INFLUENCE OF NITROGEN ALLOYING ON GALLING ...

INFLUENCE OF NITROGEN ALLOYING ON GALLINGPROPERTIES OF PM TOOL STEELS

I. Heikkila and L. SlyckeSwedish Institute for Metals Research

Stockholm

Sweden

O. SandbergUddeholm Tooling AB

Hagfors

Sweden

Abstract The objective of this study is to investigate the effect of nitrogen alloying oftool steels on their galling properties. Nitrogen is introduced to the steel com-position by solid state nitriding with ammonia gas at 550–600 ◦

C. The basematerial composition is in powder form during nitriding. Different nitrogencontents are experimented. The hard phases present in the steel are of typeM(C,N) and M6C. The materials are consolidated by HIP process. Gallingbehaviour of the produced alloys is evaluated with slider-on-sheet tribologicaltest method. As a reference for nitrogen alloyed compositions various PMtool steel grades and CVD coated tool steels are chosen. The ability of thetest materials to avoid formation of galling adherants is tested against AISI304 stainless steel with a low viscous lubricant. The test results show that ni-trogen alloying significantly increases the resistance against galling reachingequal behaviour as for CVD coated tool steels.

Keywords: carbonitride, PM tool steel, nitriding, galling, stainless steel

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256 6TH INTERNATIONAL TOOLING CONFERENCE

INTRODUCTION

Galling is one of the major causes for tool failure in sheet forming pro-cesses for stainless steel. Galling develops gradually as accumulation ofsheet material pick-ups on the tool surface during succesive forming oper-ations. The sheet material transfer process is considered to consist of aninitiation stage, a stable stage of accumulated growth, anda final stage whenthe large pick-ups start to cause aesthetic problems at the workpiece andestablish unstable friction conditions for the forming process. [1].

The tribological phenomena taking place between the tool and sheet sur-face is a complex process as surface interactions are affected by multiplefactors. The composition and surface characteristics of both the tool andsheet material play an important role as well as lubrication. Also, geometryof the tool, influencing stresses and temperature are of importance [2, 3, 4].

The focus on improving tribological properties of the sheethas been onmodifying surface topography for better lubricant retention and permeabilityat the contact area. The material properties of austenitic stainless steel areproblematic with respect to galling. The thin surfaces oxides of austeniticgrades, low thermal conductivity, high rate for work-hardening and highpercentage elongatation after fracture increase the tendency for formationof galling adherants. Nevertheless, occurrence of gallingcan successfullybe delayed by comprehensive optimatisation of the tool-sheet-lubricant in-teraction phenomena [2, 5, 6, 7, 8].

Cold work tool steels are generally used as tool material in various formingsituations. However, tool life can be increased by applyinga CVD or PVDcoating on the tool steel substrate. The good sliding properties of the coatingare lost as soon as the coatings is flaked away as a result of fatigue. Thecoating cannot be refurnished. The anti-galling properties of plain tool steelscan be improved by increasing the content hard constituents, controlling theircomposition, dimensions, shape and distribution [9, 10, 11].

PM technology offers possibilites to tailor unique microstructural fea-tures. The objective of this study is to examine the effect ofnitrogen alloyingof PM tool steel to resist galling against stainless steel. Nitrogen is intro-duced into the alloy system by nitriding gas-atomised powder particles inammonia gas. Hard carbonitrides will be formed in the strucre during con-solidation of the powder. Galling resistance is examined byslider-on-sheettribometer [12].

Influence of nitrogen alloying on galling properties of PM tool steels 257

TEST MATERIALS

The effect of nitrogen additions on the galling behaviour oftool steelswas investigated by producing alloy compositions with a varying contentof hard phases. The alloys were designed with the aid of thermodynamicequilibrium calculations by Thermo Calc softwear. The constitution of thealloys was adjusted such that two different hard phases willbe formed onmartensitic matrix at the austenisation range. The hard phases are M(C,N)carbonitride and M6C type of carbide. The letter M stands for the metalliccomponent of the carbide and it will constitute of several elements. Table 1shows the chemical composition of the designed alloys and the content ofthe hard phases according to Thermo Calc. The composed alloys were

Table 1. Compostion of nitrogen alloyed test materials and their content of hard phasesaccording to Thermo Calc

Alloy C N C Mo W V M(C,N) M6C Tot %

Vancron 1 1.28 1.34 4.37 2.80 4.10 7.84 18.3 4.0 22.3Vancron 2 0.58 3.23 4.37 3.10 4.10 8.8 24.9 3.9 28.8Vancron 3 1.96 2.82 4.18 3.20 4.80 15.20 33.1 3,9 37.0

designated as Vancron alloys.The test alloys were manufactured by gas-atomising the basealloy com-

position and later nitriding the atomised powder. Nitriding was performedas solid-state nitriding in ammonia gas at 550◦C. The nitriding equipmentconsists of a chamber charged with powder material and a heating furnacearound. The chamber is furnished with in-let channel for ammonia feed atthe bottom of the chamber and out-let channel for the exhaustgases at thetop. Figure 1 presents a schematic picture of the nitriding equipment.

The transfer process of nitrogen from gas phase into the solid materialinvolves ammonia dissolution reactions on the particle surfaces and diffusioncontrolled transfer phenomena inside the metal particles.The size of thepowder batch was 10 kg. Most of the ammonia dissolutes near the in-let atbottom of chamber. Therfore, nitrogen content will be considerably higherat the bottom of the chamber than in the top sections [13].

The nitrided powder was mixed uniformly in a rotary mill unit. The nitro-gen content of the batch was analysed. In case the analysed nitrogen contentexceeded the aimed value, an appropriate amount of unnitreded powder was


Figure 1. Nitriding equipment.

added in the mixture to achieve the desired content. In case nitrogen contentwas below the aimed value, a second nitriding was acccomplished.

The consolidation of the test materials was done by HIP process. Theconsolidated material billets were forged and soft-annealed to facilitate ma-chining. Test materials were hardened at 950–1100◦Cand tempered3 × 1

h at 560◦C. Figures 2 to 4 represent microstructures of Vancron 1 to 3 afterheat treatment.

The black phase in the structure is M(C,N) carbonitride and the whitephase is M6C carbide. The size of M(C,N) is small less than 1 µm. Thecontent of hard phases is clearly highest for Vancron 3.

The reference materials for the galling evaluation comprise of plain PMtool steel grades and coatings on a tool steel substrate. Table 2 presents thecomposition of the reference tool steels Vandis 23 and Vanadis 6. Table 2also presents the calculated amounts of hard phases by Thermo Calc at theaustenitising temterature. Both the Vancron alloys and reference tool steelswere heat treated to 62 HRC. The coatings chosen for testing were TiCN-


Figure 2. Vancron 1 in heat treated con-dition. 3000×.



Table 2. Composition of reference PM tool steels

Alloy C N Cr Mo W V MX M 7C3 M6C Tot %

Vanadis23 1.28 0.05 4.10 5.00 6.40 3.10 6.5 8.8 15.3Vanadis6 2.07 0.04 6.80 1.50 0.10 5.35 12.4 1.2 13.6


coating produced by CVD and VC-coating produced by Toyota diffusionmethod. The substrate material was Vanadis 23.

METHOD FOR GALLING EVALUATION

The tribological test methods should reproduce the existing conditions inreal industrial processes as well as possible. The slider-on-sheet tribometeris developed to simulate the tool-lubricant-sheet interaction generally pre-vailing in sheet forming operations. The test method is developed by TNOInstitute of Industrial Technology. The measurement principle is presentedin figure 5 [14].

Figure 5. Schematic picture of slider-on-sheet tribometer

A slider made of tool material is pressed against a sheet witha normalforceFn. The slider is moved in thex-direction with a specific sliding speedv. As the slider meets the end of the track the slider is lifted from the sheetand replaced over a distance of 1 mm in they-direction. The slider is returnedto the starting point in thex-direction. The normal force is addressed againand a next track is made over the same track lengthl. By this practice,sliding distance of 1 km can be accomplished on one square meter sheetmaterial. The test can be carried out as dry or lubricated tests. The testconditions used in this investigation are presented in Table 3. The normalforceFn 100 N will create a contact pressure of 900 MPa in the given slidergeometry. This is a typical pressure existing in the critical area of a drawingdie when pressing stainless steel.


Table 3. Test conditions

Test conditions ValueVelocity v 0.5 m/s, sliding

Normal forceFN 100 NSliding lengthl 1700 m

Lubricant 4-5 ml/m2 on the sheet

The normal force and friction force existing in the contact area are mea-sured with a data collection frequence of 1000 Hz. An averagevalue fornormal and friction force is calculated over each track. Since the track lengthis known, development of friction coefficient as a function of sliding distancecan be followed.

The sheet used in testing is stainless steel grade AISI 304 with a thicknessof 0,8 mm. Sliding direction is parallel to rolling direction. TheRa value ofthe sheet in rolling direction 0,10 µm and in upright direction 0,10 µm. Thelubricant is a low viscose oil Quaker Coupex 046-EP.

Roughness data of the test sliders is represented in the Table 4. The datacomprisesRa value parallel and perpendicular to sliding direction. Thesurface roughness was measured by Mitutoyo Surftest 500.

Table 4. Surface roughnessRa of the sliders perpendicular and parallel to sliding direction

Vancron 1 Vancron 2 Vancron 3 Vanadis 23 Vanadis 6 V23+TiCN V23+VC

perpendic. 0.09 0.06 0.08 0.08 0.07 0.15 0.07parallel 0.02 0.04 0.04 0.02 0.03 0.12 0.06

RESULTS

Each slider was tested in two separate experiments. The development offriction coefficient as a function of sliding distance is represented in Figs. 6,7, and 8 for the Vancron alloys, reference tool steel grades and coated vari-ants, respectively. The test was interrupted as soon as the friction coefficientreached a value of 0.2. The maximum sliding length was 2000 m.Thus, test-ing was interrupted at 2000 m eventhough friction coefficient stayed lower


than 0.2. The maximum sliding distance was applied to avoid long testingtimes.

At the start point of the test friction coefficient was low forthe uncoatedtool steel grades; in the range 0.05 to 0.07. The developmentof frictioncoefficient with an increasing sliding length varied a lot depending on thesteel grade. For the PM tool steel grades f rose rapidly beingup to 0.2 after20-50 m running. For the nitrogen alloyed materials f rose slighly after startup but remained relatively stable during the rest of the test. If the f suddenlyrose to 0.2 , the test was interrupted.

The start value for f varied from 0.07 to 0.17 for the coated sliders, thusbeing higher than in plain tool steels. The friction coefficient stayed stableduring the whole test and never exceeded 0.2. Yet, the friction coefficentstayed at high level near 0.2 for the TiCN coated variant.

During the course of testing, a transfer layer of stainless steel was devel-oped on the slider surface. Figure 9 represents an example ofsuch transferlayer at the end of the test. The Fig. 10 includes contour profile of thesheet surface at the point when testing was interrupted. Thetransfer ofsheet material to tool surface and scratching of the sheet surface are typicalcharacteristics for galling.

Figure 11 summarises the sliding lengths when f stayed below0.2 foreach test material. If f was below 0.2 at the sliding lenght of2000 m, the testwas interrupted. Thus, all the pillars in the figure 8 with L 2000 m are teststhat have been discontinued. The materials that achieved this highest slidinglength include Vancron 1 and Vancron 2, and TiCN and VC coatedvariants.It can be expected that even higher sliding distances could be achieved forthese materials if the test was continued.

DISCUSSION

The results on slider-on-sheet tribometer suggest that various tool mate-rials can be ranked with respect to their galling resistance. The differencesbetween various compositions are notably. The coated tool steels and ni-trogen alloyed tool steels showed the best resistance against galling. Thereference PM tool steel grades without coating show inferior galling resis-tance.

The content of hard phases in the reference tool steels Vanadis 23 andVanadis 6 is lower than in Vancron alloys. The Vancron alloys2 and 3 had thehighest content of carbonitride phase. They also showed thebest resistance


Figure 6. Vancron alloys: development of f as a fuction of sliding length.

Figure 7. Reference uncoated PM tool steels: development of f as a fuction of slidinglength.

Figure 8. Coated PM tool steels: development of f as a fuction of sliding length.


Figure 9. Galling adherant on Vancron 3 slider at the end of the test.100x.

Figure 10. Wear track of the sheet at the end point of the test.


against galling. The presence of carbonitride phase in highcontents and insmall particle size in the tool steel structure has a distinctly positive effecton galling resistance.

The used tribometer is developed to reproduce the tribological conditionstypically encountered in sheet metal forming. The contact pressure is inthe plastic range of the sheet. Sliding contact between the tool materialand sheet is accomplished in a such way that the slider continuously meetsa fresh sheet material. The method is flexible to adjust test parametersto resemble inquired conditions. The experiments are conducted at roomtemperature. However, this is seldom a situation in formingprocesses wherebulk temperture of the tool regularly increases to 60–70◦C [15].

The tests were performed with a low viscose lubricant. This will accen-tuate formation of galling adherants, since low viscose lubricants cannotsustain a separating lubricant film between the contacting metal surfaces athigh pressures. The surface roughness of sliders is finer than in formingtools generally. A fine surface will reduce the effect of mechanical inter-locking on galling and thus emphasize the sliding properties of tool material[16, 17].

The surface roughness of the sliders lay typically between 0.02 to 0.09.The only exception was the slider with TiCN coating which hadRa 0.12-0.15. The friction coefficient of the slider with TiCN stayednear 0.2 duringthe whole testing length. The rougher surface quality correlates well to thehigher friction level. An uneven surface produces locally high stesses at thetop of asperities thus giving higher resistance for sliding. Also, lubricationretention at the contact area is inferior.

The results on sliding length of Vancron alloys showed some scatter.Vancron 2 and 3 were able to reach sliding length of 2000 m without gallingbut also some inferior results were detected. In tribological tests, the scatterof the results is frequently high. This is due to the fact thattribologicalphenomena are complex in nature and there are many influencing parametersthat can be changed during the course of testing. Only littlechanges in suchparameters as surface properties, load, lubrication, atmosphere, materialcomposition etc. can cause drastic changes to tribologicalprocess. It shouldbe emphasized that the nitrogen alloyed compositions were produced byexperimental procedures and therefore they are likely to have inhomogenitessuch as inclusions in the structure [2].


CONCLUSIONS

Nitrogen alloying has a positive effect on galling resistance of PM toolsteels.

A high content and fine distribution of carbonitride phase inthe toolsteel structure can increase the galling resistance to samelevel as incoated tool materials.

REFERENCES

[1] E. SCHEDIN, Micro-mechanics of sheet-tool contact in sheet metal forming. PhDThesis, Royal Institute of Technolgy, 1991.

[2] S. JACOBSON, S. HOGMARK, in Tribologi- friktion, sm'örjning, och n'ötning (LiberUtbildning AB, Arl'öv, 1996) pp.5–12, 158–159.

[3] K. HOLMBERG, A. MATTHEWS, H. RONKAINEN, Finnish Journalof Tribology17(1998), No. 3–4, pp. 6–22.

[4] C. McFADDEN, C. SOTO, N. D. SPENCER, Tribology International 30(1997), No.30, pp.881–888.

[5] N.K. MYSHKIN, M.I. PETROKOVETS, S.A. CHIZHIK, Tribology International31(1998), No.1–3, pp. 79–86.

[6] S. SHEU, L. G. HECTOR, O. RICHMOND, Journal of Tribology 120(1998), pp. 517–527.

[7] A. WIHLBORG, H. THOORS, L. GUNNARSSON, Optimisation of sheet metal sur-faces with respect to friction. Swedish Institute of MetalsResearch, Report IM-2000-023.

[8] U. ENGEL, M. PFESTORF, M. GEIGER, Wear (Switzerland) 216(1998), No. 2, pp.244–250

[9] T. ARAI, Y. TSUCHIYA, Role of carbide and nitride in antigallingg property of diematerials and surface coatings. In: Metal Transfer and Galling in Metallic Systems.AIME, 1987, pp.197–216.

[10] Y TSUCHIYA, T, ARAI, S. SHIMA, Journal of the Japan Society for Technology ofPlasticity (Japan) 38(1997), No. 433, pp.31–35.

[11] K. MISYOSHI, Adhesion, friction, and wear behavior of clean metal-ceramic couples.In: International Tribology Conference. NASA 1995, pp.1–6.

[12] V.G. GAVRILJUK, H. BERNS, High Nitrogen Steels. Springer, Berlin, 1999. pp. 203–217.

[13] J. SUNDSTR'ÖM, D. EDMAN, Ammonia nitriding of Fe-7% Cr alloy powder. SwedishInstitute of Metals Research, Report IM-2001-036.


[14] / E. VAN DER HEIDE, B. HUIS IN T VELD, Lubricant selectionfor cold sheet formingof stainless steel. In: IDDRG meeting. Birmingham 1999. pp.1–9.

[15] K. V'ÁRADI, Z. N'ÉDER, K. FRIEDRICH, Wear 200(1996), pp. 55–62.

[16] R.M. MATVEEVSKII, Tribology International 28(1995),No.1, pp.51–54.

[17] S. KATO, E. MARUI, M. HASHIMOTO, Tribology Transactions 41(1998) No. 3, pp.341–349.


Figure 11. Ranking of the galling resistance of the test materials.

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