Improved creep strength and creep ductility of type 347 austenitic stainless steel through the self-healing effect of boron for creep cavitation

Composition of type 347 austenitic stainless steel was modified with the addition of boron and cerium. An improvement of creep strength coupled with creep ductility of the steel was observed with boron and cerium additions. The observation of enhanced precipitation of carbonitrides in boron-containing steel over that of boron-free steel may in part contribute to the increase in creep strength. Both grain boundary sliding and nucleation and growth of intergranular creep cavities were found to be suppressed in steel-containing boron. This results in an increase in creep strength and creep ductility. Auger electron spectroscopic analysis of the chemistry of creep cavity surfaces (exposed by breaking the creep-exposed steel specimen at liquid nitrogen temperature under impact loading) revealed the segregation of elemental boron on the creep cavity surface. Boron segregation, on the creep cavity surface in the absence of sulfur contamination, suppressed the cavity growth and provided the steel with a self-healing effect for creep cavitation. Cerium additions enabled boron to segregate on the cavity surface by effectively removing the traces of free sulfur in the matrix by the formation of ceriumoxysulfide (Ce₂O₂S).     

The mechanisms of improved creep strength in a new austenitic stainless steel

An addition of 40 ppm boron, 0.4 pct vanadium and 0.12 pct nitrogen to an austenitic stainless type steel AISI 316L (0.02 C, 18 Cr, 12 Ni, 2.7 Mo) has considerably improved the creep properties. The improved creep properties are due to a combination of the precipitation of fine stable vanadium nitrides on the dislocations and the precipitation of chromium carbides (M₂₃C₆) in the grain boundaries. The latter process is thought to be enhanced by the presence of boron and helps to improve the creep ductility. The precipitation of vanadium nitrides on the dislocations retard the creep rate. The nitrides retain their small size even after long creep testing times. A model is proposed to explain this behavior of the precipitated particles and their interactions with the dislocations. Formerly with Uddeholm AB, Hagfors, Sweden     

The mechanisms of improved creep strength in a new austenitic stainless steel

An addition of 40 ppm boron, 0.4 pct vanadium and 0.12 pct nitrogen to an austenitic stainless type steel AISI 316L (0.02 C, 18 Cr, 12 Ni, 2.7 Mo) has considerably improved the creep properties. The improved creep properties are due to a combination of the precipitation of fine stable vanadium nitrides on the dislocations and the precipitation of chromium carbides (M₂₃C₆) in the grain boundaries. The latter process is thought to be enhanced by the presence of boron and helps to improve the creep ductility. The precipitation of vanadium nitrides on the dislocations retard the creep rate. The nitrides retain their small size even after long creep testing times. A model is proposed to explain this behavior of the precipitated particles and their interactions with the dislocations.     

Twin-Boundary cavitation during creep in aged type 304 stainless steel

A transition from grain-to twin-boundary cavitation was observed in Type 304 stainless steel specimens aged at 593 and 649°C (1100 and 1200°F) and creep tested at 593°C (1100°F) and 207 MPa (30 ksi). Evidence of twin-boundary cavitation was also observed in unaged specimens tested at 649°C (1200°F) and 193 MPa (28 ksi). This same behavior was also found in aged Type 316 stainless steel. Several possible reasons have been suggested for the absence of frequently observed grain-boundary cavitation and its transition to twin-boundary cavitation.     

Experimental observations and analysis of creep cavitation in AISI type 304 stainless steel

Large numbers of creep cavities have been measured in AISI type 304 stainless steel using automatic image analysis techniques. Among other things, the data exhibit substantial amounts of scatter, and indicate that the distribution of creep cavities may not be spatially homogeneous, even in the presence of uniform stress and temperature fields. The cavity radii appear to be approximately distributed according to a conditional Weibull distribution. Indirect evidence is presented to support the view that the nucleation of cavities is heavily influenced by grain boundary sliding. Comparisons are drawn with the observed cavity radius distributions and the predictions of the analytical cavity growth model of Chen and Argon and a constrained cavity growth model due to Rice. It is found that the distribution predicted by the constrained growth model is considerably narrower than the experimental distribution, but that the model of Chen and Argon shows reasonable agreement with experiment.     

Concept of a new high‐strength austenitic stainless steel

The concept of combined addition of C and N, as persued in previous work on martensitic steels, is transferred to austenitic stainless steels in order to gain highest phase stability. Thermodynamic calculations with special respect to the influence of temperature and interstitial content (C, N or C + N) were studied in the FeCrMnNC‐system. Promising compositions like Fe‐13Cr‐17Mn and Fe‐13Cr‐21Mn revealed an extended austenitic phase field. Some appropriate alloys were investigated with regard to their microscopic and electron structure. The concentration of free electrons in the austenite as the origin of phase stabilitiy increased in the order of C, N, C + N being added. Thus, the metallic character of interatomic bonding is enhanced, which entails short range atomic order. Hence, the substitutional alloy content can be minimized.     

Effect of boron on high-temperature creep behavior of austenitic stainless steel DIN 1.4970

Samples of austenitic stainless steel DIN 1.4970 containing about 40 ppm of boron and a boron-free version of this steel were creep tested at 700 °C in both the solution-annealed and aged conditions to determine their creep ductility and strength. The microstructure before creep tests was determined using transmission electron microscopy. The distribution of boron in both the steels after solution annealing (SA) and aging was mapped by means of α-autoradiography. It has been observed that in the solution-annealed condition, the creep strength of 1.4970 steel is higher than that of the boron-free version; whereas after aging, the strength of 1.4970 steel is lower than that of the boron-free version. The creep ductilities were hardly influenced by the presence of boron. The results are discussed in terms of microstructure and boron distribution in the matrix. Formerly with the Indira Gandhi Centre for Atomic Research, Kalpakkam, India.     

Residual and trace element effects on the high-temperature creep strength of austenitic stainless steels

The heat-to-heat variation in the creep strength and ductility of austenitic stainless steels was reviewed from the viewpoint of residual and trace element effects. Based on data reported in the literature, the creep strength of unstabilized alloys such as types 304 and 316 stainless steel increased with residual element and trace element content. Niobium appeared to be the most potent strengthener. There was no direct evidence that trace elements such as sulfur and phosphorus had a deleterious effect on either strength and ductility. It was assumed that the creep strength and ductility of the unstabilized grades of austenitic stainless steels are controlled by the precipitate characteristics. It follows from this that thermomechanical treatment or residual element additions that affect the precipitate characteristics influence subsequent time dependent mechanical properties. This view is consistant with most of the information in the literature. It was concluded that more systematic studies of trace and residual element effects would be beneficial to the improvement of steels. Incorporated into the studies should be quantitative characterization of evolving precipitate morphology and composition as they are influenced by residual elements. This information should be incorporated into modeling studies of non-equilibrium segregation. Ultimately, optimum elevated-temperature strength could be developed based on a materials science approach. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Irradiation-produced defects in austenitic stainless steel

Metallurgical and Materials Transactions B - The microstructure of annealed AISI Type 304 and type 316 stainless steels has been characterized by transmission electron microscopy as a function of...     

Surface segregation in austenitic stainless steel

We report in this paper a study of surface segregation in austenitic stainless steel. Auger electron spectroscopy was used to measure segregation as a function of time and temperature. We have found that P, N, S, Cr, and Ni will all segregate to the surface. However, their presence on the surface often depends on the competitive and attractive interactions between the various elements. We show that thermodynamic data on ternary liquid iron alloys are quite valuable in predicting these interactions. We also discuss possible applications of these studies.     

Carbon paraequilibrium in austenitic stainless steel

Carburization of austenitic stainless steels under paraequilibrium conditions—i.e., at (low) temperatures where there is essentially no substitutional diffusion—leads to a family of steels with remarkable properties: enhanced hardness, resulting in improved wear behavior, enhanced fatigue, and corrosion resistance, and with essentially no loss in ductility. These enhanced properties arise from an enormous carbon solubility, which, absent carbide formation, is orders of magnitude greater than the equilibrium solubility. Using interaction parameters from the latest CALPHAD assessment of the Fe-Cr-Ni-carbon system, the authors have calculated the equilibrium and paraequilibrium carbon solubility in a model Fe-18Cr-12 Ni (wt pct) austenitic steel (essentially a model 316L composition), as well as the carbon solubility in this austenite when paraequilibrium carbide formation occurs (i.e., when carbides form in a partitionless manner). For temperatures in the range 725 to 750 K, the calculations predict a paraequilibrium carbon solubility of ～5.5 at. pct. Carburization of 316L stainless steel at these temperatures, however, results in significantly higher concentrations of carbon in solid solution—up to 12 at. pct. Much better agreement with experimental data is obtained by calculating the paraequilibrium carbon solubility using Wagner interaction parameters, taken from the most comprehensive experimental study of this system. The discrepancy between the two predicted solubilities arises because the CALPHAD Cr-carbon interaction parameters are not sufficiently exothermic at the low temperatures used for paraequilibrium carburization. After multiple paraequilibrium carburization cycles, carbide formation can occur. The carbides that form under these conditions do so in a near-partitionless manner (there is modest Ni rejection to the austenite/carbide interface) and have an unusual stoichiometry: M₅C₂ (the Hägg or η carbide).     

Acoustic damping characterization and microstructure evolution during high-temperature creep of an austenitic stainless steel

We studied the microstructure evolution of an austenitic stainless steel, Type 316L, subjected to tensile creep at 973 K through the monitoring of shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the Lorentz force mechanism is the key to establishing a monitor for microstructural change in the bulk of metals with high sensitivity. In the short interval, 60 to 70 pct of the creep life, attenuation experiences a peak, independent of the applied stress. A drastic change in dislocation mobility and rearrangement interrupted this novel phenomenon, as is supported by observations using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The EMAR exhibited the potential for the assessment of damage advance and the prediction of the remaining creep life of metals.     

Creep cavitation in 304 stainless steel

Creep cavitation in 304 stainless steel at 0.5 T_m was investigated. Two specially developed techniques were used to study the nucleation and growth of grain-boundary cavities. It was found that cavities nucleated heterogeneously throughout the creep history and those observed were well in their growth stage. Comparison of these observations with the theory for cavity nucleation requires that a high interfacial stress be present. Experiments suggest that such stress concentrations are present in the early stages of boundary sliding, and in additional transients associated with intermittent sliding of boundaries throughout the creep life. It was found that microstructural variations such as those caused by twins which strongly affect initial particle densities on boundaries can alter cavitation behavior drastically. Our results also show that wedge cracks are the result of accelerated linking of growing cavities in the triple point region of stress concentration and are not a separate phenomenon. Furthermore, at higher strain rates growth of cavities can be accelerated by grain boundary sliding. Lastly, evidence is given to support the view that in engineering alloys which contain complex phase constituents particularly along grain-boundaries, cavitation in long term service is likely to be caused by cavities nucleated in connection with a prior cold forming operation.     

奥氏体耐热不锈钢的焊接实践

通过对奥氏体耐热不锈钢的焊接性分析，提出奥氏体耐热不锈钢焊接应采取的焊接工艺及应注意的问题．     

超级奥氏体不锈钢的发展

日益增长的工业需求推动着超级奥氏体不锈钢的研发,以研发时间为序阐述超级奥氏体不锈钢3个发展阶段。第1个阶段主要是为解决硫酸介质环境的耐腐蚀性而开发的不锈钢;第2个阶段是在第1阶段研发钢的基础上添加质量分数约为0.2%的N元素、并将Mo元素质量分数增加到约6%而研发的几种耐腐蚀性能良好的超级奥氏体不锈钢;第3个阶段是在6%Mo钢的基础上将Cr、Mo、N含量都进行较大幅度的提高,其中Mo元素质量分数增加到约7%,N元素质量分数控制在0.5%左右,并加入适量Mn元素而研发出耐腐蚀性优异的超级奥氏体不锈钢。阐述了超级奥氏体不锈钢研发过程中的2个重要技术,即炉外精炼与氮合金化技术,并展望了超级奥氏体不锈钢的未来发展及推广应用。     

Nonmetallic inclusions in vacuum-treated austenitic stainless steel

The nonmetallic phase in 08X18H10T and 03X18H10 vacuum-treated austenitic stainless steel is investigated by means of a scanning electron microscope. The composition of the nonmetallic inclusions in cast samples of metal from the casting ladle, the vacuum unit, and the continuous-casting machine and samples of cold-rolled sheet (thickness 1.2 mm) is studied. In 08X18H10T steel samples from the tundish of the continuous-casting machine, the nonmetallic phase consists mainly of titanium-nitride aggregations, the crystals of which contain oxides of aluminum and titanium; a few of the nonmetallic inclusions are small globules with a titanium-oxide shell. In cold-rolled sheet, the nonmetallic inclusions are distributed over the whole cross section and consist of small (5–6 μm) crystals of titanium nitrides. Cast samples of 03X18H10 austenitic titanium-free steel are relatively free of nonmetallic inclusions. The nonmetallic phase consists mainly of small inclusions of alumocalcium silicate. In cold-rolled sheet (thickness 1.2 mm), the nonmetallic phase consists of a few very small globules (2–3 μm). The results are compared with the composition of the nonmetallic phase in analogous steel with no vacuum treatment.     

Hydrogen attack in an austenitic stainless steel

A titanium stabilized 321 grade austenitic stainless steel was exposed to hydrogen at 14 MPa pressure and at 873 K. Hydrogen attack occurred in the form of small bubbles at alloy-carbide-matrix interfaces. The damaged regions were studied by transmission electron microscopy, and the gas composition in the bubbles was analyzed. The results are discussed in terms of a thermodynamic analysis of possible hydrogen attack reactions.     

Atomic-scale characterization of multiple precipitating species in a precipitation-hardened martensitic stainless steel

Multiple precipitating species in a 2.2 GPa grade precipitation-hardened martensitic stainless steel with balanced ductility were characterized at atomic scale ...