Hardenability reduction in VN microalloyed eutectoid steels

By dilatometry, it is shown that vanadium microalloying additions to eutectoid carbon steels, which also contain an increased level of nitrogen, cause a marked decrease in hardenability when transforming from fine grained austenite.

The Effect of Boron on the Hardenability of Steel

Many attempts have been made to improve iron and steel and their alloys by the addition of boron. The re­sults obtained were not encouraging for the reason that the amount of boron used, generally from 0.2 to 2.0 per cent is altogether too high. This percentage of boron ren­ders the product hard and brittle and of late the experiments with boron in this connection have been practically abandoned.

The formation of ultrafine ferrite through static transformation in low carbon steels

A novel approach was used to produce an ultrafine grain structure in low carbon steels with a wide range of hardenability. This included warm deformation of supercooled austenite followed by reheating in the austenite region and cooling (RHA). The ultrafine ferrite structure was independent of steel composition. However, the mechanism of ferrite refinement hanged with the steel quench hardenability. In a relatively low hardenable steel, the ultrafine structure was produced through dynamic strain-induced transformation, whereas the ferrite refinement was formed by static transformation in steels with high quench hardenability. The use of a model Ni–30Fe austenitic alloy revealed that the deformation temperature has a strong effect on the nature of the intragranular defects. There was a transition temperature below which the cell dislocation structure changed to laminar microbands. It appears that the extreme refinement of ferrite is due to the formation of extensive high angle intragranular defects at these low deformation temperatures that then act as sites for static transformation.

Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100 °C/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (>0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.

The formation of ultrafine ferrite and low temperature bainite through thermomechanical processing

In the current study, a novel approach was employed to produce a unique combination of ultrafine ferrite grains and low temperature bainite in a low carbon steel with a high hardenability. The thermomechanical route included warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to bainitic transformation regime (i.e. 400-250°C). The resultant microstructure was ultrafine ferrite grains (i.e. <4μm) and very fine bainite consisting of bainitic ferrite laths with high dislocation density and retained austenite films. This microstructure offers a unique combination of ultimate tensile strength and elongation due to the presence of ductile fine ferrite grains and hard low temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy techniques.

The formation of ultrafine ferrite through static transformation in low carbon steels

A novel approach was used to produce an ultrafine grain structure in low carbon steels with a wide range of hardenability. This included warm deformation of supercooled austenite followed by reheating in the austenite region and cooling (RHA). The ultrafine ferrite structure was independent of steel composition. However, the mechanism of ferrite refinement changed with the steel quench hardenability. In a relatively low hardenable steel, the ultrafine structure was produced through dynamic strain induced transformation, whereas the ferrite refinement was formed by static transformation in steels with high quench hardenability. The use of a model Ni-30Fe austenitic alloy revealed that the deformation temperature has a strong effect on the nature of the intragranular defects. There was a transition temperature below which the cell dislocation structure changed to laminar microbands. It appears that the extreme refinement of ferrite is due to the formation of extensive high angle intragranular defects at these low deformation temperature that then act as sites for static transformation. © 2008 World Scientific Publishing Company.

Caracterização de um aço microligado ao boro e tratado termicamente utilizado na fabricação de tubos

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Desenvolvimento de tubo ARBL contendo 1Cr 0,2Mo fabricado pelo processo ERW/HF destinado à indústria de petróleo e gás

Pós-graduação em Engenharia Mecânica - FEG

Estudo das propriedades mecânicas do aço SAE 1312 modificado ao nióbio

The study of mechanical properties of high-alloy special steels is of great interest of the steel industry due to the great demand by companies that manufacture automotive components of high criticality, and also because of its high commercial value. However, the development of this type of alloy metals demand highly technical knowledge. Among these extremely important kinds of steel, the subject which is the interest of this study is the special steel modified by niobium. The manganese and niobium are the main alloying elements in the composition of these steels, both of them increase the stability of the austenite region, however, manganese increases the hardenability and tensile yield strength, and niobium increases the mechanical strength and promotes refining the grain. The mechanical characterization of steel SAE 1312 modified the niobium was made in order to gain a better understanding of the influence on the mechanical properties caused by aging at different temperatures and for different reductions in the drawing of gauge material. This characterization was made by means of tensile test and hardness. This material showed an increase in yield strength and hardness when gauge with large reductions during the wiredrawing, but when subjected to aging temperatures higher than 300 ° C had a slight loss of these properties