Characteristics of hydrogen embrittlement,stress corrosion cracking and tempered martensite embrittlement in high-strength steels

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diffraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel.     

Mechanism of embrittlement in tempered martensite

Abstract

In this paper the total driving force for the decomposition of retained austenite and martensite are calculated together with the nucleation and growth characteristics of cementite in the two phases. The results demonstrate that the driving force for the decomposition of martensite is an order of magnitude less than that of austenite. However, the driving force for cementite precipitation in martensite is two orders of magnitude greater than in austenite with a much shorter incubation period. On short term tempering cementite precipitates from martensite whereas on longer term tempering decomposition of retained austenite occurs because of the increase in driving force which is enhanced by the contraction of the martensite on decomposition. It is argued that the precipitation of cementite from the austenite results in tempered martensite embrittlement, a mechanism dependent upon the two related decomposition processes. The segregation of trace impurities or the precipitation of cementite at the grain boundaries is not a prerequisite.

MST/240     

Characterization of tempered martensite embrittlement using hardness measurement

A series of experiments were carried out on three commercial steels to explore the possibility for characterizing tempered martensite embrittlement (TME) by Rockwell, macro-, and micro-Vickers hardness tests. The results indicate distinct hardness peaks in two steels and an inflection in the other around the TME temperature. A new analytical approach for examining the slope of hardness-tempering temperature plots appears to reveal the TME phenomenon more sensitively. Dilatometric examinations substantiate that the temperature of hardness peak/inflection occurs beyond the second stage of tempering.     

The creep and fracture behaviour of tempered martensitic steels

Creep strength enhanced ferritic steels contain 9 to 12% Cr and were developed to exhibit excellent high temperature properties. These should be achieved when the microstructure exhibits a tempered martensitic matrix containing a substructure with a high dislocation density and a uniform dispersion of fine, second phase precipitates. It is interesting to note that when properly processed the typical alloy compositions for these steels provide reasonable strength but can exhibit brittle creep behaviour. The levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. The properties needed for modern design methods can only be obtained using validated procedures for both uniaxial and multiaxial testing and documentation to establish the mechanisms controlling deformation and fracture for relevant stress states.     

Hydrogen embrittlement behaviour of a cobalt-based amorphous alloy

Hydrogen embrittlement behaviour of Co₆₅Fe₅Mo₂Si₁₆B₁₂ amorphous alloy was studied at room temperature under various conditions. Hydrogen charging was carried out in 1.0 N H₂SO₄ solution at a current density of 80 mA/cm² and the susceptibility of hydrogen embrittlement determined by tensile test at a strain rate of 1.67 × 10^?5/s. The fracture stress of the amorphous alloy decreased significantly on hydrogen charging but recovered on ageing at room temperature.     

Combined equilibrium and non-equilibrium segregation treatment of temper embrittlement in low alloy steels

Abstract

Embrittlement is an important factor for low alloy ferritic steels used for components and structures in the power and petrochemical industries when exposed to a higher temperature. The embrittlement may be classified into non-hardening embrittlement and hardening embrittlement. The non-hardening embrittlement, for example temper embrittlement, originates from grain boundary segregation of impurity elements such as phosphorus. To predict this segregation behaviour, a model is established by simplifying a low alloy steel as a dilute Fe–C–Mo–P quaternary alloy and modifying previous models. This model is applied to segregation predictions in a 2.25Cr–1Mo steel subjected to a complex heat treatment cycle.     

Reverse temper embrittlement: a common source of perplexity in low alloy steels

The present paper attempts to assess the many aspects which are involved in the old, interesting and somewhat perplexing grain boundary segregation phenomenon commonly known as reverse temper embritlement (RTE). The actual mechanisms involving grain boundary failure, the mechanics of impurity segregation at grain boundaries and the speciality of grain boundaries are discussed at length . The thermodynamics of impurity segregation, which is treated as aneq uilibrium-type process, together with the segregation kinetics, which are required for an estimation of the extent of grain boundary impurity segregation in terms of time and temperature, are also considered. The various methods of portraying the effects of reverse temper embrittlement in terms of bulk chemistry, grain boundary chemistry, therm al history (tempera ture, time and impurity segregation characteristics) and material properties (micro structure, hardness and strength) are critically assessed and compared. Interms of chemistry it is shown that the extent of grain boundary segregation, viz. phosphorus monolayers, exhibits very con sistent and small datascatter trends. Consideration is given to other aspects such as thermal history, together with impurity diffusion and material properties such as hardness, tensile strength and grain size, and it is established that RT E effects can be adequately expressed in terms of (I) grain sizebulk phosphorus trends and/or (2) a grain boundary phosphorus factor.     

Static recovery of tempered lath martensite microstructures during long-term aging in 9-12% Cr heat resistant steels

Static recovery of tempered lath martensite microstructures of strength enhanced ferritic steels has been investigated during very long-term aging up to 2 × 10⁴ h at 650 °C for 3 steels containing 9 to 12% Cr. Static recovery of tempered lath martensite microstructure occurs after an incubation period for 1-2 × 10³ h in the steels. The static recovery is controlled by the loss of strengthening due to M₂₃C₆ precipitates, and disappearance of MX carbonitrides cannot be the main cause of the static recovery.     

Effects of morphology and strength of martensite on cyclic deformation behaviour in dual-phase steels

Abstract

Tension–compression cyclic deformation behaviour in dual-phase steels has been studied. Three different ferrite (α)–martensite (α′) microstructures, i.e. isolated α′-colonies dispersed in α-matrix (I), continuous α′ (C), and laminated α–α′ (L), were prepared by appropriate heat treatments, keeping the α′ volume fraction at ~0·3. The work hardening and the Bauschinger effect are found to be greater in microstructure C or L than in I when they are compared at an arbitrary forward (tension) prestrain level. An increase in the hardness of α′ enhances the Bauschinger effect and then narrows the stress–strain hysteresis loop. The stress evolved as a result of the Bauschinger stress (defined as the difference between forward prestress and backward (compression) 0·1% proof stress) is found to be almost independent of microstructure and hardness when it is compared at an arbitrarily fixed prestress level. The slip lines are very fine and relatively straight in microstructure C, but wavy in microstructure I. These findings are discussed from the standpoints of the accumulation of the average internal stress resulting from inhomogeneous plastic flow between two constituent phases and the plastic relaxation.

MST/382     

Hydrogen embrittlement in power plant steels

In power plants, several major components such as steam generator tubes, boilers, steam/water pipe lines, water box of condensers and the other auxiliary components like bolts, nuts, screws fasteners and supporting assemblies are commonly fabricated from plain carbon steels, as well as low and high alloy steels. These components often fail catastrophically due to hydrogen embrittlement. A brief overview of our current understanding of the phenomenon of such hydrogen damage in steels is presented in this paper. Case histories of failures of steel components due to hydrogen embrittlement, which are reported in literature, are briefly discussed. A phenomenological assessment of overall process of hydrogen embrittlement and classification of the various damage modes are summarized. Influence of several physical and metallurgical variables on the susceptibility of steels to hydrogen embrittlement, mechanisms of hydrogen embrittlement and current approaches to combat this problem are also presented.     

Endurance of medium-carbon steels chromized by various methods

This article describes equipment for gas and vapor phase chromizing of steel and reports the results of a study of the structure of diffused layers formed after various chromizing treatments. For equal thicknesses of the difused layers, steels chromized by the gas and vapor phase contact methods have the highest fatigue strength. The corrosion-fatigue strength of chromized steel depends on the density and thickness of the diffused layer; it reaches its highest level after gas contact chromizing.