Abstract Microalloyed steels have a maximum of 2% in alloying elements and are used in the automotive and petrochemical industries. The microstructure and mechanical properties of these steels depend on chemical composition, processing and addition of molybdenum, niobium, titanium and vanadium. To evaluate the molybdenum effect in microstructure and mechanical properties, two microalloyed steels, with similar levels of niobium, titanium and vanadium, were prepared and forged. The steels have 0.056 and 0.160 percent by weight of molybdenum. The bars were homogenized, quenched and tempered. The specimens were machined for tensile and impact tests, hardness and microstructural analyzes by confocal and scanning electron microscopy. The ThermoCalc software was used to evaluate the carbides characteristics. The results show that steel with 0.160% Mo presents greater amount of bainite and austenite retained. It’s possible that formation of carbides by the secondary hardening mechanism increase in mechanical strength and a reduction in toughness.

Superaustenitic stainless steel with high nickel percentage, chromium, molybdenum and nitrogen, has replaced austenitic AISI 304 and 316, mainly in the petrochemical industries, where high toughness, stress corrosion resistance and pitting resistance is suitable. Due to high mechanical stress in the cold deformation manufacturing, residual stresses in the pipes are common. In this case, the possibility of the corrosion process increases. Considering the area reduction after tensile tests, the objective of this study was to evaluate the effects of cold deformation on a superaustenitic steel BS EN 10283 No 14587. The steel was elaborated in an electric induction furnace and the liquid metal was poured out in sand molds. Afterward, samples cut from a specimen submitted to the tensile test were prepared for X-ray diffraction, Vickers hardness measurements, linear polarization and impedance electrochemical tests. The corrosion tests were performed in solution with 3.5% NaCl. The results show that there is a linear increase in hardness and also a decrease in corrosion resistance of the material, with increasing cold deformation. In addition, the cold work reduction was not sufficient to promote strain-induced martensite.

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.