Distribution Of MnS Inclusions Research Articles

Characterization of the morphological evolution of MnS inclusions in free-cutting steel during heating

The size, morphology and distribution of MnS inclusions in the sulfur bearing free-cutting steel are the key factors to determine the performance of the free-cutting steel. It is thus important to accurately characterize and control the morphology of the MnS inclusions in the steel. The morphology of MnS inclusions are complex during rolling due to their good plasticity. Additionally, the morphology of the MnS inclusions would change during heating due to the decomposition and mass transfer of the Mn and S in the steel matrix. The traditional metallographic for characterizing the MnS inclusions on the cross-sections only reveal the local characteristics of the inclusions and results in misunderstanding of the real morphologies. In this study, the three-dimensional (3D) morphology of the MnS inclusions in the sulfur bearing free-cutting steels is characterized by using the X-ray Micro-CT in the beamline of BL13W1 at Shanghai Synchrotron Radiation Facility (SSRF). The stereology and morphological evolution of the MnS inclusion during heating are characterized and analyzed based the 3D observation. The number of MnS inclusions increased more than twice, the length of the inclusion with sizes less than 9 μm increased from 25.79% to 71.28%. The anisotropy index was reduce from 2.30 to 1.04 due to the spheroidization of the MnS inclusions during heating. The mechanism of the morphological evolution of the MnS inclusions is discussed on basis of the local chemical potential of the MnS caused by the variation of the curvature on the surface of the inclusions.

Modification of MnS inclusion by tellurium in 38MnVS6 micro-alloyed steel

In 38MnVS6 steel, the morphology of sulfide inclusion has a strong influence on the fatigue life and machinability of the steel. In most cases, the MnS inclusions show strip morphology after rolling, which significantly affects the steel quality. Usually, the MnS inclusion with a spherical morphology is the best morphology for the steel quality. In the present work, tellurium was applied to 38MnVS6 micro-alloyed steel to control the MnS inclusion. Trace tellurium was added into 38MnVS6 steel and the effect of Te on the morphology, composition, size and distribution of MnS inclusions were investigated. Experimental results show that with the increase of Te content, the equivalent diameter and the aspect ratio of inclusion decrease strikingly, and the number of inclusions with small aspect ratio increases. The inclusions are dissociated and spherized. The SEM-EDS analysis indicates that the trace Te mainly dissolves in MnS inclusion. Once the MnS is saturated with Te, MnTe starts to generate and wraps MnS. The critical Te/S value for the formation of MnTe in the 38MnV6 steel is determined to be approximately 0.075. With the increase of Te/S ratio, the aspect ratio of MnS inclusion decreases and gradually reaches a constant level. The Te/S value in the 38MnVS6 steel corresponding to the change of aspect ratio from decreasing to constant ranges from 0.096 to 0.255. This is most likely to be caused by the saturation of Te in the MnS inclusion. After adding Te in the steel, rod-like MnS inclusion is modified to small inclusion and the smaller the MnS inclusion, the lower the aspect ratio.

Induced-Pitting Behaviors of MnS Inclusions in Steel

AbstractThis paper investigated the effects of MnS inclusions on inducing pitting in steel. Three different cooling methods were used to control the morphology, size, and distribution of MnS inclusions. AC (alternating current) impedance spectroscopy experiments and potentiodynamic polarization experiments were carried out for different sizes of MnS inclusions. The corrosion immersion experiment was conducted to study the pitting corrosion process in different morphologies of MnS inclusions. The experiment results showed that pitting corrosion occurred around MnS inclusions and pitting was induced along the junction of MnS inclusions and their surrounding steel matrix. Also, the sensitivity of inducing-pitting of MnS inclusions was changed with the change of the average size of MnS in steel and it had a critical size of MnS. The deeper the MnS inclusions were buried in steel matrix, the more difficult it was for it to dissolve and fall off. Herein, it would cause greater damage.

Distribution and Morphology of MnS Inclusions in Resulfurized Non-Quenched and Tempered Steel with Zr Addition

Distribution and morphology of MnS inclusions in as-cast ingots and as-forged bars of two Zr-bearing resulfurized non-quenched and tempered (NQT) steels have been performed. In the low Zr-bearing steel (0.001 wt%), MnS inclusions, which are teardrop-shaped or rod-like in two-dimensional (2D) morphology and dendritic or skeletal in three-dimensional (3D) morphology, are mainly distributed and segregated at the grain boundaries. While in the high Zr-bearing steel (0.0066 wt%), MnS inclusions are spherical or angular in both 2D and 3D observation and the distribution is more uniform than those in low zirconium steel. The calculated results by Thermo-Calc software show that the content of oxygen is not the direct factor that influences the morphology and distribution of MnS inclusions in medium-sulfur low-oxygen NQT steels. ZrO2 particles are ideal partcles for generation of spherical type I MnS inclusions owing to their strong nucleation capability and large amounts. However, the Zr and Al contents should be controlled cautiously to avoid generating large-sized agminated complex oxides, which are not easy to float and be removed owing to their high density. Otherwise, the ideal particles ZrO2 would decrease sharply in number and fail in offering sufficient heterogeneous nuclei for type I MnS inclusions. Besides, high proportion of complex MnS inclusions would decrease the supersaturation when pure MnS inclusions begin to precipitate, suppressing the generation of dendritic type II MnS inclusions.

Morphology, size and distribution of MnS inclusions in non-quenched and tempered steel during heat treatment

This article reports the morphology, size, and distribution evolution of MnS inclusions in non-quenched and tempered steel during heat treatment. The variation of single large-sized MnS inclusions at high temperature was observed in situ using a confocal scanning laser microscope (CSLM). The slender MnS inclusions first changed to pearl-like strings. These small-sized pearls subsequently coalesced and became closer together as the temperature increased. Large-sized MnS inclusions in non-quenched and tempered steel samples with different thermal histories were investigated with respect to the evolution of their morphology, size, and distribution. After 30 min of ovulation at 1573 K, the percentage of MnS inclusions larger than 3 µm decreased from 50.5% to 3.0%. After a 3 h soaking period, Ostwald ripening occurred. Most MnS inclusions moved from the grain boundaries to the interior. The present study demonstrates that heat treatment is an effective method of changing the morphology, size, and distribution of MnS inclusions, especially large-sized ones.

Characterisation of inclusions in resulphurised microalloyed steel

Resulphurised steels, due to their good mechanical properties and machinability, are employed in the manufacture parts of trucks and high strength components. The microstructure control of these steels can improve their toughness without losing strength. In this sense, the chemical composition, morphology and distribution of MnS achieved with different thermomechanical processes affect the mechanical properties of the steel. In this work, the morphology and distribution of MnS inclusions in resulphurised steel 15V45 were analysed. Samples of the steel in its as cast condition and after hot rolling were examined using light and scanning electron (SEM) microscopies. Microhardness measurements and chemical composition of different MnS types were carried out to evaluate the plastic behaviour of each inclusion. The SEM images have provided information about the nucleation sites and size of the inclusions. Applying a Fe–Mn–S diagram under different thermomechanical conditions, this study could predict ductility problems and crack generation during hot rolling processes.

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304 stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions. MgCl 2 was deposited onto four-point bend specimens, which were then placed in a chamber with a relative humidity of 45% and temperature of 60 °C. These test conditions were designed to reflect external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, but with the severity of the exposure conditions enhanced to allow discrimination of the effect of surface preparation in a short timescale (up to 1500 h). Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction measurements were undertaken. Stress corrosion cracks were observed for the ground and milled specimens but not for the dressed specimens, with cracks apparently originating at corrosion pits. The density of cracks increased in the order: transverse ground, milled and longitudinal ground, with the cracks notably much smaller in length for the transverse ground condition. The propensity for cracking could be linked to the high residual stress and apparent nanocrystalline microstructure at the surface. There was a greater propensity for pitting to initiate at local defect sites on the surface (laps, deeper grooves). However, the tendency was not overwhelming, suggesting that other factors such as more general roughness or the distribution of MnS inclusions had an influence, perhaps reflecting the severity of the environment.

The Effect of Ti-Zr Deoxidation on the Mechanical Properties of High Strength Low Alloy Steels

The uniformity of the mechanical properties, especially elongation and impact toughness, were compared between steel A, which was deoxidized with Ti-Zr, and steel B, which was deoxidized with Al. Microstructural observations, energy dispersive X-ray spectroscopy and X-ray diffraction analyses were conducted using an optical microscope, a scanning electron microscope and a transmission electron microscope, respectively. Results showed that sub-micron and nano-sized complex oxides were obtained by the combined deoxidation of Ti-Zr. The stability of the mechanical properties of steel A was better than that of steel B. The elongation and impact toughness of steel A were enhanced relative to those of steel B. This was attributed to spheroidization and the dispersed distribution of MnS inclusions in the matrix of steel A.

Effect of Titanium Addition on the Formation and Distribution of MnS Inclusions in Steel during Solidification.

Effect of titanium addition on the size and dispersion of MnS inclusions in Fe-0.1 C-1 Mn-0.025(mass%) base alloys has been investigated by microscopy. Titanium addition brings about a marked decrease in the size of MnS inclusions which are otherwise large and globular in Ti-free specimens. The formation mechanism of MnS inclusions is discussed on the basis of phase diagram information and thermodynamics of particle engulfment during solidification. It is shown that the reduction in size of the MnS particles consequent on Ti addition is directly the result of successive nucleation events involving (Ti,Mn)O and MnS at the solid/liquid interface of iron.

Factors influencing the rate of growth of fatigue cracks in RPV steels exposed to a simulated PWR primary water environment

Fatigue crack propagation has been studied in A533B plate exposed to simulated PWR primary coolant environment, as part of a U.K. collaborative programme. Compact tension specimens 25 mm or 50 mm thick were fatigued in a non-refreshed autoclave over the temperature range 130–290°C and frequency range 0.00167–0.1 Hz. A wide range of ΔK values were studied using stepped loading techniques on steels with nominal sulphur contents in the range 0.006–0.019%. MnS inclusion distributions were characterized by sulphur printing. It was found that a threshold stress intensity amplitude ( ΔK ThEAC), needed to be exceeded to trigger sustained environmentally assisted fatigue crack growth rates in PWR primary coolant environments. Below ΔK ThEAC crack growth was found to be transgranular with ductile striations and secondary cracking giving rates per cycle similar to those observed in air. Above ΔK ThEAC the cracking remained transgranular, but the fracture morphology changed to facets with some branching and crack growth rates became independent of ΔK. Factors found to influence ΔK ThEAc and subsequent plateau crack growth rates were fatigue cycle frequency, water chemistry and flow rate, temperature and the size and distribution of MnS inclusions in the steel. The complex interplay of these variables in relation to plant operation and the use of the ASME XI (1980) crack growth assessment procedures are discussed with reference to crack velocity parameters.