Low cycle fatigue behavior of a quenched and tempered niobium bearing HSLA steel (original) (raw)
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Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel
Metallurgical Transactions A, 1982
The fatigue crack initiation and propagation behavior of a niobium bearing I1SLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.
We describe here the precipitation behavior of copper and fine-sacle carbides during thermo-mechanical processing and isothermal aging of copper-bearing niobium-microalloyed high strength steels. During thermo-mechanical processing, precipitation of ε-copper occurs in polygonal ferrite and at the austenite-ferrite interface. In contrast, during isothermal aging, nucleation of ε-copper precipitation occurs at dislocations. In the three different chemistries investigated, the increase in strength associated with copper during aging results only in a small decrease in impact toughness, implying that copper precipitates do not seriously impair toughness, and can be considered as a viable strengthening element in microalloyed steels. Precipitation of fine-scale niobium carbides occurs extensively at dislocations and within ferrite matrix together with vanadium carbides. In the presence of titanium, titanium carbides act as a nucleus for niobium carbide formation. Irrespective of the nature of carbides, copper precipitates and carbides are mutually exclusive. .
Materials Research
The HSLA (High-Strength Low Alloy) steels are used in the production of pipes, flanges and connectors to build ducts for ore, oil and gas transport. The conventional processes are the rolling or forging. In the transport of ore and heavy oil, the abrasive particles impair the surfaces and reduce the pipelines lifetime. Therefore, besides the mechanical properties as API 5L, it is important to verify the wear resistance of these steels. In this context, two microalloyed steels were forged in the form of square bars. Thereafter, specimens of these bars were annealed at 930 ºC, quenched at 900 ºC and tempered at 600 ºC. Tensile and wear tests were performed. The results show that molybdenum and niobium present similar effects on phase transformation of steels, promoting a desired acicular ferrite/bainite microstructure and fulfill the mechanical strength of API 5L-X70 standard. The molybdenum has dominating effect in the hardenability when in solid solution, however, after tempering, thermodynamic simulation by FactSage software indicates that niobium probably promotes secondary hardening.
Materials Science and Engineering: A, 2006
We describe here the relationship between microstructure and impact toughness behavior as a function of cooling rate for industrially processed Nb-and V-microalloyed steels of almost similar yield strength (∼60 ksi). Both Nb-and V-microalloyed steels exhibited increase in toughness with increase in cooling rates during processing. However, Nb-microalloyed steels were characterized by relatively higher toughness than the V-microalloyed steels under identical processing conditions. The microstructure of Nb-and V-microalloyed steels processed at conventional cooling rate, primarily consisted of polygonal ferrite-pearlite microconstituents, while Nb-microalloyed steels besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite. The microstructure of Nb-and V-microalloyed steels processed at relatively higher cooling rate contained degenerated pearlite and lath-type (acicular) ferrite in addition to the primary ferrite-pearlite constituents. The fraction of degenerated pearlite was higher in Nb-microalloyed steels than in the V-microalloyed steels. In both Nb-and V-microalloyed steels the precipitation characteristics were similar with precipitation occurring at grain boundaries, dislocations, and in the ferrite matrix. Fine-scale (∼5-10 nm) precipitation was observed in the ferrite matrix of both the steels. The selected area diffraction (SAD) pattern analysis revealed that these fine precipitates were MC type of niobium and vanadium carbides in the respective steels and followed Baker-Nutting orientation relationship with the ferrite matrix. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels is attributed to higher fraction of degenerated pearlite in the steel.
AIP Conference Proceedings , 2019
In the present investigation, high carbon steel and Nb microalloyed steel have been subjected to hot rolling (~80% hot deformation) by varying the finish rolling temperature (FRT) followed by air cooling to room temperature. The specimens were austenitised at 1200°C for 1 h and then subjected to hot deformation with two FRTs of 800°C and 1000°C. The deformation at a higher temperature results in a continuous grain boundary network structure with apparent evidence of recrystallisation whereas, finer grains at lower deformation temperature has been observed. The volumetric percentage of ferrite has been found to be more in case of Nb microalloyed steel at a lower FRT whereas, more volume percentage of pearlite has been observed when cooled from higher deformation temperature. On the other hand, deformation at a lower temperature (800°C FRT) results in more amount of ferrite formation in the Nb microalloyed steel. The average hardness value has been found to be higher (≈270 HV30/20) for the Nb microalloyed steels at higher deformation temperature (1000°C FRT) which is attributed to the finer interlamellar spacing (≤ 100 nm). Subsequent air cooling to room temperature from a higher deformation temperature increases the ultimate tensile strength (UTS) of high carbon steel marginally but improve the UTS of Nb microalloyed steel significantly. The tensile fracture morphology reveals the abundant presence of dimples at a lower deformation temperature, indicating a ductile fracture. The fracture surface of the Nb microalloyed steel subjected to higher deformation temperature exhibits a typical river-like pattern indicating cleavage fracture. Finally, a correlation between microstructure and properties have been established.
Journal of Zhejiang University SCIENCE A, 2010
The influence of direct quenching (DQ) on microstructure and mechanical properties of 0.19C-1.7Si-1.0 Mn-0.05Nb steel was studied. The microstructure and mechanical properties of reheat quenched and tempered (RQ&T) steel plate were compared with those of direct quenched and tempered (DQ&T) steel plates which were hot rolled at different finish rolling temperatures (1173 K and 1123 K), i.e., recrystallization-controlled-rolled direct-quenched (RCR&DQ) and controlled-rolled direct-quenched (CR&DQ), respectively. The strengths generally increased in the following order: RQ&T<RCR&DQ&T< CR&DQ&T. Strength differences between the CR&DQ&T and RQ&T conditions as high as 14% were observed at the tempered temperature of 573 K. The optical microscopy of the CR&DQ&T steel showed deformed grains elongated along the rolling direction, while complete equiaxed grains were visible in RQ&T and RCR&DQ&T steels. Transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) of the DQ steels showed smaller block width and higher density of dislocations. Inheritance of austenite deformation substructure by the martensite and differences in martensite block width were ruled out as major causes for the strength differences between DQ and RQ steels.
Effect of Niobium/Molybdenum microalloying on SS316LN steel.
In recent years SS316LN microalloyed stainless steel is preferred for use as jacket material for Nb 3 Sn superconductor strands/wires. In the present investigation, microalloyed SS316LN is prepared in a vacuum induction melting furnace; Niobium and Molybdenum in their ferroalloy stage are considered as alloying element. This microalloyed steels are cast in water cooled copper mould. The tensile strength and elongation are measured and the fracture surface is studied under scanning electron microscope. It is observed that, there is a reduction of tensile strength and decrease in hardness of the steels prepared with addition of either/both the alloying elements; however there is an increase in ductility, which is helpful for cold rolling operation. From the micrographs it is observed that nitride precipitates are formed along the grain boundary, but formation of chromium carbide precipitates is reduced.
Materials Science and Engineering: A, 1996
A methodology for including the thermomechanical history in hot forming analyses is presented. A finite element formulation is employed for analysis of the inhomogeneous microstructural development. An earlier model, describing the development of the microstructure in niobium steels, is expanded to predict the final properties of the product and is integrated into the complete model of the hot compression process. Mechanical properties are obtained from a macroscopic description based on the Hall-Petch formulae. The results show that the model calculates correctly the influence of microstructure on the mechanical inhomogeneity. The analyses indicate that when all of the strengthening mechanisms are employed, the root of the sum of the squares summation method gives better agreement with experimental data than linear summation, of importance especially when the last deformation occurs in the two-phase or ferrite region. The proposed model can be used to investigate the complex behavior of a large range of microalloyed steels in hot rolling or forging processes.