Sub-Zero Treatment of Quenched Steel. I : On the Stabilization of Retained Austenite

As a fundamental study of sub-zero treatment of steel, the effects of quenching temperature, room temperature aging and tempering on the stabilization of retained austenite were examined. The stabilization of retained austenite was promoted by aging, because the martensite transformation rate due to subsequent sub-zero cooling was increased by the increase in aging, and moreover, when aging increased, the martensite transformation rate passed through a maximum. The martensite transformation due to subsequent sub-zero cooling gave rise to the burst transformation when aging increased to a certain degree. The remarkable increase in the stabilization was caused by the increase in the amount of prior martensite, though the stabilization occurred without prior martensite. It was suggested that the stabilization of austenite was caused by a mechanism similar to age-hardening

Effect of Cooling Rate on A_3 Transformation Temperatures of Iron and Iron-Nickel Binary Alloys

Effects of cooling rate on A_3 points of a few pure irons, iron-low carbon alloys and iron-nickel alloys have been examined at cooling rates between 100℃/sec and 60, 000℃/sec by using a gas quench apparatus. A_3 point of iron containing less than 0.003 wt% C falls gradually with the increase in cooling rate and continuously approaches a fixed temperature (maximum supercooled A_3), but in the iron containing more than 0.006 wt% C it falls discontinuously to the Ms temperature at the cooling rate of about 20, 000℃/sec. Maximum supercooled A_3 points of these irons are affected seriously by a very small content of carbon below 0.005 wt% C, falling remarkably from about 720℃ at 0.001 wt% C to about 650℃ at 0.003 wt% C with increasing carbon content. In the case of iron-nickel alloys it falls with increasing cooling rate and discontinuously by about 100℃ at a certain cooling rate which depends on the nickel content. The drop of the transformation temperature is independent of a further cooling rate. The critical cooling rate decreases from about 25, 000℃/sec at 1% Ni to about 2, 000℃/sec at 20% Ni. The Ms points of these alloys are in agreement, at higher nickel concentrations, with those reported previously, but are displaced to a lower temperature side at lower nickel concentration

Effect of Cyclic Heat-Treatments on the Martensitic Transformation in Iron-Nickel Binary Alloys

The effect of the cyclic heat-treatments on the martensitic transformation was examined with iron-nickel binary alloys containing 24.50 to 30.70 atomic per cent nickel from the observations of dilatation curves, microstructures and mechanical properties. Ms temperatures lowered with the increase in the number of cyclic heat-treatments so that the amount of martensite transformed also decreased. The optical and the electron microstructures of martensite after cyclic heat-treatments consisted of finer martensite plates with high dislocation density. The hardness and the tensile strength of austenite were remarkably increased by cyclic heat-treatments at an early state, but in the case of martensite they were not remarkably increased. It was suggested that these results would be explained by mechanisms based on the hardening of austenite and the partial decomposition of martensite due to the cyclic heat-treatments

Effects of Alloying Element on Supercooled A_3 Transformation of Iron

The cooling rate dependence of A_3 transformation and the maximum supercooled A_3 point have been examined on eleven kinds of iron binary alloy by using a rapid cooling apparatus. Maximum supercooled A_3 points of Fe-Mn alloys show good agreement at higher manganese concentrations with those reported previously but are displaced to lower temperatures at lower manganese concentrations. Extrapolation of the data zero content of the alloying element leads to the maximum supercooled A_3 point of about 720℃ for pure iron. The maximum supercooled A_3 points of Fe-Cr, Fe-Cu and Fe-Mo alloys descend with increasing content of alloying element and those of Fe-Al, Fe-Ti and Fe-V alloys ascend. In Fe-Co alloys, the maximum supercooled A_3 point rises up to about 40% Co and thon lowers. Si, W and Nb have little effect on the maximum supercooled A_3 temperature of iron. Some of the alloying elements have a different effect on the change of A_3 point of iron with concentration both in the non-equilibrium and in the equilibrium condition

Sub Zero Treatment of Quenched Steel. III : Effect of Aging on the Stabilization of Retained Austenite

The stabilization of retained austenite after partial transformation to martensite has been investigated using 1.12 % C, 4.98 % Ni steel. The stabilization degree was expressed by the temperature lag θ of transformation point or the amount of the stabilized austenite, δ. It has been observed that the temperature lag θ is a function of aging time. It has been found that the recovery of martensite transformation occurred during subsequent cooling after aging in the early stage of stabilization, but did not occur in the last stage of it. Accordingly the correlation between θ and δ is not a linear relation. The stabilization proceeded very rapidly when the previously transformed martensite increases to more than 65 per cent

Relationship between the Solid Phase Equilibrium and the Isothermal Martensite Transformation in Fe-Ni-Cr and Fe-Ni-Mn Alloys

The change in free-energy attendant upon the athermal and the isothermal transformations of austenite into martensite in Fe-Ni-Cr and Fe-Ni-Mn alloys was investigated. In Fe-Ni-Cr alloys containing 17 weight % Cr, the driving force necessary to initiate the athermal martensite transformation increases as Ni content increases, whereas in the isothermal transformation, this force decreases. When the Ni content in the alloys exceeds about 7 weight %, the martensite is formed isothermally at a temperature above M_s point and the driving force at this point is about 490 cal/mole. Similar phenomena were also observed in Fe-23% Ni-Mn alloy. In this alloy, the martensite is formed isothermally at a temperature above M_s point when the Mn content exceeds about 3.5 weight % and the driving force at this critical point is about 430 cal/mole. Temperatures for maximum rates of the isothermal martensite transformation in Fe-Ni-Cr and Fe-Ni-Mn alloys lower with the increases of Ni and Mn content, respectively. The driving forces for the athermal martensite transformation in the Fe-Ni-Cr and the Fe-Ni-Mn alloys used in the present work are larger than that of other alloys in which the martensite is formed only athermally. From these results, it is suggested that the nucleus of new phase in isothermal martensite transformation cannot develop into martensite plate only by the stored free energy, but probably some thermal fluctuation will play an important role

Isothermal Martensitic Transformation in Fe-Ni-Cr Alloy

The spontaneous martensitic transformations were investigated by using the Fe-base alloy containing 17 per cent Cr and 8 per cent Ni. The isothermal martensitic trans-formations occurred while it was held at above the M_s^ temperature and the T.T.T. diagram was of the form with two noses at about -100°and -135℃, respectively. According to X-ray analysis h.c.p. structure (ε) was formed at the upper nose. The driving force (ΔF^) of transformation in this alloy was 22 cal/mol. This value was derived from an assumption in which the transformation would occur owing to a very low stacking fault energy. Furthermore, from this assumption, it was suggested that the γ→ε transformation would occur at above 7 per cent Ni content. This suggestion agreed closely with practical data. The b.c.c. martensite (α\u27) grew when it was held longer in the temperature range of the upper nose. The habit plane of this α\u27 was (111)_γ plane, whereas the habit plane of the α\u27 formed isothermally in the temperature range of the lower nose and athermally at a temperature below M_s^ was (225)_γ plane

The Chemical Composition of Membrane Lipids in Acclimation to Chilling of Squash (Cucurbita moschata Duch. cv Shirogikuza) Seedlings

Chilling-tolerance of squash (Cucurbita moschata Duch. cv Shirogikuza) cotyledons was measured after 3 d of cold acclimation at 12℃. Squash seedlings grown at 12℃ (chilling-treated, CT) and 25 ℃ (non-treated, NT) were analyzed to examine whether the chilling treatment affected lipid composition and content. A slight decrease of sterol lipid/glycerophospholipid ratio was observed in CT seedlings, and although the levels of unsaturated fatty acids in phosphatidylglycerol were not significantly different between the two seedlings, the level of 3-E-hexadecenoic acid increased slightly in CT seedlings. In phosphatidylcholine, the level of linolenic acid slightly increased in the CT seedlings. In ceramide monohexoside, the levels of 8-Z- and 8-E-stereoisomerism of 4-hydroxy sphingoid bases were essentially the same between the two seedlings, while a slightly increased level of 2-hydroxy palmitic acid was observed in CT seedlings. The involvement of lipids in the change to chilling sensitivity is discussed in this experiment