Formation Of M23C6 Carbides Research Articles

Evolution of precipitate and its effect on the degradation of impact toughness in a carbon and nitrogen-controlled 316 stainless steel during thermal aging at 650 °C

Impact toughness degradation and microstructure evolution of a carbon and nitrogen-controlled 316 stainless steel during long-term aging at 650 °C for up to 8800 h were investigated. The degradation of impact toughness during aging can be divided into three stages: a rapid decrease during stage I (the first 400 h), a slow decrease during stage II (from 400 to 3200 h), and the stable stage during stage III (up to 8800 h). P enrichment at the M23C6/γ interface is induced by the rejection of P during the formation of M23C6 carbide, and the M23C6/γ interface is preferred location for crack initiation. An increase in the amount of grain boundary M23C6 carbides increases the crack initiation sites, resulting in a rapid decrease of impact toughness during stage I. The ongoing rejection of Ni and Si during the coarsening of M23C6 carbide is beneficial for the nucleation of M6C carbide. The micro-crack initiation at the M23C6/M6C and M6C/γ interfaces is introduced, and a semi-continuous network of M23C6 carbide along grain boundary promotes the crack propagation, leading to a further decrease of impact toughness during stage II. The nearly continuous precipitate morphology is induced through continued growth of grain boundary M23C6 and M6C carbides. The propagation of micro-cracks along the M6C/γ, M23C6/γ, and M23C6/M6C interfaces leads to the mostly intergranular fracture, thus the impact energy tends to be stable during stage III.

Evolution of M7C3 carbides near the solidus and the influence of Mn element on the formation of M23C6 carbides in a high carbon martensitic stainless steel 90Cr18MoV

The evolution behavior of M7C3 carbides in a 90Cr18MoV martensite stainless steel during high temperature diffusion annealing treatments at temperatures near the solidus was investigated. Results showed that only M7C3 carbides dissolved, and the dissolution of large-sized M7C3 carbides was limited when the temperature was below the solidus. However, when the temperature was above the solidus, a certain amount of liquid phase appeared around the grain boundaries. Meanwhile, two different types of precipitates appeared within the grains. And these two types of precipitates were eutectic products that mainly consisted of M7C3 carbides and bcc phase, and divorced eutectic products that mainly consisted of M23C6 carbides, respectively. Thermodynamic calculations indicated that with the increase of Mn content, the Gibbs free energy of M23C6 carbides would be lower than that of M7C3 carbides. Thus, the difference in Mn content was the main reason for the formation of those two types of precipitates, i.e., M7C3 carbides were formed in regions with low Mn content and M23C6 carbides were observed in areas with high Mn content.

Effect of microstructure on fatigue resistance of Inconel 740H and Haynes 282 nickel-based alloys at high temperature

Fatigue crack growth behavior of Inconel 740H and Haynes 282 nickel-based alloys at 20 °C, 700 °C and 750 °C was studied. Results showed increased fatigue crack growth rate for both the two alloys, while increased fatigue threshold in Inconel 740H at high temperatures. The size and distribution of MC carbides, serrated grain boundaries and twins deflected crack growth path. The strengthening γ' phase was interacted with dislocations by Orowan bypass and shear mechanisms at room temperature, while γ' was sheared in Haynes 282 and Orowan bypassed in Inconel 740H alloy at elevated temperatures. The cross slip, dislocation climbing and entanglement reduced the resistance of dislocation movement and accelerated the accumulation of damage. The higher content of Cr in Haynes 282 alloy promoted formation of M23C6 carbide while the lower content in Inconel 740H alloy promoted thicker oxide layer. The Cr content needed careful tuning as it was superior to microstructural factors for higher fatigue threshold at elevated temperatures.

SA508 low alloy steel to 316L stainless steel dissimilar metal joint made by powder metallurgy hot isostatic pressing

Joining ferritic SA508 low alloy steel (LAS) and austenitic 316 L stainless steel (SS) via powder metallurgy hot isostatic pressing (PM-HIP) was evaluated as an alternative method to welding. This study investigated the mechanical and microstructural evolutions of the bimetallic interface under different joint designs and heat treatments. The direct joining of dissimilar metal alloys by PM-HIP method resulted in two designs: 1) powder SA508 to solid bar 316 L (P508–B316L) and 2) powder 316 L solid bar SA508 (P316L-B508). In both cases, P508–B316L and P316L-demonstrated satisfactory tensile strength, however, high hardness and severe depreciation in toughness were located on the bimetallic interface. The mechanisms responsible for the detrimental mechanical properties were verified. Large oxides were observed in P508–B316L due to the prior powder boundary (PPB) oxides present in SA508 powder. The intense sensitization occurred from the formation of M23C6 carbides, consequently from the slow cooling after PM-HIP in P316L-B508. Post-HIP heat treatments were explored to reduce the distance of carbide formation; however, the heat treatment could not eliminate the carbides. The experimental results were compared to the diffusion couple simulation as a function of carbide formation with distance. The analysis also showed the high hardness at the bimetallic interface was primarily contributed by solid solution strengthening. In conclusion, the direct joining of P316L-B508 and P508–B316L via PM-HIP was deemed to be unfeasible, and a transitional material is necessary to impede the diffusion of carbon.

Effects of Cr Carbides Formation on the High Temperature Creep Property of Alloy 690 for Steam Generator Tube Material

The creep properties of Alloy 690, used as a steam generator tube material in nuclear power plants, were evaluated at 650°C, 750°C, and 850°C. The parameters of creep life prediction models were derived using the Larson-Miller (LM), Manson-Haferd (MH), and Orr-Sherby-Dorn (OSD) models, to use as mechanical properties under a virtual severe accident condition like station black out (SBO). The yield strength (YS) and creep property of Alloy 690 were compared with those of Alloy 600, and the effects of the precipitation behavior of Cr carbides on creep properties were analyzed. The YS of Alloy 600 decreased rapidly above the temperature of 750°C, but the YS of Alloy 690 decreased linearly up to the temperature of 850°C because of the formation of M23C6 carbides. The creep stress exponent (n) of Alloy 690 was between 5 and 6, and this indicated that dislocation creep was the major creep mechanism at the test temperatures. The results of creep tests were well matched with the LM, MH, and OSD models for Alloy 690, and there were no significant differences in accuracy between the models. The stress-rupture test results of Alloy 600 and Alloy 690 using the LM model showed that the decrease in creep strength with rupture time of Alloy 690 was steeper than that of Alloy 600 at high temperatures. This indicated that Alloy 690 was more susceptible to creep degradation under longterm creep conditions. The precipitation of Cr carbides in Alloy 690 increased YS, benefitting creep properties for short-term creep. However, the Cr carbides coarsened significantly under loading conditions at high temperature, and this deteriorated the creep properties for long-term creep.

Material characterization of long-term service-exposed GTD-111 nickel based superalloy

In this study, the influence of long-term service exposure on the microstructure of a GTD-111 precipitation hardened Ni-based superalloy extracted from turbine blades after 55,000 and 75,000 h without any interval rejuvenations is investigated. This alloy consists of multi phases including γ, γ′ (Ni3Al) strengthening precipitates, carbides, and eutectic phases. The long-term operation caused dramatic microstructural degradations, such as decomposition of MC carbides, formation of M23C6 carbides along grain boundaries, grain boundary oxidation and cracking, coarsening of primary γ′, dissolving of secondary γ′, and formation of TCP-type phases which profoundly impacted its failure mechanisms. The results showed that after 75,000 h of service, the grain boundaries which reached the surface were oxidized and even some led to crack initiation due to high thermal cyclic loads in the cooling holes. The obtained results also illustrate that with increasing service exposure times, the size of primary γ′ increased to about 1.5–2 µm and secondary γ′ dissolved. Dissolution and coarsening of γ′, widening of grain boundaries, decomposition of carbides, and phase transformations were analyzed by optical and scanning electron microscope and energy dispersive spectroscopy. Furthermore, the coating on the blade’s surface is investigated microstructurally to understand its severe degradation due to the high temperature and pressure such as microstructural degradation and cracking, and widening of interdiffusion layer.

Understanding carbide evolution and surface chemistry during deep cryogenic treatment in high-alloyed ferrous alloy

The study investigates the effect of deep cryogenic treatment (DCT) on a high-alloyed ferrous alloy (HAFA) and its effectiveness on carbide evolution and chemical shifts of alloying elements. With ex-situ and in-situ observations ranging from the microscopic to the nanoscopic level, we uncover the atomistic mechanism by which DCT affects carbide precipitation, resulting in a 50% increase in carbide volume fraction. Synchrotron-based scanning photoelectron microscopy provides insight into the agglomeration of carbon during exposure to DCT. We find that Mo plays a crucial role in DCT through its modification of chemical bonding states, which is postulated to originate from the loosely-formed primordial Mo2C carbides formed during exposure to cryogenic temperatures. These in turn provide energetically favorable nucleation zones that accelerate the formation of M7C3 carbides, which serve as intermediate states for the formation of M23C6 carbides, which most strongly impact the mechanical properties. These results are supported by atom probe tomography, showing the preferential formation of Mo-rich M7C3 carbides in DCT samples, resulting from greater solute mobility. This work clarifies the fundamental mechanisms on how DCT affects HAFA, solving a long-elusive problem.

Non-cube-on-cube orientation relationship between M23C6 and austenite in an austenitic stainless steel

Crystallographic orientation relationship between M23C6 carbides and austenite in the lightweight high carbon stainless steel Fe−17Cr−9Ni−6Mn−4Al−0.42C (wt.%) was investigated by diffraction analysis in scanning and transmission electron microscopes. Electron backscatter diffraction analysis in SEM indicated the existence of an orientation relationship approximated by the Kurdjumov-Sachs (K-S) orientation relationship between the M23C6 carbides induced by aging and the austenitic matrix. The observation of an orientation relationship deviating from the commonly reported cube-on-cube orientation relationship was explained by the formation of M23C6 carbides via M7C3 carbides as an intermediate phase.

Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

The relationship among B content, microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated. After aging at 800 ℃ for 1000 h, the decomposition degree of MC carbides of K4750 alloys with 0B, 0.007 wt. % B and 0.010 wt. % B were basically identical, which indicated that B has no inhibition on MC carbide decomposition during long-term aging. The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases, which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix. In addition, M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them, while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them. Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging, which was because the segregation of B at grain boundary retarded the diffusion of alloy elements, thus weakened the local fluctuation of chemical composition near grain boundary. The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa. The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy, which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.

δ-ferrite transformation mechanism and its effect on mechanical properties of 316H weld metal

The microstructure evolution and mechanical properties of the 316H stainless steel weld metals with different C contents were studied at the aging temperature of 550 °C for different aging holding time. The transformation behavior of δ-ferrite and precipitation mechanisms of M23C6 and σ phase in the as-aged weld metal were investigated. The results indicated that for the as-welded weld metal, with increasing C content, the yield and tensile strengths increased, while the elongation decreased owing to the increase of C solid solution strengthening effect. Moreover, both the high δ-ferrite content in low C weld metal and the precipitated M23C6 carbide in high C weld metal deteriorated the impact energy obviously. During the aging process, the rapid precipitation of M23C6 carbide occurred in δ-ferrite firstly owing to the high diffusion rate of C. Once the carbon is depleted by precipitation of M23C6, the slow formation of σ phase occurred through eutectoid transformation (δ → σ + γ) depending on the diffusion of Cr and Mo. Moreover, increasing C content promoted the formation of M23C6 carbides and inhibited the formation of σ phase. Therefore, increasing C content accelerated the transformation of δ-ferrite in weld metal during aging process. Furthermore, after a long enough aging time, a transformation from M23C6 to σ occurred. The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions. For the low C weld metal aged at 550 °C, with the increase of the aging time, fine M23C6 first precipitated, then coarsened, after that σ phase formed, which caused that the yield and tensile strengths first increased, then decreased, and finally increased slightly again. For the medium C weld metal, as the aging time increased, first the depletion of the solid solution C as a result the M23C6 precipitation deteriorated the strength, and then the formation of σ phase improved the strength. For the high C weld metal, with the increase of the aging time, the depletion of the solid solution C and the coarsening of the M23C6 precipitates deteriorated the strength. Furthermore, with increasing aging time, both the precipitation and coarsening of M23C6 and increasing σ phase content deteriorated the elongation and impact energy.

Study of the corrosion behaviors of 304 austenite stainless steel specimens exposed to static liquid lithium at 600 K

Investigation of corrosion behavior of stainless steel served as one kind of structure materials exposed to liquid lithium (Li) is one of the keys to apply liquid Li as potential plasma facing materials (PFM) or blanket coolant in the fusion device. Corrosion experiments of 304 austenite stainless steel (304 SS) were carried out in static liquid Li at 600 K and up to1584 h at high vacuum with pressure less than 4 × 10−4 Pa. After exposure to liquid Li, it was found that the weight of 304 SS slightly decreased with weight loss rate of 5.7 × 10−4 g/m2/h and surface hardness increased by about 50 HV. Lots of spinel-like grains and holes were observed on the surface of specimens measured by SEM. By further EDS, XRD and metallographic analyzing, it was confirmed that the main compositions of spinel-like grains were M23C6 carbides, and 304 SS produced a non-uniform corrosion behavior by preferential grain boundary attack, possibly due to the easy formation of M23C6 carbides and/or formation of Li compound at grain boundaries.

Microstructural and Phase Analysis of Service-Exposed Ni-Cr Alloy Turbine Blade

Ni-Cr superalloy has been widely used in turbomachinery equipments. Gas turbine blade made from Ni-Cr superalloys experience the effect of high temperature and stress during service which certainly cause various microstructural changes. It is found that the formation of spheroidal gamma prime (γ') phase replacing the cuboidal γ' phase and also the formation of M23C6 carbides along the grain boundaries. Numerous previous study has been conducted and reported that the microstructure of the turbine blade are being modified by service. This study involved assessing changes in blade microstructure and microhardness as a function of service time.

Failure of Alloy 625 tube stub ends – Effect of primary nitrides

This paper reports a failure analysis of Alloy 625 stub ends of ammonia cracker tubes that failed during operation after 47,000h of service operation. The failure occurred due to the formation of M23C6 carbides at grain boundaries, which made them very weak and brittle. However, the formation of M23C6 carbides at grain boundaries was surprising since they formed at temperatures around 550°C, which is much below their expected temperature of formation and occurred in a period less than half the designed life. Precipitation of the grain boundary carbides has been attributed to the presence of primary nitride particles in the present alloy instead of primary carbides which are usually observed in these alloys. Formation of nitrides consumed Ti that binds C in the form of primary MC carbides in these alloys. This left free carbon in the alloy matrix for easy formation of M23C6 carbides which otherwise form due to degeneration of primary MC carbides.

Effect of Boron on Microstructure and Impact Toughness of a 10%Cr Martensitic Steel

It was shown that in a 10% Cr martensitic steel enriched by boron this element tends to segregate within M23C6 carbides having the film-like shape and precipitated on the boundaries of prior austenite grains (PAG), mainly. It leads to a low value of Charpy V-notch impact toughness of 6 J/cm2. These carbides are highly resistant against the spheroidizing. Only the tempering at 770°C leads to the final formation of M23C6 carbides having the equiaxed shape. Concurrently, this tempering strongly decreases boron segregation. As a result, the 10% Cr martensitic steel exhibits a high value of Charpy V-notch impact toughness of 260 J/cm2.

Formation of M 23C 6-type precipitates and chromium-depleted zones in austenite stainless steel

Formation of M23C6 carbides and chromium-depleted zones in commercially available type 304L stainless steel were investigated by in situ transmission electron microscopy and analytical transmission electron microscopy. It was found that each individual small M23C6 carbide starts to grow with a clear orientation relationship with the matrix, and film-like carbide was subsequently observed at the interfaces with asymmetric Cr-depleted zones. From these experimental results, a model describing the precipitation of M23C6 and the formation of the Cr-depleted zone was proposed.

On the Precipitation Sequence in a 10%Cr Steel under Tempering

Precipitation sequence under tempering in a 10%Cr steel has been considered in details. Two different precipitation processes have been identified, concurrently. First, at 350°C, a dispersion of M2C phase with orthorhombic lattice appears. At 525°C, the M2C phase is displaced by the formation of M23C6 carbide. However, this process remains uncompleted. Dissolution of M2C phase occurs completely due to the formation and growth of more stable M6C phase as a result of tempering in the range of 650 to 770°C. Second, Nb-rich and V-rich M(C,N) carbonitrides precipitate in the temperature interval of 500–770°C. These dispersoids play a role of heterogeneous nucleation sites for precipitations of M23C6 and M6C carbides.

γ′ Precipitate Dissolution during Heat Treatment of Nimonic 115 Superalloy

In precipitation hardenable materials, it is desirable to determine the precipitate dissolution temperature for homogenizing the microstructure by controlling the size and distribution of the precipitates. In this research, differential thermal analysis, dilatometry technique, heat treatments followed by microstructure evaluation were used to determine the γ′ dissolution temperature of Nimonic 115. It is assumed that the variation of enthalpy is governed by the changes in γ′ volume fraction and γ′ concentration with time and temperature, and any contribution of the coarsening of γ′ is neglected. The values obtained for the solvus temperature of γ′ precipitates by the three methods are all in agreement indicating this temperature at approximately 1160°C. As the solvus temperature of γ′ is higher than the temperature of formation of M23C6 carbides, this is considered as the cause of occurrence of serrations at grain boundaries. During the heat treatments, grain boundary serration is observed, and the results indicate that formation of serration depends on the cooling rate, so that while furnace cooling results in boundary serration, no effect is observed by air cooling.

Further Investigations of Ductility-Dip Cracking in High Chromium, Ni-Base Filler Metals

The ductility-dip cracking (DDC) susceptibility of high-chromium, nickel-base filler metals was evaluated using the strain-to-fracture (STF) test technique. These filler metals were of the Ni-30Cr type and included INCONEL® Filler Metals 52 and 52M supplied by Special Metals Welding Products Company, and Sanicro 68HP® and Sanicro 69HP® supplied by Sandvik AB. In addition, two experimental Ni-30Cr filler metals were evaluated which contained variations in other alloy additions including Nb above 1 wt % and Mo up to 4 wt %. A wide range in DDC susceptibility was observed with these filler metals, including large heat-to-heat variations in filler metals with similar compositions. The experimental filler metals were found to be remarkably resistant to DDC. Cracking susceptibility is primarily associated with the type and form of precipitate that forms along the weld metal migrated grain boundaries. The formation of Nb-rich, M(C, N) at the end of solidification has the most profound effect on DDC, since these precipitates are most effective in pinning the boundaries. The formation of M23C6 carbides during weld cooling or subsequent reheating can also affect DDC susceptibility in these filler metals.

Retardation of σ-phase transformation in modified superalloy RR2072

A comparative investigation of σ-phase transformation in base and modified experimental alloy RR2072 (C, Hf, and B) has been undertaken. The alloys were exposed at 950 °C for 5 to 500 hours. It was found that σ-phase transformation was notably retarded in the modified alloys due to the prior formation of M23C6 carbides and Ta/Ti being tied up in the MC phase. Detailed investigation has revealed that M23C6 carbides absorb Cr, Mo, and Co, consequently retarding σ-phase transformation that demands Cr, Mo, and Co in addition to Re. In the interdendritic regions, the growth of σ-phase was physically limited by MC carbides and chemically inhibited by the very localized microsegregation.

Structure and properties of stainless steel alloyed with molybdenum

1. Raising the molybdenum content from 0.9 to 3.7% in austenitic steel with 19% Cr, 10% Mn, 7% Ni, and 0.30% N raises the ultimate strength to 93 kg/mm2 and the yield strength to 58 kg/mm2. 2. In the steel not containing molybdenum, tempering at 800°C leads to the formation of M2N and M23C6; the addition of molybdenum in amounts over 2.1% leads to preferential formation of σ phase during tempering. 3. Raising the carbon content from 0.01 to 0.11% lessens the tendency to form σ phase in austenitic steel with 2.5% Mo and leads to formation of M23C6 carbide.