A. Introduction

There are at least 70 standard grades of stainless steel and several times this number of proprietary stainless steel alloys.

This group of steels is classified as stainless steels because they are all iron-based alloys containing a least 10.5% chromium. The stainless steels that were developed around the turn of this century had only about 12% Cr, with ability to resist rusting in normal atmospheric conditions or mildly corrosive ones. However, in the intervening years many steels have been developed with much improved corrosion resistance as well as special characteristics, such that this family of steels embrace virtually all engineering applications.

Thus, stainless steels now range from essentially low-chromium iron alloys with good resistance to mildly corrosive conditions, through chromium- nickel steels with moderate corrosion and oxidation resistance plus good cryogenic toughness and creep resistance to the more highly alloyed varieties, which have very high resistance to erosion, corrosion, and oxidation.

Despite the larger number of stainless steel grades, they can be conveneintly classified into five main groups according to their metallurgical structure. Not all of these groups are relevant, to heat exchanger manufacture. However, for completeness each group is described in the following sections and only the types relevant to heat exchanger manufacturers are considered in more detail subsequently.