CORROSION PROPERTIES OF MATERIALS-CONTINUED

摘要

Precipitation (or age) hardening (PH) stainless steels have the highest strength of all the stainless steels. Elements such as copper or aluminium are added which form strengthening precipitates when the material is heated to moderately elevated temperatures (a process known as ageing). They tend to be used for specialised applications where the combination of high strength and good corrosion resistance is required. They should not be exposed to elevated temperatures so not used in welded fabrications. They are used in the aerospace and defence industries where the high strength-to-weight ratio is required along with good corrosion resistance, They are generally proprietary alloys although Type 630 (also known as 17-4 PH) is described in AS 2837. The table below gives the chemical composition and some applications of PH stainless steels. Selection of the correct type of stainless steel to be used for a particular application requires a knowledge of the service conditions to which the steel will be subjected. It is important to accurately define the environmental and operating conditions which the product will be subjected. The total environment, including start-up, shutdown and cleaning must be considered. Many expensive mistakes have been made with stainless steel because materials used for cleaning testing, etc were not compatible. The next table compares some of the more important properties which govern selection of a stainless steel from the five basic groups.