4 Oct, 2013

The Stainless Steel Family - An Overview

Stainless steel is a family of corrosion resistant steels containing chromium in which the chromium forms a passive film of chromium oxide (Cr2O3) when exposed to oxygen [1]. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium.

The film layer is impervious to water and air and quickly reforms when the surface is scratched. This protects the metal beneath – preventing further surface corrosion.  Since the layer only forms in the presence of oxygen, corrosion-resistance can be adversely affected if the component is used in a non-oxygenated environment e.g. underwater bolts on a platform support structure.

Such passivation only occurs if the proportion of chromium is high enough and is normally achieved with addition of at least 13% (by weight) chromium. Progressively higher levels of corrosion resistance and strength is achieved  by the addition of other alloying elements – each offering specific attributes in respect of strength and corrosion resistance.

Classification issues

The need to classify stainless steel has led to a fundamental problem of which method to use. Probably the best known system derives from of the Society of Automobile Engineers (SAE) e.g. 316 Cr/Ni/Mo 17/12/2. This is interpreted as stainless steel containing the proportions of 17% chromium, 12% nickel, and 2% molybdenum.

However, the waters are somewhat muddied by a variety of international and country-based systems that include EN (European Norm); and UNS (Unified Numbering System). For example, SAE 304 Cr/Ni 18/10 stainless steel is EN 1.4301 which is UNS S30400.

Stainless steels may also be graded into five basic families or phases determined by their crystalline structure: the stable phases austenitic or ferritic; a duplex mix of the two; the martensitic phase created when some steels are quenched from a high temperature; and precipitation-hardenable.

Ferritic stainless steel

In ferritic stainless steel, the iron and chromium atoms are arranged in what is termed a body-centred cubic structure in which the atoms are arranged on the corners of the cube and one in the centre (Figure 1). As well as being ferro-magnetic, ferritic stainless steel exhibits very high stress corrosion cracking resistance.

Ferritic stainless steels are plain chromium (10.5 to 18%) grades characterized by moderate corrosion resistance and poor fabrication properties. These characteristics may be improved with the addition of molybdenum; some, aluminium or titanium.

Austenitic stainless steel

With the addition of nickel, the properties change dramatically. As shown (Figure 3) the atoms are re-arranged so that they occur on the corners of the cube and also in the centre of each of the faces. In this manner it becomes what is termed austenitic stainless steel.

It can thus be seen from Table 1 that unless you are specifically looking for a ferro-magnetic material, austenitic stainless steel would be the most obvious choice. Indeed this is borne out by the fact that austenitic stainless steels account for about 70% or more of all stainless steel used worldwide – with ferritic stainless steels making up about 25%. The other families each represent less than 1% of the total market.

Austenitic stainless steels are designated by numbers in the 200 and 300 series.

Series 300

The relationship between the 300 austenitic grades is shown in Figures 4.

The basic grade 304 contains about 18% chromium and 8% nickel (often referred to as 18/8) and range through to the high alloy or ‘super austenitics’ such as 904L and 6% molybdenum grades.

Additional elements can be added such as molybdenum, titanium or copper, to modify or improve their properties, making them suitable for many critical applications involving high temperature as well as corrosion resistance. This group of steels is also suitable for cryogenic applications because the effect of the nickel content in making the steel austenitic avoids the problems of brittleness at low temperatures, which is a characteristic of other types of steel.

Generally, the 300 grade alloys are subject to crevice and pitting corrosion.

Low-carbon versions, (indicated by the letter suffix L) include 304L, 316L and 317L, in which the carbon content of the alloy is below 0.03%. This reduces the effect of ‘sensitisation’ in which chromium carbides precipitate at the grain boundaries due to the high temperatures involved in welding.  The relatively high nickel content also inhibits the brittleness exhibited by ferritic materials at low temperatures and thus makes austenitic steels suitable for cryogenic applications.

200 series

We have seen earlier how the addition of nickel is used in the creation of the classic chrome-nickel 300 series austenitic stainless steel.

The reduced nickel content of the 200 series chrome-manganese grades makes them significantly cheaper. However, depending on their chemistry, they also offer good formability (ductility) and/or strength.  Indeed, certain grades (201, 202 and 205 series) even offer about 30% higher yield strength than the classic 304-series chrome-nickel grade – allowing designers to cut weight (Table C.2).

Reducing nickel, on the other hand, reduces the maximum chromium content possible in the alloy. Less chromium means less corrosion resistance and a consequent narrowing of the range of applications for which the material is suitable.

A word of warning comes from the International Stainless Steel Forum (ISSF). Continuous pressure to cut costs, especially from the Asian market, has resulted in the development of austenitic grades ever lower in nickel and chromium, often not covered by international codes or specifications. In fact, numerous chrome-manganese grades are company-specific and identified simply by a title given to them by the producer.

Duplex stainless steels

Duplex stainless steels [6] are a mixture of austenite and ferrite microstructures that combine some of the features of each class:

  • resistance to stress corrosion cracking  – but inferior to ferritic steel;
  • superior toughness to ferritic steel – but inferior to austenitic steel;
  • roughly twice the strength of austenitic steel;
  • superior resistance to pitting, crevice corrosion and stress corrosion cracking;
  • high resistance to chloride ions attack; and
  • high weldability.

These features are achieved by adding less nickel than would be necessary for making a fully austenitic stainless steel. Thus, Grade 2304 comprises 23% chromium and 4% nickel whilst Grade 2205 comprises 22% chromium and 5% nickel – with both grades containing further minor alloying additions.

On the negative side, austenitic-ferritic duplex stainless steels are only usable between temperature limits of about -50°C and 300°C – outside which they suffer reduced toughness.

Martensitic stainless steel

Named after the German metallurgist, Adolf Martens, the martensitic Grade 400 series (Figure 5) are low carbon (0.1–1%), low nickel (less than 2%) steels containing chromium (12 to 14%) and molybdenum (0.2–1%).

Stainless steels hardened by transformation to martensite are tempered to give the desired engineering properties. At high temperatures they have an austenitic structure that is transformed into martensitic structure upon cooling to room temperature. Unfortunately, this tempering can influence corrosion susceptibility. For example, corrosion susceptibility of type 420 stainless steel is at its maximum when the alloy is tempered at temperatures in the range of 450° to 600°C. So, aalthough not as corrosion-resistant as the 200 and 300 classes, martensitic stainless steels are magnetic, extremely strong (if not a little brittle), highly machinable, and can be hardened by heat treatment.

Martensitic stainless steels are subject to both uniform and non-uniform attack in seawater. And the incubation time for non-uniform attack in even weak chlorides is often only a few hours or days.

Precipitation-hardening martensitic stainless steels

These chromium-  and nickel-containing steels can be precipitation hardened to develop very high tensile strengths. Precipitation-hardening stainless steels are usually designated by a trade name rather than by their AISI 600 series designations.

The most common grade in this group is ‘17-4 PH’, also known as Grade 630, with a composition of 17% chromium, 4% nickel, 4% copper and 0.3% niobium. The main advantage of these steels is that they can be supplied in the ‘solution treated’ condition – in which state the steel is just machinable. Following machining, forming, etc. the steel can be hardened by a single, fairly low temperature ‘ageing’ heat treatment that causes no distortion of the component.

Precipitation hardening generally results in a slight increase in corrosion susceptibility and an increased susceptibility to hydrogen embrittlement.

By: Mick Crabtree

References

1. T. Sourmail and H. K. D. H. Bhadeshia, ‘Stainless Steels’, University of Cambridge.

2. Nabil Al-Khirdaji, ‘Stainless Steel Family’, Kappa Associates International.

3. ‘Stainless Steel and Corrosion’, ArcelorMittal, Stainless Europe.

4. A.U. Malik, M. Kutty, Nadeem Ahmad Siddiqi, Ismaeel N. Andijani, and Shahreer Ahmad, ‘Corrosion Studies on SS 316 L in low pH high Chloride product water medium’, 1990.

5. ‘The Stainless Steel Family’, International Stainless Steel Forum.

6. API Technical Report 938-C: ‘Use of Duplex Stainless Steels in the Oil Refining Industry’.