Showing papers on "Bainite published in 1974"

The effect of austenitizing temperature on the microstructure and mechanical properties of as-quenched 4340 steel

Abstract: The effect of austenitizing temperature on both the plane strain fracture toughness,KIC, and the microstructure of AISI 4340 was studied. Austenitizing temperatures of 870 and 1200°C were employed. All specimens austenitized at 1200°C were furnace cooled from the higher austenitizing temperature and then oil quenched from 870°C. Transmission electron microscopy revealed an apparent large increase in the amount of retained austen-ite present in the specimens austenitized at the higher temperature. Austenitizing at 870°C resulted in virtually no retained austenite; only minor amounts were found sparsely scat-tered in those areas examined. A considerably altered microstructure was observed in specimens austenitized at 1200°C. Fairly continuous 100 to 200A thick films of retained austenite were observed between the martensite laths throughout most of the area exam-ined. Additionally, specimens austenitized at 870°C contained twinned martensite plates while those austenitized at 1200°C showed no twinning. Plane strain fracture toughness measurements exhibited an approximate 80 pct increase in toughness for specimens austen-itized at 1200°C compared to those austenitized at 870°C. The yield strength was unaffected by austenitizing temperature. The possible role of retained austenite and the elimination of twinned martensite in the enhancement of the fracture toughness of those specimens austen-itized at the higher temperature will be discussed.

Stress-Assisted and strain-induced martensites in FE-NI-C alloys

Abstract: A metallographic study was made of the martensite formed during plastic straining of metastable, austenitic Fe-Ni-C alloys withMs temperatures below 0°C. A comparison was made between this martensite and that formed during the deformation of two TRIP steels. In the Fe-Ni-C alloys two distinctly different types of martensite formed concurrently with plastic deformation. The large differences in morphology, distribution, temperature dependence, and other characteristics indicate that the two martensites form by different transformation mechanisms. The first type, stress-assisted martensite, is simply the same plate martensite that forms spontaneously belowMs except that it is somewhat finer and less regularly shaped than that formed by a temperature drop alone. This difference is due to the stress-assisted martensite forming from cold-worked austenite. The second type, strain-induced martensite, formed along the slip bands of the austenite as sheaves of fine parallel laths less than 0.5μm wide strung out on the {111}γ planes of the austenite. Electron diffraction indicated a Kurdjumov-Sachs orientation for the strain-induced martensite relative to the parent austenite. No stress-assisted, plate martensite formed in the TRIP steels; all of the martensite caused by deformation of the TRIP steels appeared identical to the strain-induced martensite of the Fe-Ni-C alloys. It is concluded that the transformation-induced ductility of the TRIP steels is a consequence of the formation of strain-induced martensite.

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

Abstract: The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the Mb temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the Mb temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain.

Carbide refining heat treatments for 52100 bearing steel

Abstract: The life of through-hardened 52100 anti-friction bearing components is improved if the excess carbides, undissolved during austenitization, are small and uniformly dispersed. One kind of carbide-refining heat treatment consists of 1) dissolving all carbides, 2) isothermally transforming the austenite to pearlite or bainite, and 3) austenitizing, quenching and tempering in the usual manner. Each step in this sequence of treatments was investigated, and the behavior of pearlitic and bainitic microstructures during subsequent austenitization was contrasted with the behavior of ferrite/spheroidized-carbide microstructures. It was shown that: 1) The usual hardening treatments given spheroidize-annealed bearing components result in an inhomogeneous microstructure, possibly due to the faster dissolution of carbides near austenite grain boundaries. 2) Austenitization of pearlite or bainite produces very uniform dispersions of ultra-fine carbides on the order of 0.1 µm diameter or less. 3) Specimens with ultra-fine carbides tend to have more retained austenite. 4) The rate of coarsening of ultra-fine carbides at austenitizing temperatures of 840°C and below, is slow enough so that conventional furnace heat treatments are satisfactory for producing this microstructure.

Tempering of the Bainite and the Bainite/Martensite Duplex Structure in a Low-Carbon Low-Alloy Steel

Abstract: The variation with tempering temperature of the mechanical properties of the bainite and the bainite/martensite duplex structure in a low-carbon highstrength steel has been investigated and compared with that of the martensite. Both the bainite and the duplex structure exhibited much lower impact transition temperatures than the martensite and this can be explained in terms of the unit crack path, which is the mean cleavage crack path.The 350° C embrittlement was more pronounced in the tempered martensite than in the tempered bainitic structures. Tempering at temperatures higher than 600° C greatly improved the toughness of each structure but decreased the strength. It is believed that this improvement in toughness by tempering is attributable to the ferrite matrix properties rather than the effect of the unit crackpath. The optimum combination of toughness and strength can be achieved by producing the tempered bainite/martensite duplex structure at all strength levels.

The Effect of the Bainite Packet Size on Toughness

Abstract: 419-42. 2. N. A. Hill and J. W. S. Jones: s NueL Mater., 1961, vol. 3, pp. 138-55. 3. D. L. Douglass: Trans. Amer. Soe. Metals, 1961, vol. 54, pp. 322-30. 4. M. Garfinkle and R. G. Garlick: Trans. TMS-AIME, 1968, vol. 242, pp. 809-14. 5. D. G. Alexander and O. N. Carlson: Trans. TMS-AIME, 1969, vol. 245, pp. 2592-3. 6. E. A. Nesbitt and H. J. Williams: J. Appl. PIjys., 1955, vol. 26, pp. 1217-21. 7. Y. lwama, M. lnagaki and T. Miyamoto: Trans. Japan. Inst. Metals, 1970, vol. 11, pp. 268-74. 8. A. J. Perry and D. J. Rowcliffe: J. Mater. ScL, 1973, vol. 8, pp. 904-7. l o w vo id s e m i c o h e r e n t

Influence of martensite formed during deformation on the mechanical behavior of Fe-Ni-C Alloys

Abstract: Tension tests were performed on metastable austenitic Fe-Ni-C alloys to study the influence of martensite formed during straining on mechanical behavior. Although both stressassisted, plate martensite and fine, lathlike strain-induced martensite were formed, only stress-assisted martensite occurred in sufficient amounts to influence mechanical behavior. The formation of stress-assisted, plate martensite raised the flow stress and the strain-hardening rate, but only after 25 to 40 pct martensite had formed. A TRIP effect was obtained,i.e., ductility was enhanced by concurrent martensite formation, but this occurred over only a narrow temperature range because of the strong temperature dependence of the austenite-to-plate martensite reaction. This is in contrast to the behavior of high-Cr austenitic steels, which exhibit a TRIP effect over a wide temperature range because of the formation of large amounts of fine strain-induced martensite, whose formation is less temperature-sensitive than that of plate martensite.

An electron-microscopical investigation on the martensitic transformation in TiNi

Abstract: An electron-microscopical investigation of the morphology of equiatomic TiNi martensite has revealed at least two types of martensite: the twinned plate martensite and a wavy martensite To our knowledge the latter has never been reported in the literature before In a thin foil, the plate martensite can be transformed into the wavy martensite within a few weeks at ambient temperature or by temperature cycling between +100° C and liquid nitrogen temperature

Bainitic ferrous alloy and method

Abstract: This invention relates to an as-worked bainitic ferrous alloy and to a novel method of processing same to obtain optimum strength and toughness. More particularly, this invention is directed to the hot working cycle of a ferrous alloy characterized by an I-T Diagram or "S" Curve having a double nose or a pearlite transformation knee of the beginning curve above a broad bainitic bay region. Such an alloys is heated to an austenitizing temperature of about 1,500 DEG to 2,200 DEG F., and subjected to a plurality of working operations at successively lower temperatures, where the final working operation is conducted after the beginning of the austenite transformation to bainite and before the complete transformation thereof.

Annealing of austenite formed by reversion from martensite in an Fe–16Cr–12Ni alloy

Abstract: An investigation has been made of the annealing behaviour of austenite produced by the reversion of partially martensitic structures in an Fe–16wt-%Cr-12wt-%Ni alloy. Martensite formed by a refrigeration treatment was rapidly heated toform ‘reversed’ austenite of high defect density and enhanced strength. The softening that occurred on annealing this reversed austenite in the range 700°–925°C involved mainly recovery rather than recrystallization. Reversed austenite which had formed from martensite induced by cold working above Ms recrystallized on annealing and softened rapidly. The As and Ar temperatures were essentially identical for reversion of martensite produced by either refrigeration or cold working.

Carbide precipitation and the reversion of martensite to austenite in a semi-austenitic stainless steel

Abstract: Carbide precipitation processes in austenitic samples of an Fe–15Cr–81/2Ni–2Mo-o·09C semi-austenitic stainless steel at 700°C have been studied by measurements of hardness, saturation magnetic intensity and by electron microscopy. The austenitic structures wereproduced by a 1250°C solution treatment or by rapid heating of partially martensitic structures to produce ‘reversed austenite’; the martensite wasformed either by refrigeration below the Ms temperature or by cold rolling at room temperature. Carbideparticles, often identified as M23C6, were observed to form in several morphologies. Some regions of martensite formed from the alloy-depleted matrix on cooling to room temperature after aging. Long aging treatments resulted in partial transformation of the austenite matrix to ferrite by a diffusional process.

The Effect of Plastic Deformation on the Martensite–Austenite Transformation in Two Fe–Ni–C Alloys

Abstract: Metallographic observations and measurements of electrical resistivity have shown that the transformation martensite → austenite on heating is progressively retarded by prior deformation of the martensite. At the heating rates used, the transformation is not martensitic but may occur by inward displacement of parts of the existing austenite/martensite interfaces.

Deforming steels - without reducing their strength

Abstract: Steels hardenable by precipitation are shaped, without reducing their strength, by deforming a bar, rod, or strip of steel to the desired curvature or straightness, while the steel is at 150-485 (205-370) degrees C. The steel is pref. perlitic. comprising a free ferrite or bainite matrix.

α系,γ系異材継手の溶接ボンドに関する研究(第2報)

Abstract: In the previous report, it was pointed out that martensite-like structure formed at weld bond of welded α-steels and γ-steels reduced the ductility of weld bond remarkably.The substance of martensite-like structure has not been cleared, because the structure is very fine.In this report, the substance of the structure was cleared up by transmission electron microscopic observation. The effect of C content in α-steels on the amount of above mentioned martensite-like structure was also investigated. Moreover, investigations were made on the hardness change of the structure with post-weld heat treatment and so on.Martensite-like structure is not formed uniformly along the whole weld bond, but formed mainly at a stagnation area where flow of the molten metal contacting with the fusion line does not occur.Martensite-like structure was identified as a mixed structure of low carbon martensite or bainite (in case of using low carbon α-steels as base metal), or high carbon martnesite or bainite (in case of using high carbon α-steels as base metal) and fine chromium carbide (Cr23C6).The amount of martensite-like structure formed at weld bond increases with increasing C content in a-steels, but it decreases with too much C.In the post-weld heat treatment at 650°C, the weld bond is softened with increasing heating time, but bigin to be hardened when heating time is over app. 2 h and hardened remarkably over app. 7 h. This hardening is caused by large amounts of Cr23C6 precipitates which are induced by the carburization from haz of α-steel.On the microscopic observation of weld bond, the hardly etched region was observed between the martensite-like structure and weld metal. This region is very hard and hardened further with post-weld heat treatment. However, the substance of the structure of this region could not be cleared up.