Three-dimensional monte carlo simulation of grain growth in the heat-affected zone of a 2.25Cr-1Mo steel weld

We report here the first three-dimensional (3-D) Monte Carlo (MC) modeling of grain growth in the heat-affected zone (HAZ) of a weldment. Computed grain-size distributions in the HAZ of a 2.25Cr-1Mo steel, for different heat inputs in the range from 1.1 to 4.8 MJ/m, were compared to independent experimental results. The simulated mean grain size for different heat inputs agreed well with the corresponding independent experimental data. The mean grain size at various locations equidistant from the fusion line were different. The predicted grain size from the 3-D model matched the experimental results more closely than that from a two-dimensional (2-D) model. When the whole calculation domain was subjected to a single thermal cycle experienced by a monitoring location within the HAZ, the computed grain size was larger than that calculated at the monitoring location by taking into account the prevailing temperature gradient due to thermal pinning. The good agreement between the simulated grain structure and the corresponding experimental results indicates significant promise for understanding grain-growth phenomena in the entire HAZ by using the MC technique.     

Cr含量对2.25Cr-1Mo钢蒸汽氧化性能的影响

对第四代核电主体结构材料2.25Cr-1Mo钢中铬含量对蒸汽氧化性能的影响进行了研究.完成了500 ℃/0.1 MPa下600h的高温蒸气氧化实验,并利用分析天平测量氧化增重,采用扫描电镜、X射线能谱和X射线衍射仪表征氧化膜的结构特征和物相.实验结果表明,Cr质量分数为1.99％和2.37％的实验钢氧化增重曲线都符合立...     

Hydrogen Attack kinetics of 2.25 Cr-1 Mo steel weld metals

The kinetics of Hydrogen Attack (HA) of the base metals and the weld metals of two Q&T 2.25 Cr-1 Mo steel weldments made by different techniques (SMAW and SAW) were studied in the temperature range 460 to 590°C (860 to 1094 °F) and 10 to 23 MPa of hydrogen. A sensitive dilatometer used to measure the rate of HA showed that the weld metals suffered HA at significantly higher rates than the base metals. The SMAW weld metal was inferior to the SAW weld metal and swelled nearly an order of magnitude faster than the base metal. This behavior is due to a significantly higher bubble density, and a resulting higher contribution of power law creep of the matrix. The SAW behavior was intermediate between those of the base metals and the SMAW. For the same hydrogen pressure the operating limit of the SMAW weld would be roughly 100°C lower than that of the base metals, and that of the SAW roughly 50°C lower.     

Dislocation damping and microstructural evolutions during creep of 2.25Cr-1Mo steels

We studied the microstructural evolution of 2.25Cr-1Mo steels subjected to tensile creep at 923 K through monitoring of shear-wave attenuation and velocity, using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the magnetostrictive mechanism is the key to establishing a monitor for microstructural change in the bulk of the metals with a high sensitivity. In the short interval, 50 to 60 pct of the creep life, attenuation experiences a peak, being independent of the applied stress. This novel phenomenon is interpreted in terms of the drastic change in dislocation mobility and rearrangement, which is supported by transmission electron microscopy (TEM) observations for dislocation structure. At this particular period, the dense dislocation structure starts to transform to subgrain boundaries, which temporally accompanies long, free dislocation, absorbing much ultrasonic energy to produce the attenuation peak. The EMAR has the potential to assess the damage advance and to predict the remaining creep life of metals.     

核能用2.25Cr-1Mo钢水蒸气氧化行为

 为了研究核电用2.25Cr-1Mo钢的抗蒸汽氧化性能,在500 ℃、0.1 MPa水蒸气条件下,对2.25Cr-1Mo耐热钢进行了600 h的氧化试验,利用分析天平测定样品氧化增重,获得氧化动力学曲线,通过扫描电镜观察分析了氧化膜的形貌和结构,结合X射线衍射和能谱分析对氧化产物进行物相分析。试验结果表明,2.25Cr-1Mo耐热钢的氧化增重曲线符合立方规律;氧化膜为双层结构,氧化膜内层较为致密的主要物相为(Fe,Cr)₃O₄尖晶石,氧化膜外层疏松多孔主要物相为Fe₃O₄和少量Fe₂O₃;腐蚀速率测定结果表明,该材料具有较好的抗蒸汽氧化性能。     

Influence of microstructural variations in the weldment on the high-temperature corrosion of 2.25Cr-1Mo steel

In order to study the influence of microstructural variation on the oxidation of the weldment of 2.25Cr-1Mo steel, regions with different microstructures were identified by optical microscopy. The weld metal, the base metal, and the heat-affected zone (HAZ), as well as the subzones within the HAZ, i.e., the intercritical (ICR), the fine-grain bainite (FGB), and the coarse-grain bainite (CGB) regions were separated from the weldment by precise steps of metallography. Transmission electron microscopic examinations for the identification of the secondary phases in microstructurally different regions and subzones have suggested that M₂₃C₆ and M₇C₃ pre-cipitates form predominantly in the subzones of HAZ, whereas the Mo₂C type of carbide forms exclusively in the weld-metal and base-metal regions of the weldment. However, population and distribution of the secondary phases were different in the three subzones of the HAZ. In order to understand the influence of these microstructural variations on the oxidation behavior, the various regions and subzones were oxidized at 773 and 873 K. The HAZ and its constituents were found to oxidize at much higher rates than the weld metal and the base metal. Relative compositions and morphologies of the scales were compared by scanning electron microscopy with energy-dispersive analyses of X-rays (SEM/EDX), and secondary ion mass spectrometry (SIMS). Scale formed over the weld metal shows a greater tendency for spallation, as suggested by tests monitoring acoustic emission. X-ray diffraction (XRD) patterns of the scales over these specimens were taken. Results of the SEM/EDX, SIMS, and XRD investigations suggest for-mation of inner scales with less Cr(i.e., less protective) over the HAZ than over the weld-metal and the base-metal regions. Variation in the Cr contents of the scales formed over the various regions is proposed to arise from the difference in microstructural features in different regions of the weldments. Formerly with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkan, India     

Hydrogen attack behavior of the heat affected zone of a 2.25Cr-1Mo steel weldment

The kinetics of hydrogen attack (HA) has been studied in the heat affected zone (HAZ) in a 2.25Cr-1Mo steel weld to determine the relative rates of attack and bubble nucleation in the HAZ, base metal, and weld metal. The HAZ was found to suffer hydrogen attack at nearly twice the rate of the base metal, but not as rapidly as the weld metal. Nucleation of bubbles does not occur during HA of the HAZ of a 2.25Cr-1Mo steel, on exposure to hydrogen pressure of 20.5 MPa or less, but does occur at higher pressures up to 31.5 MPa (4500 psi) at 550 °C, or up to 27.5 MPa (4000 psi) at 580 °C. Such nucleation results in enhancement of the HA rate by a factor of six. The weak dependence of nucleation effects on hydrogen pressure and the saturation of the nucleation effects in a short time suggest some thermally activated nucleation of fresh bubbles. Formerly with The Ohio State University.     

Prediction of type IV cracking behaviour of 2.25Cr-1Mo steel weld joint based on finite element analysis

Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint.     

Variation of the fracture mode in temper embrittled 2.25 Cr-1 Mo steel

Temper embrittled 2.25 Cr-1 Mo steel was tested by slow bending of notched specimens at various temperatures, and the fracture mode was examined by SEM fractography. Comparison of the local fracture mode with the load-displacement curves showed that intergranular fracture occurred most prominently in the region where cracking initiated, but that the fracture mode tended to change to cleavage as the cracking propagated and accelerated. When the area fraction of intergranular fracture was plotted as a function of test temperature, a maximum appeared, and the temperature of this maximum tended to increase with specimen hardness. It is argued that the gap between the cleavage fracture stress (σ _F ^CL ) and that of intergranular fracture (σ _F ^IG ) was greatest at some particular temperature, allowing a maximum amount of grain boundary fracture. However, the gap (σ _F ^CL -σ _F ^IG ) diminished as cracking accelerated, and the fracture mode tended to switch to cleavage. The contrast in behavior between temper embrittled CrMo and NiCr steels is discussed.     

Relevance of high-temperature oxidation in life assessment and microstructural degradation of Cr-Mo steel weldments

Measurement of the thickness of oxide scales that develop over high-temperature components has found an innovative application in life assessment of steam generation and handling systems. The present study is an investigation of the high-temperature corrosion and scale thickness across the weldments of a “chromium-molybdenum” steel, and reviews its possible relevance to the life assessment of the welded high-temperature components by scale thickness measurement. Results are presented of the recent investigations on the combined roles of the oxidizing environment and secondary carbide precipitation on the extent of void formation in the microstructurally different regions of weldments of the chromium-molybdenum steel. Specimens of the weld metal, heat-affected zone (HAZ), and base metal of a 2.25Cr-1Mo steel weldment were oxidized in steam. Extensive internal oxidation and oxidation-induced void formation (with a much greater intensity in the case of the HAZ) is discussed. The greater intensity of oxidation-induced void formation in the HAZ may facilitate preferential cracking in this region of the weldments and, hence, is proposed to be an important parameter in the context of the recently developed codes for life assessment of aging high-temperature components.     

Restoration of elevated temperature tensile strength in 2.25Cr-1Mo steel

Metallographic studies have been conducted on 2.25Cr-lMo steel specimens taken from pressure vessels after long term service exposures at 500 to 650 °C. The loss of strength in the 2.25Cr-lMo steel after the service exposure consists of two parts: (1) the strength loss accompanying the change in the carbide morphology as usually occurs during tempering, and (2) a strength loss which occurs without any noticeable microstructural change. This second part of the strength loss can also be produced by relatively short term thermal treatment such as step-cooling from the tempering temperature, and is reversible by retempering followed by air-cooling. This type of strength loss is associated with a reduction of molybdenum content in solid solution in the ferrite. It is hypothesized that the strength loss and recovery of 2.25Cr-lMo steel are produced by changes in the solid solution strengthening mainly due to molybdenum, carbon, and possibly nitrogen. Subsidiary of AMAX Inc.     

The role of grain size on the ductile-brittle transition of a 2.25 Pct Cr-1 Pct Mo steel

The ductile-brittle transition temperature of a 2 1/4 pct Cr-1 pct Mo steel has been meas-ured using ‘V’ notch Izod impact specimens for an unembrittled and embrittled 2 1/4 pct Cr-1 pct Mo steel with prior austenite grain sizes within the range 40 to 150 μm. The mi-crostructure of this steel was upper bainite. The variation of yield strength with grain size obeys a Hall-Petch relationship. The ductile-brittle transition temperature was found to have a pronounced grain size dependence for both unembrittled, 15 K mm^1/2, and embrittled, 19 K mm^1/2, specimens. The bainite colony size was found to vary as the prior austenite grain size. From the low temperature quasi-cleavage facet size, together with metallographic observations of crack path, it has been concluded that bainite colony size rather than prior austenite grain size is the effective grain size.     

Substructure and strengthening mechanisms in 2.25 Cr-1 Mo steel at elevated temperatures

The substructures of thermally aged, creep deformed and fatigued 2.25 Cr-1 Mo steel have been studied using optical and transmission electron microscopy. In agreement with earlier work, the substructure of the proeutectoid ferrite was found to be very stable when exposed to thermal aging or creep deformation. This stability is explained based on the tendency of molybdenum atoms to form pairs in the ferrite matrix. Nucleation and growth of additional carbide particles during creep testing was not observed. The results of these creep tests and those of Klueh have been interpreted on the basis of Mo pair stability and the affinity between molybdenum and carbon. Fatigue tests at 866 K, however, did produce a fine Mo_u2C precipitate which contributed to secondary cyclic hardening in tests lasting longer than 200 h. The alloy was found to undergo early cyclic hardening followed by abrupt softening within the first tens of cycles.     

Thermodynamic properties of carbides in 2.25Cr-1Mo steel at 985 K

Thermodynamic properties of carbides present in 2.25Cr-lMo steel were determined at 985 K by a gas flowing method with fixed CH₄/H₂ gas mixtures and by a silica capsule method with reference alloys. The carbon activity range was from 0.06 to 0.5. Total carbon content, carbide species, and Cr and Mo partitionings between the matrix and carbides were measured as a function of the carbon activity. Both M₆C and M₂₃C₆ carbides were present after 1000 to 3000 hours at the test temperature and in the carbon activity range studied. The amount of M₆C was greater in the low carbon activity range, while M₂₃C₆ carbide became the major carbide with increasing carbon activity. The M6C carbide contained Mo as a major element and Cr and Si as minor elements; approximately 13 pct of the metal constituent was (Cr + Si). The stability of M₆C carbide in this steel is significantly higher than M₆C formed in the Fe-Mo-C system. The M₂₃C₆ carbide contained Cr as a major metal component and Mo as a minor. The M₂₃C₆ carbide is more stable in an extended range of the carbon activity in 2.25Cr-lMo steel than in the Fe-Cr-C system. The presence of Si is apparently low in M₂₃C₆. Thermodynamic parameters were computed for M₆C and M₂₃C₆ carbides using a regular solution model of component carbides, FeC_x, CrC_x, and MoC_x.     

Heat treatment effects on creep and rupture behavior of annealed 2.25 Cr-1 Mo steel

The strength of 2 1/4 Cr-1 Mo steel depends on the microstructure, which, in turn, de-pends on the heat treatment. In the fully annealed and isothermally annealed conditions, the microstructure is primarily proeutectoid ferrite with varying amounts of bainite and pearlite. The relative amounts of the latter constituents depend on the cooling rates during the anneal. The creep and rupture properties were determined for steel plates (from a single heat) given three different annealing treatments: two were fully annealed, but cooled at different rates from the austenitizing temperature, and the third was iso-thermally annealed. Properties were determined at 454, 510, and 566°C. At 454 and 510°C, the cooling rate had a significant effect on the creep and rupture properties, with the ma-terial cooled fastest being the strongest. Although at 510°C strengths at short rupture times differed widely, the properties approached a common value at longer rupture times. The properties differed very little at 566°C, even for short rupture times. The effect of heat treatment was concluded to be the result of interaction solid solution hardening, a dislocation-drag process. This process gave rise to nonclassical creep curves (as op-posed to classical curves with single primary, secondary, and tertiary stages). By examining the creep-curve shape, it was possible to interpret the heat treatment effects on the creep-rupture properties.     

Effects of tempering on the carbon activity and hydrogen attack kinetics of 2.25 Cr-1 Mo steel

The variation with tempering of the carbon activity and the Hydrogen Attack rates of a Q&T 2.25 Cr-1 Mo steel (A542 C13) was studied at 550 °C. A highly sensitive capacitance dilatometer was used to measure the HA strain rates as a function of hydrogen pressure, and an equilibration technique was used for the carbon activity. Both the carbon activity and the HA strain rate decreased monotonically with the extent of tempering. A strong correlation existed between carbon activity and the HA strain rate of the samples. Excessive tempering beyond the commercial practice did not eliminate HA, and the carbon activity of a sample tempered for 500 hours at 700 °C was as high as 0.05. The high carbon activity of the excessively tempered samples is explained as due to the effects of low Cr/Fe ratio in M₂₃C₆ carbides and to less than the equilibrium Cr content next to the M₂₃C₆ resulting from the low diffusivity of Cr in α-iron at the tempering temperature of 700 °C. The methane pressure dependence of the HA strain rates suggests a grain boundary diffusion controlled growth of bubbles for hydrogen pressures up to 20 MPa at 550 °C.     

An assessment of creep deformation and fracture behavior of 2.25Cr-1Mo similar and dissimilar weld joints

The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain (FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint, at stresses ≥150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near to the weld interface. The failure occurred by extensive intergranular creep cavity formation.     

Magnetic Evaluation of Microstructure Changes in 9Cr-1Mo and 2.25Cr-1Mo Steels Using Electromagnetic Sensors

This paper presents results from a multi-frequency electromagnetic sensor used to evaluate the microstructural changes in 9Cr-1Mo and 2.25Cr-1Mo power generation steels after tempering and elevated temperature service exposure. Electromagnetic sensors detect microstructural changes in steels due to changes in the relative permeability and resistivity. It was found that the low frequency inductance value is particularly sensitive to the different relative permeability values of both steels in the different microstructural conditions. The changes in relative permeability have been quantitatively correlated with the microstructural changes due to tempering and long-term thermal exposure, in particular to changes in martensitic/bainitic lath size and number density of carbide precipitates that determine the mean free path to reversible domain wall motion. The role of these microstructural features on pinning of magnetic domain wall motion is discussed.