Environmental effects in iron aluminides

Iron aluminides based on the stoichiometric compositions of Fe₃Al and FeAl exhibit poor room temperature ductilities due to hydrogen embrittlement (HE). The presence of surface passive films reduces HE. The reduction is due to the lower rate of hydrogen liberation on the surface of iron aluminides with a passive layer. Theoretical and experimental verification for this idea are provided. The effect of addition of passivity-inducing elements Ti, Zr, V, Nb, Ta, Cr, Mo, W, Si and Ni to Fe₃Al on the thermomechanical and electrochemical behaviour has been outlined. The Cr- and Ti-alloyed intermetallics exhibited significant room temperature ductilities. Microstructural studies of the alloyed intermetallics revealed that when the addition of passivity-inducing element results in the precipitation of brittle phases with Fe and Al, they crack during the processing operation. The addition of oxygen-active elements on the embrittlement behaviour is also discussed. The effect of these additions on the potentiodynamic polarization behaviour and high temperature oxidation behaviour is also briefly addressed. Methods to minimize HE by the addition of elements that irreversibly trap hydrogen and that prevent recrystallization have also been discussed.     

Hydrogen transport and environmental embrittlement effects in iron aluminides

Environmental embrittlement types of six iron aluminides have been systematically evaluated using electrochemical hydrogen permeation measurement, hydrogen microprint technique and tensile test in this study. The results of hydrogen permeation and microprint technique show that three - disordered solid solution structure of binary and ternary iron aluminides (Fe-10Al, Fe-18Al, and Fe-18Al-5Cr) have higher effective diffusivity and permeation rate than B2 ordered structure of binary and ternary iron aluminides (Fe-28Al, Fe-28Al-5Cr, and Fe-40Al). The tensile test of six iron aluminides in air, vacuum treatment and hydrogen precharged were analyzed and concluded. Three - disordered solid solution structure of iron aluminides suffer in hydrogen environment which are quite different embrittlement mechanisms from those B2 ordered structure of iron aluminides with serious tensile loss in air by moisture induced embrittlement.     

Low Temperature Environmental Embrittlement in Ordered Intermetallics

A comprehensive review of low temperature environmental embrittlement in intermetallics is pres-ented. Moisture and hydrogen are shown to severely embrittle many intermetallics, including iron,nickel and titanium aluminides. The roles of composition, microstructure and external test variablesare emphasized. Several methods to reduce or avoid embrittlement are described.     

Fe3Al-Fe3AlC intermetallics for high temperature applications: An assessment

The advantage of carbon containing dual-phase iron aluminides is that they exhibit relative improvement in high temperature strength as well as reduction in probability of hydrogen embrittlement (HE) over the single-phase iron aluminides, which were the main impediment for the commercialisation of these materials. Owing to its practical and scientific importance a study on the dual-phase Fe₃Al-Fe₃AlC intermetallics is presented, which deals with the current status of research and developments, understanding of physical metallurgy, HE, oxidation resistance and thermal stability; and its limitation and future challenges.     

Deciphering the potentiodynamic polarization curves of iron aluminides Fe3Al and Fe3Al + Cr

The potentiodynamic polarization curves of Fe₃Al and Fe₃Al + Cr intermetallics obtained in aerated pH 4 H₂SO₄ acidic solution have been theoretically analyzed. The role of chromium in minimizing the hydrogen embrittlement (HE) of the intermetallic Fe₃Al (resulting in its poor ductility) has been addressed based on the analysis. In the case of the chromium-alloyed iron aluminide, calculations indicate that hydrogen liberation does not occur on the surface due to the shift of the corrosion mixed potential to a value nobler than the electrode potential for the hydrogen evolution reaction. This shift occurs due to the induction of passivity on alloying with Cr resulting in the formation of a passive film. The minimization of HE of iron aluminides on alloying with Cr can thus be understood.     

Effect of strain rate on behaviour of Fe3Al under tensile impact

The effect of strain rate on the behaviour of Fe₃Al has been investigated experimentally in the range 90–1300 s_-1 under tensile impact. The experimental results indicate that Fe₃Al is strain-rate sensitive and its critical strain in maximum stress increases with increasing strain rate. According to the test results for Fe₃Al in air and water, it is confirmed that environmental embrittlement induced the fracture of Fe₃Al under tensile impact. With testing at faster strain rate, further hydrogen embrittlement is suppressed. This is consistent with the micrography of the fracture surface in Fe₃Al specimens, which indicates that these iron aluminides are intrinsically quite ductile. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sliding wear resistance of iron aluminides

Room temperature dry sliding wear behaviour of iron aluminides containing 28% aluminium and various amounts of chromium has been investigated using pin on disk wear tester. The aluminides were heat treated to have orderedDO ₃structure. It was found that wear rate of the aluminides increased with the increase of applied normal load and sliding speed. Wear resistance of the aluminides increased with increase in chromium content. SEM observation of the worn surface showed that the microcutting and microploughing were the dominant sliding wear mechanisms.     

Effect of hydrogen on the mechanical behaviour of carbon-alloyed Fe3Al-based iron aluminides

The effect of hydrogen on the mechanical behaviour of two carbon-alloyed iron aluminides was studied. Weakening of some carbide-metal interfaces in the presence of hydrogen was indicated. The effect of cathodic hydrogen charging on the microstructure has also been addressed.     

铁铝金属间化合物高温硫化腐蚀研究进展

铁铝金属间化合物由于其优异的力学性能和抗高温氧化性有望成为新一代的高温材料,因此其抗高温硫化腐蚀性能就成为了人们研究的重点课题之一.影响铁铝金属间化合物高温硫化腐蚀的因素主要有:合金成分、腐蚀气氛、温度、预处理工艺等.主要从铁铝金属间化合物的硫化机理、铁铝金属间化合物中铝含量、添加元素、腐蚀气氛、腐蚀温度等方面讨论了铁铝金属间化合物硫化腐蚀的研究进展情况.     

On critical hydrogen concentration for hydrogen embrittlement of Fe3Al

The critical hydrogen concentration for hydrogen embrittlement in iron aluminide, Fe₃Al has been estimated (0.42 wppm). The estimated critical hydrogen content has been correlated to structural aspects of the decohesion mechanism of hydrogen embrittlement.     

Caustic stress corrosion cracking of a spheroidal graphite cast iron: characterisation of ex-service component

Abstract

This paper discusses observations of features suggesting grain boundary embrittlement ahead of stress corrosion crack tips in ex-service cast iron components exposed to strong caustic environments during Bayers process for alumina processing. The cracks and the neighbouring areas in the ex-service specimens were examined using conventional metallography, SEM, the extraction replica technique in the TEM and Auger electron spectroscopy. In all cases, the cracking was initiated at the surface of the steel exposed to the corrosive environment and it appeared that crack nucleation may have been aided by local stress concentrations and/or zones of local residual stress concentration. The fracture mode was almost exclusively intergranular and the crack path followed ferrite grain boundaries. There was clear evidence of a local zone of grain boundary embrittlement extending ahead of the tip of the major cracks examined. The phenomenon was established by investigating the fresh fracture surfaces produced by extending pre-existing cracks under impact loading at liquid nitrogen temperature. Auger electron spectroscopy of the fresh intergranular fracture surface failed to reveal any evidence of local elemental grain boundary segregation that might account for the observed embrittlement. In the absence of evidence of any other embrittling species on the exposed intergranular facets, there arises the possibility of hydrogen being involved in the embrittlement. The paper discusses hydrogen assisted intergranular cracking, as observed in the case of similar materials, to be the possible mechanism.     

Measurement of hydrogen and embrittlement of high strength steels

Hydrogen embrittlement of high strength steel is believed to be one of the main reasons for the cracking of prestressed concrete structures. In this study, hydrogen was generated on the steel surface by applying different fixed cathodic potentials. The steel was immersed in simulated carbonated concrete solutions with and without 0.1 M NaCl. Simultaneously, the steel was subjected to tensile loading at slow strain rate until fracture. Fractographic analysis and the measurement of the concentration of absorbed hydrogen in the iron lattice were performed. Results showed that the hydrogen atom only penetrated into the iron lattice when the steel was loaded above its yield stress. This phenomenon produced quasi-cleavage like fracture, and the strain at fracture was considerably reduced. The presence of chloride ion together with the hydrogen embrittlement did not provoke a remarkable synergic effect in the mechanical properties of high-strength steel.     

Influence of inclusions and heat treated microstructure on hydrogen assisted fracture properties of aisi 316 stainless steel

An attempt has been made to assess the influence of nonmetallic inclusions and heat treated microstructure on hydrogen assisted cracking of AISI 316 austenitic steel. The steel obtained in two levels of cleanliness was given solution annealing treatment in the temperature range of 1173–1473 K, and additional sensitization treatment of 973 K for 26 hours. Hydrogen embrittlement of this steel has been studied by charging Charpy and Compact Tension specimens of ASTM specification, with hydrogen through cathodic polarization. It is found that hydrogen embrittlement susceptibility increases with the presence of large size inclusions, larger grain and sensitized microstructure.     

Corrosion fatigue of armco iron in acid media

The nature of fracture of armco iron under conditions when corrosion fatigue and hydrogen embrittlement are equally probable was studied. It was established that armco iron fatigued in 0.1 N H₂SO₄ solution fails as a result of corrosion fatigue and that no hydrogen embrittlement takes place in this medium. The addition of 4g/l sodium sulfide to this solution produces a threefold increase in the rate of fracture.Editor's note: Both the views of the authors of this article on the role of hydrogen in corrosion fatigue of polarized metals and their explanation of experimental results are controversial. Readers are therefore invited to comment on the points raised.     

The development of Fe-Al intermetallics

The intermetallics based on aluminides have long been known for their excellent resistance to high-temperature oxidation. However, for use in structural components the poor ductility at ambient temperatures has always been felt as a stumbling block. Interest in these materials has been revived recently, after achieving some success in improving the ductility at ambient temperatures and creep at elevated temperatures in titanium aluminides. For the iron aluminides, too, similar methodologies have been attempted, namely alloying with elements such as titanium, boron, molybdenum, chromium, silicon and manganese, as well as grain refinement for improving high-temperature creep and room-temperature ductility. Raising the creep resistance close to 600 °C and improving the ambient-temperature ductility to around 6% have been the major immediate aims. Attempts are also being made to improve the high-temperature fatigue and creep properties in these materials, particularly by pushing the stability temperature of ordered D0₃ upwards. It is now visualized that once the above properties are achieved, the iron aluminides, particularly the types based on Fe₃Al, could offer themselves as excellent candidate materials For structural purposes. Their attractiveness also stems to a large extent from their low cost, as they contain only abundantly occurring materials. The present work examines two routes for introducing ductility in the Fe₃Al-based materials: one by ternary-Quarternary additions and the other by grain refinement. Structural studies have been made on materials obtained through conventional casting as well as through rapid solidification with minor alloy additions. The results confirm that Fe₃Al-based alloys, even when air-melted, are amenable to a high degree of hot working and could be made to display improved ductility at room temperature by a careful control of the chemistry. Rapidly solidified ribbons also show reasonably good bonding during high-temperature compaction. Ordering in these alloys is not suppressed even by rapid solidification.     

高强度铝合金的氢脆

氢脆是高强度铝合金在腐蚀环境中应用遇到的突出问题。本文简要介绍了高强度铝合金中的氢脆现象，氢脆特点，影响氢脆敏感性的因素以及氢脆理论等，并对这一研究领域的未来进行了展望。     

The critical stress criterion for gaseous hydrogen embrittlement

A criterion for susceptibility to hydrogen embrittlement based on the development of a critical hydrostatic stress has been proposed. The hydrostatic stress distribution ahead of plastically strained notches has been described analytically and correlated with measured susceptibility to hydrogen embrittlement. The predictions of the analysis are in good agreement with available experimental data and support the critical stress criterion for hydrogen embrittlement.     

Hydrogen embrittlement susceptibility of microstructures formed in multipass weld metal for HT780 class steel

The effect of reheating by following passes on the hydrogen embrittlement of MAG weld metal for HT780 class steels has been investigated by using specimens subjected to simulated thermal cycles. The hydrogen-charged specimens exhibited transgranular quasi-cleavage fracture and intergranular fracture along prior austenite grain boundaries on slow strain rate tensile (SSRT) tests, depending on the reheated temperature and charged hydrogen content. The reduction in elongation of hydrogen-charged specimens became more significant when intergranular fracture occurred. When specimens in as-welded state and precedently reheated at coarse grained HAZ temperature of 1,623 K were reheated at a tempering temperature of 873 K, significant amount of intergranular fracture occurred at charged hydrogen contents above 3 ppm in spite of the decrease in hardness. The specimen reheated at 1,173 K showed no intergranular fracture even after receiving the reheating at 873 K at a hydrogen content of 6 ppm, suggesting the strong influence of the prior austenite grain size on the hydrogen-induced intergranular embrittlement. The measurement of hydrogen content desorbed from the hydrogen-charged specimen at room temperature suggested that the intergranular fracture caused by the reheating at 873 K was associated with an increase in susceptibility to hydrogen embrittlement of the prior austenite grain boundary itself rather than a decrease in the amounts of trapping sites such as dislocation and retained austenite.     

Nondestructive detection of hydrogen in steel by vibration damping

The application of forced vibration measurements as a nondestructive testing method for the detection of hydrogen in steel has been investigated. The method is based on the influence of hydrogen on the internal friction of steel. Tensile test specimens of 50CrV4 steel were set into forced vibration before and after hydrogen charging. By sweeping the exciter frequency continuously over a wide range, resonances occurred at certain frequencies. The tests show close correlation between hydrogen embrittlement, hydrogen content and damping of forced vibrations in the resonance case. While the absorbed hydrogen effuses during ageing at room temperature, the resonance amplitudes increase and the hydrogen embrittlement decreases as a function of time. After an ageing period of eight days all measured values returned to their initial value before hydrogen charging. It can therefore be concluded that this method is suitable for the nondestructive testing of hydrogen-sensitive components.