Phase transformation of austenitic stainless steels as a result of cathodic hydrogen charging

The effects of cathodic hydrogen charging and aging on surface phase transformations were studied in solution treated and cold worked specimens of two austenitic stainless steels. Quantitative phase evaluation using an X-ray technique has shown that cathodic hydrogen charging and aging can result in a considerable amount of surface transformation toε andα ′ martensites. The extent of this surface transformation differs significantly from deformation-induced transformation at the same temperature, and abnormally high volume fractions ofε martensite are produced by the charging process. A minimum charging current density is necessary to induce transformation. In cold-worked samples, further surface transformation due to hydrogen charging and aging is inhibited by high volume fractions of pre-existing martensite. A. P. BENTLEY, formerly with the Department of Metallurgy and Materials Science, University of Cambridge     

The effects of deformation induced martensite on the sensitization of austenitic stainless steels

This paper compares the effects of deformation which induces martensite in austenitic stainless steel with deformation which does not on the sensitization and corrosion susceptibility of these alloys. We show that deformation which induces martensite causes rapid sensitization at temperatures below 600 °C, leads to extensive transgranular corrosion, and can produce rapid healing. The martensite is also an area of extensive carbide precipitation. Deformation alone noticeably increases the kinetics of sensitization only at temperatures where undeformed samples are readily sensitized. Without the presence of martensite, intergranular corrosion is always the predominant corrosion path, rapid healing is not observed, and most carbides precipitate along the grain boundaries.     

Damping properties of austenitic stainless steels containing strain-induced martensite

Damping properties of two austenitic stainless steel grades, EN 1.4318 and EN 1.4301, were investigated. The test materials were cold rolled to different reductions and damping capacity was measured as a function of temperature with an internal friction method. Microstructures of the test materials were studied by means of X-ray diffraction (XRD) and magnetic measurements. The results showed that damping capacity of the studied materials depended on the amounts of strain-induced ε- and α′-martensite phases. At temperatures around 0 °C, highest damping capacity was achieved with cold rolling reduction of 10 to 15 pct. This behavior is related to the existence of ε-martensite and stacking faults. Internal friction peak due to α′-martensite phase was present at the temperature of 130 °C. Strain aging heat treatment at 200 °C for 20 minutes decreased the damping capacity in the entire studied temperature range.     

Electron microscopy study of the aging and first stage of tempering of high-carbon Fe-C martensite

Transmission electron microscopy (TEM) observations of 1.95 wt pct C martensite samples, aged at 330 and 352 K for 1 and 2 hours, respectively, confirm the fact that the two steps of aging, as evidenced by Mössbauer spectroscopy kinetics study, are due to clustering and subsequent long-range ordering. Streaks are observed along 〈203〉* directions during clustering, and in addition, two types of split superstructure spots appear during ordering at the advanced stage of aging. These are tentatively explained by a 1C-2Fe-1C-5Fe-1C-5Fe sequence of carbon and iron atoms arranged along [001]α′, which gives rise to antiphase domains with a 12a superperiod within the alternating carbon-rich and carbon-depleted regions, wherea is the lattice parameter of body-centered cubic (bcc) iron. The associated formula is Fe₆C. In the first stage of tempering, the orthorhombic structure of the precipitated carbide is confirmed, while evidence of ordering as in Co₂N is lacking. The Fe₉C₄ stoichiometry, which is close to the experimental Fe_2.4C, is instead proposed forε- orη-carbide.     

Discussion of “an interpretation of the carbon redistribution process during aging of high-carbon martensite”

Y. OHMORI and I. TAMURA:Metall. Trans. A, 1992, vol. 23A,     

Comparison of hydrogen gas embrittlement of austenitic and ferritic stainless steels

Hydrogen-induced slow crack growth (SCG) was compared in austenitic and ferritic stainless steels at 0 to 125 °Cand 11 to 216 kPa of hydrogen gas. No SCG was observed for AISI 310, while AISI 301 was more susceptible to hydrogen embrittlement and had higher cracking velocity than AL 29-4-2 under the same test conditions. The kinetics of crack propagation was modeled in terms of the hydrogen transport in these alloys. This is a function of temperature, microstructure, and stress state in the embrittlement region. The relatively high cracking velocity of AISI 301 was shown to be controlled by the fast transport of hydrogen through the stress-induced α′ martensite at the crack tip and low escape rate of hydrogen through the γ phase in the surrounding region. Faster accumulation rates of hydrogen in the embrittlement region were expected for AISI 301, which led to higher cracking velocities. The mechanism of hydrogen-induced SCG was discussed based upon the concept of hydrogen-enhanced plasticity. Formerly Research Associate of the University of Illinois     

Effect of thermal aging upon the fatigue-crack propagation of austenitic stainless steels

The effect of thermal aging upon the elevated temperature fatigue-crack propagation behavior of several austenitic stainless steels was investigated using linear-elastic fracture mechanics. The steels studied were ABI Types 304, 304L, 316 (both annealed and cold-worked), and weldments in Type 304 using Type 308 filler. Aging temperatures of 1000°F and 1200°F were employed, and aging times ranged between 1500 and 6000 hours. In general, thermal aging produced beneficial results (i.e., lower crack growth rates) relative to unaged material in elevated temperature tests. It is suggested that the precipitation of the various carbide and intermetallic phases is responsible for the beneficial effect. One possible mechanism might be a blunting effect each time the crack tip encounters a second-phase particle, thereby requiring the crack to partially reinitiate itself before proceeding.     

Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic stainless steels

The effects of deformation-induced formation of martensite have been studied in metastable austenitic stainless steels. The stability of the austenite, being the critical factor in the formation of martensite, was controlled principally by varying the amounts of carbon and manganese. The formation of martensite was also affected by different test and rolling temperatures, rolling time, and various reductions in thickness. The terms “stress-induced” and “strain-induced” formation of martensite are defined. Experimental results show that low austenite stability resulted in stress-induced formation of martensite, high work-hardening rates, high tensile strengths, low “yield strengths,” and low elongation values. When the austenite was stable, plastic deformation was initiated by slip, and the work-hardening rate was too low to prevent early necking. A specific amount of strain-induced martensite led to an “optimum” work-hardening rate, resulting in high strengthand high ductility. For best results processing should be carried out aboveM _d and testing betweenM _d andM _s. Mechanical working aboveM _d had a negligible effect on the yield strength betweenM _d andM _s when the austenite stability was low, but its effect increased as the austenite became, more stable. Serrations appeared in the stress-strain curve when martensite was strain induced.     

节镍型奥氏体不锈钢中形变马氏体的产生和逆变规律

通过不同保温时间的退火处理,获得了具有不同晶粒度的节镍型奥氏体不锈钢试验材料.利用Gleeble-3000热模拟机进行不同变形程度、变形温度的冷加工,分析了变形程度、变形温度和原始晶粒度对形变马氏体含量的影响.对轧硬态节镍型奥氏体不锈钢进行不同温度和时间的热处理,研究了形变马氏体的逆变规律.结果表明,冷加工过程中,变形程度和变形温度对形变马氏体的产生有重要影响,而材料的原始晶粒度对形变马氏体含量没有显著影响.形变马氏体发生逆变的临界温度约为550℃,在800℃时,形变马氏体可以在20 s之内消除.     

The risk of hydrogen embrittlement in high-strength prestressing steels under cathodic protection

High strength prestressing steels in prestressed concrete structures are protected against corrosion due to passivation resulting from the high alkalinity of the concrete. If depassivation of the prestressing steel occurs due to the ingress of chlorides the corrosion risk can be minimized by application of cathodic protection with impressed current. The risk of hydrogen embrittlement of the prestressing steel is especially pronounced if overprotection is applied due to hydrogen evolution in the cathodic reaction. The present work considers this risk by hydrogen activity measurements under practical conditions and application of different levels of cathodic protection potentials. Information on threshold potentials in prestressed concrete structures is provided, too.     

Role of heat treatment and cathodic charging conditions on the hydrogen embrittlement of HP 7075 aluminum alloy

The response of an equiaxed-grained, high-purity 7075 aluminum alloy to hydrogen as a function of heat treatment and temperature of cathodic charging has been studied. Room-temperature (RT) tensile tests following RT or 120 °C cathodic charging revealed that hydrogen embrittlement susceptibility was dependent on both heat treatment and charging conditions. However, intergranular behavior was only observed following RT charging, limited to a zone a few grains deep from the surface. This fracture appearance has been considered in terms of the possible formation of an aluminum- or magnesium-base hydride at room temperature, compounds which are thermodynamically unstable at the 150 °C charging temperature. Reasons for the observed embrittlement following high-temperature charging are also considered. Formerly with Carnegie Mellon University.     

Studies of the orientations of fracture surfaces produced in austenitic stainless steels by stress-corrosion cracking and hydrogen embrittlement

Orientation studies have been made on several different austenitic stainless steels, using photogrammetric and electron channeling techniques. The fracture facets produced by SCC in boiling aqueous MgCl₂ (155 °C) were large and relatively flat in the case of type 310 steels, and the fracture plane was found to be at or near {100}. The transgranular stress-corrosion fractures in type 304 steels were more complex, and there was considerably more scatter in the orientation determinations. However, the orientations of the fracture facets in these steels were clearly not {100}, but fell into two distributions, one near {211} and the other near {110}. Electron diffraction studies from the fracture surfaces indicated the presence of α′ and martensites in the type 304 but not in the type 310 cases; the possibility that this was responsible for the differences in fracture planes is discussed. Studies were also made of a type 304 specimen which had failed by SCC at 289 °C. No martensitic phases were detected at the fracture surfaces in this case, and the fracture facets were large and flat, similar to those for type 310. Cleavage-like fracture surfaces were also produced in type 304 steels by hydrogen embrittlement, using both gaseous hydrogen and cathodic charging, but the facets were too small for precise orientation determination. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly with the Department of Metallurgy, University of Illinois at Urbana-Champaign. Formerly Professor of Metallurgy, University of Illinois.     

Effect of weld composition and microstructure on hydrogen assisted fracture of austenitic stainless steels

The effect of severe hydrogen environments on the tensile fracture behavior of a variety of austenitic stainless steel welds was investigated. In all cases, second phases or particulates common only to the weld microstructure were the origin of fracture initiation in hydrogen. These second phases formed as a result of microsegregation during solidification and/or solid state transformations during cooling or aging. In addition to second phases the weld microstructure matrix phase also influences fracture behavior. The fracture behavior is discussed in terms of localized chemical variations and the presence of second phases, and th interaction of dislocations with internal boundaries.     

Nature of the γ and γ∗ phases in austenitic stainless steels cathodically charged with hydrogen

P. ROZENAK, formerly with the Materials Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva     

Morphology and distribution of martensite in 301L alloy induced by different subsequent processes after prior deformation

Abstract

The influence of different processing routes: either isothermally holding at 77 K for 72 h or deforming 10% at 77 K after prior deformation to a strain level of 20% on the increment, morphology and distribution of α′ martensites has been investigated in 301L austenitic steel. The results show that the degree of stimulation of the α′ martensite transformation depends on total strain path. A detailed microstructural study by transmission electron microscopy indicates that different morphology and distribution of α′ martensites are formed as a function of strain path which reflect the effect of stress and/or strain induced α′ martensite formation.

On a examiné l’influence de différentes voies de traitement - soit maintien isotherme à 77 K pendant 72 heures ou déformation de 10% à 77 K après déformation préalable à un niveau de déformation de 20% - sur l’incrément, la morphologie et la distribution de la martensite-α′ de l’acier austénitique 301 L. Les résultats montrent que le degré de stimulation de la transformation de martensite-α′ dépend du parcours de déformation totale. Une étude détaillée de la microstructure par microscopie électronique en transmission indique que différentes morphologies et distributions de martensite-α′ sont formées en fonction du parcours de déformation, ce qui reflète l’effet de la formation de martensite-α′ induite par contrainte ou par déformation.