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.     

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.     

Structures and tempering behavior of rapidly solidified high-carbon iron alloys

High-carbon iron alloys containing carbide formers of chromium and molybdenum were rapidly solidified by means of a single roller method. In the alloy containing a high level of both chromium and molybdenum (10Cr-5Mo) and a critical carbon content of about 4 pct, the metastable phases,ε phase and austenite, are retained after solidification. Theε phase could contain a large amount of carbon in solid solution so that during tempering at about 900 K, it decomposes to very fine ferrite and carbide, which bring about an enhanced hardness of 1300 DPN. Even after tempering at a high temperature around 1100 K, the hardness hardly deteriorates due to a remarkable dispersion of fine M₃C and M₇C₃ carbides. Thus, coaddition of chromium and molybdenum is effective in obtaining high hardness. Formerly Graduate Student, Kyushu Institute of Technology     

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.     

As-cast carbides in high-speed steels

The spatial distribution and structure of as-cast carbides and the effects of W, Mo, and V content on the morphology and amount of as-cast carbides in high-speed steels were studied systematically. It was shown that increasing the Mo and decreasing the W content led to a decrease in the amount of total eutectic carbide and an increase in the MC and M₂C carbides. The eutectic morphology changed from skeletal to platelike when the content of Mo increased. The presence of V favored not only the formation of MC carbide but also the formation of M₂C carbide and reduced the formation of M₂C carbide. Increasing V led to an increase in the size of the eutectic carbides.     

Carbide reactions (M3C→M7C3→M23C6→M6C) during tempering of rapidly solidified high carbon Cr-W and Cr-Mo steels

Carbide transformations of M₃C → M₇C₃ → M₂₃C₆ → M₆C and crystallographic relationships among these carbides were examined by transmission electron microscopy. Two kinds of high carbon-chromium steels containing tungsten or molybdenum were quenched rapidly from the melts and tempered at temperatures up to 700°C. By tempering at 600°C, M₇C₃ carbides nucleated mostly on cementite/ferrite interfaces and grew inward the cementite byin- situ transformation.In-situ transformations from M₇C₃ to M₂₃C₆ and from M₂₃C₆ to M₆C were also found in these alloy steels during tempering at higher temperatures. Mutual relationships of crystal orientations among M₃C, M₇C₃, M₂₃C₆ and M₆C were decided as follows: {fx739-01}.     

Thermodynamics of the Fe-Mo-C system at 985 K

The solubility of carbon and the composition of carbides in the ferritic Fe-Mo-C system were measured at 985 K by a gas flowing method and a sealing method. The composition of alloys ranged from 0.24 pct to 2.93 pct Mo. An iron-carbon binary alloy was included in the equilibration as a reference material. The molybdenum-carbon interaction in the α-phase was analyzed by the central atoms model. The Wagner interaction coefficient was determined as ε _c ^Mo = •100 ± 2, which is a higher negative value than that in the Fe-Cr-C system at the same temperature. The carbide phase was analyzed as a regular solution of two component carbides, FeC _x and MoC _x . M₆C carbide was in equilibrium with α in the carbon activity range from 0.045 to 0.156, and M₂C carbide was in equilibrium at the carbon activity 0.51. M₆C and M₂C carbides were present at the carbon activity 0.45. Molybdenum partitioning between α- and carbide phases was measured. The standard free energies of formation of two component carbides and the interaction energy parameters were determined for M₆C and M₂C carbides.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M₂C carbides formed during the dissolution of the cementite (M₃C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M₂C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M₂C carbide, while Co increased the nucleation rate of M₂C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M₂C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.     

Strength and toughness of Fe-10ni alloys containing C,Cr, Mo,and Co

The effects of C (0.10 to 0.20 pct), Cr (0 to 3 pct), Mo (0 to 2 pct), and Co (0 to 8 pct) on the yield strength, toughness (Charpy shelf energy), and tempering behavior of martensitic lONiCr-Mo-Co steels have been investigated. Variations in the carbon content between 0.10 and 0.20 pct result in yield strengths between 160 and 210 ksi (1.1 and 1.45 GN/m²) when these steels are tempered at 900° to 1000°F (480° to 540°C) for times of 1 to 100 h. These steels exhibit a secondary-hardening peak at 900° to 1000° F (480° to 540°C) where coarse Fe₃C carbides are gradually replaced by a fine, dislocation-nucleated dispersion of (Mo, Cr)₂C carbides. Maximum toughness at a given yield strength in these steels is only obtained when they are tempered for sufficiently long times so that the coarse Fe₃C carbides are completely dissolved. Molybdenum is primarily responsible for the secondary-hardening peak observed in these steels. However, chromium additions do result in lower secondaryhardening temperatures and promote coarsening of the secondary-hardening carbide. Best combinations of strength and toughness are obtained with steels containing 2 pct Cr and 1 pct Mo. Cobalt increases the yield strength of these steels over the entire tempering range and results in a higher secondary-hardening peak. This effect of cobalt is attributed to 1) a retardation in the rate of recovery of the dislocation substructure of the martensite, 2) the formation of a finer dispersion of secondary-hardening carbides, and 3) solid-solution strengthening. The finer dispersion of secondary-hardening carbides in steels containing cobalt is favored by the finer dislocation substructure in these steels since the (Mo, Cr)₂C carbide is dislocation-nucleated. This fine dispersion of (Mo, Cr)₂C carbide combined with the high nickel content accounts for the excellent combination of strength and toughness exhibited by these steels.     

Effect of Silicon on Carbide Precipitation after Tempering of H11 Hot Work Steels

The formation of secondary carbides during tempering of H11 hot work steels at 898 K (625 °C) was studied by transmission electron microscopy (TEM) and related to the previously established effects of Si content on mechanical properties. Lower Si contents (0.05 and 0.3 pct Si) and higher Si contents (1.0 and 2.0 pct Si) were observed to yield different carbide phases and different particle distributions. Cementite particles stabilized by Cr, Mo, and V in the lower Si steels were found to be responsible for similar precipitation hardening effects in comparison to the M₂C alloy carbides in the higher Si steels. The much higher toughness of the lower Si steels was suggested to be due to a finer and more homogeneous distribution of Cr-rich M₇C₃ carbides in the interlath and interpackage regions of the quenched and tempered martensite microstructure. The present effects of Si content on the formation of alloy carbides in H11 hot work steels were found to be the result of the retarding effect of Si on the initial formation of cementite, well known from the early tempering stages in low alloy steels.     

Identification of M5C2 carbides in ex- service 1Cr-0.5Mo steels

Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M₅C₂ carbides, although some were M₃C. The M₅C₂ carbide, also known as the Hägg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M₅C₂ rods appeared to nucleate heterogeneously on M₂C carbides and persist in ferrite regions from which the needlelike M₂C carbides had disappeared. This suggests that the M₅C₂ carbide is more stable thermodynamically than M₂C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components.     

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.     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Precipitation of nanoscale carbides in Ti- Mo microalloyed steel during tempering process

The distribution, morphology and size of the carbide precipitates in as- rolled and as- tempered Ti- Mo microalloyed steels were elucidated by optical microscopy and transmission electron microscopy. The change in nanoscale precipitate during tempering and its effect on strength of test steel were analyzed by tensile test. The results revealed that substantial improvement in yield strength occurred on tempering at 600?? for 2 h because of the supersaturated precipitation and homogeneous distribution of profuse (Ti, Mo)C carbide in the matrix in average size from 5-6nm except interphase precipitation, and the precipitation volume fraction of the sample tempered at 600?? exhibited an approximate 3%-5% increase compared to the samples tempered at 650 and 700??. With increase in tempering temperature and holding time, the interphase- precipitated carbides were observed to have slightly coarsened to a maximum size of less than 8nm, but did not coarsen as much as the supersaturated carbides formed during tempering. The interphase precipitation exhibits more excellent behavior of thermal stability than supersaturated precipitation during tempering process.     

Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe₃C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo₂C (aspect ratio ranging from 10 to 25) and plate-shaped M₄C₃ carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe₃C, and interlath-retained austenite decomposed into ferrite and elongated carbide.     

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M₇C₃ carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M₂C carbides precipitate separately within the bainitic laths during slow heating. M₂₃C₆ carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M₇C₃ and, depending on the temperature, coarsening, or dissolution of M₂C carbides, respectively.     

The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls

The transformation of carbides with austenization time of a high speed steel (HSS) roll material, manufactured by a centrifugal casting method, has been studied. The correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides has also been investigated. Microstructural observations, X-ray diffraction (XRD) analysis, hardness measurements, and energy dispersive spectroscopy (EDS) have been used to characterize the carbides. The type and volume fraction of carbides were found to change with austenizing time. During austenization, the transformation of the M₃C carbides can be postulated as M₃C + γ-Fe → M₂C, with much finer nodular and rodlike MC carbides also forming through a solid-state transformation. The M₂C carbide decomposes as M₂C + γ-Fe → MC + M₇C₃ + M₆C. The decomposed carbide substantially maintains a platelike shape until the end of decomposition. The most important finding of this study is that austenization results in changes in the type, morphology, volume fraction, and distribution of carbides and that it can be controlled to produced a homogeneous distribution of hard carbides, resulting in an improvement in the wear resistance of HSS rolls. This finding may be of great use for the industrial production of HSS rolls.     

Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel

Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 °C and 600 °C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 °C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M _s ) and martensite finish (M _f ) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the M _s temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 °C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Magnetic Field-Induced Precipitation Behaviors of Alloy Carbides M2C,M3C,and M6C in a Molybdenum-Containing Steel

The effect of a 12-T high magnetic field on alloy carbide precipitation in an Fe-C-Mo alloy during tempering at an intermediate temperature was investigated. Thin foils and carbon extraction replicas of the treated specimens were examined by transmission electron microscopy (TEM). The results show that the applied high field effectively promoted the precipitation of (Fe,Mo)₆C alloy carbide. The concentration of Fe atom in Fe_6?x Mo _x C carbide is increased whereas that of Mo atom decreased when the high magnetic field was applied. However, the high magnetic field almost had no detectable influence on the atom concentration in (Fe,Mo)₂C and (Fe,Mo)₃C carbides. First principle calculations have been performed to calculate the magnetic moment per iron atom of the carbides to explore the origin of the effect of the magnetic field. The influence of the high magnetic field on the precipitation behaviors of alloy carbides was closely related to the magnetic moment of (Fe,Mo)₂C, (Fe,Mo)₃C, and (Fe,Mo)₆C. The magnetic field promotes the formation of the carbides with high total magnetic moment. The effect of the high magnetic field on the substitutional solute atom (Fe and Mo) concentration change in the three alloy carbides was attributed to their magnetization differences per Fe atom.