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1.
Some design guidelines for improving strength-toughness combinations in medium car-bon structural steels are critically reviewed.
From this, quaternary alloy development based on Fe/Cr/C steels with Mn or Ni additions for improved properties is described.
Transmission electron microscopy and X-ray analysis reveal increasing amounts of retained austenite in these alloys with Mn
content up to 2 wt pct and Ni additions at 5 wt pct after quenching from 1100°C. A corresponding improvement in toughness
properties is also found. Grain refining results in a further increase in the amount of retained austenite. In addition, the
excellent combinations of strength and toughness in these quaternary alloys are attributed to the production of dislocated
lath martensite from a homogeneous austenite phase free from undissolved alloy carbides. The question of thermal instability
of retained austenite following tempering is considered in detail and it is shown that the decomposition of retained austenite
is closely related to the ease of nucleation and growth of cementite. Thus, graphitizing alloying elements such as Ni are
beneficial in postponing the decomposition of retained austenite.
Formerly with the Lawrence Berkeley Laboratory, Berkeley, CA
This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual
meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metal-lurgy Committee
of The Metallurgical Society of AIME. 相似文献
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Donald Webster 《Metallurgical and Materials Transactions B》1971,2(7):1857-1862
It has been found possible to increase the strength and toughness of two high-strength stainless steels, AFC 77 and AFC 260, by austenitizing at temperatures that are in the range where both austenite and δ ferrite are stable. The δ ferrite is then removed by isothermal transformation in the range 1800° to 2000°F. This technique results in a greater solution of carbides and intermetallic particles and consequently in a greater amount of retained austenite than is possible at austenitizing temperatures below the δ-ferrite range. In addition, the technique permits optimum mechanical properties to be obtained over a wider compositional range. 相似文献
6.
Donald Webster 《Metallurgical and Materials Transactions B》1971,2(8):2097-2104
Compositional modifications have been made to the existing Cr?Mo?Co stainless steels to produce a steel (alloy B) which combines the high strength of AFC 77 with the toughness of AFC 260. This has been achieved by utilizing both the strengthening effect of grain refinement and the crack stopping ability of retained austenite. After tempering at 800° to 900°F alloy B possesses higher elongation than other high strength stainless steels due to the ease with which its retained austenite transforms under stress to martensite to delay necking. An explanation has been advanced for the anomalously low tensile yield strength that occurs in both alloy B and AFC 77 after tempering at 1000°F. 相似文献
7.
Effect of microstructure on strength and toughness of heat-treated low alloy structural steels 总被引:1,自引:0,他引:1
Several low alloy structural steels with different levels of Ni, Cr, and Mo and carbon contents ranging from 0.12 to 0.42
wt pct have been studied to determine the effect of transformation structures on strength and toughness. The strength and
toughness were, respectively, evaluated with the yield stress (0.2 pct proof stress) in tensile tests under ambient temperature
and ductile-brittle transition temperature (DBTT) in Charpy impact tests. The significant conclusions are as follows: well-defined
packets are observed in martensitic and lower bainitic structures, and in this case the packet diameter is the primary microstructural
parameter controlling the yield stress and DBTT. The mechanical properties are also improved to a lesser degree with decreasing
width of the lath present within the packet. If the steel has an upper bainitic structure, the packet is composed of well-defined
blocks, and the block size controls the yield stress and DBTT. 相似文献
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M. J. Roberts 《Metallurgical and Materials Transactions B》1970,1(12):3287-3294
Phase transformations in Fe?Mn alloys containing up to 9 pct Mn were studied by optical and electron transmission microscopy. Either equiaxed ferrite, massive ferrite, or massive martensite can form on cooling from austenite. The particular type of transformation product formed was found to depend on the alloy content, austenite grain size, and cooling rate. The mechanical properties of all the transformation products were evaluated using tensile and impact testing and are discussed in terms of the observed microstructural features. Yield strength and impact transition temperature were found to be relatively insensitive to manganese content but were strongly influenced by the transformation substructure and grain size of the transformed phase. In martensite it has been shown that the structural unit analogous to grain size in ferrite is the martensite packet size, which in turn is controlled by the prior austenite grain size. The fracture surface of broken impact specimens and the fracture profile were examined by means of electron and optical microscopy techniques. These fractographic observations were correlated with impact test data and microstructural observations of the various transformation products. 相似文献
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The effect of loading rate on the strength and deformation characteristics of tensile tested smoothed round bar specimens and on linear-elastic and elastic-plastic fracture toughness values was investigated. Test materials were a high toughness melt of the fine-grained structural steel 20 MnMoNi 5 5 and two model materials of reduced toughness. The strain rate was varied between ?? = 10?3 and 103 s?1 in the tensile tests, the loading rate between K? = 1 and 2 · 106 MPam1/2 s?1 in the fracture toughness tests. The true-stress true-strain curve is shifted to higher stresses with increasing strain rate. A reduction of the deformation characteristics was only observed in cases of extremely reduced toughness. However, no brittle fracture at nominal stresses below yield was found in the tensile tests with smoothed specimens. Contrary to that all fracture mechanics tests showed a reduction of the crack initiation toughness with increasing loading rate K?. 相似文献
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P. R. Howell J. V. Bee R. W. K. Honeycombe 《Metallurgical and Materials Transactions A》1979,10(9):1213-1222
This paper establishes the crystallography of the austenite-ferrite/carbide transformation in Cr bearing steels. The crystallographic
analysis, based on the use of retained austenite in the martensitic phase, is consistent with the oriented nucleation of ferrite
in austenite. Similarly, it is shown that the nature of the carbide dispersion, which precipitates in association with the
transformation, is a sensitive function of the exact austenite/ferrite crystallography. 相似文献
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The effect of austenitizing temperature upon the microstructure and mechanical properties of experimental Fe/Cr/C steels 总被引:1,自引:0,他引:1
M. F. Carlson B. V. Narasimha Rao G. Thomas 《Metallurgical and Materials Transactions A》1979,10(9):1273-1284
The present study investigates the as-quenched mechanical propertiesviz, strength, ductility and sharp notch (K
Ic
) as well as blunt notch (Charpy) toughness, of simple Fe/Cr/C alloys with and without titanium as a function of austenitizing
temperature. For the ternary Fe/Cr/C alloys the results are consistent with earlier investigations, but the fracture toughness
does not change with increasing austenitizing temperatures after 0.2 wt pct Ti is added. The titanium forms carbides (TiC)
that did not dissolve, providing a roughly constant number of crack nucleation sites, and preventing austenite grain growth
up to 1100°C. The differences in mechanical behavior, particularly the rounded notch toughness, are discussed and explained
in terms of the microstructural characteristics of the alloys, as determined by detailed electron microscopy analysis. 相似文献
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R. C. Sharma G. R. Purdy J. S. Kirkaldy 《Metallurgical and Materials Transactions A》1979,10(8):1129-1139
Velocity and spacing measurements on the pearlite reaction in three eutectoid alloys of Cr (0.4, 0.9 and 1.8 wt pct) have
been recorded and compared with growth and spacing equations based on a variety of thermodynamic and kinetic models. At high
temperatures the reaction is controlled by phase boundary diffusion of Cr while at lower temperatures, a local equilibrium
no-partition model with kinetic control by carbon diffusion best represents the data. At intermediate temperatures there is
a smooth transition rather than a sharp break betwewn the two regimes. There is a discrepancy between theory and experiment
on the slope of the spacingvs inverse under cooling plot which can be removed by assuming that the ferrite-cementite epitaxy improves with the addition
of chromium.
R. C. SHARMA, formerly with McMaster University 相似文献
17.
The thermodynamic properties of CrC6/23, CrC3/7, CrC2/3, and MoC1/2 reported in the literature have been correlated with the relative stabilities of these carbide phases derived from ternary
phase diagrams. On the basis of the free energy of formation for one of the carbides and the free energy changes for the various
equilibria occurring in ternary Mo-Cr-C system at 1300°C, free energies of formation for the stable and unstable carbide phases
have been obtained. Using this free energy data, a calculated ternary Mo-Cr-C phase diagram is presented which agrees well
with the experimentally determined one. 相似文献
18.
Structure and mechanical properties of Fe−Cr−C−Co steels 总被引:1,自引:0,他引:1
As part of a continuing program concerning the microstructures and mechanical properties of steels in which particular attention
is given to transformation substructures, the present work is concerned with martensite and bainite in Fe−Cr−C steels with
and without cobalt. Although cobalt raises theM
s temperature it does not affect the extent of twinning for the same carbon level and so M
s
temperature alone does not control transformation substructure. Thus cobalt is not effective in retaining dislocated martensite
as carbon is increased and in this regard cobalt is not beneficial to toughness. TheM
s temperatures of the steels were relatively high and hence isothermal transformation yielded mixtures of bainites and tempered
martensite depending on the temperature of transformation. The mechanical properties of the isothermally transformed steels
were inferior to those of the tempered steels due to the interference of upper bainite or (tempered) martensite during the
isothermal transformation. Thus, in the steels having highM
s temperatures the twinning tempered martensitic structure had relatively better mechanical properties compared to the isothermally
transformed steels. Attempts to produce desirable autotempered structures by air cooling (single heat treatments) were not
successful and did not improve the mechanical properties since the structure consisted of a mixture of bainite and martensite.
This paper is based upon a thesis submitted by M. RAGHAVAN in partial fulfillment of the requirements of the degree of Master
of Science at the University of California. 相似文献
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G. R. Speich D. S. Dabkowski L. F. Porter 《Metallurgical and Materials Transactions B》1973,4(1):303-315
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. 相似文献