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Materials Science and Engineering A 460–461 (2007) 542–549

Effect of surface work hardening on wear behavior of Hadfield steel

Weilin Yan a,∗, Liang Fang a, Kun Sun a, Yunhua Xu b

a State Key Laboratory for Mechanical Behavior of Materials, Xian Jiaotong University, Xian 710049, People’s Republic of Chinab Institute of Wear Resistance Materials, Xian University of Architecture & Technology, Xian 710055, People’s Republic of China

Received 27 October 2006; received in revised form 19 January 2007; accepted 22 February 2007

bstract

Shot peening has become an effective method to strengthen alloy. In this investigation, it has been used to work-harden Hadfield steel. It haseen regarded that the surface hardness of Hadfield steel has been increased greatly after shot peening. Using electron scanning microscope (SEM),igh resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD) analysis, the microstructure of shot penned surface layeras examined. It has been observed that a nanocrystalline surface layer is formed. The grain sizes of surface were decreased to 11.1–17.4 nm and

he maximum hardened layer can reach to 100 �m after the treatment. Surface hardness was also increased with increasing shot peening period.

wo-body and three-body abrasive wear experiments were carried out for work hardening and original specimens, separately. The results showed

hat the three-body wear resistance of the nanocrystallized Hadfield steel has distinctly been improved using soft abrasive particles. For harderbrasive particles like emery paper, the increased hardness and grain refinement by shot peening cannot improve two-body abrasive wear resistance. 2007 Elsevier B.V. All rights reserved.

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eywords: Hadfield steel; Work hardening; Nanocrystalline; Hardness; Abrasiv

. Introduction

Nanostructured materials, with the considerable reducedrain size and significant volume fractions of grain boundaries,ave exhibited superior mechanical properties compared to theiroarse grain types. For instance, they possess high strength andardness [1], as well as good wear resistant properties [2]. Inecent years, bulk nanostructured materials formed by severelastic deformation (SPD) have attracted the growing interest ofpecialists in materials science [3]. Synthesizing nanostructuredaterials by SPD techniques are, however, still with limited

ize shortcomings. Lu and co-workers [4–8] have got nanocrys-alline surface by ultrasonic shot peening in different metals,uch as, pure Fe, low carbon steel, 316L stainless steel alu-inum alloy 7075 and AISI 304 stainless steel. Evidently, the

urface nanocrystallization becomes another potential methodo get nanostructured materials. For Hadfield steel the previousork by the present authors [9,10] and Gavriljuk and co-workers

11,12], successfully obtained nanocrystalline and amorphous inurface layer after impact wear. In addition, nanocrystalline andmorphous have also been examined in the surface of railway

∗ Corresponding author. Tel.: +86 29 82665479; fax: +86 29 82665479.E-mail address: yanwl@eyou.com (W. Yan).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.02.094

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racks by Lojkowski et al. [13]. Those works are quite interestingnd valuable.

Is it possible to use work hardening to increase the wear resis-ance of steel? Khruschov and Richardson [14–17] reported thatncreasing the hardness of metals by work hardening has littleffect on wear resistance as shown in Fig. 1. Mechanism could bexplained as the surface layer of alloy is always work-hardenedy abrasives after running-in. The work hardened layer thick-ess generally depends on plastic deformation severity whichs decided by abrasive wear parameters. If work hardening isreviously done using parameters different to ordinary wearonditions, work hardening may give effect to the wear resis-ance of material. It has been noticed that nanostructured layerf low carbon steel and pure copper obtained by ultrasonic shoteening can reduce fatigue wear [18,19]. It has been known thatadfield steel has a strong potential ability to work hardening.

f the nanocrystallized surface layer is formed in the surface ofadfield steel before service, it should be possible to improve

brasive wear resistance in a certain wear conditions.Therefore, it becomes interesting to investigate the effect of

teel prestrain on abrasive wear. It will be significant both from

cademic and technological points of view. In this work, a nanos-ructured surface layer of Hadfield steel was synthesized by shoteening treatment. Nanocrystalline structure was characterizedy different techniques and its abrasive wear properties were

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ig. 1. Relation of relative wear resistance (ε) of two-body abrasion to hardnessH) of cold-work hardening materials [14].

tudied in comparison with coarse-grained sample. The effect ofrain refinement and hardness increasing on the abrasive wearroperties in the surface layer will be discussed.

. Experimental

Chemical compositions of test alloy in weight percentagere: 1.20 C, 12.86 Mn, 0.87 Si, 0.05 S and 0.09 P. The sam-les were solid solution treated at 1050 ◦C for 2 h, and thenere water quenched. After that heat treatment the investigated

lloy exhibited a single austenite phase, with grain sizes about00–200 �m. Samples of 40 mm × 75 mm were prepared forhot peening treatment. The samples were shot peened by aachine type S170R with a shooting area 100% sample surface.ast steel billiards was used with 0.2 mm nominal diameter and

he impacted direction to the sample surface was limited to 85◦.ir pressure was 0.18 MPa and the shooting duration was 2, 7,5, 30, 60 and 120 min, respectively. Vickers microhardness waseasured by a MH-5 microhardmeter with an applied load of

.49 N for 10 s.The abrasion test geometry was shown in Fig. 2, respectively.

mery papers with nominal grain size about 100–120 �m iniameters, Vickers hardness 1800 HV and angular geometri-

al shapes were used in the two-body wear tests. Pin diameteras 4 mm with a normal load 29.4 N. The normal pressure was,

herefore, 2.34 N/mm2. The pin was held in a loading arm past anrchimid’s track. The sample, therefore, always contacts with

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Fig. 2. Schematic of wear geometry for two

eering A 460–461 (2007) 542–549 543

resh abrasive particles. The traveling speed was varied whichncreases gradually from 0.0628 to 0.628 m/s accompanied withncreasing the Archimid’s rotating radius. It has been shownhat traveling speed has a little influence on abrasive wear. Totalraveling distance was 8.7 m. Glass particle with nominal grainize about 250–400 �m in diameters, Vickers hardness 500 HVnd angular geometrical shapes were used in the three-bodyear tests. The area of wearing surface was 15 mm × 22 mmith a normal load 9.8 N. The normal pressure was, therefore,.03 N/mm2.and the line speed of sample was about 0.48 m/s.he reciprocal value of the weight loss after 8.7 m test and 40 minear period was taken as the data of wear resistance, separately.he tests were repeated three times for one type of test sample.he worn surface morphologies were examined using a S-2700canning electron microscope (SEM). The cross-sectional pro-le of the wear was also measured by a TR-profilometer to besed for estimating the groove depth. The observation of shoteening treated surface layer morphologies was performed on a-2700 scanning electron microscope (SEM).

Cross-sectional observations of the treated sample were pre-ormed on an optical microscope. Structure of the sample surfaceas examined by X-ray diffraction (XRD) with Cu K� in X’ PertPD RRO X-ray diffractometer. The average grain size was

btained from XRD line broadening [20]. Microstructure fea-ures in surface layer were also characterized by a JEM-3010igh-resolution transmission electron microscope (HRTEM,perating at a voltage of 300 kV).

. Results and discussion

.1. Microstructure of surface layer

Fig. 3 shows a cross-sectional optical microscopicicrostructure of Hadfield steel after 60 min shot peening.evere plastic deformation can be easily observed at about0 �m deep from the top surface. Obviously, the plasticeformation is inhomogeneous. With decreasing depth, slipsecome severe.

Fig. 4 shows the X-ray diffraction patterns of the samplesefore and after shot peening treated. The Bragg-diffractional

eak broadening in the shot peening treated samples can bettributed to grain refinement and/or an increase in the atomic-evel lattice strain. It can be found that all the samples consist ofustenite without any martensite transformation occurring in the

-body and three-body abrasive wear.

544 W. Yan et al. / Materials Science and Engineering A 460–461 (2007) 542–549

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ig. 3. Cross-sectional optical microstructure of Hadfield steel after 60 min shoteening.

urface layer during shot peening treatment. Quantitative XRDeasurements indicated that the grain size in surface layer of

0 min treated sample is decreased to about 11.1–17.4 nm.Fig. 5 shows TEM observation at about 20 �m deep from

0 min shot peening treated surface. It is shown that grain sizesre decreased to nano magnitude. The grain size underneath thehot peening surface examined by TEM is in a range of a few tensf nanometers. The diffusive diffraction spots in the SAED pat-ern are identified as austenite without martensite transformationbserved.

Fig. 6 shows a TEM picture of the 60 min shot peeningreated surface. Nanostructured grains possess random crystal-ographic orientations, as indicated by SAED pattern. The grainize is about 3–10 nm, which is smaller than the XRD result11.1–17.4 nm). That might be partially attributed to the facthat the XRD collects all data of the microstructure in about�m thickness.

Surface nanocrystallization by shot peening is similar to

hose caused by those severe plastic deformation [9–12]. Theilliards provided repeated impacts on a surface at a high speednd generated high-density dislocations and deformation twins.n one hand, the resultant dislocation could be re-arranged under

Fig. 4. XRD patterns of Hadfield steel before and after shot peening.

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ig. 5. The cross-sectional TEM picture ((a) bright field and (b) dark field) ofhe microstructure of beneath 20 �m subsurface after 60 min shot peening.

tress to form dislocation networks, leading to the formationf nanocrystallites separated by diffuse grain boundaries. Innother hand, deformation twins subdivide the original grainsfficiently through introducing different boundaries whichead to form refined microstructures. In the surface layerf the Hadfield steel, nanocrystalline structure with randomrientations is formed. The formation of the nanocrystallitesan be attributed to the distinct effects: a very large strain,n extremely large strain rate and repetitive loading by shoteening. The remarkably increased strain and strain rate activatehigh density of dislocation and multisystem deformation

wins to accommodate the straining.The surface layer can be subdivided into sections along

he depth from the 60 min shot peening treated surface basedn OM and TEM observations, as well as XRD analysis: a

W. Yan et al. / Materials Science and Engineering A 460–461 (2007) 542–549 545

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ig. 6. A bright field TEM images showing planar microstructures of top surfaceayer after 60 min shot peening.

anostructured region (0–30 �m deep) and a submicro-sizednd micro-sized region with evidently plastic deformationabout 30–100 �m deep).

It becomes interesting that nanocrystalline can also be foundven though very short peening duration was done, such asand 30 min peening duration. Figs. 7 and 8 show the TEMicrostructure of surface after 2 and 30 min shot peening,

espectively. The corresponding SAED pattern indicated thatrystallographic orientations are random. The average grain

ize in the surface layer is about 40 and 30 nm for 2 and 30 minhot peening, respectively. In engineering applications, shortreatment duration is with important meaning. The suitable

ig. 7. A bright field TEM images showing planar microstructures of top surfaceayer after 2 min shot peening.

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ig. 8. A bright field TEM images showing planar microstructures of top surfaceayer after 30 min shot peening.

uration peening treatment can save time and energy in oneand, in another hand the short time treatment can also preventaterial subsurface from a lot of cracks initiation. It will be seen

n the following discussion that so-called “excessive” peeningreatment will not bring beneficial influence on improvingbrasive wear resistance.

.2. Relation of microhardness to grain refinement

Vickers hardness of Hadfield steel was measured on a cross-ectional surface before and after shot peening, as presented inig. 9. With increasing shooting duration, the hardness of the

reated sample is increased. The surface hardness for original

ample and for shooting duration 2, 7, 15, 30, 60 and 120 min are56, 400, 430, 470, 503, 676 and 774 HV, respectively. The vari-tion of hardness along depth for 60 min shot peening is shownn Fig. 10. Surface hardness with nanocrystalline is increased to

ig. 9. Hardness variation of the surface layer with shot peening duration.

546 W. Yan et al. / Materials Science and Engineering A 460–461 (2007) 542–549

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Fig. 11. Wear resistance relative to shot peening treated duration using glassparticles as abrasives.

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dpIsRcdadepth can be estimated by TEM, using Cu K� wavelength andFe extinction depth as a monitor. The depth of nanocrystallizedzone depth was estimated about 5–30 �m. It can be, therefore,

Table 1Roughness of worn surface profile abraded by glass sand

Shot peening duration (min) Ra (�m)

0 3.2302 2.9807 2.882

ig. 10. Hardness variation along depth from the surface of the sample after0 min shot peening.

76 HV, then, decreased gradually along the depth (about 256V in the matrix).As above-mentioned the original large size grains have been

ransformed to nanostructured equiaxed grains and hardness haslso been greatly increased after shot peening. The repetitivehot peening to the surface results in work hardening, whichesponses the dislocation multiplication, and finally grain refine-ent in surface layer. Yield strength (related to hardness) ofaterial, grain size d and dislocation density ρ can be shown as

ollowing expression [1,21]

y = σ0 + kyd−1/2 (1)

y = σ0 + αμbρ1/2 (2)

here σ0 and α are the constant, μ the shear modulus and b is theurgers vector. Apparently, the dislocation density can directlyelate to grain size as follows

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herefore, it is reasonable that the increase of dislocation den-ity can directly induce to grain refinement because of workardening.

.3. Abrasive wear behavior

In the three-body abrasive wear using glass sand, the shoteening duration 2, 7, 15 and 30 min treated samples helps tomprove the wear resistance of samples as shown in Fig. 11.specially, the wear resistance was increased about 72% for0 min peening duration. To our surprise, longer peening dura-ion is not beneficial to increase the wear resistance as weonsidered. If abrasive is changed to emery particles paper, thehot peening is not contributed to improvement of wear resis-ance, as shown in Fig. 12.

The SEM morphologies of the wear scar for glass particles

aper are shown in Fig. 13. It was indicated that the grooveear pattern is dominant for original untreated sample. How-

ver, for shot peening treated sample, the groove morphologyas completely disappeared. Instead plastic fatigue became the

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ig. 12. Wear resistance relative to shot peening treated duration using bondedmery paticles as abrasive paper.

ominant wear mechanism. In order to estimate groove depth, arofilometer was used to measure the roughness of worn surface.t is shown in Fig. 14 and Table 1, that the worn surface of originalample without shot peening treatment is with lager roughnessa, compared to 2 and 30 min shot peening samples. The dataan be used to roughly estimate the abrasive particle embed-ed depth. Ra was measured arranged in 2.352–3.565 �m forll types of wear samples. In addition, the nanocrystallized zone

15 2.86930 2.35260 2.99520 3.565

W. Yan et al. / Materials Science and Engineering A 460–461 (2007) 542–549 547

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ig. 13. Worn surface morphologies of before and after shot peening under glaeening duration.

oncluded that the particle embedded depth was smaller than thatf the nanocrystallized zone if shot peening was carried out.

Firstly, microstructure refinement is expected to increaseardness, which can be used to roughly explain the tendencyf abrasive wear resistance according to Rabinowicz’s abrasiveear equation [22], i.e.

= KP

H(4)

here W is the wear volume per unit sliding distance, P thepplied load, H the hardness of the worn surface and K is theear coefficient. It has been reported that nanocrystalline nickel

oating with a grain size of 13 nm made by electrodepositionas a much-enhanced abrasive wear resistance compared withhe conventional coarse-grained polycrystalline nickel [23,24].t is also noticed from Fig. 11 that wear resistance for peen-

ig. 14. Cross-sectional profile of worn surface after 60 min shot peening treatedamples by using glass particles as abrasives.

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ticle wear: (a) the original sample, (b) 30 min, (c) 60 min and (d) 120 min shot

ng duration 30 min samples was improved 72%. The increasedear resistance can be attributed to the increased hardness by

hot peening. It has been measured that the Vickers hardnessf surface for peening duration 2 and 30 min was 400 and 503V, separately. They are near to or equal to the glass particleardness (∼500 HV). During wear process the increased hard-ess keeps to effectively prevent the particles from penetratingo non nanocrystallized zone. Secondly, during wear, the sur-ace and subsurface of the sample are subjected to alternatetress and compression stress due to action of abrasive parti-les [25]. Shot peening treatment can enhances microhardnessnd yield strength [26] which decreases plastic deformation andear. Therefore, the high hardness subsurface and nanocrystal-

zed microstructure tend to increase the wear resistance of theaterial.Of course, it can easily argue that Ra data are also smaller

or 60 and 120 min shot peening samples than that of originalample without any shot peening treatment. Why are not theear amounts for 60 and 120 min peening samples decreased

s the case of 2 min or 30 min treatment samples? To investi-ate this problem, shot peening surface after 60 min durationas examined by SEM. It is found from Fig. 15 that a lot oficrocracks have been produced in surface layer because of

excessive” shot peening. Therefore, wear property will deteri-rate again after “excessive” shot peening. Data in Table 1 alsohows that roughness Ra increases again for 60 and 120 min shoteening samples, compared to samples of 30 min.

548 W. Yan et al. / Materials Science and En

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ig. 15. The cross-sectional SEM observation of the typical cracks in 60 minhot peening treated sample.

A spherical grain is taken as an example to illustrate the mov-ng patterns of abrasive particles. Considering the forces actingn the moving particle, as shown in Fig. 16, the particle contactspper and lower surface within a small area on which normalnd frictional forces are acted. It becomes convenient that theormal forces and frictional forces are simplified as a couple ofoncentrated normal force N and a frictional force F, respec-ively. The particle supports the torque Fh to have a tendencyolling and torque Ne to resist rolling.

On base of the previous work by the present authors [27], ifh > Ne, the particle will tend to roll and if Fh ≤ Ne, the parti-le will tend to slide. The higher hardness of sample, the morehallow the particle is embedded in shot peening treated surfacend the larger h/e, causing the particle easy to roll relative tohe surfaces. On the contrary, the particle becomes possible tolide relative to the surfaces. Grooving wear mechanism can,herefore, be transformed because of hardness increasing. Fromhe SEM morphologies of the wear scars (see Fig. 13), it can bebserved that micro-cutting characteristic is clear for untreatedriginal sample.

For the emery paper two-body wear test, the prestrain hard-ning cannot improve wear resistance evidently. Although theurface hardness was increased rapidly with prolong peeninguration, the hardness of emery (1800 HV) is still higher than

hat of the peening treated surface. The surface layer severelastic deformation occurred due to shot impacting, and, hence,he plastic strength accordingly declined. Generally, the plastictrength of metals also affects wear resistance properties

ig. 16. Schematic illustration of abrasive particles movement patterns for a softpper surface and a hard upper surface with the same bottom surface.

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gineering A 460–461 (2007) 542–549

In conclusion, a nanocrystalline surface layer of Hadfieldteel by shot peening can definitely improve the wear resis-ance under the conditions of soft abrasive three-body abrasionear. It becomes also necessary to reevaluate the issue that the

ffect of cold work hardening on wear resistance of material isrivial. For commercial applications shot peening treatment isn appropriate process to strengthen metal materials to improveear resistance instead of using alloying or other processes with

hanging the bulk microstructures of materials, especially in lowmpact energy wear.

. Conclusions

A nanocrystalline surface layer of a Hadfield steel is success-ully formed by means of shot peening treatment. The grain sizesn top surface layer of the treated sample are arranged about1.1–17.4 nm. The hardness can be increased gradually frombout 256 HV for original sample to about 774 HV for 120 minhot peening sample. For two-body abrasive wear when emerys used as abrasives, the increased hardness and grain refine-

ent by shot peening cannot improve wear resistance. For glassbrasive three-body abrasive wear when glass particles are useds abrasives, wear resistance can be improved by shot peen-ng, especially, the wear resistance of treated surface can bencreased by 72% for 30 min peening duration. The results indi-ated that work hardened by shot peening can have an effectn their wear resistance for soft abrasive particles conditions.he improvement of wear properties of shot peening treatedample results from work hardening and harder nanocrystallineayer.

cknowledgements

The present authors are appreciated to the financial supportrom the National High Technology Research and Developmentrogram of China (863 Program), the Ministry of Science andechnology of China (Grant 2002AA302509).

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