Unstable Austenitic Steels Research Articles

Crack path and liquid metal embrittlement specificity of austenitic steels in mercury at room temperature

A liquid metal embrittlement specificity of three austenitic steels with increasing nickel content (304 L, 316 L and 316L(N)) is studied in liquid mercury in the axisymmetric notched geometry. Only the low nickel alloys are susceptible to LME. The crack path of an austenitic steel fracture induced by liquid mercury has been elucidated at microstructural scale. Deformation induced martensite (γ(fcc) → α’(bcc)) of the low nickel steels induces numerous α’/α’ interfaces at small scale that are susceptible to be embrittled. Because the only steel that resists LME is the one that shows stability over α’ phase change due mostly to its higher nickel content, a point confirmed by X Ray fractography, it is inferred that the major factor contributing to the LME sensitivity at room temperature is the α’ phase formation in unstable austenitic steels during plastic strain. This provides a sound rationale on how to prevent mercury induced embrittlement with austenitic steels.

Crack Path and Liquid Metal Embrittlement Specificity of Austenitic Steels in Mercury at Room Temperature

A liquid metal embrittlement specificity of three austenitic steels with increasing nickel content (304L, 316L and 316L(N)) is studied in liquid mercury. Only the low nickel alloys are susceptible to LME. The crack path of an austenitic steel fracture induced by liquid mercury has been elucidated. Deformation induced martensite (γ → α’) of the low nickel steels induces numerous α’/α’ interfaces at small scale that are susceptible to be embrittled. Because the only steel that resists LME is the one that shows stability over a’ phase change due mostly to its higher nickel content, a point confirmed by X Ray fractography, it is inferred that the major factor contributing to the LME sensitivity at room temperature is the α’ phase formation in unstable austenitic steels during plastic strain. This provides a sound rationale on how to prevent mercury induced embrittlement with austenitic steels.

Influences of thermomechanical treatments on the cryogenic treatability of a slightly unstable austenitic stainless steel

ABSTRACTThe success of deep cryogenic treatment (DCT) of austenitic stainless steels (AusSSs) relies heavily on martensitic transformation. For AusSSs that are not prone to martensitic transformation, thermomechanical treatments (such as high-temperature solution treatment, plastic deformation, and sensitisation) may expedite DCT by encouraging martensite formation. This paper looks into the influences of the aforementioned thermomechanical treatments on the cryogenic treatability of a slightly unstable AusSS regarding tensile properties. Overall, no noticeable strengthening could be achieved through DCT, but a slight improvement in ductility was observed for all the thermomechanical treatment conditions mentioned above. Preliminary results indicated that the pitting corrosion resistance of the AusSS was almost unaltered by DCT.

Hydrogen-induced delayed cracking in the AISI 301 unstable austenitic steel sheet

This work aims to study the hydrogen-induced delayed cracking related to the martensitic transformation in the AISI 301 unstable austenitic stainless steel. It happens because the metastable austenite is substantially transformed into α ′ -martensite due to the deformation occurring during the clamp-forming operation. To understand this phenomenon, morphological and chemical analyses have been made. The martensite fraction in the inside bend radius was measured by X-ray Diffraction (XRD) and the breaking patterns were observed using a Scanning Electron Microscope (SEM). The clamps breaking patterns observed after delayed cracking show a brittle intergranular fracture in the bend radius. Under laboratory conditions and after hydrogen-charged clamps, delayed cracking occurred. The main fractographic feature observed in cracked clamps is the intergranular Hydrogen Embrittlement (HE). This brittle behavior is intensified by the high heterogeneous stress state in the material and the high content of martensite.

Delayed cracking in 301 austenitic steel after bending process: Martensitic transformation and hydrogen embrittlement analysis

The aim of this work is to study the delayed cracking phenomenon of the unstable austenitic stainless steels (301 grade) deformed by bending operations. During the clamp-forming operation, austenite is transformed to α ′ -martensite. To understand this phenomenon, morphological analysis and hardness measurements were made. Scanning electron microscopy (SEM) was used to perform fracture surface analysis and X-ray diffraction (XRD) to measure the amount of the martensite . The breaking patterns of these clamps obtained after delayed cracking are predominantly intergranular. This type of failure is often associated with hydrogen embrittlement. This brittle behavior is accentuated by the presence of internal stresses in the material (cold rolling, forming by bending) and the high content of martensite.

Mechanical behavior of a metastable austenitic stainless steel under simple and complex loading paths

This paper deals with the characterization and the modeling of the mechanical behavior of an unstable austenitic stainless steel that is commonly used in deep drawing processes. This steel exhibits a martensitic transformation at room temperature induced by plastic deformation during cold forming, which sometimes has an unfavorable influence on the final shape of the part. In order to improve the knowledge of this material, an extensive characterization of its mechanical behavior is performed using tensile and shear tests combined with eddy current measurements. The available material information enables the identification of a two-phase model of elastoviscoplastic type which integrates the knowledge of the initial microstructure and its evolution as a function of the inelastic strain. The model prediction is found to be accurate with respect to the experimental test results.

Residual Stress State at Different Scales in Deep Drawn Cup of Unstable Austenitic Steel

In this study, residual stresses state at different scales in the 301LN unstable austenitic steel after deep drawing was determined. The first part of the work deals with the characterization of the martensitic transformation during uniaxial loading. The austenite/martensite content which was determined by X-Ray Diffraction increases until a maximum of 0.6 for 30% strain. Internal stress distribution was determined by coupling in-situ tensile tests with sin²ψ method. As soon as martensite appears, the magnitudes of the internal stresses in this phase were found to be 400 MPa higher than in the austenite. To establish a relation between the complex loading path effect and the phase stress state, deep drawing tests were carried out for different drawing ratios. Both macroscopic tangential residual stresses and residual stresses in the martensite were determined. It appears that the macroscopic tangential residual stresses are positive and increase with increasing drawing ratios and the maximum value is located at middle height of the cup. It is about 850MPa for the Drawing Ratio (DR)=2.00. The tangential residual stresses in the martensite were found to be positive in the external face and have a same evolution as the macroscopic ones.

Delayed cracking in 301LN austenitic steel after deep drawing: Martensitic transformation and residual stress analysis

The main objective of this work is to study the delayed cracking phenomenon of the 301LN unstable austenitic steel, by determining the distribution of residual stresses after deep drawing, taking into account the phase transformation. Deep drawing for different ratios is done for two different temperatures. Cracks appear for the highest drawing ratio (DR = 2.00) in the top of the cup. The breaking patterns observed using a scanning electron microscope show ductile fracture in the middle region, and both intergranular and transgranular rupture in the edges. Martensite contents throughout the cup wall and through the thickness are determined. Increasing the martensite content was found to have a great effect on the cracking sensitivity. X-ray diffraction allows us to determine the residual stresses in the martensitic phase. These last are positive, increase with increasing drawing ratios. The maximum value is located at the middle height of the cup, it exceeds 500 MPa for the 2.00 drawing ratio, and is less than 350 MPa for the 1.89 drawing ratio.

Determination of Residual Stresses after Tensile Tests and Deep Drawing of Unstable Austenitic Steel

The main objective of this work is to contribute to the study of the 301LN unstable austenitic stainless steel by determining the distribution of residual stresses after deep drawing, taking into account the phase transformation. In the first part, kinetics of martensitic transformation are determined for uniaxial loading. Tensile tests are performed at different pre-strains at room temperature for two different strain rates. The austenite/martensite content is measured by X-ray diffraction and is coupled with the determination of residual stresses distribution. In addition, to establish a relation between the complex loading path effect and the residual stresses state, deep drawing are done for different drawing ratios for two different temperatures. Macroscopic tangential residual stresses are determined by the separation technique. It appears that the residual stresses increase with increasing drawing ratios and the maximum value is located at middle height of the cup.

Finite element analysis of the effects of martensitic phase transformation in TRIP steel sheet forming

This paper aims at investigating the effects of the martensitic phase transformation on the formability of unstable austenitic steel sheets. To this end, the constitutive model developed by Iwamoto and Tsuta (International Journal of Plasticity 2002;18:1583–606) has been implemented in the user's material subroutine of the finite element code Abaqus/Explicit. The different contributions of the martensitic transformation to the overall plastic behaviour are analysed with the aim of assessing their influence in sheet-metal forming. The effects of transformation strains, and of the stress-state dependence of the kinetics of phase transformation are critically discussed in the case of the cup drawing test. The simulation results are also compared with experimental cup tests from the literature.

A numerical analysis of the formability of unstable austenitic steels

A numerical analysis of the effects of the martensitic phase transformation on the formability of unstable austenitic steel sheets is performed using the constitutive model developed by Iwamoto and Tsuta [T. Iwamoto, T. Tsuta, Int. J. Plast. 18 (2002) 1583–1606] for TRIP steels. The effects of the transformation strains, and of the kinetics of martensitic phase transformation are analysed in the case of the cup-drawing test. The numerical predictions are finally compared with experimental cup tests on type 304 austenitic steels taken from the literature.

Production of zinc powders by alkaline treatment of lead-zinc ores

A liquid metal embrittlement specificity of three austenitic steels with increasing nickel content (304 L, 316 L and 316L(N)) is studied in liquid mercury in the axisymmetric notched geometry. Only the low nickel alloys are susceptible to LME. The crack path of an austenitic steel fracture induced by liquid mercury has been elucidated at microstructural scale. Deformation induced martensite (γ(fcc) → α’(bcc)) of the low nickel steels induces numerous α’/α’ interfaces at small scale that are susceptible to be embrittled. Because the only steel that resists LME is the one that shows stability over α’ phase change due mostly to its higher nickel content, a point confirmed by X Ray fractography, it is inferred that the major factor contributing to the LME sensitivity at room temperature is the α’ phase formation in unstable austenitic steels during plastic strain. This provides a sound rationale on how to prevent mercury induced embrittlement with austenitic steels.

545 オーステナイト系ステンレス鋼 SUS304 中のき裂近傍におけるマルテンサイト相変態

The present study has attempted to simulate the mechanical behavior of an unstable austenitic stainless steel SUS304 in which stress-induced martensitic phase transformation is to occur at room temperature. Volume fraction of α' martensite transformed in stretched SUS04 plates was measured and expressed as a function of stress level. The distribution of α' phase around a fatigue cracks was also measured in a center-cracked SUS304 plate. Computer simulations have been made on uniaxial tension tests of a center-cracked plate specimen of SUS304 stainless steel subjected to change of physical constants due to the phase transformation at room temperature. The present simulation has reproduced the distribution of α'martensite in the wake of growing crack. The distribution of α'martensite obtained by the simulation looks similar to that obtained by the experiment.

Gleichmaßdehnung?

The uniform strain is applied for characterization of workability and for linking characteristic values in materials testing. In the literature different methods are indicated for the determination of this quantity. With these methods different values of uniform strain are obtained for the same material. This paper deals with test results from unstable austenitic steels. The three dimensional and chronologic course of strain is investigated as well as the question of existence of uniform strain. The influence of the specimen geometry and the stability of the micro-structure on the deformation process of such materials is considered.

Internal hydrogen-induced subcritical crack growth in austenitic stainless steels

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocityvs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip.

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.

Role of the martensite transformation in hydrogen embrittlement of unstable austenitic steels

This paper investigates the features of the deformation martensite transformation and hydrogen embrittlement of 12Kh18N10T and 06Kh18N5G9AB steels under various conditions of hydrogen action. It was established that hydrogen has practically no influence on the short-term tensile strength in all conditions of the investigated materials. Consequently, hydrogen embrittlement of unstable austenitic steels is related to the change in intensity of formation of deformation martensite.

Study of failure for unstable austenitic steel in a hydrogen atmosphere

1. A reduction in the mechanical properties of steel 12Kh18N10T under the action of hydrogen proceeds in the pressure range 0–3 MPa both with static tests and with loading in the low-cycle fatigue region. 2. The presence of hydrogen leads to a change in failure micromechanism from ductilepitted with cyclic tension, and fatigue-grooved with low-cycle loading, to a brittle micromechanism. 3. A brittle failure mechanism occurs as result of a reduction in crack initiation energy along martensite platelet interfaces (deformation α′-phase) in the presence of hydrogen. 4. By promoting formation of microcracks in volumes where deformation martensite transformation has occurred, hydrogen prevents its continuous propagation in the rest of the volume, as a result of which its transformation is suppressed.

Hydrogen Induced Slow Crack Growth in Stable Austenitic Stainless Steels

The behavior of hydrogen induced slow crack growth in type 310 and type 16-20-10 stable austenitic stainless steels along with type 321 unstable austenitic stainless steel were investigated. It was found that slow crack growth could occur in all three types of stainless steels, and the threshold values wereK H/Kc = 0.55, 0.7, and 0.78 for type 321, 310, and 16-20-10 stainless steel respectively, when charged under load. Slow crack growth could also occur if the precharged specimens were tested under constant load in air. No slow crack growth occurred in the precharged and then out-gassed specimens. This indicates that delayed cracking in stable austenitic stainless steels is induced by hydrogen. Since there is no hydrogen induced α’ martensite in type 310 and 16-20-10 stainless steel, the existence of a’ martensite is not necessary for the occurrence of slow crack growth in the austenitic stainless steels, although it can facilitate slow crack growth. The mode of hydrogen induced delayed fracture in either the stable or unstable austenitic stainless steel is correlated with theK, value; the fracture surface is changed from ductile to brittle asK 1 is decreased.

Role of martensite transformation in forming the mechanical properties of unstable austenitic steels

1. Ductility properties of the test unstable austenitic steels increase during martensite transformation if it is preceded by plastic deformation of austenite. 2. Maximum ductility of the test steels is obtained with strictly optimum kinetics for martensite transformation which depends on the level of prior deformation. After quenching maximum ductility is obtained with a low intensity of transformation; the ductility peak is observed close to the Md point. 3. After prior deformation maximum ductility corresponds to the greatest intensity of transformation. With an increase in the degree of prior deformation the ductility peak shifts in the direction of lower temperature, and the intensity of martensite transformation corresponding to maximum ductility increases. 4. Martensite transformation may have both a strengthening and a relaxation effect. After quenching steels it mainly develops as a strengthening effect, and after deformation as a relaxation effect.