Sensitization Heat Treatment Research Articles

Behavior of β phase (Al3Mg2) in AA 5083 during friction stir welding

Friction stir welds of aluminum alloy 5083 are created at tool rotation speeds of 700, 1050, and 1600 rpm, with a fixed traveling speed of 100 mm/min. Stir welded structures, together with the as-received alloy, are subjected to a sensitization heat treatment of 175 °C for 240 h, which is known to produce severe sensitization in the as-received materials. It is demonstrated that stir welding dramatically reduces the sensitization response measured by the nitric acid mass loss test (NAMLT ASTM G67-93), suggesting that stir welding can be used to mitigate sensitization damage. The microstructures produced by the stir welding are evaluated using optical microscopy (OM), scanning electron microscopy (SEM), and an electron probe micro-analyzer (EPMA) to document the variations in the distribution of beta phase precipitates, which are believed to be responsible for this outcome.

Vacancy defects in a stress-corrosion-cracked Type 304 stainless steel investigated by positron annihilation spectroscopy

To reveal vacancy formation during the stress corrosion cracking (SCC), three factors influencing SCC in Type 304 stainless steels—sensitization heat treatment, corrosion treatment and tensile plastic deformation—were investigated by means of positron annihilation spectroscopy. Vacancy defects induced by the sensitization heat treatment and by tensile deformation were identified as monovacancies. These monovacancies were annealed within the same temperature range in which light water reactors are operated (280–320 °C). The above results allow us to conclude that such vacancy defects play an important role in high-temperature-water SCC crack propagation.

Pitting and Intergranular Corrosion Resistance of AISI Type 301LN Stainless Steels

The pitting and intergranular corrosion (IGC) resistance of AISI type 301LN stainless steels were evaluated using ASTM methods, anodic polarization, and electrochemical impedance techniques. The IGC results indicated that the microstructure of the samples after sensitization heat treatment at 675 °C for 1 h shows step or dual structure for both imported and indigenous materials indicating insignificant Cr23C6 precipitation. The results of immersion tests in boiling 6% copper sulfate + 16% sulfuric acid + copper solution for 24 h followed by the bend test (ASTM A262 Practice-E method) indicated no crack formation in any of the tested specimens. Pitting corrosion resistance carried out in 6% FeCl3 solution at different temperatures of 22 ± 2 and 50 ± 2 °C (ASTM G 48) up to the period of 72 h revealed pitting corrosion attack in all the investigated alloys. The potentiodynamic anodic polarization results in 0.5 M NaCl revealed variation in passive current density and pitting potential depending on the alloy chemistry and metallurgical condition. The passive film properties studied by electrochemical impedance spectroscopy (EIS) correlated well with the polarization results. The x-ray diffraction (XRD) results revealed the presence of austenite (γ) and martensite (α′) phases depending on the material condition. The suitability of three indigenously developed AISI type 301LN stainless steels were compared with imported type 301LN stainless steel and the results are highlighted in this article.

Isothermal Martensitic Transformation in Sensitized SUS304 Austenitic Stainless Steel at Cryogenic Temperature

We have investigated martensitic transformation behavior in a sensitized SUS304 austenitic stainless steel to know the stability of the austenitic phase at cryogenic temperatures. We found that the sensitized specimen exhibits an isothermal martensitic transformation when the specimen is held in the temperature range between 60 and 260K. The time-temperature-transformation (TTT) diagram corresponding to the formation of 0.5 vol% of α'-martensite shows a double-C curve with two noses located at about 100 and 200 K. An in-situ optical microscope observation has revealed that the double C-curve is due to two different transformation sequences. That is, the upper part of the C-curve is attributed to the direct γ (fcc) → α' (bcc) martensitic transformation and the lower part of the C-curve is due to the successive γ (fcc) → e' (hcp) → α' (bcc) martensitic transformation. The direct γ → α' transformation occurs in the vicinity of grain boundaries, while the successive γ → e' → α' transformation occurs near the center of each grain. The reason for appearing two types of isothermal transformation sequence in the sensitized SUS304 stainless steel will be due to the difference in concentration by sensitization heat-treatment.

Characterization of the sensitization degree in the AISI 304 stainless steel using spectral analysis and conventional ultrasonic techniques

Specimens of AISI 304 stainless steels exposed to sensitization and carbide solution heat treatment employing ultrasonic testing have been evaluated. The study involved measurements of the longitudinal wave velocity, attenuation and spectral analysis. Despite the large size difference between the ultrasonic wave length and the precipitated carbides, the results showed a clear attenuating effect in the sensitized specimens. This effect suggests a relation between carbide precipitation in the material and the attenuation coefficient of the ultrasonic wave. The attenuation increase is mainly attributed to the continuous distribution and possible coalescence of the carbide along the grain boundaries. Power spectra exhibit an increase of the amplitude at specific frequencies in specimens with the longest sensitization times. Ultrasonic velocity measurements did not provide significant information in order to predict any sensitization grade of the evaluated materials. The combined assessment by spectral analysis and attenuation measurements is discussed.

Effect of sensitization heat treatment on properties of Al–Mg alloy AA5083-H116

Al–Mg alloy AA5083 is a sheet and plate alloy used mainly for marine application as well as for structural components in transportation and military applications. The strength is derived from solid solution strengthening and strain hardening. The properties of as-received and sensitized samples of AA5083-H116 were investigated using microhardness measurements, tensile testing, optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Nitric Acid Mass Loss Test (NAMLT). The results show that both chemical and mechanical properties of the alloy decreased with increasing sensitization temperature and time. The deterioration in chemical property, which was measured in terms of the level of susceptibility to Intergranular Corrosion (IGC), is attributed to grain boundary precipitation of magnesium-rich particles. The loss in tensile and hardness properties is attributed to softening caused partly by decrease in Mg solute solid solution concentration with increasing sensitization time and temperature and partly by recrystallization at elevated temperatures.

Grain Boundary Precipitate Modification for Improved Intergranular Corrosion Resistance

Intergranular corrosion is a significant concern for Al-Mg alloys when subjected to a corrosive salt-water environment. To address this issue, the standard composition of a 5XXX series aluminum alloy (AA5083) was modified in an attempt to improve the alloy’s overall corrosion resistance through alloying and thermal processing. The concept being that through alloying and heat treatments, desirable precipitate phases such as τ- and/or τ-copper rich phase(s) that are known to offer corrosion resistance would potentially form that could effectively improve intergranular corrosion behavior. Therefore, the chemical composition of standard AA5083 was modified by adding various amounts of copper and zinc. Sensitization heat treatments were then performed to determine the specific conditions under which these phases would form. LOM, SEM, STEM imaging and conventional TEM were used to analyze microstructural features. Corrosion was attributed to a network of detrimental Mg-rich grain boundary precipitates in the standard alloy. Alloying with Cu and Zn can offer improved intergranular corrosion behavior. The mechanism seems to be either by delaying or eliminating precipitation at the grain boundaries.

Case reviews on the effect of microstructure on the corrosion behavior of austenitic alloys for processing and storage of nuclear waste

This article describes the corrosion behavior of special austenitic alloys for waste management applications. The special stainless steels have controlled levels of alloying and impurity elements and inclusion levels. It is shown that “active” inclusions and segregation of chromium along flow lines accelerated IGC of nonsensitized stainless steels. Concentration of Cr+6 ions in the grooves of dissolved inclusions increased the potential to the transpassive region of the material, leading to accelerated attack. It is shown that a combination of cold working and controlled solution annealing resulted in a microstructure that resisted corrosion even after a sensitization heat treatment. This imparted extra resistance to corrosion by increasing the fraction of “random” grain boundaries above a threshold value. Randomization of grain boundaries made the stainless steels resistant to sensitization, IGC, and intergranular stress corrosion cracking (IGSCC) in even hot chloride environments. The increased corrosion resistance has been attributed to connectivity of random grain boundaries. The reaction mechanism between the molten glass and the material for process pot, alloy 690, during the vitrification process has been shown to result in depletion of chromium from the reacting surfaces. A comparison is drawn between the electrochemical behavior of alloys 33 and 22 in 1 M HCl at 65 °C. It is shown that a secondary phase formed during welding of alloy 33 impaired corrosion properties in the HCl environment.

Determination of the Charge Carrier Concentration in Lead Selenide Polycrystalline Layers Using Reflectance Spectra

The possibility of using reflectance spectra in a nondestructive and contactless measurement of charge-carrier concentrations in polycrystalline lead selenide layers is shown. Changes in the carrier concentrations of such samples as a result of short-term sensitizing heat treatments are studied.

Effect of cold work on low-temperature sensitization behaviour of austenitic stainless steels

The effects of cold work and low-temperature sensitization heat treatment of non-sensitized austenitic stainless steels have been investigated and related to the cracking in nuclear power reactors. Types 304, 304L and 304LN developed martensite after 15% cold working. Heat treatment of these cold worked steels at 500 °C led to sensitization of grain boundaries and the matrix and a desensitization effect was seen in 11 days due to fast diffusion rate of chromium in martensite. Types 316L and 316LN did not develop martensite upon cold rolling due to its chemical composition suppressing the martensite transformation (due to deformation) temperature, hence these were not sensitized at 500 °C. The sensitization of the martensite phase was always accompanied by a hump in the reactivation current peak in the double loop electrochemical potentiokinetic reactivation test, thus providing a test to detect such sensitization. It was shown that bending does not produce martensite and therefore, is a better method to simulate weld heat affected zone. Bending and heating at 500 °C for 11 days led to fresh precipitation due to increased retained strain and desensitization of 304LN due to faster diffusion rate of chromium along dislocations. The as received or solution annealed 304 and 304LN with 0.15% nitrogen showed increased sensitization after heat treatment at 500 °C, indicating the presence of carbides/nitrides.

Controlling Grain Boundary Energy to Make Austenitic Stainless Steels Resistant to Intergranular Stress Corrosion Cracking

Intergranular corrosion and intergranular stress corrosion cracking are the two localized corrosion mechanisms that are of concern to the typical applications of austenitic stainless steels in industries. Until recently, the common understanding was that a higher frequency of random boundaries increases the susceptibility, caused by a sensitization heat treatment or by operating temperatures, of austenitic stainless steels to both intergranular corrosion and intergranular stress corrosion cracking. A recent study demonstrated that extreme randomization of grain boundaries leads to a considerable improvement of resistance to both sensitization and intergranular corrosion. This work is a continuation of Ref. 1 and relates the effects of grain boundary randomization to intergranular stress corrosion cracking: the results show a trend consistent with earlier observations on intergranular corrosion. It is shown that there is improvement in resistance to intergranular stress corrosion cracking with extreme randomization of grain boundaries.

Stress corrosion cracking of sensitized Type 304 stainless steel in thiosulfate solution: I. Fate of the coupling current

This work explores the fate of the coupling current generated during stress corrosion cracking of sensitized Type 304 stainless steel (304SS) in thiosulfate solution at ambient temperature. The coupling current, which is generated inside the crack, may be consumed (as required by charge conservation) by cathodic reactions at two different locations: inside the crack (e.g., on the crack flanks) and on the external surfaces. Experiments were conducted using sensitized Type 304SS compact tension specimens C(T) modified to provide external surfaces isolated from the specimen itself, so that the coupling current flowing between the crack and the external cathode could be directly monitored using a zero-resistance ammeter. The results demonstrate that significant coupling current flows from the crack to the external surface, consistent with the differential aeration hypothesis for localized corrosion. Coupling current data from specimens heat treated at 650 °C for 4, 14, and 24 h show that 4 h of sensitizing heat treatment apparently does not induce significant sensitization, while 14 h of heat treatment is sufficient to produce a fully sensitized microstructure, equivalent to that obtained after 24 h of heating. Finally, the crack was determined to grow via discrete microfracture events at the crack front with a dimension of the order of the grain size. The observations are more consistent with crack growth being due to a hydrogen induced fracture mechanism rather than slip/dissolution.

Effects of prior cold work and sensitization heat treatment on chloride stress corrosion cracking in type 304 stainless steels

The effects of prior cold work (CW) and sensitization treatment on the stress corrosion cracking (SCC) behaviour of Type 304 stainless steels have been studied in chloride solutions using electrochemical tests and magnesium chloride tests with U-bend specimen. The results indicated that the SCC behaviour of the CW steel was essentially different from that of the solution-annealed steel. Intergranular stress corrosion cracking (IGSCC) of solution-annealed material changed into a mixed mode or dominant transgranular stress corrosion cracking (TGSCC) when the degree of CW was increased. The sensitization treatment enhanced IGSCC susceptibility by shortening failure time and accelerating crack initiation and propagation rates, but this effect was different for several other degrees of CW. These effects were revised taking into consideration the electrochemical and microstructural phenomena. The most dangerous degrees of deformation for different sensitization conditions for the development of IGSCC and TGSCC processes have been determined.

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr2N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.

Effects of thermal sensitization on radiation-induced segregation in type 304 stainless steel irradiated with He-ions

Type 304 stainless steels, solution-annealed and thermally sensitized at 923 K for 0.5 to 24 h, were He-ion-irradiated up to about 4 dpa at 723 K and radiation-induced segregation (RIS) at grain boundaries was measured by EDS analysis using a FEG-TEM. Ni, Si and P were enriched, and Cr was depleted at the grain boundaries by irradiation. However, although the irradiation dose was the same for the specimens, the RIS of the elements linearly increased with the logarithm of the thermally sensitizing time, except for Cr in the specimen thermally sensitized for 24 h. The enhancement of RIS was attributed to the radiation-induced point defects having large mobility in thermally sensitized stainless steels, because of an expected decrease in C near the grain boundaries and in the matrix after the sensitization heat treatments. It was clarified by the electrochemical potentiokinetic reactivation test (EPR) that the degree of sensitization increased with the progress of Cr depletion at grain boundaries. The Cr concentration at grain boundaries in the heavily sensitized specimen was not changed, and the width of the depleted area was slightly narrowed by the irradiation. This result could be explained that the diffusion of Cr due to initial Cr concentration gradient near grain boundaries exceeded the inverse Kirkendall effect.

Introduction into the electron beam welding of austenitic 321-type stainless steel

Abstract Electron beam welding has great potential in the welding of engineering materials, since very narrow and deep penetration is possible with this process. Austenitic stainless steels are prone to inter-granular corrosion when subjected to sensitizing heat treatments between 723 and 1073 K, due to the formation of a titanium-depleted zone adjacent to a titanium-rich zone. However, during the electron beam welding of this material, the heat affected zone extends only to a narrow region across the weld pool, thus resulting in a lesser degree of defects in the weld zone. The present study is conducted to investigate the mechanical and metallurgical properties of electron beam welded austenitic 321 stainless steel. Consequently, microhardness tests across the heat-affected zone and tensile and impact testing of the resulting welds were carried out, with SEM and micro-particle induced X-ray emission (μ-PIXE) being carried out to study the microstructures and element distribution in the weld zone. In addition, analysis of the first- and second-law of thermodynamics of the welding process was carried out. It was found that the heat-affected zone increased as the workpiece thickness increased and that dendritic transverse microcracks were present at the boundary of the molten and solid regions.

Effects of Cold Work, Sensitization Treatment, and the Combination on Corrosion Behavior of Stainless Steels in Nitric Acid

Abstract Corrosion behavior of two stainless steels (SS) was investigated in nitric acid (HNO3) for the effect of cold work (CW), sensitization heat treatment (Sens.), and a combination (CW + Sens.). The corrosion rate of the solution-treated type 304 SS (UNS S30400) with extra-low carbon (type 304ELC SS [UNS S30403]) increased with time and reached constant values after 1,000 h of immersion. However, constant corrosion rates were obtained for 25% Cr-20% Ni-Nb (type 310Nb SS [UNS S31040]) from the initial stage of immersion. CW mitigated corrosion of the solution-treated SS. The effect of CW was different on the two types of SS, with the sensitization heat-treated type 304ELC SS showing higher corrosion rates and type 310Nb SS lower corrosion rates by CW. Corrosion resistance of type 310Nb SS was superior to type 304ELC SS after all treatments. Chromium concentration of the sensitization-treated type 304ELC SS was lower in the grain-boundary region than of the solution-treated one, although no chromium ca...

Effects of Laser Surface Melting on the Pitting Resistance of Sensitized Nitrogen-Bearing Type 316L Stainless Steel

Laser surface melting of sensitized nitrogen-bearing type 316L austenitic stainless steel was carried out using a pulsed ruby laser. The sensitization heat treatment was carried out at 923 K for 50, 200, 1000, and 2500 h, and the sensitized microstructure was classified according to ASTM A 262 practice A. The degree of sensitization was assessed by the electrochemical potentiokinetic reactivation (EPR) test. The critical pitting potentials of as-sensitized as well as sensitized-laser melted specimens were determined by potentiodynamic anodic polarization method in a medium containing 0.5 M NaCl and 0.5 M H2SO4 at room temperature. Results indicated that upon laser melting the pitting resistance increased significantly. This increase was attributed to the elimination of the sensitized heterogeneous microstructure by laser melting. The microscopic examination of the pitted specimens showed only micropits that developed at the interfaces of oxide/sulfide inclusions of titanium and matrix.

Cryogenic fracture behaviors and polarization characteristics according to sensitizing heat treatment on structural material of the nuclear fusion reactor

The cryogenic fracture behaviors of austenitic stainless steel HN2 developed for nuclear fusion reactor were evaluated quantitatively by using the small punch(SP) test. The electrochemical polarization test was applied to study thermal aging degradation of HN2 steel. The X-ray diffraction(XRD) analysis was conducted to detect carbides and nitrides precipitated on the grain boundary of the heat treated HN2 steel. The mechanical properties of the HN2 steel significantly decreased with increasing time and temperature of heat treatment or with decreasing testing temperature. The integrated charge(Q) obtained from electrochemical polarization test showed a good correlation with the SP energy(ESP) obtained by means of SP tests. From the results observed in the x-ray diffraction and anodic polarization curve, it was known that the material the grain boundary. Combining SP test and electrochemical polarization test, it could be useful tools to non-destructively evaluate the cryogenic fracture behaviors and the aging degradation for cryogenic structural material.

98/00370 Power production and the price of electricity: an analysis of a phase-out of Swedish nuclear power

Stabilized austenitic steels are susceptible to intergranular stress corrosion cracking (IGSCC) under boiling water reactor (BWR) conditions. This important finding for the German nuclear power station industry arises from the detection of cracks during the last 3 years in reactor hot water pipes made from titanium-stabilized steel AISI 321 in six BWRs and in reactor core components made from the niobium-stabilized steel AISI 347 in one BWR. All the observed cracks had a common feature: they had their origin in the chromium carbide precipitates at the grain boundaries and in the associated chromium-depleted region near the grain boundary. These microstructural features in the heat-affected zones of the hot water pipe weldments were caused by the heat input during deposition of the root bead. The TiC partially dissolved in the region near the fusion line and the released carbon reacted to form chromium-rich M23C6. Regarding the cracks found in the core shroud and the core grid plates, it was shown that a sensitizing heat treatment of rings taken from the same heat of steel could give rise to a microstructure susceptible to IGSCC in the region of a weldment. High carbon contents coupled with low stabilization ratios led to sensitization. Residual stresses developed during welding provided the significant contributions to the tensile stress necessary for IGSCC. With regard to the service medium, the influence of the electrochemical corrosion potential (ECP) was recognized as a dominant factor, together with the conductivity. The corrosion potential was mainly determined by the radiolytic formation of H2O2; with increasing distance from the core, the H2O2 content decreased owing to catalytic decomposition. For the pipes the problem of IGSCC could be resolved by the use of optimized steel (lower carbon content with maximum allowable stabilization ratio), by production modifications (lowest possible heat input during welding with favourable residual stress development) and by improvements in water chemistry (allowing the very low limits specified for normal operation to be exceeded for only short periods). Hydrogen control is seen as an optimal measure, whose introduction in Germany BWR plants must be considered in terms of the benefits and disadvantages. The outage times required for the replacement were dependent on the technical conditions prevailing in the individual plants and were generally justifiable. However in one plant the outage time was 1 year and in another it was more than 2 years. The intended replacement of the core shroud components has been cancelled owing to the decision to decommission the reactor for commercial reasons not exclusively related to the core shroud damage.