Cr-depleted Zone Research Articles

Effect of Heat Treatment on the Microstructure Evolution and Sensitization Behavior of High-Silicon Stainless Steel

The microstructure and sensitization behavior of high-silicon stainless steel containing 4.0 wt.% Si was studied after stabilization heat treatment followed by isothermal heat treatment. Isothermal heat treatment for 6 h was carried out on the alloy at two different temperatures, viz. 500 and 750 °C, after stabilization heat treatment at 950 °C. Phase fraction of the high-silicon stainless steel remained constant after isothermal heat treatment. Fine (Ti,C) precipitates of angular and spherical morphology were observed throughout the microstructure, in all the heat-treated sample conditions. Their number density was measured and related to the processing conditions that they were exposed to. Additionally, the presence of Cr and C was confirmed by compositional analysis, and they appeared at the grain boundary in the sample isothermally heat-treated at 750 °C for 6 h. Sensitization study of the heat-treated samples was carried out with the help of double-loop electrochemical potentiokinetic reactivation test in specially prepared acid etchant, and it was observed that degree of sensitization depended strongly on the formation of Cr-depleted zone along the grain boundary. Based on our study, we conclude that stabilization heat treatment significantly reduces the sensitization behavior, even for steel containing significant fraction of Si.

Corrosion behaviour of resistance-spot-welded high-Mn austenitic TWIP steel

ABSTRACT High-Mn twinning-induced plasticity (TWIP) steels have been proposed to meet higher passenger safety and crash-safe vehicle frame requirements demanded by the automotive industry. The corrosion behaviour of welded Fe–30Mn–5Al–0.5C–6Cr TWIP steel using the resistance-spot-welding (RSW) technique was studied. Welds showed a microstructure consisting of primary austenite dendrites and interdendritic δ-ferrite in both fusion zone (FZ) and base metal (BM), which is in agreement with an austenitic-ferritic (AF) solidification mode. The results reveal that the refinement of the microstructure in the weld region does not significantly affect the corrosion resistance of this region, having a low corrosion current density of the order of 1 μA cm–2. It was concluded that Cr-rich second phase particles along the austenite/δ-ferrite interface can lead to Cr-depleted zones, increasing pitting corrosion susceptibility. It was found that the minimum Cr content at the Cr-depleted zones is nearly independent of the δ-ferrite volume fraction.

Enhancement in intergranular corrosion resistance of the stabilised ultra-pure 430LX ferritic stainless steel by tin addition

ABSTRACTThe intergranular corrosion (IGC) susceptibility of the Ti–Nb-stabilised ultra-pure 430LX FSSs with 0–0.47 wt-% tin addition was investigated using double loop electrochemical potentiokinetic reactivation (DL-EPR) and oxalic acid etch tests. The result of DL-EPR test reveals that IGC resistance increases with the increase in tin content. The specimens with different contents of tin exhibit the same microstructural evolution based on oxalic acid etch test. M23C6 precipitates form along grain boundaries, inducing Cr-depleted zones and finally resulting in the susceptibility to IGC. Tin addition gives rise to the formation of SnO2 on the FSSs surface, thus inhibiting the attack of corrosive ions towards Cr-depleted zones.

Damage characterization of heat resistant stainless steel grate plates in an iron ore pelletizing plant

Grate plate is one of the consumable components of chain grate in pelletizing plants, which is made up of heat resistant stainless steels. Service life of grate plates has major effect on productivity and quality of pellets. Objective of this work is to study the mechanism of intergranular cracking in heat-resistant stainless steel, one of the major degradation mechanisms of grate plates, in order to suggest preventive methods. To reach this goal, plant samples were provided from used and as-cast grate plates. The samples were studied using a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS) probe and metallography techniques. The results revealed that no Cr-depletion zone was formed during solidification and also during using of the grate plates at elevated temperatures. It was also revealed that formation of interdendritic Mn, S and/or C concentrated precipitates is the reason of the intergranular crack nucleation and propagation during the service time. As a solution to prevent that defect, it is suggested to reduce the distance between the liquid impenetrable temperature (LIT) and solidus temperature during solidification (for example with decreasing the concentration of S content).

The Pitting Behavior of Newly Modified 17-4 Precipitation Hardened Stainless Steel with Different Nb, N, and Mo Contents

The effect of the addition of Nb, N, and Mo to 17-4 precipitate hardening stainless steel (17-4 PH steel) on its pitting behavior in a simulated geothermal environment was investigated using a cyclic polarization test, and the pitting potentials and the repassivation potentials of the materials were acquired to evaluate their pitting resistance. Nb and N were found to lower the pitting potential of 17-4 PH steel, while Mo increased it. In the case of repassivation potential, the addition of all these elements seemed to have a slightly negative impact. Cr-rich oxide, Nb carbonitride (Nb (C, N)), and reversed austenite were found in the materials. The element distribution of/around the Cr-rich oxide inclusion and the Nb (C, N) precipitate of the material were studied. A Cr-depleted zone was found around the inclusion/precipitate, which rendered the surrounding area to be more susceptible to pitting corrosion. Pit embryos and well-developed pits were observed after the cyclic polarization tests. Pits tended to initiate in the vicinity of the inclusion/precipitate, and the dissolution of the oxide around the pit embryo was seen, which was believed to cause the local deterioration of the electrochemical environment. Lacy cover with narrow strip-shaped holes was found above the well-developed pit, and the forming mechanism of these holes was believed to be the dissolution of martensitic.

Electrochemical Behavior and Passive Property of 13Cr Martensitic Stainless Steel in Nitric Acid Solution

Intergranular corrosion behavior of a 13Cr martensitic stainless steel (MSS) in tempered condition was studied. Tempering was done in the temperature range of 300-700 °C after austenitization, and microstructure was established using scanning and transmission electron microscopy. Electrochemical characterizations to establish localized corrosion susceptibility were performed in 5% HNO3 solution. The MSS tempered at 550 °C showed sensitization and significant localized attack during tests in HNO3. Characterization of surface film on this condition revealed an oxide with a higher defect density (through electrochemical impedance spectroscopy–Mott–Schottky analysis) and Fe content (x-ray photoelectron spectroscopy) as compared to that for the austenitized condition. The inability of sensitized MSS tempered at 550 °C to form a protective surface film was attributed to the formation of considerable nano-sized Cr-rich carbide precipitates associated with narrow Cr depletion zones. On the other hand, the MSS tempered at 300 and 700 °C did not show any localized attack in HNO3 and this was attributed to lack of sensitization at 300 °C and de-sensitization at 700 °C.

Failure Analysis of SA-213TP347H High-temperature Reheater of 600 MW Supercritical Boiler

Failure analysis of SA-213TP347H steel tube for high-temperature reheater of 600MW supercritical boiler was studied by means of macroscopic analysis, chemical composition, mechanical properties, microstructure, scanning electron microscopy and energy spectrum analysis. The results show that the chemical composition, hardness and strength of the tube samples meet the limit of ASTM A213/A213M standard. The microstructure of tube-burst sample is austenite, there are some precipitates containing Cr and Nb element at the grain boundaries. The test results for the spare part sample near elbow show that the hardness exceeds the limit of SA-213TP347H steel, there are a lot of slip lines and some non-metallic inclusions containing S in the microstructure. It is concluded that solid solution treatment missing in its manufacturing is the main reasons for the fracture, which makes Cr precipitate on the grain boundary and form the Cr-depleted zone, which is easy to cause intergranular stress corrosion and lead to the tube burst.

Intergranular corrosion behavior of low-chromium ferritic stainless steel without Cr-carbide precipitation after aging

The intergranular corrosion behavior of the low-chromium ferritic stainless steel without Cr-carbide precipitation was investigated by the methods of qualitative and quantitative corrosion testing and microstructural analysis. The intergranular corrosion susceptibility of the aged stainless steels can be attributed to the Cr-depleted zone formation induced by Cr-C co-segregation to grain boundaries before Cr-carbide nucleation. The change of intergranular corrosion morphology after aging at 450 °C from discontinuous (aged for 2 h) to semi-continuous (aged for 20 h) and to continuous (aged for 200 h) is discussed in detail.

Microstructural evolution and oxidation behaviour of CF8A austenitic stainless steel during corrosion fatigue in a simulated pressurised water reactor environment

The microstructural evolution and oxidation behaviour of CF8A stainless steel during corrosion fatigue (CF) crack initiation and propagation were investigated in a simulated pressurised water reactor environment. Under the action of alternating loads, the near-surface properties of the specimens deteriorate due to the formation of Cr-depleted zones and slip bands (SBs), and CF cracks tend to initiate from SBs on the surface of the specimens. As the CF cracks propagate, dynamic interactions are present between the stress state and the oxidation behaviour at the crack tip due to intense anodic dissolution and the redistribution of stresses and strains.

Correlation Between Cr-depleted Zone and Local Corrosion in Stainless Steels: A Review

Correlation Between Cr-depleted Zone and Local Corrosion in Stainless Steels: A Review

(Invited) Origin of Weak Points in Surface Oxides at the Atomic Scale on Cr-Containing Alloys

Stainless steels (SS) are widely used in various industrial sectors because they are protected against corrosion by an oxide film formed at the surface, only a few nanometers thick and strongly enriched in Cr(III). Despite numerous studies of stainless steel passivity at the macroscopic scale, only recent surface analytical works have attempted to characterize the nature and elucidate the origin of sub-microscopic chemical and structural heterogeneities in surface oxide layers. In this work, the topographic, structural and chemical alterations of a (100)-oriented model 304 austenitic stainless steel single crystalline surface have been investigated at the nanometer/ atomic scales during early stage oxidation by Scanning Tunneling Microscopy (STM) combined with X-ray Photoelectron Spectroscopy. The experiments were performed by exposing an oxide-free Fe-18Cr-13Ni(100) surface to oxygen. The results presented first will be on the oxide-free alloy surface structure, with STM data revealing the presence of ordered vacancies under equilibrium conditions. Then the evidence of nucleation of chromium oxide at multi-atomic surface steps will be presented. A chromium pumping effect creates Cr enrichment in the oxide nuclei at the steps, and Cr-depleted zones at the border of the oxidized steps, allowing less protective iron oxide to be formed locally. Weak points in the surface oxide layers originate from this atomic scale surface process. The further discussion of the presented data will emphasize the importance of this observation for the understanding of the role of the oxidation mechanisms in the nanoscale heterogeneity of the growing surface oxide on Cr-containing alloys such as stainless steel, providing a new insight into the mechanisms of pit initiation at weak points of passive films. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 741123, CIMNAS : Corrosion Initiation Mechanisms at Nanometric/Atomic Scale). Région Ïle-de- France is acknowledged for partial funding of the STM and XPS equipments. China Scholarship Council (CSC) is acknowledged for a scholarship to Li Ma (No. 201606380129).

Microstructural studies of the scale on Sanicro 25 after 25,000 h of oxidation in steam using advanced electron microscopy techniques

In the present study the effect of long-term oxidation at 700 °C in steam on the microstructure of Sanicro 25® (42Fe22Cr25NiWCuNbN) was investigated in detail by advanced electron microscopy, spectroscopy and FIB-SEM tomography techniques.Owing to its satisfactory creep and oxidation resistance at high temperature the steel is considered suitable for use in reheaters and superheaters in ultra-supercritical coal-fired steam boilers. Microscopic analyses were performed using electron microscopy, while the chemical composition and elemental distribution were analyzed using energy dispersive X-ray spectrometry. The SEM was used for investigation of the scale surface morphology and in-depth steel microstructure. Further analyses were conducted by (S)TEM using a S/TEM Titan Cubed G2 60-300 equipped with the ChemiSTEM (FEI) system. The FIB-SEM tomography allowed for 3D visualisation of the oxide scale surface and the boundaries between the oxide scale and the underlying steel.During steam oxidation of Sanicro 25 (up to 25,000 h) approx. 3 μm thick dual-layered scale consisting mainly of Cr2O3 plates had formed. The thicker outer layer of the scale was enriched with Si and Mn. Beneath the scale, Si underwent internal oxidation which resulted in the formation of numerous SiO2 particles at the scale/steel boundary. The growth of the Cr2O3 scale caused the formation of chromium depleted zone underneath the oxide layer. In the Cr-depleted zone, M23C6 was unstable and dissolved in the austenite. In the underlying bulk steel, M23C6, ε-Cu, Fe2W Laves phase as well as NbX and/or (Nb,Cr)N Z-phase particles had precipitated in the austenitic matrix. The study showed that Sanicro 25 steel is very resistant to steam oxidation at 700 °C, but after 25,000 h it exhibits significant microstructural and chemical composition changes (up to a depth of 20 μm) in comparison to the as-received one.

Synergetic effects of impurities and alloying element Cr on oxidation and dissolution corrosion of Ni (111) surfaces: A DFT study

Using density-functional calculations, we studied the interactions between interstitial impurities (H, O, N, S, and P) and Ni (111) surfaces doped, or not, with Cr, and studied the effect of Cr doping on the dissolution corrosion resistance of Ni(111) surfaces. The aim of this work was to study, at the atomic scale, the effects of Cr on the segregation behaviors of impurities and the synergetic effects between co-doped atoms on the resistance to dissolution corrosion of Ni (111) surfaces. The results indicate that impurities S, P, O, and H prefer to be trapped at near-surface sites, that Cr was uniformly distributed in the Ni crystal and can affect the segregation behavior of impurities S and P to move toward the surface, and it affects impurities N and O such that they shift from the surface to the subsurface. The formation of near-surface Cr nitrides (speculated to be Cr2N based on the results obtained for particular co-doped surfaces) was also noted. Introducing Cr enhances the structural stability of the Ni (111) surface and protects it from being corroded when impurities are present. The elementary processes studied afforded microscopic insights into the formation of a Cr-depleted zone, a phenomenon that leads to local corrosion of the Ni alloy surface. The results of our theoretical calculations explain some of the experimental results previously observed at the atomic scale.

Research of intergranular corrosion properties of heat affected zone for HR3C steel after aging treatment

The intergranular corrosion susceptibility of heat affeceed zone(HAZ) for HR3C steel after aging at 600°C was investigated using electrochemical double-loop potentiokinetic reactivation. Experimental results show that HAZ has intergranular corrosion susceptibility after aging at 600°C. At the initial stage precipition of M23C6 along grain boundaries occurs rapidly, which leads to formation of Cr-depleted in the grain boundary and the increase of intergranular corrosion susceptibility. With the increase of aging time, the Ra value of HAZ rapidly increase. Intergranular corrosion of HAZ after aging occur due to the precipitation of M23C6 along grain boundaries, not only the precipitation of M23C6 along grain boundaries caused poor Cr, but the precipitation of M23C6 inside the grains also results in the formation of Cr-depleted zone and subsequent corrosion.

Effects of Isothermal Aging on the Microstructure Evolution and Pitting Corrosion Resistance of Lean Duplex Stainless Steel UNS S32003

UNS S32003 materials were aged at temperatures in the range of 873 K to 1173 K (600 °C to 900 °C) for 30 minutes and 2 hours. Nitrides were found to precipitate along the ferrite/austenite phase boundaries and within the ferrite phase. The precipitation kinetics was the fastest at 1073 K (800 °C), at which temperature the Cr-depleted zones were observed to be in the vicinity of the precipitates. Precipitation and resultant Cr-depleted zone led to the highest pitting susceptibility of the UNS S32003, aged at 1073 K (800 °C), when tested in 0.6 mol/L NaCl solution. To observe the precipitation evolution, the UNS S32003 specimens were aged at 973 K (700 °C) for different time periods for up to 120 hours. Within aging time up to 24 hours at 973 K (700 °C), only nitrides precipitates were observed and an intermetallic phase was observed after 120 hours of aging along with an increase in the percentage of precipitates. Pitting potential dropped with increasing aging time and metastable pitting rate increased with longer aging time period, both indicating a negative effect of aging on pitting corrosion resistance of the lean duplex stainless steel UNS S32003.

Intergranular corrosion behavior and mechanism of the stabilized ultra-pure 430LX ferritic stainless steel

Intergranular corrosion (IGC) behavior of the stabilized ultra-pure 430LX ferritic stainless steel (FSS) was investigated by using double loop electrochemical potentiokinetic reactivation (DL-EPR) and oxalic acid etch tests to measure the susceptibility of specimens given a two-step heat treatment. The results reveal that IGC occurs in the specimens aged at the temperature range of 600–750 °C for a short time. The aging time that is required to cause IGC decreases with the increase of aging temperature. A longer aging treatment can reduce the susceptibility to IGC. The microstructural observation shows that M23C6 precipitates form along the grain boundaries, leading to the formation of Cr-depleted zones. The presence of Cr-depleted zones results in the susceptibility to IGC. However, the atoms of stabilizing elements replace chromium atoms to form MC precipitates after long-time aging treatment, resulting in the chromium replenishment of Cr-depleted zones and the reduction of the susceptibility to IGC.

Elastic Properties of FeCr20Ni8Xn (X = Mo, Nb, Ta, Ti, V, W and Zr) Austenitic Stainless Steels: A First Principles Study

Austenitic stainless steels suffer from intergranular corrosion and stress corrosion cracking when exposed to elevated temperature (500–800 °C). Under these environments, Cr-carbides and Cr-carbontrides precipitate at the grain boundaries, which results in the formation of Cr-depleted zone. In practice, alloying elements could be added into austenitic stainless steels to modify the precipitation processes. Besides the precipitation processes, the elastic properties of the iron matrix would be influenced. Using the exact muffin-tin orbitals (EMTO) method, the solute effects on the elastic properties of FeCr20Ni8 austenitic stainless steels were studied. Based on the simulated shear modulus (G) and bulk modulus (B), we proposed a design map for FeCr20Ni8 based alloys, aiming to provide a basis for the design of high-performance austenitic stainless steels.

Direct laser deposition as repair technology for a low transformation temperature alloy: Microstructure, residual stress, and properties

Fe-Cr-Ni-Mn-Mo-Nb-Si steel with low transformation temperature was fabricated by direct laser deposition (DLD) and its microstructure, residual stress and properties were investigated. The results demonstrated that the microstructure consisted of martensite laths with a lath spacing in the nano-scale range and a small amount of nano-sized polycrystalline precipitates. There was no grain boundary segregation, and the chemical elements were homogeneously distributed. The tensile strength, yielding strength, elongation and impact energy were 1275 MPa, 1032 MPa, 12% and 57 J respectively. The comprehensive mechanical properties were equivalent with those of the wrought precipitation hardening martensitic stainless steel. There was no Cr-depleted zone at the interface between the matrix and the precipitates as well as the interior at the precipitates, and the corrosion resistance was better than that of FV520B stainless steel. The neutron diffraction (ND) stress analysis results showed that the residual tensile stress level in the cladding layer was relatively low due to the effect of solid phase transition. Consequently, due to the excellent integrated mechanical properties and the residual stress state, there was no need to conduct stress relief and precipitation hardening heat treatment, thus avoiding the influence of heat treatment on the base material properties, and saving resources and funds.

Effect of shielding gas composition on intergranular corrosion of stabilized ferritic stainless steel GMA welds

The effect of shielding gas composition on intergranular corrosion of the fusion zone formed by bi-stabilized ferritic stainless steel plates (AISI 441) and ER430Ti and ER430LNb filler metals during gas metal arc welding was investigated. Double loop electrochemical potentiokinetic reactivation tests were conducted to examine the influence of the shielding gas content (Ar + 2%O2, Ar + 8%CO2 and Ar + 25%CO2) on intergranular corrosion of the ferritic stainless steel welds. It was possible to observe an increase in the tendency toward sensitization with the increase in the CO2 content in the shielding gas, especially with the ER430Ti filler metal. Analysis of the formed precipitates by means of optical microscope, scanning electron microscope an energy-dispersive X-ray spectroscopy allowed to notice that high levels of CO2 blended in the shielding gas increase the intergranular precipitates and lead to significant intergranular corrosion, which was induced by the Cr-depletion zone formation, especially for the welds produced with the ER430Ti filler metal.

The Correlation Between the Distribution/Size of Carbides and Electrochemical Behavior of 17Cr-1Ni Ferritic-Martensitic Stainless Steel

The correlation between the distribution/size of carbides and corrosion resistance of 17Cr-1Ni ferritic-martensitic stainless steel after different heat treatment temperatures was investigated by transmission electron microscope, electrochemical tests, and corrosion morphology observations. The results showed that the size of the precipitated phase decreased and corrosion resistance increased with an increase in the annealing temperature. When the tempering temperature was low (290 °C), carbides precipitated mainly at the phase boundaries due to a low degree of atomic matching and higher grain boundary energy. In this case, the polarization curve had a passivation interval and the pits were mainly initiated at the phase boundary. When the tempering temperature was higher than 400 °C, the carbides gradually precipitated in the martensite laths because the accelerated diffusion of Cr healed the Cr depletion zone at phase boundary. This outcome resulted in a polarization curve that had no passivation range and uniform corrosion occurred in martensitic region.