Choose Materials for High - Temperature Environments

tably strength, so corrosion resistance (stability) may not be the primary design consideration. Assemblies need to be strong and resilient to the unique loads and stresses imparted on them, which can include significant temperature changes and thermal gradients for many high-temperature applications. In making a choice, it is necessary to know what materials are available and to what extent they are suited to the specific application. The decision is quite involved and the choice is significantly affected by the environment and the intended use, be it a reactor vessel, tubes, supports, shields, springs, or others. Some problems may occur because of distortion and cracking caused by thermal expansion/contraction; typically, a high-temperature alloy might change 4 in./ft from ambient to 1,000°C (1,832°F). The user or designer needs to properly understand that the environment dictates the materials selection process at all stages of the process or application. For example, an alloy that performs well at the service temperature may corrode because of aqueous (dew point) corrosion at lower temperatures during off-load periods, or through some lack of design detail or poor maintenance procedures that introduce local air draughts that cool the system (e.g., at access doors, inspection ports, etc.). To provide as optimum performance as possible, it is necessary for a supplier to be aware of the application, and for the user to be aware of the general range of available materials. Otherwise, severe problems can result. For example, a catastrophic failure occurred within weeks for an ignitor, made with Type 304 stainless steel (UNS S30400, iron, 19% Cr, 9% Ni, 0.08% C). Type 304 stainless steel would have been suitable for clean oxidizing condi-