The previous question is a frequent topic of discussion among stainless steel users and few actually answer adequately with solid arguments, and in the absence of adequate knowledge on the subject, myths arise around it.
This article intends to clear up some of the most frequent doubts that will surely help users to think and make some favorable decisions.
Stainless steels are alloys based mainly on a chemical composition of three metals: Iron (Fe), Chromium (Cr) and Nickel (Ni); which provide the particular characteristics to be resistant to corrosion.
Some extra elements are intentionally added to modify a particular property, other elements are simply residual from the steel manufacturing process that are harmless or neutral to the properties of the steel; some of these elements can be Manganese (Mn), Carbon (C), Nitrogen (N), Molybdenum (Mo), Silicon (Si), Aluminum (Al) and Niobium (Nb).
The type of alloying elements and especially the amount of these will define the type of stainless steel and consequently its chemical, physical and mechanical characteristics, in table 1 we observe the typical chemical composition of some of the most commercial stainless steels and some relevant characteristics.
Typical characteristics of commercial stainless steels
It is common for many users to say that stainless steel IS NOT Magnetic, or at least that it SHOULD BE, and that when it IS magnetic, then the corrosion resistance is compromised and it will be less than a steel that does not present magnetism, there are even those who say that if it is magnetic then it will not be stainless!!, however, the above is not correct, as the corrosion resistance of a stainless steel is not related to the fact that it is or is not magnetic.
The previous discussion is typical in the purchase-sale of stainless steel scrap or scrap, where a simple test with a magnet could determine the price of a scrap lot to the convenience of the more informed.
The above requires a deeper explanation; according to the chemical composition of stainless steel, the metallurgical structures can be formed: Ferrite, Martensite and Austenite, the first two are magnetic, that is, they are attracted by the magnet, while Austenite is non-magnetic and will not be attracted by the magnet. Based on the above, stainless steels are classified into Stainless steels: Ferritic, Martensitic, Austenitic and Duplex.
The Ferritic and Martensitic will be strongly attracted by the magnet, the Austenitic will not be attracted by the magnet and the Duplex will be weakly attracted by the magnet. Chromium, ferrite promoter, is partly responsible for the magnetism of stainless steel, while Nickel, austenite promoter, causes stainless steel to be non-magnetic.
Resistance to corrosion
On the other hand, the corrosion resistance of each stainless steel is related to the presence of the alloying elements promoters of the Passive Layer that protects the surface of the steel against a corrosive medium or atmosphere, this is the reason why they are called stainless. A minimum Chromium (Cr) content of 10.5% is necessary to create this protection in steel, this element is the main responsible for the corrosion resistance of stainless steels; the most used index to compare the corrosion resistance of stainless steels is the PRE for its English acronym of Pitting Resistance Equivalent:
The higher the PRE, the greater the corrosion resistance of the steel, table 1 shows the typical values of the PRE.
Based on the above information we can understand that magnetic properties are not related to corrosion resistance, magnetism depends on the metallurgical structure and corrosion resistance is mainly a function of the amount of Cr in the alloy; we can take as an example steel 304 and compare it against 441, both steels have the same PRE=18, so they will have the same corrosion resistance, but 304 is Non-Magnetic for being austenitic (Cr+Ni), while 441 is Magnetic for being Ferritic (Cr).
Another phenomenon necessary to comment on is the following, Austenitic steels, from the 300 series, such as 301, 304, 305 or 316 are non-magnetic, or at least their attraction to the magnet is imperceptible, however, naturally these steels contain small amounts of ferrite and martensite due to the chemical composition, so a very slight magnetism could be detected.
Similarly, when austenitic stainless steels from the 300 series are subjected to cold deformation processes, such as rolling, drawing, deep drawing, bending etc., then the phenomenon known as Work Hardening will occur, where a partial transformation of Austenite into Martensite, which is magnetic, this phenomenon will produce a hardening in the steel, increasing the mechanical resistance and also the magnetism of the stainless steel. As an example, it is not rare to observe that steel 304 or 301 with Temper ½ Hard has a higher magnetism than the same material in a completely soft condition (Annealed), even when the chemical composition has not varied.
Classification of steels
Finally, we can classify steels according to the magnetism they present:
- Common carbon steel, Ferritic, Martensitic and duplex stainless steels are named Ferromagnetic, Relative Magnetic Permeability (µr) greater than unity (2 or more…), they will be "strongly" attracted by a magnet.
- Austenitic Stainless Steels in a completely soft condition (Annealed) are Paramagnetic, with relative magnetic permeability (µr) close to unity (1.01), "slightly" attracted by a magnet.
- Most non-ferrous metals, such as copper or aluminum, are Diamagnetic materials, which "are not attracted" by the magnet.
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