effect of different surface states before plasma nitriding on … · 2013-12-27 · effect of...

Surface and Coatings Technology 165(2003) 176–185

0257-8972/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00768-5

Effect of different surface states before plasma nitriding on properties andmachining behavior of M2 high-speed steel

A.daS. Rocha *, T. Strohaecker , T. Hirscha,b,c, b c

Centro de Ciencias Exatas e Tecnologicas – UNISINOS, Universidade do Vale do Rio dos Sinos, Sao Leopoldo, Brazila ˆ ´ ˜Escola de Engenharia – UFRGS, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazilb

Stiftung Institut fur Werkstofftechnik, Badgasteiner Str. 3, 28359 Bremen, Germanyc ¨

Received 6 May 2002; accepted in revised form 14 October 2002

Abstract

In the present work the effect of different surface conditions on the plasma nitriding response of AISI M2 high-speed steel wasinvestigated. Samples and drills were prepared to different surface finishes prior to plasma nitriding: ground and sandblasted.Polished samples were used as a reference surface state. The plasma nitriding was performed at temperatures of 400 and 5008Cfor two gas mixtures: 5 and 76 vol.% N in hydrogen. The surfaces were characterized before and after plasma nitriding2

concerning the microstructure, roughness, microhardness, chemical composition, phase composition and residual stress states.Machining tests were carried out with drills, during which drilling forces and flank wear were measured. A significant effect ofthe surface state prior to nitriding on the residual stress states and properties of the nitrided layer and untreated core has beenobserved. Thinner nitrided layers on ground and sandblasted samples were attributed to high compressive residual stress statesand a stress-affected diffusion of nitrogen and carbon. In the machining tests, sandblasted drills exhibited the best performance.Lower nitrogen concentrations in the gas atmosphere gave the lowest drill flank wear for sandblasted surfaces, while highernitrogen concentrations led to a reduction in drilling forces and torque.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Plasma nitriding; Surface finishes; X-Ray diffraction; Machining tests

1. Introduction

Standard nitriding processes can be used as surfacetreatments for tool steels, but plasma nitriding is espe-cially suitable because treatment temperatures wellbelow the tempering temperature can be applied and theimpinging particles of the plasma are able to destroypassive layers of high alloyed steels. This results inunchanged core properties after the treatment, besidesother advantages attributed to the plasma treatmentw1–3x.The nitrided layers in steels are usually composed of

a compound layer formed by iron-(carbo)-nitrides anda diffusion zonew1x. The compound layer(white layer)is found at the surface with a thickness of somemicrometers. In the diffusion zone the nitrogen atoms

*Corresponding author. Tel.:q49-421-218-8215; fax:q49-421-218-5333.

E-mail address: [email protected](A.daS. Rocha).

can be interstitially dissolved or precipitated as iron-(carbo-)nitrides, and most probably as(carbo-)nitridesof the main alloying elements. The nitrided layers intool steels exhibit high hardness due to dispersed alloynitrides in the matrix. High surface residual stresses aregenerated in the compound and diffusion zones, whichare the result of chemical composition gradients, stressfields around precipitates, volume changes and thermaleffects.It is well known that not only do the mechanical

strength, toughness and geometry affect the mechanicalbehavior of cutting tools, but also that the final surfacestate is of high importancew4–7x. Consequently, aneffect of the surface state before nitriding on the layerformation during nitriding and properties after nitridingcan be expected. The layer properties may be consider-ably changed, affecting the mechanical behavior of toolmaterials. The aim of the present investigation was toevaluate the effect of different surface states prior to

177A.daS. Rocha et al. / Surface and Coatings Technology 165 (2003) 176–185

Fig. 1. Size and shape of centerline drills used for machining tests.

Table 1Chemical composition of the tool material

Composition(wt.%)

C W Mo Cr V

AISI M2 standard composition 0.78–0.88 5.5–6.75 4.5–5.5 3.75–4.5 1.6–2.2Analyzed composition of samples 0.89 6.13 5.19 4.19 1.90Analyzed composition of drills 0.88 5.92 4.78 3.82 1.81

plasma nitriding on the properties of the compound layerand diffusion zone and on the behavior in machiningtests. The investigation has been subdivided into acareful microstructural analysis of samples and machin-ing tests with tools that were treated to similar surfaceconditions.

2. Experimental procedure

Samples and drills of high-speed M2 steel wereprepared with different mechanical surface treatmentsand then plasma-nitrided in the same batches. Thesurfaces were characterized before and after plasmanitriding by surface roughness measurements, glow dis-charge optical emission spectrometry(GDOES) to deter-mine chemical composition depth profiles,microhardness profiles, metallography and X-ray dif-fraction. For the different surface states, drilling testswere carried out with as-delivered drills as well as withplasma-nitrided ones.The nominal and analyzed composition of the M2

material is given in Table 1. The samples were disc-shaped with a diameter of 32 mm and a thickness of 5mm. The tools were centerline drills, as shown in Fig.1, with a geometry set by the standard DIN 333A withthe following dimensions:d s5 mm, l s65 mm and1 1

l s7 mm.2

The centerline drills received were of industrial stan-dard quality with a core hardness of 64 HRC and alreadysharpened. All samples were hardened and tempered inan industrial salt bath to the same core hardness as thedrills. After hardening and tempering, all samples werepreliminarily ground with sandpaper up to�400 grade.Next, samples and drills were prepared in batches

with different finishes:(a) polished samples;(b) groundsamples and as-delivered drills(as given by the suppli-er); and (c) sandblasted samples and drills. Polishedsamples were used as a reference state in the analysis,as several previous works demonstrated this kind ofsurface to be well suited for an analysis of layer depth,chemical composition, residual stresses and phase com-positionw8,9x. For polishing, the metallographic standardprocedure was applied, with a last polishing step with1-mm diamond paste. In order to have similar surfacestates as the tools, the samples in batch(b) were groundwith sandpaper up to�80 grade. The sandblasting ofsamples and tools of batch(c) was carried out for 30 s

using corundum(Al O ) abrasive particles with a mean2 3

size of 0.7–0.85 mm and an air pressure of 3.5 bar.For the plasma nitriding, laboratory-scale equipment

without an auxiliary heating system and two gasmixtures(5 and 76 vol.% N in hydrogen) were used.2

For all batches the pressure in the chamber was set to 5mbar and the nitriding time at the nitriding temperatureto 30 min. The time to reach the nitriding temperaturewas approximately 8 min. Nitriding temperatures of 400and 5008C were used. Fig. 2 presents a view of onecomplete batch of drills and samples. The samples andtools were symmetrically arranged following the sameprocedure for all batches. Batches were composed offour tools and four samples, plus one additional sampleand one additional tool for independent temperaturecontrol by inserted thermocouples. Due to the differentshapes, temperature differences of 10–208C were meas-ured between samples and drills. For simplification thenominal temperature presented in this paper is thatmeasured in the samples.Glow discharge optical emission spectrometry

(GDOES) w10,11x was employed for the determinationof chemical composition profiles. Phase analysis wasperformed in the grazing-incidence X-ray diffractionmode to complement results from a previous investiga-tion w12x, in which chromium and copper radiation wereapplied in the normal Bragg–Brentano geometry. Thediffraction patterns were acquired with a commercialgoniometer(Siemens D-500) using Cu-K radiation anda

a constant incidence angle of 18. All measurements were

178 A.daS. Rocha et al. / Surface and Coatings Technology 165 (2003) 176–185

Fig. 2. (a) View of the assembled batch and(b) view during plasma nitriding.

Fig. 3. Views of:(a) tip of the drill; (b) flank surfaces with indication of the cutting edges; and(c) flank wear at the main cutting edge.

executed with a primary aperture of 0.35 mm in width(0.18) and 10 mm in height. The diffraction patternswere recorded in the range of 338-2u-1508 with astep size of 0.028 and 20systep.A c-diffractometer with Cr-K radiation and thea

{ 211} diffraction lines ofa-iron were used for residualstress measurements of samples and drills. Residualstresses were then calculated from lattice strains usingthe sinc method w13–15x with macroscopic elastic2

constants of steel,Es210.000 MPa andns0.28. Thepenetration depth of Cr-K radiation in thea-irona

allowed measurements in the diffusion zone, even witha small compound layer at the surface. Residual strainmeasurements were carried out in 15 inclinationsc inthe rangey608-c-q608 for two orthogonal meas-urement directions(fs08 and 908).Residual stress profiles were obtained by X-ray strain

measurements after surface layer removal. Layer remov-al was performed by electrolytic polishing of samplesusing a solution of 80% H SO and 20% H PO .2 4 3 4

Residual stress corrections applied following the proce-

dure given by Haukw13x resulted in very small changeswhen compared to measured values.The drilling tests were performed in a numerically

controlled cutting machine using a cutting velocity of35 mymin, feed rate of 0.089 mmyturn (125 mmyminand 1400 rev.ymin) and a continuous flow of cuttingfluid. At least 600 holes per drill were executed, moni-toring the cutting forces, drilling torque and wear. Themachined material was AISI 1050 steel in a hot-rolledstate with a hardness of 244 HB(Brinell hardnessnumber) and the following chemical composition(wt.%): 0.50 C; 0.64 Mn; 0.03 P; 0.02 S; 0.19 Si; 0.003Cu; 0.024 Ni; 0.026 Cr; and 0.027 Al.Fig. 3 presents an overview(a), the cutting edges(b)

and a characteristic flank wear land at the main cuttingedge(c). The flank wear was observed under an opticalmicroscope at periodic interruptions after every 100holes with magnification of 25–50= (as shown in Fig.3c). Scanning electron microscopy(SEM) was alsoemployed for comparison. The drilling force measure-ments were carried out using a four-component piezo-


Table 2Mean roughnessR of samples for each type of surface finisha

Surface finish R (mm)a

Polished 0.010"0.005Ground with sandpaper 80 0.44"0.19Sandblasted 1.48"0.47

Results are given as mean"standard deviation.

Fig. 5. Residual stress and FWHM depth profiles for polished, groundand sandblasted samples.

Fig. 4. Topography of the rake face of(a) original ground and(b) sandblasted drills.

electric dynamometer, which provided information aboutthe feed force and torque.

3. Results and discussion

The following properties of the surface layers aredescribed before and after plasma nitriding: surfacetopography, surface residual stress states and depthprofiles, and chemical composition depth profiles. Insamples, these properties were representative of thesurface states of tools. Some measurements on toolsproved this assumption. Finally, results of the machiningtests are presented and discussed using the surfaceproperty data obtained from the samples.Table 2 summarizes the results of surface roughness

measurements. As expected, the surface roughnessRa

increases from 0.010mm for polished samples to 0.44mm for ground samples and up to 1.48mm for thesandblasted specimens. The sandblasting-induced rough-ness near the cutting edges of drills wasR s6.2"4.4a

mm, which is approximately two-fold the mean value ofthe original ground tools(R s3.2"1.1 mm). Thea

roughness of the rake face of the tools was not deter-mined. The significant increase in roughness after sand-blasting is related to the relatively large aluminum oxideparticle size(0.70–0.85 mm) and the nozzle inclinationduring peening. For peening of the drills, the nozzlewas inclined by 458 to yield better access to the cuttingedges and rake faces. The topography was positivelyaffected by changing from the characteristic grindinggrooves to a more homogeneous topography(see Fig.4). In addition, after sandblasting, overlapping materialat the cutting edges of the drills was removed.

Fig. 5 gives residual stress and FWHM(full width athalf-maximum) depth profiles for the different surfacestates. As expected, sandblasted samples exhibit thehighest compressive residual surface stresses ofy1050MPa, a maximum ofy1350 MPa below the surfaceand relative deep layer of compressive residual stresses(50 mm). In contrast, grinding and polishing led tomaximum surface residual stresses ofy620 andy150MPa, respectively. Thin layers of compressive residualstresses further characterize these surface states. FWHMdistributions also differ from each other. A value of 5.88

represents the heat-treated core of the samples and drillsat depths greater than 250mm. Towards the surface, theFWHM increases in layers with the highest compressiveresidual stresses due peening- andyor machining-inducedrecovery and rearrangement of dislocations. The remark-able increase in FWHM towards the surface was unex-pected and gives a clear indication that peening-inducedplastic deformation and work-hardening took place inthe surface layers of sandblasted material. A similar butsmaller effect can also be observed for the groundsamples. Polished surfaces present almost constant val-ues of FWHM.Figs. 6 and 7 show GDOES results of nitrogen profiles

after plasma nitriding at 5008C with gas mixtures of 5and 76 vol.% N . Differences in the depth profiles can2

be observed for the different surface treatments prior to


Fig. 6. Nitrogen profiles of samples with three different surface fin-ishes plasma-nitrided at 5008C with 5 vol.% N for 30 min.2

Fig. 7. Nitrogen profiles of samples with three different surface fin-ishes plasma nitrided at 5008C with 76 vol.% N for 30 min.2

Table 3Results of layer depth measurements

Surface Parameters Depth(mm) N (wt.%)finish

Compound layer Total layerat a depth

(SEM) GDOES OM GDOES

of 1 mm

(0.1% N)

Polished 400 8C, 5% N2 0 0 15 10.8 1.08Ground 0 0 5 8.7 1.39Sandblasted 0 0 6 11.6 2.37

Polished 400 8C, 76% N2 1.3 1.9 12 12.2 3.03Ground 1.3 1.6 8 8.7 2.50Sandblasted NM 1.4 NM 11.8 3.26

Polished 500 8C, 5% N2 0 0 18 20.2 1.67Ground 0 0 11 14.9 1.78Sandblasted 0 0 7 13.3 1.74

Polished 500 8C, 76% N2 2.1 3.3 30 30.0 6.44Ground 1.9 2.9 20 26.2 7.57Sandblasted 1.5 2.2 14 20.6 4.24

OM, optical microscopy; NM, not measured.

nitriding. The nitrogen profiles were used for evaluationof the compound layer thickness following the proceduredescribed by Rocha et al.w12x. The total layer thickness(compound layerqdiffusion zone) was taken from thediagrams at 0.1 wt.% nitrogen. In Table 3, the totallayer and compound layer thickness values are summa-rized. Measurements made by metallographic analysisand the results from the nitrogen depth profiles wereused for comparison. For plasma nitriding at 4008Cwith 5 vol.% N , the diffusion zone depths are very2

similar for the three different surface treatments. How-ever, a significantly higher nitrogen concentration at thesurface of sandblasted samples can be observed(lastcolumn). In this case, the nitrogen profile exhibits ahigher nitrogen gradient near the surface. The sametrend is found for ground samples compared to polishedones. For plasma nitriding at 5008C with both gas

mixtures, the smallest nitriding depth was observed forthe sandblasted samples, as can be observed from thenitrogen profiles in Figs. 6 and 7 and from Table 3.Considering the temperature increase from 400 to 5008C, the gain in diffusion zone depth is also remarkablysmall for sandblasted samples and gives maximumvalues for polished samples. On the other hand, onlysmall differences are found for surface nitrogen contentsof the different surface states plasma-nitrided at 5008Cwith 5 vol.% N (see Table 3). The results of chemical2

composition profiles and nitriding-layer depth measure-ments obviously show a low diffusion rate in the caseof sandblasted samples.To confirm the information for plasma-nitrided sur-

faces, Figs. 8 and 9 present X-ray diffraction patternsobtained in grazing incidence mode. A conventionaldiffraction pattern of non-nitrided samples is shown for


Fig. 8. Diffraction patterns(grazing incidence mode) for samplesplasma-nitrided with 5 vol.% N at 400 and 5008C.2

Fig. 9. Diffraction patterns(grazing incidence mode) for samplesplasma-nitrided with 76 vol.% N at 400 and 5008C.2

Table 4Variation in surface residual stresses after plasma nitriding

Parameters Surface finish RS DRS(MPa)

Non-nitrided Polished y147 –Ground y636 –Sandblasted y1064 –

400 8C, 5 vol.% N2 Polished y1104 y956.5Ground y1159 y176Sandblasted y1106 y42

400 8C, 76 vol.% N2 Polished y999 y852Ground y1198 y424Sandblasted y1055 9

500 8C, 5 vol.% N2 Polished y1196 y1049Ground y1170 y460.5Sandblasted y1107 y43

500 8C, 76 vol.% N2 Polished y1336 y1188.5Ground y1284 y458.5Sandblasted y1146 y81.5

comparison. Vertical lines mark the diffraction linepositions of the´- and g9-nitrides. Previous investiga-tions w12x demonstrated the absence of compound layersfor samples plasma-nitrided with a gas mixture of 5vol.% N , and the presence of a-compound layer for2

the gas with 76 vol.% N . The diffraction patterns here2

clearly demonstrate that the gas mixture of 5 vol.% N2

did not form a compound layer. Even for the very smallpenetration depth of the grazing incidence mode(some10ths of a nm) no ´- or g9-nitrides were found(Fig. 8)for either nitriding temperature. On the other hand, theanalysis confirmed´-compound layer formation forsamples plasma-nitrided with 76 vol.% N for all tem-2

peratures(Fig. 9). In all cases the metallographic anal-ysis showed precipitation of nitrides, carbo-nitrides orcarbides at grain boundaries for plasma nitriding with76 vol.% N and a temperature of 5008C. It is known2

that grain boundary precipitation increases with timeand temperaturew8x. By means of TEM analysis, Tierw9x found these precipitates in M2 tool steel to becementite, and similar results have been reported forlow alloy steel by Mridha and Jackw16x. The presenceof a precipitation network at grain boundaries results invery brittle surface states, as found by hardness inden-tation and wear testsw9x. The absence of grain boundaryprecipitates for temperatures lower than 5008C isassociated with a decrease in the velocity of diffusionof carbon ina-iron, an increase in energy necessary forcementite nucleationw17x and higher stability ofannealed martensite and carbides. In samples nitridedwith the gas mixture of 5 vol.% N , these precipitates2

were not observed for either of the nitriding tempera-tures. The last observation can be correlated to thehigher decarburization of the surface when no compoundlayer is being formed at the surface, as previouslydemonstratedw12x.Table 4 gives the residual stress values after plasma

nitriding, and the value ofDRS indicates the difference

between residual stresses in the surface of nitrided andnon-nitrided samples. As shown in Table 4, the highercompressive residual stresses before nitriding(see Fig.5) result in smaller increases in the compressive residualstresses after plasma nitriding. In addition to Table 4,residual stress depth profiles for polished and sandblast-ed samples are given in Fig. 10. It is obvious thatplasma nitriding of sandblasted samples at temperaturesas high as 5008C resulted in small increases in residualstresses for all depths. The depth profile consequentlyremains practically unchanged in the case of sandblastedsamples. On the other hand, for polished samples, withthe increase in nitriding temperature from 400 to 5008C, thicker layers with higher compressive residualstresses and higher nitriding depths are observed. Itshould be pointed out that for sandblasted samples thenitriding depth, even at 5008C, is smaller than thesandblasting-induced layer of compressive residualstresses(compare Table 3, Figs. 6 and 10). The nitridingdepth in this case is 18mm, approximately the depth at


Fig. 10. Residual stress depth profiles for polished and sandblastedsamples non-nitrided and plasma-nitrided at 400 and 5008C with 5vol.% N .2

Fig. 11. Mean flank wear measured for different treatments.

which the maximum peening-induced residual stressesare present. For tool steels, no relaxation of machining-induced residual stress is expected at the nitridingtemperature, and therefore the full effect of the residualstresses introduced before nitriding in diffusion can beexpected.The small increase in residual stresses in the surface

of sandblasted and ground samples after plasma nitridingdiscussed can be related to a combination of severaleffects:

1. Nitride precipitation can be modified by plastic defor-mation. This would result in a smaller increase incompressive residual stresses with increasing severityof plastic deformation.

2. Due to the superimposition of residual stresses beforeplasma nitriding with those stresses generated duringnitriding, local material resistance against yieldingcan be achieved. Therefore, the maximum residualstresses would be limited by the local resistance toplastic deformation at the nitriding temperature.

3. The surface yield strength at both room temperatureand the nitriding temperature is additionally affectedby different surface states.

Some evidence of stress effects on diffusion can befound in the literaturew18,19x. It was reported inw18xthat extremely high pressures in soft metals cause anincrease in the activation energy of diffusion, as well asa decrease in the diffusion coefficient. A description ofload stress-induced diffusion of interstitial atoms isfound in w19x: the so-called Snoek effect used for studiesof internal friction, which is the effect of tensile stressescausing diffusion of interstitial solute atoms in a direc-tion perpendicular to the applied tensile stress. Whentensile stress is applied in one direction, it causes anincrease in atom separation in the same direction and,due to the Poisson effect, a reduction in atom separationin the perpendicular direction. There will be preferentialmovement of solute atoms perpendicular to the tensile

stress. Although the Snoek effect is described for thecase of load stresses, its principles could be consideredfor macro residual stresses. In the present case ofnitrogen diffusion, the previous compressive residualstresses introduced by sandblasting act parallel to thesurface, reducing space between the solvent atoms inthis direction. In this way, those previous compressiveresidual stresses would retard nitrogen diffusion into thecore(perpendicular direction to the surface). Therefore,a lower nitriding depth is observed as a consequenceand the effect of compressive stresses in the materialcan be viewed as resistance to diffusion. Further inves-tigations are being carried out to clarify the effect ofresidual stresses on the diffusion of nitrogen duringnitriding of high-speed steel.A comparison of the properties between tools and

samples gave excellent correlation, taking into accountthe different shapes of samples and drills giving similarnitriding depths. The diffusion-zone properties of nitrid-ed tools in any aspect were similar to those found insamples. As a first result of machining tests, Fig. 11shows the mean flank wear plotted against the numberof holes for different treatments. High amounts of wearoccurred during drilling of the first 100 holes. The wearfor ground tools nitrided with 76 vol.% N at 5008C2

was markedly high. Sandblasted tools nitrided with thegas mixture of 5 vol.% N showed the smallest wear2

rate.Some of the ground tools(as delivered) presented

manufacturing defects at the cutting edges, e.g. withoverlapping material. This resulted in fracture of theplasma-nitrided cutting edges after only 100 holes, ascan be observed from Fig. 12. The analysis of sandblast-ed tools without these defects still reveals good conditionof the cutting edges after 100 holes, as shown in Fig.13. In general, for sandblasted drills a smaller level ofadhesion was observed compared to ground drills, inspite of the fact that sandblasted drills have a highersurface roughness near the cutting edges.Figs. 14 and 15 show the torque and feed force during

execution of the holes for the different surface treat-


Fig. 12. (a) Rake face and(b) cutting edge of a ground drill(notsandblasted) before testing; and(c,d) after 100 holes(drill nitridedwith 5 vol.% N at 5008C).2

Fig. 14. Mean torque for different treatment conditions. Dashed arearepresents behavior of original ground drills, nitrided and non-nitrided.

Fig. 13.(a) Rake face and(b) cutting edge of a sandblasted drill after100 holes(nitrided with 5 vol.% N at 5008C).2

Fig. 15. Mean feed force for different treatment conditions. Dashedarea represents behavior for original ground drills, nitrided and non-nitrided.

ments. The data points give mean values from tests withseveral drills. The dashed areas summarize data fororiginally ground drills. Due to effects from overlappingmaterial at the cutting edges, ground drills, in spite ofthe calculation of mean values, exhibited large scatter,and reliable differences between as-delivered and plas-ma-nitrided samples could not be detected. As shown inFig. 14, the torque generally remains constant with thenumber of holes. However, sandblasted drills nitridedwith 76 vol.% N (at both 400 and 5008C) need2

considerable less torque for the execution of holes.Among all the conditions tested, sandblasted drills

nitrided with higher nitrogen contents gave the lowestvalues for the feed force. The feed forceF is mainlyz

affected by wear of the transversal edgew20x. As aconsequence, this force increases with the number ofmachined holes, as can be observed from Fig. 15.Although the reduction in torque and feed force forsandblasted and plasma-nitrided tools(76 vol.% N ) is2

quite obvious, the flank wear for that condition did notresult in the lowest wear rate. The lowest wear rate wasobserved for sandblasted drills plasma-nitrided withouta compound layer(see Fig. 11).The results presented give a clear indication that

different aspects, such as macro- and microscopically

analyzed material states, mechanical and thermal loadingin machining tests, and finally the results of the flankwear, feed force and torque, must be taken into account.In the present investigation, sophisticated analysis of thesurface layers in samples and machining tests of drillswith similar surface properties was carried out, resultingin optimized knowledge of this complex system. Thehigher brittleness of conditions nitrided with 76 vol.%N induced by grain boundary precipitation and the2

presence of tensile residual stresses in the compoundlayer w12x are possible reasons for the higher wear ratescompared to surface treatments with 5 vol.%N . On the2

other hand, the presence of a thin compound layerreduced the cutting forces, probably by a reduction inthe friction coefficient. Finally, residual stresses alsohave some impact on wear mechanismsw21,22x. Highcompressive residual stresses in thick layers should havea beneficial effect by delaying crack initiation andgrowth. In general, sandblasting prior to nitriding isbeneficial, resulting in an equalized surface topographyand thick layers of high compressive residual stresses.In addition, the removal of overlapping material fromthe cutting edges has a positive impact on the results ofmachining tests(see Figs. 11 and 13). In spite of thefact that the nitriding depth is lower compared to ground


or even polished samples, high residual stresses are stillpresent at the same depth after plasma nitriding. Con-sequently, sandblasted drills nitrided with 76 vol.% N2

presented a reduction of approximately 15% in the meantorque and a reduction in the mean feed force in relationto non-nitrided drills. Nitrided ground drills exhibitedonly a reduction in the axial(feed) force. This isprobably associated with the correlation of axial forcesand the transversal cutting edge. The latter is geometri-cally stronger than the main cutting edges and canbenefit more from a hard and somewhat brittle layer(plasma nitriding with 76 vol.% N). In drilling, the2

main contribution to torque results from interaction ofthe workpiece material with the main cutting edgew20x.Sandblasting before plasma nitriding eliminated theoverlapping material from main cutting edges. There-fore, the compound layer generated by plasma nitridingwith 76 vol.% N reduced the torque in the case of2

sandblasted samples, but not for ground ones. Any brittleand hard compound layer around overlapping materialwould result in immediate spalling and cracking afterthe first minutes of drilling, and this results in early andhigh amounts of flank wear, as indicated by Figs. 11and 12. Therefore, in these experiments no reduction intorque was observed. These results are supported by theflank wear measurements presenting high wear for theas-delivered drills nitrided with 76 vol.% N .2

These results suggest that careful quality control oftool integrity should be carried out before PVD coatingand the use of gas mixtures with intermediate nitrogencontents to obtain the right combination of nitride layertoughness and friction reduction. Controlled sandblastingor another shot peening process should also be able toimprove the properties of the tool–workpiece system.

4. Conclusions

The surface state before plasma nitriding significantlyaffects the properties of the nitriding layer. Smallernitrided layer depths were found in ground and sand-blasted samples when compared to polished ones. Thisis primarily associated with high compressive residualstresses at and below the surface, together with someamount of plastic deformation and consequences for thesubmicrostructure of the material.Considering the machining tests, the cutting edge

integrity and surface conditions are of high importanceto obtain the best results after plasma nitriding treatment.Sandblasting of tools before plasma nitriding demon-strated interesting potential, since a considerableimprovement in tool lifetime was observed. This isattributed to the elimination of overlapping(plasticallydeformed) material from the cutting edges and theintroduction of compressive residual stresses beforenitriding, which also remain after plasma nitriding.

A reduction in feed force and torque could beobserved for plasma-nitrided sandblasted drills with an´-compound layer(plasma nitriding with gas mixtureof 76 vol.% N ), which is attributed to the low friction2

coefficient of the nitride phases. The lowest wear rateswere found for plasma-nitrided drills without a com-pound layer(use of gas mixture of 5 vol.% N), which2

is explained by the brittleness of the diffusion zone ofnitrided tools with a gas mixture of 76 vol.% N leading2

to the formation of a grain boundary network of cement-ite-like precipitates. These results indicate that controlledsurface preparation by sandblasting(or another shotpeening process) before plasma nitriding, the use of lownitriding temperatures and an intermediate gas mixturebetween 5 and 76 vol.% N can lead to process2

optimization.

Acknowledgments

One of the authors(A.daS. Rocha) would like toacknowledge CNPq from Brazil(Conselho Nacional deDesenvolvimento Cientıfico e Tecnologico) for partial´ ´financing of the project. The authors also express theirgratitude to CETEMP-SENAI of Sao Leopoldo, Brazil˜for supporting the execution of the machining tests.

References

w1x ASM Handbook, vol. 4, 10th ed., ASM, Metals Park, OH,1991, pp. 420–424.

w2x R. Grun, H.J. Gunther, Mater. Sci. Eng. A 140(1991) 435–441.¨ ¨w3x B. Edenhofer, T.J. Bewley, Heat Treatment 9(1976) 7–13.w4x S.R. Bradbury, D.B. Lewis, M. Sarwar, Surf. Coat. Technol.

85 (1996) 215–220.w5x D.B. Lewis, S.R. Bradbury, M. Sarwar, Surf. Coat. Technol.

82 (1996) 187–192.w6x Y.U. Geller, Tool Steels, MIR Publishers, Moscow, 1978, pp.

556–596.w7x G. Hoyle, High Speed Steels, Butterworth & Co Ltd, London,

1988, pp. 107–193.w8x A.daS. Rocha, Estudo da nitretacao ionica aplicada aos acos˜ ˆ¸ ¸

rapidos, MSc Dissertation, Porto Alegre, Brazil, 1996, pp.´61–78.

w9x M.A.D. Tier, Avaliacao da Resistencial ao Desgate do Aco˜ ˆ¸ ¸AISI M2 Nitretado a Plasma, PhD Thesis, Porto Alegre, Brazil,1998, pp. 89–115.

w10x Z. Weiss, K. Marshall, Thin Solid Films 308y309 (1997)382–388.

w11x R. Eckhard, P. Mayr, Mikrochim. ActawWienx I (1989)197–212.

w12x A.daS. Rocha, T. Strohaecker, V. Tomala, T. Hirsch, Surf. Coat.Technol. 115(1999) 24–31.

w13x V. Hauk, Structural and Residual Stress Analysis by Non-destructive Methods: Evaluation, Application, Assessment,Elsevier, Amsterdam, 1997, pp. 388–392.

w14x J. Lu, Handbook of Measurement of Residual Stresses, Societyfor Experimental Mechanics, Fairmont Press Inc, 1996, pp.75–84.


w15x M.R. James, J.B. Cohen, The Measurement of Residual Stress-es by X-Ray Diffraction Techniques, Academic Press Inc,1980, pp. 5–7.

w16x S. Mridha, D. Jack, Met. Sci. 16(1982) 398–404.w17x O. Kancke, O. Kubaschewski, K. Hesselmann, Thermochemi-

cal Properties of Inorganic Substances, second ed., vol. 1,Springer-Verlag, Berlin, Heidelberg, 1991, pp. 701.

w18x R.E. Smallman, Modern Physical Metallurgy, 2nd ed., Butter-worths, London, 1963, pp. 111–112.

w19x R.E. Reed-Hill, Physical Metallurgy Principles, 2nd ed., PWS-Kent Publishing Co, Boston, 1992, pp. 433–441.

w20x D. Ferraresi, Fundamentos da Usinagem dos Metais, vol. 2,3rd ed., ABM, Sao Paulo, 1975, pp. 36–38.˜

w21x D. Scott, Wear, Treatise on Materials Science and Technology,vol. 13, Academic Press, New York, 1979, pp. 449–451.

w22x E.M Trent, Metal Cutting, 1st ed., Butterworth & Co Ltd,London, 1977, pp. 120–122.

effect of different surface states before plasma nitriding on … · 2013-12-27 · effect of...

Documents