Stainless steels

T. Sourmail

Introduction

Stainless steels represent the most diverse and complex family of all steels. The list of their applications is endless: from the harsh environments of the chemical, oil production and power generation industries to street furniture or automotive trims without forgetting most cutlery, they are used either for decorative purposes and/or for their excellent resistance to corrosion.

Stainless steels are stainless because a protective layer spontaneously forms on their surfaces and reduces the rate of corrosion to almost negligible levels. Under normal conditions, this layer heals very rapidly if scratched, so that if stainless steels only suffered from uniform corrosion, they could survive for literally millions of years (Nature, 2002:415, Newman, p743).
It is generally agreed that stainlessness is obtained for additions of about 12 wt% of chromium or more, although corrosion rates continuously reduce with increasing chromium contents from 0 to this limit.

12% Cr-steels are resitant to atmospheric corrosion but are useless in acids such as HCl or H2SO4 where they exhibit a corrosion rate even greater than plain carbon steels. As will be discussed later, corrosion resistance can be greatly enhanced above that of a basic 12% Cr steel by further addition of Cr and/or use of other alloying elements such as Ni, Mo, N etc.

Corrosion resistance is of course not the only design criterion: materials cost considerations will typically favour basic, cheaper grades (12%Cr-0.1%C) rather than heavily alloyed steels. Mechanical properties must also be taken into account, as must fabrication difficulties (machinability, deformability, weldability, etc.). The number of grades is therefore seemingly infinite, with a large number of standard compositions to which manufacturers add their proprietary variants.

These are usually divided in four or five classes on the basis of their microstructures:

to which some add precipitation hardened stainless steels, although they are themselves divided into martensitic/ferritic/etc. so that they could be included in the above categorisation.

Corrosion resistance

Iron is not stable in most of the environments in which it is used. Like most non-noble metals, it does not naturally occur in its elemental form and has to be extracted from oxides. In service, the tendency is for re-formation of these oxides. Hence the rust that can be observed on most unprotected steel components.

Materials scientists and chemists often take recourse to thermodynamics to quantify and compare the stability of different phases, oxides, etc. In this case however, thermodynamics alone gives an erroneous representation of the problem: most stainless steels are used in conditions in which the dissolution of the metal is energetically favorable.
Instead, they owe their environmental resistance to the formation of a chromium oxide film which acts as a kinetic barrier: atomic transport through this layer is so slow, that the steel can be considered inert.

The passive film formed at the surface of stainless steels is extremelly thin (1-2 nm). Its chromium content depends on the bulk content, and in general, increases with the latter. The overall corrosion resistance is also enhanced as the chromium content is raised.

Corrosion can nevertheless occur if the passive film breaks down, locally or uniformly. This can happen by different mechanisms depending on the conditions of use. The most common types of corrosion are:

Phase equilibria

Main phases

As mentioned in the introduction, the designation stainless steel conventionally implies little more than a 12% Cr content. Most of the stainless steels are based on the Fe-Cr-C and Fe-Cr-Ni-C systems, but other alloying elements are also important.

Iron and its alloys can exist in two crystallographic forms (body centred or face-centred cubic). In pure iron, the f.c.c. structure exists between 910 and 1400 C, the b.c.c. structure below and above this interval (up to the melting temperature of 1539 C).
The importance of this phase-transformation in the metallurgy of steels cannot be overestimated. This transformation allow for a wide range of microstructures to be achieved by controlled heat-treatment. Mechanical properties are essentially related to microstructure, and can therefore be obtained in an extraordinarily large range of strength, toughness, etc.. Stainless steels are routinely produced with strengths from 100 MPa to largely more than 1GPa.

Knowledge of the relative stability of the b.c.c. and f.c.c. structures of iron alloys is therefore of prime concern. The history of stainless steels started with a martensitic grade (12%Cr-0.1%C) in Sheffield, UK and the austenitic 18%Cr-8%Cr in Germany ( more about the history of stainless steels). For this reason, and also because they are most often the major alloying elements, Cr and Ni have long been used as reference to quantify the influence of alloying elements on the b.c.c.<->f.c.c. phase transition: chromium additions tend to stabilise the b.c.c. phase, while nickel additions stabilise the f.c.c. one.

FeCrC isopleth
Pseudo-binary (isopleth) Fe-Cr diagram for 0.1% C. Chromium additions stabilise the b.c.c. form of iron, beyond about 18%, Fe-Cr steels no longer undergo the α/γ transition (ferritic stainless steels).

Without carbon, the limit beyond which austenite no longer forms is about 13.5 wt%. However, additions of carbon stabilise the austenite and therefore increase this limit.
Other alloying elements also affect the stability of austenite/ferrite in one direction or another. This has led to their classification as α-stabilisers or γ-stabilisers. The concept of Cr-equivalent (for α-stabilisers) and Ni-equivalent (for γ-stabilisers) is widely used in conjunction with the Schaeffler diagram to quantify their roles:
Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)
Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)
All in weight percent.

Schaeffler diagram
Schematic Schaeffler diagram. Although primarily intended for use in welding, this diagram is sometimes used in alloy design as it provides an easy way to estimate the microstructure of a stainless steel.

Modern thermodynamics calculation tools such as Thermocalc or MTDATA based on the CALPHAD method allow more rigourous determination of equilibrium phase diagrams in multicomponent systems.

Second phases

In most grades of stainless steels, alloying elements are present in quantities sufficient to cause precipitation of second phases. Most often the stable carbides, nitrides or intermetallics are of little relevance as they tend to follow a long and complex precipitation sequence.
This is because the kinetics of precipitation are largely controlled by nucleation, and nucleation itself is not necessarily easier for the most stable precipitates.
From the Fe-Cr diagram presented earlier, it appears that typical martensitic steels should exhibit ferrite and M23C6 in equilibrium at 600 C (for example). In practice, this carbide is only found after relatively long ageing. Intermediate phases are, in order, cementite, M2X and M7C3 to finally obtain M23C6.
These sequences are far more complex in heavily alloyed ferritic or austenitic stainless steels such as those used in the power generation industry. Considerable research work is being devoted to predicting quantitatively the precipitation sequences in such alloys. This is mainly because their life expectancy (about 30 years) vastly exceeds the length of laboratory tests.

Martensite formation

Most stainless steels have a high hardenability, meaning that reconstructive austenite to (ferrite + carbides) transformation is unlikely to happen unless the steel is cooled particularly slowly.
The most important features of these alloys are therefore the martensite start (Ms) and finish temperatures (Mf). For martensitic steels, the range [Mf-Ms] should be above the room temperature to ensure fully martensitic structure. On the contrary, the [Ms-Mf] range of austenitic stainless steels is often well below 0 C, which is why austenitic steels are used in cryogenic applications. Cold work can cause martensitic transformation to an extent with depends on the deformation and on the alloying composition. Heavily alloyed austenitic steels with up to 20% Cr and 25% Ni are fully stable.

The mechanisms of martensite nucleation are reasonably well understood and there are a number of models which predicts the Ms temperature with an acceptable accuracy.

Categories

On the basis of their main microstructural features, grades of stainless steels are typically divided into four categories:

Martensitic Stainless Steels

These steels still undergo the b.c.c./f.c.c. transformation of iron, although the range of austenite stability is reduced.
As for conventional steels, mechanical properties can be considerably altered by heat-treatments. Typical heat-treatments consist of austenitisation at a temperature suitable for dissolution of carbides. Stainless steels have a high hardenability, that is to say, reconstructive transformations are considerably slowed by the presence of Cr, so that a fully martensitic structure can be achieved without a severe quench. Oil or water quenching are nevertheless used with large sections so as to ensure martensite formation throughout.

Typical compositions cover 12 to 18 Cr and 0.1 to 1.2 C (wt%). As with other martensitic steels, a balance must be sought between hardness and toughness. An untempered martensitic structure typically has high hardness/yield strength but a low toughness and ductility (although the exact values depend on the carbon content). In many conditions, these are used after a tempering treatment between 600 and 750 C, which result in a lower hardness but improved toughness.

In some applications such as cutlery, surgical intruments etc., high strength is desirable and toughness/ductility of little concern. A lower temperature tempering is then used to retain most of the strength. AISI type 420 (0.15-0.4C, 1.0Mn, 1.0Si, 0.04P, 0.03S, 12-14Cr all max wt%) is a typical composition for such applications. Its proof strength in quenched and tempered condition can be in excess of 1.2 GPa. For type 440C tempered at 300 C, the proof strength can reach about 2 GPa.

The table below shows the composition and typical use of AISI standard martensitic grades:

AISI grade C Mn Si Cr Ni Mo P S Comments/Applications
410 0.15 1.0 0.5 11.5-13.0 - - 0.04 0.03 The basic composition. Used for cutlery, steam and gas turbine blades and buckets, bushings..More.
416 0.15 1.25 1.0 12.0-14.0 - 0.60 0.04 0.15 Addition of sulphur for machinability, used for screws, gears etc. 416 Se replaces suplhur by selenium. More
420 0.15-0.40 1.0 1.0 12.0-14.0 - - 0.04 0.03 Dental and surgical instruments, cutlery.. More
431 0.20 1.0 1.0 15.0-17.0 - 1.25-2.0 0.04 0.03 Enhanced corrosion resistance, high strength. More
440A 0.60-0.75 1.0 1.0 16.0-18.0 - 0.75 0.04 0.03 Ball bearings and races, gage blocks, molds and dies, cutlery, More
440B 0.75-0.95 1.0 1.0 16.0-18.0 - 0.75 0.04 0.03 As 440A, higher hardness
440C 0.95-1.20 1.0 1.0 16.0-18.0 - 0.75 0.04 0.03 As 440B, higher hardness
Standard grades of martensitic stainless steels. Comparison of grade specifications .

In additions to the standard grades, a large number of alloyed martensitic stainless steels have been developed for moderately high temperature applications. Most common additions include Mo, V and Nb. These lead to a complex precipitation sequence. A small amount (up to 2 wt%) of Ni is added which improves the toughness.
The 12Cr-Mo-V-Nb steels are used in the power generation industry, for steam turbine blades operating at temperatures around 600 C. Current research focusses on achieving service temperatures of 630-650 C under a stress of 30 MPa.

Ferritic stainless steels

Ferritic stainless steels: contain typically more chromium and/or less carbon than the martensitic grades. Both changes act towards stabilisation of ferrite against austenite so that ferrite is stable at all temperatures. Therefore, ferritic stainless steels cannot be hardened by heat-treatments as is the case of martensitic ones. They exhibit lower strength but higher ductility/toughness. Typical application may include appliances, automotive and architectural trim (i.e. decorative purposes), as the cheapest stainless steels are found in this family (aisi 409).

AISI grade C Mn Si Cr Mo P S Comments/Applications
405 0.08 1.0 1.0 11.5-14.5 - 0.04 0.03 0.1-0.3 Al
409 0.08 1.0 1.0 10.5-11.75 - 0.045 0.045 (6xC) Ti min
429 0.12 1.0 1.0 14.0-16.0 - 0.04 0.03
430 0.12 1.0 1.0 16.0-18.0 - 0.04 0.03
446 0.20 1.5 1.0 23.0-27.0 - 0.04 0.03 0.25 N
Some standard grades of ferritic stainless steels. Comparison of grade specifications .

High chromium ferritic stainless steels such as 446 are sensible to the so-called '475 C embrittlement', which is caused by the decomposition of the Fe-Cr solid solution in two phases, Fe and Cr-rich respectively. Around 475 C and below, and for Cr contents greater than about 25 wt%, this decomposition is spinodal and typically exhibits wavelength below 10 nm. As the decomposition occurs, a continuous increase of hardness is observed: for example, the hardness of an Fe-28Cr steel can increase by more than 300 Hv over an exposure 10,000 at 450 C (Ishikawa et al., Mater. Trans. JIM, 36:1995, p16-22). This results in a severe drop of impact toughness and ductility.
Addition of Ni appear to accelerate the spinodal decomposition and raise the maximum temperature at which it is observed. When post-weld heat-treatment is not possible, welding of ferritic stainless steels is usually done with a metal filler containing Ni, and there is therefore the possibility of weld embrittlement.

Austenitic stainless steels

Austenitic stainless steels: these stainless steels owe their name to their f.c.c. crystallographic structure. Typical compositions in the early 20th century were 18Cr-8Ni. The austenite in these alloys was only stable because of the relatively large carbon content, and modern equivalent usually contain up to 10.5 Ni.
These steels are often in metastable conditions at room temperature or below, and while the reconstructive formation of ferrite is not of concern, the formation of martensite can be. Most grades have a martensite start temperature (Ms) well below 0 C. However, cold work can result in formation of martensite at temperatures higher than Ms (this result in the sample becoming magnetic, while a fully austenitic structure is not). The impact of deformation on the stability of the material is conveniently quantified by the Md,30 temperature, the temperature at which the structure is 50% martensitic for 30% deformation.

The presence of nickel improves considerably the corrosion resistance when compared to the martensitic and ferritic grades.
AISI type 304 is the basic 18/8 austenitic stainless steel, so widely used that it accounts for about 50% of all stainless steel production. Other standard grades have different preferred applications; for example, type 316 which contains up to 3 % Mo, offers an improved corrosion resistance, in particular, improved pitting corrosion resistance, which makes it a material of choice for many marine applications (off-shore platforms etc..), but also for coastal environments (more on stainless steels in architecture).
In severe conditions however, 316 is not sufficient and special steels such as 254 SMO are used (example: steels used in offshore oil platforms), which contain up to 6% Mo.

AISI grade C max. Si max. Mn max. Cr Ni Mo Ti Nb Al V
301 0.15 1.00 2.00 16-18 6-8
302 0.15 1.00 2.00 17-19 8-10
304 0.08 1.00 2.00 17.5-20 8-10.5
310 0.25 1.50 2.00 24-26 19-22
316 0.08 1.00 2.00 16-18 10-14 2.0-3.0
321 0.08 1.00 2.00 17-19 9-12 5 x %C min.
347 0.08 1.00 2.00 17-19 9-13 10 x %C min.
E 1250 0.1 0.5 6.0 15.0 10.0 0.25
20/25-Nb 0.05 1.0 1.0 20.0 25.0 0.7
A 286 0.05 1.0 1.0 15.0 26.0 1.2 ~1.9 ~0.18 ~0.25
254SMO 0.02 0.8 1.0 18.5-20.5 17.5-18.5 6-6.5 ~1.9 ~0.18 ~0.25
The AISI 300 series and other examples of heat resistant austenitic stainless steels; E1250 is Esshete 1250 (Corus), 254SMO is trademark Avesta Sheffield.

Duplex stainless steels

Duplex stainless steels: duplex stainless steel typically contain 50% austenite and 50% ferrite. This confers them properties intermediate between the two types of steels:

Only one duplex steel has an AISI designation (329) so that ASTM numbers are more currently used to reffer to different grades. The archetypal stainless steel, type 2205, contains 22-23Cr, 4.5-6.5Ni and 3-3.5Mo. This grade represents 80% of all duplex stainless steel use.

Duplex stainless steels suffer from the 475 C embrittlement described earlier for ferritic stainless steels and are therefore mostly confined to applications below 300 C.

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