ASTM A262 Research Articles

Electrochemical reactivation methods applied to PM austenitic stainless steels sintered in nitrogen–hydrogen atmosphere

Exposure methods (ASTM A262 Practice A and Modified Strauss Test) and electrochemical reactivation methods such as EPR and EPRDL was applied to PM 304L and PM 316L stainless and comparisons with wrought stainless steels were made. Sintering in vacuum and nitrogen–hydrogen atmosphere was chosen. The latter was the focus of the work and the former was used on comparative basis. It was seen how nitrogen in PM stainless steels plays a relevant role both when is dissolved and when is in intermetallic precipitates. A set of heat treatments were applied: annealing and sensitizing at 375, 675 and 875 °C. The results indicate that the electrochemical reactivation methods are suitable to evaluate the degree of sensitization of PM austenitic stainless steels.

Quantitative Evaluation of Aged AISI 316L Stainless Steel Sensitization to Intergranular Corrosion: Comparison Between Microstructural Electrochemical and Analytical Methods

The evaluation of the degree of sensitization (DOS) to intergranular corrosion (IGC) of a commercial AISI 316L austenitic stainless steel aged at temperatures ranging from 550 °C to 800 °C during 100 to 80,000 hours was carried out using three different assessment methods. (1) The microstructural method coupled with the Strauss standard test (ASTM A262). This method establishes the kinetics of the precipitation phenomenon under different aging conditions, by transmission electronic microscope (TEM) examination of thin foils and electron diffraction. The subsequent chromium-depleted zones are characterized by X-ray microanalysis using scanning transmission electronic microscope (STEM). The superimposition of microstructural time-temperature-precipitation (TTP) and ASTM A262 time-temperature-sensitization (TTS) diagrams provides the relationship between aged microstructure and IGC. Moreover, by considering the chromium-depleted zone characteristics, sensitization and desensitization criteria could be established. (2) The electrochemical method involving the double loop–electrochemical potentiokinetic reactivation (DL-EPR) test. The operating conditions of this test were initially optimized using the experimental design method on the bases of the reliability, the selectivity, and the reproducibility of test responses for both annealed and sensitized steels. The TTS diagram of the AISI 316L stainless steel was established using this method. This diagram offers a quantitative assessment of the DOS and a possibility to appreciate the time-temperature equivalence of the IGC sensitization and desensitization. (3) The analytical method based on the chromium diffusion models. Using the IGC sensitization and desensitization criteria established by the microstructural method, numerical solving of the chromium diffusion equations leads to a calculated AISI 316L TTS diagram. Comparison of these three methods gives a clear advantage to the nondestructive DL-EPR test when it is used with its optimized operating conditions. This quantitative method is simple to perform; it is fast, reliable, economical, and presents the best ability to detect the lowest DOS to IGC. For these reasons, this method can be considered as a serious candidate for IGC checking of stainless steel components of industrial plants.

Sensitization behaviour of modified 316N and 316L stainless steel weld metals after complex annealing and stress relieving cycles

Sensitization behaviour of austenitic stainless steel weld metals prepared using indigenously developed modified 316N (C=0.05%; N=0.12%) and 316L (C=0.02%; N=0.07%) electrodes was studied. Detailed optical and scanning electron microscopic examination was carried out to understand the microstructural changes occurring in the weld metal during isothermal exposure at various temperatures ranging from 500°C to 850°C (773–1123K). Based on these studies the mechanism of sensitization in the austenite–ferrite weld metal has been explained. Time–temperature–sensitization (TTS) diagrams were established using ASTM A262 Practice E test. From the TTS diagrams, critical cooling rate (CCR) above which there is no risk of sensitization was calculated for both materials. The heating/cooling rates to be followed for avoiding sensitization during heat treatment cycles consisting of solution-annealing and stress-relieving in fabrication of welded components of AISI 316LN stainless steel (SS) were estimated taking into account the soaking time and the number of times the component undergoes thermal excursions in the sensitization regime. The results were validated by performing controlled heating and cooling heat treatment trials on welded specimens.

Use of eddy current testing method in detection and evaluation of sensitisation and intergranular corrosion in austenitic stainless steels

This paper deals with the applicability of eddy current testing (ECT) technique to assess and quantify sensitisation and intergranular corrosion (IGC) in thermally aged AISI type 316 stainless steel. Chemical and electrochemical tests specified by ASTM A262 Practices A and E (Strauss test) and G108, and ECT, were used to quantify degree of sensitisation (DOS). The DOS was categorised based on severity of the crack after bend test in increasing order as unaffected, fissured, cracked, and broken. The eddy current (EC) amplitude increased with increasing DOS in both the as-aged and Strauss tested conditions. The EC amplitude in as-aged condition was smaller than in Strauss tested condition. Empirical relationships were established between ECT amplitude and other DOS parameters.

Intergranular corrosion damage evaluation through laser scattering technique

AISI type 316 austenitic stainless steel in sensitized condition was subjected to ASTM A262 practice A test (electrolytic etching in 10% ammonium persulphate at 1 A/cm 2) for various durations from 10 s to 90 s. The different degrees of intergranular corrosion attack produced on these tested specimens were evaluated by laser scattering technique using a 1 mW He–Ne laser. The scattering intensity pattern of the laser beam incident on the specimen was acquired using a CCD camera and transferred to a computer for further analysis. The specular intensity of the scattering pattern and its full width at half maximum (FWHM) was calculated. The results indicated that the specular intensity decreased and the FWHM increased as the degree of IGC attack increased. A good correlation was found between the extent of IGC with the laser scattering parameters.

Sensitization evaluation of the austenitic stainless steel AISI 304L, 316L, 321 and 347

This work presents a systematic investigation of the influence of time and temperature in the sensitization of stainless steel AISI 304L, AISI 316L, AISI 321 and AISI 347 pipes used in petroleum refining plants. The sensitization was assessed by Scanning Electron Microscopy (SEM) according to ASTM A-262 and by the Double Loop Electrochemical Potentiokinetic Reactivation test (DLEPR). The results showed that all steels did not present sensitization at operating temperature (380°C) in the desulfurizers process, but the temperature of 500°C was critical to the appearing of sensitization for the both low carbon stainless steels and AISI 321 SS, while for the AISI 347 the critical temperature was 550°C. The stabilized steels confirmed to be more resistant to sensitization than the low carbon stainless steels, and niobium showed to be more efficient stabilizing agent than titanium.

EBSD investigation of intergranular corrosion attack on low interstitial stainless steel

AbstractGrain boundary effect on corrosion behaviour of low interstitial AISI 316LN austenitic stainless steel after homogenization and heat treatment at 700 and 800°C was investigated by means of Electron Backscattered Diffraction (EBSD) technique. Rapid oxalic acid etch test (ASTM A262‐ practice A) was used to determine steel susceptibility to intergranular corrosion. An attempt to quantify oxalic acid etch test results was also made.Beneficial effect of low carbon and nitrogen content on grain boundary precipitation and corrosion was observed. No changes neither in grain orientations nor in grain boundary types between sensitised and non‐sensitised steel states using orientation measurements were recorded. Twin boundaries were found in all samples with highest amount and only small occurrence changes. CSL categorizations showed only little increase of special boundary fractions with increasing time and temperature of heat treatment. Irrespective to annealing conditions a continued network of random boundaries was retained. Additionally, based on misorientation measurement of 50 grooved grain boundaries, a tendency for preferred attack of high angle boundaries (30–55°) was noted.

Evaluación del grado de sensibilización en el acero inoxidable AISI

Austenitic stainless steel, when heat-treated at 550-850 °C, became susceptible to intergranular corrosion in acids. This phenomenon, know as sensitization, it is result from the precipitation of chrome carbides in the grain boundary, making these areas less resistant to corrosion. Two different electrochemical reactivation tests are compared with a destructive test and related to the classification of its respective microstructures. It was established a quantitative methodology to evaluate the degree of sensitization in AISI 304 and also to compare the correspondence of the results with the data of the automatic and portable EPR device for non-destructive field measurement of the degree of sensitization. The used electrochemical techniques were the EPR (Electrochemical Potentiokinetic Reactivation) or single loop test and the PRP (Pasivation Reactivation Potentiokinetic) or double loop test. The destructive test used was boiling, 120 h ferric sulfate-50 % sulfuric acid, according to the standard practices ASTM A-262 practices B. The classifications of each structures were according to the standard practices ASTM A-262 practices B.

Intergranular stress corrosion cracking behavior of types 308 and 316 stainless steel weld metals in a simulated boiling water reactor environment

The sensitization behavior of types 308 and 316 stainless steel weld metals as internal overlays for reactor pressure vessels (RPVs) was studied with respect to the effects of postweld heat treatment (PWHT) at about 600 °C during RPV fabrication and low-temperature aging during operation. For the study, a criterion for the rate of intergranular corrosion (IGC) for detecting the susceptibility to intergranular stress corrosion cracking (IGSCC) in high-temperature oxygenated pure water was established by quantitatively evaluating the results from a modified ASTM A262E test. A criterion for expecting satisfactory resistance to IGSCC was found to be an IGC rate of about 1 µm/h. Type 308 weld metal can be sensitized as indicated by an IGC rate >1 µm/h, and can be healed, as indicated by an IGC rate <1 µm/h, depending on the length of PWHT. However, this healed weld metal can be resensitized by exposing it to 500 °C×24 h; in this condition, it shows a relatively high susceptibility to IGSCC. On the contrary, type 316 weld metal was almost immune to sensitization under the same heat treatments. By transmission electron microscopy (TEM), its excellent resistance was attributable to carbon fixation by molybdenum carbide precipitation within ferrite phases during PWHT. Reheat embrittlement of type 316 weld metal was also examined.

Assessment of intergranular corrosion in AISI Type 316L stainless steel weldments

This paper deals with the applicability of various techniques for the assessment and quantification of sensitisation in AISI Type 316L welds. Welded joints of AISI Type 316L stainless steel were aged at 973 K for periods of up to 200 h. The base and weld metal components of the aged joints were then assessed for susceptibility to sensitisation and intergranular corrosion (IGC) by using various tests specified by ASTM A262, Practices A and E, and ASTM G108 (the electrochemical potentiokinetic reactivation (EPR) test). The possibility of using eddy current testing (ECT) to detect sensitisation and IGC was also assessed. The use of ASTM A262 Practice A and E tests indicated sensitisation in base metal aged for 20 h and above. Aged weld metals showed no failure in these tests. Tensile tests on the weld joints before and after exposure to Cu–CuSO4-H2SO4 solution did not indicate any differences in the tensile properties. Double loop EPR tests indicated a significant increase in the ratios of charge and peak current densities on reactivation to activation after aging the welded joint for 20 h and above. However, weld metal showed no change in the ratios of the above two parameters. Following doubts about the suitability of the EPR test for on line corrosion monitoring, the ECT technique was investigated in order to assess its suitability for the detection and quantification of sensitisation. The ratio of eddy current amplitudes after and before exposure to Cu–CuSO4-H2SO4 solution was used as an assessment criterion. A significant increase in this ratio was observed on aging the base metal for more than 20 h. No significant change was observed in the ratios of eddy current amplitudes for weld metal. The ECT results correlated very well with the findings of the EPR and ASTM Practice E tests. This indicates that ECT holds promise as an on line monitoring tool for sensitisation and IGC.

Sensitization behavior of type 308 stainless steel weld metals after postweld heat treatment and low-temperature aging and its relation to microstructure

Type 308 weld metal overlaid on a low-alloy steel by a submerged arc welding (SAW) process using wide strip electrodes can become sensitized by not only postweld heat treatments (PWHTs) at around 600 °C but also low-temperature aging (LTA) at 500 °C for 24 hours after PWHT. A slow strain-rate test (SSRT) in oxygenated, high-temperature pure water revealed that type 308 SAW weld metals after PWHT were susceptible to intergranular stress corrosion cracking (IGSCC) when sensitized such that the intergranular corrosion (IGC) rate in the modified ASTM A262E test exceeds 1 µm/h. The effect of microstructure (ferrite phase distribution) on the sensitization behavior was evaluated using a microstructural parameter NLβ−γ. Though mostly useful, the average NLβ−γ was not always accurate in distinguishing IGC-susceptible, SAW overlay weld metals. However, local NLβ−γ, newly introduced to deal with a rather large fluctuation in ferrite distribution within SAW overlay weld metals, was partly able to explain the IGC susceptibility of a theoretically resistant heat with a high average NLβ−γ. The beneficial effect of surface remelting by plasma arc on the IGC resistance was described.

Improvement of Intergranular Corrosion Resistance of Type 316 Stainless Steel by Laser Surface Melting

Laser surface melting (LSM) on sensitised austenitic stainless steel with various degrees of cold work (5, 10, 15, 20, and 25%) was carried out by using a continuous wave CO2 laser with a maximum power 450 W. The stainless steel was sensitised at 898 K for 10, 20, and 50 h durations. The ASTM A262 Practice A test was used to examine the microstructures of the specimens before and after LSM, while the single loop electrochemical potentiokinetic reactivation (SL-EPR) technique was used to quantify the degree of sensitisation (DOS). A trend is noted between the level of cold work in the stainless steel and the degree of desensitisation by LSM and it is inferred that laser surface melting can be effectively used as a method to improve the intergranular corrosion resistance of austenitic stainless steel.

Carbon and Nitrogen Effects on Sensitization Resistance of Type 347 Stainless Steels

Abstract The sensitization resistance of the stabilized Type 347 (UNS S34700) stainless steels (SS) having different carbon and nitrogen contents was evaluated using the double-loop electrochemical potentiokinetic reactivation (DL-EPR) and ASTM A262, practice E methods. Samples were heat-treated to simulate the coarse grain region of the heat-affected zone (HAZ) during welding and subsequently aged at 948 K for different times. For comparison, DL-EPR tests also were conducted for the unstabilized steels, Type 316 (UNS S31600) and Type 304 (UNS S30400). For Type 347 SS, a criterion for sensitization in accordance with the ASTM A262, practice E test corresponded to a condition of > 10% reactivation ratio (peak reactivation current [Ir] /peak activation current [Ia]) from DL-EPR test. The carbon and nitrogen effects on the sensitization resistance of the Type 347 SS were explained in view of the observed microstructures. By introducing a resistance parameter based on the chemical composition, sensitization r...

Effect of Prior Cold Work on Intergranular and Transgranular Corrosion in Type 304 Stainless Steels: Quantitative Discrimination by Image Analysis

Stainless steel (AISI 304 [UNS S30400]) was evaluated as a function of prior cold work and several thermal sensitization treatments. Traditional experimental techniques such as ASTM A262, practice A; modified Strauss test; electrochemical potentiokinetic reactivation (EPR); and electrochemical potentiokinetic reactivation double-loop (EPRDL) were used. Microstructural studies also were conducted. Analysis of the results showed the presence of transgranular attack (TGA) and intergranular attack (IGA) and a transition phenomenon between them. The IGA and TGA contributions to this transition phenomenon were analyzed and quantified by a new procedure based on quantitative metallography performed by image analysis. Using this new methodology, it was possible to determine the most dangerous degrees of deformation for the development of intergranular corrosion (IGC), transgranular corrosion (TGC), intergranular stress corrosion cracking (IGSCC), and transgranular stress corrosion cracking (TGSCC) for different sensitization conditions.

Desensitization of type 316 stainless steel by laser surface melting

Laser surface melting (LSM) on sensitized austenitic stainless steel of cold‐work 25 per cent was carried out by using a continuous wave CO2 laser of power 5kW and traverse speed 20mm/sec of the beam. The stainless steel was sensitized at 898K for 10, 20 and 50hr durations. ASTM A262 Practice A test was used to examine the microstructures of the specimens before and after LSM while single loop electrochemical potentiokinetic reactivation (SL‐EPR) technique was used to quantify the degree of sensitization (DOS). A trend is noted between the cold‐work of stainless steel and the degree of desensitization by LSM.

Nitrogen Bearing Austenitic Stainless Steels for Surgical Implants

Nitrogen addition promotes substantial improvements on general and localized corrosion performance of stainless steels. In recent times high Nitrogen (up to 0.6 wt%) and Mn bearing super austenitic stainless steel has been studied for medical applications due to its low Ni content, the so called body friendly alloys. 18%Cr, 0.4%N and 15%Mn stainless steels were cast either from electrolytic or commercial master alloys in induction furnace, forged, solubilized at 1423K for 3 hours and water quenched. Delta ferrite and carbide precipitate free structures were observed. Potentiodynamic corrosion tests of all alloys in 0.5M sulfuric acid solution presented passive current density as low as those observed for AISI 316L stainless steel. Intergranular corrosion susceptibility testing, according to ASTM A262 - Practice A, were performed and samples made of electrolytic raw material presented step structures with no ditches at grain boundaries, pointing out no sensitization. Mechanical testing presented high yield and tensile strengths with 60% elongation. These properties can be attributed to the Nitrogen action on stacking fault energy of austenitic structure of the alloy. These results fulfill orthopedic implant materials application requirements. In vivo biological compatibility tests for two months showed no adverse reactions, no necrosis and no evidence of corrosion products on implanted materials.

Laser surface melting for improving intergranular corrosion resistance of cold‐worked and sensitised type 316 stainless steel

This study used laser melting techniques to improve the intergranular corrosion resistance of cold worked and sensitised stainless steel surfaces. Type 316 stainless steel specimens, cold worked to 5 per cent, 10 per cent and 20 per cent reductions in thickness values, were sensitised at 923K for 25 hours. These specimens were laser‐surface‐melted by using a 300W Nd:YAG pulsed laser, and tested according to ASTM A262 practice A and practice E tests. The results of the practice A test showed that a cellular‐dendritic structure was present in the laser‐melted region in contrast to a typical ditch microstructure observed for sensitised unmelted specimens and a ditch structure was not present in the melt‐affected zone (MAZ). The hardness measurements across the melted, MAZ, and unmelted zones showed significant variations in their values. The results of the practice E tests showed no intergranular cracks for laser‐melted specimens while the unmelted specimens (5 and 10 per cent cold working) failed the test through significant cracking. The improvement in IGC resistance is attributed to the dissolution of M23C6 carbides and the homogenisation of chromium‐depleted regions.

Corrosion resistance of sintered austenitic stainless steels in boiling nitric acid

A comparative study of the corrosion resistance in boiling nitric acid has been carried out for P/M and cast AISI 304L and AISI 316L stainless steels. The influence of sensitization and acid concentration on the corrosion rate have been determined. The results are discussed with reference to ASTM A262 (Huey test). Scanning electron microscopy was used to study the microstructural characteristics and the corrosion products. It is shown that the corrosion phenomenon in P/M steels is almost entirely due to pore induced crevices; a possible corrosion mechanism for the materials is proposed.

Influence of thermal aging on the intergranular corrosion resistance of types 304LN and 316LN stainless steels

Intergranular corrosion (IGC) resistance of types 304LN and 316LN stainless steels (SS) thermally aged at 823, 873, and 923 K for various durations was assessed by ASTM A262 practice A test (electrolytic etch test) and electrochemical potentiodynamic reactivation (EPR) test. The results indicated that the type 316LN SS has significantly improved IGC resistance compared to 304LN SS. Based on the results of these tests, time-temperature-sensitization (TTS) diagrams were developed for both alloys. The secondary precipitates formed during thermal aging treatments were electrochemically extracted and analyzed by X-ray diffraction (XRD) to determine the types of precipitates formed during the aging treatments. The results indicated that the precipitates were mostly of M23C6 carbides.

High Nitrogen Steels. Relationship between Pitting and Intergranular Corrosion of Nitrogen-bearing Austenitic Stainless Steels.

Nitrogen-bearing types 304 and 316 stainless steels thermally aged at 823, 873 and 923 K for various durations were investigated to understand the relation between the sensitized microstructure (intergranular corrosion) and pitting corrosion resistance. The sensitized microstructure was assessed as per ASTM A262 practice A (electrolytic etch test) and electrochemical potentiokinetic reactivation (EPR) tests; and pitting corrosion resistance was evaluated in an acidic chloride medium by potentiodynamic anodic polarization method. The results indicated that the pitting resistance decreased as the degree of sesitization increased, and this was attributed to the formation of a heterogeneous microstructure consisting of Cr-rich M23C6 carbides, Cr-depleted regions and the carbide/matrix interfaces resulting from the sensitizing treatment. Pitting attack was found at triple points, grain boundaries and precipitate/matrix interfaces. Time-temperature-sensitization-pitting diagrams were developed for both the alloys based on the results, interrelating the sensitized microstructure and the pitting attack.