When plastically deformed, traditional austenitic stainless steels with 18% chromium and 8% nickel undergo a martensitic phase transformation. This is due to the fact that the austenite phase is not thermodynamically stable at room temperature. As seen in the phase diagram in Fig 1 the phases expected at room temperature (in true equilibrium) are austenite (γ), ferrite (α) and carbides.

Owing to the unique combination of properties of stainless alloys they are indispensable in modern medicine. The required materials properties include good corrosions resistance in body fluids, biocompatibility, good strength to-weight ratio, fatigue resistance and a low magnetic susceptibility. The term biocompatibility is not well-defined but can be described in simple terms as the ability to perform adequately in a specific situation in the human body.

An Achilles heel of austenitic stainless steels is the susceptibility to stress corrosion cracking (SCC). However, when the nickel concentration exceeds about 20% considerable improvement in the resistance to stress corrosion is observed (Fig 1). Nickel-rich austenitic stainless steels (NiASS), therefore, deserve to be treated as an own family. In fact, for nickel concentrations beyond about 30% the resistance to stress corrosion is comparable to that of duplex and ferritic stainless steels.

An Achilles heel of austenitic stainless steels is the susceptibility to stress corrosion cracking (SCC). However, when the nickel concentration exceeds about 20% considerable improvement in the resistance to stress corrosion is observed (Fig 1). Nickel-rich austenitic stainless steels (NiASS), therefore, deserve to be treated as an own family. In fact, for nickel concentrations beyond about 30% the resistance to stress corrosion is comparable to that of duplex and ferritic stainless steels.

Martensite, the hardest structural component in steels, is responsible for the high hardness in many sharp edge tools. The formation of martensite is a remarkable phenomenon in materials science and smiths have known for at least 3000 years how to make use of it to produce implements such as knives, axes and ploughs. Therefore, martensite has played a crucial role in the development of our civilization.

Number three 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 month features duplex stainless steels which combine the properties of austenite with the properties of ferrite.

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.