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1.
This paper reports a study of tempered martensite embrittlement in a Ni-Cr steel doped with 0.01 wt pct S. The segregation
of sulfur to the grain boundaries and the associated embrittlement of this material are very dependent upon the austenitizing
temperature. If the austenitizing temperature is below 1050 °C very little embrittlement and very little intergranular fracture
are observed because sulfur remains precipitated as chromium sulfide. At higher austenitizing temperatures the sulfides dissolve
and sulfur segregates to the grain boundaries. Because of the high bulk content, the sulfur concentration at the grain boundaries
becomes great enough for the sulfides to reprecipitate there. This leads to low energy intergranular ductile fracture. However,
some sulfur remains unprecipitated at the boundary and can lower the cohesive strength across the boundary. When plate-like
cementite precipitates at the grain boundary during tempering heat treatments at 300 to 400 °C, the combination of the carbides
and the unprecipitated sulfur causes intergranular fracture and tempered martensite embrittlement. 相似文献
2.
Yoshiyuki Tomita 《Metallurgical and Materials Transactions A》1988,19(10):2513-2521
Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain
fracture toughness (K
IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different
mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron
microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows:
in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural
factor controllingK
IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet
is composed of well-defined blocks, and the block size controls theK
IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase
bainite have a significant effect on theK
IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite,
dramatically improves theK
IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the
parent martensite is associated with tempered martensite, it significantly lowers the KIC. 相似文献
3.
The structure and mechanical properties of tempered martensite and lower bainite were investigated in a series of high purity 0.25 pct C steels with varying amounts of nickel and manganese. The martensites in 0.25 C-5 Ni?Fe and 0.25 C-3 Mn?Fe alloys were mainly untwinned, while those in 0.25 C-5 Ni-7 Mn?Fe and 0.25 C-7 Mn?Fe alloys were heavily twinned. Manganese appears to promote carbide precipitation along the lath boundaries in tempered martensite. At equivalent yield and ultimate tensile strength levels, the tempered martensite of lower manganese steels showed better impact toughness than the tempered martensite of higher manganese steels. The impact toughness (compared at similar strength levels) of untwinned tempered martensite of 0.25 pct C steel with Widmanstatten precipitation of carbide was higher than that of lower bainite, which showed unidirectional carbides. The reasons for the difference in impact toughness between the alloys, and also between the structures are rationalized in terms of internal twinning, grain boundary precipitation and carbide morphology together with other microstructural features. 相似文献
4.
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
5.
A. Reguly T. R. Strohaecker G. Krauss D. K. Matlock 《Metallurgical and Materials Transactions A》2004,35(1):153-162
Charpy V-notch (CVN) specimens from experimental heats of 5160 steel containing 0.001 and 0.034 mass pct phosphorus were austenitized
at temperatures between 830 °C and 1100 °C, quenched to martensite, and tempered at temperatures between 100 °C and 500 °C.
Scanning electron microscopy (SEM) was used to characterize the fracture surfaces of tested CVN specimens and carbide formation
on prior austenite grain boundaries. Quench embrittlement, the susceptibility to intergranular fracture in as-quenched and
low-temperature tempered high-carbon steels due to cementite formation as affected by phosphorus segregation on austenite
grain boundaries, developed readily in specimens of the high phosphorus steel austenitized at all temperatures. The low phosphorus
steel developed quench embrittlement only after austenitizing at 1100 °C. Intergranular fractures correlated with low room-temperature
CVN impact toughness. The results are discussed with respect to the dissolution of carbides during austenitizing and the effect
of phosphorus on grain boundary, carbide formation, and stability. 相似文献
6.
Effect of Morphology for Novel Bainite/Martensite Dual-Phase High Strength Steel on Its Hydrogen Embrittlement Susceptibility 总被引:2,自引:0,他引:2
CHANG Kai di GU Jia lin FANG Hong sheng BAI Bing zhe ZHANG Wen zheng YANG Zhi gang LIU Dong yu ZHANG Chi 《钢铁研究学报(英文版)》2001,8(1):37-40
It is very imperative to study the novel highstrength materials with high purity,high uniformityand refined grain,for improving the utilization rateand quality of materials.However,improving thestrength will reduce the resistance of materials tohydrogen embrittlement. For example,in lastcentury,some hydrogen embrittlements in thelanding gear of plane were reported[1] . When yieldstrength exceeds 1380 MPa,high strength steelsbeing used now exhibit strong susceptibility tohydrogen embrittlement… 相似文献
7.
B. London D. V. Nelson J. C. Shyne 《Metallurgical and Materials Transactions A》1988,19(10):2497-2502
Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels
was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively.
Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate
increased at a given ΔK and ΔKth decreased. Threshold values varied from 2.8 MPa m1/2 (200 °C temper) to 9.5 MPa m1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain
boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced
transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation
in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms.
The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the
threshold values for all tempering temperatures except 200 °C.
Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA.
Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA. 相似文献
8.
M. Sarikaya A. K. Jhingan G. THOMAS 《Metallurgical and Materials Transactions A》1983,14(5):1121-1133
Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Some in situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M3C films upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe3C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The situation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which occurs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can occur in the same steels, depending on the tempering conditions. 相似文献
9.
10.
In this study two different heat treatments were conducted on an X 37 Cr Mo V 5‐1 hot‐work tool steel, resulting either in a tempered fully martensitic matrix or a matrix almost consisting of tempered bainite. Short‐term creep tests were performed at a high stress level of 800 MPa and at temperatures in the range from 450 °C to 500 °C. Creep specimens consisting of a tempered fully martensitic microstructure exhibited a three times longer creep‐to‐rupture time, than those consisting of a tempered almost bainitic microstructure. Microstructural investigations of creep specimens were performed by transmission electron microscopy. Results of these investigations revealed that due to a lower cooling rate, which is necessary to form bainite, the tempered bainitic microstructure consists of large former bainitic plates, whereas tempered martensite shows fine former martensitic laths. Tempered bainite also exhibits a higher number density of large M3C, M7C3 and MC carbides than tempered martensite. Small M2C carbides appear in both microstructures in the same quantity, however, nanometer‐sized MC carbides could only be found in tempered martensite. Thus poor short‐term creep behavior of the tempered almost bainitic microstructure can be explained by the lesser amount of strengthening relevant precipitates, a smaller size‐effect due to distance of bainitic interfaces as well as lower solid solution hardening. 相似文献
11.
通过热处理制备出具有回火马氏体组织、下贝氏体组织以及粒状贝氏体组织的718钢,利用光学显微镜、扫描电子显微镜、X射线衍射仪、万能拉伸实验机比较其显微组织及力学性能。同时借助高速铣削实验及光学轮廓仪,研究力学性能以及组织结构对切削性能的影响。结果表明,当切削速度低于145 m·min?1时,贝氏体组织类型比回火马氏体组织更易切削,切削贝氏体组织比切削回火马氏体组织的刀具使用寿命高30%~40%。当切削速度高于165 m·min?1时,马氏体组织发生了加工软化现象,刀具使用寿命提高,切削性能上升。粒状贝氏体组织加工表面因为严重的刀具黏附而出现背脊纹路,马氏体组织具有最佳的切削表面粗糙度。综合考虑之下,三种组织的综合切削性能从高到低排序为:下贝氏体组织、马氏体组织、粒状贝氏体组织,采用300 ℃等温淬火工艺可以有效提升718塑料模具钢的综合切削性能。 相似文献
12.
Tempered martensite embrittlement in phosphorus doped steels 总被引:1,自引:0,他引:1
In this paper the effect of phosphorus on tempered martensite embrittlement of Ni−Cr steels is reported. It is shown that
the measured degree of embrittlement depends on the phosphorus concentration, test temperature, grain size, and austenitizing
temperature. Although reducing the prior austenite grain size tends to reduce the observed embrittlement, this can be offset
by the fact that the low austenitizing temperatures used to produce the fine grain size cause an increased amount of impurity
segregation. It is further shown that bulk phosphorus concentrations below 100 wppm may be required to avoid embrittlement
of this type in ultra-high strength steels. 相似文献
13.
M. Sarikaya A. K. Jhingan G. Thomas 《Metallurgical and Materials Transactions A》1983,14(6):1121-1133
Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M3C filMs upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe3C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions. 相似文献
14.
This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which
were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures
that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K
Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of
plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation
fields ahead of cracks, suggesting thatK
Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The
trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation
of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating
the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels
studied can be explained quantitatively using different fracture models. 相似文献
15.
The microstructural characteristics of the hot‐worked and subsequently tempered tool steel grade X38CrMoV5‐1 was studied as a function of the cooling rate using transmission electron microscopy and three‐dimensional atom probe. According to the continuous cooling transformation diagram different cooling rates were chosen to adjust a fully martensitic or mixed microstructure consisting of martensite and bainite. The sample with the highest cooling rate exhibited a martensitic structure with nanometre sized secondary hardening carbides of the type M3C, M2C, M7C3, and MC. M3C and M2C were not stable and transformed to M7C3 as the cooling rate decreased. Furthermore, with decreasing cooling rates an increasing number of M7C3 precipitates are particularly present at former austenite grain boundaries as well as martensite and bainite lath boundaries, which strongly affects the mechanical properties. 相似文献
16.
Mechanical properties of 0.40 pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite 总被引:3,自引:0,他引:3
A study has been systematically made of the effect of bainite on the mechanical properties of a commercial Japanese 0.40 pct
C-Ni-Cr-Mo high strength steel (AISI 4340 type) having a mixed structure of martensite and bainite. Isothermal transformation
of lower bainite at 593 K, which appeared in acicular form and partitioned prior austenite grains, in association with tempered
marprovided provided a better combination of strength and fracture ductility, improving true notch tensile strength (TNTS)
and fracture appearance transition temperature (FATT) in Charpy impact tests. This occurred regardless of the volume fraction
of lower bainite present and/or the tempering conditions employed to create a difference in strength between the two phases.
Upper bainite which was isothermally transformed at 673 K appeared as masses that filled prior austenite grains and had a
very detrimental effect on the strength and fracture ductility of the steel. Significant damage occurred to TNTS and FATT,
irrespective of the volume fraction of upper bainite present and/or the tempering conditions employed when the upper bainite
was associated with tempered martensite. However, when the above two types of bainite appeared in the same size, shape, and
distribution within tempered martensite approximately equalized to the strength of the bainite, a similar trend or a marked
similarity was observed between the tensile properties of the mixed structures and the volume fraction of bainite. From the
above results, it is assumed that the mechanical properties of high strength steels having a mixed structure of martensite
and bainite are affected more strongly by the size, shape, and distribution of bainite within martensite than by the difference
in strength between martensite and bainite or by the type of mixed bainite present. The remarkable effects of the size, shape,
and distribution of bainite within martensite on the mechanical properties of the steel are briefly discussed in terms of
the modified law of mixtures, metallographic examinations, and the analyses of stress-strain diagrams. 相似文献
17.
An investigation into the mechanisms of tempered martensite embrittlement (TME), also know as “500°F” or “350°C” or one-step
temper embrittlement, has been made in commercial, ultra-high strength 4340 and Si-modified 4340 (300-M) alloy steels, with
particular focus given to the role of interlath films of retained austenite. Studies were performed on the variation of i)
strength and toughness, and ii) the morphology, volume fraction and thermal and mechanical stability of retained austenite,
as a function of tempering temperature, following oil-quenching, isothermal holding, and continuous air cooling from the austenitizing
temperature. TME was observed as a decrease in bothK
Ic and Charpy V-notch impact energy after tempering around 300°C in 4340 and 425°C in 300-M, where the mechanisms of fracture
were either interlath cleavage or largely transgranular cleavage. The embrittlement was found to be concurrent with the interlath
precipitation of cementite during temperingand the consequent mechanical instability of interlath films of retained austenite during subsequent loading. The role of silicon
in 300-M was seen to retard these processes and hence retard TME to higher tempering temperatures than for 4340. The magnitude
of the embrittlement was found to be significantly greater in microstructures containing increasing volume fractions of retained
austenite. Specifically, in 300-M the decrease inK
Ic, due to TME, was a 5 MPa√m in oil quenched structures with less than 4 pct austenite, compared to a massive decrease of 70
MPa√m in slowly (air) cooled structures containing 25 pct austenite. A complete mechanism of tempered martensite embrittlement
is proposed involving i) precipitation of interlath cementite due to partial thermal decomposition of interlath films of retained
austenite, and ii) subsequent deformation-induced transformation on loading of remaining interlath austenite, destabilized
by carbon depletion from carbide precipitation. The deterioration in toughness, associated with TME, is therefore ascribed
to the embrittling effect of i) interlath cementite precipitates and ii) an interlath layer of mechanically-transformed austenite,i.e., untempered martensite. The presence of residual impurity elements in prior austenite grain boundaries, having segregated
there during austenitization, may accentuate this process by providing an alternative weak path for fracture. The relative
importance of these effects is discussed.
Formerly with the Lawrence Berkeley Laboratory, University of California. 相似文献
18.
S. Takayama T. Ogura Shin-Cheng Fu C. J. McMahon 《Metallurgical and Materials Transactions A》1980,11(9):1513-1530
A study was made of quenched and tempered laboratory heats of 3.5 Ni-1.7 Cr steels doped with P, Sn, or Si in which the hardness,
the grain size, and the extent of intergranular segregation of the metalloid dopents were varied. The segregation was accomplished
by aging treatments of various times at 480 °C. It was found that the increases in ductile-brittle transition temperature
(ΔTT) were determined completely by the above three variables and that they can be combined in an “embrittlement equation” that
permits the calculation of theΔTT that will occur from a particular degree of segregation in a steel of a given hardness and grain size. The embrittlement
equation contains coefficients which must be determined empirically for each type of steel and embrittling element; however,
the equation is derived by a straight-forward Taylor series expansion of theTT as a function of the three variables. Each term of the resulting equation can be justified from physical considerations.
The embrittlement equation can serve as a rational basis for analysis of a given type of steel or for comparisons between
types. In particular, it allows one to account for variations in hardness and grain size, and to determine the benefits to
be gained from control of these two factors.
Formerly with the Department of Materials Science and Engineering, University of Pennsylvania. 相似文献
19.
《Canadian Metallurgical Quarterly》2013,52(1):169-175
AbstractThe case of intergranular embrittlement of steels induced by grain boundary segregation of impurities is briefly reviewed. Particular emphasis is placed on segregation in the austenite and the resulting tempered martensite embrittlement with some suggested ways of controlling them from an alloy design approach. Achievement of this goal would also improve resistance to other forms of intergranular embrittlement in very high-strength steels based on tempered martensite microstructure where impurity segregation in the austenite appears to govern many of the intergranular fracture processes. Résumé Le cas de la fragilisation intergranulaire des aciers induite par la ségrégation d'impuretes aux joints de grains dans l'austénite est succintement examiné. Une attention particulière est dévolue à la ségrégation dans l'austénite et à la fragilisation de la martensite revenue qui s'ensuit tout en suggérant des moyens pour éviter ces effetset ce lors de l'élaboration des alliages. En suivant ces suggestions on pourrait aussi augmenter la résistance à d'autres formes de fragilisation intergranulaire dans les aciers à très hautes résistances tablant sur une microstructure de martensite revenue, où la ségrégation des impuretés dans l'austenite semble déterminer plusieurs des processus de rupture intergranulaire. 相似文献
20.
W. T. Shieh 《Metallurgical and Materials Transactions B》1974,5(5):1069-1085
A method is described for the transmission and scanning electron microscope study of the relationship between the microstructure
and the fracture properties of two quenched and tempered, electron beam melted, modified SAE 4620 steels consisting of tempered
low carbon martensite. Among all the microstructure constituents considered, the constituentR (randomly oriented, “tempered low carbon martensite, TLCM”) achieved the highest probability for dimple fracture. The thick
TLCM laths (designated as the microstructure constituent II) exhibited higher probability of dimple plus quasi-dimple mode
of fracture than the thin laths (I). It is concluded that the steel EB1035 derived the high toughness from a) the high concentration
of the “high toughness” microstructure constituentsR and II, b) “non-embrittled” prior austenite grain boundaries with 50 pct probability for smooth plus quasi-smooth mode and
50 pct dimple plus quasi-dimple mode of intergranular fracture. In contrast, besides having low content ofR and II, the steel EB1014 displayed “completely embrittled” prior austenite grain boundaries with 100 pct probability for
smooth plus quasi-smooth intergranular fracture. The conclusions derived from the microconstituentsR, II and I seemed to reflect the “embrittling” effect of decreased spacings between the pseudo twin related laths and between
the lath boundary cementite films, and the “toughening” effect of the randomly oriented laths. Auger spectra obtained from
the fracture surface before and after sputtering is analyzed to determine the presence of grain boundary sulfur segregation. 相似文献