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The second in a series of eight columns throughout 2017 on the topic of the seven families of stainless steels; their characteristics, complementary properties and the wide variety of applications from the smallest of items destined for the human body to large scale constructions in the process industry. This article focuses on ferritic stainless steels which are so resistant to stress corrosion.

This column is the first in a series of eight columns on the topic of the seven families of stainless steels; their characteristics, complementary properties and the astonishingly wide variety of applications ranging from tiny stents in the human body to large scale constructions in the process industry. It is natural to start with the family of austenitic stainless steels because of its large impact on our society.

In my previous column, I promised to give an overview of the role of molybdenum and tungsten in duplex stainless steels (DSS). They are neighbors of chromium in the periodic table (Group 6B) and even more potent than chromium in preventing pitting, which is shown in the familiar PRE-relation:

PRE = %Cr + 3.3×[%Mo + 0.5%W] + 16%N

Designing a steel that is stainless was considered impossible in the beginning of the 1900’s. A renowned German chemist, G Mars, maintained the opinion that creating a stainless steel is impossible because iron is not a noble metal and its oxides are thermodynamically more stable than the pure metal. The year was 1911 but, by the irony of fate, the two first stainless steels were launched the following year.

Some nitrogen has been present in steel as long as steel-making has taken place. This has been unintentional and, in practice, there have been no means of removing or controlling the nitrogen concentration. It was not until the advent of modern process metallurgical techniques such as AOD, VOD and CLU during the 1970’s that nitrogen could be added to the steel melt in a controlled way and nitrogen, therefore, became an important alloying element.

In this series of articles, the role of the elements nickel, manganese, chromium, molybdenum and nitrogen in the design of duplex stainless steels will be described. An optimum balance between austenite and ferrite is accomplished by a clever use of the alloying elements.