Harm of long-term use of steel at elevated temperatures

When the mechanical properties of a metal change, its microstructure alters, and the creep rate increases.

For example, when the pipe wall is exposed to high-temperature flue gas, if it does not receive adequate cooling, its operating temperature may exceed the design value or it may get damaged after exceeding the allowable operating time; this condition is known as overheating. The damage caused by short-term overheating results from the reduced strength of the pipes due to high temperatures; this can lead to blockages inside the pipes, water shortages, disruptions in water circulation, or film boiling. In most cases of damage resulting from short-term overheating, obvious elongation and contraction deformations are observed, with the breakage edges taking on a blade-like shape ； Thinning of the tube wall is not noticeable unless the overheating temperature exceeds the phase transition temperature AC3, causing the ferrite in the steel to transform into austenite. High-temperature creep, also known as medium- to long-term overheating, occurs when steel is exposed to temperatures above its creep temperature for an extended period, resulting in changes in its microstructure ； These include: spheroidization of pearlite, graphitization in carbon steel and molybdenum steel, aggregation of carbides, and precipitation of the σ-phase in austenitic steel, all of which reduce the intergranular strength of the metal and lead to its degradation. This type of damage does not result in a significant thinning of the pipe wall, and the thick-lipped cracks are characteristic of high-temperature creep.

Under high-temperature conditions, the diffusion activity of metal atoms increases, causing the microstructure of the steel to keep changing. This, in turn, causes changes in the properties of the steel. The higher the temperature, the greater the ability of atoms to diffuse. The longer steel is used at high temperatures, the more atoms diffuse, and thus the greater the changes in the steel’s structural structure. In steel that operates under high-temperature conditions for extended periods, the main harmful structural changes that occur include spheroidization of pearlite, graphitization, and depletion of alloying elements in the solid solution. As high temperatures increase the diffusion ability of alloying element atoms, it leads to a transfer process of these alloying elements between the solid solution and carbide phases. Those alloying elements that strengthen the solid solution, such as chromium, molybdenum, manganese, etc., will continuously dissolve out, while the amount of alloying elements in the carbide phases gradually increases; in other words, the alloying elements shift from the solid solution to the carbides, resulting in a depletion of these elements in the solid solution. As a result of the transfer of alloying elements, the high-temperature strength of the material (creep limit and endurance strength) decreases.

At room temperature, the microstructure of steel is generally quite stable. However, under high-temperature conditions, the diffusion activity of metal atoms increases, causing the microstructure of the steel to keep changing. This, in turn, causes changes in the properties of the steel. The higher the temperature, the greater the ability of atoms to diffuse. The longer steel is used at high temperatures, the more atoms diffuse, and thus the greater the changes in the steel’s structural structure. In steel that operates under high-temperature conditions for extended periods, the main harmful structural changes that occur include spheroidization of pearlite, graphitization, and depletion of alloying elements in the solid solution.