Bainitic Temperature Research Articles

Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.MST/526

Erosion behavior of high silicon bainitic structures: I: Austempered ductile cast iron

The erosion behavior of unalloyed austempered ductile cast iron has been studied as a function of austempering time for irons austempered in the upper bainitic temperature range (370 °C). A minimum erosion rate and iron matrix hardness occurs at the end of the stage II austempering time, where the microstructure consists of bainitic ferrite and retained austenite. Experiments show that erosion induces transformation of retained austenite at early times of stage II austempering and that maximum erosion resistance correlates with maximum work hardening of the substrate during the erosion process. Scanning electron microscopy studies of the eroded surfaces and the substrate revealed that the material removal in erosion occurs because of the cutting, ploughing and flake formation mechanisms which depend upon the microstructures and their mechanical properties.

Erosion behavior of high silicon bainitic structures: II: High silicon steels

The erosion behavior of two austempered high silicon (2.5 wt.% Si) steels, one containing 1 wt.% Mn and the other 1 wt.% Ni, has been compared with the mechanical property variations after austempering for various times in the upper bainite temperature range (420 °C). For both steels the erosion resistance is a maximum for austempering times near the end of stage II. The erosion resistance correlates directly with the tensile toughness and with the percentage elongation divided by the percentage reduction in area and inversely with the hardness in all three stages of austempering. It is shown that the strain-hardening coefficient peaks at the minimum erosion rate and that the high strain-hardening character of the retained austenite in these austempered steels is beneficial to erosion resistance. Comparison with the austempered cast irons in Part I of this study shows that the graphite nodules of the cast irons have no significant effect upon erosion rate.

An approximate approach for the calculation ofM s in iron-base alloys

Having estimated the critical driving force associated with martensitic transformation,ΔG α→M, as $$\Delta G^{\alpha \to M} = 2.1 \sigma + 900$$ whereσ is the yield strength of austenite atM s, in MN m−2, we can directly deduce theM s by the following equation: $$\Delta G^{\gamma \to {\rm M}} |_{M_S } = \Delta G^{\gamma \to \alpha } + \Delta G^{\alpha \to M} = 0.$$ The calculatedM s are in good agreement with the experimental results in Fe-C, Fe-Ni-C and Fe-Cr-C, and are consistent with part of the data in Fe-Ni, Fe-Cr and Fe-Mn alloys. Some higher “M s” determined in previous works may be identified asM a,M s of surface martensite or bainitic temperature. TheM s of pure iron is about 800 K. TheM s in Fe-C can be approximately expressed as $$M_S (^\circ {\text{C}}) = 520 {\text{--- }}\left[ {{\text{\% C}}} \right]{\text{ }}x 320.$$ In Fe-X, the effect of the alloying element onM s depends on its effect onT 0 and on the strengthening of austenite. An approach for calculation of ΔG γ→α in Fe-X-C is suggested. Thus dM s/dx c in Fe-X-C is found to increase with the decrease of the activity coefficient of carbon in austenite.

Structu re-property relationships in commercial low-alloy bainitic-austenitic steel with high strength, ductility, and toughness

AbstractA commercial low-alloy steel exhibiting combinations of mechanical properties superior to those of many other highly alloyed steels has been designed. The good properties are achieved by a carbide-free bainitic–austenitic structure, which can be produced by isothermal heat treatment of high-carbon silicon-alloyed steels in the upper bainite temperature range. Structure–property relationships are evaluated in order to find the optimum combination of heat treatment and alloy content. The effective grain size, the austenite volume fraction, and the austenite morphology are important factors controlling the properties. The mechanical stability of the austenite is of little importance.

Prebainitic phenomena in AISI 86B80 and 0.8C-5.3Ni steel

An ultrasonic investigation shols that the prebainitic kinetics of AISI 86B80 and 0.8C-5.3Ni steels follol t 2 3 and or t 1 time lals in the loler bainite temperature region. The data anallsis indicates that these kinetics signal carbon segregation to dislocations. The simple time lals are interrupted at the bainite start time. These results strongll suggest that the loler bainite reaction is initiated bl the formation of supersaturated ferrite through carbon segregation and the subseluent local martensitic transformation.

Effects of Tensile and Compressive Stresses, Applied During Isothermal Holding of Steel, on the Amount of Retained Austenite

It is well known that when steel is hot-quenched to the temperature range of lower bainite formation and is partially transformed isothermally, the residual austenite is stabilized and austenite retained (γR) at room temperature is increased. In previous reports, however, the author indicated the existence of the effect of stress on the formation of martensite nuclei in addition to the hitherto known effect of stress on the driving force of martensite transformation, and assumed the coherent embryos of martensite formed due to Bain’s strain to explain the reverse effect of tension and compression upon the transformation. In the present study, the author has investigated how austenite is stabilized when steel is held isothermally in the temperature range of unstable austenite under the load of tension or compression. The experimental results obtained are as follows; (1) When steel samples are held for the incubation time, there is no change in the amount of γR irrespective of the holding temperature and the application of stress of positive or negative sign. Therefore, this substantiates the results obtained in the previous report that “the amount of γR is dependent upon the temperature range in which the stress is applied”. (2) In the bainite temperature range the incubation time is shortened as a result of the shear stress effect irrespective of its sign. At room temperature, however, tensile stress increases γR and compressive stress decreases γR (as in case of the stress effect during continuous cooling disclosed in the previous report), owing to the reverse effect of stresses, depending on whether the sign is positive or negative, and on the number of nuclei formed after holding the samples for a certain time. Thus it is confirmed here again that there is an effect of stress on the number of martensite nuclei.