Papers by Keyword: Tempering

Abstract: Economically alloyed steels for critical details of drill pipes have been studied. Effect of microalloying on structure and properties has been investigated. The article shows that boron and niobium additives change the structure and properties of Cr-Mn-Mo steels after quenching and high tempering. Methods of optical and electron microscopy have been used. Basic mechanical properties and impact strength of investigated steels are determined. Optical and electronic fractography has been carried out. The quantitative content of the ductile component is determined in steel fracture. It is shown that steel microalloying leads to a substantial structure refinement. This is due to the influence of niobium on the austenite grain value. An increase in the amount of carbide particles leads to structure refinement with an increased molybdenum content. Boron microadditives allow obtaining the tempered martensite structure throughout the product section. This provides an increase in both the strength and ductile properties. Combined microalloying of chromium-manganese-molybdenum steel with additions of boron up to 0.005 % and niobium up to 0.05 % makes it possible to increase the strength and reduce the tendency to brittle fractures significantly. The nature of the fracture becomes completely ductile. Distinct cleavage fracture surface feature “river patterns” are observed in unmodified steels. Сleavage facetes are large enough, it proves the presence of large grains in the steel. Microalloying changes the destruction mechanism, it becomes a ductile “dimple rupture”. An increase in the molybdenum content to 0.6 % makes it possible to obtain strength above 1100 MPa in microalloyed steel.

Abstract: The effect of quenching from 900°C (20 min exposure) and different tempering in the 250-650°C (for 1 hour) interval, as well as additionally preliminary carburization for 8 hours at 930°C, followed by a similar heat treatment on abrasive and shock-abrasive wear of low-carbon manganese (10-24%Mn) steels, phase composition and mechanical properties was studied. It was confirmed that an increase in the manganese reduces the abrasive wear resistance and increases the impact-abrasive wear resistance. The expediency of carburization of low-carbon manganese steels is shown in order to obtain the residual austenite in the structure which amount and stability must be optimized in relation to specific abrasive impact characterized by the dynamic ratio with taking into account the chemical composition.

Abstract: We examined the effects of tempering process and alloying elements on the microstucture, tensile properties, bendability and impact property of direct quenched (DQ), and re-austenitizing and quenched (RQ) low-carbon martensitic steels. For this purpose, four low carbon martensitic steels (Fe-0.07C-1.8Mn-Cr-Nb-Ti-B) were selected. We have investigated the effects of tempering temperature and alloying elements of chromium (Cr), titanium (Ti) and niobium (Nb) on mechanical properties and microstructures. Mechanical properties and microstructures were analyzed as well using tensile test, V-bending test, charpy V-notched impact test and electron microscopy for DQ, DQ and tempered (DQ-T), RQ and RQ and tempered (RQ-T) low-carbon martensitic steels. It has been found that the as-quenched microstructures of the DQ and RQ specimens were fully martensitic structure. Prior austenite grain size and effective grain size after quenching were larger in the case of RQ steel. In both cases, tempering made the needle-shaped carbides. It is shown that the strength decreased when the tempering temperature increased. The strengths of the DQ and DQ-T steels were 30~50MPa higher than those of the RQ and RQ-T steels. Despite the higher strength of the DQ and DQ-T states, both had similar impact properties with the RQ and RQ-T states. However, the impact properties of the Nb added RQ and RQ-T steels with fine martensite morphology exhibited higher than those of DQ and DQ-T steels.

Abstract: The aim of this study was to determine the effect of non-isothermal tempering on microstructure evolution in large-size slabs. Using high-resolution dilatometry, three different cooling rates (from 0.08 to 3°C/s) representative of different regions from the surface to the core of the slab were experimentally simulated, and then tempering was carried out for each starting microstructure. A combination of light and electron microscopy and X-ray diffraction analyses were employed to accurately analyze different phenomena occurring during the tempering process, specially, the identification of different microstructures (bainite, martensite and retained austenite), and the determination of the percentage of retained austenite for each experimental condition were considered. Experimental results revealed that the microstructure after the cooling rate of 0.08°C/s consisted of bainite and some retained austenite. For the cooling rate of 0.3°C/s, martensite plus bainite was detected, and when the cooling rate was increased to 3°C/s, a martensitic microstructure was obtained. Analysis of dilatometry curves indicated that tempering behavior varied significantly with the starting microstructure. Martensite tempering was accompanied with a length decrease due to the decomposition of medium-carbon martensite to low-carbon martensite plus carbides. Tempering of bainite and retained austenite resulted in a remarkable length increase.

Abstract: Commercial production of high strength steel plates by the quenching and tempering (Q&T) route requires control of alloy design and heat treatment parameters to achieve the desired strength and toughness through thickness. Plates with different thicknesses (up to approximately 100 mm) are produced for applications in the energy and power or lifting and excavation sectors. For thick plate the difference in cooling rate through thickness affects the as-quenched microstructure with martensite, auto-tempered martensite and lower and/or upper bainite being present. The different as-quenched microstructures can show a different response to tempering which affects the final strength and toughness.In this study the starting microstructure of a low alloy 0.17 wt% C Q&T steel has been varied using isothermal heat treatment at 430 °C to create mixed martensite and lower bainite microstructures (nominally 25:75; 50:50 and 75:25 percentages). The effects of tempering at 600 °C for times between 0.5 and 16 hours on the carbide precipitates and hardness of the mixed microstructures have been investigated and compared to the tempering response of single phase (martensite and lower bainite) microstructures. It has been found that the hardness decrease due to tempering is larger in the martensitic structure than the bainitic structure due to more rapid carbide coarsening. The as-quenched hardness of the mixed microstructures can be predicted by a rule of mixtures using the single phase properties. The tempering response of the mixed microstructures is discussed.

Abstract: Low alloy quench and tempered (Q&T) steels plates up to 100mm thick are used in applications such as cranes and earth movers due to their combination of high strength and toughness. In order to ensure that appropriate tempering conditions are used to give optimum properties through thickness in Q&T steels it is desirable to be able to predict the effect of composition and tempering conditions (time and temperatures) on the microstructure and hence the hardness evolution. In this paper, the types and coarsening rates of carbides formed in a low alloyed Q&T steel have been investigated on tempering at 600 °C. It has been found that in the as-quenched condition auto-tempered martensite is present with needle-shaped epsilon and cementite particles, whilst after different tempering times (1 - 16 hours) cementite becomes the stable phase with an elliptical shape, which coarsens with time. Besides, the coarsening of inter-lath cementite with a faster rate is independent from that of intra-lath ones.

Abstract: The major challenge in a heat-resistant steel is to generate thermally stable microstructures that allow increasing the operating temperature, which will improve the thermal efficiency of the power plant without diminishing strength or time to rupture. The strengthening mechanism in tempered martensitic 9Cr steels comes mainly from the combination of solid solution effect and of precipitation hardening by fine MX carbo-nitrides, which enhance the sub-boundary hardening. This work is focused on the effect of ausforming processing on MX nanoprecipitation, on both their distribution and number density, during the subsequent tempering heat treatment. The creep strength at 700 oC was evaluated by small punch creep tests. The creep results after ausforming were compared to those obtained after conventional heat treatment concluding, in general, that ausforming boosts the creep strength of the steel at 700 oC. Therefore, conventional ausforming thermomechanical treatment is a promising processing route to raise the operating temperature of 9Cr heat-resistant steels.

Abstract: Hot rolled structural plate steels with yield strength of 700 MPa are an excellent choice for a variety of demanding applications that require excellent toughness and welding properties. SSAB has developed novel hot-rolled plate steels that are produced using precision controlled rolling and an innovative cooling and tempering strategy that ensures invariable mechanical properties in the width and the length directions of the plate. The recently developed steel meets or exceeds the requirements of EN 10025-6 for the S690QL grade. The minimum yield strength (ReH) is between 630 MPa and 690 MPa depending on plate the thickness, and the interval of tensile strength is 760 - 940 MPa, while the minimum elongation at fracture is 14 %. Further, an impact energy of 69 J at-40 °C on transverse V-notch specimen is guaranteed. The novel grades represent superior bendability and surface quality, weldability with excellent HAZ strength and toughness with very low CET and CEV values, exceptional consistency of properties within a plate guaranteed by close tolerances. In addition to the excellent formability, the novel hot rolled steels exhibit greatly improved toughness properties which provides for good resistance to fracture. These outstanding properties are achieved through carefully controlled manufacturing processes. In the present study, a sophisticated recrystallization based hot rolling process optimization method is presented. With the final aim to improve the impact toughness of the novel steel, recrystallization analyses and modelling of austenite grain size development through the rolling pass schedule is performed.

Abstract: The paper provides the comparison of vibromechanical technology for stabilization of bearing rings and thermal tempering on the following parameters: performance, energy consumption, and magnitude of residual stresses after treatment. To assess the effectiveness of residual stress relaxation through the usage of vibromechanical energy, there were conducted experimental studies on a specially designed and manufactured prototype of the equipment. The results of experimental studies show that the energy consumption during vibromechanical stabilization is ten times less, and performance is several times higher than during the thermal tempering. Moreover, vibromechanical stabilization allows more effective residual stresses relaxation. The costs for capital investment can be reduced, as the cost of equipment for vibromechanical stabilization is ten times less than the cost of electric furnaces.

Abstract: The high strength aerospace steel alloyed with Cr, Mn, Si, Ni, W and Mo was studied. The austenite transformations under continuous cooling conditions were investigated using the dilatometer analysis at the cooling rates 0.1...30 °C/s. The mechanical properties of the studied steel were determined after the conventional quenching and tempering heat treatment. The dependences of the mechanical properties on the tempering temperature were obtained. The novel quenching and partitioning heat treatment was applied to the steel under consideration. The microstructure and the mechanical properties were studied after three different modes of the quenching and partitioning (QP) treatment: single-stage QP, two-stage QP and single-stage QP with subsequent tempering (QPT).