Available online at ScienceDirect. Procedia Engineering 192 (2017 )

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1 Available online at ScienceDirect Procedia Engineering 192 (2017 ) TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport Fatigue resistance and influence of cutting technology on the mechanical properties of modern steels used in the automotive industry Robert Ulewicz a *, František Nový b a Czestochowa University of Technology, Al. Armii Krajowej 19B, Czestochowa , Poland b University of Zilina, Univerzitná 1, Žilina, Slovak Republic Abstract Automotive industry is the one of the most rapidly developing sector of engineering. Using of new, progressive materials can make significant benefits because of growing durability and reducing weight of structural parts, which can lead to the materials and fuel savings. In this article authors present basic mechanical properties of high strength low alloyed steel (HSLA) and compares several types of different cutting technics in terms of changing mechanical properties in the areas near the cuts. The fatigue properties of Hardox 400 steel in the high and ultrahigh-cycle range were determined also. Authors compares results of their own experimental works and subsequently discus these result and their possible effect on the design process. The paper includes analysis of application possibilities new types steels in the design of structural parts of semi-trailers chassis The The Authors. Published by Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of TRANSCOM 2017: International scientific conference on Peer-review sustainable, under modern responsibility and safe of transport. the scientific committee of TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport Keywords: HSLA; HARDOX 400; fatigue; technology of cutting 1. Introduction The automotive industry, as one of the most dynamically developing branches of the world economy, has set the direction of expansion of new design solutions using modern materials. Significant reduction in the weight of cars as * Corresponding author. Tel.: ; fax address: ulewicz@zim.pcz.pl The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the scientific committee of TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport doi: /j.proeng

2 900 Robert Ulewicz and František Nový / Procedia Engineering 192 ( 2017 ) well as semi-trailers in the period of past few years was the result of the development of new types of steels, in particular fine-grained and other High Strength Low Alloy steels (HSLA steels). HSLA steels are characterized by good mechanical properties obtained as a result of grain refinement and precipitation hardening caused by addition of small amount of alloying elements [1, 2, 3]. Changing requirements concerning transporting cargo with the use of truck car kit are forcing designers to decrease tare weight of semi-trailers and increasing their durability and safety [4, 5]. New materials require new technologies of mechanical treatment which will not reduce the properties of the base material. There is a very strong relation between new requirements, material, technology and production processes of semitrailers (Fig.1). The article presents the results of own research in the field of fatigue research and the impact of chosen processing technologies on the mechanical properties of fine-grained Hardox 400 steel. 2. The characterization of HSLA steel Hardox 400 Fig.1. The scheme in designing a new product - (car semi-trailer). Fine-grained steels are characterized by improved mechanical properties in comparison with the conventional steels [6, 7, 8]. Both, the yield strength as well as the fatigue strength for steel with a finer grain proved to be significantly better, and the impact resistance increased dramatically (Table 1). The use of more durable and also lighter chassis will contribute to improving efficiency of fuel use in cars, as well as reducing carbon dioxide emissions. Hardox steels are characterized by high resistance to abrasive wear, the possibility of machining with special tools, good weldability, high mechanical properties and resistance to impact loads. Above mentioned properties of Hardox steels are obtained by the precisely chosen chemical composition and reduced content of harmful admixtures (P and S). Currently, volume of Hardox steels used in design of structural components of trailers and semitrailers grows

3 Robert Ulewicz and František Nový / Procedia Engineering 192 ( 2017 ) rapidly. As an example of successful application of Hardox steels can be listed WIELTON S.A., company from Wielun in Poland, which is using Hardox steels as a standard material for tippers structural components. Table 1. The chemical composition and mechanical properties of Hardox 400 steel C (wt%) Si (wt%) Mn (wt%) P (wt%) S (wt%) Cr (wt%) Ni (wt%) Mo (wt%) B (wt%) R p0.2 (MPa) R m (MPa) A5 (%) KV -40 (J) Fig. 2. Microstructure of Hardox 400 steel in a state as delivered, after being etched with 1% Nital Hardox 400 steel in delivery state have the microstructure of tempered martensite with some amount of retained austenite (Fig. 2). The structure of Hardox 400 steel demonstrates great homogeneity throughout the cross-section, there were observed only a few small inclusions iregullarly distributed in the microstructure of tested specimens. Within the same type Hardox steels have different carbon and alloying additions content (Cr, Ni, Mo). This variation depends on the sheet thickness. In this manner, with constant content of the boron there is controlled hardenability of these materials (homogeneous sheet structure). The microstructure composition Hardox steels determine in addition to the carbon content also addition of alloying elements: nickel, manganese, chromium, molybdenum and also boron. Their impact on the microstructure composition and mechanical properties of Hardox steels is different, and their quantities are carefully chosen to strict regulation of mechanical and technological properties of these steels [9]. 3. Fatigue properties of Hardox 400 steel One of the most important factors determining the possibility of using new types of steels for structural components of semi-trailers are the fatigue properties [10 13]. This is a very important factor that can determine the safety design of the semi-trailer chassis. In the recent years, demands on the fatigue properties of materials for structural parts has changed, and typical fatigue limits (defined by N f=10 7 ) are not sufficient enough, due to demands for safety operation for much longer times and with much higher velocities, which logically caused an increase of number of loading cycles, to which is the vehicle exposed throughout its life. For this reason, designing of structural components of vehicles must be supported by experimentally verified fatigue properties in very-high cycle region. In this study were determined fatigue properties of Hardox 400 HSLA steel. Fatigue tests were carried out in high and ultrahigh-cycle range. Fatigue tests in high-cycle area were carried out by using the Rotoflex testing machine implementing load in rotating bending mode (parameter of cycle asymmetry was R = -1, the specimens were loaded with the frequency 30 Hz). The results of carried out experiments were plotted as the dependence of the number of

4 902 Robert Ulewicz and František Nový / Procedia Engineering 192 ( 2017 ) cycles to failure on the applied load amplitude of the specimen N= f(σ a). Determination of fatigue properties in ultrahigh-cycle range was done by using the ultrasonic resonant machine KAUP-ZU. Specimens were loaded with the cyclic loading with sinusoidal character by the symmetric tension-compression loading (R=-1), with the loading frequency f 20 khz. Both studies were carried out at ambient temperature T = 20 C ± 10 C. In case of low frequency testing (f 30 Hz) can be clearly observed the occurrence of the significant fatigue limit for Hardox 400 steel at the level of 490 MPa, (see Fig. 3). Testing in the ultrahigh-cycle area shows continuous decrease of fatigue limit with increasing number of cycles. In the range from N f=10 7 to N f=10 10 was this decrease approximately 110MPa, when are these values compared with the values for standard fatigue limit (N f=10 7 ). This decrease is quite significant, and it must be accepted in the process of structural components designing of the transport vehicles. Fig. 3. The fatigue life of Hardox 400 steel; rotaring bending in the high-cycle interval (Rotoflex, f = 30 Hz, T = 20 C ± 10 C, R = -1), the fatigue test using high frequencies (KAUP-ZU f = 20 khz, T = 20 C ± 10 C, R = -1) 4. Effect of cutting technology on the mechanical properties of Hardox 400 steel Fatigue properties, especially in the high cycle and the ultrahigh-cycle area depend mostly on the surface state of tested material. In the those range of fatigue, the stage of initiation of fatigue crack consume more than 90% of the all number of cycles, so it means, that initiation stage is the most important stage of fatigue process in the high and ultrahigh cycle region thus properties of surface and subsurface layer (what is the origin of fatigue cracks in this range of cyclic loading) of the testing materials determines the fatigue properties in this regions. Technological processes used in the production of the structural component can significantly affect the properties of surface layer. The newly developed HSLA steels, such a Hardox 400, are available in the form of plates. In the production, this plates are cutting to the final shape of produced components, and depending on the technology of the cutting, the surface layer in the edge of this cut-offs is influenced by the cutting process. The amount of the influence and the degree of the changes of the mechanical properties in those areas strongly depends on the cutting technology, because this can significantly affect the structure of the HSLA steel and thus the resulting mechanical properties and fatigue properties. In the production, the most widespread cutting technology is the plasma or laser cutting. This technologies have very high productivity and satisfactory properties (as accuracy, roughness,...) but on the other hand, they introduce some heat to the material, and this could affect the microstructure in the near-cut areas. In the Fig.4, there are shown our results of measuring of the hardness profile (HV 0.5) in the cutting zone for some cutting technologies (plasma cutting, laser cutting, guillotine cutting, water jet cutting) of Hardox 400 steel.

5 Robert Ulewicz and František Nový / Procedia Engineering 192 ( 2017 ) Fig. 4. Distribution of hardness in the zone of: Plasma cutting - Messer HyPerformance HPR130, laser cutting - TruLaser 5060, guillotine cutting - Durma 3016, water cutting -Waterjet-2060 As is shown in the Fig.4, it is clearly visible, that most appropriate methods for cutting Hardox 400 steel are guillotine cutting and water jet cutting, because of very small influence on the mechanical properties in the near-cut areas. Using of plasma cutting and laser cutting, some heat is introduced to the material, and this cause local quenching or tempering (depending on the used method). This affected area cause inhomogeneity of mechanical properties and they could serve as the preferred places for initiation of fatigue crack and this could significant decrease the fatigue properties of the Hardox 400 steel. This effect increases with increasing number of cycles, where the material is highly sensitive to any inhomogeneous behavior during cyclic fatigue loading. 5. Discussion and conclusions The obtained results are the basis for the development of new application possibilities for high strength fine-grained steels for structural components of machines and structures, as well as to improve simulation models of load and operating of semi-trailers. In the article there are presented in a very narrow range studies issues associated with the use of new materials and the need to adapt the technology. In case of Hardox steel fatigue tests showed the possibility of use for structural elements tippers, due their very good properties even in the high and ultrahigh cycle region. Investigation of the influence of cutting technology on the change of the material properties has shown that the best technology for cutting of Hardox 400 steel is water jet cutting, due to lowest effect on the mechanical properties. However, due to the high costs and low speed of cutting this technology is not optimal. This is why the most commonly used technology is the laser cutting, which guarantees the achievement of good quality parameters of the cut at optimum cost, but have some influence on the changes of microstructure and mechanical properties, so this influence must be considered in designing process, as it could decrease the fatigue properties of this structural components. Acknowledgements The research was supported by Scientific Grand Agency of Ministry of Education of Slovak Republic and Slovak Academy of Science, grant No. 1/0951/17, and project Research Centre of the University of Žilina, ITMS The authors are grateful to Eng. Pawel Szataniak, PhD., (Wielton S.A., Company) for comments about the technical aspects of semitrailers production process.

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