Steels for bearings

The evolution of microstructure and texture during cold rolling of commercial-purity titanium (CP-Ti) was studied with particular reference to deformation twinning and dislocation slip. For low to intermediate deformation up to 40% in thickness reduction, the external strain was accommodated by slip and deformation twinning. In this stage, both compressive ( { 1 1 2 ¯ 2 } 〈 1 1 2 ¯ 3 ¯ 〉 ) and tensile ( { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 1 ¯ 〉 ) twins, as well as, secondary twins and tertiary twins were activated in the grains of favorable orientation, and this resulted in a heterogeneous microstructure in which grains were refined in local areas. For heavy deformation, between 60 and 90%, slip overrode twinning and shear bands developed. The crystal texture of deformed specimens was weakened by twinning but was strengthened by slip, resulting in a split-basal texture in heavily deformed specimens.

Effect of deformation twinning on microstructure and texture evolution during cold rolling of CP-titanium

THE only addition to this issue of “Steel and its Heat Treatment” is a chapter on nitriding, contributed by Dr. V. O. Homerberg. In the space of sixteen pages a brief outline is given of the nature and conduct of the nitriding process, of the steels suitable for nitriding, and of their properties after treatment. Otherwise the book remains the same, although eight years have passed since the preparation of the previous edition. There is still, therefore, no reference to the notched-bar impact tests to which British metallurgists rightly attach so much importance, to grain size control (although this originated in America), to the newer types of stainless steels, or to manganese -molybdenum steels.Steel and its Heat TreatmentD. K.BullensBy. Third edition, rewritten and reset. Pp. xiii + 580. (New York: John Wiley and Sons, Inc.; London: Chapman and Hall, Ltd., 1935.) 25s. net.

Steel and its heat treatment

Modeling the dynamic recrystallization under multi-stage hot deformation

In this work, the application of the Quenching and Partitioning (Q&P) process to two low-carbon steels has led to the development of a new kind of steel microstructure formed by laths of martensite separated by films of intercritical ferrite and retained austenite. The chemical compositions of the steels have been specially designed for this process, containing 3.5 wt.% Mn to retard the formation of bainite and combinations of Si and Al to avoid cementite precipitation. The microstructural changes occurring during the application of the heat treatments are discussed in terms of the current knowledge of the Q&P process and the experimental observations. A significant amount of retained austenite has been obtained in both steels after application of appropriate heat treatments, especially in the steel alloyed with higher amount of Si, in which the volume fraction of retained austenite reached values up to 0.19. Tensile tests in some selected specimens of both materials have shown outstanding combinations of strength and ductility, indicating that the designed Q&P steels are a promising candidate for the development of a new generation of advanced high strength steels.

New low carbon Q&P steels containing film-like intercritical ferrite

We describe here the effects of quenching and partitioning (Q&P) process on the evolution of microstructure and consequent changes in hardness in a set of medium-carbon and high-carbon steels containing varying percentage of chromium, manganese, and silicon, with the aim to advance our understanding of Q&P process. The study suggests that in medium-carbon steels, higher partitioning of carbon from martensite to retained austenite and stabilization of austenite occurs when martensite has a higher supersaturation of carbon after quenching, which is obtained at low quench temperature. Another important aspect that emerges from the study is that the transition ( ɛ ) carbide decreases hardness and its formation is promoted in medium-carbon steels with higher silicon-content such that the precipitation occurs at lower temperature of 250 °C. However, in contrast to medium-carbon steels, the high chromium content in high-carbon steels has a negative impact on the Q&P process because of the formation of large cementite during the spheroidizing treatment that reduces the ability of austenite to be enriched with carbon. The decrease in hardness in high-carbon steels during partitioning is a cumulative effect of austenite stabilization, softening of martensite, and decrease in carbon supersaturation of martensite.

Microstructure–hardness relationship in quenched and partitioned medium-carbon and high-carbon steels containing silicon

In this paper, we have developed the equations for predicting austenite grain size (AGS) of microalloyed medium carbon steel during hot rolling Dynamic recrystallization, which plays a major role in reducing the flow stress and AGS, was described by modifying Avrami's equation In addition, for better AGS prediction of material during rolling, the fraction of dynamic recrystallization was expressed as a function of strain together with the Zener–Hollomon parameter Torsion tests were performed in the temperature range of 900–1100 °C and the strain rate range of 50×10 −2 –50×10 0 s −1 to study the softening behavior To examine the validity of the developed equation, we have then applied it to a four-pass continuous deformation process simulated by hot torsion test For the exact prediction of microstructural evolution during hot torsion test, the change of temperature was calculated using finite difference method It was found that the calculated grain size of the alloy is in good agreement with the observed grain size It is expected that a fine grained steel showing high strength and high toughness can be obtained by controlling deformation conditions on the basis of the newly developed equation

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

The aim of the present study is to evaluate the influence of nano-sized carbides upon tensile behavior in UFG medium-carbon steels and to develop a material with improved tensile properties. UFG medium-carbon steels with fine carbides were successfully fabricated by multi-pass caliber rolling at 773 K. Alloying chromium and molybdenum resulted in thinner pearlitic lamellae, which were transformed into finer particles after severe plastic deformation. The UFG steel containing the alloying elements exhibited superior tensile properties, which was attributed to the enhanced strain hardening rate by the imbedded finer particles. Subsequent annealing induced growth of grains and particles, which also recovered elongation at the expense of strength. All UFG steels investigated here showed a yield-point phenomenon due to the decreased hardening rate and lack of mobile dislocations and their sources. The deteriorating effect of particle growth overwhelmed the improving effect of grain growth after annealing of the UFG medium-carbon steel, leading to a reduced strain hardening rate. This resulted in a positive correlation between a grain size and Luders elongation in the investigated UFG steels.

Enhancing tensile properties of ultrafine-grained medium-carbon steel utilizing fine carbides

The effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels has been investigated on the basis of microstructural analysis and thermodynamic calculations. The results showed that an increase of silicon content in high carbon chromium bearing steels retards the spheoridization of cementite. The thermodynamic calculations revealed that the shrinkage of the austenite phase field in bearing steels with increasing silicon content gave rise to an increase of volume fraction of cementite at an annealing temperature, possibly resulting in incomplete spheroidization. Furthermore, due to the low solubility of silicon in cementite, an increase of silicon content can raise the activity or chemical potential of carbon atoms in austenite at the austenite/cementite interfaces. Consequently, the difference in chemical potential of carbon atoms at the interfaces would be reduced with increasing silicon content, causing a decrease of the driving force for their diffusion from cementite to austenite.

Duk-Lak Lee

Papers

Microstructure–hardness relationship in quenched and partitioned medium-carbon and high-carbon steels containing silicon

Modeling of AGS and recrystallized fraction of microalloyed medium carbon steel during hot deformation

Enhancing tensile properties of ultrafine-grained medium-carbon steel utilizing fine carbides

Effect of silicon on the spheroidization of cementite in hypereutectoid high carbon chromium bearing steels

Surface wrinkle defect of carbon steel in the hot bar rolling process