Precipitation Behavior Of Carbides Research Articles

Multiple Precipitation Behavior of Niobium Carbide and Copper in Martensitic Steel

The multiple precipitation behavior of NbC and Cu particles in martensitic structure was investigated by using 0.05C-0.46Nb-2Cu-1.5Mn steel (NbC-Cu steel). Additionally, 0.05C-0.45Nb-2Mn steel (NbC steel) and 2Cu-5Mn steel (Cu steel) were also prepared to examine the respective precipitation behaviors of NbC and Cu. Aging treatment at 873K after quenching revealed that these steels exhibit typical age hardening. Comparing the NbC steel and Cu steel in the precipitation rate, the Cu precipitated much faster than the NbC. On the other hand, the peak hardness in NbC-Cu steel is higher than that by the respective precipitations in NbC steel and Cu steel. Besides, the aging time for the peak hardness in NbC-Cu steel was between those in NbC steel and Cu steel. This suggests that the NbC and Cu particles were separately precipitated within martensite matrix and each of them contributed to the hardening in NbC-Cu steel. As a result of TEM investigation for crystallographic characteristics of the precipitates, the NbC and Cu particles had different crystallographic orientation relationship with tempered martensite matrix: Baker-Nutting relationship for NbC particle and Kurdjumov-Sachs relationship for Cu particle.

Evolution of Grain Boundary Carbide and Cr-depletion Region in Alloy 690 during Isothermal Treatment

The precipitation behavior of grain boundary carbides and the evolution of Cr-depletion region in Ni-based superalloy 690 after solid solution treatment (1 020 ℃/15 min) followed by heat treatment at different temperatures (650 ℃,715 ℃,800 ℃) for various times (1～40 h) are studied by means of transmission electron microscopy.Based on the analysis of microstructure,an optimal heat treatment is given.It is concluded from the present investigation that carbides precipitate on both sides of the grain boundary at 800 ℃.In addition,the distribution of Cr near grain boundary is most smooth at the treatment of 715 ℃/10～20 h.As a whole,the optimal heat treatment process is 751 ℃/10～20 h.

Sub-zero treatments of AISI D2 steel: Part I. Microstructure and hardness

Microstructure, hardness and wear behavior of AISI D2 steel subjected to varied sub-zero treatments have been examined with reference to conventional heat treatment. Part I of this work presents the variations of microstructure and hardness, whereas part II deals with the wear behavior. The sub-zero treatments studied are cold treatment, shallow cryogenic treatment and deep cryogenic treatment. The developed microstructures have been characterized by XRD, optical microscopy and SEM examinations coupled with EDX and image analyses. Macrohardness and microhardness of the specimens have been evaluated by Vickers indentation technique. The obtained results infer that (i) retained austenite content is reduced by cold treatment, but is almost completely eliminated by both shallow and deep cryogenic treatments, (ii) the sub-zero treatments modify the precipitation behavior of secondary carbides; lower the temperature of sub-zero treatment higher is the degree of modification, (iii) the deep cryogenic treatment refines the secondary carbides, increases their amount and population density, and leads to more uniform distribution, and (iv) bulk hardness increases marginally but apparent hardness of the matrix improves considerably by deep cryogenic treatment.

Correlation of microstructure with wear behaviour of deep cryogenically treated AISI D2 steel

This study aims to reveal the underlying mechanisms responsible for the enhancement of wear resistance of AISI D2 steel by deep cryogenic treatment (DCT) through in-depth microstructural analyses, and thereby attempt to correlate microstructure with wear behaviour of DCT specimens with reference to that of conventional heat treatment (CHT) and cold treatment (CT). Microstructural characterizations of the differently treated specimens have been done by image analyses of optical and SEM photographs, XRD, and EDX analyses, whereas wear behaviour has been characterized by wear rate, wear resistance, and analyses of worn surfaces, wear debris and subsurfaces. The results indicate that DCT markedly enhances the wear resistance of the selected steel compared to CHT and CT. Formation of white layer and its subsequent delamination have been identified as the operative wear mechanisms; the extent of these phenomena and the consequent wear rate is dependent on the type of treatment that determines the microstructures. The wear behaviour can be unambiguously correlated with the modifications in the precipitation behaviour of secondary carbides and reduction in the retained austenite content of the microstructure, which are the governing mechanisms for the improved of wear resistance of tool steels by DCT.

Dependence of Precipitation Behavior and Creep Strength on Cr Content in High Cr Ferritic Heat Resistant Steels

It is known that high temperature tensile strength increases with increasing Cr content in Cr containing heat resistant steels. Recently, however, it was found that long-term creep strength decreased with increasing Cr content in the heat resistant steels containing 8.5-12%Cr. In this study, precipitation behavior of M23C6 carbide and the Z phase after creep tests was investigated using two kinds of high Cr ferritic steels (9Cr and 10.5Cr). As a result, 10.5Cr steel exhibited larger average particle size of M23C6 than 9Cr steel irrespective of creep stress levels, but the amount of M23C6 carbide was almost the same in both steels. On the other hand, the amount of the Z phase became large in 10.5Cr steel compared with 9Cr steel. These experimental results indicate that high level of Cr content accelerates precipitation and coalescence rate of both M23C6 carbide and the Z phase, resulting in degradation of long term creep strength in 10.5 Cr steel compared to 9Cr steel.

Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel

The influence of deep cryogenic processing in between quenching and tempering (QCT) on the carbide precipitation and the tribological behavior of a commercial AISI D2 steel has been examined. The developed microstructures have been characterized with an emphasis to understand the influence of QCT vis-à-vis conventional quenching and tempering (QT) on the nature, size, morphology, and distribution of carbide particles. The mechanical properties such as hardness and wear resistance of the samples treated by QT and QCT have been evaluated employing Vickers indentation and sliding wear techniques, respectively. It has been demonstrated that deep cryogenic treatment leads to considerable microstructural changes which result in enhanced tribological properties.

Precipitation behavior of the lower bainitic carbide in a medium-carbon steel containing Si, Mn and Mo

The fine microstructure, crystallographic features and ɛ-carbides precipitation behavior of lower bainite produced by isothermal transformation in a medium-carbon steel containing Si, Mn and Mo were investigated using transmission electronic microscopy. It was found that the microstructure produced by isothermal reaction at 320 °C was composed of a large amount of plate-like lower bainite with retained austenite embedded between the plates, and ɛ-carbides precipitated within them. Midrib and subunits were readily visible in the lower bainite plate. The bainite plate kept a G-T orientation relationship (OR) with the austenite. Selected area electron diffraction patterns of “three phases in four variants” and analysis indicated that two variants of ɛ-carbides precipitated in a single bainitic ferrite plate. The two (or three) variants of ɛ-carbides can simultaneously keep a Jack OR with its “bainite matrix”, while keeping no fixed OR with the austenite. The crystallographic features of ɛ-carbides precipitated within the bainite were the same as those observed in tempered martensite. The results indicated that the bainitic transformation bore an analogy to the martensitic one in carbide precipitation.

4020 水素放出スペクトル解析によるタービンロータ鋼の材質劣化評価(S25-4 損傷評価,S25 材料・構造の非弾性挙動・高温強度・損傷評価)

The thermal desorption spectroscopic analysis after the hydrogen charging was applied to the interrupted creep specimens to investigate the effect of creep on the changes in hydrogen desorption characteristic of high Cr turbine rotor steel. In addition, the measured spectra were decomposed to two different spectra and their changes with aging and creep were investigated to correlate the variations in spectrum with the microstructural changes. Experimental results revealed that the applied stress had no large influence on the change in hydrogen spectrum and the amount of hydrogen desorbed at temperatures ranging from R.T. to 250℃ decreased significantly with increasing creep life fraction at the early stage of life. The changes in decomposed spectra in the low and high temperature regions were also discussed from the standpoints of the variations in lath-martensite structure and precipitation behavior of carbides and Laves phase, respectively.

Effect of carbon concentration on precipitation behavior of M23C6 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.

Influence of laser surface melting on the susceptibility to intergranular corrosion of sensitized Alloy 600

The effects of laser surface melting (LSM) on the intergranular corrosion of the sensitized Alloy 600 were studied by the double loop electrochemical potentiokinetic reactivation (DL-EPR) method in de-aerated 0.01 M H 2SO 4+20 ppm KSCN at a scan rate of 0.5 mV/s at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM, and the sensitized Alloy 600 after LSM also exhibited a relatively low DOS, comparing with that of the sensitized but not laser treated Alloy 600. From the microscopic examination, it was found that the microstructural changes induced by the laser treatment, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement in resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to intergranular stress corrosion cracking of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination.

Double loop electrochemical potentiokinetic reactivation test of Nickel-base Alloy 600 surface-melted by a CO2 laser beam

Ni-base Alloy 600 has been widely used as a steam generator (S/G) tubing material in nuclear power plants because of its good mechanical and corrosion properties at high temperatures. However, degradations of S/G tubes due to intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) during normal operation have been frequently reported. In particular, Alloy 600 can be very susceptible to IGA/IGSCC in some sulfur-bearing environments by sensitization. In this paper, the beneficial effects of laser surface melting (LSM) on intergranular corrosion of the sensitized Alloy 600 is presented from the results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) test. The DL-EPR test was performed in de-aerated 0.01 M H2SO4+20 ppm KSCN at a scan rate of 0.5 m V/sec at room temperature. The degree of sensitization (DOS) of the sensitized Alloy 600 measured from the DL-EPR test was considerably reduced by LSM. The sensitized Alloy 600 after LSM also exhibited a relatively low DOS, compared with that of the sensitized but not laser treated Alloy 600. From the microscopic observation, it was found that the microstructural changes brought about by the LSM process, especially changes in the precipitation behavior of grain boundary Cr-rich carbides, caused the improvement of resistance to intergranular corrosion of the laser treated Alloy 600. The resistance to IGSCC of the laser treated Alloy 600 in sulfur-bearing environments was also discussed from the results of measured DOS and microstructural examination.

Ortho-equilibrium and para-equilibrium phase diagrams for interstitial / substitutional iron alloys

A mathematical model for the evaluation of compositionally constrained thermodynamic equilibrium has recently been implemented into the computer program MatCalc. This model is applied to the calculation of para-equilibrium phase diagrams for some ternary model iron alloy systems FeXC, with X = Mn, Ni, Cr and Mo. The results are compared to the corresponding full equilibrium (ortho-equilibrium) phase diagrams and the impact of each element on the austenite / ferrite / carbide transformation in steels is analyzed. The para-equilibrium phase diagrams are considerably more simple than the potentially complex ortho-equilibrium phase diagrams, showing cementite formation as the only stable carbide under para-equilibrium conditions. The driving forces for precipitation of cementite and the complex chromium carbides in the FeCrC system are evaluated as a function of the precipitate composition. The evaluation of the driving forces under para-equilibrium conditions predicts carbide precipitation behavior that agrees with experimental findings.

Effects of sensitization treatment on the evolution of Cr carbides in rapidly solidified Ni-base Alloy 600 by a CO2 laser beam

Abstract The effects of heat treatment on the precipitation of Cr carbides in Ni-base Alloy 600 laser surface melted by a CO2 laser beam were investigated with microscopic equipment. Aging treatment at 600°C for 24 h induced precipitation of Cr-rich M23C6 and Cr7C3 carbides on some high angle grain boundaries, and the minimum Cr concentration along the grain boundary was measured to be about 12 wt%. In particular, the Cr-rich M23C6 was precipitated on TiN and MgS type particles surrounded by tangled dislocations along the cell boundaries in the laser melted zone. The precipitation behaviors of Cr carbides in the laser surface melted and subsequently sensitized Alloy 600 were analyzed and then compared with those of the conventionally processed, solution annealed and then identically sensitized Alloy 600 from the viewpoint of different microstructural features.

Effects of strain and grain size on carbide precipitation and corrosion sensitization behavior in 304 stainless steel

The simultaneous effects of strain and grain size on carbide precipitation and sensitization development in 304 stainless steel having a constant carbon composition of 0.051% were studied. Three different grain sizes (150 μm, 40 μm, and 15 μm) were uniaxially deformed to 10% and 20% strain, and aged in the sensitization ranges of 625°C and 775°C. 150 μm samples were also strained to 20% in liquid nitrogen and aged between 0.1 and 10 h at 670°C. At 625°C, straining of the 150 μm grain size material produced a systematic increase in carbide precipitation, sensitization development, and desensitization. Transmission electron microscopy (TEM) revealed a greater continuity of carbides along the grain boundaries in the strained samples vs the unstrained samples. At the 15 μm grain size, these strain effects were found to be far less prominent with the sensitization-desensitization process occurring much faster, and accelerated at the higher aging temperature of 775°C, i.e. sensitization-desensitization envelopes were shifted to shorter aging times. Liquid nitrogen straining produced a sub-micron, 2-phase γ/α′, sub-micron grain structure which demonstrated an almost instantaneous sensitization-desensitization behavior, when aged at 670°C, which was predicted by extrapolation of the aging data for the larger range of grain sizes (∼ 15 to 150 μm) and the 625 and 775°C aging temperatures.

On the precipitation of TiC in liquid iron by reactions between different phases

In order to study the in situ formation of carbide-reinforced metal matrix composites and the inoculation reactions of some low alloy steels, titanium carbide precipitation in liquid iron was investigated. TiC particles were formed directly in liquid iron solution through reactions between different phases. The high-temperature reactions studied in the present work are reactions between (a) solid graphite and liquid Fe-Ti alloys, (b) Fe-Ti and Fe-C liquid alloys, and (c) pure solid titanium and liquid Fe-C alloys. Small samples for these reactions have been processed using a mirror furnace facility in a precisely controlled manner. Samples of Fe-Ti alloys reacted with graphite were also processed by a high frequency induction furnace and other facilities. The precipitation behavior of titanium carbide in various reactions and the distribution of carbide particles in iron melts were investigated. Different growth morphologies of the carbide were observed under various conditions. The effects of convection on the reactions were observed and discussed, and by processing the samples with various orientations relative to gravity, various patterns of carbide distribution were obtained.

ＴＭＣＰ鋼ＨＡＺのＰＷＨＴ過程におけるミクロ組織と機械的性質との相関

Correlation between Heat-Affected Zone (HAZ) microstructures and the mechanical properties during Post-Weld Heat Treatment (PWHT) on two types of Thermo-Mechanical Control Process steels (TMCP steels), especially the relation between the stress relaxation behavior and high temperature deformability was investigated. Simulated-weld heat treatment was carried out using a weld-heat cycle simulator with a maximum temperature of 1623 K. PWHT was done at a heating rate of 55.6 K/ks. The mechanical properties during the PWHT process were evaluated by means of the constant-strain rate test on heating and the constant-load test on heating. The influence of PWHT on the reducing tendency of ductility was discussed from the viewpoint of precipitation behavior of inter-or transgranular carbides and the deformability of the matrix on each HAZ microstructure. The results are summarized as follows :(1) The stress relaxation behavior of the specimens of bainitic structure on both AcC type (Controlled rolled+Accelerated Cooled) and Non AcC type (only Controlled Rolled) TMCP steels were delayed clearly above 600 K. The stress relaxation behavior of the Non AcC type TMCP steel was further delayed to the higher temperature range during PWHT than that of the AcC type TMCP steel.(2) High temperature deformability of the specimens of bainitic structure on both TMCP steels had a remarkable reducing tendency of ductility in the range of 850-900 K. The Non AcC type TMCP steel had a remarkable reducing tendency of ductility than that of the AcC type TMCP steel.(3) The reducing tendency of ductility was caused by the difference in precipitation behavior of inter-or transgranular carbides in each HAZ microstructure during PWHT process. The tendency of the Non AcC type TMCP steel which contained more alloying elements occurred more strongly because of precipitation of alloying carbides.(4) When the high temperature ductility or embrittlement of HAZ microstructure of the TMCP steels was evaluated, it was very important to measure the deformability of the material during the PWHT process. An evaluating method of ductility of TMCP steels was proposed using the relation between deformability of material and the displacement of each microstructure during PWHT process.

Correlations between microstructure and mechanical properties during PWHT of TMCP steel weld HAZ

Summary This paper describes an investigation of the correlation between HAZ microstructures amd mechanical properties in the post‐weld heat treatment (PWHT) of two types of thermomechanical control process steels (TMCP steels), especially the relationship between the stress relaxation behaviour and high‐temperature deformability. Simulated weld heat treatment was performed with a welding thermal cycle simulator at a maximum temperature of 1623 K. PWHT was performed at a heating rate of 55.6 K/ksec. The mechanical properties in the PWHT process were evaluated by rising‐temperature constant‐strain rate tests and rising‐temperature constant‐load tests. The effect of PWHT in reducing ductility is discussed from the perspective of the precipitation behaviour of intergranular and transgranular carbides and the associated deformability of the matrix in each HAZ structure. The results obtained may be summarised as follows: The results obtained during measurement of the stress relaxation behaviour in the rising‐t...

Phase instability and toughness change during high temperature exposure of various steels for the first wall structural materials of a fusion reactor

The objective of the present research is to clarify the phase instability, particularly, the precipitation behavior of carbide and nitride during the long term aging in the non-irradiation state of the materials proposed for the first wall structural component of fusion reactors, such as a type 316 austenitic steel, its modified steels, ferritic heat resisting steels and reduced radio-activation materials. The effect of the precipitation behavior on the toughness is also investigated. It is noticed that the toughness was much deteriorated by the formation of large amounts of coarse carbides within grains and on grain boundaries during 2.88 × 10 4 ks (8000 h) aging at 873 K and that intergranular fracture occurred by the impact test at room temperature even in the type 316 steel.

Stress relaxation behaviour in heat affected zones of plain carbon steels during reheat treatment

Summary This paper describes an investigation into the effect of the microstructure in the heat affected zone of plain carbon steels (S50C, S25C, S15C, and SUYB steels) on the stress relaxation behaviour during reheat treatment. The HAZ structures of the plain carbon steels were produced by synthetic heat treatment. Two types of microstructure were produced in the synthetic HAZ by air cooling (AC) or water quenching (WQ) treatment. Stress relaxation tests were conducted by the three‐point bending test and constant‐strain rate test under a rising temperature. The deformability of the synthetic HAZ structures at high temperature during the reheat treatment process was examined by the constant‐load test under a rising temperature. The relationship between the precipitation behaviour of carbides and the change in the carbon concentration in the matrix of the synthetic HAZ were investigated during the reheat treatment process. The effect of this relationship on the stress relaxation behaviour is discussed in t...

High Temperature Strength and Aging Behavior of 12%Cr-15%Mn Austenitic Steels

High Mn-Cr austenitic steels are still considered to be an important high temperature structural material from the point of view of reduced radio-activation. The objective of the present study is to make a fundamental research of mechanical properties and microstructure of 12%Cr-15%Mn austenitic steels. Especially the effects of alloying elements of V and Ti on the mechanical properties and microstructure evolution of high Mn-Cr steels were studied. Precipitation behaviors of carbides, nitrides and σ phase are investigated and their remarkable effects on the high temperature strength are found. The addition of V was very effective for strengthening the materials with the precipitation of fine VN. Ti was also found to be beneficial for the improvement of high temperature strength properties