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Dry Austempering Heat Treatment Process: Interactions Between Process Parameters, Microstructural, and Mechanical Properties

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Dry Austempering Heat Treatment Process: Interactions Between Process Parameters, Microstructural, and Mechanical Properties Trattamenti termici Dry austempering heat treatment process: interactions between process parameters, microstructural, and mechanical properties E. Dabrock, L. Hagymasi, T. Krug, F. Sarfert, E. Kerscher The new heat treatment process dry austempering, which is a vacuum austenitization process followed by high pressure gas quenching, leads to changes in process flow and process parameters compared to the austempering process in a salt bath. Therefore, the interaction between the process parameters and the generated microstructure is not completely understood. For that reason, a series of tests were carried out with systematically varied process parame- ters using bearing steel 100Cr6. This paper is focused on the effect of the austenitizing temperature and the differences between a one-step and a two-step transformation process on the mechanical properties. So far, hardness measurements and cyclic load increase tests as well as microstructure analysis with scanning electron microscopy were performed. In addition, in situ XRD (iXRD) measurements were used to understand the bainitic transformation kinetics. KEYWORDS: BAInITe - AuSTempeRIng - HIgH pReSSuRe gAS quenCHIng - DRy AuSTempeRIng - 100CR6 - In SITu XRD - vacuum pRoCeSS INTRODUCTION The development and realization of concepts to optimize engines The components of today’s modern fuel injection systems are ex- in terms of performance and fuel-consumption poses a higher posed to high pressure load cycles, which result in high require- thermo-mechanical load on the components due to the resulting ments regarding the mechanical and microstructural properties increase in pressure. Here, the classic bainitic heat treatment with of the materials. Steels with bainitic microstructure are the first salt bath quenching is increasingly reaching its limits. The use of choice to satisfy these demands. atmospheric technology during the austenitization process can Such microstructures ensure a good combination of high strength cause a negative influence on the near-surface material compo- and sufficient toughness at a low percentage of retained austen- sition due to decarburization or carburization and oxidation, ite. Bainitic heat treatment also results in a low distortion and a thus, an additional hard machining is necessary. This, again, higher plasticity in comparison to martensite. The currently ap- leads to a decrease in the load capacity of the components. In plied technology for the generation of a bainitic microstructure addition, especially for injection systems, many components, uses an atmospheric austenitization furnace followed by a salt such as valve bodies or valve pieces, have blind holes and bath quenching. bores with a high length-to-diameter ratio, thus it is becoming more and more difficult to clean those complex geometries once the salt bath treatment is done. The toxic salt bath is also hazard- ous in both operation and disposal under environmental aspects. The so-called dry austempering, which is a vacuum proc- E. Dabrock , F. Sarfert ess followed by a high pressure gas quenching, avoids these Robert Bosch gmbH, Renningen, germany drawbacks and also offers the potential of high process control and almost in-line capability [1]. The distortion is also minimized L. Hagymasi, T. Krug through a variable quenching intensity. The process conditions Robert Bosch gmbH, Stuttgart, germany enable optimized charging concepts with adequate positioning of the components thus allowing a targeted fluid flow with re- spect to critical component areas and result in a lower scatter per E. Kerscher batch. Also due to the lower heat transfer coefficient, the process materials Testing, university of Kaiserslautern, is limited to a smaller component volume for a given material Kaiserslautern, germany compared to the salt bath process. The new process equipment for the dry austempering heat treat- ment leads to changes in process flow and process parameters. Both differ significantly compared to the austempering process in La Metallurgia Italiana - n. 10 2015 35 Memorie salt bath. Therefore, the interactions between the new heat treat- There are two kinds of cementite precipitations in lower bainite: ment process and the generated microstructure are not com- those which precipitate inside the supersaturated ferrite (in a pletely understood. For that reason, a series of tests were carried single crystallographic variant of about 60° to the growth direc- out with systematically varied process parameters. The aim of the tion of the ferrite platelets) and others which grow from investigation is to understand the interactions between process the carbon-enriched retained austenite between the bainitic fer- parameters and the microstructural and mechanical properties. rite platelets. Due to the reduced diffusion of the carbon in the This paper is focused on the effect of the austenitizing temper- austenite, as a consequence of the low temperature, fewer and ature and the differences between a one-step and a two-step finer carbides precipitate between the needles in lower bainite transformation process on the mechanical properties. than in upper bainite. The absence of these coarse and brittle In situ X-ray diffraction measurements show the effects of these particles makes the lower bainite interesting for industrial ap- process parameters on the transformation kinetics. This kinetics plications. is related to the resulting mechanical and material proper- Increasing austenitizing temperature increases the level of dis- ties obtained from hardness measurements, cyclic load increase solved carbon in the austenite. Dis-solved carbon is the main in- tests, and microstructural investigations by scanning electron fluencing factor with respect to the morphology of bainite. With microscope. in-creasing carbon content in austenite the growth of the bainite needles stops earlier because the difference between the gibb´s BAINITIC TRANSFORMATION energies of austenite and ferrite is lower. Accordingly, the nee- There are several different models to explain the bainite transfor- dles become thinner and more numerous [4]. In addition, the mation [2]. The scientific preponderance is in favor of a displacive incubation period is prolonged, the bainite start temperature theory of transformation [3]. The driving force behind the trans- is lowered and the martensite start temperature decreases [5; 6]. formation is given by the difference between the free energies Raised carbon content also supports the formation of carbides of the parent phase and the product phase. The transformation from both the austenite and ferrite. of austenite to ferrite of the same composition has a thermody- Because of the incomplete reaction phenomenon and the slow namic driving force as long as the austenite has a higher free diffusion of carbon into austenite, the transformation to an al- energy than the ferrite. The transformation starts with a subunit most retained austenite-free condition takes a long time which of bainitic ferrite which nucleates with carbon partitioning at is not practical for industrial application. Therefore, a two- the austenite grain boundaries. This subunit grows displacively. step bainitic transformation (increasing of the transformation The large strains associated with the shape change cannot be temperature after a certain time) is applied in the industry to sustained by the austenite, which has a lower strength at high reduce the process time. It was already shown that the two-step temperatures. So the growth is halted by plastic relaxation in the transformation results in a more ductile material behavior with adjacent austenite. In comparison to the martensitic transforma- improved strength properties [5; 7; 8]. This is due to a relaxation tion, carbon diffuses in the adjacent austenite during the bainitic of the transformation induced distortions at higher tempera- transformation due to the high transformation temperatures. The tures. The consequence for components is that the stress next plate has to grow from a carbon-enriched austenite. This fields are more homogeneous, which results in a better ductility. process continuous until diffusionless transformation becomes impossible at the so called T0-boundary, at which austenite and EXPERIMENTAL ferrite have equal gibbs free energy and zero driving force [3]. Material and samples A further transformation can only occur by increasing the free For the investigations, specimens were manufactured from energy of the austenite, for example, by reducing the carbon con- the bearing steel 100Cr6 (1.3505, AISI 52100) with a high tent due to carbide precipitation from the austenite. cleanliness level (fine size and distribution of non-metallic in- Depending on the transition temperature and the related pre- clusions) produced by ingot casting. The steel billets were cipitation kinetics, a differentiation between upper and lower hot-rolled to bars of 14 mm in diameter and spheroidized bainite is done. At low transformation temperatures, no annealed. This steel is frequently applied in automotive significant fraction of the carbon can diffuse away from the fer- industry, e.g. for diesel injection components. The chemical rite into the austenite due to the low diffusivity of the carbon and composition of this steel is shown in Table 1. the high transformation speed. There is thus initially a martensitic The specimens for mechanical testing were hard machined after transformation of austenite at nearly full carbon supersaturation the heat treatment process. The
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