A NEW LOOK AT MICRO WEAR IN CAST IRON BRAKE ROTORS

G. Miskinis and W. Powell (retired)ThyssenKrupp Waupaca, Waupaca. WI, USA

Copyright © 2009 American Foundry Society

Abstract

Wear in cast iron is generally a macro phenomenon,measured in grams of weight loss or millimeters of thicknesslost in a test specimen. Traditional wear test methods employloads that are relatively high and of a short duration, in theautomotive industry NVH (Noise. Vibration and Harshness)is a primaiy concern with automotive driveline and chassiscomponents. Recent NVH issues on some specific vehicleswere determined to be caused by brake rotor thicknessvariation as observed during actual on-road tests after a shortperiod in service. Changes in brake rotor thickness wereobserved in isolated areas of the brake plates during routinedynamometer testing of disc brake rotors. A new test wasdeveloped to determine the cause of the thickness variations.Numerous studies were conducted to reduce what was first

thought to be thermal growth. Thermal growth reduction wasinvestigated by looking at casting residual stresses, pearlitedecomposition reactions, elevating Martensite transitiontemperatures, pearlite spacing, matrix hardness, and alloylevels affecting all of these. Micro wear was determined tobe the cause for brake rotor thickness variation. The microwear was in turn found to be a function of the graphite densitydifferences around the rotor brake plate diameter caused bylocalized cooling rates. Casting process simulation studieswere conducted to determine the effects of gating .systems oncooling profiles. Changes were made to the gating system,minimizing the cooling rate differences.Keywords: wear, NVH (Noise. Vibration and Harshness),residual stress, thickness variation

Introduction

To prove useful in any application, a material needs to havespecific and measurable properties, which can be both un-derstood and altered. Wear is one such property in cast iron.Standardized tests allow engineers to quantify the abilityof cast iron to withstand frictional forces applied tlirough asecond material. Cast Iron being very heterogeneous in itsnature provides a complex structure with soft graphite, softferrite, varying degrees of pearlite hardness and a vast arrayof carbide structures.

Standard wear tests place a test piece in contact with a sec-ond material through a predetermined load. Usually one ofthe two materials is rotated and the other held stationary forthe duration of the test. The material under investigation is

typically weighed before and after the testing period withthe results reported as the difference of the before and afterweights, or weight loss. At best, the test allows only a rank-ing of materials and is not a prediction of in-service perfor-mance. To compensate for this shortfall, most engineeringgroups have developed component or application specificwear tests that can take into account in-service parameterslike geometry, mating materials, temperature, environment,loads and application cycles, etc. With these specific weartests, more useful data can be obtained and service life canbe accurately estimated.

Automotive brake rotors are subjected to more or less in-dustry-standardized tests. The On-Brake Disc ThicknessVariation (OBDTV) is an automotive wear test that utilizesan actual vehicle specific brake rotor design coupled with

inboard Runoitf Outtward Rwiout - ;

0 90

Figure 1. Disc thickness variation plot.

International Journal of Metalcasting/Fall 09 59

the matching brake caliper and brake friction pads for thatvehicle. Brake forces are also specific to the application tobe tested. Rather than measuring weight loss of the brakerotor before and after the test, the change in brake surfacelocation is measured for each brake surface. Before begin-ning the test the as-machined surface profile is measuredand results graphed. The plotted x-axis is 360 degrees ofrotor radius and the y-axis is the position of the brake platesurface measured in microns. The predominant testingmethod is to conduct 6500 brake events and conclude thetest. A second set of measurements is made and graphedon the original plot. Figure I is a typical test report. Theupper heavy line is the inboard facing brake plate surfaceand the lower heavy line is the outboard facing brake sur-face. If the lines remain parallel then only lateral runout isnoted. If the lines converge then there has been localizedwear, if the lines diverge, then material growth or DTV"Bump" is observed. Some lateral runout is normal and aproperly functioning brake and caliper can accommodatesome degree of warpage.

Dynamometer test data has been correlated to acceptablevehicle performance and a maximum amount of thick-ness variation in the test rotor has been long established.As light trucks have been increasingly utilized as eitherthe second vehicle in a family, or in many households.the primary family vehicle, suspension systems haveevolved to yield a more auto-like ride. The demand forimprovement in ride quality and the need to cope withthe increasing weight of accessories and comfort featurescreated conditions where the driver has more feedbackfrom anomalies in the braking system. The traditionallyaccepted rotor thickness change or lateral runout no lon-ger provided an acceptable gauge of suspension systemperformance was the conclusion of the automotive en-gineering group. Brake disc thickness variation neededto be reduced. Wfiat was unknown was what the actualcause of the variation in thickness was. Was the changein thickness caused by residual stress induced distortion,elevated temperature growth, localized wear, or micro-structure variation? A series of programs were initiatedto determine the cause of rotor dimensional changes.

DTV Causes Explored

One potential cause of thickness variation was unequal wearrates around the rotor caused by either a stress related phe-nomenon and the associated distortion and associated wearfrom rubbing brake pads called Off Brake Wear. Consider-ing the first case, all castings solidify with some degree ofresidual stress: this is especially true of disc brake rotors. Adisc brake rotor is composed of a series of rings and cylin-ders with varying diameters and wall thicknesses. After so-lidification of the metal, solid-state linear contraction beginsas the casting cools to room temperature. Adjoining sec-tions will cool at different cooling rates and produce differ-ent final dimensions, inducing a variety of residual stressesthroughout the casting. Riser and ingate attachment areasfurther alter the local stress state by influencing the coolingrate of the section they are attached to.

Two possible solutions for reducing casting residual stresswere investigated. The first method applied a conventionalstress relief heat treatment cycle to the casting. The stressrelief treatment used a continuous belt, temperature zonecontrolled furnace under a Nitrogen atmosphere. The cycleconsisted of an initial 60 minute ramp up to 566°C ( 1050°F),followed by a hold at that temperature for 90 minutes andconcluded with a \2\°C/\\T (250°F^r) controlled cool to149° C (300°F) at furnace exit to 21°C {70°F) still air. Ap-plying a stress relief cycle to the rotor would have been theleast disruptive to the foundry process, but greatly affectedthe machining process fiow in terms of transportation, inven-tory management and significantly increased the final cost.Stress relief trials were conducted on both raw and partiallymachined castings: neither method resulted in a reduction inthickness variation.

The second stress control method investigated was the in-troduction of specific alloys to the iron. Additions of up to0.50% Chromium, 0.50% Copper and 0.50% Molybdenumraise stress relaxation temperatures. By adding above al-loys alone or in combination, it was thought to be possibleto raise the stress relaxation point well above the normalpeak operating temperature range of 427 to 538'̂ C (800 to

Riser

In-GatesCasting ga&iglocations

Figure 2. Position dependent disc thickness variation.

60 International Journal of Metalcasting/Fall 09

1000°F) under severe disc brake fade conditions and avoidthe associated dimensional change. Under normal drivingconditions, the normal operating temperature is seldomabove 260°C (500°F). The addition of alloys was againless invasive to the casting process, but added significantmanufacturing costs in terms of the alloys added and thereduction in machine tool life as a result of the higher cast-ing hardness. The increases in physical properties andperceived associated wear resistance provided by highermatrix hardness produced no significant reduction in discihickness variation.

Another potential growth phenomena examined was ther-mally induced microstructure change. Two possible mech-anisms were considered. The first was the phase changeand subsequent volume change that occurs when Pearlitedecomposes to Ferrite. This reaction can be retarded byfirst ensuring a stable Pearlile structure is created with asuitable alloy addition of Copper or Chromium. Elevatedtemperature stability of Pearl ite could also be accomplishedby tbe addition of Molybdenum. Tbe use of Molybdenumfor this purpose is well documented in tbe literature.' -Tbesecond source of thermally induced growth was from theformation of Martensite. which occurred after an elevatedtemperature excursion above the Austenite formation tem-perature, followed by rapid back-quenebing provided by

tbe lower temperature of tbe bulk of the brake disc. TheMartensite phase occupies more space than the Pearlitefrom which it has transformed. This is tbe usual initia-tion of tbe tbermal cracking found on brake drums and ro-tors.' Alloys serve to reduce the MS (Martensite Start)Temperature; therefore the introduction of alloys would beconducive to reducing growth by this mechanism. Dyna-mometer testing demonstrated that neither tbe addition, northe subtraction of alloy affected OBDTV.

It was concluded that tbe change in tbickness was prob-ably a wear issue ratber tban a thermal growth issue. Wearoccurred over most of the rotor, but not at selected, local-ized positions. Close observation of numerous test plotsshowed tbat tbe major ebange in rotor thickness occurredusually only in one location. Correlating the test reportplot to the casting orientation in tbe mold with respect tothe gating system revealed tbat an area of non-wear primar-ily occurred in tbe area of tbe riser and to a lesser degreewhere ingates were positioned. Figure 2 shows this clearlywhen notations of gating position are added to the radialdirection of [he graph. At the riser location a thickeningof the rotor is noted. To a lesser degree each ingate can bealso observed as a thickening of tbe rotor. Tbe observedchange of dimension was named. Position Dependant DiscTbickness Variation.

Figure 3. Tradltionai riser simulation of graphite coarseness.

2mm

0

•

10mm

1

Measurement Locations Sample Preparation

Figure 4. Microstructure evaluation areas.Computar Density Calculation

International Journal of Metalcasting/Fall 09 61

Considering that both flake graphite fonnation and castinggating systems are interrelated thermal events, solidificationanalyses were performed. Figure 3 clearly shows that therotor outer diameter is not thermally symmetrical just aftersolidification. The riser, and to a lesser extent the in-gates,can clearly be seen to fonn a coarser structure analogouswith higher temperature and subsequently lower cooling ratethan the majority ofthe rotor outer diameter.

The in-gates and the riser thermally functioned as theywere intended to do. The Ín-ga!es heated the sand dur-ing the pouring period and the riser stayed open for di-rectional solidification of the casting. Both areas werenow hotter than the remainder of the rotor perimeter. Thehigher temperature and lengthened solidification time al-lowed for greater graphite flake growth. This phenom-enon has long been recognized as a source of physical

Table 1. Traditional versus ModifiedRiser Graphite Flake Density

SAMPLELOCATION

1:002:003:004:005:006:007:008:009:0010:0011:0012:00

AVERAGEMINIMUMMAXIMUM%ABOVE%BELOW

TRADITIONAL RISERFLAKES/MM^

INBOARD2803073172983043023153193203 U290228300228320

6.8%23.9%

OUTBOARD260302321288292274291277291312257214282214321

14.0%24.0%

MODIFIED RISERFLAKES/MM^

INBOARD300295322294312315320308331325328334315294334

5.9%6.8%

OUTBOARC275309326329311283308290272308295292300272329

9.7%9J%

Graphite Flake DensityTraditional Riser Gating

500

450

400

350

300

250

200

150

100

50

-B

OB

1 2 3 4 5 6 7

Cloclc Position

Figure 5. Traditional riser graphite flake density.

10 11 12

62 International Journal of Metalcasting/Fall 09

property variations at riser locations, usually manifestedas lower tensile strength than as seen in the balance of thecasting. In brakes, the small variation in graphite size orflake density was not previously deemed detrimental toperformance based on the typical service cycles the partwas subjected to.

Sections were cut from 12 evenly spaced segments aroundihe rotor brake plates. A standard depth of 2mm (0.080") be-low the as-cast surface and a radial distance of 10mm (0.8")from the casting outer diameter were selected for evaluation.The microstructure sample orientation and locations are il-lustrated in Figure 4.

Detailed microstructure investigations were made with theaid of computerized image analysis systems. The programshad the ability to analyze and characterize the graphitestructure in terms of flake length (tip to tip or serpentinelength), count per unit area (density), perimeter and area.Most outputs provided some reasonable correlation, but forease of data collection and speed. Graphite Density provedto be the most useful calculation. The graphite density wasexpressed in terms of Flakes per Square Millimeter. Asshown in Table I. the differences between each locationcan be expressed in the simple relationship of the percentdifference of flake density between any two locations.Graphing the resulting flake density clearly shows a cor-

Figure 6, Modified riser simulation of graphite coarseness.

Graphite Flake DensityModified Riser Gating

500

450

400

350

"c 300

250

200

150

100

50

BOB

5 6 7 8

CkKk Position10 11 12

Figure 7, Modified riser graphite flake density.

International Journal of Metalcasting/Fall 09 63

relation between disc thickness variation caused by unevenwear and fiake density, as shown in Figures 2 and 5.

Changing the .solidification pattern proved much more diffi-cult than understanding why wear occurred. Risers performa variety of functions beyond supplying feed metal. This isespecially true with vertically poured molds. Risers bleedoff colder iron, pass mold gasses to the mold exterior, bleedoff bits of molding sand and core debris and allow slag toflow clear of the mold cavity. To simply not riser a moldwas not an acceptable answer. Again casting process simu-lation proved to be a valuable tool. With numerous itera-tions of different riser connections, one could simply lookat the thermal profile just after solidification to determine ifany thermal imbalance was evident. Figure 6 is a Magmathermal profile of a brake rotor with a modified riser contact.In this case, the convention live-feed riser (riser filled fromgating system) was replaced by an out-flow riser of reducedmodulus. The modified riser and contact has nullified thelocalized thennal effect of the previous riser arrangement.Note that there is very little difference in predicted graphitecoarseness around the rotor perimeter.

Multi-location microanalysis proved that graphite densitywas very uniform wilh less than 20% variation in graphitedensity as shown in Table I. Long duration dynamometerwear tests confirmed no discernable differences in wearwere detected. A comparison of the traditional riser ex-ample in Figure 5 and the modified riser example shownin Figure 7 illustrated the improvement in graphite flakedensity. The effect of the reduction in graphite flakedensity is illustrated in Figure 8 in which the rotor test-ed generated no significant disc thickness variation. Thepercent variation in graphite density is negligible aroundthe entire rotor. In the absence of evidence of prolongedoperation at elevated temperature witnessed by matrixphase Iransformation or lack of positive response to stressrelaxation, it is readily apparent that larger flake densitieswere responsible for the higher wear rates in select areas.

This effect was consistent with the observations made byLeach and Borland while investigating the unlubricatedwear of gray iron^

Conclusions and Final Comments

Brake roughness due to On-Brake Disc Thickness Variationis a selective wear phenomenon that has been proven to beposition dependent due to the influence of traditional gat-ing elements, namely risers and ingates. Through the use ofcasting simulation, an accurate assessment of the effect ofgating elements on the resulting graphite structure or coarse-ness can be made. Overall variation in Graphite Flake Den-sity was found to be a useful tool in predicting the propensityof OBDTV or "Bump" generation.

The Japanese brake company Kiriu carried out an indepen-dent and parallel study of disc brake thickness variation. Dr.T. Okamura reported to the Japanese Society of AutomotiveEngineers that "non-uniformity of material properties, dueto the different cooling rates with a disc during solidifica-tion in the casting process, caused local wear and increasedDTV (disc thickness variation)"."' Kiriu also developed botha rapid wear test, casting design strategy and gating systemto reduce graphite density variations within a rotor, but werenot detailed in the paper.'' A Flake Graphite Structure In-dex (K-FGI) evaluation method developed by the authorsrequired an extensive analysis consisting of 240 evaluationsper braking surface. Although the exact test location, allow-able degree of variation, casting design changes and gatingwere not defined in the work, it is apparent that commongoals were sought. It is the author's opinion that the com-bined use of casting process simulation and a similar, butless intensive graphite evaluation is a useful tool in predict-ing OBDTV potential of a brake disc.

Acknowledgements

Figures 1,2,4 and 8 courtesy of the Robert Bosch Corporation.

Intioard Runout Outboard Runout —i

a.aÖ

0 90 Degrees

Figure 8. Modified riser disc thickness variation.370 3S0

64 Intemationai Journal of Metalcasting/Fall 09

REFERENCES

1. Janowak J.F., et al.. "Technical Advances in Cast Iron 4.Metallurgy". 48th International Foundry Congress,Varna. Bulgaria, (1981).

2. Gundlach R.B., "Elevated-Temperature Properties of 5.Alloyed Gray Irons for Diesel Engine Components",AFS International Cast Metals Journal, pp 11-20. 6.(September, 1979).

3. Angus H.T., et al., "Conditions Leading to Failure in

Cast Iron Brakes", B.C.l.R.A. Journal, vol. 14. No. 4,pp 371-385 (1966).Leach P.W. and Borland D.W., "The UnlubricatedWear of Flake Graphite Cast Iron", Wear, 85, pp 257-266. 1983.Okamura T., et al., High-Precision Brake Discs toReduce Judder, SAE 2005-01-3924.Okamura T.. et al., Design for Homogeneity of BrakeDiscs to Reduce Judder, from R&D to Manufacturing,SAE 2007-01-3961.

International Journal of Metalcasting/Fall 09 65