The aging behavior of homogenized type 308 and 308CRE stainless steel

The aging behavior of type 308 stainless steel and type 308 stainless steel with controlled residual elements (308CRE) was studied. The alloys were initially homogenized, resulting in a fully austenitic structure in the type 308 alloy and a duplex, austenite plus ferrite structure in the type 308CRE alloy. The materials were subsequently aged at 550, 650, 750, and 850 °C for times up to 10,000 hours. Aging of the type 308 steel resulted in precipitation of chromium-rich M₂₃C₆, primarily along grain boundaries, followed by sigma-phase formation after long aging times (≥5000 hours). The aging response of the duplex type 3O8CRE steel was somewhat more complicated. Although the titaniumrich carbides, nitrides, and sulfides produced during homogenization remained stable during subsequent aging, some additional precipitation was found. G-phase formed within the residual ferrite but eventually redissolved. The ferrite partially dissolved during early aging and later transformed to sigma phase. The sigma-phase formed significantly faster in the aged type 308CRE alloy. The results are summarized in the form of TTT diagrams. A comparison is made with a similar aging study on the same alloys but starting with the as-welded condition.     

Effect of long-term service exposure at elevated temperature on microstructural changes of 5Cr-0.5Mo steels

Effects of long-term service exposure at elevated temperature on microstructural changes have been studied for both virgin and service-exposed process heater tube pipes of 5Cr-0.5Mo steels used in oil refineries. Samples selected for this study had experienced a nominal temperature range of 450 °C to 500 °C for about 20 to 25 years. Two different initial virgin microstructures were taken and designated by steel A and steel B. The virgin microstructure of steel A exhibited fine platelets of fibrous or hairlike M₂C carbides within the ferrite grains and occasionally irregularly shaped M₂₃C₆, both along the grain boundaries and at the grain interiors, and very few spheroidally shaped M₃C, either along the grain boundaries or at the grain interiors. The size, shape, position, distribution, and type of carbides in virgin steel A changed significantly due to 220,000 hours of service exposure in the temperature range of 450 °C to 500 °C. Massive M₂₃C₆ carbides precipitated along the grain boundaries. In addition, regular geometrically shaped M₂₃C₆ carbides, such as hexagonal, square, and triangular type, were observed to form at the grain interiors. The virgin steel B microstructure exhibited predominantly M₂₃C₆ carbides, either along the grain boundaries or at the lath boundaries. Occasionally, fine platelets of M₂C carbides were also observed within the laths. The position, shape, distribution, and type of carbides did not change significantly due to 172,000 hours of service exposure in the temperature range of 450 °C to 500 °C. The average interparticle spacings of the carbides increased from 0.35 to 1.2 μm due to 172,000 hours of exposure.     

Evolution of Precipitates of S31042 Heat Resistant Steel During 700 °C Aging

The evolution of precipitates of S31042 steel during 700 °C aging was investigated by using a scanning electron microscope, a transmission electron microscope, and electron energy spectrum technology. The various combinations of M₂₃C₆, MX, NbCrN, and σ and G phases in the steel were found at different aging states. In the beginning of aging, M₂₃C₆ precipitates swiftly along the grain boundaries. When the aging time exceeds 6000 h, precipitated M₂₃C₆ carbides along the grain boundaries turn to be granular. It was found that Si element segregates to grain boundaries during above process, which may enhance the granular shape of M₂₃C₆ carbides and its transformation to σ and G phases. When the aging time exceeds 10000 h, various shaped σ phase and granular G phase appear along the grain boundaries and there are no continuous M₂₃C₆ carbides along the grain boundaries. Meanwhile, a large quantity of granular M₂₃C₆ carbides and a minor amount of G phase precipitate near the grain boundaries. Based on the segregation of silicon to the grain boundaries, a precipitation evolution model during aging was concluded.     

Study of Property Degradation of T23 Heat‐resistant Steel based on Microstructural Evolution during Creep

In order to study the microstructural evolution and the effect on property degradation of T23 heat‐resistance steel (2.25Cr‐1.6W‐V‐Nb‐B‐N) during creep, creep rupture specimens were investigated at 823K, 873K and 923K. The microstuctural evolution was examined by optical, scanning and transmission electron microscopy. It has been noted that the creep property degradation of T23 is related to the decrease of dislocation density due to the recovery and recrystallization of the bainitic ferrite matrix and the martensite in the carbon‐rich islands, the coarsening of M₂₃C₆ carbides, and even the transformation from M₂₃C₆ to M₆C. Coarsening of M₂₃C₆ is the dominating effect during short‐term creep whereas recovery and recrystallization is the key factor for long‐term creep. Property degradation is advanced at higher temperature due to the quicker recovery and recrystallization.     

Creep inhibition at grain boundaries in a stainless steel

The inhibition of creep in a vanadium alloyed austenitic stainless steel has been found to rely upon heterogeneous precipitation of vanadium nitride both on matrix and grain bound-ary dislocations together with the formation of M₂₃C₆ grain boundary carbides. The effect of these precipitation reactions on the separate stages of the creep test has been investi-gated microstructurally, particularly with reference to the processes occurring at grain boundaries. Dislocation configurations in grain boundaries, especially in the region of grain boundary carbides are modified by the presence of vanadium nitride precipitates. It is suggested that the roles of M₂₃C₆ and vanadium nitride precipitation during creep of this steel are interrelated and complimentary in reducing creep rates and extending creep ductility.     

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.     

Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel

An analytical electron microscopy study was undertaken in order to characterize intergranular and matrix precipitation accompanying intermediate temperature aging in NITRONIC 50, a nitrogen-strengthened austenitic stainless steel. Extensive precipitation on most grain boundaries had occurred after aging for 24 hours at 675 °C. The primary intergranular phase at that time was Cr-rich M₂₃C₆, and energy dispersive spectra taken on grain boundary segments between these carbides indicated Cr-depletion and Fe- and Ni-enhancement relative to the matrix. After aging for 336 and 1008 hours at 675 °C, M₆C (eta-carbide) precipitates were also present on grain boundaries. These precipitates were distinguished from M₂₃C₆ on the basis of their lattice parameters and chemistries, with M₆C containing less Cr and Fe, and more Ni, Mo, and Si than M₂₃C₆. The differences in chemistry were clarified by a statistical treatment of the spectra. The statistical analysis also showed that precipitates with a range of chemistries between M₂₃C₆ and M₆C coexisted with these phases on the grain boundaries. Associated with this shift in precipitate stoichiometry was an increase in the average concentration of Cr and a decrease in the average concentration of Ni at the grain boundaries. Intergranular sigma phase was also observed after times 24 hours at 675 °C, with sigma precipitating on grain boundaries containing carbides. Intragranular precipitates observed to be stable up to 1008 hours at 675 °C included Z-phase, a complex nitride which had formed during solution annealing; M₇C₃ carbides, which nucleated at Z-phase/austenite interfaces; M₂₃C₆ carbides, which precipitated on incoherent twin boundaries; and Cr-rich MN precipitates, which nucleated on dislocations.     

Role of Tungsten in the Tempered Martensite Embrittlement of a Modified 9 Pct Cr Steel

The effect of tempering on the mechanical properties and fracture behavior of two 3 pct Co-modified 9 pct Cr steels with 2 and 3 wt pct W was examined. Both steels were ductile in tension tests and tough under impact tests in high-temperature tempered conditions. At T ≤ 923 K (650 °C), the addition of 1 wt pct W led to low toughness and pronounced embrittlement. The 9Cr2W steel was tough after low-temperature tempering up to 723 K (450 °C). At 798 K (525 °C), the decomposition of retained austenite induced the formation of discontinuous and continuous films of M₂₃C₆ carbides along boundaries in the 9Cr2W and the 9Cr3W steels, respectively, which led to tempered martensite embrittlement (TME). In the 9Cr2W steel, the discontinuous boundary films played a role of crack initiation sites, and the absorption energy was 24 J cm^?2. In the 9Cr3W steel, continuous films provided a fracture path along the boundaries of prior austenite grains (PAG) and interlath boundaries in addition that caused the drop of impact energy to 6 J cm^?2. Tempering at 1023 K (750 °C) completely eliminated TME by spheroidization and the growth of M₂₃C₆ carbides, and both steels exhibited high values of adsorbed energy of ≥230 J cm^?2. The addition of 1 wt pct W extended the temperature domain of TME up to 923 K (650 °C) through the formation of W segregations at boundaries that hindered the spheroidization of M₂₃C₆ carbides.     

Effect of carbon concentration on precipitation behavior of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M₂₃C₆ carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M₂₃C₆ carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M₂₃C₆ carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M₂₃C₆ carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.     

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     

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M₂₃C₆ type carbides, having high chromium content, and M₆C type carbides, having high molybdenum content, co-exist on the grain boundaries. M₂₃C₆ type carbides, however, are quantitatively predominant. Furthermore, M₆C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.     

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M₇C₃ carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M₂C carbides precipitate separately within the bainitic laths during slow heating. M₂₃C₆ carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M₇C₃ and, depending on the temperature, coarsening, or dissolution of M₂C carbides, respectively.     

Instabilities in stabilized austenitic stainless steels

The effect of aging on the precipitation of grain boundary phases in three austenitic stainless steels (AISI 347, 347AP, and an experimental steel stabilized with hafnium) was investigated. Aging was performed both on bulk steels as well as on samples which were subjected to a thermal treatment to simulate the coarse grain region of the heat affected zone (HAZ) during welding. Aging of the bulk steels at 866 K for 8000 hours resulted in the precipitation of Cr₂₃C₆ carbides, σ, and Fe₂Nb phases; the propensity for precipitation was least for the hafnium-stabilized steel. Weld simulation of the HAZ resulted in dissolution of the phases present in the as-received 347 and 347AP steels, leading to grain coarsening. Subsequent aging caused extensive grain boundary Cr₂₃C₆ carbides and inhomogeneous matrix precipitation. In addition, steel 347AP formed a precipitate free zone (PFZ) along the grain boundaries. The steel containing hafnium showed the best microstructural stability to aging and welding. Formerly with Exxon Research and Engineering Company.     

Mechanism of the formation of lamellar M23C6 at and near twin boundaries in austenitic stainless steels

Precipitation characteristics of M₂₃C₆ during aging at 800 °C were studied by transmission electron microscopes in two austenitic stainless steels, A (type AISI 316L) and B (type DIN 4981), which had been quenched in water after their solution treatment at high temperatures (1150 °C and 1275 °C, respectively). After precipitation at grain boundaries, M₂₃C₆ precipitated at incoherent and coherent boundaries of twins and inside austenite grains. Close to an incoherent twin boundary on either side of the boundary, M₂₃C₆ mostly grew as elongated plates, although elsewhere in austenite matrix these grew usually as equiaxed particles. The plates lied on planes parallel to the twinning plane and were aligned unidirectionally along the axis of intersection of the twinning plane and a {110} plane perpendicular to the twinning plane. On coherent twin boundaries, similar plates of M₂₃C₆ formed along with some equiaxed particles. Existing models for the mechanism of formation of these lamellar carbides fail to explain the observed features of these carbides formed around different twins. It is suggested that the residual stress localized in the vicinity of twin boundaries in quenched specimens influences the nucleation and growth of these carbides during aging, resulting in their specific morphology.     

Microstructure Evolution and Pinning of Boundaries by Precipitates in a 9 pct Cr Heat Resistant Steel During Creep

Structural changes in a 9 pct Cr martensitic steel during a creep test at 923 K (720 °C) under the applied stress of 118 MPa were examined. The tempered martensite lath structure (TMLS) was characterized by M₂₃C₆-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm. The M₂₃C₆ particles were located on the packet/block/lath boundaries, whereas the MX precipitates were distributed homogeneously throughout TMLS. TMLS in the grip portion of the crept specimen changed scarcely during the tests. In contrast, the structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. The formation of a well-defined subgrain structure in the gauge section was accompanied by the coarsening of M₂₃C₆ carbides and precipitations of Laves phase during creep. The most pronounced structural changes occurred just at the beginning of the tertiary creep regime, which was interpreted as a result of the change in the mechanism of grain boundary pinning by precipitates.     

On the Solidification of a H23 Tool Steel

The H23 tool steel contains high concentration of carbide forming elements, which affect the microstructure and mechanical properties. This present study described the microstructure and mechanical properties of the as cast H23 tool steel. The steel was prepared by vacuum induction melting. The microstructural investigation used XRD and electron microscope. The nano hardness and elastic moduli of matrix and carbide were also measured. The results show that the as cast microstructure consisted of ferrite matrix and M₆C, MC and M₂₃C₆ carbides. The eutectic M₆C carbides had two different morphologies owing to different growth mechanisms. There was agreement between the experimental results and the calculated solidification path for the H23 tool steel regarding the presence of carbides in the microstructure. The nanohardness and elastic moduli of ferrite matrix and M₆C carbides were respectively 4.2 ± 0.2 and 10.6 ± 1.2 and 198.3 ± 10.2 and 253.5 ± 11.7 GPa.     

Effect of Si on δ-Ferrite/γ-Austenite Transformation and M23C6 Precipitation During Solidification of X10CrAlSi18 Ferritic Heat-Resistant Stainless Steel

Herein, the δ-ferrite/γ-austenite transformation and the precipitation behavior of M₂₃C₆ carbides in X10CrAlSi18 ferritic heat-resistant stainless steel (FHSS) with various Si contents at a cooling rate of 100 °C min⁻¹ using confocal scanning laser microscopy (CSLM) are investigated. The findings reveal that γ-austenite preferentially forms along the δ-ferrite phase boundaries, and it progressively precipitates into the δ-ferrite phase as the temperature decreases. The increase in the Si content reduces the δ-ferrite/γ-austenite transformation temperature. It also inhibits the martensite transformation in the subsequent cooling process, decreasing the volume fraction of γ-austenite/martensite. M₂₃C₆ carbides are mostly found at the δ-ferrite and γ-austenite/martensite phase boundaries. Meanwhile, the nucleation of M₂₃C₆ carbides becomes more difficult as the volume fraction of γ-austenite/martensite decreases. Furthermore, the complex solidification mechanism of the nucleus is addressed.     

Extended X‐Ray Absorption Fine Structure Investigation of Carbon Stabilized Expanded Austenite and Carbides in Stainless Steel AISI 316

Low temperature carburized AISI 316 stainless steel ‐ carbon expanded austenite ‐ was investigated with EXAFS and synchrotron diffraction together with synthesized carbides of the type M₃C₂, M₇C₃ and M₂₃C₆. It was found that the chemical environment of carbon expanded austenite is not associated with any of the investigated carbides, that carbon has a strong affinity for chromium, i.e. short range order, and that carbon is in solid solution.     

Carbides in M-50 high speed steel

The carbides in M-50 high speed tool steel were studied in detail. The dissolution of carbides as a function of austenitizing temperature, and their precipitation as a function of tempering temperature were characterized by X-ray diffraction and microchemical analysis. The carbides in the annealed steel are M₂₃C₆, M₆C, M₂C, and MC. Upon austenitizing, with increasing temperatures, the carbides dissolve in the order: M₂₃C₆, metastable M₂C, M₆C, and MC. The residual carbides in the heat treated steel are MC and stable M₂C. The solvus temperatures of M₂₃C₆ and M₆C were determined. Upon tempering the hardened steel, with increasing tempering temperatures, carbides precipitate in the order: M₂₃C₆, metastable M₂C, MC, and M₆C. It is shown that the composition of the precipitated metastable M₂C is different from that of the residual stable M₂C and it varies with the tempering temperature.