S.S.M. Tavares

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.

Mechanical properties of a quenched and tempered dual phase steel

Low carbon steels can be properly heat treated to produce a high strength dual phase structure of ferrite plus martensite. The heat treatment may consist of an intercritical annealing (between A{sub 3} and A{sub 1}) followed by rapid cooling to promote the austenite{r_arrow}martensite transformation. In this paper, mechanical properties of a heat treated dual phase steel were investigated as a function of the martensite volume fraction. The changes promoted by tempering at 100 C, 200 C and 300 C were measured and investigated. Residual stresses introduced by quenching from the intercritical temperature and the changes occurred during the tempering were also measured.

Deleterious phases precipitation on superduplex stainless steel UNS S32750: characterization by light optical and scanning electron microscopy

Deleterious phases precipitation in superduplex stainless steels is the main concern in fabrication by welding and hot forming of this class of material. Sigma, chi and secondary austenite phases are considered deleterious phases because they produce negative effects on corrosion resistance. Besides, sigma and chi phases also promote strong decrease of toughness. In the present work, the precipitations of sigma, chi and secondary austenite under aging in the 800-950 °C interval were studied in two UNS S32750 steels with different grain sizes. The deleterious phases could be quantified by light optical microscopy, with no distinction between them. Scanning electron microscopy was used to distinguish the individual phases in various aging conditions. The results elucidate the influence of the aging temperature and grain size on the kinetics precipitation and morphology of deleterious phases. The kinetics of deleterious phases is higher in the fine grained material in the initial stage of aging, but the maximum amount of deleterious phases is higher in the coarse grained steel.

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.

Effects of low-temperature aging on AISI 444 steel

The consequences of aging at 400 and 475 °C on the mechanical properties, corrosion resistance, and magnetic properties of the ferritic stainless steel (SS) AISI 444 were investigated. Age hardening was measured as a function of aging time at both temperatures and was found to be more intense at 475 °C. The localized corrosion susceptibility increased, while the impact toughness decreased with aging time. These two effects were also more important at 475 °C. Unlike duplex SSs, AISI 444 did not present any variation in coercive force or Curie temperature with aging time. The effects on the Mössbauer spectra were also determined and analyzed.

Analysis of the nanopores produced in nickel and palladium by high hydrogen pressure

Abstract Samples of pure nickel and palladium were submitted to a high hydrogen pressure (HHP) of 3.5 GPa at 800 °C for 5 h. Analysis of the resulting structural modification was performed using X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and small-angle X-ray scattering (SAXS), the latter specifically for Ni. The formation of superabundant vacancies (SAVs) was observed in the structure in both cases. For Pd, the pores, which formed by the coalescence of vacancies, had dimensions of 20–30 nm when present in the interior of the metal and 1–3 μm when condensed at the surface. The pores were seen to be dispersed homogeneously across the surface of Pd. For Ni, however, pores were created preferentially at the grain boundaries, which promoted significant decohesion in the metal. The distribution of pores induced by heat treatment of Ni subjected to HHP was determined by SAXS analysis and two populations of pores, with population mean diameters of 50 and 250 A, were observed.

Influence of the starting condition on the kinetics of sensitization and loss of toughness in an AISI 304 steel

This work describes the investigation of the embrittlement of AISI 304 steel sensitized at 650°C by Charpy impact test, comparing two starting conditions: (1) mill annealed and machined (MA-M); and (2) solution treated at 1050°C by 1 h followed by oil quenching (ST). The degree of sensitization for both samples was assessed by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and by Double Loop Electrochemical Potentiodynamic Reactivation test. The results showed that MA-M samples undergo more severe and rapid embrittlement than ST ones and a higher kinetics of sensitization due to small strains concentrated in grain boundaries and α′ martensite phase produced during the machining operations. The martensite phase is found to be quite stable at the sensitization treatment at 650°C. The increase of microvoids nucleation at the grain boundaries seems to be the mechanism of embrittlement in the sensitized 304 steel.

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.

Naphthenic corrosion resistance, mechanical properties and microstructure evolution of experimental Cr-Mo steels with high Mo content

One method to face the effects of naphthenic acid corrosion in petroleun refining plants is to use alloys with good resistance to this kind of corrosion. For this purpose, molybdenum additions to chromium containing steels are specially recommended. In this work, experimental Fe-9Cr-xMo (x = 5, 7 and 9 wt. (%)) ingots were cast, forged and hot rolled before being tested in a naphthenic acid environment. Evolution of the mechanical properties with composition is presented. The precipitation and dissolution of phases were investigated with the help of Thermocalc and electron backscattered diffraction (EBSD).

Study of deleterious phase precipitation under continuous cooling of superduplex stainless steel UNS S32750

Abstract Duplex and superduplex stainless steels have high strength and high corrosion resistance with a crescent demand in chemical and petrochemical industries. The best mechanical properties of this family of steels are obtained with approximately equal parts of ferrite and austenite. Grain refinement increases the toughness and resistance to environmentally assisted cracking. The precipitation of tertiary phases, such as σ, χ and secondary austenite, decreases the mechanical and corrosion resistance properties. The σ phase is the most dangerous of these deleterious phases and the one that precipitates in higher amounts. The majority of research works on deleterious phases in duplex and superduplex steels evaluate the kinetics of precipitation in specimens isothermally treated. In the present work, the precipitation of deleterious phases during continuous cooling from 1100°C was evaluated by optical and scanning electron microcopies, hardness measurements and electrochemical corrosion tests. Two superduplex stainless steel steels UNS S32750 with similar composition but quite different grain sizes were analysed in order to evaluate the influence of this microstructural feature.