Wear of aluminium bronze in sliding contact with lubricated stainless steel sheet material

Wear of aluminium bronze in sliding contact with lubricated stainless steel sheet material

Wear 261 (2006) 68–73 Wear of aluminium bronze in sliding contact with lubricated stainless steel sheet material E. van der Heide a,∗ , E.D. Stam a ,...

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Wear 261 (2006) 68–73

Wear of aluminium bronze in sliding contact with lubricated stainless steel sheet material E. van der Heide a,∗ , E.D. Stam a , H. Giraud b , G. Lovato b , N. Akdut c , F. Clarysse c , P. Caenen d , I. Heikill¨a e a

TNO, PO Box 6235, 5600 HE Eindhoven, The Netherlands Arcelor, Ugine&ALZ, PO Box 1, 71130 Gueugnon, France c Arcelor, OCAS, J. Kennedylaan 3, B-9060 Zelzate, Belgium d Arcelor, Ugine&ALZ, Genk-Zuid, Zone 6A, B-3600 Genk, Belgium e SIMR, Drottning Kristinas V¨ ag 48, SE-11428 Stockholm, Sweden b

Accepted 13 September 2005 Available online 3 November 2005

Abstract Aluminium bronze, well known for its good sliding properties, is frequently applied as tool material in sheet metal forming (SMF) of stainless steel, e.g. for the production of washing, refrigeration and cooking equipment. The limited hardness of the material makes it, however, sensitive to tool wear that is: volumetric wear of the tool due to sliding contact with the sheet material. Conventional wear tests like the rubber wheel abrasion test or the Taber abrader test cannot be used to simulate the interaction of the tooling with lubricated sheet material. Dedicated tribo tests are therefore conducted with the slider-on-sheet test. The aim of the research is to measure the specific wear rate of aluminium bronze at SMF-like conditions. Experimental results showed a pronounced influence of lubricant selection and sheet material selection. The measured specific wear rate varied from 10−8 mm3 /N m for a smooth stainless steel sheet quality to 10−6 mm3 /N m for a rough surface quality. © 2005 Elsevier B.V. All rights reserved. Keywords: Lubricated wear; Abrasive wear; Sheet metal forming; Slider-on-sheet tribometer

1. Introduction

wear rate k by Eq. (2).

Aluminium bronze, well known for its good sliding properties, is frequently applied as tool material in sheet metal forming (SMF) of stainless steel, e.g. for the production of washing, refrigeration and cooking equipment. The limited hardness of the material makes it, however, sensitive to tool wear that is: wear of the tool due to sliding contact with the sheet material. Volumetric wear of a forming tool, is first visible at spots with high local pressure, such as exists at the die radius in an axisymmetric deep drawing process. Combined experimental and FEM work [1] suggests that wear of a drawing die can be modelled with an Archard type of wear equation, here presented in a non-dimensional form [2] by Eq. (1) and in terms of the specific

Kw =



Corresponding author. Tel.: +31 40 265 0421; fax: +31 40 265 0302. E-mail address: [email protected] (E. van der Heide).

0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.09.023

VHsoft sFn

k (mm3 /N m) =

(1) V sFn

(2)

where Kw represents the coefficient of wear, V the wear volume, s the sliding distance, Fn the applied normal force and Hsoft the hardness of the softest contact partner. Eq. (1) suggests that volume loss of a forming tool is inversely proportional to the hardness of the tool material. Conventional (abrasive) wear tests like the rubber wheel abrasion test or the Taber abrader test, confirmed that the amount of abrasive wear decreases as hardness increases, see for example Table 1 in which the specific wear rate is given for some common tool materials. The presented absolute values are in the severe wear regime, k = 10−5 to 10−3 mm3 /N m. Conventional wear tests cannot be used to simulate the interaction of the tooling with lubricated sheet material. Near future

E. van der Heide et al. / Wear 261 (2006) 68–73

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Table 1 Specific wear rate for three ductile cast iron grades, aluminium bronze (see Section 2.1 for details) and tool steel WN 1.2379 (61 HRC), measured with a rubber wheel abrasion test similar to ASTM G65 at Fn = 100 N using quartz sand as abrasive (900 ± 50 HV, grain size 0.2–0.5 mm, rounded shape) [3]

k (10−6 mm3 /N m)

GGG 40

GGG 60

GGG 70L

Al. bronze

WN 1.2379

1357

1030

859

356

46

changes, such as the introduction of chlorine-free lubrication and the introduction of new sheet surface textures can therefore not be addressed with these methods. In order to measure the specific wear rate of aluminium bronze at SMF-like conditions, a series of dedicated tribo tests were performed using the TNO1 slider-on-sheet tribometer [4]. Previously gained test results [5] with this method on zinc coated deep draw steel showed that the specific wear rate of a soft tool material could change two orders of magnitude as a function of the sheet’s surface quality. Furthermore it was shown that the results agreed very well with results gained from an industrial set-up using the same tool and sheet materials. 2. Experimental 2.1. Materials Experiments were conducted on 1.0 mm austenitic stainless steel AISI 304 finish 2B, 0.5 mm ferritic stainless steel AISI 430, finish Bright Annealed (BA) and finish Electro Discharge Texturing (EDT), and 1.0 mm ferritic stainless steel AISI 409, finish 2D. The nominal composition of the sheet materials is given in Table 2. Sheet material AISI 304 2B was delivered in two batches; batch I and batch II (see Table 3). The other materials were delivered and taken from one batch of the specific sheet material. Roughness measurements with a Mitutoyo Surf test 301 SJ mechanical stylus were conducted to get indicative values for the sheet surface roughness. The presented values in Table 3 should be used carefully, keeping in mind likely variations in roughness from sheet to sheet and variations as a function of the angle to the rolling direction. The presented results in Table 3 point at a difference between the two batches of AISI 304 2B. The tribological experiments were performed parallel to the rolling direction of the sheet. Table 3 also shows the indicative hardness values for the sheet material, measured at a cross-section of the sheet. The sliders used for the experiments were made out of a frequently used aluminium bronze [6]. The selected aluminium bronze alloy consisted of 6.1 wt.% Fe, 15.2 wt.% Al and 2.1 wt.% Mn, balanced by copper. It is produced with a matrix hardness of 400 HV0.2 . Local hard phases (580–680 HV0.02 ) were visible after etching with a FeCl3 /HCl solution, see Fig. 1. The sliders were polished with abrasive paper to a resulting Ra of 0.03 ␮m parallel and 0.03 ␮m perpendicular to the sliding direction. The Ra values were calculated based on a cut-off length of 0.25 mm and a measuring length of 0.75 mm because of the sharp 1

TNO: Netherlands Organisation for Applied Scientific Research.

radii. The dimensions of the slider were Ø 44 mm × 8 mm with a radius of 6 mm perpendicular to the sliding direction. The lubricants, five in total, were all delivered by Quaker Chemical BV. Draw 42 (K), a high performance lubricant for heavy cold forming applications was taken as reference lubricant. The chlorine content of Draw 42 (K) is below 35 wt.% and it does not contain sulphur. Lubricants Draw 140 EL, Quaker 7000 RN and Draw 220-VP were selected as alternative, chlorine-free lubricants. Draw 140 EL is described as a high performance deep drawing lubricant. It consisted of a blend of synthetic esters, mineral oil and additives. The chlorine content was below 0.05 wt.%, the sulphur content below 0.3 wt.% Draw 220-VP is a relative new drawing fluid, composed of a blend of vegetable polymers. The 7000 RN lubricant is frequently used as cutting and grinding fluid. It is a mixture of mineral oil, water, soap and additives. Table 4 gives the nominal viscosity η and density ρ of the lubricants. All sheets were cleaned with acetone before applying the lubricant to the sheet with a roller. The amount of lubricant used was 4–5 ml/m2 . 2.2. Test method Wear resistance of forming tool materials was tested with a model wear test, using the TNO slider-on-sheet tribometer. A detail of the tribometer is given in Fig. 2, showing the sliding contact between the double curved ring and sheet material. The slider is pushed against the sheet material with a normal force Fn and makes a track with a length l on the sheet, with a velocity v. Then the slider is lifted from the sheet and returns to the starting line. The slider moves 1 mm perpendicular to the sliding direction and makes a new track. During a track, the normal force and the resulting friction force are measured. The tribometer is described in detail in [4].

Fig. 1. Light microscopy image of aluminium bronze tool material after etching. Local hard phases are visible as dark areas.

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Table 2 Nominal composition of the sheet materials (wt.%) Sheet materials

C

Si

Mn

P

S

Cr

Ni

Ti

AISI 304 AISI 430 AISI 409

≤0.08 ≤0.12 ≤0.03

≤1.00 ≤1.00 ≤1.00

≤2.00 ≤1 ≤1

0.045 0.040 0.040

≤0.03 0.03 0.015

18.0–20.0 16.0–18.0 10.5–12.5

8.00–10.5 0 –

– – ≤0.65

Table 3 Indicative roughness (␮m) and hardness values for the sheet materials Sheet materials

Ra

Rq

Ry [DIN]

Rz [DIN]

Rt

Rp

HV1

AISI 304 2B, batch I AISI 304 2B, batch II AISI 430 BA AISI 430 EDT AISI 409 2D

0.28 0.10 0.04 2.33 0.15

0.39 0.15 0.05 3.01 0.21

2.99 1.54 0.44 17.56 2.76

2.46 1.24 0.31 13.65 1.66

3.07 1.62 0.45 18.42 2.87

0.61 0.28 0.15 5.25 0.9

172 203 157 165 121

Table 4 Density and viscosity of the selected lubricants

15 ◦ C

(g/cm3 )

Density at Viscosity at 40 ◦ C (cSt) Viscosity (Pa s)

Draw 42

Draw 42 K

7000 RN

Draw 220-VP

Draw 140 EL

1.171 490 0.574

1.120 120 0.134

1.010 71 0.072

0.930 220 0.205

0.885 162 0.143

After the experiment, the semi-axes of the contact ellipse were determined in the direction of sliding (a) and perpendicular to the direction of sliding (b) by profilometry. The volume (V) removed by the sliding action is estimated using Eq. (3), assuming the slider wears off flat, in which the product π·a(γ)·b(γ) represents the worn area at a certain worn height γ, laying between zero and the actual height of the worn volume (h). In this case R1 = 22 mm and R2 = 6 mm. The height h of the wear volume at centre of the scar, follows from the measured semi-

axis’ and the geometry of the slider.    h a(γ) = 2R1 γ − γ 2  V =π a(γ) · b(γ) dγ, b(γ) = 2R2 γ − γ 2 0

(3)

The parameters used for the slider-on-sheet experiments were based on previous research conducted in collaboration with industry. The experiments were performed at a normal force Fn of 100 N and a sliding velocity v of 0.5 m/s. These conditions correspond to critical contact situation in sheet metal forming such as exist at the radius of an deep drawing die or at the radii of draw beads [4,5]. 3. Results and discussion

Fig. 2. Detail of the TNO slider-on-sheet tribometer.

Austenitic stainless steel AISI 304 2B lubricated with Draw 42 K and processed with aluminium bronze tooling, represents a tribo system that is frequently used in metal forming applications that are sensitive to galling. Typical slider-on-sheet friction data for this combination are given in Fig. 3. Friction started at a relative low level and decreased as a function of the sliding distance from a coefficient of friction f = 0.08 to a value of f = 0.03 at 1500 m sliding distance. Inspection of the slider revealed an elliptically shaped wear scar with a = 0.665 mm and b = 1.315 mm. The specific wear rate for the aluminium bronze slider is 3 × 10−7 mm3 /N m, which is in the mild wear regime. This corresponds well with industrial practise, where a specific wear rate of 1 × 10−6 mm3 /N m is generally regarded as an upper limit for engineering surfaces in sliding contacts.

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Fig. 3. Coefficient of friction as a function of the sliding distance for aluminium bronze in combination with AISI 304 2B lubricated with Draw 42 K. Fn = 100 N, v = 0.5 m/s.

Profile measurements on the wear scar of the aluminium bronze slider showed a wear pattern with grooves parallel to the direction of sliding, as depicted in Fig. 4. Scanning electron microscopy (SEM) confirms the presences of grooves, and shows small lumps of tool material near the edges, see Fig. 5. Scratches are visible in Fig. 5, up to a width of 5 ␮m. The mechanism of wear, although not fully understood by the authors, is most likely related to a ploughing action of (deformed) sheet asperities. High contact pressures at asperity level can work harden the stainless steel surface and increase the hardness of the deformed roughness plateaus. The thin hard layer of chromiumoxides, present at the stainless steel sheet surface, could further increase the severity of the sheet’s ploughing action. Yet, more work is required to verify this. Wear of the slider clearly implies a non-constant contact pressure during the experiment. The average contact pressure pavg during the experiment given in Fig. 3 decreases from the 500 MPa range at the start of the experiment, see [5], to about 8 MPa at the end of the experiment. The reduction in pressure could well explain the differences in friction between the start and the end of the experiment if one assumes mixed lubrication conditions both at the start and at end of the experiment. Wear of the slider successively increases the hydrodynamic action of the lubricant film and moves the tribological

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Fig. 5. SEM image of the wear scar on the aluminium bronze slider showing grooves parallel to the direction of sliding and small lumps of tool material near the edges of the grooves.

system to higher H = ηv/pavg values, which, with a Stribeck curve in mind, generally corresponds to lower friction levels. Wear of the slider further implies that the edge of the wear scar next to the previously made track on the sheet, will be in contact with run-in sheet material if the half width of the contact ellipse becomes larger than 0.5 mm. For the experiment described in Fig. 3 this was the case at a sliding distance of 533 m (assuming constant volumetric wear as a function of the sliding distance). Such contact can be avoided by increasing the distance between the tracks from the presently used 1 mm to, e.g. 2 mm. The introduction of chlorine-free lubrication is expected to affect the specific wear rate measured for the aluminium bronze slider. Experiments were performed at standard conditions for the selected alternative lubricants. The lifetime was in all cases limited by volumetric wear of the tool material. No signs of galling were detected within the maximum sliding distance of 1500 m. Corresponding specific wear rates are given in Table 5. Table 5 shows that the wear rate of the aluminium bronze slider varied more than a factor 10 as a function of the selected lubricant in sliding contacts for AISI 304 2B. Draw 42, the viscous chlorinated reference lubricant, performed quite well both in absolute terms (k < 1 × 10−6 mm3 /N m) as in relation to the

Fig. 4. Detail of the wear scar on the aluminium bronze slider.

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Table 5 Specific wear rates for the aluminium bronze slider in combination with lubricated AISI 304 2B sheet and lubricated AISI 430 BA sheet material Lubricant

k-value 10−6 mm−3 /N m AISI 304 2B Batch I

140-EL 140 EL 42 42 K 7000 RN 220-VP

0.95 1.11 0.19 0.33

k-value 10−6 mm−3 /N m AISI 430 BA

Batch II

0.09 2.8 2.3

0.051 0.065 Not measured 0.020 0.091 0.094

other lubricants. Lubricant 7000 RN and 220-VP did not sufficiently protect the aluminium bronze slider surface in case k = 1 × 10−6 mm3 /N m is taken as upper limit. The wear rate in combination with lubricated AISI 430 BA varied less than a factor 5 as a function of the selected lubricant. Draw 42 K performed best, both in absolute terms (k  1 × 10−6 mm3 /N m) as in relation to the other lubricants. Lubricant 140-EL, 7000 RN and 220-VP also sufficiently protected the aluminium bronze slider surface. An additional experiment was performed to measure the effect of the batch differences for AISI 304 2B. The specific wear rate measured with this batch in combination with Draw 42 K was significantly lower that is: 0.09 × 10−6 mm3 /N m. From the results with Draw 140 EL it is concluded that an acceptable scatter in calculated k-values exist for measurements performed within one batch of sheet material. Caution is, however, required when multiple batches are used for the experiments. Straightforward application of mixed lubrication theory [8] predicts a larger separation of two surfaces in sliding contact, in case a more viscous lubricant is selected. It seems fair to assume that the specific wear rate of aluminium bronze will decrease with increasing lubricant viscosity, since separation will prevent contact between sheet and tool surface asperities. The results with 7000 RN and Draw 42, with viscosities of, respectively, 72 and 574 mPa s, confirm this hypothesis. A closer look, however see, e.g. Fig. 6, reveals that this relation cannot be

Fig. 7. Worn volume as a function of the product of normal force and sliding distance. The slope of the trend line, drawn using linear regression, corresponds with the specific wear rate: k = 0.7 10−6 mm3 /N m for 2D and k = 1 × 10−6 mm3 /N m for EDT. Fn = 100 N, v = 0.5 m/s, lubricant Draw 42 K.

generalised. Reducing the dynamic viscosity of Draw 42 from 574 to 134 mPa s, as in Draw 42 K, does not change the specific wear rate of aluminium bronze significantly. A possible explanation for this can be found by taking into account the additive package of the lubricants, which is dominated for both lubricants by chlorine containing compounds. Although the viscosity of the lubricant is adapted, one can expect that the same type of boundary layers are present in the sliding contact. The experimental results suggest that boundary layers created in the sliding contact between sheet and tool surface by (anti-wear) additives can compensate for the effect that reduced viscosity has on the specific wear rate. The specific wear rate measured in combination with lubricated AISI 430 BA was much lower than the specific wear rate measured in combination with lubricated AISI 304 2B, for all the selected lubricants. This difference could possibly be related to the roughness of the sheet materials. Furthermore it is demonstrated by the experiments with AISI 304 2B from batch I and II that the wear rate was lowest for the smoothest batch. More experimental evidence for the suggested relation between the specific wear rate and sheet roughness is given in Fig. 7, which summarises the results of experiments with Draw 42 K lubricated AISI 430 material with finish EDT and with Draw 42 K lubricated AISI 409 material with finish 2D. Wear was clearly affected by the surface finish of the sheet material. Finish 2D, a factor 3–4 rougher in terms of Ra compared to the BA finish, increased the specific wear rate of the tool from 10−8 to 10−7 mm3 /N m. Measurements with finish EDT, a further increase in roughness by a factor 15 in terms of Ra , again resulted in an increase in specific wear rate, that is to 1 × 10−6 mm3 /N m. Fig. 7 also indicates that the wear volume was proportional to the product of sliding distance and normal force. The same relation was found for slider-on-sheet experiments on zinc coated steel and soft tool materials [5]. 4. Conclusions

Fig. 6. Specific wear rates for the aluminium bronze slider in combination with lubricated AISI 304 2B as a function of the dynamic viscosity of the selected lubricant.

• The selected aluminium bronze tool material was sensitive to volumetric wear in sliding contact with lubricated stainless steel sheet material.

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• The absolute values found in the experiments with the slideron-sheet test were in the mild wear regime, k = 10−8 to 10−6 mm3 /N m, and correspond better with metal forming applications than conventional (abrasive) wear tests with wear rates in the order of 10−5 to 10−3 mm3 /N m. From the results it is concluded that an acceptable scatter in calculated kvalues exists for measurements performed within one batch of sheet material, but that caution is required when comparing values for a sheet material type taken from different batches. • Experimental results showed a pronounced influence of lubricant selection and sheet material selection on the measured specific wear rate. Replacement of the chlorinated reference oil Draw 42 by one of the selected alternative lubricants is expected to increase tool wear for aluminium bronze in forming applications of ferritic stainless steel AISI 430 BA and in forming applications of austenitic stainless steel AISI 304 2B. Tool wear is also expected to increase in case of the introduction of rough sheet surfaces. The measured specific wear rate varied from 10−8 mm3 /N m for a smooth stainless steel sheet quality to 10−6 mm3 /N m for a rough surface quality.

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Acknowledgement This work is done within the framework of the ECSC project ‘High volume forming of stainless steel with easy to clean lubricants’, contract number 7210-PR-307. References [1] M. Eriksen, T. Wanheim, Wear optimisation in deep drawing, in: K. Dohda, T. Nakamura, W.R.D. Wilson, (Eds.), Proceedings of the first International Conference on Tribology in Manufacturing Processes ’97, Gifu, Japan, 1997, pp. 128–133. [2] M.C. Shaw, Dimensional analysis for wear systems, Wear 43 (1977) 263–266. [3] H.M. Slot, A.J. van der Borden, Abrasive wear resistance of selected forming tool materials, TNO report Div400, 2001, p. 1065. [4] E. van der Heide, A.J. Huis in ’t Veld, D.J. Schipper, The effect of lubricant selection on galling in a model wear test, Wear 251 (2001) 973–979. [5] E. van der Heide, M. Burlat, P.J. Bolt, D.J. Schipper, Wear of soft tool materials in sliding contact with zinc-coated steel sheet, J. Mater. Process. Technol. 141 (2003) 197–201. [6] http://www.ampcometal.net/metal/produits.htm. [8] E.R.M. Gelinck, D.J. Schipper, Calculation of Stribeck curves for line contacts, Tribol. Int. 33 (2000) 175–181.