In order to control the machinability and mechanical property of resulfurized free-cutting steel, the high temperature experiments at laboratory and thermodynamic analysis were carried out to ...investigate the formation and evolution of inclusions in the processes of refining and solidification. Furthermore, the migration behavior of sulfur was also assessed between the inclusions and the liquid steel. The results showed that the liquid oxysulfide inclusion (Mn,Si)x(O,S)y with trace amount of sulfur formed after FeS addition at 1600°C. As sulfur segregated in the residual liquid steel during solidification, sulfur rapidly migrated into (Mn,Si)x(O,S)y from the liquid steel at the initial stage of solidification. However, sulfur migrated out from (Mn,Si)x(O,S)y at the middle stage of solidification because MnS precipitated from the supersaturated liquid steel. After the steel solidified completely, sulfur continued to migrate out from the liquid oxysulfide inclusion (Mn,Si)x(O,S)y. When the temperature decreased to 1250°C, the liquid inclusion (Mn,Si)x(O,S)y transformed to two solid phases of (Mn,Si)xOy and MnS. Finally, the complex inclusion composed of (Mn,Si)xOy and MnS formed.
This work continued to discuss the forms of Bi particles accompanying sulphide, and analysed the effect of Bi on tensile and machining properties in 1215MS free-cutting steel. The machining and ...tensile tests were carried out based on the as-cast steels. The effect of Bi on the size and hardness of sulphide was investigated. Results show that at 200 ppm Bi or more, the normalised frequency of the large-sized sulphide inclusions with area of above 50 μm
2
was maximum, 0.35. In Bi-containing steels, the forms of sulphide inclusions were summarized that: clustering, rodlike, long strip, spindle, ellipsoidal, and spherical shape. Most Bi particles adhering to sulphide inclusions mainly existed in steels. The average hardness of steel matrix and the inclusions in 240 ppm Bi steel were 103 and 116 kgf mm
−1
, respectively. Bi added to steel increased the ductility and improved the surface finish of the machining steels.
This study investigates the in-situ evolution of the morphology of heated microstructures and sulfides, and to detect the machinability of Bi-bearing high S, low C free-cutting steels. The results ...show that Bi particles soften, melt, and vaporize during heating process. Its temperature varies slower than that of steel, and causes extensive local expansion of austenite grains. Heat absorbed by small sulfides leads to their volume expansion and formation of larger sulfides by fusion, whereas heat absorbed by large sulfides causes enlargement in dependent spherical or ellipsoidal shapes. During the heating process, at temperatures between 1500°C and 1550°C, the nominal coarsening speed of the tail wave is higher than that of the triple junction, but their movement speeds are similar. The addition of Bi particles to steel enhances its machinability. The cutting heat melts Bi particles to form liquid Bi films, resulting in rapid chip breaking during cutting.
In order to research the effect pattern of MnS inclusions on free-cutting steel, we study the microstructure evolution, the damage mechanism and the mechanical properties in free-cutting steel in the ...presence of MnS inclusions. Spindle shaped MnS is added as inclusions within the free-cutting steel. The mechanical properties were found to change when inclusions were present. The gained results show that the formation of voids causing fracture starts from the interface inside the matrix close to the MnS. From the point of view of nanocomposite strength, the main effect of MnS inclusions is related to stress concentration, leading to the effect of increased stresses near the interface between the interior of the matrix and the inclusions. The inclusions have lower Young's modulus and lower dislocation activity, resulting in smaller deformation of the alloy system, larger interfacial stress concentrations and earlier hole formation. The maximum strain and stress regions of the alloy also appear near the MnS inclusions, which leads to the formation of defects near the MnS inclusions and then fracture of the alloy. MnS inclusions adversely affect the tensile properties of the alloy, such as Young's modulus, yield stress and yield strain. By comparing the stress-strain curves of single crystal iron and alloy containing MnS inclusions, it is indicated that the yield strength of the latter decreases. Slip bands and dislocation lines are first generated around the MnS inclusion, and the phase transition is induced from the original single BCC structure to FCC, HCP and amorphous structures, and the atoms of FCC, HCP and amorphous structures increase with increasing strain, while those of BCC structure decrease, especially after yield strain. This study is significant for understanding the effect of inclusions on the mechanical laws and fracture mechanisms of the alloy.
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•We study investigates the mechanical properties in steel in the presence of MnS inclusions.•When MnS inclusions are present, the location of the cavities is associated with the location of the inclusions.•The presence of MnS inclusions, has a significant effect on the mechanical properties of the alloy.
MnS precipitates in resulfurized free-cutting steels are elongated in the rolling direction and then have a large size in length. It is difficult to accurately obtain the characteristics especially ...full length of MnS by two-dimensional observation. Three-dimensional morphology of MnS in a Ca-treated resulfurized free-cutting steel was investigated by using tensile method and scanning electron microscopy. Compared with the statistical results of MnS in two-dimension, the size in three-dimension was larger. In two-dimension image, the length of MnS was in the range of ~100 μm, and the percent of MnS with size in the rage of 50~100 μm was about 1.15%. In three-dimension image, however, the length of MnS observed was in the range of ~325 μm, and the percent of MnS with size larger than 100 μm was about 5.73%. The results provide an insight into the role of fracture surface analysis in MnS characterization in resulfurized free-cutting steels.
The free-cutting steel is one of the essential structural steels in electrical appliance industry and automotive industry. Most of the steel contains Pb, as solid lubricant, chip breaker, tool edge ...stabilizer and tool life extender. MnS grains also play the same role of Pb. However, Pb is toxic so that it will be prohibited to add to the steel in near future. Steel industry has tried to replace Pb with other element(s) and/or compound(s). The trials are still going on. This study proposes a candidate of the workability parameters for free-cutting steels with/without Pb. Investigated steels were loaded quasi-statically (strain rate: 1×10-3 s-1) and dynamically (1×103 s-1) up to fracture. Half of the specimens were pre-fatigued to form easier slip situation in steel matrix. After fracture, thin steel matrix penetrated into plenty of MnS grains. For the steel with Pb, the ratios of the number of such penetrations to that of total grains maintained almost the same value for all loading conditions. The ratios were varied with the loading conditions for the steel without Pb. These facts meant that Pb was a key element for easier slip of steel matrix. Therefore, this ratio had a feasibility to be one of the workability parameters for the free-cutting steel.
Manganese sulfide (MnS) inclusion in steel is related to the machinability of steel. Controlling the type and shape of MnS inclusion has been a problem for the steelmaking process for a long time. ...The effect of tellurium (Te) treatment on the control of MnS inclusion is studied with experiments and discussion of sequential solidification. The result shows that Te addition to the high- and low-sulfur steel dominantly changes the size and shape of MnS inclusions in steel. The aspect ratio of inclusions in cast steel changes with Te/S ranges from 0 to 1.9, which offers an effective way to control the MnS type and thus to improve the machinability and mechanical properties of steel. With an excessive Te addition, no further improvement could be observed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS).
In this paper, the influence of bismuth content and heating rate on the morphology of MnS inclusions in bismuth-containing free-cutting steel during heating was investigated through in situ ...observation experiments and 3D electrolytic corrosion experiments. By observing the microscopic morphology of inclusions in the original sample, it was found that MnS inclusions in the sample were rod-shaped, spherical, irregular, small in size, and mostly clustered at the grain boundary in the form of chains and divergences. With the addition of bismuth, MnS inclusions of a larger size appear in the steel, and the inclusions distributed at grain boundaries are also reduced. When bismuth (0.010~0.020%) is added to the steel, MnS is mainly spherical and uniformly dispersed in the steel matrix. If the bismuth content is too high, the inclusions aggregate. Through in situ observations of the inclusions in the sample, it was found that the addition of bismuth in the heating process delays the appearance of ferrite grain boundaries and contributes to the spheroidization of MnS inclusions. Mn and S elements can fully diffuse slowly in the matrix with a heating rate below 1 °C/s and a long holding time (300 s), which provides the possibility for the spheroidization of MnS inclusions.
The development of free-cutting steel is inseparable from the development of environmentally friendly alloy elements and the control of inclusions shape. Alloying elements can affect the composition, ...morphology, size, and distribution of inclusion, which are the main factors affecting the machinability of free-cutting steel. This study selected sulfur free-cutting steel with different chemical compositions as the research object to examine the effects of bismuth and bismuth tellurium on sulfur-containing free-cutting steel through electrolytic corrosion experiments, metallographic microscopy, scanning electron microscopy, energy spectrum, and electron backscattering analyzer. The results showed that the microstructures of free-cutting-steel containing sulfur, free-cutting steel containing sulfur bismuth, and free-cutting steel containing sulfur bismuth tellurium are composed of ferrite, pearlite, and inclusions. The inclusions in sulfur-containing free-cutting steel are chain, cluster, and a few dotted MnS. The inclusions in sulfur-bismuth free-cutting steel are point and a few dotted MnS. After the addition of Te, the number of dotted inclusions is reduced, while the number of chain and cluster inclusions is increased. Most of the inclusions in bismuth-containing free-cutting steel are flake inclusions, and the class II MnS change into class III MnS, which is beneficial for improving the free-cutting property of steel and to reduce anisotropy. With the addition of Te, MnS of other shapes, such as heart, water drop, butterfly, etc. of a length–width ratio of less than 4 also appeared as MnS and MnTe complex inclusions, and the fusiform manganese sulfide accounted for most of the steel. Both Bi and Te had modification effects on MnS.
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