Comparative Procedure

The comparative procedure for materials selection involves selecting a cheap, tolerant and well-understood material and investigating to what extent its properties fall short of what is required for the component to operate satisfactorily.

A typical example, and one for which this procedure is extremely suitable is the specification of a material for chemical process plant. Suppose it is proposed to fabricate a tank using carbon steel which is cheap, readily procured, easily fabricated, ductile and, therefore, tolerant of flaws and geometrical irregularities and corrodes uniformly at a predictable rate. If carbon steel is found to be unsatisfactory then undesirable property or properties of the carbon steel can be modified. The necessary change may impair other properties but will do so in a predictable way. Thus:

 (a) Improved corrosion resistance may be obtained by the use of a stee1 with a higher chromium, and possibly a higher nickel content. This will increase cost and probably also delivery time, render design and fabrication more difficult and may enhance sensitivity to localised corrosion.

 (b) Improved strength may be obtained by the use of a steel with increased carbon and alloy content with drawbacks similar to those that apply to the case of the improved corrosion-resistance material.

 (c) A higher temperature of operation may require the use of a creep resisting steel, again with similar drawbacks to (a).

 (d) Operating at a lower temperature may require a steel with guaranteed low- temperature properties or may, at the limit, require an aluminium alloy.

Evidently this procedure, with the exception where a change necessitates the use of a completely different material, involves changes which are progressive, and whose effects can be foreseen. Therefore the chances of encountering some unexpected drawback are minimised and the requirement for component testing is also minimised.