ABSTRACT
Thermal fatigue is only one element in the complex of problems that may face a design engineer, but it is of increasing importance. Constantly growing demands for new or better products frequently lead to intensified thermal regimes in industrial processes, while the trend to their integration and operation on a continuous basis vastly increases the cost of interrupting production by breakdown or the need to replace components. This also applies in the case of the very large individual units that are now constructed in the interest of technological efficiency-blast furnaces with a volume of 5000 m3, convertors handling 400 to 600 tons of metal at one time, ladles and mixer-type ladles with a capacity of, say, 500 tons, and giant presses for hot forging. The highly efficient continuous casting process combines the features of great size and integrated processing. In such equipment the scale factor itself, as well as the required operating conditions, generally contribute to the thermal fatigue problem; the temperature differences across thick sections create intense stresses. Any item that operates at a high temperature may be subjected to significant temperature variation in use and major temperature variation at startup and shut down. Shutdown, particularly emergency shutdown, usually produces the most rapid temperature changes and the most dangerous stresses. Useful machines and structures are never built simply to get hot and cold. The elements exposed to thermal fatigue must support loads and transmit forces that may be constant or variable and in many instances are also exposed to corrosion and erosion. Hence machinery and structures subjected to elevated and varying temperatures are exposed to many mechanisms of failure, among which pure thermal fatigue may have a dominant or subsidiary, but still significant role. Methods of estimating the durability of components when mechanical fatigue, creep, and thermal fatigue occur simultaneously were discussed in Chapter 6.
