About Stainless Steel
Who invented stainless steel and when?
The answer to this ambiguous question is unclear. While Harry Brearley is often credited as the inventor, it is clear that many people preceded him along the road to its discovery. One could probably go back to the 1820’s with the work of Englishmen Michael Faraday and James Stoddard on Wootz steel as well as the work of Frenchman Pierre Berthier.
The 1870’s saw J.E.T. Woods and J. Clark discover an acid and weather resistant alloy. Many others contributed- Julius Baur, Henri-Ami Brustlein, Hans Goldschmidt, Leon Guillet, P. Monnartz and W. Borchers- to name just a few. But it was Harry Brearley that created a steel with 12.8% chromium and 0.24% carbon, arguably, the first piece of modern stainless steel. That was in 1913, though one should bear in mind that stainless steel was not properly defined until 1911.
Fast forward to today, there are five basic categories of stainless steel: austenitic, ferritic, duplex, martensitic and precipitation hardening. Within these basic categories are many grades of stainless steel, each with different properties and abilities to perform different functions. Selecting the correct grade of stainless to perform a specific function is of critical importance.
Austenitic grades are the most widely used group of stainless steels. When nickel is added to stainless steel to the required level, the crystalline structure changes from ferrite to austenite stainless steel.
Austenitic stainless steel is 18% chromium and 8% nickel. This is knife and fork stainless that you commonly see stamped 18/8. Another name for this steel is 304 grade. If molybdenum is added to around the 2% level, it becomes 316 grade, with a corresponding increase in corrosion resistance.
Ferritic stainless steels have no nickel present and are magnetic. They are high in chromium, typically from 10.5 to 18% and have a relatively low carbon content. Domestic appliances and builders’ hardware are commonly made using this type of stainless. Weldability of these steels is quite poor.
Duplex Stainless Steel
Duplex stainless steels have a microstructure of roughly equal amounts of ferrite and austenite and are often referred to as ferritic-austenitic stainless steels- hence the description, duplex. The chromium content varies from 18 to 28%, while the nickel content of 4.5 to 8% is not high enough to develop a fully austenitic crystal structure. Most grades of duplex stainless contain between 2.5 and 4% of molybdenum as well as small quantities of nitrogen. This has the effect of increasing strength and the resistance to pit corrosion. You will find duplex stainless in heat exchangers, desalination plants and the offshore industry.
Martensitic stainless steels were one of the first produced industrially. Carbon content is fairly high and the chromium content varied from 12-18%. It was the ideal steel for cutlery as it is able to be heat treated. Corrosion resistance, however, is quite moderate.
Precipitation hardening stainless steels belong to a family of corrosion resistant alloys that can be heat treated to quite high tensile strengths, (by stainless steel standards). There are three main types- low carbon martensitic, semi-austenitic and austenitic. 17/4 PH can be found in our Cromox range of stainless products and it is a martensitic steel. Corrosion resistance of this steel is not very high, somewhat similar to 18/8 stainless.
Why Use a Duplex Stainless Anchor Chain?
To answer this question, we have to go back to the basic reason of why we like stainless steel- it has resistance to corrosion. This resistance to attack is because of the chromium rich oxide film on the surface of the steel. This invisible, inert and passive film adheres to the metal and protects against corrosion. When damaged by abrasion, it is self- repairing in the presence of oxygen. Surface finish should be mentioned at this point as well- a polished surface is more corrosion resistant than a brushed finish, for example.
AISI 316L is commonly used for anchor chain. In many cases, it will perform its function well. If the chain is used intermittently and is constantly in and out of the seawater, there should be little problem in using AISI 316L. It gets more complicated if the anchor chain is left immersed in seawater for longer periods of time. Pitting corrosion is the enemy of stainless steel that spends a lot of time in seawater. Pit corrosion can lead to catastrophic failure of the chain, often at a time of least convenience. Duplex stainless resists pit corrosion more than AISI 316 stainless. This is due to the fact that AISI 318LN has a higher PREN value. AISI 316 has a PREN of around 23.1 to 28.5, while AISI 318LN has a PREN of around 30.8 to 38.1. It is generally considered that stainless steels with a PREN above 32 are resistant to seawater corrosion.
Pitting corrosion is an electrochemical oxidation-reduction process that occurs within localised holes on the surface of a metal that is coated with a passive film. It is a form of galvanic corrosion where the chromium in the passive layer is dissolved, leaving only the corrosion prone iron. In seawater, as the chromium is dissolved, the electrically driven chlorides bore into the stainless steel and create a smooth wall spherical pit. The residual solution that is left in the pit is ferric chloride, ( FeCl3). Ferric chloride is highly corrosive to stainless steel.
Duplex stainless steels also provide increased resistance against stress corrosion cracking (SCC). When tensile stress combines with a specific corrosive environment, the net effect can result in cracking failure. SCC usually starts with pitting or crevice corrosion as a precursor to the formation of a stress concentrator. By using a duplex stainless in sea water, the likelihood of pitting corrosion is greatly reduced, thereby reducing the flow on effect of such pits.
The pitting resistance equivalent number, PREN, is a measure of the relative resistance of stainless steel to pitting corrosion in an environment that contains chloride- namely sea water in this context. The elements chromium, molybdenum and nitrogen each play a crucial role and this is illustrated by the fact that the formula for the PREN value involves these three elements.
PREN = 1 x % Cr + 3.3 x % Mo + 16 x % N
Even though it is not an absolute value, it is useful for comparing stainless steels from within the same family.