Deformation induced pearlite transformation and dynamic spheroidization in a eutectoid steel

The microstructure evolution of a eutectoid steel during the deformation induced pearlite transformation of undercooled austenite was investigated by uniaxial hot compression simulation experiment. The effects of different deformation degree, deformation rates and deformation temperature on the deformation induced pearlite transformation were explored. The results indicate that the induced pearlite transformation can occur rapidly during the deformation, for the stress accelerates phase transition. With the increase of the deformation degree, the dislocation density and phase transition driving force in the microstructure are improved, accelerating the occurrence of phase transition and the process of cementite spheroidization. For the diffusion- controlled phase transition, the deformation rates decrease to prolong the deformation time, so the carbon atoms can diffuse sufficiently to obtain spheroidized cementite. At lower deformation temperature from A1 to Ar1, significant refinement of the fragmentation of cementite occurs due to the increase of supercooling and spheroidized time. The ultrafine microstructure of cementite particles can be obtained through the high deformation degree, low deformation rates and low deformation temperature. It is also observed that the pro- eutectoid ferrite nucleates along the austenite boundary in the process of deformation.     

The pearlite reaction

A critical appraisal of theory and experiments for both isothermal and forced velocity pearlite is presented. It is concluded for binary systems that both the theoretical models for volume diffusion and boundary diffusion control are well-advanced and adequate for the purposes of experimental test. However, some ambiguity remains in the boundary diffusion model with respect to the thermodynamics of the boundary ”phase” region, so it is still not possible to predict absolute rates of transformation. The theoretical problem for ternary pearlites is also well understood, although rigorous theory seems intractable. A new perturbation procedure for definition of the optimal steady-state spacing is presented and amplified for both isothermal and forced velocity pearlite, and for both volume and boundary diffusion models. In terms of the critical spacing S_c for isothermal pearlite and the spacing at minimum undercooling S_m for forced velocity pearlite the predicted stability points are as follows: {fx2777-1} For isothermal pearlite these perturbation results correspond closely to the state of maximum entropy production rate while for forced velocity pearlite the correspondence is also satisfactory. A detailed analysis of the data leads us to reaffirm the author’s conclusions that the eutectoid reactions in Cu-12 pct Al and some related ternary alloys reported by Asundi and West are controlled by volume diffusion and that the eutectoid reaction in Al-78 Zn reported by Cheetham and Ridley is controlled by boundary diffusion. We conclude further after careful analysis that the pearlite reaction in Fe-0.8 C is controlled for the higher temperatures by volume diffusion of carbon in austenite. We are also led to state that the pearlite transformations in Fe-C-Mn and Fe-C-Ni occur for the most part in a nopartition regime and are therefore controlled by volume diffusion of carbon in austenite, while the transformations in Fe-C-Cr and Fe-C-Mo, being forced by thermodynamics to sustain partition of chromium and molybdenum, are controlled by phase boundary diffusion of the latter elements. nt]mis|M. P. PULS, formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

Forced velocity pearlite

A high purity Fe-C alloy of eutectoid composition has been transformed from homogenized austenite in two ways: isothermally and at forced constant-velocity. The latter transformation mode, forced over five decades of velocity, produces structures with characteristic dimensions ranging over nearly two orders of magnitude. There is a consistent alignment of the transformation product in the heat-flow direction at all velocities, but the classical pearlite morphology undergoes a microstructural transition at the higher velocities. A spacing-velocity relationship for regular lamellar pearlite was measured as V^0.41±o.02λν = constant.     

Kinetics of pearlite spheroidizations

A study of the kinetics of pearlite spheroidization under static annealing conditions was carried out in two materials — AISI 1080 steel and pure Fe-C alloy. A stereological “shape factor”,F, defined asF =S _v ^p/3• K_m, was introduced for the kinetic study. The significance of this shape factor in relation to the geometrical characters of lamellar structures is discussed. For constant temperature a linear relation betweenF and the logarithm of time was obtained. Analysis of the time and temperature dependencies for a constant shape factor gave an activation energy of 70 kcal/mole for AISI 1080 steel and 58 kcal/mole for Fe-C alloy which indicates that volume diffusion of Fe in ferrite is the rate-controlling mechanism. The modified fault migration theory, which was developed from the mechanism study of this research, was applied to predict the kinetics of the pearlite spheroidization. For both the AISI 1080 and the Fe-C alloy experimental results have a good match with the theoretical prediction.     

Fracture of steels containing pearlite

The relative effects of pearlite and spherodite on ductile, cleavage, and fatigue failure are summarized. Neither the cleavage strength nor the fatigue endurance limit appear to depend directly on cementite contentper se. Spherodized steels cleave less readily than ferrite/pearlite steels. Ductile fracture resistance is lowered considerably by both types of cementite, pearlite being more deleterious. Ferrite/pearlite steels appear to exhibit slower fatigue crack growth rates at low stress intensity levels than high strength steels. At high stress intensity levels the behavior is reversed. Slip-incuded cracking of carbide lamellae appears easier than that of spherodized carbides. In ductile fracture situations the crack spreads progressively through a pearlite colony via preferential cracking of carbides and rupture of the intervening ferrite accompanied by large local shear strains. Fatigue fracture proceeds with formation of frequent branches, preferentially along the pearlite colony interface. This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

Hot deformation-enhanced globularization of pearlite

A plain high-carbon steel (0.74% C) was used to study the globularization (spheroidization and coalescence) of pearlite. In order to induce and/or enhance this process, specimens were deformed in the temperature range of austenite-to-pearlite transformation. It was found that only hot deformation applied after the completion of pearlite transformation leads to spheroidization of pearlite. The rate of this process is accelerated by a factor of 10⁴ compared with annealing without deformation. The kinetics of pearlite spheroidization are dependent on the pearlite lamellae thickness as well as pearlite orientation with respect to the direction of deformation. The final microstructure is, compared with that after a conventional soft annealing, considerably finer, the strength properties thus being largely retained.     

Thermomechanical treatments of a 1050 pearlite steel

A study of the microstructures and mechanical properties of thermomechanically treated 1050 steel has been carried out. The materials were first transformed to fine and coarse lamellar pearlite structures. These samples were reduced 75 pct in thickness by cold rolling, then heat treated at 780 °C (up-quenching) or 650 °C (annealing) for different periods of time, followed by air-cooling. The mechanical properties were found to be vastly improved by suitable thermomechanical treatments. A significant improvement in yield strength was accompanied by moderate increase in ultimate tensile strength, and the elongations were maintained above 15 pct in 3 cm gage length. The optimal mechanical properties were developed between 30 and 120 seconds during 780 °C up-quenching for coarse pearlite samples and 100 to 400 seconds for both annealed coarse and fine pearlite specimens. In these instances the relatively long duration of heat treatment time shall facilitate the industrial processing of the steel. The improvement in mechanical properties was correlated with distinct microstructures in pearlite regions, which were fine subgrains (or grains) less than 1 μm in size and dispersed globular carbide particles along the subgrain (or grain) boundaries. Formerly with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan.     

Ferrite: Cementite crystallography in pearlite

The interlamellar crystallography of pearlite in iron-carbon alloys is examined in Fe-0.8C and Fe-0.81C-12Mn steels with attention given to the Bagaryatsky, Pitsch-Petch, and Isaichev orientation relationships and their related atomic habit planes. A survey of the overall literature reveals confusion about the crystallographic uniqueness in these orientation relationships. The present article focuses on this uniqueness and on habit planes between the pearlitic ferrite and cementite, from which the crystallography of ferrous pearlite orientation relationships is proposed to emanate. Techniques to characterize and identify orientation relationships unambiguously are developed. Habit planes are shown to be consistently obtained, even during curvature, with all deviations accommodated by microscopic steps.     

The effect of alloying elements on pearlite growth

The model developed by Sharma 1 and Sharmaet al. 2 for the mechanism of pearlite growth in ternary Fe-C-Cr alloys has been extended to other systems. Detailed thermodynamic and semi-empirical kinetic calculations have been carried out to determine pearlite growth rates for a number of Fe-C-X alloys (X = Mn, Cr, Ni, Si) and compared with the experimental data from the literature. It has been shown that the pearlite growth is, in general, controlled by the alloying element boundary diffusion at low supersaturations and carbon volume diffusion at high supersaturations, as predicted by the model. The transition temperature between the two mechanisms is a function of the amount and type of alloying element. Formerly a at the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India     

Behavior of pearlite of various morphologies during cyclic tension

The structural evolution of hypereutectoid U10 steel with a pearlitic structure of various types (fine lamellar or partly spheroidized pearlite) is studied during fatigue loading. The fracture of these structures is considered using fractography data. The specific features of the structural transformations and the changes in the dislocation structure of the U10 steel are revealed during cyclic tension in the high-cycle fatigue region at a significant distance (10 mm) from a fatigue fracture surface. Substantial structural changes are shown to occur in U10 steel with various pearlitic structures during high-cycle fatigue tests (tension at a stress amplitude in a cycle lower than the macroscopic yield strength). These are the fragmentation, partial dissolution, and spheroidization of cementite lamellae and the polygonization of the ferrite component. The relationship between the type of fracture surface and the type of structure formed upon fatigue loading is found.     

The effect of morphology on the strength of pearlite

The effects of various morphological features on the strength of high-purity pearlite were studied. A continuous-cooling mode of transformation from different austenitizing temperatures was used to produce variations in average nodule diameter and minimum interlamellar spacing. It was found that, for a constant transformation temperature, nodule size was directly related to prior austenite grain size. On the other hand, minimum interlamellar spacing is controlled by transformation temperature, independent of prior austenite grain size and nodule size. Both the yield strength and fracture stress of pearlite was found to be inversely proportional to interlamellar spacing and independent of prior austenite grain size and nodule size.     

The effect of morphology on the strength of pearlite

The effects of various morphological features on the strength of high-purity pearlite were studied. A continuous-cooling mode of transformation from different austenitizing temperatures was used to produce variations in average nodule diameter and minimum interlamellar spacing. It was found that, for a constant transformation temperature, nodule size was directly related to prior austenite grain size. On the other hand, minimum interlamellar spacing is controlled by transformation temperature, independent of prior austenite grain size and nodule size. Both the yield strength and fracture stress of pearlite was found to be inversely proportional to interlamellar spacing and independent of prior austenite grain size and nodule size.     

Effect of alloys additions on the interlamellar spacing of pearlite

The spacing of pearlite was measured to investigate the effect of, alloying elements on this parameter using high purity plain carbon, 2 % Co, 2 % Si, 1 % Cr and 0.3 % Mo eutectoid steels. The investigation revealed that the addition of these alloying elements increases the spacing of pearlite at a given undercooling.     

低碳钢中伪珠光体分析

 利用光学显微镜检测了抗拉强度超过标准上限的低碳钢的金相组织,试样中伪珠光体体积分数达到50%以上,明显高于正常组织中的珠光体体积分数,因此具有显著的珠光体强化效果;并且伪珠光体存在明显的退化现象。对比分析生产工艺后认为,层流冷却过程中对先共析铁素体析出的抑制以及终冷温度低于空冷的[Ar1]是产生这种退化显著的伪珠光体组织的必要条件。结合生产实践分析了影响先共析铁素体析出的因素。     

Effect of continuous cooling on the morphology and kinetics of pearlite

Metallurgical and Materials Transactions A - The effect of variations in cooling rate on the morphology and kinetics of pearlite was studied and was contrasted with the isothermal and isovelocity...