in the higher temperature range of 350–400°C to obtain upper ausferrite. It is subsequently engineered to improve the surface properties and obtain the fatigue and tribological characteristics required by the intended application,

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

and unalloyed ADI [4–8].

on an isothermal transformation diagram [2].

**2. Shot peening**

using processes such as shot peening.

compressive stresses on the surface of a component.

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].

**Figure 2.** (a) Temperature-time plot for a typical austempering treatment and (b) austempering treatment superimposed on an isothermal transformation diagram [2].

austenite (*Vγ*), having high carbon content (*Cγ*) and provide a bending fatigue strength in the region between 200 and 500 MPa as reported in various studies carried out on both alloyed and unalloyed ADI [4–8].

When ADI engineering components require high toughness and ductility at the core of the component coupled with high bending fatigue strength and good tribological characteristics at the surface, the ADI can be first austempered in the higher temperature range of 350–400°C to obtain upper ausferrite. It is subsequently engineered to improve the surface properties and obtain the fatigue and tribological characteristics required by the intended application, using processes such as shot peening.
