General principles for selection of stainless steels

Introduction

Stainless steels are defined as iron alloys with a minimum of 10.5% chromium.
Other alloying elements are added to enhance their structure and properties, but fundamentally, stainless steels are considered for selection as steels with corrosion resistant properties.

In economic terms they can compete with higher cost engineering metals and alloys based on nickel or titanium, whilst offering a range of corrosion resisting properties suitable for a wide range of applications. They have better strength than most polymer products (GRP), are readily repairable and 'recyclable' at the end of their useful life.

When considering stainless the most important features are: -

    · Corrosion (or oxidation) resistance
    · Mechanical & physical properties
    · Available forming, fabrication & joining techniques
    · Environmental & material costs (including total life cycle cost)
The basic approach is to select a grade with as low a cost as possible, but the required corrosion resistance. Other considerations such as strength and hardenability are secondary.

 

Corrosion resistance

Chromium (Cr) content sets stainless steels apart from other steels.
The unique self-repairing 'passive' surface layer on the steel is due to the chromium.
Commercially available grades have around 11% chromium as a minimum. These can be either ferritic or martensitic, depending on carbon range control.

Increasing chromium enhances corrosion and oxidation resistance, so a 17% Cr 430 (1.4016) ferritic would be expected to be an improvement over the '410S' (1.4000) types.
Similarly martensitic 431 (1.4057) at 15% Cr can be expected to have better corrosion resistance than the 12% Cr 420 (1.4021 / 1.4028) types.
Chromium levels over 20% provide improved 'aqueous' corrosion resistance for the duplex and higher alloyed austenitics and also forms the basis of the good elevated temperature oxidation resistance of ferritic and austenitic heat resisting grades, such as the quite rare ferritic 446 (25% Cr) or the more widely used 25 % Cr, 20% nickel (Ni) austenitic 310 (1.4845) grade.

In addition to this basic 'rule', nickel (Ni) widens the scope of environments that stainless steels can 'handle'.
The 2% Ni addition to the 431 (1.4057) martensitic type improves corrosion resistance marginally but its main purpose is to improve the impact toughness of the steel.  Additions of between about 4.5% and 6.5% Ni are made in forming the duplex types. The austenitics have ranges from about 7% to over 20%.
The corrosion resistance is not simply related to nickel level however. It would be wrong to assume that a 304 (1.4301) with its 8% Ni therefore has better corrosion resistance that a 1.4462 duplex with only 5% Ni.

More specific alloy additions are also made with the specific aim of enhancing corrosion resistance.
These include molybdenum (Mo) and nitrogen (N) for pitting and crevice corrosion resistance. The 316 types are the main Mo bearing austenitics. Many of the currently available duplex grades contain additions of both Mo and N.

Copper is also used to enhance corrosion resistance in some 'common', but hazardous, environments such as 'intermediate' concentration ranges of sulphuric acid. Grades containing copper include the austenitic 904L (1.4539) type and some 25% Cr "superduplex" steels such as 1.4501 and 1.4507.

Mechanical and physical properties

Basic mechanical strength increases with alloy additions, but the atomic structure differences of the various groups of stainless steels has a more important effect.

Only the martensitic stainless steels are hardenable by heat treatment, like other alloy steels. Precipitation hardening stainless steels are strengthened by heat treatment, but use a different mechanism to the martensitic types. Very small particles are formed by the appropriate heat treatment and act as the strengthening agent in the steel matrix.
The ferritic, austenitic and duplex types cannot be strengthened or hardened by heat treatment, but respond to varying degrees to cold working as a strengthening mechanism.

Ferritic types have useful mechanical properties at ambient temperatures, but have limited ductility, compared to the austenitics. They are not suitable for cryogenic applications due to loss of impact toughness and lose strength at elevated temperatures over about 600 °C, although have been used for applications such as automotive exhaust systems very successfully.

Austenitic types, with their characteristic face centred cube 'fcc' atomic arrangement, have quite distinct properties. Mechanically they are more ductile and impact tough at cryogenic temperatures.
The main physical property difference from the other types of stainless steel is that they are 'non-magnetic' i.e. have low relative magnetic permeability, provided they are fully softened. They also have lower thermal conductivity and higher thermal expansion rates than the other stainless steel types.

Duplex types, which have a 'mixed' structure of austenite and ferrite, share some of the properties of those types, but, fundamentally are mechanically stronger than either ferritic or austenitic types.

Forming, fabrication and joining techniques

Depending on their type and heat-treated condition, wrought stainless steels are formable and machinable. Stainless steels can also be cast or forged into shape.
Most of the available types and grades can be joined by use of appropriate 'thermal' methods including soldering, brazing and welding.

Austenitics are suitable for a wide range of applications involving flat product forming (pressing, drawing, stretch forming, spinning etc).
Although ferritics and duplex types are also useful for these forming methods, the excellent ductility and work hardening characteristic of the austenitics make them a better choice.

Formability of the austenitic types is controlled through the nickel level.
The 301 (1.4310) grade which has a 'low' nickel content, around 7% and so work hardens when cold worked, enabling it to be use for pressed 'stiffening' panels.
In contrast nickel levels of around 8.0% make the steel ideally suited to stretch forming operations, for example in the manufacture of stainless steel sinks. Higher nickel levels around 9.0% are required for deep drawing.

Martensitics are not readily formable, but are used extensively for blanking in the manufacture of cutting blades.

Most stainless steel types can be machined by conventional methods, provided allowance is made for their strength and work hardening characteristics.
Techniques involving control of feed and speed to undercut work hardening layers with good lubrication and cooling systems are usually sufficient.
Where high production volume systems are employed, machining enhanced grades may be needed.
In this respect, stainless steels are treated in similar ways to other alloy steels, sulphur additions being the traditional approach in grades like 303 (1.4305). Controlled cleanness types are now also available for enhanced machinability.

Most stainless steels can be soldered or brazed, provided care is taken in surface preparation and fluxes are selected to avoid the natural surface oxidising properties being a problem in these thermal processes.
The strength and corrosion resistance of such joints does not match the full potential of the stainless steel being joined, however.

To optimise joint strength and corrosion resistance, most stainless steels can be welded using a wide range of techniques.
The weldablity of the ferritic and duplex types is good, whilst the austenitic types are classed as excellent for welding. The lower carbon martensitics can be welded with care but grades such as the 17% Cr, 1% carbon, 440 types (1.4125) are not suitable for welding.

Summary of the main advantages of the stainless steel types

Type Examples Advantages Disadvantages
Ferritic 410S, 430, 446 Low cost, moderate corrosion resistance & good formability Limited corrosion resistance, formabilty & elevated temperature strength compared to austenitics
Austenitic 304,316 Widely available, good general corrosion resistance, good cryogenic toughness. Excellent formability & weldability Work hardening can limit formability & machinability. Limited resistance to stress corrosion cracking
Duplex 1.4462 Good stress corrosion cracking resistance, good mechanical strength in annealed condition Application temperature range more restricted than austenitics
Martensitics 420, 431 Hardenable by heat treatment Corrosion resistance compared to austenitics & formability compared to ferritics limited. Weldability limited.
Precipitation hardening 17/4PH Hardenable by heat treatment, but with better corrosion resistance than martensitics Limited availability, corrosion resistance, formability & weldability restricted compared to austenitics

Other 'Families' of stainless steels

There is a wide range of stainless steel types.
Special grades with enhanced compositions have been developed and are available that minimise the short comings of any particular type.
These include: -

    · Super ferritics
    · Super austenitics
    · Super duplexes
    · Low carbon weldable martensitics
    · Austenitic precipitation hardening types

 

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