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Tribology International 42 (2009) 634–641

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

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journal homepage: www.elsevier.com/locate/triboint

Effect of surface nanocrystallization on abrasive wear propertiesin Hadfield steel

Weilin Yan a,b,�, Liang Fang a, Zhanguang Zheng b, Kun Sun a, Yunhua Xu c

a State Key Laboratory for Mechanical Behavior of Materials, Xian Jiaotong University, Xian 710049, People’s Republic of Chinab College of Mechanical Engineering, Guangxi University, Nanning 530004, People’s Republic of Chinac Institute of Wear Resistance Material, Xian University of Architecture & Technology, Xian 710055, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 30 July 2006

Accepted 27 August 2008Available online 7 October 2008

Keywords:

Hadfield steel

Work hardening

Nanocrystalline

Abrasive wear

9X/$ - see front matter & 2008 Elsevier Ltd. A

016/j.triboint.2008.08.012

esponding author. Tel./fax: +86 29 82665479.

ail address: yanwl@eyou.com (W.L. Yan).

a b s t r a c t

A nanocrystalline surface layer was synthesized on a Hadfield steel by shot peening treatment. The

microstructure evolvement of the surface layer of the shot peening treated sample was characterized by

optical microscopy, X-ray diffraction and high-resolution transmission electron microscopy. It has been

shown that the grain sizes in surface layer were decreased to 11.1–17.4 nm after 60 min shot peening

duration. Surface hardness was also increased greatly. Two- and three-body abrasive wear experiments

were carried out for work hardening and original specimens, separately. The results showed that the

wear resistance of the nanocrystallized Hadfield steel has distinctly been improved in case of soft

particles used as two-body wear abrasives or light impact load applied for impact abrasive wear.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Nanostructured materials, with the considerable reduced grainsize and significant volume fractions of grain boundaries, haveexhibited superior mechanical properties compared to theircoarse grain types. For instance, they possess high strength andhardness [1], as well as good wear resistant properties [2]. Inrecent years, Lu et al. have got surface nanocrystalline micro-structures of different metals and alloys by ultrasonic shotpeening treatment [3–7]. The technique of surface nanocrystalli-zation is expected to afford a new approach for applications ofnanostructured materials. In fact, wear becomes one of mainfailure patterns for engineering materials. From another view-point, the optimization of the surface structures and propertiescan effectively improve the global service behavior of materials,which gives people another choice to improve wear service life,besides alloying metals.

Hadfield steel is known for a high resistance to heavy impactwear caused by rapid work hardening. However, it is not benefit tolight impact wear conditions. Is it possible to use work hardeningto increase the wear resistance of Hadfield steel? Khruschov andRichardson et al. [8–11] reported that increasing the hardness ofmetals by work hardening has little effect on abrasive wearresistance, as shown in Fig. 1. In the previous work by the presentauthors [12] has examined that a nanocrystallized surface layer is

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produced in the surface of Hadfield steel after impact wear andthe wear resistance of the steel has been remarkably improved. Ifthe nanocrystallized surface layer is formed in the surface ofHadfield steel before service, it becomes also possible to improveHadfield steel abrasive wear resistance in a certain wearconditions.

Therefore, it becomes interesting to investigate the effect ofsteel prestrain on abrasive wear. It will be significant both fromacademic and technological points of view. In this work, ananostructured surface layer was synthesized on a Hadfield steelby shot peening treatment. Nanocrystalline structure was char-acterized by different techniques and its abrasive wear propertieswere studied in comparison with coarse-grained sample. Theeffect of grain refinement and hardness increasing on the abrasivewear properties in the surface layer will be discussed.

2. Experimental procedures

Chemical compositions of test alloy in weight percentage are:1.20 C, 12.86 Mn, 0.87 Si, 0.05 S and 0.09 P. The samples were solidsolution treated at 1050 1C for 2 h, and then were water quenched.After that heat treatment the investigated alloy exhibited a singleaustenite phase, with grain sizes about 100–200mm. Samples of40�75 mm2 were prepared for shot peening treatment. Thesamples were shot peened treatment by a machine type S170Rwith a shooting area 100% sample surface. Cast steel billiards wasused with 0.2 mm nominal diameter and the impacted directionto the sample surface was limited to 851. Air pressure was

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Fig. 1. Relation of relative wear resistance (e) to hardness (H) using fixed abrasive

grain paper: cold-work hardening materials [8].

Fig. 2. Cross-sectional optical microstructure of Hadfield steel for 60 min shot

peening treated.

Fig. 3. XRD patterns of Hadfield steel before and after shot peening treated.

Fig. 4. The cross-sectional TEM observation of the microstructure of the subsur-

face layer of about 20mm deep from 60 min shot peening treated surface.

W.L. Yan et al. / Tribology International 42 (2009) 634–641 635

0.18 MPa and the shooting duration 2, 3, 60 and 120 min.Cross-sectional observations of the treated sample were pre-formed on an optical microscope (OM). Structure of the samplesurface was analyzed by X-ray diffraction (XRD) with CuKa in a X’Pert MPD RRO X-ray diffractometer. The average crystallite sizewas obtained from XRD line broadening [18]. Microstructurefeatures in surface layer were characterized by a JEM-3010 high-resolution transmission electron microscope (HREM, at 300 kV),respectively. The Vickers microhardness was measured by a MH-5microhardmeter with an applied load of 0.49 N for 10 s.

The two-body wear tests were carried out by pin-on disc weartester type ML-10. Glass papers and emery papers with nominalgrain size about 100 and 120mm in diameters, Vickers hardness500, 1800 HV and angular geometrical shapes were used in thewear tests. Pin diameter was 4 mm with a normal load 29.4 N. Thenormal pressure was, therefore, 2.34 N/mm2. The pin was held in aloading arm past an Archimid’s track. The sample, therefore,always contacts with fresh abrasive particles. The traveling speedwas varied which increases gradually from 0.0628 to 0.628 m/saccompanied with increasing the Archimid’s rotating radius. It has

been shown that traveling speed has a little influence on abrasivewear. Total traveling distance was 8.7 m. The three-body weartests were carried out by a impact wear test apparatus type MLD-10. Quartz sand with 250–400mm in size and Vickers hardness900–1200 HV was used in the wear tests. The impact load was 0.5and 3.0 J with a frequency 200 /min. The reciprocal value of theweight loss after 8.7 m test and 8000 cycle impacts was taken aswear resistance. The tests were repeated three times for onetype of test sample. The worn surface morphologies wereexamined using a S-2700 scanning electron microscope (SEM).The cross-sectional profile of the wear was also measured by aTR-profilometer to be used for estimating the groove depth. Theobservation of shot peening treated surface layer morphologieswas performed in a JSM-6700F field emission scanning micro-scope (FEM).

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W.L. Yan et al. / Tribology International 42 (2009) 634–641636

3. Results and discussions

3.1. Microstructure of surface layer

Fig. 2 shows a cross-sectional optical microscopic microstruc-ture of Hadfield steel after 60 min shot peening. Severe plasticdeformation evidences can be easily observed at about 100mmdeep from the top surface. Obviously, the plastic deformation wasinhomogeneous. With increasing depth, irregular multiple slips

Fig. 5. A bright field cross-sectional tem images in the 60 min shot peening treated

surface.

Fig. 6. A plane-view HRTEM image of the nanostructure in the 60 min shot

peening treated surface.

near the surface were gradually replaced by single slips. The bendsof slip lines was also observed near the surface for different grains.

Fig. 3 shows the XRD patterns of the samples before and aftershot peening treated. The Bragg-diffractional peak broadening inthe shot peening treated sample may be attributed to grainrefinement and/or an increase in the atomic-level lattice strain. Itcan be found that all the samples consist of austenite without anymartensite transformation occurring in the surface layer duringshot peening treatment. Quantitative XRD measurements indicate

Fig. 7. A bright field cross-sectional tem image in the 2 min shot peening treated

surface.

Fig. 8. A bright field cross-sectional tem image in the 30 min shot peening treated

surface.

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Fig. 11. Wear resistance relative to shot peening treated duration in glass paper.

W.L. Yan et al. / Tribology International 42 (2009) 634–641 637

that the grain size in surface layer of 60 min treated sample hasbeen decreased to about 11.1–17.4 nm.

Fig. 4 shows a TEM observation at about 20mm deep from60 min shot peening treated surface. It is shown that grain sizeshave already been decreased to nano-magnitude. The grain sizewas about 30–80 nm. The diffusive diffraction spots in the SAEDpattern are identified as the austenite without martensite.

Fig. 5 shows the TEM microstructure of the 60 min shotpeening treated surface. Nanostructured grains with randomcrystallographic orientations are easily recognized, as indicatedby the SAED pattern. The grain size was about 3–10 nm, which issmaller than the XRD result (11.1–17.4 nm). That might bepartially attributed to the fact that the XRD collects all data ofthe microstructure in about 5mm thickness. A close observation ofthe nanocrystalline in surface was performed by using HRTEM, asdepicted in Fig. 6.

Surface nanocrystallization by shot peening is similar to thosecaused by those severe plastic deformation [12–14]. The billiardsprovided repeated impacts on a surface at a high speed andgenerated high-density dislocations and deformation twins. Inone hand, the resultant dislocation could be re-arranged understress to form dislocation networks, leading to the formation of

Fig. 9. Hardness variation of the surface layer with the shot peening treated

duration.

Fig. 10. Hardness variation along depth from the surface of the sample after the

60 min shot peening treated.

Fig. 12. Wear resistance relative to shot peening treated duration in emery paper.

Fig. 13. Wear resistance relative to shot peening period in 0.5 J impact load.

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nanocrystallites separated by diffuse grain boundaries. In anotherhand, deformation twins subdivide the original grains efficientlythrough introducing different boundaries which lead to formrefined microstructures. In the surface layer of the Hadfield steel,nanocrystalline structure with random orientations is formed. Theformation of the nanocrystallites can be attributed to the distincteffects: a very large strain, an extremely large strain rate andrepetitive loading by shot peening. The remarkably increasedstrain and strain rate activate a high density of dislocation andmultisystem deformation twins to accommodate the straining.

The surface layer can be subdivided into sections along thedepth from the 60 min shot peening treated surface based on OMand TEM observations, as well as XRD analysis: a nanostructuredregion (0–30mm deep) and a submicro-sized and micro-sizedregion with evidently plastic deformation (about 30–100mmdeep).

It becomes interesting that nanocrystalline can also be foundeven though very short peening duration was done, such as,2 and 30 min peening duration. Figs. 7 and 8 show the tem

Fig. 14. Wear resistance relative to shot peening period in 0.5 J impact load.

Fig. 15. Worn surface morphologies of before and after shot peening using glass

microstructure of surface after 2 and 30 min shot peening,respectively. Nanocrystalline with random crystallographic orien-tations can be easily recognized. The average grain size in thesurface layer was about 40 and 30 nm for 2 and 30 min shotpeening, respectively. In engineering applications, short treatmentduration is with important meaning. The suitable durationpeening treatment can save time and energy in one hand, inanother hand the short time treatment can also prevent materialsubsurface from a lot of cracks initiation. It will be seen in thefollowing discussion that so-called ‘‘excessive’’ peening treatmentwill not bring beneficial influence on improving abrasive wearresistance.

3.2. Abrasive wear behavior

The hardness of Hadfield steel was measured on a cross-sectional surface before and after shot peening, as presented inFig. 9. With increasing shooting duration, the hardness of thetreated sample was increased. The surface hardness for originalsample and for shooting duration 2, 30, 60 and 120 min were 256,400, 503, 676 and 774 HV, respectively. The hardness along depthfor 60 min shot peening is shown in Fig. 10. Surface hardness withnanocrystalline was increased to 676 HV, then, decreased gradu-ally along the depth (about 256 HV in the matrix). The increasedhardness may be attributed to grain refinement in the surfacelayer following the Hall-Petch relationship [1,15].

In the two-body abrasive wear using glass paper, the shootpeening duration 2 and 30 min treated samples helps to improvethe wear resistance of samples as shown in Fig. 11. Especiallythe wear resistance was increased about 70 percent for 2 minpeening duration. To our surprise, longer peening duration is notbenefit to increase the wear resistance as we considered. Ifabrasive paper is changed to emery particles paper the shotpeening is not contributed to improvement of wear resistance, asshown in Fig. 12.

Impact abrasive wear tests were carried out to evaluate theinfluence of impact load on the wear of treated Hadfield steel. Itcan be shown in Fig. 13 that the wear resistance under 0.5 J impactload for shot peening samples is better than that of original

paper wear: (a) the original sample, (b) 2 min and (c) 60 min shot peening.

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Table 1Profile data of worn surface after glass paper wear test

Shot peening duration (min) Ra (mm)

0 1.161

2 0.787

30 0.974

60 1.293

120 1.815

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sample in 2 and 30 min shot peening duration. Longer peeningduration will have not helps to increase the wear resistance ofHadfield steel. For 3.0 J impact load as shown in Fig. 14, however,any shot peening duration will have not good influence on thewear resistance.

The SEM morphologies of the wear scar for glass particlespaper are shown in Fig. 15. It is indicated that the groove wearpattern was dominant. In order to estimate groove depth, aprofilometer was used to measure the roughness of worn surface.It is shown in Fig. 16 and Table 1, that the worn surface of originalsample without shot peening treatment is with lager roughnessRa, compared to 2 and 30 min shot peening samples. The data canbe used to roughly estimate the abrasive particle embedded

Fig. 16. Cross-sectional profile of worn surface made on before and after shot

peening treated samples under glass paper wear.

depth. Ra was measured arranged in 0.787–1.815mm for all typesof wear samples. In addition, the nanocrystallized zone depth canbe estimated by TEM, (using) CuKa wavelength and Fe extinctiondepth as a monitor. The depth of nanocrystallized zone depth wasestimated about 5–30mm. It can be, therefore, concluded that theparticle embedded depth was smaller than that of the nanocrys-tallized zone if shot peening was carried out.

Firstly, microstructure refinement is expected to increasehardness, which can be used to roughly explain the tendency ofabrasive wear resistance according to Rabinowicz’s abrasive wearequation [16], i.e.

W ¼ KP

H(1)

where W is wear volume per unit sliding distance; P is appliedload; H is hardness of the worn surface and K is wear coefficient. Ithas been reported that nanocrystalline nickel coating with a grainsize of 13 nm made by electrodeposition has a much-enhancedabrasive wear resistance compared with the conventional coarse-grained polycrystalline nickel [17,18]. It is also noticed from Fig. 10that wear resistance for peening duration 2 min samples wasimproved 70%. The increased wear resistance can be attributed tothe increased hardness by shot peening. It has been measured thatthe Vickers hardness of surface for peening duration 2 and 30 minwas 400 and 503 HV, separately. They are near to or equal to theglass particle hardness (�500 HV). During wear process theincreased hardness keeps to effectively prevent the particles frompenetrating to non-nanocrystallized zone. Secondly, during wear,the surface and subsurface of the sample are subjected toalternate stress and compression stress due to action of abrasiveparticles [19]. Shot peening treatment can enhances microhard-ness and yield strength which decreases plastic deformation andwear [20]. Therefore, the high hardness subsurface and nanocrys-talized microstructure tend to increase the wear resistance of thematerial.

Of course, it can easily argue that Ra data are also smaller for60 and 120 min shot peening samples than that of original samplewithout any shot peening treatment. Why are not the wearamounts for 60 and 120 min peening samples decreased as thecase of 2 or 30 min treatment samples? To investigate thisproblem, Vickers hardness of samples was measured in a cross-sectional surface after wear. The surface hardness for original,2 and 120 min shot peening sample were, 517.3, 540 and 487 HV,respectively. It was found that the worn surface hardness foruntreated sample and for 2 min treated sample is increased afterwear, compared to surface before wear tests. The increment ofhardness for 2 min treated sample is smaller than untreatedsample, but the hardness value is higher than that of untreatedsample, which infers work hardening degree is not enough foruntreated sample, resulting severe wear of material. 120 mintreated sample is quite the contrary. Its surface hardness afterwear is decreased, instead of increase. That phenomenon is oftenthe case that spalling occurs. The hardened nanocrystalline layeris dug off at the initial stage of wear test, resulting in wearincreasing. The worn surface morphology of 60 min shot peeningsample was examined by SEM (see Fig. 15) after wear tests.

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Fig. 17. The cross-sectional SEM observation of the typical cracks in 60 min shot

peening treated sample.

Fig. 18. The cross-sectional FEM observation of the typical cracks in 60 min shot

peening treated sample.

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Subsurface spalling cannot be observed in that case. It is,therefore, judged that spalling may occur at the initial stage ofwear test. For this reason shot peening surface after 60 minduration was examined by SEM and FEM. It is found from Figs. 17and 18 that a lot of microcracks have been produced in subsurfacebecause of ‘‘excessive’’ shot peening. Therefore, wear property willdeteriorate again after ‘‘excessive’’ shot peening. Data in Table 1also shows that roughness Ra increases again for 60 and 120 minshot peening samples, compared to samples of 2 min.

For the emery paper two-body wear test, the prestrainhardening cannot improve wear resistance evidently. Althoughthe surface hardness was increased rapidly with prolong peeningduration, the hardness of emery (1800 HV) is still higher than thatof the peening treated surface. The surface layer severe plasticdeformation occurred due to shot impacting, and, hence, theplastic strength accordingly declined. Generally, the plasticstrength of metals also affects wear resistance properties

For the impact wear, light load can improve the wearresistance, compared to heavy impact load as shown in Figs. 13and 14, separately. That can be explained as the prestrainincreases the initial energy of material. If absorbed energyduring impact wear excesses the energy of crack initiation and

propagation the prestrain will bring a harmful effect on increasingwear resistance in heavy impact wear conditions. On the contrary,if impact load is light which is not enough to cause complete thework hardening of steel, a certain prestrain treatment will bebenefit to improve the wear resistance. It is the case for 0.5 Jimpact wear tests.

In conclusion, a nanocrystalline surface layer of Hadfield steelby shot peening can definitely improve the wear resistance underthe conditions of soft abrasive two-body abrasion and light loadimpact wear. It becomes also necessary to reevaluate the issuethat the effect of cold work hardening on wear resistance ofmaterial is trivial. For commercial applications shot peeningtreatment is an appropriate process to strengthen metal materialsto improve wear resistance instead of using alloying or otherprocesses with changing the bulk microstructures of materials.

4. Conclusions

A nanocrystalline surface layer is successfully fabricated inHadfield steel by a controlled shot peening treatment. The grainsize for the 60 min treated surface is arranged in 11.1–17.4 nm. Thesurface hardness increases gradually from about 256 HV fororiginal sample to about 774 HV after 120 min peening treatment.For harder abrasive particles like emery paper, the increasedhardness and grain refinement by shot peening cannot improvetwo-body abrasive wear resistance. For soft particles like glasspaper, surface work hardening can bring about the increase ofwear resistance by 70% for 2 min peening duration. In thesituations of impact wear, the light impact load will also improvethe wear resistance to some extent. The heavy impact load cannotbe benefit to increasing wear resistance, vise versa. It has alsobeen noticed that excessive shot peening treatment cannot furtherincrease wear resistance because of the induced microcracks insubsurface.

Acknowledgments

The present authors appreciate the financial support fromthe National High Technology Research and Development Pro-gram of China (863 Program), the Ministry of Science andTechnology of China (Grant 2002AA302509) and the ScienceDevelopment Initial Funding of Guangxi University (GrantX071044).

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