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1.
This study was aimed at developing low-alloy steels for nuclear reactor pressure vessels by investigating the effects of alloying
elements on mechanical and fracture properties of base metals and heat-affected zones (HAZs). Four steels whose compositions
were variations of the composition specification for SA 508 steel (class 3) were fabricated by vacuum-induction melting and
heat treatment, and their tensile properties and Charpy impact toughness were evaluated. Microstructural analyses indicated
that coarse M 3C-type carbides and fine M 2C-type carbides were precipitated along lath boundaries and inside laths, respectively. In the steels having decreased carbon
content and increased molybdenum content, the amount of fine M 2C carbides was greatly increased, while that of coarse M 3C carbides was decreased, thereby leading to the improvement of tensile properties and impact toughness. Their simulated HAZs
also had sufficient impact toughness after postweld heat treatment (PWHT). These findings suggested that the low-alloy steels
with high strength and toughness could be processed by decreasing carbon and manganese contents and by increasing molybdenum
content. 相似文献
2.
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 M 2C carbides formed during the dissolution of the cementite (M 3C). 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 M 2C 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 M 2C carbide, while Co increased the nucleation rate of M 2C 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 M 2C 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. 相似文献
3.
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/m 2) 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 Fe 3C 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 Fe 3C 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. 相似文献
4.
The tempering behavior of simulated coarse-grained (CG) heat-affected zones (HAZs) in two ferritic alloy steels, 2.25Cr-1Mo
and HCM2S, was investigated. The hardness of HCM2S was found to be stable at longer times and higher temperatures than the
2.25Cr-1Mo steel, even though the “as-welded” hardnesses were approximately equal. Both materials reached a peak secondary
hardness after tempering for 5 hours at 575 °C. The increase in hardness of the 2.25Cr-1Mo steel was due to precipitation
of Fe-rich M 3C carbides within the prior-austenite grains, whereas the secondary hardening in HCM2S was due to a fine dispersion of intragranular,
W-rich carbides. The HCM2S steel retained its hardness at longer times and higher temperatures than 2.25Cr-1Mo steel, because
of the precipitation of intragranular, W-rich carbides and V-rich MC carbides that stabilized the lath structure. This study
shows that HCM2S should not be heat treated in the same way as 2.25Cr-1Mo steel and also provides a basis for defining the
postweld heat treatment (PWHT) of HCM2S. 相似文献
5.
Thermally stable TiN particles can effectively pin austenite grain boundaries in weld heat-affected zones (HAZs), thereby
improving toughness, but can also act as cleavage initiators. The HAZs simulated in a GLEEBLE 1500 TCS using two peak temperatures
( T
p
) and three cooling times (Δ ∼
8/5) have determined the effects of matrix microstructure and TiN particle distribution on the fracture toughness (crack tip
opening displacement (CTOD)) of three steels microalloyed with 0.006, 0.045, and 0.1 wt pct Ti. Coarse TiN (0.5 to 6 μm) particles are identified in steels with the two higher levels of Ti, and fine Ti(C, N) (35 to 500 nm) particles were present
in all three steels. Large prior austenite grain size caused by higher T
p
decreased fracture toughness considerably in steels containing coarse TiN particles but had little effect in their absence.
Fracture toughness was largely independent of matrix microstructure in the presence of coarse particles. Cleavage fracture
initiation was observed to occur at coarse TiN particles in the samples with a large prior austenite grain size. Alloy thermodynamics
have been used to rationalize the influence of Ti content on TiN formation and its size. 相似文献
6.
This study is concerned with effects of complex oxides on acicular ferrite (AF) formation, tensile and Charpy impact properties, and fracture toughness in heat affected zones (HAZs) of oxide-containing API X80 linepipe steels. Three steels were fabricated by adding Mg and O 2 to form oxides, and various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs. The no. of oxides increased with increasing amount of Mg and O 2, while the volume fraction of AF present in the steel HAZs increased with increasing the no. of oxides. The strengths of the HAZ specimens were generally higher than those of the base metals because of the formation of hard microstructures of bainitic ferrite and granular bainite. When the total Charpy absorbed energy was divided into the fracture initiation and propagation energies, the fracture initiation energy was maintained constant at about 75 J at room temperature, irrespective of volume fraction of AF. The fracture propagation energy rapidly increased from 75 to 150 J and saturated when the volume fraction of AF exceeded 30 pct. At 253 K (?20 °C), the total absorbed energy increased with increasing volume fraction of AF, as the cleavage fracture was changed to the ductile fracture when the volume fraction of AF exceeded 45 pct. Thus, 45 vol pct of AF at least was needed to improve the Charpy impact energy, which could be achieved by forming a no. of oxides. The fracture toughness increased with increasing the no. of oxides because of the increased volume fraction of AF formed around oxides. The fracture toughness did not show a visible correlation with the Charpy absorbed energy at room temperature, because toughness properties obtained from these two toughness testing methods had different significations in view of fracture mechanics. 相似文献
7.
The fractures of three model alloys, imitating by their chemical composition the matrixes of the quenched high-speed steels of various Mo: W relations were analyzed. According to the measurements of the stress intensity factor KIc and the differences in the precipitation processes of carbides it was found out that the higher fracture toughness of the matrix of the molybdenum high-speed steels than on the tungsten ones is the results of the differences in the kinetics of precipitation from the martensite matrix of these steels during tempering. After tempering at 250 and 650°C the percentage of the intergranular fracture increases with the increase of the relation of Mo to W in the model alloys of the high-speed steel matrix. This is probably the result of higher precipitation rate of the M 3C carbide (at 250°C) and the MC and M 6C carbides (at 650°C) in the privileged regions along the grain boundaries. The change of the character of the model alloy fractures after tempering at 450°C from the completely transgranular one in the tungsten alloy to the nearly completely intergranular one in the molybdenum alloy indicates that the coherent precipitation processes responsible for the secondary hardness effect in the tungsten matrix begin at a lower temperature than in the molybdenum matrix. After tempering for the maximum secondary hardness the matrix fractures of the high-speed steels reveal a transgranular character regardless the relation of Mo to W. The higher fracture toughness of the Mo matrix can be the result of the start of the coherent precipitation processes at a higher temperature and their intensity which can, respectively, influence the size of these precipitations, their shape and the degree of dispersion. The transgranular character of the fractures of the S 6-5-2 type high-speed steel in the whole range tempering temperatures results from the presence of the undissolved carbides which while cracking in the region of stress concentration can constitute flaws of critical size which form the path of easy cracking through the grains. The transgranular cracking of the matrix of the real high-speed steels does not change the adventageous influence of molybdenum upon their fracture toughness. On the other hand, the carbides, undissolved during austenitizing, whose size distribution in the molybdenum steels from the point of view of cracking mechanics seems to be unsatisfactory, influence significantly the fracture toughness of these steels. 相似文献
8.
The authors evaluated the effect of the volume fraction and the dispersion rate of cementite on fracture toughness of ferrite. The investigations were performed at -196°C on five types of carbon steels containing 0.028–1.22% of C in which cementite was coagulated at 700°C for 1–8 h from the quenched state. It was determined that the fracture toughness of steel increases very strongly up to the content of carbides of about 7% by volume. At the same time, hardness and strength of these steels grow. First of all, this is the result of size reduction of ferrite grains by fine carbides. These carbides, distributed almost exclusively on grain boundaries, can only participate in the transmission of the crack to the neighbouring grain. At larger contents of carbides, their dispersion rate decreases while their number in the grain volume grows. Fine carbides from inside of the grains set the path of easy cracking on the boundaries with the ferritic matrix while the coarse carbides crack in front of the fracture. As a result, the steel fracture toughness decreases. The fracture development by means of carbides is less harmful than on the carbide/matrix boundaries. 相似文献
9.
The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated
in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass
hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission
gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both
Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition
of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic
phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited
fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed
in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the
ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides
had one variant of the B-N relationship ((100) MC//(100) ferrite, [011] MC//[010] ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation
sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus,
believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic
transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles. 相似文献
10.
The nondestructive evaluation procedure for detecting in-service materials degradation of low-alloy 2.25Cr-1Mo and CrMoV steels
by the electrochemical method has been investigated. The results can be summarized as follows. (1) For 2.25Cr-1Mo steels,
the peak current mainly caused by the selective dissolution of coarse carbides M 6C appears at ∼+100 mV during potentiodynamic polarization measurements in dilute sodium molybdate solution. This peak value
of current density, ΔI p, can be chosen as a reflective parameter of an amount of coarse carbides M 6C and shows excellent correlations both with shifts in fracture appearance transition temperature (FATT) caused by carbide
coarsening and with hardness change. Actual operational temperature can be estimated from operational period, since the Larson-Miller
time-temperature parameter (LMP) value of materials has a unique relationship with ΔI p values. (2) For CrMoV steels, the evaluation of temper embrittlement of CrMoV cast steel by a novel electrochemical technique
is described. Intergranular corrosion (IGC) occurs only on temper-embrittled samples during anodic polarization process in
calcium nitrate solution. The characteristic changes in polarization curves attributed to IGC have an excellent correlation
with shifts in FATT caused by temper embrittlement. 相似文献
11.
Newly developed low-carbon 10 wt pct Ni-Mo-Cr-V martensitic steels rely on the Ni-enriched, thermally stable austenite [formed via multistep intercritical Quench-Lamellarization-Tempering ( QLT)-treatment] for their superior mechanical properties, specifically ballistic resistance. Critical to the thermal stability of austenite is its composition, which can be severely affected in the weld heat-affected zones (HAZs) and thus needs investigations. This article represents the first study of the nanoscale redistributions of C, Ni, and Mn in single-pass HAZ microstructures of QLT-treated 10 wt pct Ni steels. Local compositions of Ni-rich regions (representative of austenite compositions) in the HAZs are determined using site-specific 3-D atom-probe tomography (APT). Martensite-start temperatures are then calculated for these compositions, employing the Ghosh-Olson thermodynamic and kinetics approach. These calculations predict that austenite (present at high temperatures) in the HAZs is susceptible to a martensitic transformation upon cooling to room temperature, unlike the austenite in the QLT-treated base-metal. While C in the QLT-treated base-metal is consumed primarily in MC and M 2C-type carbide precipitates (M is Mo, Cr, V), its higher concentration in the Ni-rich regions in the HAZs indicates the dissolution of carbide precipitates, particularly M 2C carbide precipitates. The role of M 2C carbide precipitates and austenite stability is discussed in relation to the increase in microhardness values observed in the HAZs, relative to the QLT-treated base-metal. Insights gained from this research on austenite stability and carbide precipitation in the single-pass HAZ microstructures will assist in designing multiple weld cycles for these novel 10 wt pct Ni steels. 相似文献
12.
The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without
cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at
673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties
were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence
of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of
interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of
transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness
of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which
had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M s but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at
673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably
Mo 2C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated
the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions. 相似文献
13.
The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness
in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and
ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M 7C 3 carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing
a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These
large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating
the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure
of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries
and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear
resistance by forming additional M 2C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate
compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum. 相似文献
14.
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 M 2C 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 M 7C 3 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. 相似文献
15.
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
16.
The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without
cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at
673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties
were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence
of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of
interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of
transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness
of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which
had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M
S
but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at
673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably
Mo 2C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated
the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions. 相似文献
17.
The effects of heat treatment and of the presence of primary carbides on the fracture toughness, K Ic and the fatigue crack growth rates, da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values of K Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix. 相似文献
18.
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
20.
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 M 2C 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 M 2C carbide and reduced the formation of M 2C carbide. Increasing V led to an increase in the size of the eutectic carbides. 相似文献
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