Bin Xuand Yucheng Cai

（1．School of Materials Science and Engineering，Shandong University，Jinan 250061，China；2．School of Materials Science and Engineering，Shandong Jianzhu University，Jinan 250101，China）

Surface Nanocrystallization of Steel 20 Induced by Fast Multiple Rotation Rolling

Xingdong Yuan1，2，Bin Xu2?and Yucheng Cai2

（1．School of Materials Science and Engineering，Shandong University，Jinan 250061，China；2．School of Materials Science and Engineering，Shandong Jianzhu University，Jinan 250101，China）

In order to expand the application of steel 20 in precision device，fast multiple rotation rolling（FMRR）is applied to fabricate a nanostructured layer on the surface of steel 20.The FMRR samples are then Cr-Rare earth-boronized under low-temperature.The microstructure of the top surface layer is characterized by transmission electron microscopy（TEM）.Microhardness of the top surface is measured by a Vickers microhardness tester.The boride layer is characterized by using scanning electron microscopy（SEM）. Experimental results show that a nanostructured layer with their grain size range from 200 to 400 nm is obtained in the top surface layer.The microhardness of FMRR sample changes gradiently along the depth from about 274HV in the top surface layer to about 159HV in the matrix，which is nearly 1.7 times harder than that of the original sample.The penetrating rate is enhanced significantly when the FMRR samples are Cr-Rare earthboronized at 600℃for 6 h.Thickness of the boride layer increases to around 20 μm，which is nearly twice thicker than that of the original sample.

fast multiple rotation rolling；steel 20；surface nanocrystallization；dislocation；Cr-Rare earthboronizing

1 Introduction

Over the past decades，many researchers have paid more attention tonanocrystalline materials since they possess properties and behaviors that are different from，and often superior to，those of conventional coarse-grained materials.Various synthesis techniques have been developed to fabricate nanocrystalline microstructures on many materials.A nanometergrained surface layer without porosity and contamination was fabricated by means of surface mechanical attrition treatment（SMAT）in a pure iron plate［1］. Experimental results showed that the average grain size in the top surface layer is about 10-25 nm，and the diffusivity of Cr in the nanocrystalline Fe is significantly improved.Ultrasonic peening（UP）was used to fabricate a nanocrystalline surface layer on an AISI-321 stainless steel［2］.The results showed that a nanocrystalline surface layer with the grain size of about 10 nm at the top surface is obtained.A gradient nanomicro-structure in the surface layer of bulk pure copper was synthesized by using surface mechanical grinding treatment（SMGT）at cryogenic temperatures.The results showed that the average grain size in the topmost surface is about 22 nm［3］.The synthesis of a nanostructured surface layer on a low carbon steel by using a high-energy shot peening（HESP）was reported by Liu et al.The results indicated that the thickness of the nanostructured surface layer is more than 20 mm，and the average grain size in the top surface layer is as small as a few nanometers［4］.A nanocrystalline layer with grain size of about 30-40 nm is fabricated on the surface of 7050 aluminum alloy by a new technique called high pressure shot peening（HPSP）which is the combination of common shot peening equipment with a pressurizing vessel［5］.Plates of Inconel 718 in precipitated state have been subjected to ultrasonic shot peening（USP）with different parameters［6］.The results showed that surface nanocrystallization may not be induced at all if the impacting energy of the balls is not large enough.

Properties of nanostructure layerhave been investigated by many researchers.An et al.［7］fabricated a refined Fe／Ni alloy surface layer on a pure iron plate.It was obtained that the properties of the alloy layer are remarkably improved，which is attributed to the alloy layer with smaller grains.Wang et al.［8］investigated the effect of shot peening on corrosion resistance of the steel.The results showedthat corrosion resistance of 1Cr18Ni9Ti stainless steel in the chlorine-ion-contained solution is markedly enhanced.Li et al.［9］found that the SMAT process can improve the fatigue strength for the carbon steel by as much as 13.1%.Properties of the chromized SMAT sample were measured，in comparison with those of the chromized coarse-grained samples［10］.Experimental results showed that microhardness，wear and corrosion resistances of the chromized SMAT sample are improved markedly.The role of surface mechanical attrition treatment（SMAT）on pack boronizing of AISI H11 type tool steel was discussed［11］.The results showed that the diffusion of boron during boronizing is promoted and the case depth and hardness of the borided layer increase and it was suggested that SMAT can be used as a pretreatment for boronizing of H11 tool steel.A nanostructured surface layer of about 20 μm thick was obtained by Wang et al.by means of SMAT［12］.Experimental results indicated that a much thicker Cr-diffusion layer is obtained in the SMAT sample than in the coarse-grained one and the formation temperature of chromium compounds is found to be much lower.The refinement mechanism is also studied and the grain subdivision into the subgrains is found to be the main mechanism responsible for grain refinement［13-15］.

There are some problems in these processes proposed above，however，which have hindered the widespread application of surface nanocrystallization technologies.Such as limited nanostructured layer thicknesses，low processing efficiencies and structural inhomogeneity in the surface layer［3］.Fast multiple rotation rolling（FMRR），a novel and efficient surface nanocrystallization technique，was developed by Chui et al.［16］with the aim to achieve a nanostructured surface layer in metal surface.In this paper，F(xiàn)MRR technique was applied to fabricate a nanostructured layer on the surface of steel 20.The FMRR samples were then Cr-Rare earth-boronized under lowtemperature.

2 Experimental

2.1 Experimental Materials

The main material for the experimentwas steel 20 with the dimension of 10 mm×10 mm×10 mm.The Cr-Rare earth-boronizing process was performed under 600℃×6 h.

2.2 Experimental Methods

The surface nanocrystallization for steel 20 was performed by using the FMRR equipment（as shown in Fig.1）.The processing parameters were as follows：p＝4.2 MPa（pressure），ω＝1 600 r／min（rotational speed）.The duration of the sample treated was 60 min. Transmission electron microscopy（TEM）was used to analyze nanostructures and local high-density dislocations induced by the surface nanocrystallization. Microhardness testing was performed on the surface of samples with a Vickers microhardness tester using a load of 15 g for 5 s.Three indentations were made on each condition to obtain an average value of the microhardness.The FMRR samples were then Cr-Rare earth-boronized at 600℃for 6 h in a penetrating tank which was cylindrical with the inside diameter of 100 mm and the height of 100 mm.Scanning electron microscopy（JSM-6380LA）was used to characterize the morphology of the boride layer.

Fig.1 Schematic diagram of FMRR device

3 Results and Discussion

3.1 Microstructure Characterization of Surface Layer

Fig.2 shows a TEM bright field image（Fig.2（a）），a darkfield image and the corresponding selected area electron diffraction（SAED）pattern（Fig.2（b））in the top surface layer of FMRR specimen.It can be seen that the grains in the top surface layer are equiaxed nano-grains with their grain size range from 200 to 400 nm.It is believed that dislocation activities are motivated in the original coarse grain during plastic deformation.With increasing strains，more and more dislocations formed and accumulated，and dislocation rearrangement and annihilation occurred，result in transforming into subboundaries separating individual cell.The grain size can be refined to submicrometer scale after the repeating of the same process.

Fig.2（b）shows that the SAED pattern is composed of continuous diffraction rings，confirming that the grains of the FMRR treated specimen have been broken down to nano-grains with random in crystallographic orientations.

TEM image of the top surface layer of FMRR sampleis shown in Fig.3.High-density dislocations are observed in the top surface layer of FMRR specimen.It is believed that a high stacking fault energy is obtained by sever plastic deformation in the top surface of FMRR sample，which results in the cross-slip of dislocations and the increase in density of dislocations.

3.2 Microhardness Measurement

The variation ofmicrohardness of FMRR samplewith the distance from surface is indicated in Fig.4.It can be seen that the microhardness of FMRR sample changes gradiently along the depth from about 274HV in the top surface layer to about 159HV in the matrix，which is nearly 1.7 times harder than that of the original sample.This may be related to rapid increased grains size and rapid decreased strain.

Fig.2 TEM images of the top surface layer of FMRR sample

Fig.3 TEM image of the top surface layer of FMRR sample

3.3 Morphology of the Boride Layer

SEM images of the boride layer on steel 20 under 600℃for 6 h before and after FMRR treatment are shown in Figs.5 and 6.It can be seen from Fig.5 that the layer is thin（approximately 10 μm）and loose，and not attached well onto the substrate，which is due to the weak activity of boron atoms caused by low reaction temperature.Fig.6 shows the SEM image of the boride layer of FMRR samples on steel 20.It can be seen that the morphology of the layer is saw-toothed and the thickness of the boride layer becomes thicker to approximately 20 μm.The boride layer is continuous，dense and uniform，and adheres well to the metallic substrate.

Fig.4 Variation of microhardness of FMRR sample with the distance from surface

Fig.5 SEM image of the boride layer on steel 20 under 600℃for 6 h before FMRR treatment

The variation of thickness of boride layer with treatment duration is shown in Table 1.It can be seen that thickness of the boride layer increases to around 20 μm，which is nearly twice thicker than that of the original sample.The penetrating rate is enhanced significantly when the FMRR samples are Cr-Rare earth-boronized at 600℃for 6 h.The grains in the top surface of steel 20 become smaller and the grainboundaries increase after FMRR treatment，which provide more channels for the dispersion of boron atoms.Meanwhile，the high-density dislocations left on the local surface of steel 20 lower the activation energy of dispersion for boron atoms，which is beneficial to the enhancement of penetrating speed.

Fig.6 SEM images of the boride layer of FMRR samples on steel 20 under 600℃for 6 h

Table 1 Variation of thickness of boride layer with treatment duration

4 Conclusions

1）The grains in the top surface layer of FMRR samples are equiaxed nano-grains with their grain size range from 200 to 400 nm and are random in crystallographic orientations.

2）Microhardness of FMRR sample changes gradiently along the depth from about 274HV in the top surface layer to about 159HV in the matrix，which is nearly 1.7 times harder than that of the original sample.

3）Thickness of the boride layer after FMRR treatment increases to around 20 μm，which is nearly twice thicker than that of the original sample.The penetrating rate is enhanced significantly.

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TG156.87

：

：1005-9113（2015）05-0038-04

10.11916／j.issn.1005-9113.2015.05.006

2014-10-15.

?Corresponding author.E-mail：xubin＠sdjzu.edu.cn；yxdhit＠sdjzu.edu.cn.