Carbide Evolution in Temper Embrittled NiCrMoV Bainitic Steel

Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement.     

NiCrMoV钢中铌和锡的非平衡晶界共偏聚

 结合偏聚理论的最新进展,对R.D.K.Misra等认为Ni和Sn的晶界偏聚动力学曲线相互平行,而且2种元素可同时达到一个晶界浓度极大值的实验结果重新进行了分析。分析结果证实,Ni和Sn在恒温过程中发生的是一非平衡晶界共偏聚现象,Ni的非平衡偏聚是由Sn的非平衡偏聚引起的。     

Carbide precipitation,grain boundary segregation,and temper embrittlement in NiCrMoV rotor steels

This paper presents a study of carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor steels. One of the steels was high purity, one was doped with phosphorus, one was doped with tin, and one was commercial purity. In addition, two NiCrV steels, one high purity and one doped with phosphorus, were examined. Carbide precipitation was studied with analytical electron microscopy. It was found that after one hour of tempering at 600 ‡C only M₃C carbides were precipitated in the NiCrMoV steels. These were very rich in iron. As the tempering time increased, the chromium content of the M₃C carbides increased significantly, but their size did not change. Chromium rich M₇C₃ precipitates began to form after 20 hours of tempering, and after 50 hours of tempering Mo-rich M₂C carbides were precipitated. Also, after 100 hours of tempering, the matrix formed bands rich in M₃C or M₇C₃ and M₂C particles. Tempering occurred more rapidly in the NiCrV steels. Grain boundary segregation was studied with Auger electron spectroscopy. It was found that the amount of phosphorus and tin segregation that occurred during a step-cooling heat treatment after tempering was less if a short time tempering treatment had been used. It will be proposed that this result occurs because the low temperature tempering treatments leave more carbon in the matrix. Carbon then compctes with phosphorus and tin for sites at grain boundaries. This compctition appears to affect phosphorus segregation more than tin segregation. In addition to these two impurity elements, molybdenum and nickel segregated during low temperature aging. The presence of molybdenum in the steel did not appear to affect phosphorus segregation. Finally, it will be shown that all of the steels that contain phosphorus and/or tin exhibit some degree of temper embrittlement when they are aged at 520 ‡C or are given a step-cooling heat treatment. Of the NiCrMoV steels, the phosphorus-doped steel showed the least embrittlement and the commercial purity steel the most. The phosphorus-doped NiCrV steel was also more susceptible to temper embrittlement than the phosphorus-doped NiCrMoV steel. This latter difference was attributed to molybdenum improving grain boundary cohesion. It was also found that as the segregation of phosphorus or tin to the grain boundaries increased, the measured embrittlement and the amount of intergranular fracture increased. However, there was a large amount of scatter in all of these data and the trends were only qualitative. All parts of this study are compared in detail to others in the literature, and general trends that can be discerned from all of these results are presented. Formerly with the University of Pennsylvania, Department of Materials Science, Philadelphia, PA     

Quench embrittlement of hardened 5160 steel as a function of austenitizing temperature

Charpy V-notch (CVN) specimens from experimental heats of 5160 steel containing 0.001 and 0.034 mass pct phosphorus were austenitized at temperatures between 830 °C and 1100 °C, quenched to martensite, and tempered at temperatures between 100 °C and 500 °C. Scanning electron microscopy (SEM) was used to characterize the fracture surfaces of tested CVN specimens and carbide formation on prior austenite grain boundaries. Quench embrittlement, the susceptibility to intergranular fracture in as-quenched and low-temperature tempered high-carbon steels due to cementite formation as affected by phosphorus segregation on austenite grain boundaries, developed readily in specimens of the high phosphorus steel austenitized at all temperatures. The low phosphorus steel developed quench embrittlement only after austenitizing at 1100 °C. Intergranular fractures correlated with low room-temperature CVN impact toughness. The results are discussed with respect to the dissolution of carbides during austenitizing and the effect of phosphorus on grain boundary, carbide formation, and stability.     

Correlation of microstructure and tempered martensite embrittlement in two 4340 steels

This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K _Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation fields ahead of cracks, suggesting thatK _Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels studied can be explained quantitatively using different fracture models.     

Crack growth from internal hydrogen—temperature and microstructural effects in 4340 steel

Internal hydrogen effects on stage II crack growth rates in AISI 4340 steel have been studied as a function of test temperature. A model is developed that is physically based in that classical thermodynamics relates to solubility and trapping and Fick’s second law controls hydrogen transport. Both of these are microstructurally related to how trapping affects both the crack initiation site and diffusion to it. For two tempered conditions of 4340 steel, it is shown that there is a test temperature,T ₀, for stage II crack growth, above which the crack does not grow. The fractography associated with test temperatures approachingT ₀ tends toward 100 pct intergranular for both 1340 MPa and 1620 MPa strength levels. At lower test temperatures, there is as much as 50 pct microvoid coalescence or 30 pct quasi-cleavage. In the lower strength condition, hydrogen traps at oxysulfide particles with a binding energy near 75 kJ/mol. Where these intersect the prior austenite grain boundaries, this promotes fingers of intergranular fracture which later triggers tearing of 100 μm size ligaments by microvoid coalescence. For the higher strength material, it is proposed that hydrogen traps along martensite lath intersections with prior austenite grain boundaries, the binding energy being near 27 kJ/mol. This promotes 1 μm size striations along intergranular facets. In both cases the fractography is consistent with a proposed model of stress field concentration of hydrogen, further concentration along trap sites, fracture nucleation at trap sites, and local, discontinuous fracture instabilities.     

Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

The microstructure,morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5 Mo stainless steel during the tempering process were analyzed using optical microscope(OM),transmission electron microscope(TEM),X-ray diffraction(XRD)and scanning transmission electron microscope(STEM).The results show that the microstructure of the tempered steel is mainly composed of tempered martensite,retained austenite,and delta ferrite.In the case of samples tempered from 500 to 700 ℃,the precipitates are mainly M_(23)C_6,which precipitate along the lath martensite boundaries.The precipitate content increases with the tempering temperature.During the tempering process,the content of retained austenite initially increases and then decreases,the maximum content of retained austenite being 29 vol.% upon tempering at 600 ℃.TEM analysis of the tested steel reveals two morphology types of retained austenite.One is thin film-like retained austenite that exists along the martensite lath boundary.The other is blocky austenite located on packet at the boundary and the original austenite grain boundary.To further understand the stability of reversed austenite,the Ni content in reversed austenite was measured using STEM.Results show a significant difference in nickel concentrations between reversed austenite and martensite.     

Texture,Grain Boundary Characterization and Segregation of Phosphorus in an Annealed Interstitial Free Steel

The {111} fiND plane texture, grain boundary characterization and grain boundary segregation of phos- phorus are investigated for the phosphorus added high strength interstitial free （IF） steel annealed at 810℃ for 60 s to 180 s. The results show that the maximum volume fraction of {111} fiND plane texture is about 85% and the grain boundary Segregation peak of phosphorus is about 14 at. % for the steel annealed at 810℃ for 120 s. The ∑3 and other low-∑CSL （coincident site lattice） boundaries are lowest for the steel annealed for 120 s. Segregation of phosphorus is also found at low angle grain boundaries.     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

Aging embrittlement and grain boundary

“Clean” 3.5NiCrMoV steels with limited contents in trace elements (P, Sn, As, Sb) are commonly provided for manufacturing big rotor shafts. The possible increase in temperature in future steam turbines has promoted the development of “superclean” steels characterized by an extra drastic decrease of manganese and silicon contents. Their higher cost in comparison to “clean” steels leads to concern above which temperature they must be considered as mandatory for resisting aging embrittlement in operation. 3.5NiCrMoV “clean” steel samples (Mn = 0.30 pct; Si = 0.10 pct) were aged at 300 °C, 350 °C, and 400 °C for 10,000 hours up to 30,000 hours. No embrittlement results from aging at 300 °C and 350 °C, but holding at 400 °C is highly detrimental. Auger spectroscopy confirms that, when aging at 400 °C, phosphorus is the main embrittling trace element. It is suggested that grain boundary embrittlement is associated with the building of a layer that contains, on the one hand, Ni and P and, on the other hand, Mo and Cr. Head of the Testing and Head of the Testing and Head of the Testing and     

Tempered martensite embrittlement and intergranular fracture in an ultra-high strength sulfur doped steel

This paper reports a study of tempered martensite embrittlement in a Ni-Cr steel doped with 0.01 wt pct S. The segregation of sulfur to the grain boundaries and the associated embrittlement of this material are very dependent upon the austenitizing temperature. If the austenitizing temperature is below 1050 °C very little embrittlement and very little intergranular fracture are observed because sulfur remains precipitated as chromium sulfide. At higher austenitizing temperatures the sulfides dissolve and sulfur segregates to the grain boundaries. Because of the high bulk content, the sulfur concentration at the grain boundaries becomes great enough for the sulfides to reprecipitate there. This leads to low energy intergranular ductile fracture. However, some sulfur remains unprecipitated at the boundary and can lower the cohesive strength across the boundary. When plate-like cementite precipitates at the grain boundary during tempering heat treatments at 300 to 400 °C, the combination of the carbides and the unprecipitated sulfur causes intergranular fracture and tempered martensite embrittlement.     

Kinetics of phosphorus segregation in 2.7Cr-0.7Mo-0.3V steels with different phosphorus contents

The phosphorus grain boundary segregation kinetics during tempering at 680°C and aging at 500°C of 2.7Cr-0.7Mo-0.3V steels with phosphorus mass contents of 0.004, 0.014, and 0.027 % was investigated. To determine the grain boundary concentrations of phosphorus the Auger electron spectroscopy was used. Chemical compositions of carbide particles were determined by means of EDX/STEM. Xu Tingdong's and McLean's models of non-equilibrium and equilibrium segregations, respectively, were used to analyze experimental data. It was shown that a phosphorus grain boundary enrichment during tempering was mainly caused by non-equilibrium segregation. During aging the mechanism of the equilibrium grain boundary segregation was prevalent. Slow phosphorus segregation kinetics was observed in the experimental steels during aging.     

The micro-mechanisms of tempered martensite embrittlement in 4340-type steels

A study of the micro-mechanisms of tempered martensite embrittlement was made on a series of 4340-type steels in which the contents of manganese, silicon, and trace impurities, especially phosphorus and sulfur, were varied. One plain-carbon steel was also examined. The study employed Charpy impact tests and four-point slow-bend tests coupled with an elastic-plastic stress analysis, as well as scanning electron fractography, Auger electron spectroscopy, transmission electron microscopy of extraction replicas, and magnetic measurements of the transformation of retained austenite. The results indicate that in these steels the TME phenomenon is an intergranular embrittlement problem caused by carbide precipitation on prior austenite grain boundaries which are already weakened by segregated phosphorus and sulfur. The transformation of intragranular retained austenite is concluded not to be of primary significance in the TME in these steels, although it may contribute to the magnitude of the TME toughness trough.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel—IV. Precipitation behaviour and distribution of elements at grain boundaries

The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and “Mo-free AISI 316L” type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29–530°C/s from 800, 1075 or 1250°C. Tetragonal M₂B, M₅B₃ and M₃B₂, all rich in Fe, Cr and Mo, precipitated in the “high B” (40 ppm) AISI 316L steel whereas orthorhombic M₂B, rich in Cr and Fe, was found in the “Mo-free steel” with 23 ppm B. In the “high B steel” a thin (<2 nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M₉B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field. “Non-equilibrium phases” might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed.     

磷元素在钢中的晶界偏聚

磷元素在钢中的晶界偏聚可以分为两类,即平衡偏聚和非平衡偏聚.介绍了这两类偏聚现象以及偏聚理论.以磷对晶界结合力、晶界扩散和晶界能的影响机理为基础,分析了磷元素对钢力学性能的影响和磷在钢中发生晶界偏聚的原因.     

Caustic stress corrosion cracking of NiCrMoV rotor steels—The effects of impurity segregation and variation in alloy composition

This paper reports a study of the effects of phosphorus, tin, and molybdenum on the caustic stress corrosion cracking susceptibility of NiCrMoV rotor steels. Constant load tests were performed on these steels in 9M NaOH at 98 ± 1 °C at a controlled potential of either -800 mV_Hg/Hgo or -400 mV_Hg/Hgo. Times to failure were measured. The results show that at a potential of -400 mV_Hg/Hgo the segregation of phosphorus to grain boundaries lowers the resistance of these steels to caustic stress corrosion cracking. When molybdenum is removed from a steel that has phosphorus segregated to the grain boundaries, the steel’s resistance to stress corrosion cracking is improved. High purity alloys, both with and without molybdenum, show very good resistance to caustic cracking at this potential. At-800 mV_Hg/Hgo segregated phophorus has no effect; only molybdenum additions lower the resistance of the steel to caustic stress corrosion cracking. Segregated tin has little effect at either potential. Metallographic examination shows that one explanation for these results is that molybdenum and phosphorus, probably as anions precipitated from solution, aid in passivating the sides of the crack and thus help keep the crack tip sharp. This sharpness will increase the speed with which the crack will propagate through the sample. Furthermore, removal of molybdenum greatly increases the number of cracks which nucleate. This higher crack density would increase the relative area of the anode to the cathode and thus act to decrease the crack growth rate. Formerly with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA.     

Nonequilibrium grain-boundary cosegregation of nitrogen and chromium in NiCrMoV steel

It is concluded in this article that nonequilibrium grain-boundary cosegregation (NCGS) of nitrogen and chromium occurs in NiCrMoV steel. That conclusion is reached from experimental observations of the parallel segregation isotherms and the maximum coverage of Cr and N at grain boundaries during the isotherms. This means that the nonequilibrium segregation of Cr induces that of N, in NiCrMoV steel.