Effect of extrinsic grain-boundary dislocations on M23C6 precipitate nucleation in an austenitic stainless steel

From a comparison of the isothermal precipitation curve for M₂₃C₆ grain-boundary precipitates in an austenitic stainless steel and of the spreading times of extrinsic grain-boundary dislocations (EGBDs), it has been shown that M₂₃C₆ precipitates cannot directly nucleate on EGBD lines lying on random high-angle grain boundaries. This process can occur only on coherent and incoherent twin and other special boundaries.     

Effect of thermal exposure on the stability of carbides in Ni-Cr-W based superalloy

The microstructure evolutions of Ni-Cr-W based superalloy during thermal exposure have been investigated systematically. M₆C carbides in the alloy decompose into M₂₃C₆ carbides at temperatures from 650 to 1000 °C due to its high content of Cr. The M₆C carbides decompose dramatically from 800 to 900 °C. At temperatures up to 1000 °C, a few M₂₃C₆ carbides form on the surface of M₆C carbides. The decomposition behavior of primary M₆C can be explained by the following reaction: M₆C → M₂₃C₆ + Me (W, Ni, Cr, Mo). At temperatures below 900 °C, coarse lamellar M₂₃C₆ carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000 °C, discrete M₂₃C₆ carbides precipitate at the grain boundaries. Moreover, there are lots of small M₂₃C₆ particles precipitated around M₆C carbides from 650 to 1000 °C.     

Precipitation and thermal instability of M23C6 carbide in cast Ni-base superalloy K452

During long-term thermal exposure M₂₃C₆ dendritically precipitates from the supersaturated γ matrix and then changes in shape from flower-like dendrite to irregular block to regular polyhedron. The reduction in the interfacial energy is responsible for the morphological transition, but the stable morphology of the carbide is principally determined by the anisotropy of the interfacial energy between the carbide and the γ' in the M₂₃C₆-γ' cell. In addition, M₂₃C₆ has a negligible influence on the tensile property of the thermally exposed alloy even though it can block the mobile dislocations and stacking faults to some extent.     

On precipitation in an Fe-Cr-Ni-Mn-Mo-W-V-Nb-N alloy,following prolonged ageing at 650°C

The main existing intergranular phase in the alloy observed is thick lamellar and small plate-like M₆C. Owing to the presence of M₆C precipitates, intergranular M₂₃C₆ nucleates at these locations and coherently precipitates to form a thin lamellar and large-sized filmy single crystal. The misfit of M₂₃C₆ with M₆C obtained from the moiré pattern is 3%. Intragranular M₆C and M₂₃C₆ twin along their own planes (111), and the resulting twins keep a coherent relationship with austenite. The misfit of VC precipitated along dislocations with the matrix is smaller than that of dispersed VC with the matrix. Intragranular Laves phase together with intragranular M₆C and VC strengthens the alloy grains.     

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M₇Ca₃ carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M₂₃C₆ precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M₂₃C₆ and M₇C₃ secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.     

Formation of the reversed austenite during intercritical tempering in a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Formation of the reversed austenite obtained by intercritical tempering has been studied via transmission electron microscopy (TEM) in a Fe-13%Cr-4%Ni-Mo low carbon martensitic stainless steel. It is found that the precipitation of M₂₃C₆ carbides along the martensite lath boundaries will result in Ni-enrichment in the adjacent region. The reversed austenite forms with the Ni-enrichment region as the nucleation sites, keeps a cube-cube orientation relationship with the M₂₃C₆ carbides and bears the Kurdjumov-Sachs (K-S) relationship with the martensite. Moreover, the reversed austenite formed inside the martensite laths is also confirmed. The mechanism for formation of the reversed austenite is discussed in detail.     

The relationship between sensitisation and small punch test behaviour in a high-nitrogen austenitic stainless steel at cryogenic temperatures

Austenitic stainless components used in nuclear fusion reactors must be capable of maintaining reasonable mechanical properties to thermal ageing caused by welding and in‐service. Recently, high‐nitrogen (High‐N) austenitic stainless steels (SS) are receiving increased attention because of their strength advantages, but they have been found to be susceptible to dichromium nitride (Cr₂N) precipitation during thermal exposure at 823–1073 K. The susceptibility to sensitisation at thermal ageing temperature for high‐N austenitic SS is examined using the single‐loop electrochemical potentiokinetic reactivation (EPR) test. High‐N SS were found to be susceptible to sensitisation caused by grain boundary precipitation of Cr₂N, with the degree of sensitisation increasing systematically with ageing time and temperature. In particular, it was found that the precipitates, which effected sensitisation, were changed from carbides (M₂₃C₆) to nitrides (Cr₂N) with increasing ageing time and temperature. The deterioration of mechanical properties associated with thermal ageing in high‐N SS was investigated by a small punch (SP) test using miniature specimens at cryogenic temperatures. Results indicated that the degradation of mechanical properties in this alloy was caused by a decrease of cohesive strength resulting from carbides (Cr₂₃C₆) and nitrides (Cr₂N) precipitated in grain boundaries.     

Evolution of M23C6 phase in HR3C steel aged at 650 °C

The evolution of the morphology, composition, and particle size of the M₂₃C₆ phase in HR3C steel aged at 650 °C was analysed, by means of metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results showed that the Cr/Fe ratio in the M₂₃C₆ phase, in the form of irregular square and long strip, increased with the ageing time. Fe was gradually replaced by Cr in the M₂₃C₆ lattice. The M₂₃C₆ phase gradually evolved and then became stable after ageing for 2000 h.     

Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants

Abstract

It is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M₂₃C₆. Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the fine dispersion of precipitates along boundaries. The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M₂₃C₆ carbides near prior austenite grain boundaries, which precipitate during tempering and are less fine, are effective for preventing the long-term degradation of creep strength and for improving long-term creep strength. This can be achieved by the addition of boron. The steels considered in this paper exhibit higher creep strength at 650 °C than existing high-strength steels used for thick section boiler components.     

Formation of Re-containing Carbides in a Second Generation Directionally Solidi¯ed Ni Base Superalloy

The precipitates at grain boundary in a directionally solidified Ni base superalloy after heat treatment, aging at 975°C, and creep rupture test have been characterized. Besides the primary MC carbides and fine particles of μ phase, the Re-containing M₂₃C₆ was observed. The precipitation kinetics revealed that the formation of M₂₃C₆ was associated with the dissolution of μ phase and MC carbides. TEM image shows that the continuous precipitation of M₂₃C₆ particles effectively hinders the dislocation movement and strengthens the grain boundaries. The high strength of the alloy suggests that M₂₃C₆ carbides are beneficial to the properties although Re as an important matrix strengthening element was consumed.     

Correlation between grain boundary misorientation and M23C6 precipitation behaviors in a wrought Ni-based superalloy

The correlation between the grain boundary misorientation and the precipitation behaviors of intergranular M₂₃C₆ carbides in a wrought Ni–Cr–W superalloy was investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that the grain boundaries with a misorientation angle less than 20°, as well as all coincidence site lattice (CSL) boundaries, are immune to precipitation of the M₂₃C₆ carbides; in contrast, the random high-angle grain boundaries with a misorientation angle of 20°–40° provide preferential precipitation sites of the M₂₃C₆ carbides at the random high-angle grain boundaries with a higher misorientation angle of 55°–60°/[2 2 3] turn to retard precipitation of M₂₃C₆ carbides owing to their nature like the Σ3 grain boundaries and retard the precipitation of M₂₃C₆ carbides. The low-angle and certain random grain boundary segments induced by twins were found to interrupt the precipitation of the M₂₃C₆ carbides along the high-angle grain boundaries.     

Study on microstructure and properties of centrifugal casting 35Cr45NiNb+MA furnace tubes during service

In the present study, the microstructure evolution of 35Cr45NiNb+MA ethylene cracking furnace tubes during service and its effect on the properties were investigated. According to the results, in the early stage of service, the skeletal M₇C₃ and vermicular NbC were transformed into blocky M₂₃C₆ and G phase (Ni₁₆Nb₆Si₇), respectively, accompanied with many dispersed M₂₃C₆ secondary carbides. With the extension of service time, M₂₃C₆ carbides on the grain boundaries were transformed into M₇C₃ with high stacking fault structure and coarsened, and the blocky G phase was transformed into granular NbC. The yield strength and ultimate tensile strength of the aging furnace tubes decreased by 9%-18%, yet the elongation after fracture decreased significantly from 10.0%-14.0% to 3.0%-5.0%. The hardness of the carburized zone of the carburized tubes increased by 10%-17%, and the rupture time decreased by 45%-75% under the test condition of 1100℃ and 16MPa. Finally, the evolution map was summarized.     

Influence of grain boundary carbides on mechanical properties of high nitrogen austenitic stainless steel

Precipitation behavior of grain boundary carbides and its influence on mechanical properties and fracture mechanism of the high nitrogen austenitic stainless steel produced by different processing methods were studied. The simulation software Thermo-calc was applied to analyze the effects of element content on precipitation of carbides. The results show that hot-rolled plate has higher strength, but solution-treated one followed by water quenching has excellent combination of strength and ductility (toughness). M₂₃C₆ is the main precipitate and deteriorates the toughness of the steel obviously when it precipitates along grain boundaries. In this case, intergranular fracture is the predominant failure mechanism and the fracture surface is characterized by the shape of rock candy. The toughness at −40 °C is decreased by 53% when small amount of carbides precipitates during sand cooling process after solution treatment. The simulation results exhibit that with the decrease of C content, both the precipitation quantity and precipitation temperature of M₂₃C₆ decrease. Cr and N have no influence on precipitation quantity of M₂₃C₆, but the precipitation temperature will increase with the increase of Cr and the decrease of N.     

Unraveling the origin of strain-induced precipitation of M23C6 in the plastically deformed 347 Austenite stainless steel

In this research, the phenomenon of strain-induced precipitation of M₂₃ (M = Cr, Fe, Mo) C₆ precipitates in 347 austenite stainless steel was systematically studied. During annealing at 650 °C, the experimental results exhibited significantly earlier formation of M₂₃C₆ precipitates in the plastically strained specimen, than in the no-strain specimen. The various microstructural characterizations showed that the accelerated formation of M₂₃C₆ precipitates occurred around deformation bands that were generated by deformation twinning. Extensive investigation suggested that the strain-induced precipitation of M₂₃C₆ is attributed to the formation of strain-induced α′ martensite during plastic deformation near deformation bands.     

Precipitation in 9Cr–1Mo steel after creep deformation

The carbides present after creep testing a 9Cr–1Mo steel at 566 °C over a range of stress levels giving rupture times of up to 7300 h have been characterized and identified using a transmission electron microscopy, energy-dispersive X-ray spectroscopy and electron diffraction. The initial carbide precipitates present were M₇C_3, (NbV)C and VC and it was determined that M₆C carbide precipitates were present in all specimens after elevated temperature exposure for greater than approximately 1700 h. No precipitation of M₂₃C₆ was detected. The evolutionary sequence from the initially present carbides during high temperature exposure involved the formation of the stable M₆C carbide directly, without the intermediate formation of M₂₃C₆, as is reported to occur in other Cr–Mo steels.     

High temperature creep behaviour of an Ni-Cr-W-B alloy

The high-temperature creep behaviour of a solid-solution strengthened Ni-Cr-W-B alloy was studied, with emphasis on microstructural parameters. Creep strength was determined from tests conducted at 925°C/40 MPa. Various techniques of analytical electron microscopy were used to characterize the microstructure and microchemical composition. A number of microstructural parameters which promote creep strength, including (1) pinning of grain boundaries by tungsten-rich M₆C carbide, (2) relatively low stacking-fault energy, and (3) boron segregation to M₂₃C₆ carbide, were identified. However, their beneficial effects were suppressed by the initial presence of discontinuously precipitated M₂₃C₆ carbide at grain boundaries which accelerated intergranular cracking. Suppression of the discontinuous grainboundary reaction and a significant improvement in creep strength could be achieved by a proper heat treatment which appeared to induce a sufficiently high defect density promoting intragranular carbide precipitation. Competition between intergranular and intragranular precipitation was found to be influenced by an external stress. Strengthening by intragranular carbide precipitates appeared to occur by an attractive interaction with dislocations. Dislocations bowing out at subboundaries, cross-slip, motion of jogged screw dislocations and generation of dislocations at high-angle grain boundaries appeared to operate simultaneously as strain-producing mechanisms during steady-state creep.     

Some effects of heat treatment on grain boundary chemistry and precipitation in type 316 steel

Abstract

The effects of solution treatment temperature and cooling rate, carbon content, and aging at 600°C on the precipitation and grain boundary composition of AISI type 316 stainless steel were investigated. No matrix precipitation was observed even in a high carbon cast aged for 1000 h. Carbides of M₂₃C₆ type were the only precipitates observed and these formed on grain boundaries. Nucleation of a second phase in these solution treated specimens was difficult, despite the carbon supersaturation. The grain boundary regions were investigated using high spatial resolution X-ray analysis to show that chromium depleted and nickel enriched zones formed when grain boundary carbides occurred. The behaviour of molybdenum was more complex; it segregated strongly to grain boundaries either during a slow cool from the solution treatment temperature or during aging, but was also incorporated into any carbides that formed.

MST/890     

Grain boundary Cr-depleted zones in Ti and Nb stabilized austenitic stainless steels

The development of Cr-depleted zones due to the precipitation of M₂₃C₆ at grain boundaries during the ageing of niobium and titanium stabilized austenitic stainless steels (base composition Fe 17 % Cr 12.5% Ni) at 750 C has been studied by STEM/EDX. The chromium composition profiles in high angle grain boundaries were found to be similar both in the vicinity of intergranular M₂₃C₆ carbides and at boundary segments between them, i.e. the whole grain boundary area acted as a collector plate to supply the growing M₂₃C₆ with chromium. Cr-depleted zones did not develop at coherent twin boundaries in the same way as at ordinary high angle boundaries. No major difference in the width or depth of the depleted zones was observed for the niobium and titanium stabilized alloys except for a weak persistence at 100 h in the niobium containing alloy. The error function solution of Fick's second law for diffusion was found to give a good approximation of the depleted zones in the early stages of ageing.     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.     

Lifetime extension prediction of the rejuvenated first stage gas turbine blades

ABSTRACT

This paper presents a novel method for the first stage gas turbine blades lifetime extension based on degraded GTD-111 substrates hardness evolution after the rejuvenation heat treatments (RHTs) process. Moreover, the RHT impact on directionally solidified (DS) GTD-111 nickel-based suparalloys homogenisation and blades hardness properties were revealed and discussed. The measured hardness values indicate that turbine blades hardness progress is closely related to the nucleation of topologically close packed (TCP) phases, irregular γ’ (Ni₃, Al) precipitates growth, and Cr-rich M₂₃C₆ Coarsening at grain boundaries. Aspects like γ′/γ eutectic regions rafting, cooling holes oxidation attacks, and formation of cavities within γ-matrix were also identified and analysed.