J.M. Neto

Magnetic property changes during embrittlement of a duplex stainless steel

Abstract Changes in magnetic properties were used to investigate the ferrite decomposition that occurs in wrought duplex stainless steel (DSS) UNS S31803 at high (800°C) and low temperatures (475°C). At 800°C the saturation and residual induction, the coercive force and the differential permeability decrease with time, due to the increase in the austenite content. Firstly, secondary austenite (γ2) forms in the α/α grain boundaries causing embrittlement, but not hardening. Ferrite then decomposes into σ and austenite phases (α→σ+γ2), producing hardening and severe embrittlement. During embrittlement at 475°C, only a small increase in the coercivity was observed, even on aging up to 500 h. On the other hand, the Curie temperature increased with aging time, as a consequence of spinodal decomposition (α→α+α′). The results obtained show that the mechanical property changes of DSS UNS S31803 due to exposures at 475 and 800°C may be monitored by magnetic measurements.

475 °C Embrittlement in a duplex stainless steel UNS S31803

The susceptibility of a duplex stainless steel UNS S31803 to thermal embrittlement at 475 °C was evaluated by means of mechanical tests (impact energy and hardness), magnetic measurements (hysteresis and thermomagnetic analysis) and scanning electron microscopy. The results show that the material undergoes severe embrittlement and hardening in the first 100 h. The corrosion resistance of the ferrite phase in a 10%HNO3 + 0.05%HF solution deteriorated after 500 h of ageing. The Curie temperature (Tc) was the most sensitive magnetic property to the microstructural changes that promote embrittlement. Tc increases with ageing time due to the progressive reduction of chromium in the Fe-rich matrix during spinodal decomposition.

Magnetization of lithiated magnetite, LixFe3O4

Abstracts are not published in this journal

Magnetic and mechanical hardening of Fe-based alloys

Abstract The aim of this work was the analysis of the influence of precipitates on the magnetic and mechanical properties, during the thermal treatment of a duplex stainless steel at 475 and 550°C, as well as during the thermal aging at 610°C of a Fe–20Mo–5Ni–0.12C alloy. In the duplex stainless steel it was verified that the precipitation of the α′ phase inside the α phase increases the coercive force, H c , and increases the mechanical hardness of the α phase, for the heat treatment at 475°C. In this process the mechanical hardness of the γ phase remains constant. After the heat treatment at 550°C the mechanical hardness of the α phase and the coercive force of the stainless steel remain practically constant. For the Fe–20Mo–5Ni–0.12C alloy the thermal aging at 610°C results in simultaneous magnetic and mechanical hardening. In both cases (steel and alloy) the magnetic and mechanical hardening curves are similar.

Mössbauer studies on high-temperature lithiated magnetite, LixFe3O4

Abstract The influence of the insertion of lithium ions in magnetite by high-temperature techniques are studied by Mossbauer spectroscopy at room temperature. The results show: (i) the existence of two non-interacting phases, one corresponding to the pure magnetite and the other to a paramagnetic lithiated phase; (ii) that the mechanisms of electron hopping remain practically unaffected by the insertion of Li up to x ≈ 0.20. The results, when compared with those obtained by electrochemical Li insertion, show good agreement.

The Observation of Phase Transition in LiF3O4 by Mössbauer Spectroscopy

The compound LiFe3O4 is a novel material derived from magnetite by the insertion of Li+ in the spinel structure. As a consequence of the insertion, there is a change of the crystal structure from spinel to rock salt type, with all octahedral sites filled and the tetrahedral ones empty. A model is proposed in which the electron hopping between B-sites is little affected by the Li insertion. Our results also indicate some interesting features: (a) the compound is paramagnetic at room temperature and the Mössbauer spectra can be fitted with four different sites (as expected from the crystallography); (b) it undergoes a kind of structural phase transition in the temperature region 80 K<T<100 K manifested by relaxation and a large broadening of the linewidths. For temperatures between 100 and 125 K there is anomalous behavior of the hyperfine parameters. For 80 K one sees again the same pattern observed for temperatures above the transition region, i.e., the presence of four well-resolved paramagnetic doublets, characterizing the region ranging from 80 to 125 K as a reentrant one. The antiferromagnetic phase transition is expected to occur around 50 K. This reentrant region where the phase transition occurs is about the same as the region where the Verwey transition in the magnetite is observed. Measurements with other techniques confirm this assumption. To our knowledge, it is the first time that such a behavior is observed in materials related to magnetite.

Magnetic properties and thermomagnetic analysis of a Fe-20Mo-5Ni-0.075C alloy

A Fe-20Mo-5Ni-0.075C (wt %) alloy for magnets was prepared by induction melting under vacuum. The material was hot rolled, solution treated and aged at 610°C for different periods of time. The magnetic properties (Hc, Br, Bs and (BH)max) were measured and compared with some commercial alloys containing cobalt. The precipitation of Mo-rich phases and the decrease of the ferrite lattice parameter during ageing were detected by X-ray diffraction. The thermomagnetic analysis (TMA) was carried out in the solution treated samples and aged samples. The behaviour of TMA curves was explained with the help of X-ray analysis.

Magnetic hardening in a hot-rolled Fe-20Mo-5Ni-0.12C alloy

Abstract The magnetic properties in an Fe-20Mo-5Ni-0.12C alloy, hot-rolled, solution treated at 1230°C and aged at 610°C for different times were measured. These properties were compared with those of the same alloy previously cold-rolled, solution treated at the same temperature and aged under the same conditions. The results for the same alloy without carbon, obtained by other authors, have also been considered. The alloy presented magnetic hardening during aging, reaching a H c value of 401 Oe after 240 min. of aging. The precipitation hardening was accompanied by thermomagnetic analysis (TMA) in the range 25–800°C and X-ray diffraction.

High-temperature substitutional lithiation of magnetite

The reaction products of Li2CO3 and Fe2O3 have been studied by X-ray diffraction for the compositional range 0≤LiFe≤0.2. Under oxidizing conditions (750°C in air) the products were LiFe5O8 and Fe2O3. Reduction in H2 at 450°C yielded a single, apparently cation—deficient spinel phase for 0≤LiFe≤0.05; the spinel phase was mixed with a rocksalt phase for 0.1≤LiFe≤0.2. Further reduction (780°C in Ar in a carbon furnace) yielded Fe3O4 and a rocksalt phase. The spinel/rocksalt fraction extrapolates to zero at a rocksalt composition LiFe2.5O4. This LiFe2.5O4 phase, with space group Fm3m, differs from the product of room—temperature lithiation of Fe3O4, which retains the [Fe2]O4 spinel framework with space group Fd3m and so prevents a completely random distribution of Li on the octahedral sites.

Texture development and magnetic anisotropy in semi-hard magnetic Fe20Mo5Ni alloys

This work analyzes the crystallographic texture of cold rolled and magnetic aged samples of Fe20Mo5Ni with 0.020, 0.057 and 0.092% of carbon. Results were compared with the increase of magnetic anisotropy represented by the relation between remanence (B{sub r}) and saturation induction (B{sub s}). Results show that these alloys have a sharp {l_brace}100{r_brace} texture component after magnetic hardening that increases with rolling reduction. Magnetic anisotropy increases with this component.