Manganese sulfide inclusions

Figure 5. Elongated manganese sulfide inclusion in aluminum killed steel after hot...

What happened The German and Japanese steel metallurgists had understood much earlier in the game, during the sixties, the paramount importance of eliminating the manganese sulfide inclusions. Their equipment and highly quality-oriented minds allowed them to tackle the difficult orders ahead of everybody else. 

Figure 2.25 SEM view of the fracture surface of a low-carbon steel specimen broken in tension, showing ductile dimples, local quasicleavage, and manganese sulfide inclusions. (Reprinted with permission from ASM International. All rights reserved www.asminternational.org)...

It is important to control the surface area of manganese sulfide inclusions. During the rolling process for pipeline manufacture, for example, these inclusions get flattened to form platelets. This ino-eases the available surface area for hydrogen adsorption and therefore increases material susceptibility to HIC. One way to control this is the addition of calcium to ladle steel. Calcium reduces the oxygen and sulfur content of the steel. Calcium also results in the spheroidization of sulfide inclusions, thereby decreasing the available surface area for adsorption of atomic hydrogen. 

Park, J. O., Suter, T., and Bohni, H., Role of Manganese Sulfide Inclusions on Pit Irritiation of Super Austenitic Stairrless Steels, Corrosion, Vol. 59, 2003, p. 59. 

The shape of manganese sulfide inclusions can be partially altered by cross rolling. This method is being used on plate mills for producing high quality steel. Even when this technique is used, the sulfide shape control is incomplete because the inclusions still retain a biaxial ellipsoidal shape. Furthermore, cross rolling on modern continuous hot strip mills is currently impractical. 

Among the interesting results of such endeavors for 304 stainless steel is a stress upon inclusions involving manganese sulfide and manganese oxide. Surprisingly, traces ofCu in the steel are detected and form flowerlike deposits prior to the initiation of a pit. Metallic inclusions tend to dissolve anodically in 0.1 MNaCl at 400 mV SCE. There is some evidence for their readsorption. However, to be the cause ofpit initiation, the inclusions have to be at least 0.7 jum in size. It may be that the local current density for dissolution at smaller inclusions is too high to sustain CF adsorption there. 

Inclusions Inclusions are three-dimensional defects consisting of soluble particles of foreign material in the metal. Voids, a three-dimensional defect, are empty or gas-filled spaces within the metal. Metal oxides, sulfides, and silicates are common inclusions. For example, manganese sulfide in stainless steel provides a favorable site for pitting corrosion. 

Iron and manganese occur in a number of soil minerals. Sodium and chlorine (as chloride) occur naturally in soil and are transported as atmospheric particulate matter from marine sprays (see Chapter 10). Some of the other micronutrients and trace elements are found in primary (unweathered) minerals that occur in soil. Boron is substituted isomorphically for Si in some micas and is present in tourmaline, a mineral with the formula NaMg3AlgB3Sig027(0H,F)4. Copper is isomorphically substituted for other elements in feldspars, amphiboles, olivines, p5Toxenes, and micas it also occurs as trace levels of copper sulfides in silicate minerals. Molybdenum occurs as molybdenite (M0S2). Vanadium is isomorphically substituted for Fe or A1 in oxides, pyroxenes, amphiboles, and micas. Zinc is present as the result of isomorphic substitution for Mg, Fe, and Mn in oxides, amphiboles, olivines, and pyroxenes and as trace zinc sulfide in silicates. Other trace elements that occur as specific minerals, sulfide inclusions, or by isomorphic substitution for other elements in minerals are chromium, cobalt, arsenic, selenium, nickel, lead, and cadmium. 

The shape of sulfide inclusions can be altered from elongated to globular by chemical means. This is accomplished by adding an element which forms a high melting point sulfide, more stable than manganese sulfide and not as readily deformable at hot rolling temperatures. 

Microprobe analysis of the steels with rare earths indicate that the manganese sulfides which would otherwise fuse on heating to form film-type or work-elongated inclusions are absent and that the inclusions contained in the steel may be either rare earth oxides or rare earth sulfides, both of which are globular in nature. 

Some Instances of the Effect of the Inclusions 9.5.1 Titanium and Manganese Sulfides... 

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