While stainless steel is perceived as not rusting, it can exhibit destructive corrosion when exposed to harsh usage environments. Since various corrosion phenomena such as pitting, intergranular corrosion, stress corrosion cracking, and galvanic corrosion can occur depending on the steel grade or environment, the selection of steel grade is a very important factor.

This chapter deals with heat resistant steel, so I will describe it in terms of temperature.

The 304 grade (18Cr-8Ni), which is representative of 300 series austenitic steel, shows near-universal performance in general usage environments, so it can be encountered everywhere in daily life. However, in specific temperature ranges, it reacts particularly sensitively and corrosion progresses, which is called intergranular corrosion (IGC) that occurs in the sensitization temperature range.

The sensitization temperature varies by data, but generally refers to the 425℃~815℃ range. When this temperature range is passed through for a long time, chromium carbides (Cr₂₃C₆) precipitate at grain boundaries, which are the microstructure of the metal, causing chromium depletion. In other words, stainless steel is an alloy iron that has 10.5% or more chromium added to form a passive film (Cr₂O₃) on the surface to improve corrosion resistance. If chromium is depleted, it loses its corrosion resistance and corrosion occurs along grain boundaries.

Heat resistant steel is used to improve this. There is a method of alloying a lot of nickel, which boasts excellent corrosion resistance, but since nickel is an expensive raw material, there is also a method of preventing chromium depletion by adding and subtracting metal elements according to the usage temperature range.

Simply put, it is important to manage so that chromium carbides (Cr₂₃C₆) do not occur. If the sensitization temperature range is passed through quickly, low carbon content steel grades are used from the beginning, or rapid cooling after welding, chromium carbide generation would be suppressed, but if it is used in a continuous temperature range, there is no choice but to use the method of adding alloying elements.

Chromium carbide formation occurs because chromium is too friendly with carbon from the beginning. Therefore, alloying elements that are more friendly with carbon can be added, and at this time, titanium (Ti) or niobium (Nb) is added. As shown in the table, when titanium (Ti) is added, it becomes 321 grade, and when niobium (Nb) is added, it becomes 347 grade. 347 grade can be a better choice than 321 grade in nitric acid environments, and other properties are almost similar.

Selecting appropriate steel grades according to the usage temperature range is an important factor in terms of maintenance and cost, but special steel grades are not products that follow low prices. Since the usage environment is harsh, it should be treated as a consumable, so it is also important to select the longest possible usage period. After all, maintenance and replacement time is also transferred to cost.

If the normal usage temperature is 1100℃ or higher, 310S grade is recommended. It is used in high-temperature harsh and repetitive environments such as iron/steel making facilities and heat treatment facilities, which is why that expensive nickel (Ni) accounts for 20% by weight. Since the price is 3-4 times more expensive than 304 grade, there is a tendency to prefer products with long usage periods and proven products.