Optimized Thermomechanical Treatment for Strong and Ductile Martensitic Steels
Araz Ardehali Barani, Dirk Ponge, Radko Kaspar, Dierk Raabe
Nowadays, quenched and tempered, medium carbon martensitic steels are widely used for applications where high mechanical strength is the main design driver. But besides strength in most engineering applications a good ductility, toughness or fatigue and corrosion behaviour are demanded. To obtain a good compromise between strength and toughness the as quenched martensite is reheated to temperatures between 150 and 700°C to obtain the required level of toughness without a dramatic loss in strength. The strength (wort doppelt verwendet – stil verbessern) increases with decreasing tempering temperature while the ductility or toughness decreases. Thus, higher strength levels can not be achieved by conventional heat treatment methods that only offer a few possibilities to design the material properties. Additionally, most commercial steels contain impurities that influence the toughness and ductility significantly. The well known embrittlement phenomenon observed around 350°C is such an example where grain boundary segregation of impurity elements together with carbide films at grain boundaries deteriorate the mechanical properties of the material [1-5].
Martensite forms by a displacive transformation from the parent austenite phase. Therefore, the grain and dislocation structure, or in general the defect structure of the austenite is inherited to the product phase, because the relationship between neighbouring atoms does not change. Thus the microstructure of the austenite is very crucial for the final properties of the martensite. Research at our Institute has shown that austenite deformation prior to quenching can lead to an increase in strength without adverse effects on ductility or toughness. For a medium carbon chromium vanadium steel Peters and Wettlaufer demonstrated [6-9] that by deforming the austenite, its grain size and grain substructure can be controlled in such a way to produce high strength martensitic steels with excellent ductility, toughness and endurance limit.
The silicon chromium steel 55SiCr6 (Fe-0.55C-1.4Si-0.65Cr-0.65Mn (mass%)) was selected for our investigations. Since the early 90ies most of the automotive coil springs in Europe are made of this steel grade. It exhibits high strength and good sag resistance [12-13]. To study the sensibility to harmful elements the content of phosphorous (0.0023 to 0.0213 mass %), of copper (0.169 to 0.540 mass %), and of tin (0.0193 to 0.060 mass %) was systematically varied. The ratio of tin to copper was always kept constant at 0.1. The experimental alloys used were prepared from high-purity base materials as 70kg vacuum induction melts. Bars were rolled after annealing at 1100°C for heat treatment and thermomechanical treatment experiments. All the treatments were carried out using the large scale 2.5MN hot press at Max-Planck-Institut für Eisenforschung [14-15].
It was reported earlier  that the maximum in strength and ductility of martensite is achieved when prior to quenching the austenite is deformed around 900°C with a strain slightly below the peak strain . To understand the effect of recrystallised and non-recrystallised portions of the microstructure on the mechanical properties including the effect of the impurity elements, we kept the deformation strain (0.4) and rate (5/s) constant and varied the deformation temperature. The microstructure was observed after quenching.
Fig. 1 summarizes the results for the conventional heat treatment. The well-known dependence of ductility and strength with tempering temperature was observed. Additionally the effect of the impurity elements was determined for various tempering temperatures and processes. A strong reduction of the ductility is caused by
Fig. 1 Tensile properties of quenched and tempered 55SiCr6
Fig. 2 Combination of strength and ductility for 55SiCr6 with 0.54 Cu and 0.054 Sn (mass %)
addition of phosphorous or the addition of copper and tin. In Fig. 2 the combination of ductility and strength is shown for the melt with the highest copper and tin concentration. The reduction of area of conventionally heat treated samples is relatively low at all tempering temperatures and only after tempering at 450°C it reaches values above 25 %. To ascertain the effect of the austenite microstructure and condition on the resulting mechanical properties samples were deformed at various temperatures prior to quenching and tempered at 350 and 400°C. All deformed samples exhibit a higher ductility than the conventionally heat treated samples tempered at the same temperatures. A deformation can improve the ductility above the value obtained after conventional heat treatment and tempering at 450°C. Fig. 3 compares the fracture surfaces of a conventional heat treated sample tempered at 350°C with a thermomechanically treated sample tempered at the same temperature. The first fails in an intergranular brittle way while the second shows no sign of embrittlement. It can be shown that deformation of the austenite can refine or eliminate the formation of carbide films at prior austenite grain boundaries (Fig. 4).
Fig. 3 Scanning electron micrographs of fracture surfaces of 55SiCr6 samples with 0.54Cu and 0.054 Sn (mass %) tempered at 350°C. a) conventionally heat treatment, b) thermomechanical treatment
Fig. 4 Scanning electron micrographs of 55SiCr6 samples with 0.54Cu tempered at 350°C. left) conventional heat treatment, centre and right) thermomechanical treatment
Two austenite conditions, a recrystallised and a non recrystallised condition corresponding to deformations at 850 and 750°C respectively, were selected for further investigation. Samples were austenitized at temperatures between 900 and 1000°C and then deformed at the aforementioned temperatures to produce the two austenite conditions. The mechanical properties of these samples after tempering at 300°C are compared for the steel with the highest phosphorous concentration (Fig. 5). Independent of austenitization temperature, the reduction of area of the TMT samples is at least three times higher than after conventional heat treatment. The reduction of area of the recrystallised austenite modification (TMTRX) is independent of austenitization temperature. After deformation at 850°C the austenite recrystallises and grain refinement occurs. Independent of austenitization temperature the recrystallisation after deformation at 850°C led always to a similar austenitic grain size. The reduction of area of the non-recrystallised austenite modification (TMTNRX) increases with decreasing austenitization temperature. The austenite grain size is dependent on the austenitization temperature. The deformation only elongates the austenite grains and produces a work-hardened austenite. It does not refine the grain size. From the observations an optimized thermomechanical treatment was tested for the entire composition range: A first deformation is carried out at 850°C to refine the austenite grains followed by a second deformation at 750°C to produce the desired work-hardened defect structure. The enhancement of ductility for the same strength level (above 2200 MPa), i.e. same tempering temperature (300°C) is presented in Fig. 6. This two-step thermomechanical treatment delivers mechanical properties that are not sensitive to impurity element content (within the range tested).
Fig. 5 Reduction Influence of austenitization temperature upon ductility for CHT and TMT. 55SiCr6 with 0.021mass % phosphorous
Fig. 6 Reduction of area of CHT- and two-step TMT samples after tempering at 300°C. Steel 55SiCr6 with different P-content.
Applying a deformation prior to quenching improves the mechanical properties of quenched and tempered martensitic steels. The austenite microstructure prior to quenching has a strong influence upon the size distribution of the martensitic units (i.e. packets, blocks, lath), the dispersion of carbides during tempering, and the carbide morphology at the prior austenite grain boundaries. Furthermore, the microstructure can be tailored in such a way to minimize the sensitivity to embrittlement by reducing the segregation driving force or by maximizing the grain boundary area, and by doing so decreasing the grain boundary concentration of the impurity elements. Thereby, a significant improvement of both strength and ductility is possible at the same time.
The results show that not only deformation at low temperatures but even at deformation temperatures above the recrystallisation temperature lead to remarkable improvements of the properties. A higher deformation temperature reduces the difficulties of applying thermomechanical treatment in technical production processes. Addition of microalloying elements like vanadium can further be used to refine the austenite grain size during austenitization or enhance the possibilities of microstructure control by a thermomechanical treatment.
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