Integration of Gas Nitrocarburising and Oxidising in a Mass Production Line for Brake Pistons

TRANSACTIONS OF MATERIALS AND HEAT TREATMENTPROCEEDINGS OF THE 14 IFHTSE CONGRESSOctobeIntegration of Gas Nitrocarburising and Oxidising in a Mass Production Line forBrake pistonsGero Walkowiak, Dieter Muller", Uwe Zeibig1. Bodycote Warmebehandlung GmbH, Ebersbach, Germany2. Bodycote Warmebehandlung gmbH, Ebersbach, Germany3. Sander Umformtechnik gmbH Co K Renchen-UIm, GermanyAbstract: The combination of wear-and corrosion resistance is a demand to a multitude of automotive parts. Several metalliccoatings as hard/soft chromium or electroless nickel have been the conventional surface protection e. g. on parts as brakepistons and shock absorber or gas spring piston rods. The Corr-l-Dur process-a special gasnitrocarburising and oxidisingtechnology. has a huge potential not only in substituting those coatings but also in delivering surfaces with higher qualityThe benefits of the replacement of soft chromium with Corm-I-Durand the implementation of the process in the customersproduction line with an annual capacity of 17 million brake pistons is described in this presentation, Starting with the demaron wear- and corrosion resistance the way of process development and the design of applicative equipment is shown. Thresult is a tailor-made surface solution with superior quality for deep drawn brake pistons. Two main disadvantageoutgassing of the chromium layer and reduced corrosion resistance on the inner diameter-are eliminated by the replacementwith Corr-I-Dur, The process-sure achievement of enhanced corrosion resistance, uniform layer thickness, negligibledistortion as well as environmental and economical advantages make Corr-"an excellence choice for this applicationKey words: Corr-I-Dur nitrocarburising, oxidising, corrosion resistance, brake piston, mass productionA REASONABLE MATERIAL CHOICE is one of the established a commercial gasnitrocarburising andfundamentals of the success of the production of com- oxidising technology that reproducibly delivers an ex-ponents. Damages can appear during service by corro- cellent corrosion resistancesion, wear and fatigue failure1. 2. Layer ConstitutionWhen looking for a sufficient corrosion resistance,Layers produced by gasnitrocarburisingan oxidisingommonly stainless steels or coatings as chromium or processes generally consist of three zones:electroless nickel are considered. Recent experiencesThe transition to the substrate is built by the diffu-has shown, that in many cases a nitrocarburising andsion layer. This layer has a thickness of 0, 1 to 1, 0oxidising treatment is the most reasonable choice frommm. Nitride precipications and interstitially dis-the economical and technical point of viewsolved nitrogen enhance the hardness and theEspecially if corrosion and wear protection is desiredfatigue strength of this areacombined with a high strength of the base material,The compound layer which consists of E-andnitrocarburising and oxidising treatments more andY(carbo)nitrides. The thickness of this layer ismore become first choice, especially for parts that arebetween 5 and 30umproduced in high quantities.The outer layer is the oxide layer(0,5-3um). Thechemical composition is Fe] O4(magnetite).1. Corrosion Protection by NitrocarburisingIt is well known that nitriding or nitrocarburising canThe compound layer has a high hardness and deliversenhance the corrosion resistance of low alloyed steelsgood wear resistance, especially if it mainly consists ofe-nitrides. Under special conditions the oxide layer canBut the results in corrosion tests do not meet the usual work as a passive layer on the compound layer. It wasspecifications for parts that are exposed to corrosiveshown, that a sufficient concentration of nitrogen plusenvironment. A subsequent oxidation especially of carbon in the compound layer is the key to a goodnitrocarburised surfaces can give a further distinct improvement of the corrosion resistance [1-3orrosion protection[2-5]. As these concentrations aredirectly influenced by the process parameters hugeThe best known commercial nitrocarburising and discrepancies between gasnitrocarburising andoxidising processes are based on salt bath nitrocarbuising technology. Nowadays oxidation ot gasnitrocar-. process which is established by Bodycote has to fulfilronmental aspects. Common processes indeed reach certain standards concerning furnaces and processparameters, so it reproducible delivers an excellentabout 96 hours in salt spray test but cannot fulfil the corrosion resistanceincreasing demands of 240 hours or moreWith the so called Corr-I-Dur process, Bodycote has中国煤化工CNMHG406PROCEEDINGS OF THE 14IFHTSE CONGRESSOctober 20042. Fields of ApplicationProducing the surface in an environmentally comTypical applications are automotive parts on which aibleCorr-l-Dur is a suitable process for the treatment ofcombiantion of wear- and corrosion resistance is the deep drawn brake pistons. To keep the core hardnessrequired. In a lot of cases the Corr-I-Durtreatmentreplaces traditional coatings as chromium or electrolessof the material, the process has to run at reducednickel. On many parts customers changed the treatment temperatures compared to a standard process.Thefrom gasnitrocarburising to Corr-I-Dur, adopting the maximum allowed value was found in annealing tests atpossibility of an additional corrosion resistancedifferent temperatures. For a good corrosion resistance itIn this work we present the substitution of a chreis very important to control the process in that way that amium coating on brake pistons for automotive industry compound layer with a sufficient nitrogen content isby a special Corr-I-Dur treatment, and its introduction produced, in spite of the reduced treatment temperaturesThis could be obtained by optimised gas compositionsinto mass production3. Process Development for Brake Pistonson the inner diameter because CorT-I-Dur is based upongasnitrocarburisingStart of the development of the new surface for brakeWe decided to run the nitrocarburising process in apistons was a new type of brake pistons developed by batch process rather than in a continuous process, as weour customer. These brake pistons are manufactured in a found out, that retort fumaces can deliver a highernew deep drawing/stretching process. They have the quality, especially a superior corrosion resistance.advantage of up to 35% less mass compared to casted or 3.2. Resultscold forged parts. Picture I shows a brake piston in aThe feasibility was first checked with small quantitiesbrake caliper.of brake pistons. It was shown, that the Corr-I-Durtreatment could fulfil all requirements. After positiveresults we ran full batches with quantities comparable tothe oncoming serial production. The batches wereaccompanied by testing of all relvant propertiesPicture 2 shows a cross section of a Corr-I-durtreated brake piston.Picture 1: Longitudinal section of brake caliper withbrake piston3. 1 Requirements upon the Pistonse In order to skip a new investment in a chrome plating Picture 2: Nital etched cross section of a Comr-I-Durhe our customer looked out for a new process with the treated brake pistonfollowing specificationEnhance of corrosion resistance compared toThe corrosion resistance was checked in neutral salthromium layer Corrosion resistance must also be spray test according to DIN 50021-SS(ASTM Bpresent on the inner diameter, because in service117-73). The parts were placed horizontally in the testchamber. The test conditions are listed in Table 1his area is most affected by the corrosive envi-Table 1: Parameters of neutral salt spray testImprovement of wear resistanceaccording to DIN 50021-SS(ASTM B 117-73)The raw part has an enhanced hardness by workhardening. No distinct drop of this core hardnesMass-% NaCl5O+5gprnduring the surface treatment process is allowedGood adhesion of the surface layer. Ablation of theAlready after one cycle the chromium brake pistonslayer can cause failure of the brakeshowed distinct corrosion. After 36 h the inner diameterNo hydrogen intake during the process. Outgassing was totally corroded. The Corr-l-Dur treated brakeduring service leads to failure of the brakepistons were free of any corrosion after 60h( Picture 3),Keeping the dimensional tolerances after the treat-H中国煤化工 icture 4). The pIcCNMHGTRANSACTIONS OF MATERIALS AND HEAT TREATMENTthe Corr-l-Dur treated pistons especially on the inner with aggravated conditions concening exposure to weardiameter compared to the chromium plated pistonsand corrosion. This test was carried out by the endcustomer, a world leading supplier of brake systems.4. Implementation into Serial ProductionThe brake pistons are manufactured by ErdrichUmformtechnik, a subsidary of Sander Umformtechnik,from unalloyed steel sheets in a new deep drawing/stretching process in a multiple stage press. In fol-lowing interlinked steps the blank develops to a finishedart. All working, machining and grinding steps as wellas the cleaning before the heat treatment are carried outin a continuous floThe cleaned parts are batched on a special designedjig by a industrial robot. At this stage, the productionprocess is switched from a continuous to a batch processThe automatisation device and the jigs have tocorrespond exactly to avoid a brakedown of the production flow. This critical step of production was de-veloped in close cooperation between our customer andThe loaded jig is taken over by Bodycote to carry orthe heat treatment process. After inspection of theincoming pistons the jigs are loaded in one of sevenhorizontal retort furnaces where the Corr-I-Dur treatment is carried out Picture 5 shows the retort furnacesPicture 3: Brake Pistons after 60h neutral salt spray test for the Cor - -dur treatment. For economical reasonsDIN 50021-SSthe batch has a high mass and is very dense packed. ThusAbove: Corr-I-Dur treatedthe uniformity of the batch can only be achieved throughBelow: Chromium plateda special process guiding and sufficient gas agitation inHeating tests in brake fluid showed, that the brakthe furnace as well as the use of suitable jigspistons do not contain any hydrogen. Even worst caseals show that a hydrogen intake during the procesThe investigations showed, that all demands on thesurface of the brake pistons could be fulfilled by theCorr- -Dur treatment.5: Furnaces for Corr-l-DurAfter the Corr- -Dur treatment the furnaces arePicture 4: Inner diameter of brake pistons after neutral unloaded. As the pistons are critical parts each batch issalt spray test.spot checked for core- and surface hardness, layerLeft: Chromium plated, 36 hthickness and layer hardness, roughness and diameter asRight: Corr-I-Dur treated, 60hwell as corrosion rein neutral salt spray testBrake pistons which were Corr-I-Dur" treated in aunloaded similar tofull batch successfully passed a long-time in-service test中国煤化工 steps including a0o%CNMHGPROCEEDINGS OF THE 14 IFHTSE CONGRESSOctober 2004diameter testing are carried out by the customerIn 2002 Bodycote started the Corr-I-Dur treatmentfor brake pistons with a capacity of 8. 5 Million brake Schmit. Thomas Kh. G and Rauch B Vergleichende Korro-pistons, integrated in the production site of Erdrichonsuntersuchungen an Nitridschichten auf unlegiertemUmformtechnikSahl,HTM42(1987)6,356-362Meanwhile our customer doubled the capacity by2 Ebersbach U. Naumann J. Zimdars H. Spies H-J.EinfluB derduplicating the production line, again with the integratedWasserdampfbehandlung auf das Kormosionsverhalten vonnitriertem und nitrocarburiertem Stahl 20Mn Cr5, TeillEinfluB der Anwarmatmosphare, Anwarmdauer und Halte-5 Conclusionsdauer,HTM52(1997)3,178-1883 Ebersbach U Naumann J. Zimdars H Spies H-J EinfluB derCorr-I-Dur is an excellent choice for automotiveWasserdampfbehandlung auf das Korrosionsverhalten vonparts which are exposed to wear and corrosion. Thenitriertem und nitrocarburiertem Stahl 20MnCr5. Teil2.corrosion resistance of Corr- treated deep drawnEinfluB des sauerstoffpartialdrucks und desteel brake pistons is superior to the former chromiumtemperatur, H(1998)1,5662coating and the process is ecologically friendly. The 4 Walkowiak G and wagner M. Corr-I-Dur Nitrocarburizingimplementation of the Corr-I-Dur treatment of brakeand Oxidizing: Correlation between Layer Constitution andpistons in our customers mass production line is a typi-Corrosion Resistance, Proceedings of 8th Seminar of thecal example how to heat treat huge quantities of partsIFHTSE 2001, 12-14 September, Dubrovnik-Cavtat, Croatia,economically with a high and even quality201-207Mittemeijer E.J. and Colijn P F. Oberflachenoxidation vonNitrierschichten: Uber die Ursachen der Verbesserung derSpecial thanks to everyone who has been active in this projectKorrosionsfestigkeit, HTM 40(1985)2, 77-79nd works to make the serial production smoothly running中国煤化工CNMHG