Ultra-pure Ferritic Stainless Steel Research Articles

Sensitization characteristics for type 445 ultra-pure ferritic stainless steel with an electrochemical reactivation method

The sensitization characteristics of type 445 ultra-pure ferritic stainless steel (FSS) under different aging conditions were investigated by the optimized double loop electrochemical potentiodynamic reactivation (DL-EPR) test and microstructure observation. The solution 2 mol L−1 H2SO4 + 0.01 mol L−1 KSCN +0.5 mol L−1 NaCl and a scan rate of 100 mV min−1 are suitable for the DL-EPR test of 445 FSS. With the change of aging temperature from 500 to 900 °C, the degree of sensitization (DOS) slightly increases at first and then decreases, which reaches a maximum value at about 700 °C. Under the aging condition at 700 °C for 10 min to 2 h, the DOS value gradually increases, but it is less than 3% due to the low intergranular corrosion susceptibility. The aging treatment at 700 °C results in the precipitation of Laves phase and (Nb, Ti)(C, N), but no Cr-carbides. The sensitization may be related to the segregation of Cr atoms around (Nb, Ti)-precipitates at the grain boundaries.

Refining mechanism for TiN and solidification structure through a nucleating chain in magnesium-yttrium-treated ultrapure ferritic stainless steel

To investigate the refining phenomena of TiN inclusion and macro-/microstructure through heterogeneous nucleation of δ-Fe at TiN encapsulating oxide during solidification in ultrapure ferritic stainless steel, five samples were prepared, denoted as the initial, low-yttrium-treated, moderate-yttrium-treated, high-yttrium-treated, and magnesium-yttrium-treated samples. Based on the results of TiN inclusions, the minimum value of the average size (2.61 μm) and the maximum value of the number density (346.70 mm−2) were observed after magnesium-yttrium (Mg–Y) treatment. Combined with the mechanism analysis, these results reveal that the TiN can be refined because of the following three reasons: (ⅰ) the pure oxides, possessing low planar and/or linear disregistry with TiN; (ⅱ) the complex oxides, including the phase, which possesses extremely low disregistry with TiN; and/or (ⅲ) the increasing number of the oxides. By comparison, finer macrostructures were yielded with the addition of Y-/Mg–Y-based modifiers because of the decreasing nucleation barrier, and the finest macrostructure with equiaxed grain ratio of 100% and average grain size of 1.099 mm was observed in the Mg–Y-treated sample. Coupled with the results of TiN inclusions, therefore, the refining efficiency of macrostructure increases with the number of TiN. Referring to the results of differential scanning colorimetry, a general rule of solidification for metallic materials has been revealed. In addition, the in-situ nucleation-growth of δ-Fe was observed using high temperature laser confocal microscopy, and the microstructures were refined after Y/Mg–Y treatment. Eventually, the interfacial wetting-lattice mismatch heterogeneous nucleation model is successfully applied to evaluate the refining phenomenon of macro-/microstructures.

A Dynamic Decarburization Model in Vacuum Oxygen Decarburization Process for Ultrapure Ferritic Stainless Steel

A Dynamic Decarburization Model in Vacuum Oxygen Decarburization Process for Ultrapure Ferritic Stainless Steel

Mechanism for TiN refinement and resultant heterogeneous nucleation of δ-Fe in magnesium-rare earth-treated ultrapure ferritic stainless steel

The effect of magnesium rare-earth (Mg-RE) treatments on TiN epitaxial nucleation to refine the as-cast structure of ultrapure ferritic stainless steel was investigated. Four samples with different melt treatments were prepared, namely an initial sample, a low-RE-treated sample, a high-RE-treated sample, and a Mg-RE-treated sample. The results reveal that TiN can be refined after the RE/Mg-RE treatment because of the co-crystallization of TiN and RE-Mg-Al-O. Moreover, the size of the inclusions increased with the RE content owing to the decreasing number of oxides. Compared to the low-RE treated sample, the average inclusion size of the Mg-RE-treated sample was further reduced because of the presence of more oxides. These results suggest that the number of oxides is a key factor in TiN refinement. However, few changes were observed in the 3D morphology of the composite TiN under such circumstances. In addition, compared with the Initial sample, the RE/Mg-RE treatment yielded finer solidification structures, indicating that the addition of RE/Mg-RE modifiers decreased the nucleation barrier. The solidification structure coarsened with increasing RE content because of the decrease in the inclusion number density, particularly for TiN. The Mg-RE treatment yielded the finest solidification structure with completely equiaxed grains. In addition, high temperature laser confocal microscopy was successfully applied to observe the in-situ nucleation-growth of δ-Fe, showing that the average grain size after Mg-RE treatment decreased by 38.09%. To reveal these phenomena, a new interfacial wetting-lattice mismatch heterogeneous nucleation model is established, accounting for the high-temperature wetting angle (θ1). Using differential scanning calorimetry, the upper limit of the heterogeneous nucleation rate (Iheter.) of δ-Fe was determined from the number density of all the TiN inclusions. Crystallization of primary δ-Fe and Iheter. depend on the critical degree of undercooling (∆Tc∗).

A Dynamic Decarburization Model in Vacuum Oxygen Decarburization Process for Ultrapure Ferritic Stainless Steel

A mathematical model incorporating dynamic parameters is proposed to investigate the overall decarburization process during the vacuum oxygen refining (VOD) process for ultrapure ferritic stainless steel. This paper also conducted a precise theoretical calculation of the C‐Cr equilibrium. Moreover, the top‐blowing oxygen distribution is categorized into six directions: [C] oxidation, alloy component oxidation, CO bubble oxidation above the bath surface, bath surface oxidation (FeO), dissolution in the molten steel ([O]), and escape into the exhausted gas. The FeO and Cr2O3 oxide are intermediate product of oxygen to oxidize [C] remaining in the molten steel during the VCD stage. The modified approach of Gibbs free energies determines the critical carbon concentration by the distribution ratio of carbon to the top‐blowing oxygen. The top‐blowing oxygen primarily consumed by CO‐CO2 bubble (49–55%) and Cr2O3 (26–33%). Additionally, the model considered operational aspects, the system's heat balance, and changes in overall metal‐slag conditions during the refining process. The exhausted gas composition and end‐point carbon concentration align well with industrial data. Within a certain range (approximately up to 0.50%), increasing initial carbon concentration can enhance overall decarburization efficiency.

Effects of hydrogen charge on the intergranular corrosion of high-defect-density ultrapure ferrite stainless steel

This research scrutinizes the intricate effects of hydrogen charging on crystal defects and passive film properties in high-defect-density 443 ultrapure ferritic stainless steel. It discerns that with non-saturated hydrogen at hydrogen traps, escalated hydrogen charging current density results in increased hydrogen capture. These trapped atoms fill the defects, thus mitigating the stress level and fostering the growth of chromium oxides within the passive film. Consequently, the alloy’s resistance to intergranular corrosion is enhanced.

Characteristics of SEN clogging and adhesive behavior of oxide inclusion during continuous casting of Ti-stabilized ultra-pure ferritic stainless steels

Characteristics of SEN clogging and adhesive behavior of oxide inclusion during continuous casting of Ti-stabilized ultra-pure ferritic stainless steels

Effects of grain boundary characteristics changing with cold rolling deformation on intergranular corrosion resistance of 443 ultra-pure ferritic stainless steel

The relationship between evolution in grain boundary characteristics and intergranular corrosion properties of cold rolled 443 ultra-pure ferritic stainless steel was investigated. The results showed that low-angle (LA) grain boundaries, special grain boundaries (SGB), and high-angle (HA) grain boundaries had a high intergranular corrosion sensitivity after cold deformation in reducing acid medium. In oxidative acid medium, the LA and SGB boundaries had a high intergranular corrosion resistance in comparison to HA boundaries, and their inhibitory effect on the corrosion continuance of the dynamic recrystallization high-angle (RHA) grain boundaries can improve the corrosion resistance of the RHA grain boundaries.

New Findings on Intergranular Corrosion Mechanism of 445J2 Ultra-Pure Ferritic Stainless Steel

A criterion for characterizing the intergranular corrosion sensitivity of 445J2 ultra-pure ferritic stainless steel was established using microstructure analysis and electrochemical impedance spectroscopy (EIS). The differences in the characterization results for the intergranular corrosion sensitivity of 445J2 stainless steel by EIS and the double loop-electrochemical potentiokinetic reactivation test were analyzed. The Mo-rich intermetallic compound did not cause depleted chromium zones in the sensitization temperature range of 500°C to 700°C but caused a surrounding elastic stress field. It was determined that the intergranular corrosion in 445J2 is due to the elastic stress field caused by the precipitated phase.

Effects of titanium content on the large precipitates in 443 ultra-pure ferritic stainless steel

ABSTRACT The effects of titanium contents on the precipitates of 443 ultra-pure ferritic stainless steel during solidification were determined experimentally and compared with a thermodynamic simulation software, that is, Thermo-Calc. Ultra-pure ferritic stainless steel was cast in a vacuum. The experimental results demonstrated that two types of precipitates during solidification exist in the as-cast ingot according to the compositional characteristics, that is, Al2O3–TiN–NbC and TiN–NbC. The number density of TiN–NbC is larger than the Al2O3–TiN–NbC with the Al2O3 core. The number density of the precipitates increases with the Ti content increasing. These precipitates are generated during solidification. The precipitating process could be well predicted through Thermo-Calc software. The Al2O3 forms first in the liquid steel before solidification, TiN appears later in the solidification process, NbC lastly below 1000°C. The titanium content variation from 0.10% to 0.29% has no apparent effect on the liquidus and precipitation temperatures of TiN, but it distinctly reduces the solidus temperature from 1422°C to 1378°C. The higher Ti content in 443 steel leads to TiN precipitates and Al2O3–TiN particles amount increase during solidification. The disregistry theory and thermodynamic calculation indicate that the Al2O3 could act as the nucleus of TiN and the TiN could act as the nucleus of NbC.

Comparison of feasibility, microstructure and performance of hybrid laser arc, activated flux tungsten inert gas and friction stir welding for thick plate of innovative ultra-pure ferritic stainless steel

Comparison of feasibility, microstructure and performance of hybrid laser arc, activated flux tungsten inert gas and friction stir welding for thick plate of innovative ultra-pure ferritic stainless steel

Wind resistance performance of a continuous welding stainless steel roof under static ultimate wind loading with testing and simulation methods

Ultrapure ferritic stainless steel provides a new generation of long-span metal roof systems with continuous welding technology, which exhibits many unknown behaviors during wind excitation. This study focuses on the wind-resistant capacity of a new continuous welding stainless steel roof (CWSSR) system. Full-scale testing on the welding joints and the CWSSR system is performed under uniaxial tension and static ultimate wind uplift loadings, respectively. A finite element model is developed with mesh refinement optimization and is further validated with the testing results, which provides a reliable way of investigating the parameter effect on the wind-induced structural responses, namely, the width and thickness of the roof sheeting and welding height. Research results show that the CWSSR system has predominant wind-resistant performance and can bear an ultimate wind uplift loading of 10.4 kPa without observable failures. The welding joints achieve equivalent mechanical behaviors as those of base material is produced with the current of 65 A. Independent structural responses can be found for the roof sheeting of the CWSSR system, and the maximum displacement appears at the middle of the roof sheeting, while the maximum stress appears at the connection supports between the roof sheeting with a significant stress concentration effect. The responses of the CWSSR system are greatly influenced by the width and thickness of the roof sheeting but are less influenced by the welding height.

Preparation of high corrosion resistant solar absorbing coating on the surface of ferritic stainless steel by utilizing chemical coloring

The corrosion resistance of the solar selective coating (SSC) has become the biggest bottleneck of solar energy applications. Ultra-pure ferritic stainless steel was used as an SSC substrate for the first time, and chemical coloring was improved and applied to the preparation of SSC. The coloring liquid is Na2H4Cr2O9-H2SO4 with the additives of ZnSO4 and Cr2(SO4)3, the solar absorptance of 0.9343, and thermal emittance of 0.1124. After Neutral Salt Spray test (NSS) for 240 h, PC is 0.0432 < 0.05, indicating the high durability.

Thermal Stability of Chromium-Iron Oxidation Mixture Cermet-Based Solar Selective Absorbing Coatings.

The solar selective absorber coating (SSAC) are at the core of the efficient solar-thermal system. In this paper, for the first time, the Chromium-iron oxidation mixture cermet was successfully prepared on the surface of ultra-pure ferritic stainless steel by chemical coloring as SSAC. The coating surface has an optical trap structure, and the chromium-iron oxidation mixture cermet is used as an absorption layer to realize solar-thermal conversion. The solar absorptance (AM1.5) of the coating reached 93.66, and the thermal emittance was less than 13. After thermal shock tests at 25/300 °C done 32 times (accumulated 812.8 h), the Performance Criterion (PC) of the coating was 0.01375 < 0.05, showing outstanding thermal stability.

Improved mechanical properties of aluminum modified ultra-pure 429 ferritic stainless steels after welding

The microstructure and welding performance of ultra-pure 429 ferritic stainless steels with modified aluminum contents have been investigated. Room temperature tensile mechanical properties after welding improve apparently due to the precipitate of nano-sized TiN and core-shell structure Al2O3-TiN particles which play the important role of precipitate strengthening by dislocation pinning of the weld joint. In steels without the addition of aluminum, although the ultimate tensile strength hardly varies before and after welding, the ultimate tensile strength significantly improves after welding especially for high-Al steel, and laser welding and tungsten inner gas welding show hardly any influence on room temperature tensile strength. The effect of fine grain strengthening is very obvious after laser welding, which contributes to the significantly increased value of micro-hardness of the weld joint. The aluminum modified 429 ultra-pure ferritic stainless steels show good welding performance in consideration of room temperature mechanical properties.

Inclusions evolution during the LF refining process of 439 ultra-pure ferritic stainless steel

The morphology, composition, size, and number of inclusions in 439 ultra-pure ferritic stainless steel samples were analyzed using an automatic scanning electron microscope combined with an energy-dispersive X-ray spectrometer. In addition, the appropriate contents of titanium, aluminum, and calcium were analyzed through the coupling of thermodynamics calculation and experimental results. CaO-Al2O3-MgO inclusions existed in the 439 steel before Ti additions in the ladle furnace (LF) refining process. After Ti addition in the LF refining process, the inclusions were transformed into CaO-Al2O3-MgO-TiOx inclusions. The evolution of these inclusions was consistent with thermodynamic calculation, which indicated that when the Al, Ca, and Ti contents were within a reasonable range, Ca treatment could significantly modify the aluminate and spinel to form CaO-Al2O3-MgO liquid inclusions. In addition, the compositions of inclusions after the addition of titanium were mostly located in the Al2O3-TiOx stable phase. The collision of the CaO-Al2O3-MgO liquid inclusions and Al2O3-TiOx inclusions resulted in the modification of the CaO-Al2O3-MgO-TiOx inclusions. The compositions of most inclusions were located in the liquid zone. The control range of the aluminum, calcium, and titanium contents was obtained: logAl% ≥ 1.481logTi% − 0.7166, Ca% ≥ 34.926(Al%)3 − 3.3056(Al%)2 + 0.1112(Al%) − 0.0003.

Investigation on ultra-pure ferritic stainless steel 436L susceptibility to intergranular corrosion using optimised double loop electrochemical potentiokinetic reactivation method

ABSTRACTThe susceptibility of 17Cr ferrite stainless steel to intergranular corrosion (IGC) was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the microstructural characterisation. The results show that the optimised DL-EPR test condition for 17Cr ferrite stainless steel is l M H2SO4 + 0.006 M KSCN solution with a scan rate of 0.1 V min−1 at 25°C The severe IGC occurs in the temperature range of 500–650°C and the nose temperature locates at approximately 600°C. The Ir/Ia value rises up to 16.99% when heat treated at 600°C for 30 min. After aging treatment, M23C6 precipitates are detected using transmission electron microscopy, resulting in the presence of the Cr depletion zone. However, the degree of IGC for 436L is extremely limited due to the Cr element diffusion form the matrix despite prolonging the aging time to 5 h.

Precipitation Behavior of Ultra-pure Ferritic Stainless Steel During Hot Deformation

Precipitation Behavior of Ultra-pure Ferritic Stainless Steel During Hot Deformation

The Quantitative Effect of Mo and Cu on the Stress Corrosion Cracking and Pitting Corrosion Behavior of Ultra-Pure Ferritic Stainless Steels

The Quantitative Effect of Mo and Cu on the Stress Corrosion Cracking and Pitting Corrosion Behavior of Ultra-Pure Ferritic Stainless Steels

Effects of Microalloying and Heat-Treatment Temperature on the Toughness of 26Cr–3.5Mo Super Ferritic Stainless Steels

The effects of Ni content and heat-treatment process on the toughness of a super ferritic stainless steel with 26 wt% Cr and 3.5 wt% Mo were investigated. It was found that with the increase of Ni content, the Charpy impact toughness improved remarkably, and transformed from cleavage brittle fracture to the most ductile fracture. There were no obvious differences between the high- and low-Ni contents on the microstructure and mechanical properties since the addition of Ni did not influence crystal structure, phase composition, and precipitation of ultra pure ferritic stainless steels. Meanwhile, the heat-treatment process was a key point to maintain a high level of toughness by optimizing structure and removing detrimental precipitation, i.e., chi phase.