The improvement of back-up rolls properties in terms of wear resistance is driven by the need of longer and longer rolling sessions aimed to increase productivity and reduce costs. Chemical composition effect on contact fatigue phenomena, bringing to the occurrence of macroscopic damages named spalling, have been studied and tested in laboratory. Moreover, the removal by grinding operations of damaged portion of rolls surface should be not sufficient to restore the initial performances of material. Experimental tests showed that a portion of material below the damaged one keeps memory of the last fatigue cycle, and has to be removed

A. Di Schino

P. E. Di Nunzio

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345METALURGIJA 56 (2017) 3-4, 345-348P. E. DI NUNZIO, A. DI SCHINOCONTACT FATIGUE PHENOMENA IN BACK-UP ROLLS OF ALLOYED STEELSReceived – Prispjelo: 2017-01-05Accepted – Prihvaćeno: 2017-05-10Original Scientific Paper – Izvorni znanstveni radP. E. Di Nunzio, Centro Sviluppo Materiali SpA, Via di Castel Romano, Roma, ItalyA. Di Schino, Università di Perugia, Dipartimento di Ingegneria, Pe-rugia. ItalyThe improvement of back-up rolls properties in terms of wear resistance is driven by the need of longer and longer rolling sessions aimed to increase productivity and reduce costs. Chemical composition effect on contact fatigue phenomena, bringing to the occurrence of macroscopic damages named spalling, have been studied and tested in laboratory. Moreover, the removal by grinding operations of damaged portion of rolls surface should be not suffi-cient to restore the initial performances of material. Experimental tests showed that a portion of material below the damaged one keeps memory of the last fatigue cycle, and has to be removed.Key words: alloyed steel, back-up rolls, fatigue, microstructure, surfaceINTRODUCTIONThe worldwide increasing competition between steel producers, linked with emerging countries produc-ers growth, requires the individuation of all possible solutions to save money and reduce production costs. The elongation of back-up rolls rolling campaign, re-ducing the number of plant stops, brings to a longer life of rolls, and represents one possible way of improve-ment. Many papers have been published related to the metallurgical design and to the hot deformation behav-iour of back-up rolls [1-4], but not so large research have been carried out about the relations between mi-crostructure and surface fatigue phenomena. Some re-sults are reported in literature showing that the finely precipitated carbides at different matrix during heat treatment process strongly influence mechanical prop-erties of the backup roll. In particular, the spheroidized pearlite at the inner regions which consists of large globular or rod-like M7C3 and a little of small globular M23C6 shows increased toughness and fracture resist-ance properties than those of the lamellar pearlite with lamellar M23C6 and a little percentage of globular M7C3 [5]. In framework of the relations between microstruc-ture and surface fatigue properties it is important to evaluate the effect of different metallurgical mecha-nisms (carbide precipitation, microstructure) [6] on sur-face fatigue phenomena which are responsible of rolls surface damaging named spalling This paper reports the effect of microstructure on contact fatigue phenomena of back-up rolls rolling campaign duration and grinding operations.ISSN 0543-5846METABK 56(3-4) 345-348 (2017)UDC – UDK 669.15:621.867.6:620.178.3:620.18:620.197.6=111EXPERIMENTALBack up rolls are manufactured by a forging process followed by a quenching and tempering (Q&T) heat treatment, aimed to obtain a target hardness of 450 HV in the surface. The component, after heat treatment, needs to be characterized by good toughness levels in the roll bulk and necks, and elevated wear /fatigue be-haviour on the barrel surface. Starting from 3 % Cr steel for back up rolls (Steel A), the metallurgical design of new solutions for this kind of products is based on the introduction of some chemical composition variants aimed to activate different steel hardening metallurgical mechanism which could improve both surface fatigue resistance and toughness. Mechanism under considera-tions are the precipitation of V carbides (Steel B) which are known to be quite effective in enhancing wear re-sistance, and toughness improvement by lowering Mo content and raising Mn up to 1,5 % (Steel C) (see Table 1). Steels were processed by thermo mechanical rolling followed by Q&T on pilot plant, in order to obtain the target mechanical strength.Surface fatigue resistance was evaluated by a Ring-on-Ring tribometer. According to the experimental set-up, a ring is pressed against a cylinder, the two speci-mens rotating at assigned angular velocities. The two Table 1  Chemical composition of the steels under investigation (other elements: N, S, P) / wt. %Steel A Steel B Steel CC 0,45 0,45 0,45Si 0,35 0,35 0,35Mn 0,75 0,75 1,50Ni - - -Cr 3,00 3,00 3,00Mo 0,80 0,80 0,30V - 0,13 -346P. E. DI NUNZIO et al.: CONTACT FATIGUE PHENOMENA IN BACK-UP ROLLS OF ALLOYED STEELS METALURGIJA 56 (2017) 3-4, 345-348samples can be machined, treated, and finished in such a way to present the surface characteristics similar to the ones of the roll surface to be simulated.The normal load and the two velocities can be set within some limits imposed by the test machine.The tribometer is able to control the normal load from 0 N up to 5 000 N and the temperature from 0 up to 1 200 °C. The cylinder angular speed can be con-trolled from 50 to 1 200 rpm, while the ring (counter part) angular speed can assume values from 0 to 500 rpm. The two axes can be moved depending on the sam-ple dimensions. During these tests the diameter of test sample ring used was between 68 mm and 70 mm. The parameters are controlled by a dedicated software. Ro-tation can have either the same or the opposite verses. In this investigation the tests have been performed at room temperature and with no lubrication. Normal load dur-ing tests is set to obtain a desired maximum Hertzian contact pressure, which ranged between 710 MPa and 1 230 MPa during the test campaign. Air cooling has been ensured in order to prevent heating and uncontrolled oxidation phenomena on the contact track. Each mate-rial is characterized obtaining the equivalent “S-N sur-face fatigue” curve. Using as reference value the yield limit of the material, a first test load is chosen and the test is performed under pure rolling conditions or with a slipping percentage if required. After a certain amount of rolling cycles, the test is stopped and the sample made by the testing material is observed by Scanning Electron Microscope (SEM) in order to evaluate the presence of surface fatigue damage. The test is stopped if, for instance, a dimension of 100 microns pitting is reached. A reduced test load is then fixed and the proce-dure repeated until enough points are available to trace the surface fatigue characteristic of the material. A ref-erence ring is generally used as counterpart for all the tests; the diameter of the used ring is 76 mm.Microstructure was analysed by Electron Back Scat-tered Diffraction (EBSD) technique. About 100 mm x 100 mm areas were scanned. From these measurements some microstructural characteristics of the material can be estimated, e.g. misorientations and types of grain boundaries, crystallographic orientations, etc., directly related to materials strength/toughness. EBSD is a very accurate method to measure the grain size, even for mi-crostructures as fine as tempered bainite and tempered martensite. In this paper subgrains and packets were de-fined as microstructural features with misorientation lower than 5° and higher than 15°, respectively [7-8].RESULTS AND DISCUSSIONMaterials production and microstructural assessmentThe three steels reported in Table 1 were cast on laboratory scale. 80 kg weight ingots were cast and then hot rolled, simulating an industrial forging process. Hot Figure 1 Tempering behavior of the considered steelsFigure 2  Microstructures of the considered steel after Q&T (EBSD results)Steel ASteel BSteel C347P. E. DI NUNZIO et al.: CONTACT FATIGUE PHENOMENA IN BACK-UP ROLLS OF ALLOYED STEELSMETALURGIJA 56 (2017) 3-4, 345-348rolled plates were then quenched and tempered. In order to identify the suitable tempering temperature to obtain the target hardness, materials were tempered in a wide temperature range (Figure 1). Due to precipitation hard-ening, steel B requires higher tempering temperature in order to obtain the same target hardness, as expected.Microstructures and relevant hardness profiles of the three considered steels after Q&T are reported on Figure 2. In all cases, a tempered bainitic microstruc-ture was observed by EBSD.No differences in terms of packet/cell sizes are found between the three materials, so that similar ten-sile/toughness behavior have to be expected as obtained by different metallurgical mechanism hence process pa-rameters (e.g. different tempering temperature). Moreo-ver, due to the high intrinsic steel hardenability quite homogeneous through thickness hardness profiles were found (Figure 3).Contact fatigue resistanceSteels reported in Table 1 were tested by tribometer by means of a standard ring on ring test. Specimens were machined from each hot rolled steel and then tested in the tribometer using a reference counterpart ring of nitrided cromium steel. A first testing campaign was performed using sliding conditions, air cooling and three different load conditions (710 MPa, 897 MPa and 1 230 MPa) in order to define a damage curve for each steel. The results of this test are summarized in Figure 4.Steel ASteel BSteel CFigure 3  Through thickness hardness profiles of the considered steel after Q&TFigure 4 Ring on ring results after first test campaignThe variant B, based on V carbides precipitation hardening, is quite more expensive than the basic com-position, but the advantages in terms of test duration and contact fatigue resistance are repetitive and con-stant at all hertzian pressures tested. The variant C, based on partial substitution of Mo by Mn content in-crease, which is cheaper than the basic composition, shows a very interesting behavior at low pressures but at high pressures has the same resistance to contact fa-tigue damages of the standard steel.Examples of SEM analysis of specimen surfaces cor-responding to pitting initiation are reported in Figure 5.In order to understand the effect of machining on the wear resistance, steel A was machine grinded reducing the radius of 0,15 mm, 0,50 mm and 1,0 mm respectively, and then tested at 897 MPa of maximum Hertzian pres-sure (the intermediate value of the previously executed tests). Results of Ring on Ring tests show a decrease of the damage resistance of the steel after machining: in Figure 6 a comparison of the effective number of cycles needed to damage steel A in different machining condi-tions is reported. This behavior has been observed also on the other two steels; the comparison between Steel A and Steel C in two conditions is reported in Figure 7 as an example (as received condition and radius reduced of 0,15 mm are shown). The test parameters were set to maintain an Hertzian pressure of 897 MPa in all trials.CONCLUSIONSNew steel variants for back-up rolls steels have been designed, aimed to improve rolls surface fatigue thus 348P. E. DI NUNZIO et al.: CONTACT FATIGUE PHENOMENA IN BACK-UP ROLLS OF ALLOYED STEELS METALURGIJA 56 (2017) 3-4, 345-348position, but the advantages in terms of test duration and contact fatigue resistance are repetitive and con-stant at all hertzian pressures tested.The variant C, based on partial substitution of Mo by Mn content increase, which is cheaper than the basic composition, shows a very interesting behavior at low pressures but at high pressures has the same resistance to contact fatigue damages of the standard steel.These results show that the knowledge of the influ-ence of different hardening mechanisms on the contact fatigue phenomena enables the design of the proper chemical composition and heat treatment, in terms of cost and performances, depending on the work condi-tions of the back-up rolls.REFERENCES[1]  C. Imbert, N.D. Ryan, H.J. McQueen: Hot workability of three grades of tool steels, Metallurgical Transactions A 15 (1984) 1855-1864. DOI: 10.1007/BF02664899[2]  S.Mengaroni, F. Cianetti, F. Curbis, A. Di Schino, A. Fa-brizi, M. Calderini, E. Evangelista, Hot deformation and microstructure evolution analysis by hot torsion tests of a 3 % Cr steel, Metallurgia Italiana 107 (2015), 11-14.[3]  F. Curbis, S.Mengaroni, M. Calderini, S Neri, E. Evangeli-sta, A. Di Schino, M. Paura, Alloying elements effect on hardenability and tempering of back up rolls steels with re-duced Mo content, Metallurgia Italiana 105 (2013) 23-28.[4]  S. Mengaroni, F. Cianetti, M. Calderini, S. Neri, E. Evan-gelista, A. Di Schino, H.J. McQueen, Tool steels: forging simulation and microstructure evolution of large scale in-got Acta Physica Polonica 128 (2015) 629-632. DOI: 10.12693/APhysPolA.128.629[5]  X.Y. Song, H.J. Zhang, L.C. Fu, H.B. Yang, K. Yang, L. Zh, A review of strategies for carbon steel design, Mate-rials Science and Engineering A, 677 (2016) 465-468. DOI: 10.1016/j.msea.2016.09.079[6]  G. Bregliozzi, A. Di Schino, J.M. Kenny, H. Haefke, The influence of atmospheric humidity and grain size on  friction and wear of steel, Materials Letters, 57 (2003) 4505-4508. DOI: 10.1016/S0167-577X(03)00351-3[7]  A. Di Schino, C. Guarnaschelli, Effect of microstructure on cleavage resistance of high strength quenched and tem-pered steels Materials Letters 63 (2009) 1968-1972 DOI: 10.1016/j.matlet.2009.06.032[8]  K. Zhum Gahr, Microstructure and wear of materials, Else-vier, 1987.Note: The responsible for English language is: Elisabetta Petricci, ItalyFigure 7  Damage comparison before and after machining on Steel A and Steel CFigure 6 Ring on ring tests on Steel A after machiningFigure 5  SEM image of a typical damage on ring surface (Steel A, Steel B)Steel B after testing at 710 MPa for 1.04*106 cyclesSteel A after testing at 710 MPa for 8.00*105 cyclesSteel B after testing at 710 MPa for 1.00*106 cyclesallowing elongated rolling campaign improving pro-ductivity and reducing costs. Starting from the basic 3 % Cr steel chemical compositions variants were de-signed. The effects of steel chemical composition on contact fatigue phenomena, that bring to the occurrence of macroscopic damages named spalling, have been evaluated. Both steel variants let to improve contact fa-tigue resistance even if with different behavior.The variant B, based on V carbides precipitation hardening, is quite more expensive than the basic com-

Contact fatigue phenomena in back-up rolls of alloyed steels

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