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Mechanical Properties of Engineering Materials

The selection criteria used by materials engineers in choosing from a group of materials includes a list of qualities that are either desirable or necessary. Unfortunately, the optimum properties associated with each selection criteria can seldom be all found in a single material, especially when the operating conditions become aggressive. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.

A wide variety of iron and nickel-based materials are used for pressure vessels, piping, fittings, valves and other equipment in process industries. The most common of these is plain carbon steel. Although it is often used in applications up to 482-516oC, most of its use is limited to 316-343oC due to loss of strength and susceptibility to oxidation and other forms of corrosion at higher temperature.

Ferritic alloys, with additions consisting primarily of chromium (0.5-9%) and molybdenum (0.5-1%), are most commonly used at temperatures up to 650oC. Their comparative cost, higher strength, oxidation and sulfidation resistance and their particular resistance to hydrogen, for example, result in their being the material of choice. However, these low alloy steels have inadequate corrosion resistance to many other elevated temperature environments for which more highly alloyed Ni-Cr-Fe alloys are required.

For applications for which carbon or low alloy steels are not suitable, the most common choice of material is from within the 18 Cr-8Ni austenitic group of stainless steels. These alloys and the 18Cr-12Ni steels are favored for their corrosion resistance in many environments and their oxidation resistance at temperatures up to 816oC. Above 650oC their falling strength becomes a consideration and more heat resistant alloys must often be used.

Most chemical process equipment is designed and fabricated to the requirements of specific pressure vessel and piping codes. These codes include only approved materials and establish the basis for and the setting of allowable stresses. Thus, the mechanical properties of a material are usually the first criteria that materials engineers apply in the selection process. This is especially important for applications at temperatures in the creep range where a minor difference in operating temperature can significantly affect the load carrying ability of the material.