High Nitrogen Martensitic Steels A New Family of Martensitic Corrosion Resistant Steels for Improved Aerospace Bearing Performance

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-338 The Society shall not be responsible for statements or opinions advanced in papers or in dis cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only If the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright 199 by ASME High Nitrogen Martensitic Steels A New Family of Martensitic Corrosion Resistant Steels for Improved Aerospace Bearing Performance WERNER TROJANS Material Mechanics, Research and Development Center FAG Kugelfischer Georg Schafer KGaA LT-P-AC, Postfach 160 D-870 Schweinfurt, Germany ABSTRACT The need for greater reliablity of corrosion resistant bearings, especially in aerospace and aircraft applications, has led to the development of Cr-steels with 1 96 Mo. A partial exchange of carbon by nitrogen improves corrosion resistance, mechanical properties and bearing life as compared to conventional stainless bearing steels. KEYWORDS Bearing steel, Corrosion resistance, Nitrogen alloyed 1. INTRODUCTION In aircraft engine applications conventional steels like SAE 5100, M50 and M50NiL are most widely used for bearings. These steels offer a good fatigue life capability but no corrosion resistance. well as mechanical properties and bearing performance.. STEELMAKING The adjusting of a nitrogen content up to 0.4 % in martensitic steels is impossible by conventional melting practices. Following Sievert's law this content is reached by applying a pressure of up to 40 bar during the remelting in a 0 ton electroslag remelting furnace (PESR) [4]. 3. TEST 3.1 STEELS Two different nitrogen alloyed steels were produced by PESR. The so called X15 had C and N, the so called X30 contained C and N. Table 1 compares the chemical compositions of test steels. As a consequence, unexpected corrosion is one of the main reasons for the rejection of bearings [1] If corrosion is expected, the steel AISI 440 C is applied. This steel offers a limited corrosion resistance and a sufficient [] hardness of 58 HRC min. The fatigue life capability is reduced as compared to the other SAE 5100 * X 65 Cr C X steels [3]. During recent years a new family of high strength corrosion resistant steels was developed. The basis is a Cr steel with 1 96 Mo. A partial exchange of carbon by nitrogen is the main difference to conventional steels. This paper gives an overwiev of the new steels. Corrosion resistance is included as X 30 M50 * * = average analysis Cr TAB. 1: Chemical Composition of Test Steels (Wt %), Main Elements V Presented at the International Gas Turbine and Aeroengine Congress and Exposition Cologne, Germany June 1-4, 199 Downloaded From: on Terms of Use:

2 60 Typical microstructures in the as quenched condition are presented in Fig. 1. Steels like AISI 440 C and M 50 contain large 59 eutectic carbides. With decreasing carbon content, amount and size of these carbides can be reduced, but they still are present. In X 65 Cr 13 carbides of pm size are common. Both X-alloys are free of these large carbides Tempering Temperature ( C) FIG. Heat Treatment Behavior C 50mm FIG. 1 Carbides in Corrosion Resistant Steels, as Hardened Hot hardnesses of the X-steels, AISI 440 C and M 50 are compared in Fig. 3. While the behavior of the low tempered steels is similar up to 00 C [39 F], hot hardness of X 15 remains relative high up to 350 C (66 F), as it is known from the high temperature bearing steel M RESPONSE TO HEAT TREATMENT Steels like AISI 440 C and SAE 5100 are known to reach 58 HRC min. In the low tempered condition, M 50 a hardness of 60 HRC min. in the high tempered condition. As can be seen from Fig. a, X 30 reaches the 58 HRC-goal in the low tempered condition, while X 15 has to be tempered in the region of 450 C (84 F), Fig. b `-' O 45 austenitized: 30 min quenched : oil deep frozen: 80*C1h Temperature [ C] FIG. 3 Hot Hardness after 1 hour at Temperature The maximum hardness of X15 remained constant after 1000 hrs at 400 C (75 F) Austenitizing Temperature ( C ) A further improvement in secondary hardening is brought about by adding elements like vanadium or niobium, Fig. b. More research work will be done on the secondary hardening. Downloaded From: on Terms of Use:

3 3.3 CORROSION RESISTANCE Dynamic current density-potential measurements of X-steels reveal a dinstinct drop of the passive current density and a raise of the break-through potential if compared to conventional steel, Fig C _15. treanment 1010.C. 30 min oil ( MK'. 30 min) MC 150.C. h air )00 00.C. h air XI3 450 "C, 3a 1 h air strength deformation 0.% -yield ultimate (MPa) (MPa) (%) X 65 Cr Young's mod. (GPa) 440 C X X TAB. : Tensile Properties of Corrosion Resistant Steels (Average of 5) X POten1101 U in mv FIG. 4 Current Density-Potential Curves of Corrosion Resistant Steels in a 1-n HSO4 Solution The superior resistance of X-steels against pitting corrosion is also demonstrated by salt spray testing according to DIN SS (35 C (95 F), 5-1 NaC1 in water) Fig. 5 shows X-15 and 440 C rings after 4 hrs in the test chamber. While the 440 C ring exhibits extensive corrosion, no indication could be seen on X-steel rings. This was still true after 30 hrs of test. In compression all the steels exhibit a yield strength in the range of 1870 to 040 MPa and an ultimate strength between 500 and 650 MPa. Impact energy tests of unnotched specimens (4 x 10 mm cross section) are summerized in fig. 6. Independent of test temperature, toughness of 440 C remains low. The X-steels and the X 65 Cr 13 increase in impact energy significantly. 140 E 10 FIG. 5 Test Rings after 4 h Salt Spray (35 C, 5 % NaC1 in water) 3.4 MECHANICAL PROPERTIES 1 I 100p 60 a 40 E 0... X X 65 Cr 13 X Z6' 0, Temperature [ 4C]. 440 C FIG. 6 Impact Energy Testing of Unnotched Flat Specimens (Average of 5) 00 The results of tensile tests (average of 5 specimens) are given in Table. The X-steels surpass the conventional corrosion resistant steels in strength and ductility. As far as fracture toughness is concerned the values for X-steels were found to be in the same range as compared to other conventional through hardening steels, 16-3 MPa m. 3 Downloaded From: on Terms of Use:

4 3.5 BEARING TESTS Bearing life testing was done under conditions given in Table 3. Interrupted Run Test bearings: Angular Contact Ball Bearing (bore diameter 5 mm) Hertzian Pressure: 800 MPa Rotating Speed: rpm Lubrication: Mobile Velocite No., 65 C Calculated Life: 50 hrs acc. DINISO 81 Test A: Continuos run, poor lubrication Test B: Interrupted run with N x (4h salt spray + 4h run) poor lubrication Test C: Continuos run, full lubrication (EHD) TAB. 3: Conditions for Bearing Life Tests, 5 Rings per Serie Tests A and B were runs under poor lubrication (i. e. partial metal to metal contact in the rolling contact zone). Test C was done under full elasto-hydrodynamic condition (EHD, i. e. no metal to metal contact). Inner rings of conventional and X-steels were tested against balls and outer rings made of steel 100 Cr 6 (SAE 5100). The inner ring was designed to be the life limiting part. The calculation of unfactored bearing life (DIN ISO 81) amounted to 50 hrs for a 10 %- failure probability (L10-life). The results clearly demonstrate the superior performance of the N-alloyed steels as well in the continuous run, test A, Fig. 7, as in the interrupted run with intermittent salt spraying, test B, Fig. 8. FIG. 8 Overrolling Test with Intermittent Salt Spray, 10 % of 440 C Rings Failed within 1 Cycle, 100 % within 4 Cycles. No Failure on X-Steels after 8 Cycles (End of Test) In test C (EHD-condition) no failure occured on X 15 or X 30 rings after more than 000 hrs of continuous overrolling (test continued). For the steels M 50 and SAE 5100 a L10-life of 1300 hrs is considered a good result. Wear resistance of the steels were tested by running different combinations of rings and balls without lubrication under a low Hertzian pressure. As can be taken from Fig. 9, X 15- rings wear less than M 50-rings. In combination with silicon nitride balls, the mass loss is as low as thin on dense chromium-coated M 50-rings o ie BEARING Mgli, RACES TDC, iliv COATED.". CI RACES X 5 V 0, 4.11,r.--"-- CERAMIC BALLS CI e 5 r, RACES X15 440C BALLS BEAR NG M WEAR MASS OF OR+IR+BALLS (mg) FIG. 9 Mass Loss of Angular Contact Ball Bearings after Operation as a Function of Material Combination, Hertzian Pressure 865 MPa (15 KSI), Rotation: 500 rpm Duration: 50 h, Lubrication: Dry FIG. 7 L10-Life in Continuos Overrolling under Poor Lubrication L. Downloaded From: on Terms of Use:

5 4. DISCUSSION The corrosion tests demonstrated a superior corrosion resistance of the nitrogen alloyed steels. Although the positive influence of nitrogen in austenitic stainless steels has been used for a long time the mechanism is not exactly known. An explanation could be: a) A local corrosion around nitrides is missing [4]. Nitrides, being precipitated during heat treatment, contain less Cr and Mo than carbides [5]. A higher amount of these elements in the matrix increases passivation and decreases corrosion. b] Nitrogen atoms segregate at the surface and form CrN. These Nitrides react - for example - with water to ammoniak and ammonium. Ions of ammonium raise the local ph-value. This results in a lower corrosion rate [6]. c] Nitrogen is reported to react with Mo. A kind of film is built up [7]. Expecially in the case of pitting corrosion, this film prevents further metal dissolution and favours passivation. The exellent performance of the new nitrogen steels in the tensile test as well as in bearing life testing mainly is based on the absence of large carbides. These carbides act as weak points [8] and stress raisers. In analogy to the effect of hard inclusions, the local stress could exceed the strength of the steel, though the nominal load is relative low [9]. Very often carbides are the origin of cracks and pittings in the raceway [3]. Furthermore, the absence of large carbides favours a low operational noise of the bearings [10]. 5. CONCLUSIONS The results shown above demonstrate, that the X-steels are ready for application. They are already included in varios test programs for bearings and linear motion systems. [3] P. K. Pearson, T. W.Dickson: The role of carbides in performance of high-alloy bearing steels, in: Effect of steel manufactoring process on the quality of bearing steels, ASTM STP 987, J.J.C. Hoo, Ed., American Society for Testing and Materials, Philadelphia, 1988, pp [4] G. Stein, J. Menzel, H. Dorr: Nitrogen alloying under pressure, in: High nitrogen steels, Proceedings of HNS, 1988, Inst. of Metals, London 1989, pp [5] J. Lueg: Nitrogen alloyed tool steels, VDI-Fortschr.-Ber. 188, Reihe 5, 1990, VDI-Verlag, DUsseldorf [6] R. Kieffer, F. Benesovsky: Hartstoffe, Springer Verlag, Wien, 1965 [7] C. Clayton, K. Martin: Evidence of Anodic Segregation of Nitrogen in High Nitrogen Stainless Steels and its Influence on Passivity. Proc. "International Conference on High Nitrogen Steels, HNS 88", The Institute of Metals, May 1988, Lille (F), pp [8] R. Newman et. al.: Mechanism of Passivation in Stainles Steels Containing High Concentrations of Nitrogen, Proc. "9th International Congress on Metallic Corrosion", Toronto 1984, Vol. 3, pp [9] W. Trojahn: "Geflige und Eigenschaften ledeburitischer Chromstdhle mit Niob und Titan ", Fortschr.-Ber. VDI, Reihe 5, Nr. 90, VDI-Verlag, 1985 [10] H.-J. Biihmer: Rolling contact fatigue, in: Research-a Basis for Products of the Future, FAG-Publication NO WL 4005 EA, Schweinfurt (Germany), 1991, pp [11] A. Hirao, W. A. Kuhn: Kugellager aus nichtrostendem Stahl mit verbesserten Materialeigenschaften, Antriebstechnik 8 (1989), Nr. 9, pp In addition, more work is being performed to improve the hot hardness feature of the X 15 steel. REFERENCES [1] E. Steinhardt: Design and Beaviour of Aero Engine Bearings, MTU Focus 1989 [] FAG-Standard catalogue 41510DA: Life calculation with regards to capacity ratings, p Downloaded From: on Terms of Use: