886 Chiang Mai J. Sci. 2013; 40(5)

Chiang Mai J. Sci. 2013; 40(5) : 886-897http://it.science.cmu.ac.th/ejournal/Contributed Paper

Increasing the Durability of Pressure Dies by ModernSurface Treatment MethodsJacek Sawicki [a]*, Marek Gorecki [b], Lukasz Kaczmarek [a], Zbigniew Gawronski [a],Konrad Dybowski [a], Robert Pietrasik [a] and Wojciech Pawlak [a][a] Institute of Material Science and Engineering, Lodz University of Technology,

Stefanowskiego 1/15, 90-924 Lodz, Poland.[b] New York University, Division: Information Technology, Weaver, 251 Mercer Street, 201,

New York, NY 10003, USA.*Author for correspondence; e-mail: jacek.sawicki@p.lodz.pl

Received: 27 July 2012Accepted: 18 October 2012

ABSTRACTThe objective of this work was to select the optimum technological surface

layer for casting cores and ejector pins used in pressure dies in the process of castingaluminum alloys. The controlled measurements of: microhardness, wear and frictioncoefficients, internal stress distribution, as well as superposition of internal and realstress, resulting from computer simulation of the casting process, were performed foreach sample. In order to experimentally determine the influence of the selected treatmentmethods on the state of stress of the material, the selected material and technologicalsolutions were tested under service conditions. The optimal surface treatment, bothfrom the point of view of durability and cost-effectiveness, was obtained in the case ofmultiplex technologies including sulfonitriding treatment followed by the depositionof a novel low-friction MoS2(Ti,W) coating or economical FineLPN vacuum nitridingfollowed by TiN coating deposition.

Keywords: nitrided and sulfonitrided, low friction coatings, friction and wear, pressuredies, residual stresses

1. INTRODUCTIONDie durability largely depends on the

correct selection of materials, heat treatment,as well as appropriately formed surfacelayers preventing the occurrence of macro-cracks, and at the same time delaying theoccurrence and propagation of thermalcracks for as long as possible. Increasingexperience drawn from practice [1-4, 11, 15,16,19] proves the value of several subsequentoperations involving advanced vacuum heat

treatment and surface engineering, namely,quenching, tempering, nitriding, PVDcoating, and multiplex technologies,employed in improving the workingsurfaces of dies. The ability to carry outsome of these operations in the sameapparatus would make manufacturingmuch easier and cheaper. There have beenseveral reports about attempts to combineion nitriding and PVD plating in the same

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vacuum chamber [23,24]. Contemporarymultipurpose vacuum furnaces canintegrate precise austenizing, high pressuregas quenching, single or multiple tempering,and also low-pressure nitriding [7,25,26].The application of these techniquescontributes to a significant increase in diedurability through strict limitation ofthermal fatigue, erosion, cavitation, andadhesion of the casting to the die. Aninappropriately selected surface layer canprecipitate quick and severe damage to thedie instead of increasing its durability. Thecauses of premature damage to diessubjected to surface treatment stem fromthe uncontrolled superposition of internalstresses originating from surface layerformation and those initiated by externalforces generated cyclically during die use.

In this study, the economical vacuumnitriding technology FineLPN and newlydeveloped low-friction MoS2(Ti,W)coating were used to create a duplextechnology. The detailed studies on theobtained surface layers properties, namelythe measurements of microhardness, wearand friction coefficient, residual stressdistribution, were done. Additionally, theexperimental verification of the proposedtechnology was done. As a result theoptimal technological surface layer forcasting cores and ejector pins used inpressure dies in the process of castingaluminum alloys was chosen.

2. EXPERIMENTALThe material used in the studied

samples, in casting cores, and in ejector pinswas tool steel for hot work 1.2343(X38CrMoV5-1). The surface layer wasimproved with four types of surfacetreatment (apart from the traditionaltreatment method):a) FineLPN [7] vacuum nitriding, which

is a combination of typical heattreatment with precise, multi-segment low-pressure “boost-diffusion” nitriding;

b) SULFONIT sulfonitriding [8,9] whichgives rise to a thin layer of iron sulfideFeS (along with the nitrided layer) in thesurface layer of the treated material,which offers protection against scuffing;

c) duplex technology (FineLPN nitriding+PVD), where a TiN coating is depositedon the diffusion nitrided layer – theprocess of nitriding was conducted insuch a way as to form a very thin layerof γ’ nitrides on the surface to ensuregood adhesion of the TiN layer to thesubstrate [20];

d) duplex technologies (SULFONITsulfonitriding+PVD; FineLPN nitriding+PVD), where a low-friction MoS2(Ti,W)coating composed of molybdenumdisulfide with the addition of titaniumand tungsten was deposited onsulfonitrided and nitrided diffusionlayers [12].

The technologies and process para-meters used were selected on the basis ofexperiments conducted at the Institute ofMaterial Science and Engineering, LodzUniversity of Technology, taking intoconsideration results from studies on theirinfluence on functional properties [8,10,12-14, 20-22]. Based on previous experiencein the field of imparting functionalproperties to surface layers, the followingsolutions were selected for further analysis(Table 1-2):• for casting cores subjected to forces

resulting from overcoming the forces ofadhesion of the casting to the pressuredie surface: (A), (B), (B+E);

• for ejector pins subjected to the forcesof friction against guiding holes: (A),(A+C), (A+C+D), (B), (B+D).

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Table 2. Parameters of PVD coatings deposition.Coating’s type

Pa 3,2×10-3 2,7×10-1

oC below 300 below 150

No. Parameter UnitTiN MoS2(Ti,W)

PVD deposition Filtered Cathodic Arc Magnetronmethod - Evaporation (FCAE) Sputtering (MS)

1. Residual pressure Pa 2,7×10-3

2. Gas - Nitrogen Argon3. Pressure during

deposition4. Working gas flow sccm 14 N/A5. Time of deposition ks 10,86. Deposition

temperature7. Plasma source power kW 1.8 1

Treatment (designation) Parameters of treatmentsThermal treatment (A) Quenching temperature: 1303K

Tempering temperature: 873KTemper time: 2×2h.Atmosphere : vacuum

FineLPN vacuum nitriding (B) Temperature: 813K, Time: 3hPressure: 0.025barGas mixture: NH3+N2+H2

SULFONIT sulfonitriding (C) Temperature 813 K, Time: 5hPressure: 1barGas mixture: NH3+N2+H2+S

MoS2(Ti,W) coating deposition (D) See table 2TiN coating deposition (E) See table 2

Table 1. Selected technologies of thermal and surface processing.

Samples, which had the form of discswith a diameter of 25 mm and a thicknessof 6 mm, were measured in terms of thedistribution of internal stresses. Also, themicrohardness and thickness of thetechnological surface layers of the sampleswas measured following the processes oftechnological treatment. Additionally,friction tests were performed on samplesselected for pressure die ejector pins.

Measurements of internal stresses inthe strengthened surface layers wereconducted by the X-ray method using a

PROTO iXRD diffractometer. The valuesof angles ψ and θ were set for the X-raybeam penetration depth to be in the rangeof 10 to 20 μm. Microhardness wasmeasured with a CLEMEX microhardnesstester by the Vickers method at a load of20 g (for TiN and MoS2(Ti,W) coatings andfor γ’ nitride layers) and 100 g for diffusionlayers. Friction tests in a “ball-on-disc”geometry with point contact friction wereconducted using a zirconium dioxide(ZrO2) ball with a diameter of 6.35 mm.For all the tests, the friction radius was

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7 mm, the sliding rate 0.1 m/s, and the load10 N. Tribological tests were conducted ata relative humidity of 20 ± 2 g/m3 and anambient temperature of 297 ± 2 K. Thefriction path was 1 km. After conductingfriction tests, profilograms were made forthe friction tracks and wear coefficientswere determined on their basis. Additionally,roughness parameters for the surface ofsamples with and without coatings weredetermined using a Hommel-Werke T8000profile meter.

A superposition of internal stressesand those resulting from external forces(as determined in a computer simulationof the casting process) was performed forcasting cores [17,18]. The optimization ofsurface treatment for elements workingunder conditions of contact and thermalfatigue is conducted in the following way[5,6]: an element is subjected to surfacetreatment using the studied methods, theinternal stresses in the layers formed aredetermined, and, after verification ofresults, the distribution of internal stressesin those layers is plotted as a function ofdistance from the surface. Subsequently,based on the technical specification of themachine in which a given element is used,a spectrum of external forces for extremeworking conditions is modeled for thiselement and a discrete model is constructed.

The model is then applied to determinestresses arising due to external forces.Subsequently, stresses from external andinternal forces after treatment aresuperposed, and the resultant stresses areplotted as a function of the range of theiroperation in the analyzed area of theelement. Analysis of these plots enables theselection of the optimal method of surfacetreatment. This process of surface treatmentselection accounts for the anisotropy ofelastic properties and makes it possible toidentify local discontinuities, whichconstitute stress concentration points.

In order to experimentally determinethe influence of the selected treatmentmethods on the state of stress of thematerial, the selected material and techno-logical solutions were tested under serviceconditions. Verification was conductedusing actual elements (casting cores andejector pins used in aluminum pressure diecasting), which were subjected to mechanicalforces resulting from overcoming theforces of adhesion of the casting to the diesurface and the forces of friction on ejectorpins against guiding holes.

3. RESULTSThe parameters of the technological

surface layers of the monitored elementsare given in Table 3.

Table 3. Parameters obtained for selected treatment methods of technological surfacelayers.

ParameterA B B+E A+C A+C+D B+D

Core hardness [HV] 445 435 435 440 440 435Surface / sub-surface - 870 (γ’) 1605(TiN) 520 520hardness [HV] 930 960 560 (MoS2(Ti,W)) (MoS2(Ti,W))

560 860Technological surfacelayer thickness [�m] - 80 80 150 150 80Surface layer 2 5 3 3thickness [�m] - (γ’) (TiN) - (MoS2(Ti,W)) (MoS2(Ti,W))Stresses in the surfacelayer [MPa] ~ 0 -740 -1460 -650 -620 -650

Treatment method

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3.1 Superposition of StressesA dynamic state of stress model of the

pressure die was prepared on the basis of adesigned element – a representation(simplified in terms of shape) of an actualcasting manufactured by the companyWIFAMA-PREXER POLAND. Acomplete analysis of the behavior of aliquid aluminum stream inside the pressuredie and the distribution of actual stresses

occurring on the surface of the castingcores and inside the aluminum stream werepresented in previous works [17,18]. In thiswork, for a time step selected from thesimulation, a superposition of stresses wascomputed for a point most exposed tothermal and mechanical fatigue (Figure 1)in order to determine the actual workingconditions of the casting cores.

Figure 1. Map of longitudinal stresses in the pressure die for selected time steps [17,18];a) model of the pressure casting die, b) stress field inside the pressure casting die forselected time steps c) chosen place and path in relation to which the superposition of thestress was occurred from external loads.

a) b) c)

The measurement area S1 defined alongthe surface (Figure 1c) is a very characteristicarea in the casting core, as it is exposed todirect action of hot liquid metal and

Figure 2. Superposition of actual and internal stress in casting cores.

subjected to the greatest tensile stresses,which are extremely detrimental to fatiguelife (Figure 2).

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As Figure 2 shows, under the influenceof a hot metal stream undesirable tensilestresses are generated on the surface of themeasurement area S1. This is extremelydetrimental to fatigue strength and mayconsequently lead to the development of anet of microcracks in that area. This processmay be significantly delayed by producingcompressive stresses in the surface area. Thesum total of those stresses and the tensilestresses would yield a favorable spectrumstresses in the die. To verify the aboveproposition, a superposition of actualstresses and internal stresses resulting fromsurface treatment was calculated for themeasurement area S1. Figure 2 shows thatin the case of typical heat treatment (A), inthe absence of internal stresses (the internalstresses in the surface area were of the orderof 0 MPa), the distribution of stresses isequivalent to the S1 path. In the case ofsurface treatment such as nitriding (B) orduplex layer formation (B+E), thespectrum of resultant stresses remainsentirely in the range of negative values.It follows from the above that nitridedand duplex layers formed in the courseof surface treatment could significantlydelay the nucleation and propagation ofthermo-mechanical cracks, thus improvingthe durability of the pressure die. Theultimate selection of a surface treatmentmethod for casting cores should be precededby an in-depth analysis of their locationwithin the die and of the operatingconditions to which they are subjected.

3.2. Friction TestsFigure 3 presents dynamic friction

coefficients for all the studied treatmentmethods. Furthermore, Figure 4a shows ahistogram of wear coefficients under dryfriction conditions, and Figure 4b a histo-gram for the roughness parameter Ra.

Friction tests during which dynamic(transient) friction coefficients wererecorded revealed differences in frictioncoefficients for steel subjected to differenttreatment methods. The highest frictioncoefficient (0.95) was recorded for the steelsample (A). In the case of nitrided (B) andsulfonitrided (A+C) samples, thecoefficient decreased to approximately 0.35,while the course of change seems to be moreuniform for sulfonitrided layers. This maybe explained by the presence of a layer ofiron sulfides with a hexagonal structure andsubstantial anisotropy, which gives rise toeasy glide planes and, consequently, to aself-lubricating film between the sample andcountersample. On the other hand, it seemssurprising that the friction coefficient forthe sulfonitrided layer was slightly higherthan that for the nitrided layer, at least inthe initial period of friction.

Friction coefficient tests conductedunder the same conditions as above forsamples coated with MoS2(Ti,W) show thatthis coating makes it possible to reduce thedry friction coefficient to less than 0.1. Forthe nitrided and sulfonitrided samples, thefriction coefficients were 0.075 and 0.083,respectively, which means that their valueswere lower by about fifteen times infriction with a zirconium countersampleas compared to samples subjected only tothermal treatment.

The wear coefficients presented inFigure 4 reveal that the nitrided samplewas most prone to wear, followed by thesulfonitrided sample. The sample subjectedto heat treatment only showed the leastwear. Similar results were obtained forcoated substrates. In friction tests usinga ZrO2 ball, the wear coefficient was ofthe order of 10–15 m3/Nm for non-coatedsamples. This value was by an order ofmagnitude smaller for samples covered

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with low-friction coatings. To explain thistribological behavior, a roughness test wasconducted (Figure 4b), which revealed thatdiffusion treatment methods lead to anincreased surface area. The Ra parameterfor polished samples deteriorated by afactor of about 3 upon nitriding and 4.5upon sulfonitriding. However, the use of

the duplex technology consisting ofsulfonitriding followed by deposition of alow-friction MoS2(Ti,W) coating improvedthe adhesion of the molybdenum disulfidecoating to steel substrates while preservinga low friction coefficient and high resistanceto friction wear.

Figure 4. Histograms of wear coefficients (a) and roughness parameters Ra (b) for theinvestigated coating systems.

Figure 3. Coefficients of dry friction for steel X38CrMoV5-1 after: (a) A, A+C, Btreatment, (b) A+C+D, B+D treatment.

a)

b)

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3.3 In-service TestingThe obtained modeling results were

confirmed by in-service testing of castingcores (Figure 5), whose operating conditionscorresponded to the spectrum of external

loads assumed in the superposition ofstresses. Additional tests were conductedon ejector pins, which were primarilysubjected to the forces of friction againstguiding holes.

The choice of elements to be examinedwas dictated by ease of assembly anddisassembly for the purposes of monitoringof their surface condition and assessingthe different technological surface layersolutions under identical operating con-ditions. The monitored casting cores wereperfectly suited to X-ray measurement

(using a PROTO iXRD device) aimed atassessing the state of stress of core materialafter a predefined number of work cycles.Measurements were conducted after 5,000,10,000, and 55,000 shots. Changes in thestress values over the above-mentioned timeof operation of pressure dies with castingcores are presented in Figure 6.

Figure 6. Changes in stresses on the surface of casting cores subjected to varioussurface treatment processes as a function of the number of work cycles of the pressurecasting die.

Figure 5. Casting cores subjected to in-service examination.

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The degree of wear of particular pressuredie cores found upon examination wasdifferent, and the wear trends correspondedto the level of stresses. The study showed adominant influence of compressive stresseson the durability of casting cores subjectedto different surface treatment technologies.In the case of cores subjected to thermaltreatment (A) consisting of hardening and

tempering, molten aluminum adhered tothe surface of the casting cores, and thesoldering phenomenon increased with thenumber of work cycles performed bythe pressure die. In the case of castingcores subjected to nitriding or duplextechnologies, this phenomenon did notoccur at all.

Figure 7. Ejector pins subjected to in-service examination.

A visual examination of ejector pins(Figure 7) shows that elements subjectedto treatment (A) were ruined afterundergoing the same number of cycles(10,000) as ejector pins subjected totreatments (A+C), (A+C+D), (B), and(B+D). The increase in the service life ofthe casting cores obtained thanks to theduplex technology followed from thelower coefficients of friction between theejector pin guides and the hardened surfacelayers.

4. DISCUSSIONDie casting practice shows that the

state of stresses introduced to the surfacelayer of pressure die elements by treatment

processes changes over the time of service.This is confirmed by the monitoring, atimportant verification points, of stresses inthe casting cores that were subjected totreatment according to the proposedtechnologies. Analysis of the diagrampresented in Figure 6 shows that thepurposefully formed surface layerssubstantially delay the formation of fatiguecracks. This results from the fact that theresultant spectrum of stresses obtained forthe surface area remains in the range ofnegative values (Figure 2). This eliminatesthe need to anneal the dies several timesover during the operating life, whichsignificantly reduces the costs of production.In the case of during heat treatment (A),

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it should be expected that incipient fatiguemicrocracks will arise very early becausethe resultant value of tensile stressesapproaches the tensile strength of thematerial.

In the case of elements primarilysubjected to friction forces (Figure 4),a very important issue is to preserve a lowfriction coefficient on the friction facesand hence ensure high resistance to frictionwear. A visual examination of the surfaceof ejector pins confirms that the use ofduplex technologies (A+C+D) and (B+D)extends the useful life of these elements ofpressure dies thanks to the application ofthe MoS2(Ti,W) low-friction coating, whichdiminishes the dry friction coefficient tounder 0.1.

The use of new technologies not onlyincreases the service life of pressure dieelements, but also shortens the duration ofthe treatment process and reduces thenumber of pieces of equipment needed toperform the treatment (e.g., in the case ofFineLPN nitriding). This in turn translatesinto both economic and environmentalbenefits.

5. CONCLUSIONS1) Stress distribution that has been obtained

may be used as a criterion for choosingtechnological surface layer. It is consideredan important parameter for furtherindustrial implementation, as it enablessuperposition of stress in surface layersof die elements.

2) The stresses superposition enabled notonly to estimate their values under realconditions, but also their character. Asa result it was possible to estimate thetensile stresses range, as this kind of stressis believed to be undesired when fatiguestrength is considered.

3) The obtained stresses for the die treated

with the proposed technology maintaintheir character as compressive stresseseven after several thousand injections,while only heat-treated elementsdemonstrate the tensile stresses which arevery undesirable from the durabilitypoint of view.

4) The presence of compressive stressesgenerated in the surface layer of thepressure casting dies by using duplextechnology significantly increases theirdurability.

ACKNOWLEDGEMENTSThis work was financially supported

by the Ministry of Science and HigherEducation of Poland (No N507 411036).

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