I. Mejía

Modeling of the hot deformation behavior of boron microalloyed steels under uniaxial hot-compression conditions

Abstract The present study shows that the hot deformation behavior of boron microalloyed steels can be quantitatively described by constitutive equations. These equations take into account both dynamic recovery and recrystallization phenomena. They have been fitted using experimental data taken from hot compression tests of four boron microalloyed steels in order to determine their characteristic parameters. The tests were carried out over a wide range of temperatures (950, 1000, 1050 and 1100°C) and strain rates (10−3, 10−2 and 10−1 s−1). The analysis of the characteristic parameters of the constitutive equations describing the hot flow behavior of these steels shows that boron additions play a major role in softening mechanisms rather than on hardening. A quantification of the boron effect is also presented. The experimental data were compared with the predictions of the proposed model and an excellent agreement between measured and predicted values for all boron micro-alloyed steels over a wide range of temperatures and strain rates was obtained.

Determination of the Work Hardening Exponent by the Hollomon and Differential Crussard-Jaoul Analyses of Cold Drawn Ferrite-Pearlite Steels

This research work analyses the effect of cold working level produced by drawing, on the work hardening exponent of 0.18 and 0.43 % C ferrite-pearlite steels. Such analysis is carried out by means of true stress-true strain curves derived from uniaxial tension tests. The work hardening exponent behaviour was determined by using Hollomon and differential Crussard-Jaoul models. It is found that the work hardening exponent decreases as a function of the applied cold-drawing level, and negative values were obtained when differential analysis is used. The results indicate that the Hollomon analysis shows some deviations from the experimentally determined true stress - true strain curves while the differential Crussard-Jaoul analysis fits better when two work hardening exponents are considered. This analysis establishes two exponents for different stages of plastic deformation which are determined by the sharp slope change in the plot of ln (d σ/d ε) - ln ε.

Effect of boron on microstructure of directionally solidified high chromium white irons

Abstract This work analyses the effect of boron additions (up to 195 ppm) in a directionally solidified high chromium white iron (17Cr–2·5C). Owing to its high level of segregation during solidification, boron additions have been able to change the solidification mechanism and hence to modify the microstructure of the iron. Microprobe analysis in a scanning electron microscope was used to determine the chemical distribution within the phases forming the microstructure. Boron was observed to segregate mainly to the austenite/carbide interface, it was also detected within the eutectic carbide and rarely as a boron compound. It was found that boron additions decreased the amount of proeutectic matrix and promoted a microstructural refinement by moving the chemical composition of the alloy towards the eutectic. An increase in the carbide volume fraction was measured as boron increased; however, no increase in the density of carbides was observed. These phenomena are discussed in terms of the boron segregation during solidification.

Effect of rare-earth metals on the hot strength of HSLA steels

Abstract An experimental study was done on the effect of rare-earth metals on a high-strength low-alloy steel. The work was focused in deriving the influence on Ce and La on the hot-working behavior. For this purpose, uniaxial hot-compression tests were carried out in a wide range of temperatures and strain rates. The effect of the rare-earth metals was determined by comparison of the characteristic parameters, describing the constitutive equations of the high-temperature response of the steel with, and without, rare-earth metals. The results showed that rare-earth metals were playing a major significant role on hardening mechanisms rather than on softening by dynamic recovery. On the contrary, rare-earth metals were able to delay the onset of dynamic recrystallization. All the present experimental results suggested that the latter roles are played by solid solution strengthening, through a solute drag effect, and not by precipitated particles.

Boron effect on the precipitation of secondary carbides during destabilization of a high-chromium white iron

Abstract This work analyses the secondary carbides precipitation during the destabilization of a 17% Cr White iron containing 195 ppm boron. The experimental iron was characterised in the as-cast conditions to have comparable parameters with the heat-treated samples. Destabilisation heat treatments were undertaken at temperatures of 825, 900 and 975 °C for 25 min; each sample was air-cooled after this soaking time. Characterisation was undertaken by optical and electronic microscopy, image analysis and energy dispersive spectroscopy microanalysis, hardness and microhardness were also evaluated. It was found that the volume fraction of secondary carbides precipitated is always higher for the lowest destabilisation temperature (825 °C) due to the lower carbon solubility in austenite at low temperatures. A much higher precipitation for the irons containing boron than that for the iron without boron at any destabilisation temperature was also noticed. For the iron containing boron, a density of 23 carbide particles per square micron was measured when destabilised at 825 °C, and it decreased to about 10 particles per square micron when destabilised at 975 °C. In the case of the alloy without boron additions, about 10 carbides per square micron were counted when destabilised at 825 °C and about 5 when destabilised at 975 °C. Higher volumes of carbides precipitation implies higher values of bulk hardness and microhardness in the alloys. The results suggest that boron works as a nucleant for the precipitation of secondary carbides; this is discussed in terms of the limited solubility of boron in iron and the formation of boron-rich precipitates found in the iron in the as-cast conditions.

Nanoparticle and Intermetallic Formation in Dissimilar Friction Welds Produced with Silver Interlayers

The introduction of a silver interlayer during MMC/AISI 304 stainless steel friction welding promoted the formation of intermetallic compounds and silver nanoparticles. It is suggested that nanocrystal formation and the formation of Ag3Al intermetallics regions resulted from mechanical mixing during the friction welding operation. The nanoparticles were analyzed by a Philips FEG Tecnai F20. Experimental Procedure The chemical compositions of the MMC and AISI 304 stainless steel base materials are indicated in Table 1. All dissimilar friction welds were made using 19-mm bars 0f diameter of AISI 304 stainless steel and Al 6061/Al2O3 (W6.A.l0A-T6) base material containing 10 vol.% Al2O3 particles. The contacting surfaces of the steel and MMC substrates were machined perpendicular to the axes of the as-received bars. The stainless steel test bars were polished using 1µm diamond particles and coated with 5 µm-thick nickel strike layers.

Can Young’s Modulus of Metallic Alloys Change with Plastic Deformation?

The information in the basic references about the relation between elastic constants and particularly Young’s modulus (E) behavior and plastic deformation indicates that this parameter is constant or almost constant. At the beginning of the XX century several authors indicated that E of some metals decreased when cold deformation increased and detected reductions up to 15% in steels, aluminum, copper, brass... In the last years this behavior is taking into account during the finite-element analysis of sheet metal stamping or other plastic deformation processes. This work includes an extensive review of papers of our research team and of other authors related with the behavior of Young’s modulus during plastic deformation of some metallic alloys. This parameter can diminish up to 10% by plastic deformation (tension test) in iron, aluminum, and stainless steel (UNS S 30403) but remains practically unaltered in aluminum alloys deformed before or after aging. Results of Young’s modulus in nanostructured copper and copper alloys determined by ultrasonic technique are also presented. Additional results of Young’s modulus of UNS G10180 and UNS G10430 steels measured during loading and unloading steps in tension test are also included. High differences in the E values were detected between both steps.

Effect of Copper Additions on Secondary Carbide Precipitation in High Chromium with Cast Iron

This work analyzes the secondary carbide precipitation during the destabilization heat treatment of a 15 % Cr white iron and copper additions between 0.5 and 2 wt%. The destabilization heat treatments were undertaken at temperatures of 850, 950, and 1050 ∘C between 5 and 480 min. Characterization was carried out by X-ray diffraction (XRD), optical and electron microscopy, image analysis, and energy-dispersive spectroscopy (EDS) microanalysis; the resultant hardness was also measured. It was found that the copper addition resulted in an increase in discontinuity of the eutectic carbide in as-cast conditions. After the heat treatment, the number of precipitated carbides increased with the copper addition, and they were found smaller. There were an increase of precipitation for lower destabilization temperatures and large soaking times.