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 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.     

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。     

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.     

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.     

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.     

Stress-assisted hydrogen attack cracking in 2.25Cr-1Mo steels at elevated temperatures

Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500 °C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K_l = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K_l ^6.5. Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K_l rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement. Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University     

Carbide Precipitation During Tempering of a Tool Steel Subjected to Deep Cryogenic Treatment

Using transmission electron microscopy, Mössbauer spectroscopy, and measurements of hardness, the carbide precipitation during tempering of steel X153CrMoV12 containing (mass pct) 1.55C, 11.90Cr, 0.70V, and 0.86Mo is studied after three treatments: quenching at RT and deep cryogenic treatment, DCT, at 77 K or 123 K (?196 °C or ?150 °C). In contrast to some previous studies, no fine carbide precipitation after long-time holding at cryogenic temperatures is detected. After quenching at room temperature, RT, the transient ε(ε′) carbide is precipitated between 373 K and 473 K (100 °C and 200 °C) and transformed to cementite starting from 573 K (300 °C). In case of DCT at 123 K (?150 °C), only fine cementite particles are detected after tempering at 373 K (200 °C) with their delayed coarsening at higher temperatures. Dissolution of cementite and precipitation of alloying element carbides proceed at 773 K (500 °C) after quenching at RT, although some undissolved cementite plates can also be observed. After DCT at 123 K (?150 °C), the transient ε(ε′) carbide is not precipitated during tempering, which is attributed to the intensive isothermal martensitic transformation accompanied by plastic deformation. In this case, cementite is the only carbide phase precipitated in the temperature range of 573 K to 773 K (300 °C to 500 °C). If DCT is carried out at 77 K (?196 °C), the ε(ε′) carbide is found after tempering at 373 K to 473 K (100 °C to 200 °C). Coarse cementite particles and the absence of alloying element carbides constitute a feature of steel subjected to DCT and tempering at 773 K (500 °C). As a result, a decreased secondary hardness is obtained in comparison with the steel quenched at RT. According to Mössbauer studies, the structure after DCT and tempering at 773 K (500 °C) is characterized by the decreased fraction of the retained austenite and clustering of alloying elements in the α solid solution. It is suggested that a competition between the strain-induced transformation of the retained austenite and carbide precipitation during the wear can control the life of steel tools.     

Effect of Constraint on Creep Behavior of 9Cr-1Mo Steel

The effect of constraint on creep rupture behavior of 9Cr-1Mo steel has been investigated. The constraint was introduced by incorporating a circumferential U-notch in a plain cylindrical creep specimen of 5 mm diameter. The degree of constraint was increased by decreasing the notch root radius from 5 to 0.25 mm. Creep tests were conducted on plain and notched specimens at stresses in the range of 110 to 210 MPa at 873 K (600 °C). The creep rupture life of the steel was found to increase under constrained conditions, which increased with the increase in degree of constraint and applied stress, and tended to saturate at a higher degree of constraint. The creep rupture ductility (pct reduction in area) of the steel was found to be lower under constrained conditions. The decrease in creep ductility was more pronounced at a higher degree of constraint and lower applied stresses. Scanning electron microscopic studies revealed a change in fracture behavior with stress and degree of constraint. The fracture surface appearance for relatively lower constrained specimens at higher stresses was predominantly transgranular dimple. Creep cavitation-induced intergranular brittle fracture near the notch root was observed for specimens having a higher degree of constraint at relatively lower stresses. The creep rupture life of the steel under constrained conditions has been predicted based on the estimation of damage evolution by continuum damage mechanics coupled with finite element analysis of the triaxial state of stress across the notch. It was found that the creep rupture life of the steel under constrained conditions was predominantly governed by the von-Mises stress and the principal stress became progressively important with increase in the degree of constraint and decrease in applied stress.     

Austenite Grain Growth in a 2.25Cr-1Mo Vanadium-Free Steel Accounting for Zener Pinning and Solute Drag: Experimental Study and Modeling

Metallurgical and Materials Transactions A - Austenite grain size has been experimentally determined for various austenitization temperatures and times in a 2.25Cr-1Mo vanadium-free steel. Three...     

Evolution of Carbide Precipitates in 1.25Cr-0.5Mo Steel During Simulated Postweld Heat Treatment

In the present work, the effect of simulated postweld heat treatment (SPWHT) on the microstructure of 1.25Cr-0.5Mo steel was systematically studied. Microstructure is mainly composed of ferrites and carbides after quenching-tempering and SPWHT. Outstanding features of carbides, including distribution, composition, morphology, and size, are identified by means of an electron-probe microanalyzer and a transmission electron microscope. The results show that the carbides coarsen and gather along grain boundaries and M₇C₃ transforms into M₂₃C₆ in situ.     

The effect of titanium on creep strength in 2.25 Pct Cr-1 Pct Mo steels

The effect of a low level of titanium on the microstructure and creep properties of 2.25 pct Cr-1 pct Mo steels has been examined as a function of carbon content and austenitizing temperature. The addition of 0.04 wt pct titanium resulted in a dramatic increase in creep strength at 565 °C, and this was found to be associated with the presence in the microstructure of very small (50 to 100 Å) titanium-bearing precipitates based upon both TiC and Mo₂C. The variation of the minimum creep rate with carbon content and austenitizing treatment was explained in terms of the solubility of TiC in austenite. The titanium-bearing carbides have an important effect on microstructural stability and on the maintenance of creep strength, but it is also apparent that solid solution strengthening by molybdenum can make a significant contribution to creep strength at low carbon levels (0.02 wt pct).