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Vacuum 101 (2014) 177e183

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Vacuum

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A comparative study on gray and nodular cast irons surface meltedby plasma beam

Xiu Cheng, Shubing Hu*, Wulin Song, Xuesong XiongState Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

a r t i c l e i n f o

Article history:Received 18 April 2013Received in revised form15 August 2013Accepted 17 August 2013

Keywords:Cast ironsPlasma beamSurface meltingDepth of molten surfaceSurface roughness

* Corresponding author. Tel.: þ86 13995667466; faE-mail address: hushubing@163.com (S. Hu).

0042-207X/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.vacuum.2013.08.012

a b s t r a c t

In this study, gray cast iron (GCI) and nodular cast iron (NCI) were surface melted by plasma beam toremove the negative effect of the near-surface graphite phases. Afterward, the modified surfaces wereanalyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated thatthe molten surface had no pores and cracks, composing of inter-dendrites and eutectics with a hyper-eutectic structure. The cross section micro-hardness as well as the depth of the molten surface wasmeasured. Cementite, martensite, and retained austenite co-existed in the molten surface, improving themicro-hardness from about 315 HV0.1 of the substrate specimens to an average of 1004 HV0.1 and933 HV0.1 of the molten surface on GCI and NCI specimens treated at the main arc current 55 � 0.5 A,respectively. Generally, the depth of molten surface on the treated NCI specimen was deeper than that onthe treated GCI specimen under the same plasma parameters. Besides, the molten surface on the treatedNCI specimen had lower mean surface roughness (Ra) under the same plasma parameters owing to thedifferent graphite shape in cast irons. At the main arc current 55 � 0.5 A, the mean surface roughness ofthe treated GCI was 153 � 5 mm, while that of the treated NCI was 83 � 4 mm.

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1. Introduction

Cast irons have excellent castability, wear resistance, goodmachinability and are relatively low cost when compared withalloy steels with similar mechanical properties [1e5].

Cast irons can be divided into gray cast iron, vermicular cast ironand nodular cast iron by their different graphite shapes. Gray castiron (GCI) and nodular cast iron (NCI) are widely used in machineconstructions, drawing dies and so on.

Tong et al. [6] studied the effect of graphite shape on thermalfatigue resistance of cast ironwith biomimetic non-smooth surface.They observed that thermal fatigue resistance of samples with thesame kind of surface (smooth or non-smooth) all of graphite shapewere sorted as nodular cast iron > vermicular cast iron > gray castiron. The effect of graphite morphologies on the tribologicalbehavior of austempered cast iron was discussed by Ghaderi et al.[7]. They concluded that austempered nodular cast iron showed thehighest impact energy followed by compact and gray cast iron andaustempered compact cast iron showed the highest wear resistanceprobably due to its graphite morphology which controlled crackpropagation and thermal conductivity. Their results have also

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shown that the graphite shape played an important role in theproperties of cast irons.

The microstructure of cast irons consisted of graphite phasesbuilt in the iron matrix. The continuity of the iron matrix wasinterrupted by the graphite phases. The near-surface graphitephases acted as a source of crack nucleation under impact condi-tions and could cause crack growth [1]. Furthermore, the graphiteexposed to the atmosphere might cause graphitic corrosion [2].Therefore the near-surface graphite phases of the cast ironsmust beeliminated [1].

Recently, high energy beams such as electron beam [3,8,9], laserbeam [10e13] and plasma beam [14e17] have been used exten-sively to modify the surface of cast irons by the melting hardeningprocess. In this technique, the cast iron surface was partially meltedand rapidly solidified. The rapid cooling during solidification led tothe formation of large amount of eutectic cementite instead of thesoft graphite.

Plasma beam is actually an extremely high-temperature flowusually possessing a power density of an approx 109 W/m2, whichallows rapid heating of almost every kind of solid material to itsmelting or evaporating point. It is a kind of rapid, non-equilibriummetallurgical process. The heating efficiency of plasma beam(about 85%) could be much higher than a laser beam (about 30%)heating of material surface [16].

Fig. 1. The starting metals (a) GCI; (b) NCI, F: ferrite and P: pearlite.

X. Cheng et al. / Vacuum 101 (2014) 177e183178

Plasma beam provides some significant advantages for iron-carbon alloys which included selective hardening, minimum partdistortion, controllable case depth and a refinement of the struc-ture [18,19]. As a high energyeintensity heating source, plasmabeam is widely used for surface hardening to improve the corro-sion and wear resistance of the surface modified ferrous workpieces [20].

Benyounis et al. [11] studied surface melting of nodular cast ironbyNdeYAG laser and TIG, while Liu and Previtali [21] paid attentionto laser surface treatment of gray cast iron by high power diodelaser. Both papers have only employed one kind of cast iron used asa substrate for surface melting. The difference about surfacemelting between nodular cast iron and gray cast iron had been notinvolved.

The aim of this work was to use a plasma beam for surfacemelting of GCI and NCI specimens. The rapid cooling of the moltensurface enabled the formation of the white cast iron, which wasvery hard and wear resistant, on the surface of nodular and graycast irons. Furthermore, the effects of graphite shape on the hard-ening behavior of GCI and NCI have been investigated.

2. The materials and experimental

2.1. The starting materials

The starting materials of GCI (HT300) and NCI (QT600) wereproduced by Dongfeng company. Experimental specimens weremachined into plates in the dimension of 120mm� 80mm� 20mmfor plasma surface melting treatment. GCI specimen consisted ofgraphite flake and pearlite as shown in Fig. 1(a). The dimension inlengthof graphiteflakewasover 1000 mm. Fig.1(b)was amicrographof the NCI specimen which consisted of graphite nodules withaverage nodule diameter of 30 mm surrounded by ferrite and somepearlite. Their chemical compositions were given in Table 1.

2.2. Experimental

2.2.1. Plasma surface melting processPlasma surface melting was carried out by means of a hand-

made set-up for combined plasma beam of transferred and non-

Table 1Chemical compositions of starting materials (wt.%).

C Si Mn P S Re Mg Fe

GCI 3.3 1.7 1.0 0.18 0.10 e e BalanceNCI 3.4 2.5 0.5 0.08 0.02 0.02 0.03 Balance

transferred DC arc as shown in Fig. 2. Atmospheric dense plasmawas generated by the equipment and the ionization degree was lessthan 0.1%. The plasma pressure was larger than an atmosphericpressure (0.1 MPa).

The specimen and nozzle served as the anode, and the tungstenneedle the cathode (Fig. 2). The nozzle diameter selected for thetest was 2 mm, and the distance between the nozzle and thespecimen 4 mm. The plasma torch was controlled by a small vari-able speed DC motor; thus the motion speed of the torch could beadjusted and kept constant throughout the test. Argon used in theprocess served the functions of both plasma gas and shielding gas.

Before testing, the experimental specimens were ground up tothe mean surface roughness Ra ¼ 8 mm and induction heated up toabout 250 �C before surface melting, in order to reduce thermalshock, thus reducing the tendency to crack.

The main process parameters used in the tests are listed inTable 2. After surface melting, the specimens were left to cool toroom temperature in still air. Specimens containing molten sur-face and base material were machined into the block with thedimensions of 10 mm in length, 10 mm in width for furtheranalysis.

Fig. 2. Schematic diagram of the plasma surface melting equipment.

Table 2Process parameters for plasma surface melting.

The nozzle diameter 2 mmThe distance between the

nozzle and specimen4 mm

Argon of flow rate forplasma gas

1 L/min

Argon of flow rate forshielding gas

1 L/min

Plasma pressure >0.1 MPaPlasma torch moving

speed640 mm/min

Overlapping rate 50%Arc voltage 30 w 48 VMain arc current 40 � 0.5 A, 45 � 0.5 A, 50 � 0.5 A, 55 � 0.5 A,

60 � 0.5 A, 65 � 0.5 A, 70 � 0.5 A

Fig. 4. Relation between the main arc current and the depth of the molten surface.

X. Cheng et al. / Vacuum 101 (2014) 177e183 179

2.2.2. The characterization of the microstructureThe microstructure of the modified surface and the starting

materials was analyzed by Quanta 200 scanning electron micro-scopy (SEM). The phase structures of the modified surfaces wereanalyzed by X0 Pert PRO with Cu-Ka radiation (l ¼ 1.5406 �A)generated at 40 kV and 40 mA, and a scanning speed of 0.25�/min.

The cross section micro-hardness distribution of the treatedspecimens was conducted with a microvickers indentator (DHV-1000) under the load of 100 gf.

2.2.3. The measurement of depth and surface roughnessThe cross section of the specimens treated by plasma beam from

surface to the inner could be separated into three layers: themoltensurface, the heat-affected zone and the un-molten zone (Fig. 3). Thedepth measurements of the molten surface were made by ImagePro Plus software on polished and etched transverse section at themid-width of each specimen treated by plasma beam as shown inFig. 3. The surface roughness (in terms of arithmetical mean devi-ation, Ra) of the specimen was measured by a laser scanningconfocal microscope (Keyence VK-X200, 0.0005e1000 mm) from asampling area of around 10 � 10 mm2. The scanning distance wasabout 10 mm with the longitudinal and horizontal directions withrespect of the traveling plasma torch.

3. Results and discussion

The depth of modified surface wasmeasured according to Fig. 3.Fig. 4 presented the relationship between the arc current and thedepth of modified surface. It could be concluded that the increasein the current led to an increase in the depth of the molten surface.At the constant voltage, the increase of current led to an increase inthe heat input and consequently increased the depth of themodified surface by TIG (tungsten inert gas shielded welding) [11].And the depth of molten pool increased as the increase of energyintensity of the jet flow because the laminar jet-flow temperatureincreased with the arc current [16]. Although the depth of hard-ened surface increased with increasing arc current, the surface

Fig. 3. Schematic diagram of cross section view of the specimen surface melted byplasma beam.

roughness and defects in the surface layer increased also with theincreasing current [16]. Consequently, the arc current should bemoderate. Besides, it was obvious in the figure that the depth ofthe molten surface on the treated NCI specimen was deeper thanthat on the treated GCI one under the same parameters.

Plasma surface melting technique was a treatment techniqueunder nonhomogeneous and non-equilibrium condition resultingin multi-phase formation. XRD analysis of the modified surfacefabricated by plasma beam at current 55 � 0.5 A revealed the ex-istence of phases: a-Fe, g-Fe (austenite), Martensite, Fe3C as seen inFig. 5(a) and (b). The formation of Fe3C in the resolidified zone wasattributed to the dissolution of graphite during melting thatspreads throughout the iron matrix. The quick cooling rate of themolten pool resulted in a super-saturated austenite with carbonatoms.

The GCI and NCI specimens treated by plasmamelting process at55 � 0.5 A were cross-sectioned. The cross section views wereshown in Fig. 6. It revealed that different microstructures wereemerged in the molten surface layer and interface region comparedwith the as-received starting materials (Fig. 1). Also visible in thesefigures was the irregular shape of the interface, which was seen inboth the plasma surface melted specimens. This could be inter-preted as the modified surface encroached into the inner of spec-imens. It was the material in the vicinity of the graphite phases,whichmelted initially; this occurred, because of carbon enrichment

Fig. 5. XRD patterns of the molten surface at current 55 � 0.5 A on (a) the treated GCIspecimen; (b) the treated NCI specimen.

Fig. 6. Cross section view of surface melted specimens at current 55 � 0.5 A (a) the treated GCI specimen; (b) the treated NCI specimen.

Fig. 7. Microstructure of molten surface at current 55 � 0.5 A on (a) the treated GCI specimen; (b) the treated NCI specimen.

X. Cheng et al. / Vacuum 101 (2014) 177e183180

near the graphite phases locally lowered the melting point of theferrous area.

Fig. 6 also showed that the modified surface obtained by plasmabeam on NCI was far deeper than that on GCI. The modified surfaceon GCI specimen (Fig. 6(a)) was about 323 mm deep, which was120 mm less than the modified surface on NCI specimen (Fig. 6(b),the depth of the modified surface was 443 mm). This was consistentwith Fig. 4.

Examination of the molten surface at high magnificent using ahigh resolution scanning electron microscopy showed a dendriticstructure throughout the modified surface (Fig. 7(a) and (b)). It wasevident from Fig. 7(a) and (b) that the graphite phases had

Fig. 8. Micro-hardness profile of surface melted specimen at 55 � 0.

disappeared in the molten bath and rapid self-quenching hadsuppressed the reformation of graphite in favor of the formation ofwhite cast iron. Elimination of graphite nodules was also observedby Karamıs and Yıldızlı [1] and Gulzar et al. [3]. The molten surfacesshowed very fine structure consisting of dendrites and eutectics.These dendrites were characterized by long primary and secondaryarms, which indicated a considerable amount of a high cooling rateduring solidification. The larger dendrite and more uniformmicrostructure appeared in modified surface on NCI specimen asshown in Fig. 7(b).

The changes in micro-hardness from the outermost surface tothe inner were measured as shown in Fig. 8(a) and (b). Both of the

5 A (a) the treated GCI specimen; (b) the treated NCI specimen.

Fig. 9. Microstructure of HAZ in surface melted specimen at current 55 � 0.5 A (a) the treated GCI specimen; (b) the treated NCI specimen.

Fig. 10. Crystallographic lattice of graphite in cast irons [22,31].

X. Cheng et al. / Vacuum 101 (2014) 177e183 181

molten surfaces have a much higher micro-hardness values thanthose of the substrate specimen did, which may be mainly attrib-uted to the formation of primary Fe3C and martensite hard phases(Fig. 5) in the modified surface as well as the formation of finedendrites and the ledeburite structure (Fig. 7).

The average micro-hardness of the molten surface on NCI wasfound to be about 933 HV0.1, which was 3 times higher than that ofthe substrate as shown in Fig. 8(b), while the hardness of modifiedsurface on GCI was higher, about 1004 HV0.1.

The depth of the modified surface in GCI specimen was about350m (Fig. 8(a)), which was 100 mm less than that in NCI one(Fig. 8(b)). The wide of the heat-affected zone in GCI was also lessthan that of in NCI comparing Fig. 8(a) with (b).

Fig. 11. eutectic reaction happened along length direction of the flake in the HAZ (a) Graphperpendicular to the heat flow direction.

In heat-affected zone, partial dissolution of the graphiteoccurred and the trace of dissolved graphitewas visible as shown inFig. 9(a) and (b). It could be concluded from Fig. 9(a) and (b) that thefarther the distance from the treated surface, the more the remaingraphite. The results indicated that heat input would decrease withthe increasing of distance from the treated surface. More micro-defects appeared in the heat-affected zone of the treated GCI dueto the long graphite flake comparing Fig. 9(a) with (b).

In light of the above results, it could be concluded that the depthof the treated NCI specimens was deeper than that on treated GCIspecimens under the same plasma parameters. A clear distinctionin thermal conductivity between GCI and NCI led to the differencein terms of graphite morphology which controls thermal conduc-tivity of the cast irons [7].

With the same content of carbon and the temperature, thethermal conductivity/diffusivity was clearly affected by themorphology of graphite [22]. The graphite was present as flakes inGCI, while the graphite shape in NCI is spheroidal.

The graphite built in the cast irons had a hexagonal lattice asshown in Fig. 10. A strongly anisotropic thermal conductivity ofgraphite had been reported [22e25]. The thermal conductivityalong the C axis is 84W/(m K) and that in the basal planes is 419W/(m K) which is four times larger than the former [22,25e30]. The Caxis is along the radial direction of the graphite nodule and the baseplane and C axis are along the length and thickness direction of theflake graphite, respectively [22].

In graphite flakes, heat propagation in the basal plane is domi-nant, while heat propagation along the C axis is dominant inspheroidal graphite. A longitudinal alignment of the basal planes inthe graphite flakes created a more efficient heat propagation thanthe tangential alignment of the basal planes (C axis) in spheroidal

ite flake position parallel to the heat flow direction; (b) and (c) Graphite flake position

Fig. 12. The eutectic reaction happened around the nodule in the HAZ of the treated NCI specimen.

X. Cheng et al. / Vacuum 101 (2014) 177e183182

graphite. This was also proved in Figs. 11 and 12. In the case ofFig. 11, eutectic reaction happened along length direction of theflake. The eutectic cell was located in both sides of flake when thegraphite flake position was parallel to the heat transfer direction asshown in Fig. 11(a), while the eutectic reaction happened above theflake (Fig. 11(b) and (c)) when it was perpendicular to the heattransfer direction. In this case, the graphite flake suppressed theeutectic reaction due to that heat propagation in the basal planewas dominant in graphite flake. In the case of Fig. 12, there waseutectic cell around the spheroidal graphite. No suppression toeutectic reaction happened owing to heat transfer law in thespheroidal graphite. Comparing with Fig. 1(a) and (b), it could befound that the dimension in length of graphite flakes was over1000 mm which was far larger than the diameter of graphite nod-ules (30 mm). Thus, there were more eutectic reactions in NCI thanthat of in GCI under the same plasma surface melting parameters.The depth of the molten surface increased with the increasing ofthe eutectic reactions.

The graphite has a higher thermal conductivity (129 W/(m K))[22], while the thermal conductivity of the metal matrix is only39 W/(m K) [22]. The graphite flake in GCI connected each other asshown in Fig. 1(a). However, Fig. 1(b) showed that the spheroidalgraphite in NCI was independent. The eutectic cells in GCI exhibitedan extensive three dimensional interconnected network of graphite

Fig. 13. The mean surface roughness of the molten specimens changes with the mainarc current.

as a heat conduction channel (Fig. 1(a)). In contrast, NCI attainedlarger mean free paths of matrix connecting the graphite nodules(Fig. 1(b)), and the heat had to propagate longer distances in themetal matrix having lower thermal conductivity [22].

Consequently, the thermal conductivity of GCI is up to 45e67W/(m K), even 90 W/(m K) [22] due to its graphite flake and inter-connected network of graphite as a heat conduction channel asshown in Fig. 1(a), while NCI has the lower values, only 25e42 W/(m K) because of its independent spheroidal graphite and heatconduction mainly by metal matrix (Fig. 1(b)).

The higher the thermal conductivity, the higher the thermaldiffusivity. With the same heat input, GCI with higher thermaldiffusivity would gain less energy to melt the specimens and themolten bath would be shallow. The shallower bath meant the lesscase depth. Compared with GCI, NCI having lower diffusivity couldcapture more energy to melt the specimens. Therefore, the moltensurface layer of NCI specimen treated by plasma beam was deeperthan that of GCI under the same parameters.

The mean surface roughness of the surface melted specimenincreased essentially with increasing arc current, as shown inFig. 13. The result was consistent with the reference [16]. However,the mean surface roughness of the treated NCI specimens showedlower under the same plasma current.

Fig.14was the surfacemorphologies heated by the plasma beamat 55 � 0.5 A. Both of the plasma beam heated specimens showedcontinuous and uniform solidified trace, and the molten pool wasnot broken. The trace surface on the treated NCI specimen showedlower roughness compared with Fig. 14(a) and (b).

Fig. 14. Top view of the treated specimens at 55 � 0.5 A (a) the treated GCI specimen;(b) the treated NCI specimen.

Fig. 15. (a) the graphite flake built in iron matrix; (b) schematic diagram of the edge effect.

X. Cheng et al. / Vacuum 101 (2014) 177e183 183

It could be concluded from Figs.13 and 14 that surface quality onthe treated NCI specimen was better than that on the treated GCIone under the same plasma parameters.

The graphite acted as a “crack” as shown in Fig. 15(a) in castirons. There were amount of edges due to the existence of the“crack”. In the case of plasmamelting, heat may first melt the edgesto form the collapse due to edge effect as shown in Fig. 15(b). Thecollapse would be presented as a micro-defect. It was obvious inFig. 9(a). And the plasma arc might be towed by the edges whichresulted in the loss of energy.

From the Fig. 1, it was found that graphite flake was far larger inlength than that of diameter of the graphite nodule. Thereweremoregraphite cracks in the GCI than that in NCI. The more serious edgeeffect may be happened, the more collapse and defects may be pre-sent. The surface roughness increased due to the more collapse. Andthe rapid cooling rate in GCI specimen due to its higher thermalconductivity also led to the improvement of surface roughness.Hence, the surface roughnesson the treatedGCI specimenwashigher.

4. Conclusion

Surface melting of GCI and NCI by plasma beam led to thedissolution of the near-surface graphite and re-solidification of awhite cast iron structure. XRD of the molten surface revealed theexistence phases of a-Fe (ferrite), g-Fe (austenite), martensite,cementite (Fe3C).

Micro-hardness distributed in molten surface layer was farhigher than that in substrate specimen due to the grain refinementand the carbides. The molten surface on GCI treated by plasmabeam had the highest micro-hardness.

The shape of the graphite affected the case depth of cast ironssurface modified by the plasma beam. Under the same plasmaparameters, the case depth on treated NCI was deeper than that ontreated GCI. The main factor was that the thermal conductivity/thermal diffusivity of GCI with graphite flake were higher than thatof NCI with spheroidal graphite. The extended network of highlyconducting graphite in GCI increased the heat conductivity, whilethe lower thermal conductivity was attained in NCI because NCIattained larger mean free paths of matrix connecting the graphite.Besides, better surface quality on NCI specimen surface melted byplasma beamwas obtained comparing with the GCI ones treated bythe same plasma parameters due to the different graphite shape.

Acknowledgments

This work was supported by the Province and Ministry coop-eration of university-industry collaboration (2010090200047) andthe National Natural Science Foundation of China (No. 51375005).The authors express their sincere thanks for the help of analyticaland testing center of Huazhong University of Science and Tech-nology. The authors also wished to thank the editors and reviewersfor their constructive suggestions and comments.

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