A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.     

Hydrogen embrittlement susceptibility and permeability of two ultra-high strength steels

Slow displacement rate tensile tests were carried out in a saturated H₂S solution to investigate the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of two ultra-high strength steels (PH 13-8 Mo stainless steel and T-200 maraging steel). Hydrogen permeation properties were determined by an electrochemical permeation method. The results of permeation tests indicated that over-aged specimens showed a lower diffusivity/hydrogen flux and higher solubility than those solution-annealed. The great increase in reverted austenite (irreversible hydrogen traps) together with numerous precipitates at the expense of dislocations (reversible) in the over-aged specimen led to such a change in permeability. Ordinary tensile tests indicated that four tested specimens had roughly the same yield strength level. Hence, the hydrogen embrittlement susceptibility of the material could be related to their permeation properties. The uniform distribution of strong hydrogen traps in over-aged specimens instead of weak traps in the solution-annealed impeded the hydrogen transport toward the strained region, thus, the resistance to sulfide stress corrosion cracking was improved in over-aged specimens.     

The effects of cold rolling and sensitisation on hydrogen embrittlement of AISI 304L welds

The notched tensile strength (NTS) of 304L stainless steel welds subjected to cold rolling (20% thickness reduction) and sensitisation treatment (600 °C/10 h) was measured in a MgCl₂ (40 wt.%) solution at 80 °C. The NTS loss, which is used for comparing the susceptibility to hydrogen embrittlement (HE), was consistent with the extent of embrittled area on the fracture surface of various welds. The HE susceptibility of the weld decreased after cold working but increased with the additional sensitisation treatment of the cold-rolled welds. Besides, cracks tended to propagate along the skeletal boundaries for all welds tested in the MgCl₂ solution.     

Effect of hydrogen environment on the notched tensile properties of T-250 maraging steel annealed by laser treatment

In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H₂S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.     

Notched tensile tests of cold-rolled 304L stainless steel in 40 wt.% 80 °C MgCl2 solution

The effects of cold-rolling (20% thickness reduction) and sensitization treatment (600 °C/10 h) on the microstructure, tensile properties and susceptibility to stress corrosion cracking of 304 stainless steel in 80 °C MgCl₂ (40 wt.%) solution were investigated. The increase in hydrogen traps, which retarded hydrogen diffusion to the strained region, accounted for the low loss in notched tensile strength (NTS) of such a cold-rolled specimen, as compared to the solution-treated specimen in the corrosive environment. By contrast, the high NTS loss of sensitized specimens in MgCl₂ solution was attributed mainly to the formation of stress-induced martensite near grain boundary regions.     

Embrittlement of T-200 maraging steel in a hydrogen sulfide solution

Slow displacement rate tensile tests were carried out to investigate the effect of hydrogen embrittlement (HE) on notched tensile strength (NTS) and fracture characteristics of aged T-200 maraging steel. Hydrogen diffusivity, permeation flux and apparent hydrogen solubility were determined by an electrochemical permeation method, and correlated with the HE susceptibility and microstructures of the specimens. The results indicated that all aged specimens were susceptible to HE in the saturated H₂S solution, to different degrees. The susceptibility in the decreasing order of severity was observed to be under-aged, peak-aged, and over-aged conditions. The main trend was that the specimen with the highest diffusivity and permeation flux of hydrogen had the greatest NTS loss. Reverted austenite, if present in the microstructure, acted as irreversible traps for hydrogen and hence, improved the HE resistance. At similar strength and hydrogen solubility level, the more reverted austenite the less susceptibility to HE of specimens was resulted. The detailed microstructures of distinct specimens and their performances in hydrogen-containing environments are discussed.     

The effect of grain size and aging on hydrogen embrittlement of a maraging steel

Notched tensile tests were conducted under a slow displacement rate to evaluate the influences of grain size and aging on hydrogen embrittlement (HE) of T-200 maraging steel. In addition, an electrochemical permeation method was employed to measure the effective diffusivity (D_eff) and apparent solubility (C_app) for hydrogen of various heat-treated specimens. The results indicated that the aged (482 °C/4 h) specimens comprised of numerous precipitates led to a raised C_app and a decreased hydrogen diffusivity as compared to those of the solution-treated ones. The solution-treated specimens were resistant to gaseous HE, whereas aged specimens were susceptible to it, implying the strength level was the controlling factor to affect the HE susceptibility of the specimens. Nevertheless, all specimens suffered from sulfide stress corrosion cracking (SSCC) severely but to different degrees. The aged specimens were more likely to form intergranular (IG) fractures in H₂S but quasi-cleavage (QC) in H₂. For the solution-treated specimens, a fine-grained structure was susceptible to HE in H₂S and revealed mainly QC that differed from the IG fracture of the coarse-grained one. The fracture mode of the specimens could also be related to the transport path and / or the supply of hydrogen to the plastic zone of notched specimens in hydrogen-containing environments.     

Notch tensile properties of laser-surface-annealed 17-4 PH stainless steel in hydrogen-related environments

Slow displacement rate tensile tests were performed to determine the notched tensile strength (NTS) of 17-4 PH stainless steel with various microstructures in hydrogen-related environments. Solution-annealed (SA), peak-aged (H900), over-aged (H1025), and laser-annealed (LA) specimens were included in the study. Based on the results of NTS in air, the NTS loss in both gaseous hydrogen and H₂S-saturated solution was used to access the detrimental effects of hydrogen in 17-4 PH steel subjected to different treatments. Electrochemical permeation tests were also employed to determine the hydrogen permeation characteristics of the 17-4 PH steel plate with various microstructures. The result indicates that all the specimens have low NTS loss in gaseous hydrogen but significantly suffer from sulfide stress corrosion cracking (SCC), especially for the soft SA specimen. It was deduced that high hydrogen diffusivity and less trapped hydrogen atoms in the SA matrix provided rapid transport of massive hydrogen atoms into highly stressed region, and deteriorated the NTS tested in the saturated sulfide solution. On the other hand, H1025 specimen consists of the blocky austenite together with Cu-rich precipitates uniformly distributed in the grain interior; dense and coarse precipitates are also observed along prior austenite grain boundaries. Hydrogen atoms tend to be trapped along grain boundaries, and lead to the formation of intergranular fracture for H1025 specimen tested in the H₂S solution. Fine and homogeneously distributed precipitates in the H900 matrix result in uniformly trapping of hydrogen atoms, so it behaves superior properties than other specimens. The decohesion of precipitate/matrix interfaces induces quasi-cleavage fracture of the H900 specimen tested in H₂S solution. Finally, the application of laser-annealing treatment on the H900 specimen cannot improve its resistance to sulfide SCC, because the laser-annealed zone is susceptible to hydrogen embrittlement in the H₂S solution.     

Hydrogen embrittlement of a Ti-strengthened 250 grade maraging steel

Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 °C (800 °F) underaged specimens are very sensitive to strain rate; the 482 °C (900 °F) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 °C (1100 °F) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni₃Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.     

Effects of electrical discharge surface modification of superalloy Haynes 230 with aluminum and molybdenum on oxidation behavior

The effects of the electrical discharge alloying (EDA) process on improving the high temperature oxidation resistance of the Ni-based superalloy Haynes 230 have been investigated. The 85 at.% Al and 15 at.% Mo composite electrode provided the surface alloying materials. An Al-rich layer is produced on the surface of the EDA specimen alloyed with positive electrode polarity, whereas, many discontinuous piled layers are attached to the surface of the EDA superalloy when negative electrode polarity is selected. The oxidation resistance of the specimen alloyed with positive electrode polarity is better than that of the unalloyed superalloy, and the effective temperature of oxidation resistance of the alloyed layer can be achieved to 1100 °C. Conversely, the oxidation resistance of the other EDA specimen alloyed with negative electrode polarity is even worse than that of the unalloyed superalloy.     

Early high temperature oxidation behaviour of Ti(C,N)-based cermets in air

Isothermal oxidation behaviour of two Ti(C,N)-based cermets (TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32-x)Ni-xCr, x = 0 and 6.4 wt%) was investigated in air at 800-1100 °C up to 2 h. Mass gains exhibited neither linear nor parabolic law during isothermal oxidation. The oxide scales formed at 800-1100 °C were multi-layered, consisting of NiO outerlayer, NiTiO₃ interlayer and TiO₂-based innerlayer. The internal oxidation zones formed at 1000-1100 °C consisted of Ti-based, Ni-based and Mo-based complex oxides. Cermet with 6.4 wt% Cr exhibited superior oxidation resistance, due to the presences of Cr_0.17Mo_0.83O₂ in TiO₂-based innerlayer of the oxide scale and Cr-rich Ti-based complex oxide in the internal oxidation zone.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

Effect of sintering temperatures on corrosion and wear properties of sol-gel alumina coatings on surface pre-treated mild steel

Sol-gel alumina coatings were developed on the surface pre-treated (zinc-phosphated) mild steel substrate and subsequently sintered at 300 °C, 400 °C and 500 °C. The alumina sol was synthesised using aluminium iso-propoxide as a precursor material. FTIR of the boehmite (AlOOH) gel sintered at above-mentioned temperatures was employed to identify the presence of various functional groups. The microstructural features and the phase analysis of the sol-gel coated specimens were carried out using SEM and XRD respectively. The corrosion resistance of the sol-gel alumina coatings was evaluated by electrochemical measurement in 3.5% NaCl solution at room temperature. The abrasive wear behaviour of the sol-gel coated specimens was measured in two body (high stress) conditions. The experimental results revealed that the sol-gel coated specimen sintered at 400 °C has superior wear and corrosion resistance properties as compared to the sol-gel coated specimen sintered at 300 °C. However, the sol-gel coated specimen sintered at 500 °C has exhibited a very poor corrosion and wear resistance properties. Poor performance of the sol-gel coatings sintered at 500 °C could be explained to be due to (i) the presence of numerous cracks (ii) absence of organic groups in the coating.     

Effect of thermal cycles on the corrosion and mechanical properties of UNS S31803 duplex stainless steel

The microstructure and properties of UNS S31803 duplex stainless steel had been investigated following isothermally aging in a temperature range of 450–1000 °C for 10 min, which simulates a fairly long time relative to a weld thermal cycle. The results showed that specimens aged between 600 and 950 °C are subjected to precipitation of secondary phases, such as Cr₂N, σ and χ. And the most significant reduction in pitting corrosion resistance and impact energy occurs at 850 °C. Moreover, at aging temperatures below 600 °C, austenite is the preferred site for pitting initiation, which changes to precipitates modified ferrite phase at higher temperatures above 600 °C.     

Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

Comparative investigation of Al- and Cr-doped TiSiCN coatings

The aim of this work was a comparative investigation of the structure and properties of Al- and Cr-doped TiSiCN coatings deposited by magnetron sputtering of composite TiAlSiCN and TiCrSiCN targets produced by self-propagating high-temperature synthesis method. Based on X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy data, the Al- and Cr-doped TiSiCN coatings possessed nanocomposite structures (Ti,Al)(C,N)/a-(Si,C) and (Ti,Cr)(C,N)/a-SiC_xN_y/a-C with cubic crystallites embedded in an amorphous matrix. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum at 1000, 1100, 1200, and 1300 °C or in air at 1000 °C for 1 h. The results obtained show that the hardness of the Al-doped TiSiCN coatings increased from 41 to 46 GPa, reaching maximum at 1000 °C, and then slightly decreased to 38 GPa at 1300 °C. The Cr-doped TiSiCN coatings demonstrated high thermal stability up to 1100 °C with hardness above 34 GPa. Although both Al- and Cr-doped TiSiCN coatings possessed improved oxidation resistance up to 1000 °C, the TiAlSiCN coatings were more oxidation resistant than their TiCrSiCN counterparts. The TiCrSiCN coatings showed better tribological characteristics both at 25 and 700 °C and superior cutting performance compared with the TiAlSiCN coatings.     

Tensile properties and fracture toughness of 18Ni (250 grade) maraging steel weldments

Abstract

Gas tungsten arc welding was performed on 18Ni (250 grade) maraging steel sheet using two different filler wires: one of the same composition as the base material and the other containing more cobalt and aluminium and less molybdenum and titanium. Weld specimens were then aged under four different sets of conditions. After metallographic characterisation, mechanical properties including hardness, tensile strength and ductility, and fracture toughness were evaluated. Results showed that use of the matching filler material led to lower strength but higher ductility than in the other case; this was attributed to the presence of reverted austenite in the former (caused by segregation, especially of molybdenum) at the fusion zone substructure boundaries. In both types of weld metal, a re-solution treatment followed by aging at 480°C resulted in optimum tensile properties. Fracture toughness of the aged weldments was in general close to that of the parent material aged at 480°C; some deterioration occurred only when welds with pronounced segregation were made at high temperature.     

Phase composition of high-chromium white cast iron

Conclusion The change in hardness after annealing of cast iron and after normalization at temperatures up to 1100° is due to the structural condition of phase, while the reduction of the hardness when the normalization temperature is raised from 1100 to 1200° is due to the appearance of austenite. The presence of a substantial amount of austenite also explains the low hardness of the cast iron in the cast condition as compared with the hardness after normalization at 1000–1100°. The high hardness of the cast iron normalized at 1000–1100° may be due partially to the precipitation of secondary carbides.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 59–61, December, 1977.     

Effect of nitrogen addition on the stress corrosion cracking behavior of 904 L stainless steel welds in 288 °C deaerated water

This paper concerns with the effect of nitrogen addition to 904 L stainless steel (SS) welds on their stress corrosion cracking (SCC) behavior in high temperature (288 °C) and high pressure (1050 psi) water of high oxygen content (100 ppb) and high conductivity (2.5 μS/cm). For this study, 316 L SS base plate TIG welded with 904 L SS filler wire and with nitrogen contents of 0.027, 0.058 and 0.095 wt.% were used. Flat pin-loaded tensile specimens were fabricated from transverse welds, with the weld in the gauge length. Slow strain rate tests (SSRT) were carried out at a strain rate of 2.2 × 10⁻⁶ s⁻¹. The study shows that the samples, when tested in air, failed at the weld fusion zone for 0.027 and 0.058 wt.% N and at the base metal for 0.095 wt.% N. In the environment, the samples failed in the base metal except the one with least nitrogen content (0.027 wt.%). With nitrogen addition, as the failure location shifted to the base alloy, the weld seemed to acquire SCC resistance and became even more resistant than the base alloy.     

Evaluation of susceptibility of high strength steels to delayed fracture by using cyclic corrosion test and slow strain rate test

To evaluate susceptibilities of high strength steels to delayed fracture, slow strain rate tests (SSRT) of notched bar specimens of AISI 4135 with tensile strengths of 1300 and 1500 MPa and boron-bearing steel with 1300 MPa have been performed after cyclic corrosion test (CCT). During SSRT the humidity around the specimen was kept high to keep absorbed diffusible hydrogen. The fracture stresses of AISI 4135 steels decreased with increment of diffusible hydrogen content which increased with CCT cycles. Their delayed fracture susceptibilities could be successfully evaluated in consideration of both influence of hydrogen content on mechanical property and hydrogen entry.