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What is Stainless Steel

[The following extract is taken from a technical information document supplied by the Australian Stainless Steel Development Association with regard to the 200 Series.]

Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium.

All stainless steels have a high resistance to corrosion. This resistance to attack is due to the naturally occurring chromium-rich oxide film formed on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. The film is rapidly self repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired.

Grade 200 Series - CrMn Grade

Nickel prices have been relatively high over the last couple of years. As a result, there has been increased interest in low-nickel or no-nickel grades of stainless steel. One such family of stainless steels is the 200-series and use of these has doubled this decade. They
have become popular in China and South East Asia, particularly.

However, this has not been without problems. Because the 200-series grades are austenitic, they are not magnetic and therefore very difficult to distinguish from the widely used 300-series grades, such as grade 304, which are also non-magnetic. This has led to confusion in the marketplace, including cases of incorrect labelling, etc, with 200-series material being sold as grade 304.

Most growth in 200-series use over recent years has been in low-nickel and therefore low  chromium versions which have less corrosion resistance than grade 304.

The end result has been corrosion failures in some applications and dissatisfied customers. In addition, there are concerns that this 200-series material may contaminate the existing stainless steel recycling circuit which is based on grade 304. The 200-series are a technically valid family of stainless steels but, like all stainless steel grades, they have their limitations. If you are considering the use of a 200-series material then you should ensure that you have all the necessary information that you require to make a rational judgment – mechanical and physical properties, corrosion performance in your environment etc. And it is strongly recommended that you deal only with reputable and knowledgeable suppliers who can provide high quality material of known origin.

Austenitic stainless steels

The term austenitic describes another type of crystal structure. Some elements have the ability to change ferrite to austenite when they are added into stainless steel. They are called austenite formers and the best known and most widely used of these is nickel (Ni). If sufficient Ni is added, the crystal structure changes from ferrite to a structure where there is one atom at each corner of the cube and one in the middle of each face – this “face centred cubic” structure is called “austenite”:

When Ni is used to create the austenite structure, the resulting stainless steels are known as the 300-series. The most common of these is grade 304 which contains approximately 18% Cr and 8% Ni. This 8% Ni is the minimum amount of Ni which can be added into an 18% Cr stainless steel in order to change all the ferrite to austenite. Another common 300-series grade is grade 316 which is essentially grade 304 with 2% molybdenum (Mo) added to improve corrosion resistance. Ni is not the only element which can change ferrite into austenite.

Nitrogen (N) is also very powerful at doing this. But it is a gas and can only be added in limited amounts before problems arise, such as the formation of chromium nitrides and even gas porosity. However, manganese (Mn) is also an austenite former and has the additional characteristic that it allows more N to be added. So Mn and N, sometimes with copper (Cu), may be used to replace some of the Ni and the resulting stainless steels are known as the 200-series.

They are commonly referred to as the CrMn stainless steels, although it is the N which has
the greatest effect in forming austenite. A combination of Mn and N is normally not
sufficient to change all the ferrite to austenite so some Ni is still added, although in a smaller amount compared to what would be used in a 300-series grade.

The austenitic stainless steels have the general characteristics:

  • Usually more expensive than the ferritic grades (the 200-series are usually cheaper
    than the 300-series)
  • Good formability and weldability –generally much better than the ferritic grades
  • Excellent toughness (impact resistance) even to very low, cryogenic temperatures – ferritic grades have poor low temperature toughness
  • Not magnetic (although some small degree of magnetism can develop when cold worked, such as in a bolt or at a bent edge)

The combination of good formability and weldability mean that the austenitic grades
have good fabricability – they can easily be used to manufacture all manner of items. It
is for this reason that the 300-series dominate stainless steel use worldwide.

Types of 200-Series Stainless Steel

Because of the increased use of CrMn grades over the last couple of years, there is a tendency to think of them as a new development. However, they have been around since the 1930s and a lot of development work was done in the 1940s and 1950s, particularly in USA, because of a shortage of nickel at the time. Useful alloys which came out of that work were registered with the American Iron and Steel Institute (AISI) and given AISI numbers (such as 201) and Unified Numbering System (UNS) numbers (such as S20100).

 

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