Modelling of transition from upper to lower bainite in multi-component system

A model for estimating the upper to lower bainite transition has been developed for the iron–carbon–manganese–chromium–silicon alloy system by comparing the time required to decarburise a supersaturated bainitic ferrite platelet and that needed for the start of cementite precipitation in the ferrite. The problem is treated as a competition between the decarburisation time and the kinetics of cementite precipitation. Lower bainite is induced when the latter process is faster. The time for forming a volume fraction of 0.01 of the equilibrium amount of cementite is taken as the precipitation start time. The model was calibrated using experimental data from the iron–carbon system, and verified with the iron–carbon–manganese–molybdenum system and an experimental steel currently being developed for high-strength applications.     

Microstructure and microhardness of pearlitic steel after laser shock processing and annealing

The microstructure evolution of the high carbon pearlitic steel after laser shock processing (LSP) with different laser pulse energy and high temperature annealing was investigated. After LSP, the cementite lamella were bent, fractured and broken into granules. Fragmentation and dissolution of the cementite lamella were enhanced by increasing the laser pulse energy. Results show that the ferrite lattice parameter increased due to carbon atom dissolution in the ferrite matrix, and the corresponding ferrite X-ray diffraction peaks shifted significantly towards the smaller diffraction angles. After annealing at 650°C for 30?min, an ultrafine duplex microstructure (ferrite+cementite) was formed on the surface. After LSP with a high energy, equiaxed ferrite grains were refined to 400?nm and the cementite lamella were fully spheroidised with the particle diameter of ～150?nm. The corresponding grain size of ferrite and cementite under low pulse energy was 500 and 300?nm respectively. After annealing, the ferrite peaks significantly shifted towards the higher diffraction angles, and the ferrite lattice parameter decreased. The microhardness initially increases after LSP and then slightly decreases after subsequent annealing but remained higher than without LSP.     

Warm multiaxial forging of AISI 1016 steel

In this work, the microstructural evolution in AISI 1016 steel processed by using warm multiaxial forging technique is studied. With increase in multiaxial forging strain, a finer substructure evolved. Structural evolution in pearlite phase is addressed in detail considering the strain paths and strain rate. Pearlitic cementite fragmented into ultrafine particles of about 100–300 nm size. Warm multiaxial forging process also dispersed the ultrafine cementite particles into the ferrite matrix. Based on the grain boundary characterization and textural evolution, mechanism of ferrite grain refinement is explained. Up to six strain steps, crystallographic slip is the dominant mode of deformation and grain subdivision and recovery is the mechanism for ferrite grain refinement. At nine strain steps, dominant deformation mechanism appears to be grain boundary sliding and random grain rotation. After nine strain steps, initial grains of average 17 μm size reduced to submicron sized grains with the fraction of high angle grain boundaries exceeding 0.7. Double-n behavior is observed during tensile testing of some multiaxially forged steels. Tensile strength and hardness values of multiaxially forged steel increased by more than 100% after eighteen warm multiaxial forging strain steps, whereas ductility reduced by only about 30%.     

Microstructural evolution and mechanical behavior of warm multi-axially forged HSLA steel

Detailed studies are conducted on the microstructural evolution and mechanical behavior of a high strength low alloy (HSLA) steel processed by warm multi-axial forging (MAF). After nine MAF strain steps, the initial ferrite grains of average 13-μm size reduced to submicron-sized grains with over 0.7 fraction of high angle grain boundaries. Pearlitic cementite is fragmented and refined to about 50–100 nm size particles. The microstructure evolution with respect to fraction of HAGB with increase in number of strain steps is more sluggish in HSLA steel as compared to plain carbon steel of comparable carbon content. This is ascribed to the Zener pinning effect of (Ti, Nb) carbide particles. Tensile strength and hardness values of MAFed steel increased by more than 45 and 58 %, respectively, after nine warm MAF strain steps, whereas the fracture strain was reduced from 21 to 12 %.     

Microstructure and mechanical properties of UFG medium carbon steel processed by HPT at increased temperature

Using the high pressure torsion (HPT) deformation method the medium carbon steel (AISI 1045) was the experimental material used to conduct the deformation process. The torsion deformation experiment was performed at increased temperature of 400 °C. The influence of deformation processing parameters, resolved shear strain γ (number of turns N = 1–6) and applied pressure p (constant pressure of 7 GPa), was evaluated by microstructure analysis and mechanical properties. The strength behaviour was assessed by microhardness measurements across the disc to detect the positional hardening, by tensile tests and in situ measured torque. In situ measurement of torque during deformation allows characterizing the changes in mechanical properties due to the large shear deformation developed across the disc. To obtain absolute values of strength the ultimate tensile strength was measured in radial direction with respect to the deformed sample. From each deformed disc two sub-sized tensile test specimens with gauge length of 2.5 mm were machined. The tensile strength in samples increased markedly with the number of turns. The hardness measured at disc edge gradually increases as straining increases until it saturates after 2–3 turns. However, the hardness values at edge were different from those measured in disc centre and for applied straining no saturation was reached across the disc. The SEM and TEM investigations were carried out to analyze the fine microstructure evolution regarding the strain introduced. To follow the difference in strain distribution across the deformed disc the microstructure analysis was performed at edge and central site of the disc in order to evaluate the effect of the strain distribution. TEM investigation confirmed the increasing misorientation even in very small grains, the fragmentation and dissolution of the cementite lamellae, (diffuse cementite/ferrite boundaries), the alignment of the fragments to the shear plane with increasing deformation. Indistinct deformation of ferrite and preserved cementite lamellae morphology were found at the centre of the disc.     

The role of carbon segregation on nanocrystallisation of pearlitic steels processed by severe plastic deformation

The nanostructure and the carbon distribution in a pearlitic steel processed by torsion under high pressure was investigated by three-dimensional atom probe. In the early stage of deformation (shear strain of 62), off-stoichiometry cementite was analysed close to interphase boundaries and a strong segregation of carbon atoms along dislocation cell boundaries was observed in the ferrite. At a shear strain of 300, only few nanoscaled off-stoichiometry cementite particles remain and a nanoscaled equiaxed grain structure with a grain size of about 20 nm was revealed. 3D-AP data clearly point out a strong segregation of carbon atoms along grain boundaries. The influence of this carbon atom segregation on the nanostructure formation is discussed and a scenario accounting for the nanocrystallisation during severe plastic deformation is proposed.     

The influence of diffusion of carbon in ferrite as well as in austenite on a model of reaustenitization from ferrite/cementite mixtures in Fe-C steels

The kinetics of formation of austenite from ferrite and cementite mixtures has been modelled by assuming the local equilibrium at the planar phase interfaces. The exact solutions to the diffusion equations governing the volume diffusion of carbon in austenite and ferrite are presented. The concurrent motions of the two interfaces are calculated via solving a set of transcendental equations derived from the flux balance conditions. At low isothermal transformation temperatures, it is found that the time required for reaustenitization is slightly greater than the time previously calculated with no diffusion of carbon in ferrite.     

Effect of intercritical temperature on the strain hardening of dual-phase bainite/ferrite steel

Medium-carbon dual-phase bainite/ferrite steels with different bainite and carbide volume fractions were prepared via intercritical annealing and austempering. Bainite dominated the microstructure at high intercritical temperatures, and carbides and ferrite phase dominated at low temperatures. Carbides existed in two types, undissolved alloy–cementite and vanadium carbide. The ferrite grain size was reduced to 1.22?±?0.7 µm at 790°C. Modified Crussard–Jaoul (MC–J) model was used to analyse the strain hardening process, and the highest strain hardening rate was obtained at 755°C. The high carbide fraction at 755°C increased the ferrite dislocation density and enhanced the ferrite phase strength. The high dislocation density in ferrite and the small strength difference in bainite and ferrite enhanced the strain hardening rate at 755°C.     

Influence of torsion deformation on microstructure of cold-drawn pearlitic steel wire

A comparative microstructural analyse of cold-drawn pearlitic steel wires in as-drawn and after an additional torsion deformation states is presented in this paper. During torsion the temperature of the wire increases to attain 90 °C. Then the microstructure of wires is the result of different events effects, as initial drawing, temperature increase and torsion deformation. Individually or in association, both events influence the stress level and nature in ferrite and cementite lamellae, modify the kinetic of cementite decomposition and change the dislocation mobility in cementite and ferrite. Carbon atoms migration from cementite to ferrite is affected by these thermomechanical treatments inducing a modification of dislocation pinning by carbon atoms and lamellae interfaces. The phases’ determination and quantification, associated with the carbon content variation in each phase was investigated by Mössbauer spectroscopy. The evolution of the pearlitic steel wires microstructure will be discussed point-by-point, as a function of applied deformation nature.     

Fabrication of nanostructured pearlite steel wires using electropulsing

This work reports the refinement of pearlite structure into nanostructure using electropulsing. Nanostructured pearlitic steel wires possess nanoscale lamellae or nanoscale grain microstructures. Fabrication of nanostructures by severe plastic deformation and lamellar to grain transformation have been investigated. It is suggested that an aligned pearlite structure is preferred in severe plastic deformation. The lamellar to grain transformation is controlled by diffusion of carbon within cementite and also from cementite to ferrite phases. Carbon mobility is changed by mechanical, thermal and electrical states. The interface between nanoscale sub-grains in the ferrite phase has considerable carbon content. Numerical calculations and experimental observations demonstrated these mechanisms.     

Microstructural development in non-oriented lamination steels

The isothermal decomposition of austenite in two commercial low carbon (0.04 w/o) steels has been examined using scanning electron microscopy and transmission electron microscopy. Particular emphasis has been placed on analysing the pearlite reaction and the development of massive films of cementite at pro-eutectoid ferrite/pearlite interfaces. Similarly, grain boundary precipitation of cementite has been investigated. The results strongly support the contention that films of cementite at ferrite/pearlite interfaces form predominantly by a coarsening process. In addition, it is shown that grain boundary precipitation of cementite can occur from super-saturated ferrite or from the decomposition of austenite. Examination of the early stages of the pearlite reaction has provided evidence that multiple nucleation of cementite can be a necessary precursor to the development of a pearlite colony.     

Hybrid Friction Stir Welding of High-carbon Steel

A high-carbon steel joint,SK5(0.84 wt% C),was successfully welded by friction stir welding(FSW),both without and with a gas torch,in order to control the cooling rate during welding.After welding,the weld zone comprised gray and black regions,corresponding to microstructural variation:a martensite structure and a duplex structure of ferrite and cementite,respectively.The volume fraction of the martensite structure and the Vickers hardness in the welds were decreased with the using of the gas torch,which was related with the lower cooling rate.     

Suppression of hydrogen-induced blisters in SK4 carbon steel alloy by friction stir processing

The feasibility of using friction stir-processing technique as a microstructural refining method for preventing hydrogen-induced blisters of SK4 carbon steel alloy containing microstructure of hard cementite particles in a ductile ferrite matrix was examined. Amount and size of hydrogen-induced blisters decreased in the stirred layers with increasing rotation speed and completely disappeared when rotation speed attained 400 rpm because of the formation of a hard and fine microstructure consisting of pearlite, martensite, and retained austenite instead of that containing hard spheroidized cementite particles in a soft ferrite matrix. Interfaces between ferrite matrix and cementite particles were the preferential sites for the hydrogen blisters initiation.     

Effects of vanadium addition on nucleation and growth of pearlite in high carbon steel

Abstract

The influence of vanadium addition on the microstructure of high carbon steels has been investigated. A careful examination of the initial stages of austenite decomposi~ion has been made, using a range of high resolution metallographic techniques. It has been confirmed that vanadium addition results in the formation of grain boundary ferrite films, even in the eutectoid composition range. It is argued that this ferrite is the product of eutectoid transformation, and is not proeutectoid ferrite. This is because the first event is the nucleation of carbide particles along the grain boundaries. These carbides have been identified mainly as cementite. The presence of vanadium appears to change the morphology and distribution of the grain boundary cementite, so that rather than forming a grain boundary network, the cementite occurs in the form of a high density of small discrete particles along the boundaries. It is proposed that this occurs because vanadium increases the driving force for cementite nucleation. The formation of the grain boundary cementite depletes the surrounding region of carbon and encourages the formation of ferrite, but because of their discrete and fine dispersion, the cementite particles are engulfed by the more voluminous ferrite phase. In such regions, the onset of afully cooperative growth regime is delayed. Pearliteforms later at the ferrite/austenite interfaces.

MST/1923     

Origin and mechanism of torsion fracture in cold-drawn pearlitic steel wires

The structural and mechanical factors that control the torsion fracture behavior of cold-drawn eutectoid steel wires are examined. Two types of the fracture are identified; namely, flat- and cleavage-type. Torsion cracks are found to initiate in ferrite and propagate along the ferrite/cementite interface. The shear stress distribution within the wires is affected not only by the applied torque, but also by the residual stress. The maximum shear stress occurs halfway from center to the surface, where the cracks initiated. The growth of torsion cracks is sensitive to the orientation of cementite lamellas in pearlite grains. The influence of thermal history on the occurrence of cleavage fracture is ascertained, with the assistance of atom probe. It shows that the cleavage fracture results from a decrease in dislocation mobility, caused by thermally activated diffusion of carbon atoms into ferrite.     

Wear of Laying Head Pipes in a Wire Rod Mill

The failure analysis on the premature wear of a “Laying Head Pipe” in a Wire Rod Mill has been presented. The hot-rolled wire rods subsequent to finish rolling pass through the “Laying Head Pipe” which rotates and lays the wire rods in the form of coils for air cooling to achieve the final properties. A worn-out pipe and a thermo-mechanically treated (TMT) re-bar have been analyzed. The material of the pipe is ASTM A335-P5 grade of seamless alloy steel pipe used for high temperature service. The microstructures of the wear groove of the alloy steel pipe show predominantly ferrite and globular cementite/carbide particles along with scales, while that away from the wear groove shows coarse tempered martensite matrix. EDS analysis confirms the presence of alloy carbides near the wear groove. Microhardness profile shows reduction in hardness toward the inner surface of pipe; hardness at the inner surface of the pipe becomes lower than the surface hardness of the TMT re-bar, exhibiting tempered martensite matrix. Softening at the inner surface of the pipe wall occurs due to a rise in temperature/ over-tempering in contact with the passing hot wire rods (900 °C) which causes transformation of martensite into ferrite and coarse globular cementite.     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

超快冷对碳素钢中渗碳体析出强化行为的影响*

用超快速冷却技术并控制轧后冷却温度, 研究了3种碳含量不同的碳素钢热轧后组织中渗碳体的析出行为和强化机制。结果表明, 在超快速冷却条件下0.04%C和0.5%C(质量分数, 下同)实验钢的主要强化方式分别是细化晶粒和细化珠光体片层间距, 没有纳米级渗碳体颗粒析出, 而在0.17%C实验钢的组织中则有大量弥散的纳米级渗碳体析出, 颗粒直径范围为10-100 nm, 通过超快速冷却技术实现了在不添加微合金元素的条件下纳米级渗碳体的析出。随着超快速冷却终冷温度的降低纳米渗碳体的析出强化作用使0.17%C钢的屈服强度提高110 MPa, 强化效果明显。在超快速冷却的工艺基础上若继续采用形变热处理工艺, 可进一步提高0.17%C实验钢的位错密度, 促进渗碳体均匀形核, 实现纳米级渗碳体颗粒在整个组织中更加均匀弥散的分布, 达到更好的均匀强化效果。在超快速冷却和形变热处理工艺条件下0.17%C钢的屈服强度可达到650 MPa以上, 强化效果提高300 MPa以上。     

Study on dislocation slips in ferrite and deformation of cementite in cold drawn pearlitic steel wires from medium to high strain

Abstract

The deformation of cementite was studied via optical microscopy and SEM in the longitudinal sections of pearlitic steel wires from medium to high strain. The cementite shows good deformability, the angle between the deformation direction of cementite and drawing direction decreases with increasing strain, and finally the deformation directions of cementite turn to the drawing axis at high strains. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, S-bands and cracks across cementite observed parallel to either {110}_α-Fe or {112}_α-Fe plane traces determined by the largest Schmid factors.     

钢材相似显微组织的分析与鉴别

在日常钢材的显微组织检验中,由于网状铁素体和网状二次渗碳体、针状铁素体和针状渗碳体、马氏体及其回火产物（回火马氏体、回火屈氏体和回火索氏体）、未溶铁素体和先共析铁素体、淬火屈氏体和回火屈氏体等显微组织的显微特征非常相似,容易出现误判。以常用碳钢为例,运用显微组织观察、显微硬度测试、化学试剂侵蚀等金相检验的普通手段,通过对显微组织形貌中的组织形态、分布、颜色进行直观比较,总结归纳出了上述几种相似显微组织的鉴别方法,以供金相检验人员参考。