Superferritic Stainless Steel Research Articles

Effect of Subsequent Continued Heating After Hot Rolling on the Microstructures and Mechanical Properties of Hot-Rolled Super Ferritic Stainless Steels

Effect of Subsequent Continued Heating After Hot Rolling on the Microstructures and Mechanical Properties of Hot-Rolled Super Ferritic Stainless Steels

EFFECT OF Ce ON THE MICROSTRUCTURE AND PROPERTIES OF 27Cr-4Mo-2Ni SUPER-FERRITIC STAINLESS STEEL WITH 800 °C AGING

The influence of Ce additions on the microstructure and properties of 27Cr-4Mo-2Ni super-ferritic stainless steel during 800 °C aging was systematically investigated via thermodynamic calculations, microstructural characterizations, impact and corrosion measurements. The results indicated that with an increase of Ce content from 0.00 w/% to 0.050 w/%, Al2O3 wrapped by TiNb(CN) were gradually modified into CeAlO3 wrapped by TiNb(CN), spheroidal Ce2O3, and Ce2O2S, the ferrite grain was noticeable refined, the  phase changed from continuously distributed with a coarser size to dispersedly distributed with a tiny size accompanied by an increase in number, the impact toughness of super-ferritic stainless was improved at 25 °C, and the cleavage facets at the morphology of a fully brittle fracture becomes smaller, the uniform corrosion resistance of super-ferritic stainless steel to 40 vol.% H2SO4 were enhanced especially at 60 °C.

Effect of recrystallization degree on properties of passive film of super ferritic stainless steel S44660

Abstract The effect of the recrystallization degree on the properties of passive films formed in 0.1 M HNO3 solution for super ferritic stainless steel S44660 was examined in this study. The initial specimens, in their cold-rolled state, showed a high dislocation density, as observed through electron backscatter diffraction (EBSD) experiments. Analysis of potentiodynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS) measurements suggested that with the increase of recrystallization degree, the corrosion current density reduced and the corrosion potential increased. As revealed by Mott–Schottky analysis, the passive film showed a dual structure of n-type and p-type semiconductors, with the carrier density of the passive film decreasing as the recrystallization degree increased. X-ray photoelectron spectroscopy (XPS) provided insights into the film composition, indicating that the Fe2O3 and Cr2O3 content, which improved the stability of the passive film, increased with the degree of recrystallization. In summary, the increase in recrystallization degree reduced the number of defects in the microstructure, thereby creating favorable conditions for the formation of highly protective passive films. The passive film formed after complete recrystallization exhibited enhanced corrosion resistance.

Hydrogen Embrittlement of 27Cr-4Mo-2Ni Super Ferritic Stainless Steel.

The effect of hydrogen content on the deformation and fracture behavior of 27Cr-4Mo-2Ni super ferritic stainless steel (SFSS) was investigated in this study. It was shown that the plasticity and yield strength of SFSS were very susceptible to hydrogen content. The introduction of hydrogen led to a significant decrease in elongation and a concurrent increase in yield strength. Nevertheless, a critical threshold was identified in the elongation reduction, after which the elongation remained approximately constant even with more hydrogen introduced, while the yield strength exhibited a monotonic increase with increasing hydrogen content within the experimental range, attributed to the pinning effect of the hydrogen Cottrell atmosphere on dislocations. Furthermore, the hydrogen-charged SFSS shows an apparent drop in flow stress after upper yielding and a reduced work hardening rate during the subsequent plastic deformation. The more hydrogen is charged, the more the flow stress drops, and the lower the work hardening rate becomes.

Effects of Laves phase precipitation embrittlement and its dissolution kinetics in super-ferritic stainless steels on the ductile-brittle transition

This study simulated the Laves phase precipitation embrittlement of super-ferritic stainless steels (SFSSs). The dissolution kinetics of the Laves phase, grain coarsening, and their effects on the mechanical properties and ductile-brittle transition during dissolution heat treatment were subsequently studied. The Laves phase dissolution adhered to the Johnson-Mehl-Avram-Kolmogorov variation expression, exhibiting an activation energy of approximately 368 kJ mol−1. High-temperature reheating (1100 °C) resulted in severe grain coarsening, leading to brittle fracture. A dissolution treatment of only 0.5 h at 1050 °C resulted in a high yield strength of 407.8 MPa, large elongation of 29.6%, and impact toughness of 237.2 J/cm2 because the entirety of the Laves phase was dissolved and no obvious grain growth was observed. This indicates that dissolution heat treatment is a simple and feasible method for recovering the ductility and toughness of Laves phase precipitation-embrittled SFSSs.

Effects of Al additions on precipitation, recrystallization and mechanical properties of hot-rolled ultra-super ferritic stainless steels

In this study, 1.0 wt%Al element was added into the traditional 26.8Cr-3.7Mo–2Ni super-ferritic stainless steel to tailor the brittle phase precipitation, recrystallization microstructure and mechanical property via solution annealing at temperatures of 950–1150 °C. The experimental results demonstrated that hot-rolled microstructure for 1.0Al sample consisted of surface fine grain region and core elongated grains together with TiN, NbC and Laves phase, while none of these fine grains appeared in 0Al samples. After annealing below 1050 °C, both σ-phase and Laves phase were formed in Al-free samples, while only abundant Laves phase precipitated in 1.0Al sample indicating the inhibition effect of Al on the σ-phase and the promotion effect on Laves phase. When annealing above 1050 °C, recrystallization was completed and neither σ-phase nor Laves phase were observed. Relatively stable mean grain size was produced due to the Laves phase pinning effect during annealing at 1050 °C, while the Nb/Al solute drag effect bore responsibility for the situation at 1050–1150 °C. Sudden grain coarsening phenomenon appeared after annealing above 1150 °C. Al additions gave a solution strengthening and intensive Laves phase precipitation strengthening for the designed alloys, and good comprehensive mechanical properties were obtained demonstrating the probability of developing of the Al-modified ultra-super stainless steels.

The precipitation tendency and stacking fault energy of Al-modified ultra-super ferritic stainless steels: An experimental and first-principles study

Super ferritic stainless steels (SFSS) are prone to the precipitation of multiple brittle second phases, increasing the production difficulty and restricting their applications. In this study, Al is added to super ferritic stainless steel to facilitate industrial production with controlled precipitation of brittle phases. The precipitation and recrystallization of the Al-modified super ferrite stainless steel are investigated through experimental and first principles approaches. It is found the addition of Al can inhibit σ-phase and promote the precipitation of Laves phase. It also increases the surface energy of BCC-Fe and reduces the unstable stacking fault energy, leading to an increase in the ductility of the SFSS. The results provide theoretical guidance for further improvements of mechanical properties of Al-modified ultra-super ferrite stainless steel.

Mechanical characterization and evaluation of pitting corrosion resistance of a superferritic stainless steel model alloy 25Cr–6Mo–5Ni

Abstract The mechanical properties and corrosion resistance of annealed superferritic stainless steel model alloy 25Cr–6Mo–5Ni were investigated in 0.6 M NaCl solution. The microstructure consisted of a ferrite matrix and eutectoid phase with a lamellar structure distributed at grain boundaries and within the ferrite grains with a spherical morphology. Tensile and impact results suggested brittle behavior of the model alloy. Fractography analysis revealed typical cleavage facets, river patterns and micro-cracks at grain boundaries and across the ferrite grains. Pitting corrosion began within the eutectoid phase, which contains in chrome depletion zones. Electrochemical impedance spectroscopy measurements suggest that the breakdown of passive films was more susceptible in the eutectoid phase.

Effects of cooling manner on Laves phase precipitation and mechanical properties of Al modified super ferritic stainless steels

The solution treating and cooling experiments were conducted to study the microstructure and mechanical properties of Al modified super ferritic stainless steels with various cooling methods. The results demonstrated that the grain was characterized by the stratified distribution. The grain coarsened gradually with the decrease in cooling rate. And the surface grain coarsened obviously, Nb (C, N) particles were precipitated in the samples in different cooling manners. However, the Laves phase was distributed both at the grain boundaries and in the grains in the furnace-cooled specimen, and the area fraction of Laves phase increased significantly with the increase of Al content. With the cooling rate decreased, the yield strength, ultimate tensile strength and Vickers hardness of the Al-free samples increased, while the elongation decreased, and the furnace-cooled specimens showed brittle fracture during the tensile test. The addition of Al also increased yield strength, ultimate tensile strength and Vickers hardness and deteriorated plasticity of samples with different cooling manners. Meanwhile, the plasticity of furnace-cooled samples was more sensitive to addition of Al than water-quenched and air-cooled specimens. The analysis results showed that the plastic deterioration of furnace-cooled samples was mainly related to the coarsening of Laves phase at grain boundaries. Hence, the precipitation of large size Laves phase at grain boundaries should be inhibited in the process of industrial production.

A novel precipitation mechanism of Laves phase in Fe-30Cr-2Mo super ferritic stainless steel: In-situ phase transformation

Laves phase is generally prejudicial to the performance of ferritic stainless steels, so understanding the underlying formation mechanism should be technologically useful. Here we demonstrate a new precipitation behavior of Laves phase in Fe-30Cr-2Mo super ferritic stainless steel by using transmission electron microscopy. It is revealed that the MC carbides firstly precipitate adhere to the M2SC carbosulphides with an orientation relationship (OR) of (0001)M2SC//(111¯)MC and [112¯0]M2SC//[011]MC. Subsequently, the interfaces between MC carbides and ferrite matrix provide preferential nucleation sites for Laves phases, which hold OR with MC as follows: (111¯)MC//(0003)Laves and [011]MC//[112¯0]Laves. Notably, the transformation from MC to Laves phases is an in-situ process based on the dissolution of MC carbide, which supplies the niobium element for nucleation and growth of Laves phase. This work enriches the recognition of the precipitation mechanism of Laves phase, and should therefore facilitate the design of high-performance ferritic stainless steels.

Effects of Cooling Manner on Laves Phase Precipitation and Mechanical Properties of Al Modified Super Ferritic Stainless Steels

Effects of Cooling Manner on Laves Phase Precipitation and Mechanical Properties of Al Modified Super Ferritic Stainless Steels

Corrosion behaviour of super ferritic stainless steel 020Cr25MoCuNbTi in the waste phosphoric acid of a surface treatment process

The corrosion behaviour of super ferritic stainless steel 020Cr25MoCuNbTi in waste phosphoric acid during the surface treatment process was investigated between 25 and 80 ℃. After 24 h of immersion, 020Cr25MoCuNbTi is found to be spontaneously passivated at all temperatures. The corrosion resistance increases with immersion time and decreases with temperature. The passive film is mainly composed of chromium oxide and hydroxide in addition to phosphates or polyphosphates. The super ferritic stainless steel 020Cr25MoCuNbTi is revealed to show excellent resistance to waste phosphoric acid corrosion.

The Effect of Holding Time on Dissimilar Transient Liquid-Phase-Bonded Properties of Super-Ferritic Stainless Steel 446 to Martensitic Stainless Steel 410 Using a Nickel-Based Interlayer.

The dissimilar joining of martensitic and ferritic stainless steels have been developed that needs corrosion resistance and enhanced mechanical properties. In this study, the transient liquid-phase bonding of martensitic stainless steel 410 and super-ferritic stainless steel 446 was conducted with a nickel-based amorphous interlayer (BNi-2) at constant temperature (1050 °C) and increasing times of 1, 15, 30, 45, and 60 min. For characterization of the TLP-bonded samples, optical microscopy and scanning emission microscopy equipped with energy-dispersive X-ray spectroscopy were used. To investigate the mechanical properties of TLP-bonded samples, the shear strength test method was used. Finally, the X-ray diffraction method was used for microstructural investigation and phase identification. The microstructural study showed that the microstructure of base metals changed: the martensitic structure transited to tempered martensite, including ferrite + cementite colonies, and the delta phase in super-ferritic stainless steel dissolved in the matrix. During the transient liquid-phase bonding, the aggregation of boron due to its diffusion to base metals resulted in the precipitation of a secondary phase, including iron-chromium-rich borides with blocky and needle-like morphologies at the interface of the molten interlayer and base metals. On the other hand, the segregation of boron in the bonding zone resulted from a low solubility limit, and the distribution coefficient has induced some destructive and brittle phases, such as nickel-rich (Ni3B) and chromium-rich boride (CrB/Cr2B). By increasing the time, significant amounts of boron have been diffused to a base metal, and diffusion-induced isothermal solidification has happened, such that the isothermal solidification of the assembly has been completed under the 1050 °C/60 min condition. The distribution of the hardness profile is relatively uniform at the bonding zone after completing isothermal solidification, except the diffusion-affected zone, which has a higher hardness. The shear strength test showed that increasing the holding time was effective in achieving the strength near the base metals such that the maximum shear strength of about 472 MPa was achieved.

Microstructure and Properties of Super-Ferritic Stainless Steels Used for Marine Construction after Fire Exposure

In this paper, the super-ferritic stainless steels (SFSSs) were simulating treated in a fire accident, and their mechanical properties and corrosion performance before/after fire exposure were studied. The experimental results demonstrated that SFSSs have a good combination property with the tensile strength, total elongation, and corrosion rate in FeCl3 solution of ∼600 MPa, ∼22.5%, and 0.03 mm/a. After fire exposure, the mechanical properties and corrosion performance were affected based on the exposure temperature due to the precipitation of intermetallic compounds. When the fire exposure temperature was 700–800°C, the plasticity was exasperated and the value decreased to only 1.5% at the condition of 800°C × 4 h due to the bulk σ-phase precipitation at grain boundaries. Meanwhile, the brittle fracture patterns emerged, and the corrosion rate in FeCl3 solution increased to 0.4 mm/a. When fire exposure takes place at 600°C for less than 4 h, the mechanical properties and the corrosion performance were little affected. Thus, it was possible to use SFSSs in marine environments.

The dissolution behavior of σ-phase and the plasticity recovery of precipitation-embrittlement super-ferritic stainless steel

In this study, a dissolution heat treatment was applied planning to recover the plasticity of an embrittled super-ferritic stainless steel caused by the precipitation of σ-phase, χ-phase and Laves phase. The dissolution behaviors of the brittle precipitation phase, and the concerned grain coarsening phenomena, as well as their effects on the mechanical properties were investigated. It was observed that the χ-phase dissolved first then did the σ-phase and therefore the Laves phase. The σ-phase dissolution kinetics showed a strong dependence on the reheating temperature. A weak grain growth phenomenon was found during the reheating process at 1100 °C. The Laves phase re-formation was observed during reheating at 1000 °C–1050 °C, which was generated by the Nb and Mo atoms diffusion. The plasticity of the tested steels was recovered after dissolution treatment at the temperature range of 1050 °C–1100 °C, during which the fracture mode was remodeled from brittle fracture to toughness fracture. The dissolution treatment at the temperature of >1050 °C was effective to recover the plasticity of precipitation- embrittlement SFSSs.

Analysis of the electrochemical behavior of a super ferritic stainless steel model alloy containing 0.07 % boron

Abstract For the present research, a super ferritic stainless steel model alloy with 4.4 wt.% nickel addition was produced. Due to the increased nickel content, the chromium content was also increased to 25.7 wt.% in order to maintain the ferritic phase stability, including a molybdenum content of 4.6 wt.% to increase the corrosion resistance. A boron content of 0.07 wt.% was also added in order to increase temperability, toughness and stability of grain boundaries. The effect of solution annealing treatments on the microstructure and corrosion resistance of the 0.07 wt.% boron model alloy was studied at temperatures of 1150 °C and 1200 °C for different times. After the solution treatments, there was an increase in the corrosion resistance of the alloy and also for the critical pitting temperature, showing that the solution treatments performed were efficient for the alloy to achieve good properties for industrial applications.

High cycle fatigue life improvement of superferritic stainless steel by laser shock peening without coating

The excellent mechanical properties, high thermal conductivity and corrosion resistance combined with moderate cost make superferritic stainless steels (SFSS) commercially attractive. Though these steels are finding increase uses in many applications where fatigue loadings are applied, few studies are devoted to study this theme. Even fewer works aim to investigate the fatigue improvement by surface treatments. Laser shock peening without coating (LSPwC), is a surface modification technology using high-energy laser with no absorbent coating that increases the fatigue life in many alloys. However, though LSPwC in SFSS UNS44600 has reported to produce beneficial compression residual stresses (CRSs) and strain-hardening surface layer, it also causes intergranular corrosion which avoids the improvement of low fatigue life (LCF). Thus, attempting to discern if this result remains in the HCF regimen, the present paper is focused on studying the influence of LSPwC on the high cycle life (HCF) of SFSS UNS S44600. The results showed that LSPwC enhances the HCF of SFSS UNS S44600, being the fatigue life improvement more pronounced with higher pulse density. This can be rationalized considering the main role of the induced CRS in slowing occurrence and growth of crack in corroded grains on HCF regimen. The examination of fracture surface by scanning electron microscopy (SEM) substantiates these fatigue results.

Understanding the pitting mechanism of super ferritic stainless steel in bromide solutions: The role of Ti/Nb–Mo precipitates with a core–shell structure

The core–shell structure of Ti/Nb precipitates in UNS S44660 was revealed by focused ion beam–transmission electron microscopy (FIB–TEM). In the bromide solution, the Nb in the shell is preferentially and selectively eluted, leaving a micro-crevice. The combined effect of the strong anodic adsorption of Br− and the large strain field in the shell promotes the free dissolution of residual Mo. Micro-crevices surround the Ti-rich nucleus and expand into pits. Consequently, pitting preferentially initiates at the Ti/Nb precipitates, which was verified by scratching electrode tests and immersion tests in bromide solutions.

In-situ study on cavitation erosion behavior of super ferritic stainless steel

Revealing the cavitation erosion initiation and propagation in super ferritic stainless steels is important both scientifically and commercially. The initial microstructure of a Cr30Mo2 steel was observed through electron backscatter diffraction. Specimens were cavitation erosion tested for 2–12 h. The specimen surfaces were analyzed by scanning electron microscopy and optical profilometry. Cavitation erosion damage initiated at persistent slip bands and pits in grains with low Taylor factors. Material removal propagated along {100} planes and could continue across grain boundaries lower than ~30°. Grain boundaries with higher misorientation angles tend to retard cavitation erosion propagation. Finally, a mechanism was proposed for cavitation erosion in super ferritic stainless steel.

Hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel

The hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel were investigated at the temperature range of 950–1150 °C and strain rate varying from 0.01 to 10 s−1. A strain compensated constitutive equation based on the Arrhenius-type model was established to predict the flow stress. The hot processing map based on the dynamic materials model was achieved to identify the optimum processing parameters. In addition, the features of microstructure evolution combined with the processing map were systematically investigated. The experimental results revealed that the flow stress increased with decreasing deformation temperature or increasing strain rate. Dynamic recovery was confirmed to be the predominant softening mechanism. The values of flow stress predicted by the strain compensated constitutive equation agreed well with the experimental values. The extent of dynamic recrystallization and recrystallized grain size increased with increasing deformation temperature or decreasing strain rate, and the continuous dynamic recrystallization was attributed to be the predominant mechanism of recrystallization during hot deformation. The optimum hot working parameters were determined to be the deformation temperature of 1070–1150 °C and strain rate of 0.1–1 s−1 with a peak power dissipation efficiency of 42%.