Berenice Mendonça Gonzalez

The influence of copper addition on the formability of AISI 304 stainless steel

The formability of AISI 304 austenitic stainless steel can be enhanced when it is designed to transform to martensite under appropriate strain levels, gaining additional plasticity (transformation induced plasticity). In this work, the effects of a partial substitution of Ni by Cu on the formability of AISI 304 steel were analyzed in terms of microstructure changes during tensile straining. Conventional metallographic techniques aided by scanning probe microscopy were employed to identify and characterize the stress-induced martensitic phases. The results obtained are discussed in terms of the relationship between chemical composition and the stability of austenite, taking into account the stacking fault energy and the characteristics of stress-induced martensite in these steels. It has been observed that the partial substitution of Ni by Cu gave rise to an increase in the strain level required to induce martensitic transformation, resulting in higher maximum uniform elongation and better stretch formability.

Measurement of fine pearlite interlamellar spacing by atomic force microscopy

Pearlite interlamellar spacing is an important parameter controlling ductility and strain hardening of carbon steels. Fine pearlite is the appropriate initial microstructure for drawing high carbon steel with exponential strain hardening rate, leading to high final tensile strengths. The majority of optical and electron microscopy methods for measuring interlamellar spacing present difficulties when applied to fine microstructures. Atomic force microscopy (AFM) was employed to investigate pearlitic steels lead patented at 510 °C and then cold drawn to 86% reduction in area. Conventional specimen preparation techniques for optical metallography were appropriated to produce high resolution AFM images, on which measurements of minimum interlamellar spacing, in good agreement with spacings estimated using the Embury–Fisher model, were easily performed.

Strain aging of AISI 430 ferritic stainless steel

High stress corrosion resistance and relatively low production costs are the main reasons for the increasing application of AISI 430 ferritic stainless steels. Problems such as loss of ductility and toughness when exposed to elevated temperatures, occurring in these steels, for instance, during welding, are being dealt with, and the main challenge now is to explore attractive, non-conventional properties that the material might exhibit, in order to offer alternative choices for engineering design. This was the idea underlying the present work, in which the phenomenology and kinetics of strain aging in a commercial AISI 430 stainless steel were characterized, by means of electrical resistivity measurements and tensile tests. The values of the kinetic parameters were determined based on the changes in electrical resistivity, a technique which has been used for many years to study the effects of impurities and phase transitions in metals and alloys. Measurements of electrical resistivity performed at the liquid nitrogen temperature have been employed to characterize the kinetics of strain aging in drawn high carbon steels, disclosing important phenomena related to the decomposition of cementite in this class of steels.

Martensite Formation and Recrystallization Behavior in 17Mn0.06C2Si3Al1Ni TRIP/TWIP Steel after Hot and Cold Rolling

This work evaluates the evolution of the microstructure and its influence on the mechanical behavior of steel containing 17% Mn, 0.06% C, 2% Si, 3% Al, and 1% Ni after hot rolling at 1070°C, cold rolling with 44% reduction, and annealing at 700°C for different time periods. The resultant athermal, strain-induced martensite and austenite grains were analyzed by optical and scanning electron microscopy (SEM). The volume fractions of the g, e, and α’ phases of martensite were confirmed by X-ray diffraction, dilatometry, and SEM-electron backscatter diffraction (EBSD) techniques. It was found that cold reduction results in the formation of more a’ martensite. The Vickers microhardness values were higher for the cold-rolled condition and lower for recrystallized samples, as expected. However, this reduction is counterbalanced by the formation of athermal e and a’ martensite during the cooling process. The sizes of the recrystallized grains change exponentially during their growth and remain within 1–3 mm. The yield and tensile strength of the hot-rolled steel reach values close to 250 and 800 MPa, respectively, with a total elongation of 40%, which demonstrates the high work-hardening rate of the steel.

Comparison of Microstructure and Mechanical Behavior of the Ferritic Stainless Steels ASTM 430 Stabilized with Niobium and ASTM 439 Stabilized with Niobium and Titanium

The use of Ferritic Stainless Steels has become indispensable due its lower cost and the possibility to replace austenitic stainless steels in many applications. In this study, cold rolled sheets of two stabilized ferritic stainless steels with 85% thickness reduction were annealed by applying a heating rate of 24 oC/s and a soaking time of 24 s. The niobium stabilized ferritic stainless steel type ASTM 430 (430Nb) was annealed at 880 oC while the niobium and titanium bi-stabilized steel ASTM 439 was annealed at 925 oC. The annealed samples were tensile tested and due to the smaller grain size, steel 430Nb, showed a higher yield stress and a higher total elongation. Concerning drawability the steel ASTM 439 presented a better performance with higher average R-value, lower planar anisotropy coefficient and a greater value for Limit Drawing Ratio (LDR). These results are explained in terms of the differences in size and volume fraction of precipitates between the two steels.

Effect of Retained Austenite on Impact Toughness of the Multi-Phase Bainitic-Martensitic Steel

The new class of bainitic steels can present toughness at room temperature greater than traditional quenched and tempered martensitic steel. This is because the microstructure of steel with high Si content (≈1.5wt%) submitted to bainitic transformation is compose of fine plates of bainitic ferrite separated by retained austenite. The inhibition of cementite precipitation leads to the improvement of toughness. The presence of cementite facilitates the nucleation of cracks. Moreover, the blocks of retained austenite are undesirable. This morphology is rather unstable and tends to transform into hard and brittle untempered martensite under the influence of small stress, contributing to a low toughness. However, it was observed in this work that the greater the volume fraction of retained austenite, the greater is the toughness (10-24 J) for multi-phase steel. The values of toughness were independent whether the retained austenite is present on film or block forms. The decrease of toughness values was observed by the tempered samples after the bainitic transformation (10-14 J). This occurred because the blocks of retained austenite decomposed into carbides, martensite and/or bainite.

Strain aging behaviour of cold rolled multiphase steel

Abstract The bake hardening behaviour of cold rolled multiphase steel, processed to a minimum yield strength of 250 MPa and a minimum tensile strength of 450 MPa, was evaluated within a range of tensile prestrain of up to 20% by means of aging experiments and tensile tests. The influence of room temperature aging time on the steel’s mechanical properties and Lüders strain was also examined. It was found that, after reaching a peak value at a prestrain level of between 0·5 and 1·0%, the bake hardening parameter began to decrease upon reaching higher levels of prestrain. This behaviour was explained in terms of changes in the dislocation structure and dislocation interaction with dissolved carbon atoms. Concerning the natural aging behaviour, no significant influence of room temperature aging time on the mechanical properties could be detected. Moreover, the appearance of Lüders strain was not observed during a 6 month evaluation period, indicating that the investigated steel possesses a natural non-aging behaviour.

Fatigue Life Curves of NiTi Alloys – The Z Effect

Superelasticity is closely related to shape memory effect. It refers to the property presented by some materials submitted to large strains (usually up to about 8%) to restore their original shape immediately after unloading without the need of heating. This phenomenon results directly from a diffusionless transformation of the material from an austenitic to a martensitic phase (martensitic transformation). The recovering mechanism is the reverse transformation, from martensite to austenite. This paper compares fatigue live curves obtained in bending-rotation fatigue tests carried out on wires of NiTi alloys with three different microstructures, stable austenite, unstable austenite (superelastic), and stable martensite. These curves are also compared to data from the literature. The tests were strain controlled and the wires were submitted to strain amplitudes from 0.6% to 12.0%. To minimize changes in material properties, the wire temperature was monitored using a thermocouple and controlled by its rotation speed. For strain amplitudes up to 4%, the εa-Nf curve for superelastic wires was consistent with those reported in the literature, closely approaching the curve of the stable austenite wire. For higher strain amplitudes, fatigue life of superelastic wires increased with strain until it approached the fatigue life curve of stable martensitic wire. This unusual behavior results in a “Z-shaped” curve for high strain values. It is possibly linked to the changes in microstructure and fatigue properties that occur when the superelastic material is deformed.

Effect of Inter-Pass Ageing during Cold Rolling on Magnetic Properties of Fe-3%Si

The effects of inter-pass ageing temperature during cold rolling on structure, magnetic properties and mechanical properties of high permeability grain oriented electrical steel was studied. The samples were processed in a single-stage cold rolling to 0.27 mm thickness, with 88 % reduction, without and with inter-pass ageing treatment in order to determine the magnetic properties. To determine the changes in mechanical properties due to strain ageing, the samples underwent overlapped ageing, after pre-strain by rolling, under the same conditions of inter-pass ageing and then were subjected to tensile test. The effect of strain ageing was more pronounced in the thickness of 0.7 mm and the largest variation in yield strength was at 200°C in all evaluated thicknesses. At this temperature the largest amount of {110} orientation after primary recrystallization was also observed, as well as the lowest final grain size and consequently the best results of core loss. The magnetic induction had almost no alterations.

Influence of Annealing Heating Rate on Nb Ferritic Stainless Steel Microstructure and Texture

The cold rolled band of the niobium stabilized type ASTM 430 ferritic stainless steel with 85 % thickness reduction was annealed with heating rates of 0.10, 6.8, 23.5 and 41.5 °C/s and a soaking time of 24 s. The changes in microstructure and texture were followed by interruptions in the annealing cycle at temperatures of 780, 830 and 880 °C. Annealing at the lower heating rate was more effective for the development of γ-fiber than the annealing performed with high heating rate. The increased rate of heating provided an increase in the onset recrystallization temperature, a reduction in average grain diameter and a more homogeneous distribution throughout the thickness. The specimens with higher volume fraction of the γ-fiber annealed with low heating rate showed a high average coefficient of anisotropy R =1.99.