Nonequilibrium solute segregation to austenite grain boundaries in low alloy ferritic and austenitic steels

Nonequilibrium segregations of substitutional solute and impurity elements to austenite grain boundaries during cooling from the austenitizing temperature have been measured in thin foil specimens of ferritic 2.25 pct Cr 1 pct Mo, 2.25 pct Cr 1 pct Mo 0.08 pct Sn and austenitic Type 316 steels using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometry (STEM-EDS). The results are discussed and compared with the predictions of a theoretical analysis which describes the segregation in terms of a quench-induced diffusion of vacancy-solute atom pairs to the grain boundaries.     

Nonequilibrium solute segregation to austenite grain

Nonequilibrium segregations of substitutional solute and impurity elements to austenite grain boundaries during cooling from the austenitizing temperature have been measured in thin foil specimens of ferritic 2.25 pct Cr 1 pct Mo, 2.25 pct Cr 1 pct Mo 0.08 pct Sn and austenitic Type 316 steels using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometry (STEM-EDS). The results are discussed and compared with the predictions of a theoretical analysis which describes the segregation in terms of a quench-induced diffusion of vacancy-solute atom pairs to the grain boundaries.     

The structure of tempered martensite and its susceptibility to hydrogen stress cracking

A series of 4130 steels modified with 0.50 pct Mo and 0.75 pct Mo were tempered at temperatures between 300 and 700 °C for one hour. The changes in the carbide dispersion and matrix substructure produced by tempering were measured by transmission electron microscopy. These measurements were correlated with resistance to hydrogen stress cracking produced by cathodic charging of specimens in three-point bending. Scanning electron microscopy showed that specimens tempered between 300 and 500 °C failed by intergranular cracking while those tempered at higher temperatures failed by a transgranular fracture mode. Auger electron spectroscopy showed that the intergranular fracture was associated with hydrogen interaction with P segregation and carbide formation at prior austenite grain boundaries. Transgranular cracking was initiated at inclusion particles from which cracks propagated to produce flat fracture zones extending over several prior austenite grains. The 4130 steels modified with higher Mo content resisted tempering and showed better hydrogen stress cracking resistance than did the unmodified 4130 steel. The transition in fracture mode is attributed to a decohesion mechanism in the low temperature tempered samples and a pressure mechanism in the highly tempered samples.     

Chromium depletion and martensite formation at grain boundaries in sensitised austenitic stainless steel

A microstructural and compositional investigation of grain boundary precipitation and martensite formation in sensitised 304 stainless steel has been conducted. Grain boundary depletion of chromium has been quantified in terms of sensitisation time, temperature and boundary type by energy dispersive X-ray microanalysis in the transmission electron microscope. Chromium depleted profiles measured in grain boundary vicinities are sometimes asymmetrical and correlate with the expected profiles generated by growth of semicoherent and incoherent carbide interfaces. The depletion of chromium promotes martensite formation within near-grain boundary regions and this transformation has been directly studied by in situ cold stage microscopy down to − 150°C. Transformation occurs at the most severely depleted boundaries and initiation is favoured at slip band-boundary intersection points and along grain boundaries whose plane orientation matches that of the martensite habit plane. The preferential formation of grain boundary martensite could be an important factor in the stress corrosion and environment sensitive failure of this material.     

The influence of temper embrittlement on the stress corrosion susceptibility of Fe-3 wt.% Ni alloys

The influence of a temper embrittling heat treatment on the intergranular stress corrosion susceptibility of an Fe-3 wt.% Ni alloy containing minor additions of tin or phosphorus has been investigated. Microanalysis of the grain boundary regions has been carried out using a scanning transmission electron microscope (STEM) fitted with an energy dispersive X-ray spectrometer. Measured changes in stress corrosion cracking behaviour, resulting from embrittlement at 823 K, have been correlated with changes in the electrochemical properties of the grain boundary regions. Crack growth has been interpreted in terms of an anodic dissolution process.     

Reversible temper embrittlement of rotor steels

The susceptibility to temper embrittlement of eight different rotor steels has been studied in terms of the effects of composition, of cooling rate from tempering temperature, of isothermal aging, of steel-making practice and of strength level and tempering temperature. The Ni Cr Mo V steels tested showed increasing susceptibility to temper embrittlement with increasing nickel content. The normally marked susceptibility of a high phosphorus 3 pct Cr Mo steel was eliminated by the removal of manganese. Embrittlement in a 3 pct Ni Cr Mo V steel was caused by the equilibrium segregation of solute atoms to the prior austenite grain boundaries. Two Cr Mo V steels tested were not susceptible to temper embrittlement. Electroslag remelting and refining had very little effect on the susceptibility of the steels tested. Strength level and tempering temperature had no effect on the degree of embrittlement of the 3 pct Ni Cr Mo V disc steel. The possibilities of remedial action include an adjustment of the post tempering cooling rate, to optimize the conflicting interests of minimum temper embrittlement and adequate stress relief, and the production of very low manganese rotor steels.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Retained Austenite and Tempered Martensite Embrittlement in Medium Carbon Steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Some in situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M₃C films upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe₃C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The situation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which occurs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can occur in the same steels, depending on the tempering conditions.     

Studies on Nb Microalloying of 13Cr Super Martensitic Stainless Steel

The effect of Nb microalloying on microstructure, mechanical properties, and pitting corrosion properties of quenched and tempered 13?pct Cr-5?pct Ni-0.02?pct C martensitic stainless steels with different Mo and N contents was investigated. The microstructure, density, and dispersion of high-angle boundaries, nanoscale precipitates, and amount of retained austenite were characterized by using electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction to correlate with properties. The results show that the combined effects of lowering nitrogen content in 13?pct Cr-5?pct Ni-1~2?pct Mo-0.02?pct C steels to 0.01?wt pct, and adding 0.1?pct Nb are to decrease the amount of Cr-rich precipitates, as Nb preferentially combines with residual carbon and nitrogen to form carbonitrides, suppressing the formation of Cr₂N and Cr₂₃C₆. Austenite grain refinement can be achieved by Nb microalloying through proper heat treatment. If the nitrogen content is kept high, then Cr-rich precipitates would occur irrespective of microalloying addition. The NbN would also occur at high temperature, which will act as substrate for nucleation of coarse precipitates during subsequent tempering, impairing the toughness of the steel. It was shown that the addition of Nb to low interstitial super martensitic stainless steel retards the formation of reversed austenite and results in the formation of nanoscale precipitates (5 to 15?nm), which contribute to a significant increase in strength. More importantly, the pitting corrosion resistance was found to increase with Nb addition. This is attributed to suppression of Cr-rich precipitates, which can cause local depletion of Cr in the matrix and the initiation of pitting corrosion.     

The effects of microstructure,strength level,and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.     

The nucleation of aluminum grains in alloys of aluminum with titanium and boron

The microstructure of a ternary alloy, Al-5 wt pct Ti, 1 wt pct B, has been examined by optical and electron transmission microscopy, by selected area diffraction, and electron probe microscopy, by selected area diffraction, and electron probe microanalysis. Particles of Al₃Ti are found at the center of grains and there exist preferred epitaxial orientations between this compound and the surrounding aluminum. Particles containing titanium and boron occur at aluminum grain boundaries and have no preferred configurations with respect to the aluminum or to one another. It is concluded that the active heterogeneous nuclei are therefore Al₃Ti and that particles of TiB₂, AlB₂, or a ternary compound are not active in this alloy. Grain size measurements in binary Al-Ti alloys suggest that particles of a nucleating phase must be present at concentrations as low as 0.01 wt pct Ti, and it is suggested that these could be Al₃Ti if the existing binary phase diagram Al-Ti is in error.     

The tempering of low carbon steels containing tungsten

The tempering behavior of three steels each containing 0.20 pct C and having tungsten contents of 2.1, 3.9 and 5.9 pct has been followed by thermomagnetic analysis and electron microscopy. Using a Sucksmith Balance, the proportions of autotempered carbide and retained austenite in as-quenched specimens were estimated, and the amount of cementite precipitated upon subsequent tempering measured accurately. Solution of tungsten in cementite during tempering was monitored by observing changes in Curie temperature. The magnetic nature of alloy carbides precipitating at high temperatures allowed tentative identification and this directed and assisted the electron microscopy study which provided new information on the morphology of tungsten carbides. J. B. LUPTON, formerly Research Student, Metallurgy Department, University of Sheffield, England, S. MURPHY, formerly Research Fellow, Metallurgy Department, University of Sheffield,     

On the nature of the electrochemically synthesized hard Fe-0.96 mass pct C alloy film

A hard Fe-0.96 mass pct C alloy with a hardness value of 810 HV has been electrochemically synthesized from a ferrous sulfate bath containing a small amount of citric acid and L-ascorbic acid The nature of the alloy has been investigated by a number of techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), M?ssbauer spectroscopy, differential scanning calorimetry (DSC), and magnetic measurements. The decomposition behavior of the alloy is also studied and compared with that of thermally prepared martensite. It has been found that the electrochemically deposited Fe-C alloy exists in a state that is ahead of the freshly quenched state of martensite. It is suggested that the state of the electrochemically deposited Fe-0.96 mass pct C alloy corresponds to the state of thermal martensite, which had been heated to stage I of tempering.     

The formation of metastable phases by mechanical alloying in the aluminum and copper system

A wide composition range of aluminum-copper alloys has been prepared by mechanical alloying (MA). The microstructure and phase formation have been investigated by X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy. The primary grain size of the powders subjected to ball milling for the aluminum-rich compositions is less than 30 nm. The solid solubility of coppe in aluminum can reach 2.7 at. pct Cu and that of aluminum in copper can reach 18.0 at. pct Al. Over a wide composition range, formation of a nonequilibrium distorted body-centered cubic (bcc) phase which possesses some ordering has been confirmed.     

Precipitation of austenite particles at grain boundaries during aging of Fe-Mn-Ni steel

Precipitation of austenite particles at grain and lath boundaries after aging treatment of a Fe-8Mn-7Ni alloy was investigated by selected area electron diffraction (SAD), X-ray energy dispersive spectrometry (EDS) in a scanning transmission electron microscope (STEM), and high-resolution (HRTEM) analysis. High spatial-resolution (2 to 5 nm) EDS analysis revealed no significant segregation of alloying elements at grain boundaries but the precipitation of very fine particles of Mn- and Ni-rich phase. Detailed EDS, SAD, and HRTEM analyses all confirmed that these particles are austenite phase, which have a Kurdjumov-Sachs (K-S) orientation relationship with one of the adjacent grain. The concentration of Mn and Ni in austenite, measured by EDS, varied from ∼15 pct to a maximum of ∼30 pct. Low-voltage scanning electron microscopy (SEM) fractographs also revealed the presence of very fine, second-phase precipitates on the fracture surface of the embrittled alloys.     

Retained austenite and tempered martensite embrittlement in medium carbon steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M₃C filM_s upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe₃C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions.     

Study of Aging-Induced Degradation of Fracture Resistance of Alloy 617 Toward High-Temperature Applications

For the Alloy 617, the effect of aging on the fracture energy degradation has been investigated after aging for different time periods at 1023 K (750 °C). A sharp reduction in impact energy (by ~55 pct vis-à-vis the as-received material) after 1000 hours of aging, as evaluated from room-temperature Charpy impact tests, has been observed. Further aging up to 10,000 hours has led to a degradation of fracture energy up to ~78 pct. Fractographic examinations using scanning electron microscopy (SEM) have revealed a change in fracture mode from fibrous-ductile for the un-aged material to intergranular mode for the aged one. The extent of intergranular fracture increases with the increasing aging time, indicating a tendency of the material to undergo grain boundary embrittlement over long-term aging. Analysis of the transmission electron microscopy (TEM) micrographs along with selected area diffraction (SAD) patterns for the samples aged at 10,000 hours revealed finely dispersed γ′ precipitates of size 30 to 40 nm, rich in Al and Ti, along with extensive precipitation of M₂₃C₆ at the grain boundaries. In addition, the presence of Ni₃Si of size in the range of 110 to 120 nm also has been noticed. The extensive precipitation of M₂₃C₆ at the grain boundaries have been considered as a major reason for aging-induced embrittlement of this material.     

A study of the early stages of tempering in an Fe-1.2 Pct alloy

Mössbauer effect spectroscopy has been used to study changes in the microstructure of an Fe-1.22. wt pct C alloy due to tempering between 373 and 523 K. The orthorhombic transition carbide, η-Fe₂C, was identified by transmission electron microscopy and the similarity of ∈-carbide electron diffraction patterns to η-carbide diffraction patterns is noted. Systematic changes in the Mosbauer parameters of martensite and austenite are presented for the various stages of tempering. The same amount of C remains randomly dissolved in the retained austenite throughout tempering and some C is retained in the martensite throughout the range of transition carbide formation. Two sets of Mössbauer parameters corresponding to magnetic phases other than martensite and cementite have been found. These parameters may come from η-carbide, but alternative interpretations are presented.     

Microstructure and microsegregation effects in the intergranular corrosion of austenitic stainless steel

Intergranular corrosion in austenitic stainless steel was studied using transmission electron microscopy of corroded thin foils and electron probe microanalysis of bulk specimens. In the sensitized material, since carbides remained unattacked in corroded grain boundaries after exposure to a boiling copper sulfate-sulfuric acid solution, the location and severity of corrosion could be directly observed in relation to individual carbide particles for various precipitate morphologies. After the sensitized material was exposed to a potassium dichromate nitric acid solution, carbides were consistently absent from corroded grain boundaries as the particles themselves apparently became susceptible to attack in this environment. Chemical composition inhomogeneities were measured for nickel and chromium in the commercially annealed material and found to become more pronounced upon sensitization heat treatments. Such inhomogeneities can result in chemical composition differences across grain boundaries, which in turn can lead to electrochemical action that may adversely affect intergranular corrosion behavior. N. C. BARBI, formerly Graduate Student, Rensselaer Polytechnic Institute, Troy, N. Y. 12181     

Prevention of the intergranular fracture by addition of silicon and aluminum to a high-purity Fe-0.2 Pct P alloy with a trace of boron

The effect of addition of 0.25 pct silicon and aluminum on the intergranular fracture and the grain-boundary segregation of solutes in a high-purity Fe-0.2 pct P alloy with a trace of boron has been investigated by impact test, tensile test at low temperatures, optical and scanning electron microscopy, and Auger electron spectroscopy (AES). The addition of 0.25 pct silicon and aluminum remarkably reduces the susceptibility of an Fe-0.2 pct P alloy to the intergranular fracture and decreases the ductile-brittle transition temperature (DBTT). The addition of silicon and aluminum causes considerable segregation of boron to grain boundaries and, simulta-neously, remarkably decreases the segregation of phosphorus in the Fe-0.2 pct P alloy. The cause for these effects is discussed. Formerly Professor, Institute for Materials Research, Tohoku University