AN IMPROVEMENT OF CLOSED DIE FORGING OF HIGH STRENGTH STEEL PRODUCTS AND A STUDY OF ITS WELDING SUITABILITY

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1 AN IMPROVEMENT OF CLOSED DIE FORGING OF HIGH STRENGTH STEEL PRODUCTS AND A STUDY OF ITS WELDING SUITABILITY Ing. Khodr Ibrahim, West Bohemia University, Univerzitni 8, Pilsen Czech Republic ABSTRACT In automotive industry, there is an increasing demand for components of steels with improved mechanical properties. Typically, this includes steels with high strength with a combination of adequate ductility. Processing a material to obtain high strength and high ductility at the same time used to be rather difficult. A new heat treatment method with a term of Q&P process (Quenching and Partitioning), leads to achieve a combination of martensite and retained austenite with strength above 2000 MPa and an elongation of %. The morphology and distribution of retained austenite leads to the resulting mechanical properties. The distribution and morphology and the volume fraction of retained austenite were examined using diffraction analysis (SAED) and bright and dark-field illumination and X-ray diffraction. Welding of HSS is a widely common subject for discussion on a global scale. These processes cause mechanical properties of the weld metal and the heat-affected zone to decrease, which may weaken the quality of the fabricated structure. The effects of electron beam welding on mechanical properties of an AHSS steel had been studied. KEYWORDS AHSS, Q&P process, SAED, closed die forging, VEBW, GFaP. INTRODUCTION Due to the increasing necessities of various industrial applications for steel such as closed-die steel forgings, with better mechanical properties like high strength and good ductility, few experimental advanced high strength steel have been developed. Closed-die steel forging products are made by hot forging preforms, then it is converted into a desired part shape using plastic deformation processes in impression closed dies. A typical microstructure after quenching consists of martensite. It exhibits high strength but it has very poor ductility. This causes problems in parts which lead to a failure in service under a various operating load. Hence, parts that contain martensite are normally tempered after quenching. Today, a useful modern metallurgical procedures are available for achieving higher ductility. They include long-time lowtemperature austempering, the process of TRIP steels, and the Q&P process. Long-time low-temperature austempering can produce tensile strengths of up to 2500 MPa and hardness levels of HV10 [1]. Long-time low-temperature austempering is characterised by long holding times of several tens of hours and low temperatures. The resulting microstructure consists of very fine bainitic ferrite [1]. The concept of TRIP steels depends on achieving a mixture of bainite, ferrite and retained austenite formed by intercritical annealing and isothermal holding at the bainitic transformation temperature during a controlled cooling process [2,3].The third method Q&P which is characterised by rapid cooling from the austenite region to a temperature between the Ms and Mf [4,5]. For this purpose the experimental steels were designed to achieve a lower transformation temperature of Ms and Mf which leads to obtain a higher volume fraction of retained austenite and achieving a suitable forgeability [6,7,8]. On the other hand a welding suitability test with vacuum electron beam welding for similar joints was used to test the effects of VEBW on the mechanical properties of this steel. This steel was used to produce a hollow stock which was manufactured in the GFaP (Gas Forming and Partitioning). The use of such steel has some limitations because of the results of conventional welding processes. Electron beam welding is a very reliable process for making deep, narrow and parallel welds with a small heat affected zone with a comparison with conventional welding processes [9]. EXPEREMENTAL PROGRAMME The use of the Q&P process in practice is depending on the ability to interrupt the quenching between the Ms and Mf temperatures. With this challenge in mind, four new experimental steels have been proposed. Their special chemistries were designed to reduce the Ms and Mf temperatures (Tab. 1). In all of these first four experimental steels, the Ms and Mf temperatures were depressed through the addition of manganese. Manganese is a main element for austenite-formation which has an effect on transformations during heat treatment of steel. Manganese stabilises austenite, increases the solubility of carbon in austenite. Its diffusion rate is very low. To increase the material s strength, silicon and chromium have been added as well. Silicon was chosen in order to prevent carbide formation and thus to provide adequate supersaturation of martensite with carbon. Molybdenum was employed to both reduce the Ms and Mf temperatures and shift the start of ferritic and pearlitic transformations towards lower cooling rates. Nickel was added in small amounts to

2 stabilise austenite during cooling, to enhance hardenability and to provide solid solution strengthening. The carbon content was the same in all steels between 0.42 and 0.43 %. C Mn Si Cr Ni Mo Ms [C ] Mf [C ] AHSS AHSS AHSS AHSS AHSS Table 1: Chemical composition of experimental steels AHSS 1-5 [w%] In the AHSS-1 steel, the manganese level was 2.5 % and the silicon level was 2 %. The JMatPro software was employed for calculating the approximate transformation temperatures. The calculated results of Ms and Mf temperatures in this steel were 218 C and 88 C, respectively. In order to detect the effect of molybdenum on microstructural evolution and the shift in transformation temperatures in the AHSS-2 steel, its content was increased to 0.16 %. This increase in molybdenum content, however, has not altered the Ms and Mf temperatures in any essential way. The Ms temperature was 214 C and the Mf was 83 C. In AHSS-3, the nickel level was set at 0.5 %, to achieve the desired hardenability and to depress the martensitic transformation temperatures. The Ms and Mf temperatures were 209 C and 78 C, respectively. In the AHSS-4, the molybdenum content was increased along with the nickel content. The combination of these elements led to the lowest transformation temperatures: Ms=204 C and Mf=73 C. In the AHSS-5 as its shown that the Mn content is less than the rest experimental steels and this steel was chosen to create a hollow stock. Application of Q&P process to heat treatment First, a heat treatment schedule for the forged parts has been developed. The development involved testing of an austenitising temperature TA, three different quenching temperatures QT, and various carbon partitioning temperatures PT, using small specimens in a thermomechanical simulator MTS 810 (Material Test System). (Tab. 2) lists the parameters of the physical simulation, the resulting elongation values A5mm and the fractions of retained austenite RA in the matrix. Based on the results of this modelling, one experimental steel was chosen AHSS-3. To verify the process, two complex-shaped closed-die forgings were made of these steel (Fig. 1) with three forging steps. Two steel bars with different dimensions were used for the forging process, with diameters of 45 mm, 31 mm and length of 160 mm, 180 mm. These forging products were then heat-treated and their microstructures and mechanical properties were evaluated using small flat specimens for tensile test that were machined from the forged parts, its dimensions were 5 mm of reduced section and cross section mm. TA[ C]/tA [s] Cooling rate [ C/s] QT [ C] PT [ C] /tpt [s] A 5mm [%] UTS (Rm) [MPa] RA [%] 850/ / / / / / / / Table 2: Heat treatment schedules

3 Figure 1: Demonstration products: forgings of AHSS steels heat-treated using the Q&P process Vacuum electron beam welding process In this experiment, the high-strength steel AHSS-5 was chosen for the electron beam welding process and (table 1) shows its chemical composition. It was processed by GFaP and supplied in the form of hollow stock. The hollow stock was manufactured in the GFaP (Gas Forming and Partitioning) technology chain which comprised hot internal high pressure forming, press hardening and then Q&P process. Four specimens of mm were cut from the hollow stock to study the mechanical properties and the welding suitability. These specimens were then electron beam-welded and (table 3) shows the electron beam welding parameters. Thickness (mm) Sheet dimensions (mm) Weld method Penetration Current (ma) Accelerating voltage (kv) Working distance (mm) 2 50*80 butt middle Table 3: Welding parameters Average values were calculated from the measured data. The ultimate strength of the input stock was approximately 2100 MPa and the A5mm elongation was more than 15 % (Tab. 4).To allow the mechanical properties before and after welding to be compared, reduced tension test pieces were cut from the welded specimens (Fig. 2). Figure 2: Miniature tension test piece with a weld Rp0.2 [MPa] UTS (Rm) [MPa] A5mm [%] HV10 [-] Initial condition Electron beam welded Table 4: Tensile test data for the stock before and after electron beam welding DISCUSSION OF RESULTS The best results of the thermomechanical simulation were acquired with the cooling rate of 1 C/s. The ultimate strength was in the range of MPa; the elongation was 15 % which is a result of the highest achieved volume fraction of retained austenite in the martensitic matrix up to 18 % by volume. Up on these findings, a low cooling rate has been recommended for processing the forgings. Heating was done in a furnace to the austenite region, the temperature was approximately 850 C. Quenching with hot oil ended at the temperature of 150 C. Austenite was stabilised at 200 C, thanks to carbon which migrates from the super-saturated martensite to austenite. After the heat treatment of the forgings, the microstructure was studied and mechanical tests were carried out. Electron microscopy proved the presence of

4 martensitic microstructure with a small amount of bainite (Figs. 3). Quenching with hot oil led to an ultimate strength of up to 2200 MPa and an elongation of approximately 12 %. The fraction of retained austenite in the martensitic matrix was 15 %. Retained austenite in the martensitic matrix was identified using dark-field illumination and selected diffraction spots. It was found along prior austenite grain boundaries, subgrain boundaries and boundaries of martensite laths (figure 4). (a) (b) Figure 3: Optical micrograph (a) and scanning electron micrograph (b) of martensitic structure with a small proportion of bainite. After quenching in an oil bath at the temperature of 150 C and austenite stabilisation in a furnace at 200 C (a) (b) Figure 4: Distribution of retained austenite under dark field illumination (a), and (b) Diffractogram of the martensitic matrix with marked spots taken to verify the presence of austenite. Zone axis of diffraction: z : [3 5 6]-AHSS-3 For the welding results, examination of the microstructure using optical microscopy revealed the presence of coarsegrained structure in the weld, with columnar grains aligned in the direction of heat dissipation into the base material. Fractographic characterisation of the fracture surfaces (Fig. 5) was carried out to estimate the influence of welding on the fracture behaviour of the material. Macroscopic examination of the fractures did not observe any relations of plastic deformation. In specimens A and B, cup fractures were found. In C and D specimens, the macroscopic appearance of the fracture showed the river pattern. Fractographic examination of the centres of fracture surfaces proved the presence of ductile fracture. AHSS-5. D. Beam welding AHSS-5. B Figure 5: Fractographic examination of fracture surfaces

5 For the purpose of evaluate the effect of welding on the microstructure and mechanical properties related to it, microhardness measurement was carried out along lines across the specimens. The lines started near the fracture in the weld and ended on the surface of the base material. In the AHSS-5 A - Beam welding specimen, a drop in hardness in the fracture location was detected. In the HAZ, the hardness increased again, up to the HAZ boundary. Beyond it, the hardness dropped again, possibly due to the tempering of the material (figure 6). The microstructure in the fracture location revealed signs of network along grain boundaries. The network had higher levels of chromium and silicon. In the other specimen, no decrease in hardness in the fracture location was observed, unlike in the specimen AHSS-5 A - Beam welding. However, the drop in hardness on the HAZ boundary was the same for other samples which it could be due to that the microstructure had been tempered as well. AHSS-5. A. Beam welding CONCLUSION Figure 6: Line measurement of microhardness of the welded specimen The heat treatment of closed die forging parts made of developed experimental steels led to martensitic microstructures with a fraction of stabilised retained austenite. The sequence of heat treatment was based on the quenching and partitioning process (Q&P). First, physical simulation was carried out on specimens. The findings were then translated into the processing of real-life demonstration closed die forging products. The physical simulation led to strengths of up to 2150 MPa and A5mm elongation levels of approximately 12 %. The amount, morphology and distribution of retained austenite have crucial impact on the resulting mechanical properties, an examination by X-ray diffraction and diffraction analysis (SAED) and bright and dark-field illumination have been employed. The retained austenite fraction was up to 15 %. After the findings and parameters were translated into the real-life process, a simple heat treatment of real-life forgings led to equivalent values of mechanical properties. The processing was carried out in hot quenching oil at 150 C and in a furnace at the partitioning temperature of 200 C. Using electron beam welding, specimens of an AHSS-5 have been welded. This AHSS had an ultimate strength of 2100 MPa and A5mm elongation of more than 15 %. The results of tensile test showed that the material retained high ultimate strength of more than 1780 MPa with an A5mm elongation level of more than 6 %. Ductile fracture was observed in all specimens. The HAZ in all specimens contained a mixture of martensite and bainite. The results of physical simulation and real-life processing and welding tests open new opportunities for closed-die forging products which have a good forgeability welding suitability. The key tasks for the future involve an optimisation of the cooling rate, a correct choice of cooling process and media and also finding out a way to apply an electron beam welding with more productivity. High ultimate and fatigue strengths will permit the designs of forged parts to be altered to thinner walls and applications with smaller thickness and weight and more complex shapes.

6 ACKNOWLEDGMENT This paper includes results created within the project SGS New Martensitic Structures - Process Parameters and Properties. The project is subsidised from specific resources of the state budget for research and development. REFERENCES [1] X.Y. Long, F.C. Zhang, J. Kang, Low-temperature bainite in low-carbon steel, Materials and Science Engineering, 594 (2014), , doi: /j.msea [2] H.K.D.H. Bhadeshia, S.R. Honeycombe. The Bainite Reaction. In Steels: Microstructure and Properties, 3rd edn, Butterworth-Heineman: Oxford, (2006), , ISBN: [3] Caballero, FG., Bhadeshia, H. K.D.H. Very strong bainite, Current Opinion in Solid state and Materials Science, Vol. 8, Issues 3-4, 2004, pp [4] L. Kucerova 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, vol. 18, pp , [5] J. Speer et al., Carbon Partitioning into Austenite after Martensite Transformation, Acta Materialia, vol. 51, pp , May [6] B. Mašek, H. Jirková, D. Hauserová, L. Kučerová, D. Klauberová, The Effect of Mn and Si on the Properties of Advanced High Strength Steels Processed by Quenching and Partitioning, Materials Science Forum, (2010), 94-97, doi: / [7] 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 Silicon. Materials Science Forum, 2012, Vols , pp [8] JIRKOVÁ, H. et al.: Influence of metastable retained austenite on macro and micromechanical properties of steel processed by the Q-P process, Journal of Alloys and Compounds, available online, Journal of Alloys and Compounds, 615 (2014) pp.163 S168 [9] ASM INTERNATIONAL HANDBOOK COMMITTEE, OLSON, D. L.; et al., Eds. ASM Handbook: Volume 6: Welding, Brazing, and Soldering, 10th ed.; ASM International: University of California, 1993.