Showing papers on "Bainite published in 1970"

The Influence of Austenite Strength Upon the Austenite-Martensite Transformation in Alloy Steels

Abstract: The resistance of austenite to plastic deformation (austenite flow stress) was measured using a high temperature tensile apparatus. The flow stress was then correlated with the Ms temperature as determined magnetically during subsequent cooling. In one part of the study, the flow stress of the austenite was varied only by work hardening the austenite, allowing the austenite composition, which is known to affect Ms, to be held constant. A decrease in Ms temperature with increasing austenite flow stress was observed. This observation was supported by the observation of a decrease in the amount of austenite transformed at 25°C. In the other part of the study, a series of alloy steels of different chemical compositions was tested. A decrease in Ms temperature with increasing austenite flow stress was again observed. Strengthening of austenite by plastic deformation was shown not to change the chemical driving force for transformation. The effect of deformation on Ms temperature thus results from its influence on either the nucleation or the growth process. While the effect of austenite deformation on martensite nucleation is uncertain, specific nucleation models can account for only approximately one-third of the nonchemical free energy change which accompanies transformation. A proposal, consistent with the observations, was made that the energy expended for the deformation of austenite during martensite plate growth could reasonably account for a substantial part of the nonchemical free energy change.

Effect of Transformation Substructure on the Strength and Toughness of Fe-Mn Alloys

Abstract: 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.

The influence of structural parameters on the yield strength of tempered martensite and lower bainite.

Abstract: : The contributions to the yield strength of tempered martensitic and bainitic structures was examined in 4340 steel. The principal factors that contribute to the reduction in yield strength with tempering in the range from 600 to 1000F are carbide coarsening and enlargement of the cellular substructure. The yield strength of both tempered martensitic and bainitic structures can be described in terms of cell size and dispersoid distribution by a single relationship in which the Orowan model is employed for the contribution from dispersion hardening and the Langford-Cohen model for that from cell size. (Author)

Strain tempering of bainite

Abstract: In the present investigation the strain tempering of bainite has been carried out with (EN 24) a medium carbon low alloy steel. The specimens were austenitized at 850°C and isothermally transformed at 300°, 360°, and 400°C to produce bainite, strained and tempered in the range 100° to 400°C. The bainite formed at 360°C has been specifically examined by thermomagnetic analysis to confirm quantitatively the solution of ∈ carbide. Thermomagnetic analysis has shown that the carbide phase in bainite formed even at 360°C is a mixture of ∈ carbide and cementite. It has been found that with lowering of bainite transformation temperature, the strain tempering treatment results in higher strength consistent with good ductility. The present investigation favors the carbon dislocation trapping model for the mechanism of strain tempering of bainite, similar to that proposed for the strain tempering of martensite.

Austenite ferromagnetism and martensite morphology

Abstract: The Curie temperature of the austenite, the martensite-start temperature, and martensite morphology have been determined in a series of nil-carbon Fe−Ni and Fe−Ni−Co alloys. For these alloys, austenite ferromagnetism aboveMs is a necessary, but not sufficient, condition for the formation of lenticular rather than packet martensite. In contrast to Fe−Ni alloys where lenticular martensite only forms below ≈O°C, some of the Fe−Ni−Co alloys transform to this structure at temperatures up to ≈200°C. The results support the hypothesis that the resistance of austenite to plastic deformation affects the habit plane and thus morphology of the martensite which forms.

Martensite stainless steel

Abstract: THE PROPOSED INVENTION RELATES TO A MATERIAL FOR WELDING-WIRE FOR STEELS, WHICH MATERIAL COMPRISES CARBON, MANGANESE, SILICON, NICKEL, CHROMIUM, TITANIUM, ALUMINUM, MOLYBDENUM, ZIRCONIUM, CALCIUM, BORON AND IRON.

Isothermal transformation of austenite under high pressure

Abstract: 1. High pressure slows down the transformation of austenite. 2. The structure of bainite and pearlite obtained under high pressure is no different from that of ordinary bainite and pearlite obtained at atmospheric pressure. 3. Pearlite obtained under pressure has larger distances between platelets than pearlite obtained at the same temperature under atmospheric pressure. 4. The pearlitic region broadens under pressure, extending to lower temperatures than under atmospheric pressure, which is probably due to suppression of the bainitic transformation under high pressure. Low-temperature pearlite obtained under pressure is very fine; the cementite platelets are broken up. 5. The variation of the mechanical properties of pearlite obtained under pressure conform with the changes in its structure — coarse pearlite has a low ductility, the strength and ductility increase with decreasing distance between platelets of pearlite.

The Effect of Delta-Ferrite on the Properties of Low-Carbon Martensite Stainless Steels

Abstract: : The formation of structurally free delta-ferrite in martensitic stainless steel greatly impairs its mechanical properties. Tests along the fiber showed lower notch toughness values, particularly at low testing temperatures; the cold-brittleness considerably increased while the work of crack propagation decreased in tests across the fiber and, particularly in the third direction (over the thickness of the plate), delta-ferrite was found to have a drastically negative effect on the mechanical properties. The brittleness of martensitic steel with the appearance of delta-ferrite is caused by the low resistance of ferrite to brittle failure, and apparently, the increase in martensite ductility due to the increase in carbon content in the martensite following its redistribution between the austenite and the delta-ferrite.

Some observations on the relationship between microstructure and mechanical properties in large cylindrical gun tube forgings.

Abstract: : A series of laboratory isothermal and continuous cooling heat-treatments were employed to develop and characterize the low temperature transformation products or microstructures which could be present in commercially produced large gun tube forgings The tensile mechanical properties, hardness and Charpy V-notch impact transition curves were determined for each of the various microstructures produced Of the three microstructures (martensite and two bainites) evaluated, tempered martensite produced the best combination of strength and toughness Continuous cooling heat treating studies were used to demonstrate that a fully martensitic microstructure could be produced at the mid-radius of full size large gun tube forgings Low yield strengths and impact energies were correlated with the tempered bainitic structures produced by transforming or quenching the gun steel forgings too slowly (Author)

Metallographic technique for the development of microstructural constituents in gun steel.

Abstract: : An investigation was undertaken to develop etching procedures to clearly distinguish certain metallographic features of large forgings. The investigation demonstrates that (1) good microstructure resolution was obtained in untempered martensite using a 2% nital etchant, (2) either 25% sodium bisulfite in water or 4% picral plus hydrochloric acid yielded the best results for both tempered martensite and a duplex structure of tempered martensite and tempered lower bainite, and (3) an aqueous solution of 1% picric acid and 7% sodium tridecylbenzene sulfonate proved highly satisfactory in revealing both the prior austenitic grain boundaries and the macrostructure. (Author)

The influence of grain size and stress on the morphology of a 300-grade maraging steel

Abstract: The effects of austenite grain size and externally applied stress on the morphology of martensite formation for a 300-grade maraging steel were investigated. Austenite grain sizes ranging from ASTM 14 to 5 were examined. The growth pattern of the martensite was revealed by a selective aging treatment that involved heating to 815° C (austenitizing), cooling to 184° C (about 50 pct transformation to martensite), reheating to 400° C (partial aging of the martensite), and finally, cooling to room temperature (balance of austenite transforms). Blocky martensite formed in the fine-grain austenite, whereas for the coarse-grain size, a stringer-like structure developed. Electron transmission studies showed that the individual martensite units (laths or platelets) were similar in size for both types of morphologies. In both cases, the length of these units corresponded closely to the spacing between the twin boundaries or grain boundaries of the fine-grain specimens. Differences in morphology for different grain sizes are explained in terms of the relative ratios between the size of the martensite unit and the distance between boundaries intersecting the path of growing platelets. The application of an external stress to a coarse-grain specimen results in the delineation of the austenite annealing twins that normally cannot be readily detected. It is proposed that the application of a stress causes a preferential acceleration of the transformation in one of the two differently oriented, twin-related regions of an austenite grain. This argument is based on the differences in the maximum resolved shear stress for the most favorable orientation of the (112) {111} shear variants in each of the various possible twin-related austenite crystal orientations. Hardness and tensile data were also obtained. The absence of any significant variation in these properties for the different grain sizes is attributed to the similarity in size of the martensite units.

A rapid quenching optical hot-stage microscope

Abstract: An optical hot-stage microscope has been developed to study the growth kinetics of individual bainite needles in plain carbon and low alloy steels. A description of the apparatus is given, together with typical results obtained.