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Why Not Use Stainless Steel Pipes and Tubes for Solution Treatment and Annealing of Water Heaters?

Stainless steel is softened through solution treatment. Typically, the stainless steel pipe tubing used in water heaters is heated to around 950℃ to 1150℃ and held for a period of time to ensure that carbides and various alloy elements are completely and uniformly dissolved in the austenite. This is followed by water quenching to obtain a pure austenite structure without giving carbon and other alloy elements time to precipitate. This process is called solution treatment. There are three reasons for using solution treatment on stainless steel pipes for water heaters.

To make the structure and composition of the stainless steel pipe tubing used in water heaters uniform.

This is particularly important for raw materials, as different rolling temperatures and cooling rates of different parts of the hot-rolled wire rod can result in inconsistent structure. At high temperatures, atomic activity increases and solution occurs, resulting in chemical components becoming uniform, and a homogeneous single-phase structure is obtained after rapid cooling.

To eliminate work hardening of the stainless steel pipe tubing and facilitate further cold working.

Through solution treatment, deformed lattices are restored, elongated and broken crystals are re-crystallized, internal stress is eliminated, the tensile strength of the stainless steel pipe is reduced, and the elongation rate is increased.

To restore the inherent corrosion resistance of the stainless steel pipe tubing used in water heaters.

Carbide precipitation and lattice defects caused by cold working reduce the corrosion resistance of stainless steel. After solution treatment, the corrosion resistance of the steel pipe is restored to its best state. For stainless steel pipes used in water heaters, the three elements of solution treatment are temperature, holding time and cooling rate. The solution temperature is mainly determined by the chemical composition.

Generally, for brands with many types and high content of alloy elements, the solution temperature should be correspondingly increased. In particular, for steels with high manganese, molybdenum, nickel and silicon content, only by raising the solution temperature can they be completely dissolved to achieve the softening effect. However, when the solution temperature is high, carbides of stabilized elements that stabilize carbon in austenite are dissolved completely and precipitate at grain boundaries in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. To prevent the decomposition of carbides of stabilized elements without dissolution, the lower limit of the solution temperature is usually used.

As the saying goes, stainless steel does not rust easily. In fact, some stainless steel tubes and pipes used in water heaters have both rust resistance and acid resistance (corrosion resistance). The corrosion resistance of stainless steel is due to the formation of a chromium-rich oxide film (passivation film) on its surface. Both stainless and corrosion resistance are relative. Experimental results show that the corrosion resistance of steel in the atmosphere, water, and other weak media as well as in nitric acid and other oxidizing media increases with an increasing chromium content in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel changes suddenly, namely from easy to rust to difficult to rust, and from non-corrosive to corrosion-resistant.

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