Introduction to grain size and inclusions in metal materials for pressure vessels

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I. Grain size: It is well known that the grain size of steel has a significant impact on its mechanical properties. At room temperature, the smaller the grain size of steel, the higher its strength and toughness. Conversely, coarse grains tend to reduce the strength of steel, especially its toughness. Grain size includes initial grain size and actual grain size, etc. 1. Initial grain size: When the steel is heated to the Ae3 critical point, the austenite grains at the moment just after the transformation of pearlite into austenite are very small; these are commonly referred to as the initial grain size. 2. Actual grain size: The actual size of the grains in steel under normal conditions, as well as the actual grain size obtained in the steel or components after various heat treatments (heating). The grain size of commonly used steel grades is divided into 16 levels (1–16). The smaller the grain size grade, the coarser the grains; the larger the grain size grade, the finer the grains. There are many factors that affect the actual grain size of steel. The heating temperature and holding time play a decisive role; higher temperatures and longer holding times result in larger grains. In addition, alloying elements, the initial microstructural state, hot working, heat treatment, etc., also have an impact on the actual grain size of steel. The grain size and its determination methods shall comply with GB/T 6394-2017 \"Methods for determining the average grain size of metals\". II. Inclusions: During the high-temperature smelting and casting process of steel, it is inevitable that some non-metallic inclusions, namely non-metallic compounds, will appear. Mainly some oxides, sulfides, and silicates, etc. They are present in steel, and although their concentration is very low, their impact on the properties of the steel cannot be ignored. The degree of this impact depends on factors such as the type, size, quantity, and distribution pattern of the inclusions. Therefore, it is very important to detect non-metallic inclusions in steel. Usually, testing and grading are carried out in accordance with GB/T10561—2005 \"Determination of the content of non-metallic inclusions in steel\" in order to determine whether the steel meets the requirements. Common internal inclusions in steel include brittle inclusions (oxides and brittle silicates) and plastic inclusions (sulfides and deformable silicates), among others. 1. Oxides: The common oxide in steel is Al2O3, which is a fine, refractory, highly hard, and brittle inclusion formed during the deoxidation of aluminum. Such inclusions do not deform after hot processing, but instead distribute along the processing direction in the form of short linear shapes and granular bands. It has a significant impact on the fatigue properties of steel. 2. Sulfides: The common sulfides in steel are FeS, MnS, and their solid solutions (MnS-FeS). Due to its presence, it causes the steel to become thermally brittle during the heating process. 3. Silicates: Common silicates in steel include ferrosilicate (2FeO-SiO2), manganosilicate (2MnO-SiO2), and ferromanganesosilicate (mFeO-nMnO-pSiO2), among others. It comes in two types: brittle silicates and malleable silicates.