Fine structure of low-carbon steel after electrolytic plasma treatment

This work shows the results of research of the fine and dislocation structure of the transition layer of 18CrNi3Mo low-carbon steel after the influence of electrolytic plasma. Conducted research has shown that the modified steel layer, as a result of carbonitriding, was multiphase. Quantitative estimates were made for carbonitride М23(С,N)6 in various morphological components of α-martensite and on average by material in the transition layer of nitro-cemented steel. It was established that α-phase is tempered martensite after nitrocementation. Released martensite is represented by batch, or lath and lamellar low-temperature and high-temperature martensite. Inside the tempered martensitic crystals, lamellar cementite precipitates are simultaneously present, and residual austenite is found along the boundaries of the martensitic rails and plates of low-temperature martensite. It was determined that inside the crystals of all morphological components of α-martensite there are particles of carbonitride М23(С,N)6.

Correlation between Microstructures and Ductility Parameters of Cold Drawn Hyper-Eutectoid Steel Wires with Different Drawing Strains and Post-Deformation Annealing Conditions

The relationship between microstructures and ductility parameters, including reduction of area, elongation to failure, occurrence of delamination, and number of turns to failure in torsion, in hypereutectoid pearlitic steel wires was investigated. The transformed steel wires at 620 °C were successively dry-drawn to drawing strains from 0.40 to 2.38. To examine the effects of hot-dip galvanizing conditions, post-deformation annealing was performed on cold drawn steel wires (ε = 0.99, 1.59, and 2.38) with a different heating time of 30–3600 s at 500 °C in a salt bath. In cold drawn wires, elongation to failure dropped due to the formation of dislocation substructures, decreased slowly due to the increase of dislocation density, and saturated with drawing strain. During annealing, elongation to failure increased due to recovery, and saturated with annealing time. The variation of elongation to failure in cold drawn and annealed steel wires would depend on the distribution of dislocations in lamellar ferrite. The orientation of lamellar cementite and the shape of cementite particles would become an effective factor controlling number of turns to failure in torsion of cold drawn and annealed steel wires. The orientation and shape of lamellar cementite would become microstructural features controlling reduction of area of cold drawn and annealed steel wires. The density of dislocations contributed to reduction of area to some extent.

Effects of Wire Drawing and Annealing Conditions on Torsional Ductility of Cold Drawn and Annealed Hyper-Eutectoid Steel Wires

The effects of microstructural features on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires were investigated. The patented wire rods were successively dry drawn to ε = 0.79 (54.7%) ~ 2.38 (90.7%). To examine the effects of hot-dip galvanizing conditions on torsional ductility, steel wires with ε = 1.95 were annealed at 500 °C for 30 s for ~1 h in a salt bath. In cold drawn wires, the number of turns to failure increased steadily, showing the maximum peak, and then decreased with drawing strain. During the post-deformation annealing at 500 °C, torsional ductility of steel wires decreased with annealing time, except for the rapid drop due to the occurrence of delamination for 10 s annealing. The decrease of the number of turns to failure would be attributed to the microstructural evolutions, accompanying the spheroidization and growth of cementite particles and the recovery of ferrite in cold drawn steel wires. From the relationship between microstructural evolution and torsional ductility, it was found that among microstructural features, the shape and orientation of lamellar cementite showed the significant effect on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires.

Improvement of Mechanical Properties and Structure Modifications of Low Carbon Steel by Inoculations with Nano-Size Silicon Nitride

AISI 1020 steel is considered to be one of the most applicable structural steels, in particular in the cold drawn form. Heating of this grade of steel prior to AC1 must have been applied to spheroidize the lamellar cementite, and consequently enhances the cold formability character of the steel. Si3N4 nanoinoculation of this grade of steel has been used in this study, where it is added to the molten steel, in order to avoid the high cost long term heat treatment process prior to cold deformation process. Optical microscopy and SEM have been used to evaluate the morphology of cementite after nanoinoculation process with Si3N4. Tensile properties of nanoinoculated steels have been investigated. Finally, wear adhesive resistance of investigated samples has been evaluated. The obtained results showed a great enhancement in the mechanical properties, strength, ductility and adhesive wear resistance, as a result of the nucleation of cementite into a new spheroidal phase and grain refinement by Si3N4 inoculation and allow usage of AISI 1020 steels inoculated by nanoinoculant Si3N4 in further technological applications.

Effects of Mn, Si and Cr Addition on the Spheroidization of Cementite in Hypereutectoid Fe-1mass%C Steel

Effect of Mn, Si and Cr on spheroidization of cementite in Fe-1mass%C steel has been investigated over a range of austenitizing temperatures. In Fe-1C steel, a fully spheroidized structure is obtained but some large cementite particles are formed. The addition of 1.5 mass% Si or Cr accelerates spheroidization of cementite. An addition of Cr remarkably refine the cementite particle size, but the influence of Si addition on the cementite particle size is not remarkable. A fully spheroidized structure fails to develop in steel with the addition of 1.5% Mn under the condition used in present study. Some lamellar cementite still exist in the 1.5Mn steel. The pearlite-promoting effect of Mn is possibly attributed to the inhomogeneous distribution of cementite particles during the intercritical austenitization.

Magnetic Field-Induced Granular Pearlite at Early Stages of Phase Transformation

ffects of high magnetic fields (HMF) up to 19.81T on pearlite phase transformation are studied by examination of the microstructures of a Fe-0.47C-2.3Si-3.2Mn (wt %) alloy partially isothermally processed above the eutectoid temperature. The results show that granular pearlite (GP) can be obtained at earlier transformation stages. The evolution of the granular pearlite is always accompanied by the formation of lamellar pearlite. TEM analysis reveals the existence of sub-grain boundaries within GP colonies and indicates that the nucleation of ferrite matrix in GP belongs to multiple nucleation mechanism. Most of carbides at the early stage of pearlite formation are found to precipitate at the α/γ interface--the growing front of ferrite phases, and some of coarse carbides can further develop into thin lamellar cementite.

Cementite Spheroidization of Drawn Wire during Isothermal Treatment

Lamellar cementite will be spheroidized in drawn pearlitic steel wire during galvanization process. To understand the evolution of the microstructure in this process, effects of isothermal time on microstructure of drawn pearlitic steel wires were investigated by using scanning electron microscope (SEM), transmission electron microscope (TEM) and DSC Technique. Experimental results showed that the lamellar cementite would transform to spheroidized cementite during the isothermal treatment. During the heating process, no endothermic or exothermic peak existed in pearlitic strand, while an obvious exothermic peak appeared in cold drawn pearlitic wire at about 380°C. It results from the spheroidization of lamellar cementite. The dislocation density was very low in pearlitic strand, but the dislocation density increased shapely after drawing. During the isothermal treatment at 450°C, the high dislocation density zone disappeared and some cementite became spheroidized. The cementite spheroidization phenomena first began at the boundary of pearlitic blocks or grains, and then in the high dislocation density zone in pearlitic blocks.