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1.
In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which cointain alloying elements, such as Mo, W and Cr, contributing to
the secondary hardening by forming M 2C type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening,
while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and
considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition,
because the cementite of M 3C type was stabilized at higher temperatures and the formation of M 2C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the
secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation
for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition. 相似文献
2.
In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which contain alloying elements, such as Mo, W and Cr, contributing to
the secondary hardening by forming M 2C-type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening,
while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and
considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition,
because the cementite of M 3C type was stabilized at higher temperatures and the formation of M 2C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the
secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation
for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition. 相似文献
3.
This article considers five different microstructures of a tempered martensitic 0.34C, 3Ni-1.3Cr-0.4Mo-0.1V steel through
various heat treatments, including double austenitization (DA) treatments, and how the impact toughnesses are influenced by
microstructure. Of the four mechanisms considered to explain the beneficial effect of DA treatment, the roles of retained
austenite, grain-boundary embrittlement by impurity segregation, and matrix flow stress are discounted. The 50 pct fracture
appearance transition temperature (FATT) of this steel is found to be dependent on both the grain size and the carbide dissolution.
The conventionally treated steel contains mainly platelike M 3C carbides. The DA treatment helps to dissolve VC carbides and coarsen and spheroidize M 3C carbides in favor of the precipitation of short rodlike M 7C 3 carbides with a lower aspect ratio. The improvement of impact toughness (upper shelf energy, ductile-to-brittle transition
temperature (DBTT), and lower shelf energy) by DA treatment, explained in detail, is attributed to a change of this material’s
tensile and work-hardening behavior affected by a variation of carbide morphology. 相似文献
4.
In order to understand the influence of high-temperature aging effects and those of the superimposed creep stress on the microstructural
variations in a 1.25Cr-0.5Mo steel, the shoulder as well as gage portions of specimens subjected to stress-rupture tests at
520 °C and 560 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the microstructure
of the steel consists of 90 pct ferrite and 10 pct bainite, and M 3C is the only carbide present in bainite and at a few ferrite grain boundaries. On aging at 520 °C for 5442 hours, Cr 2N precipitates in a fibrous form at ferrite-bainite interfaces, and the creep stress has enhanced this mode of precipitation.
On holding for 13,928 hours at 520 °C, fibrous carbide is still present but its composition has changed to Mo 2C, while the superimposed creep stress has promoted the precipitation of Mo 2C needles with fine globular precipitates of M 23C 6. Aging at 560 °C for 1854 or 10,338 hours has resulted in the precipitation of longer Mo 2C needles and ellipsoidal M 23C 6 carbide precipitation; the superimposed creep stress has resulted in a more dense precipitation of shorter needles in both
cases. There is some recovery of bainitic ferrite at 560 °C, though the cementite coarsening is negligible. 相似文献
5.
Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement. 相似文献
6.
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, M 23C 6 and M 7C 3 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, M 2C and M 6C, 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. 相似文献
7.
This paper presents a study of carbide precipitation, grain boundary segregation, and temper embrittlement in NiCrMoV rotor
steels. One of the steels was high purity, one was doped with phosphorus, one was doped with tin, and one was commercial purity.
In addition, two NiCrV steels, one high purity and one doped with phosphorus, were examined. Carbide precipitation was studied
with analytical electron microscopy. It was found that after one hour of tempering at 600 ‡C only M 3C carbides were precipitated in the NiCrMoV steels. These were very rich in iron. As the tempering time increased, the chromium
content of the M 3C carbides increased significantly, but their size did not change. Chromium rich M 7C 3 precipitates began to form after 20 hours of tempering, and after 50 hours of tempering Mo-rich M 2C carbides were precipitated. Also, after 100 hours of tempering, the matrix formed bands rich in M 3C or M 7C 3 and M 2C particles. Tempering occurred more rapidly in the NiCrV steels. Grain boundary segregation was studied with Auger electron
spectroscopy. It was found that the amount of phosphorus and tin segregation that occurred during a step-cooling heat treatment
after tempering was less if a short time tempering treatment had been used. It will be proposed that this result occurs because
the low temperature tempering treatments leave more carbon in the matrix. Carbon then compctes with phosphorus and tin for
sites at grain boundaries. This compctition appears to affect phosphorus segregation more than tin segregation. In addition
to these two impurity elements, molybdenum and nickel segregated during low temperature aging. The presence of molybdenum
in the steel did not appear to affect phosphorus segregation. Finally, it will be shown that all of the steels that contain
phosphorus and/or tin exhibit some degree of temper embrittlement when they are aged at 520 ‡C or are given a step-cooling
heat treatment. Of the NiCrMoV steels, the phosphorus-doped steel showed the least embrittlement and the commercial purity
steel the most. The phosphorus-doped NiCrV steel was also more susceptible to temper embrittlement than the phosphorus-doped
NiCrMoV steel. This latter difference was attributed to molybdenum improving grain boundary cohesion. It was also found that
as the segregation of phosphorus or tin to the grain boundaries increased, the measured embrittlement and the amount of intergranular
fracture increased. However, there was a large amount of scatter in all of these data and the trends were only qualitative.
All parts of this study are compared in detail to others in the literature, and general trends that can be discerned from
all of these results are presented.
Formerly with the University of Pennsylvania, Department of Materials Science, Philadelphia, PA 相似文献
8.
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 M 2C 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 M 6C 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 M 6C carbides. The amount of MC and M 2C 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. 相似文献
9.
Specimens of 2 1/4 Cr?1 Mo steel that had been decarburized in sodium or aged in an inert atmosphere for 26,500 h at 839 K were metallurgically examined with optical and transmission electron microscopy. The carbide particle density of the decarburized and aged specimens was considerably less after 26,500 h than after 10,000 h, with the density in the aged specimen being considerably greater than that in the decarburized specimen. For the aged specimen, the density decrease was a result of Ostwald ripening, and the total amount of carbide was not noticeably affected. Decarburization, however, resulted in a gradient in carbide density, with the smallest density near the surface. Carbides were electrolytically extracted and identified by X-ray diffraction. From the before-test material, which was in the annealed condition and had a ferrite-pearlite microstructure, 1.4 wt pct carbide was obtained. This sample contained M 3C, M 23C 6, and M 6C, with M 23C 6 constituting the major portion. In the aged specimen, 2.3 wt pct of precipitate was extracted and contained 60 pct M 6C, the balance M 23C 6. The decarburized specimen contained two regions with different carbide contents. The first 20 pct of the 1.6 mm thick (0.063 in.) specimen yielded 1.3 wt pct precipitate, which contained 94 pct M 6C, the balance M 23C 6. The interior of the specimen contained 1.6 wt pct precipitate, 86 pct M 6C. 相似文献
10.
Rapid solidification by electron beam surface melting of a Mo-base high-speed steel (M7) has produced microstructural features
different from those observed in the conventionally processed material. As a result of rapid solidification, the volume percent
of the carbide phases formed has decreased sharply and has resulted in the formation of M 2C and M 23C 6 carbide phases, while in the conventionally processed material, M 6C and MC carbides were present. Microanalysis of the extracted carbides formed by electron beam melting has yielded an intriguing
finding. M 23C 6 is found to be unusually rich in molybdenum, tungsten, and vanadium; the concentration of (Mo + W), for instance, is approximately
60 wt pct. The corresponding values for Fe and Cr are surprisingly low (6 wt pct Cr and 1 wt pct Fe). This is in marked contrast
with carbides encountered in the conventionally processed high-speed steel, where Cr and Fe are the major constituents. The
shift in composition of the carbide phases could be attributed to the accelerated evaporation of chromium during surface melting
as compared to the evaporation of Mo, W, and V.
formerly Research Associate, University of Connecticut 相似文献
11.
Thermodynamic properties of carbides present in 2.25Cr-lMo steel were determined at 985 K by a gas flowing method with fixed
CH 4/H 2 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 M 6C and M 23C 6 carbides were present after 1000 to 3000 hours at the test temperature and in the carbon activity range studied. The amount
of M 6C was greater in the low carbon activity range, while M 23C 6 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 M 6C carbide in this steel is significantly higher than M 6C formed in the Fe-Mo-C system. The M 23C 6 carbide contained Cr as a major metal component and Mo as a minor. The M 23C 6 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 M 23C 6. Thermodynamic parameters were computed for M 6C and M 23C 6 carbides using a regular solution model of component carbides, FeC x, CrC x, and MoC x. 相似文献
12.
The precipitation sequence in a modified A-286 Fe-base superalloy (nominally Fe + 30 pct Ni + 15 pct Cr + 2.0 pct Ti + 1.25 pct Mo + 0.2 pct Al) was examined by optical and electron microscopy and electrical resistance measurements. Secondary phases which precipitate during aging were found to be γ′ in the matrix; (Ti, Mo)C and a boride, M 3B 2, at grain boundaries; and 17 in three morphologies, i.e., cellular, Widmanstätten, and platelets inside γ′ particles. The exact aging sequence depends upon time, temperature, and pre-age condition. Composite time-temperature-precipitation (TTP) diagrams were developed which show the precipitation sequence for two pre-age conditions, viz., solution treated and high energy rate forged (HERFed). It was found that HERFing prior to aging accelerates γ′ and (Ti, Mo)C precipitation, whereas it has negligible effect on cellular 17 and M 3B 2 precipitation. HERFing also promotes the precipitation of Widmanstätten 17 and 17 platelets inside γ′ particles. 相似文献
13.
The effects of additions of 0.7 pct Mn and/or 0.6 pct Si on the temper embrittlement behavior of 2.25 Cr-1 Mo steel at several
hardness levels were determined. As in Part I, the tendency for the P segregation was dependent upon the Mo concentration
in the ferrite, which is controlled by the types of carbides formed during heat treatment or aging. Additions of either Mn
or Si increase the fraction of grain boundaries which adsorb P (although they do not increase the P concentration on the embrittled
boundaries significantly) thereby raising the amount of temper embrittlement as measured by the transition temperature shift.
Mn and Si appear to act independently and their effects appear to be additive; this is rationalized in terms of their expected
influences on the segregation free energy for P in Fe. 相似文献
14.
Bolts made of 2
1/4 CrMoV steel became gradually embrittled and exhibited an increasing fraction of intercrystalline fracture after long exposure
(more than 25,000 h) at 540 °C. This has been found to be caused by the segregation of phosphorus to the prior austenite grain
boundaries. This was accompanied by the depletion of molybdenum in the ferrite down to about 0.3 pct owing to the formation
of a molybdenum-containing carbide Fe 3Mo 3C(M 6C). Reheating of the embrittled bolts to 680 °C removed the segregation of phosphorus at the grain boundaries and therefore
also removed the intercrystalline embrittlement. However, the molybdenum content of the ferrite was lowered further to about
0.2 pct; hence the scavenging of phosphorus by molybdenum was further reduced, and therefore the embrittling tendency of these
bolts during reuse was increased. Even after reaustenitizing, quenching and tempering, the service life of these bolts was
found to be shorter than in the original condition; this is presumed to be due to an inhomogeneity of molybdenum in the austenite
not removed by the reaustenitizing treatment. 相似文献
15.
In this article, the effects of Mo contents of 0.25 and 0.50 pct on the martensitic structure and carbide precipitation behavior of the experimental steels were investigated and their effects on strength, toughness, and fatigue strength were studied. The results showed that the martensitic blocks and laths were refined and the dislocation density increased with the addition of Mo contents of 0.25 and 0.50 pct. Meanwhile, the amount of carbides increased and the size of carbides decreased in tempered steels. The refinement of carbides is due to the increment of nucleation sites resulting from martensitic structure refinement and the dislocation density increment. Besides, the improvement of thermal stability of M23C6 carbides enriched with Mo also contributes to carbide refinement. With the addition of 0.50 pct Mo, the strength was improved and the toughness did not deteriorate compared with the steel 0.25 pct Mo. Meanwhile, the fatigue strength was also significantly improved with the addition of 0.50 pct Mo. 相似文献
16.
Rod-shaped precipitates up to 6 μm} long and 0.25 μm 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 M 5C 2 carbides, although some were M 3C. The M 5C 2 carbide, also known as the Hägg or X-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M 5C 2 rods appeared to nucleate heterogeneously on M 2C carbides and persist in ferrite regions from which the needlelike M 2C carbides had disappeared. This suggests that the M 5C 2 carbide is more stable thermodynamically than M 2C 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. 相似文献
17.
Metallographie studies have been conducted on 1Cr-0.5Mo steel “taken from a pressure vessel which had been in service for
20 years in a hydrogenous environment at 524 °C. The original microstructure of the steel, reproduced by reheat treatment
of the exposed material, consisted of proeutectoid ferrite and tempered bainite, the carbides being mainly cementite. The
service exposure caused precipitation of needle-like M 2C carbides in the ferritic regions and M 7C 3 carbides in the vicinity of the original cementite particles. Chromium and molybdenum moved from solid solution to the carbides
during the service exposure with 72 pct and 32 pct of the total chromium and molybdenum contents, respectively, remaining
in solid solution after service for 20 years.
Formerly with AMAX Materials Research Center (formerly Climax Molybdenum Company of Michigan) 相似文献
18.
The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch
(CVN) impact and compact tension plane strain fracture toughness ( K
1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered
martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures
between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was
always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage
across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture,
apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated
with TME in the high P steel. K
IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of
a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture
zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C). 相似文献
19.
The effect of carbide precipitation on creep and creep rate curves was investigated for 10Cr-30Mn austenitic steel containing
0.003 to 0.55 wt pct carbon. After solution annealing, the specimens were subjected to creep testing at 873 K for up to 30
Ms (8300 hours). In the low-carbon steels containing below 0.1 wt pct carbon, where carbide precipitation scarcely occurred,
the decrease in creep rate with time in the transient creep region was described by log έ = A - (1/3) log t, where A is a
constant depending on stress and carbon concentration. On the other hand, in the high-carbon steels containing above 0.2 wt
pct carbon, where extensive precipitation of M 23C 6 occurred, the creep rate decreased significantly at long times above 3 to 5 ks (1 hour), deviating from the preceding equation
for the low-carbon steels. The Johnson-Mehl equation with the time exponent n = 2/3 provided a reasonable approximation for
the significant decrease in creep rate at long times. This resulted from a stress-induced precipitation of M 23C 6 on dislocation lines produced by creep deformation. The rate constant of the Johnson-Mehl equation depended on carbon concentration
but not on stress levels examined. 相似文献
20.
The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and “Mo-free AISI 316L” type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29–530°C/s from 800, 1075 or 1250°C. Tetragonal M 2B, M 5B 3 and M 3B 2, all rich in Fe, Cr and Mo, precipitated in the “high B” (40 ppm) AISI 316L steel whereas orthorhombic M 2B, rich in Cr and Fe, was found in the “Mo-free steel” with 23 ppm B. In the “high B steel” a thin (<2 nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M 9B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field. “Non-equilibrium phases” might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed. 相似文献
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