COMBINATION OF INTERNAL HIGH PRESSURE FORMING AND Q-P PROCESS FOR PRODUCTION OF HOLLOW PRODUCTS FROM AHS STEEL

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1 COMBINATION OF INTERNAL HIGH PRESSURE FORMING AND Q-P PROCESS FOR PRODUCTION OF HOLLOW PRODUCTS FROM AHS STEEL Bohuslav Mašek a, Ivan Vorel a, Hana Jirková a, Petr Kurka b a University of West Bohemia in Pilsen, Research Centre of Forming Technology FORTECH, Univerzitni 8, Pilsen, Czech Republic, EU, masekb@vctt.zcu.cz b Fraunhofer Institute for Machine Tools and Forming Technology IWU, Germany, Petr.Kurka@iwu.fraunhofer.de Abstract All sectors of industry experience high demand for shaped products with as good mechanical properties as possible at low costs. Automotive industry, in addition, requires that the parts are of lightweight construction. Consequently, new types of materials and processes have to be combined to design new production chains capable to meet this demand. For instance, there are high-strength low-alloyed steels, whose final properties are attained by advanced heat treating techniques. One of such techniques is the Q&P process which can deliver excellent ultimate strengths exceeding 2000 MPa at a sufficient elongation level of 10 %. When combined with an unconventional forming method, it allows complex-shaped parts with outstanding mechanical properties to be made. One example of such combined procedure is the sequence of internal high pressure forming, hot stamping and Q&P processing. In the present study, thin-walled hollow stock was processed using such a combined procedure. After stepwise optimization of processing parameters, products with martensitic structure and a small amount of bainite were obtained. In all locations of the product which were tested, the ultimate strength exceeded 1950 MPa and elongation reached 15 %. Keywords: Q&P Process, hot stamping, internal high pressure forming 1. INTRODUCTION Advanced high-strength steels (AHSS) find use in a wide range of industrial applications. Their excellent mechanical properties allow the mass of the engineering parts made of them to be reduced. It is desirable to seek new processing options for these materials and to explore combinations of various processes in sophisticated production chains. One of such effective processes is Quenching and Partitioning (Q&P). 1.1 Q&P Processing Q&P processing is an advanced heat treating technique for high-strength steels [1], (Obr. 1). It includes austenitizing and quenching to a temperature (QT) between the Ms and Mf temperatures to obtain a mixture of martensite and retained austenite in the product [2,3]. It also relies on carbon diffusion processes taking place between the Ms and Mf temperatures. Obr. 1 Schematic of Q&P process [4]

2 Carbon from the super-saturated martensite migrates to retained austenite during isothermal holding at the partitioning temperature (PT), making the austenite stable [4].For the retained austenite to remain in the microstructure, it is essential that carbon does not precipitate from martensite and form carbides during isothermal holding at the partitioning temperature. To prevent this, the steel should contain silicon, aluminium or phosphorus additions. The critical cooling rate can be reduced by adding chromium and manganese which shift the pearlitic and bainitic transformation curves towards longer times. However, the amount of chromium should not suppress the retarding effect that silicon has on carbide formation. Manganese, silicon, chromium and aluminium have favourable effect on solid solution strengthening. Manganese, an austenite stabilizer, effectively prevents formation of free ferrite in the microstructure and thus contributes to strengthening. Silicon, on the other hand, promotes free ferrite formation and stabilizes retained austenite [5,6]. Q&P process, an advanced heat treating technique, can be combined with unconventional forming procedures. One of available ways to manufacturing complex-shaped parts with excellent mechanical properties is the use of a new technology chain relying on internal high pressure forming, press-hardening and Q&P processing (Obr. 2). [7] Obr. 2 Technology chain combining internal high pressure forming, hot stamping and Q&P process 1.2 Internal High Pressure Forming Forming by internal pressure of nitrogen gas is an unconventional process for making complexshaped hollow products. The heated stock is enclosed in a die and shaped by internal pressure of nitrogen gas (Obr. 3). The process can be altered by varying the die opening time, the period after which the die opens and the product is removed at the corresponding temperature. Obr. 3 Forming of holloq parts by internal gas pres sure 2. EXPERIMENTAL PROGRAMME In the experimental programme, internal high pressure forming was trialled in conjunction with Q&P processing on low-alloyed high-strength 42SiCr steel (Tab. 1). The main alloying elements of the material were manganese, silicon and chromium. Tab. 1 Chemical composition of 42SiCr Steel [weight %] C Si Mn Cr Mo Nb P S Ms Mf

3 The key phase transformation temperatures, Ms and Mf, were tentatively determined by calculation. The calculated Ms temperature was 289 C and the Mf was 178 C (Tab. 1). The microstructure of the tubular stock consisted of ferrite and pearlite, having a hardness of 295 HV10, ultimate strength of 981 MPa and elongation of 30 %. 2.1 Process Trials The tubular stock diameter, length and wall thickness were 43 mm, 380 mm and 4 mm, respectively (Obr. 4). The stock was heated for 25 minutes in furnace at 915 C. This high temperature guaranteed full austenitizing. The stock was then placed in a die at the ambient temperature. After the die was closed, the stock was pressurized with nitrogen gas at 700 bar. The contact of the expanded stock with the die walls led to rapid cooling of the material. In order to change the final temperatures in the workpiece quenching process, the die opening times were varied from 5 to 20 seconds. Once the products were removed from the die at temperatures between 180 and 250 C, two process routes were used in order to map the effect of the Q&P process on the resulting microstructure and mechanical properties. The first route only involved air cooling, whereas in the second process route the product was immediately placed in a furnace at 260 C and held for 25 or 35 minutes to allow carbon partitioning and austenite stabilization. Obr. 4 Experimental stock and the resulting shape after processing 3. RESULTS AND DISCUSSION By means of the varied die-opening times, products with different microstructures and properties were obtained. Two products, each coming from one of the two process routes, were selected for characterization and mechanical property measurement. The first route was represented by a product with the die-opening time of 15 s (Q210 C Cair), which was quenched to 210 C and air cooled. The second route with the Q&P process was represented by a product with the same die-opening time (Q200 C P200 C) and with the quenching temperature of 200 C. The partitioning process for stabilizing retained austenite took place in furnace at 260 C for 25 minutes (Tab. 2). Tab. 2 Parameters of experimental heat treatment Austenitization QT PT Partitioning time [min] (Q 210 C C air) (Q 200 C P 200 C) Samples for metallographic observation and measurement of mechanical properties were taken from both products after the process. The locations for taking samples were selected so as to explore the effect of the process and the sample orientation on mechanical properties. Therefore, samples were taken from the

4 gripped ends, from areas where the product cross-section, and thus the strain was the largest and from transition zones between different diameters and shapes (Obr. 5). A total of 14 test specimens were taken from each product. Average values were calculated from data for identical locations. Obr. 5 Sampling locations on the product for measurement of local mechanical properties 3.1 Mechanical Properties In order to fit the tension test specimen in all locations of the product, the size of miniature test specimes was selected; with a gauge length of 5 mm and a cross-section of mm (Obr. 6). Obr. 6 Specimen for mechanical testing The product which was removed from the die at 210 C and cooled in air (Tab. 3) exhibited very high strength. Strengths in all measured locations exceeded 2260 MPa at the A5mm elongation level of more than 10 %. The lowest strength, 2263 MPa, and elongation of 12 % were found in the transitional zone of the largest cross-section (Obr. 4, location T3/5). It is likely that this area did not come into full contact with the die wall, and therefore did not cool as rapidly as others. This location also had the lowest hardness: 667 HV10. Other areas of the product did not show equally strong effect of the location on mechanical properties. Tab. 3 Rerults of tensile testing of the semiproduct (Q210 C - Cair) Position R p0.2 R m A 5mm [%] HV10 [-] T3/ T3/ T3/ T3/ T3/ T3/

5 Products of the second route with the Q&P process exhibited ultimate strength levels which were MPa lower than those of the first route (Tab. 4). This difference is due to the martensite tempering process which took place during the isothermal hold at 260 C in the furnace. The lowest strength of the Q&P processed part was found in the same location as in the air cooled product. In this case, the strength and elongation were 1914 MPa and 20 %, respectively (Obr. 5, location T7/5). In other locations, the ultimate strength was close to 2000 MPa. On the other hand, the elongation levels upon the Q&P process were substantially higher: between 17 and 20 %. Tab. 4 Results of tensile testing of the semiproduct (Q210 C - Cair) upon air cooling Position R p0.2 R m The tests were complemented with fracture surface observation. Ductile fractures with dimples were found in all cases (Obr. 7). No signs of brittle fracture were detected even in the locations of the air-cooled product with the highest strength. A 5mm [%] HV10 [-] T7/ T7/ T7/ T7/ T7/ T7/ Obr. 7 Fracture surfaces: a) air cooled product (Q 210 C - C air), location T3/3; b) Q&P processed product (Q 210 C - C air), location T7/3 In order to obtain complete data on mechanical properties along the entire length of the product, HV10 hardness profiles were measured (Obr. 8). Hardness profiles of both products showed similar character. They reflected the cooling rate, which was governed by the contact between the stock and the die, and the reduction in the wall thickness due to forming. The subsequent heat treatment only affected hardness values but not the shape of the hardness profile along the product axis. The highest hardness was found on both ends, in areas by which the stock was held in the die. The Obr. 8 Hardness profile along the axis of the product lowest hardness was found in the transition area of the largest diameter section. These results are in agreement with the measured strength levels.

6 3.2 Assessment of Microstructure Microstructure observation was performed in the same locations as mechanical testing. The product quenched to 200 C and air cooled (Q210 C Cair) contained martensite with a small amount of bainite in almost all examined locations (Obr. 9). The areas which did not come into full contact with the die wall contained an additional small amount of free ferrite which formed due to the lower cooling rate (Obr. 10). Obr. 9 Product (Q210 C - Cair) with air cooling: martenzite-bainite microstructure (position T3/2) Obr. 10 Product (Q210 C - Cair) with air cooling: martenzite-bainite with small fraction of ferrite (position T3/12) The Q&P-processed product contained a mixture of martensite and bainite. The proportion of free ferrite was very small. Ferrite was again found in locations which lacked the full contact with the die wall (Obr. 11, 12). Obr. 11 Product (Q210 C - Cair) with Q-P proces: Martenzite-bainite microstructure (position T7/2) Obr. 12 Product (Q210 C - Cair) with Q-P proces: Martenzite-bainite with small fraction of ferrite (position T7/12) 4. CONCLUSION Hot forming by internal pressure of gas followed by press hardening and heat treatment was used for making complex-shaped hollow products without discontinuities and with high-quality surface. In the product (Q210 C Cair), which was air cooled after removal from the die, the die opening time of 10 seconds led to quenching to the temperature of 210 C. This is 79 C higher than the calculated Ms temperature. The resulting microstructure was a mixture of martensite, bainite and free ferrite.

7 The product of the otherwise identical route which, however, included Q&P processing (Q200 C P200 C) had a temperature of 200 C when extracted from the die. Carbon partitioning by diffusion took place during 25 minutes in furnace at 260 C. The resulting microstructure consisted of tempered martensite, bainite and a small amount of free ferrite. The isothermal holding in furnace led to a lower strength of approx MPa in the Q&P processed product (the product which cooled in air showed 2300 MPa). Holding at the partitioning temperature had a significant impact on the elongation value, which was 18 % (in the air cooled product, it was 10 %). The same trend was observed in hardness values. The strength levels just below 2000 MPa and elongation values A5mm=18 % achieved by a simple process in complex-shaped parts made of steel with cost-efficient chemistry can be considered a promising outcome. Given the low complexity of the manufacturing process used, they offer attractive potential for efficient production of new-generation products. ACKNOWLEDGMENTÍ This paper includes results created within the projects CZ.1.05/3.1.00/ Technological Verification of R&D Results II, individual activity Hollow Shafts for Passenger Cars Produced by Heat Treatment with the Integration of Q-P Process and CZ.1.05/2.1.00/ West-Bohemian Centre of Materials and Metallurgy. The projects are subsidised by the Ministry of Education, Youth and Sports from the European Regional Development Fund and resources of the state budget of the Czech Republic. Further the research was supported from the project SGS Support of students research activities in materials engineering field. REFERENCES [1] KUČEROVÁ, L. et al. Comparison of Microstructures and Properties Obtained After Different Heat Treatment Strategies of High Strength Low Alloyed Steel. Journal of Iron and Steel Research International, 2011, vol. 18, s [2] SPEER, J. et al. Carbon Partitioning into Austenite after Martensite Transformation. Acta Materialia, May 2003, vol. 51, s [3] SPEER, J. et al. The Quenching and Partitioning Process: Background and Recent PRogress. Materials Research, 2005, vol. 8 (4), s [4] TSUCHIYMA, T. et al. Quenching and Partitioning treatment of a low-carbon martensitic stainless steel. Materials Science and Engineering A, January 2012, vol. 532, s [5] JIRKOVÁ, H., KUČEROVÁ, L., MAŠEK, B. Effect of Quenching and Partitioning Temperatures in the Q-P Process on the Properties of AHSS with Various Amounts of Manganese and Cilicon. Materials Science Forum, January 2012, vols , s [6] KUČEROVÁ, L. et all. Optimization of Q-P Process Parameters with Regard to Final Microstrucutres and PRoperties. Annals of DAAAM For 2009 & Proceedings of the 20th International DAAAM Symposium, 2009, vol. 20, s [7] MAŠEK, B. et al. Improvement of Mechanical Properties of automotive Components Using Hot Stamping with Integrated Q-P Process. Journal of Iron and Steel Research International, May 2011, vol. 18 (1-2), s