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Effect of shot peening on the fatigue performance of ductile iron castings

S. Ji, K. Roberts, Z. Fan

Department of Materials Engineering

Brunel University

Uxbridge, Middlesex, UB8 3PH

UK

(shouxun.ji@brunel.ac.uk)

Abstract

Ductile iron is a commonly used structural material. However the unsatisfactory fatigue

performance has limited its application for some dynamic loads. Shot peening is a mechanical

surface modification process, which can extend the fatigue life of materials by introduction of

working hardening and compressive layer on surface and removal of the surface

irregularities. Results of the influence of shot peening treatment on ductile iron castings with

as-cast surface and machined surface are presented. The results showed that the shot peening

on ductile iron could double the fatigue life for as-cast surface and increase by 4 times for

machined surface. It is believed that shot peening affects fatigue life through the reduction of

the density and the length of the cracks formed on the surface of specimens of ductile iron.

Keywords: ductile iron, fatigue, surface treatment, shot peening, micro-cracks

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1. Introduction

Ductile iron is one of the most commonly used structural materials in the world because of its

good combination of low cost, design flexibility, good strength-to-weight ratio, toughness,

wear resistance and fatigue performance [1,2]. It offers this good combination of properties

with the excellent castability, thus possessing unique production advantages [3]. However,

the fatigue performance of ductile iron is not sufficiently good in some applications for

dynamic loads. A lot of trials have been undertaken to improve the fatigue life of ductile iron

in the last few decades [1], but the industrially applicable processes are still rare because of

the limitation of the gains in properties and operation flexibility. However, little is known

about its fatigue in response to shot peening surface treatment, especially for the directly

shot-peened as-cast surface, even though some results have been found on the austempered

ductile iron with other processes [4,5,6,7]. As it involves blasting specimen surface with high

velocity steel or glass particles, shot peening puts the specimen’s interior in a state of tension

while the surface, including a thin layer of sub-surface material, is in compression [8]. This is

considered to be an effective life enhancement process because of the compressive residual

stress and its effect, which is to delay surface cracking [9, 10]. Therefore, this paper aims to

introduce the effect of shot peening on the fatigue life of ductile iron. Four different kinds of

surface conditions, as-cast, shot-peened as-cast, machined, machined and shot-peened were

used to examine the fatigue life, microstructural and fracture features.

2. Experimental

The ductile iron castings were produced in a foundry for the current project. A medium

frequency coreless induction furnace was used to melt the alloys and superheat to 1500oC.

The melt was taped into a preheated shank ladle of 20kg capacity, containing the nodulising

alloys and inoculation alloys with sandwich techniques. Another kind of inoculating granular

additive of ferrosilicon alloys was added as stream inoculation during pouring the treated

melt into the mould. The pouring temperature of the melt was controlled at 1450oC. A plate

type casting with a dimension of 225×150×40 mm were produced in the moulds

manufactured by silica sand with 1% furan resin binder and 0.40% hardener (based on sand).

The typical chemical composition of the produced casting was 3.54wt.%C, 2.41wt.%Si,

0.53wt.%Mn, <0.02wt.%P, <0.01wt.%S and 0.03wt.% of retained magnesium. The numbers

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and the nodularity of graphite nodules in casting section were >200mm-2 and >0.90,

respectively. The main mechanical properties of the ductile iron include tensile strength of

500MPa, 2% proof stress of 320MPa, elongation of 7% and typical hardness from HB170 to

HB230.

The produced castings were treated with blasting in a foundry, following the industrial

castings. Then the castings were randomly classified into four groups for specimen

production. One group of castings was retained without further surface treatment, which was

defined as as-cast surface (AC). The specimens, as schematically shown in Fig. 1, were

machined from the bottom of the castings. Only one specimen was cut off from one casting.

The bottom surface of the specimens was retained for surface treatment and other surfaces

were machined to a same quality. Three types of further surface treatments used in this study

were shot peened as-cast surface (PC), machined surface (MS), and machined and shot

peened surface (MP). For the machined surface, 1.5mm of metal was cut off at the bottom of

the casting surface.

Shot peening was performed by means of an injector-type system. Peening intensity was

measured using an A Almen strip. The main parameters of shot peening are summarised in

Table 1. The surface roughness was used to define the surface condition, which was

measured by a profilometer. The measured Ra values for different surface conditions were

17.0 µm for AC, 7.3µm for PC, 4.3µm MS and 3.7 for MP, respectively.

The specimens were fatigued to failure in a bending machine at a frequency of 25Hz using

10-20 specimens for S-N curves and the laboratory temperature was 20oC. The

microstructural observations were taken for tested specimens by means of optical microscope

and scanning electron microscope (SEM). SEM also observed the fracture characteristics of

specimens after failure.

3. Results

The relationship between the bending stress and the number of cycles to failure is shown in

Fig. 2. It is evident that the shot peening treatment can significantly improve the fatigue life

of the ductile iron. The numbers of fatigue life of the specimens with shot-peened surface

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were obviously higher than that of the surface without shot peening treatment, including

machined surface and as-cast surface, even though the roughness of shot-peened as-cast

surface is higher than that of machined surface. The average increasing range of the fatigue

life of the specimens was double between as-cast surface and shot-peened as-cast surface and

4 times between machined surface and shot-peened and machined surface. The fatigue life of

the specimens with shot-peened as-cast surface was higher than those with machined surface.

At low stresses, the fatigue life of the specimens with shot-peened as-cast surface was close

to those with machined surface. Both of them were much higher that the specimens with as-

cast surface and lower than the specimens with machined and shot-peened surface.

Fig. 3 shows the crack developed after bending test on the unetched section of ductile iron

with different surface conditions. The results illustrated that shot peening can drastically

reduce the number and penetrating depth of the micro-cracks on the casting surface and sub-

surface. For the specimens with as-cast surface, as shown in Fig. 3a, a lot of deeply

penetrated cracks were found on and near the surface. The graphite nodules on and near the

casting surface were also apparently distorted, which were prolonged along the crack

direction. After shot peening treatment, as shown in Fig. 3b, the numbers of the cracks on the

surface of the specimens were considerably reduced. The depth and the length of the cracks

penetrating into the specimens were also smaller than those with as-cast surface. Meanwhile,

the distortion of graphite nodules near the surface of specimen was pronouncedly reduced.

The smaller distortion of the graphite nodules and reduced micro-cracks indicated that the

matrix was hardened by shot peening. For the machined surface, as shown in Fig. 3c, small

and short cracks were found to distribute randomly on the surface and sub-surface of the

specimens and the graphite nodule distortion was quite small. For the machined and shot-

peened surface, as shown in Fig. 3d, the numbers of the cracks were further reduced and the

distortion of graphite nodules was not pronounced in the micrograph.

Fig. 4 shows the microstructures of ductile iron with different surface conditions, which

illustrated the variation of the deformation of ferrite and pearlite in the matrix after the

bending fatigue life test. Both ferrite and pearlite in the matrix were apparently distorted near

the as-cast surface in the specimens, as shown in Fig. 4a. The distortion of the ferrite and

pearlite in the matrix was reduced for the specimens with shot-peened as-cast surface

(Fig.4b). This could further indicate the existence of the harden layer on the shot-peened

surface. The distortion of ferrite and pearlite in the matrix of specimens with machined

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surface (Fig. 4c) and shot-peened and machined surface (Fig. 4d) was not apparent. The

results in Fig. 4 revealed that the distortion of the ferrite and pearlite near the shot peened

surface is different to those with as-cast surface.

Fig. 5 shows the fractographical micrographs of ductile iron with different surface conditions.

For specimens with as-cast surface, the fracture surface contains mainly the faceted brittle

areas. The cleavage cracks were visible on the fracture surface of the specimens (Fig. 5a).

The fracture surface of shot-peened as-cast specimens had less cleavage cracks and more

dimpled ductile areas (Fig. 5b). The similar tendency could be found in Fig. 5c and Fig. 5d,

corresponding to the machined surface and machined and shot-peened surface, respectively.

The faceted brittle fracture was also clearly visible in the shot peened layer near the surface in

both Fig. 5b and 5d.

4. Discussion

The results illustrated that shot peening on the surface of ductile iron castings improves their

fatigue life significantly. The fatigue life of the specimens with shot peening can be doubled

for as-cast surface and increased by 4 times for machined surface. The experimental

observations also found that the increase of the fatigue life can be attributed the reduction of

the cracks on the surface and sub-surface of the specimens. These can be further intercepted

by the existence of surface hardening and compressive stresses and the removal of

irregularities on the casting surface introduced by shot peening.

Numbers of cycles to failure of thin specimens (4mm) are governed by the initiation and

growth of small cracks, and residual stresses. The crack density developed in the early stage

of fatigue life increases with cycling due to the nucleation of additional cracks. Because of

this and the growth of some existing cracks, the cracks spacing is presumed to decrease

continuously and approach its steady value when material attains its saturation. Subsequently,

a few of the cracks could link up to form a critical crack that in turn can propagate to failure

in negligible cycles. As cracks start at the surface, a concern of course is its condition of

surface. Being in as-cast state, the surface is rough and usually contains some inclusions.

There are many graphite nodules distributed in the matrix. These lead the development of

stress concentration and further promote crack nucleation and propagation across the

specimen section. As a result, it has a lower fatigue life at all stresses (Fig. 2&3a). The

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machined surface cut off the rough layer, which gives rise to an improved feature. So the

fatigue life is longer than those of as-cast surface (Fig.2). But the existence of graphite

nodules can be the source of crack nucleation, as shown in Fig. 3 and 4. So the fatigue life is

still limited.

The improvement of shot peening on the fatigue life can be attributed to three aspects, surface

hardening, and the removal of irregularities on the casting surface and the existence of

compressive residual stress. Shot peening is a classic method known to bring about working

hardening in the surface region of materials, which could considerably improve the fatigue

strength [11]. The working hardening can explained by the dislocation mechanism of crystal-

lattice transformation, which responds to the low-temperature phase transformation of ferrite

and pearlite to influence the mechanical properties. The significantly increased hardness near

the surface region of materials has been found in different materials [12, 13].

It is also conceivable that the removal of irregularities on the casting surface can improve the

fatigue life because most cracks start at surface (Fig. 3), especially for the as-cast surface.

The shot peening on as-cast surface can drastically reduce its surface roughness Ra from 17.0

µm to 7.3µm. Some irregularities and inclusions on the casting surface are peened off. So the

opportunities of crack nucleation from these surface irregularities and inclusions is reduced.

The fatigue performance is therefore improved. Similar behaviours have been noted for other

alloys during shot peening treatment [14].

Another important factor to improve the fatigue life is the existence of compressive residual

stress developed in the ductile iron by shot peening. A high-level compressive residual stress

exists up to the depths of 400-500µm with a maximum values of –450MPa at the surface

layer [15]. When applying a higher level fatigue stress, the compressive residual stress on the

surface is rapidly decreased and the tensile stress occurs through the concentration of stress at

dents or irregularities on the surface or at graphite nodules existing immediately below the

surface. As a result, the cracks can be nucleated and propagate from the surface. In this case,

the fatigue life is mainly determined by the value of the stress concentration. The role of the

existed compressive residual stress on the surface of ductile iron is thus limited. When

applying a lower level fatigue stress, the compressive residual stress at the surface area does

not decrease significantly or can last for a longer cycling time, which could prevent the

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nucleation and propagation of the cracks. With the increase of cycling, the cracks can be

nucleated from the internal defects such as graphite nodules and inclusions in the matrix and

the irregularities on the surface of castings [15,16]. In this case, the compressive residual

stress is expected to play an important role to extend the fatigue life.

It is difficult to exactly define the role of three aspects, the working hardening on surface, the

removal of surface irregularities and the existence of compressive residual stress on surface

and sub-surface on the improvement of the fatigue performance of the shot-peened casting

surface.

5. Summary

Shot peening on the surface of ductile iron castings can significantly extend their fatigue life.

Compared to conventional as-cast surface and machined surface of castings, numbers of

cycles to failure of thin specimens are doubly increased for shot-peened as-cast surface and

increased by 4 times for shot-peened and machined surface, respectively. It is believed that

shot peening affects fatigue life through the retardation of crack nucleation and growth

because of the introduction of working hardening and compressive stresses on the surface and

sub-surface and removal surface irregularities of the ductile iron castings.

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Table 1. The parameters of shot peening treatment for ductile iron castings

Pressure Shot size Distance Vertical speed Rotating speed Coverage Intensity

3.0kg/cm2 0.80mm

(S330)

200mm 240 mm/min 30 rpm > 85% 0.30 A

30m

m

φ7mm

90mm

10mm

R32mm

δ=4mm

Fig. 1. The schematic diagram of the specimen for fatigue life tests.

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150

200

250

300

350

400

450

1.E+04 1.E+05 1.E+06 1.E+07

Number of cycles to failure

Stre

ss (M

Pa)

ACPCMSMP

Fig. 2. S-N curves of ductile iron with as-cast surface (AC), shot-peened as-cast surface (PC),

machined surface (MS), and machined and shot-peened surface (MP).

104 105 106 107

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Fig. 3. The Un-etched optical micrographs showing the cracks and the distortion of graphite

nodules in failed ductile iron specimens at a fatigue stress of 360MPa with different surface

qualities. (a) as-cast surface (AC), (b) shot-peened as-cast surface (PC), (c) machined surface

(MS), and (d) machined and shot-peened surface (MP).

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Fig. 4. The optical micrographs etched with 1% nital showing the microstructures of in failed

ductile iron specimens at a fatigue stress of 360MPa with different surface qualities. (a) as-

cast surface (AC), (b) shot-peened as-cast surface (PC), (c) machined surface (MS), and (d)

machined and shot-peened surface (MP).

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Fig.5. The SEM micrographs showing the fracture surface after fatigue failure of ductile iron

specimens at a fatigue stress of 360MPa with different surface qualities. (a) as-cast surface

(AC), (b) shot-peened as-cast surface (PC), (c) machined surface (MS), and (d) machined

and shot-peened surface (MP).

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