Tempering of 2.25 Pct Cr-1 Pct Mo Low Carbon Steels

The effect of carbon level on the tempering behavior at 700°C of 2.25 pct Cr-1 pct Mo steels having typical weld metal compositions has been investigated using analytical electron microscopy and X-ray diffraction techniques. The morphology, crystallography and chemistry, of each of the various types of carbides observed, has been established. It has been shown that each carbide type can be readily identified in terms of the relative heights of the EPMA spectra peaks for iron, chromium, molybdenum, and silicon. A decrease in the carbon level of the steel increases the rate at which the carbide precipitation reactions proceed, and also influences the final product. Of the carbides detected, M₂₃C₆ and M₇C₃ were found to be chromium-based, and their compositions were independent of both the carbon level of the steel and the tempering time. The molybdenum-based carbides, M₂C and M₆C, however, showed an increase in their molybdenum contents as the tempering time was increased. The rate of this increase became greater as the carbon content of the steel was lowered.     

Evolution and Coarsening of Carbides in 2.25Cr-1Mo Steel Weld Metal During High Temperature Tempering

Transformation and coarsening of carbides in 2.25Cr-1Mo steel weld metal during tempering at 700 ℃ for different time intervals ranging from 1 to 150 h were examined by transmission electron microscopy and scanning electron microscopy. M3C carbides were observed in the as-welded specimens and when tempered, the precipitates were mainly composed of M3C, M7C3, and M23C6 carbides. A sequence for corresponding carbide transformation during tempering with initial precipitation of M3C and the subsequent precipitation of M7C3 and M23C6 was proposed. The precipitation of M7C3 with higher chromium content was the main factor contributing to the decrease in coarsening rate of precipitates after prolonged tempering. The decrease in hardness of the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening owing to the coarsening of carbides.     

Evolution of Carbide Precipitates in 2.25Cr-1Mo Steel during Long-Term Service in a Power Plant

Carbide precipitation from the steel matrix during long-term high-temperature exposure can adversely affect the fracture toughness and high-temperature creep resistance of materials with implications on the performance of power plant components. In the present work, carbide evolution in 2.25Cr-1Mo steel after long-term aging during service was investigated. Boiler pipe samples of this steel were removed from a supercritical water-cooled coal-fired power plant after service times of 17 and 28 years and a mean operational temperature of 810 K (537 °C). The carbide precipitation and coarsening effects were studied using the carbon extraction replica technique followed by analysis using transmission electron microscopy and energy dispersive X-ray spectroscopy. The carbides extracted using an electrolytic technique were also analyzed using X-ray diffraction to evaluate phase transformations of the carbides during long-term service. Small ball punch and Vickers hardness were used to evaluate the changes in mechanical performance after long-term aging during service.     

Mechanism of Embrittlement and De-Embrittlement for 2.25Cr-1Mo Steel

 The constant embrittlement curve for constant segregation concentration on grain boundary of impurity element P and relationship between equilibrium grain boundary segregation concentration and operation time for 225Cr-1Mo steel were derived based on the theory of equilibrium grain boundary segregation. The mechanism of step-cooling test and mechanism of de-embrittlement for 225Cr-1Mo steel were explained. The segregation rate will increase but equilibrium grain boundary segregation concentration of impurity element P will decrease as temperature increases in the range of temper embrittlement temperature. There is one critical temperature of embrittlement corresponding to each embrittlement degree. When the further heat treating temperature is higher than critical temperature, the heat treating will become a de-embrittlement process; otherwise, it will be an embrittlement process. The critical temperature of embrittlement will shift to the direction of low temperature as further embrittlement. As a result, some stages of step-cooling test would change into a de-embrittlement process. The grain boundary desegregation function of impurity element P was deduced based on the theory of element diffusion, and the theoretical calculation and experimental results show that the further embrittlement or de-embrittlement mechanism can be interpreted qualitatively and quantitatively by combining the theory of equilibrium grain boundary segregation with constant embrittlement curve.     

2.25Cr-1Mo钢低硅冶炼方法浅析

舞钢炼钢厂现有一台90t超高功率电弧炉、两台LF精炼炉和一台VD炉，工艺流程为UHP→LF→VD→CC(IC)，主要生产硅含量≥0．15％系列钢种。为满足市场要求，生产了一批低硅钢种，在生产过程中存在一些问题，即部分炉次在精炼过程中涨[Si]严重，不能保证成品[Si]≤     

抗氢2.25Cr-1Mo钢热处理工艺与性能研究

本文主要研究了2.25C r-1M o钢正火处理后显微组织和回火过程中碳化物相对钢的强韧性的影响,奥氏体化处理后进行冷却(加速冷却和空冷),得到的显微组织为粒状贝氏体和先共析铁素体。对于2.25C r-1M o厚钢板,显微组织和碳化物相的变化是造成2.25C r-1M o钢强韧性能变化的主要原因。     

作用应力对2.25Cr-1Mo合金钢回火脆性的影响

在146.7MPa的作用应力下,对加氢反应器材料2.25Cr-1Mo合金钢(%:0.15C、2.32Cr、0.95Mo、0.011S、0.009P、0.0068As、0.0035Sb、0.0079Sn、0.01V)进行468℃125h和400h的等温回火脆化试验。根据加氢反应器母材试块脱脆、脆化和应力作用3种状态冲击功和温度关系曲线,得出各状态回火脆性转变温度VTr54.2(℃)值和回火脆化度ΔVTr54.2(℃)。结果表明,温度和等温时间是导致材料回火脆化的主要因素,作用应力对2.25Cr-1Mo钢回火脆性的影响不显著。     

2.25Cr-1Mo合金钢回火脆化过时效现象的研究

研究了2.25Cr-1Mo合金钢在回火温度下时效处理时,发生回火脆化和回火脆化的过时效反偏聚现象。应用非平衡偏聚的动力学理论,确定了成分(%)为0.15C、2.32Cr、0.95Mo、0.009P的2.25Cr-1Mo合金钢在650℃140 h回火时钢中磷的非平衡晶界偏聚规律。试验得出,2.25Cr-1Mo合金钢650℃磷非平衡偏聚的临界时间t_c为20 h,晶界磷的原子浓度,由初始状态的0.016%提高至最大值2.79%。根据试验数据进行的动力学计算,得出的计算曲线与试验结果吻合,验证了非平衡晶界偏聚空位-复合体模型的扩散机制。     

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.     

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...     

Atom Probe Tomography Analysis of Precipitation during Tempering of a Nanostructured Bainitic Steel

Carbon distribution during tempering of a nanostructured bainitic steel was analyzed by atom probe tomography (APT). Three different types of particles are detected on samples tempered at 673 K (400 °C) for 30 minutes: lower bainite cementite with a carbon content of ~25 at. pct, ε-carbides with a carbon content close to 30 at. pct, and carbon clusters, small features with a carbon content of ~14 at. pct indicative of a stage of tempering prior to precipitation of ε-carbide. After tempering at 773 K (500 °C) for 30 minutes, the ε-carbide-to-cementite transition was observed. Solute concentration profiles across carbide/ferrite interfaces showed the distribution of substitutional elements in ε-carbide and cementite for all the tempering conditions.     

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.     

回火时间对0．16C－10Ni－14Co－1Cr－1Mo钢和0．23C－12Ni－14Co－3Cr－1Mo钢组织与性能的影响

研究了０．１６Ｃ－１０Ｎｉ－１４Ｃｏ－１Ｃｒ－１Ｍｏ（１６ＮｉＣｏ）钢在５１０℃和０．２３℃－１２Ｎｉ－１４Ｃｏ－３Ｃｒ－１Ｍｏ（２３ＮｉＣｏ）钢在４８２℃回火的组织与性能。随回火时间的延长，强度、硬度的降低主要是由于Ｍ_２Ｃ的粗化及与基体共格性降低所致。Ｍ_２Ｃ的粗化速度２３ＮｉＣｏ钢要小于１６ＮｉＣｏ钢。     

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.     

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.     

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。     

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.     

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.     

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.