Austenitic stainless steels They are stainless steels with main phase that of the austenite (γ-Fe). Austenite is the allotropic form of iron, which crystallizes according to the face centered cubic system (FCC). Austenitic stainless steels contain very low quantity of carbon (usually <0,08% C, but some contain up to 0,15% C) and at least 16% Cr. The austenite is stabilized with the addition of Ni or and Mn, and remains into stable phase in all the temperature breadth from the alloy’s melting point until well below 0℃. Due to the austenite’s non-magnetic nature, austenitic stainless steels are not magnetic.

The most common austenitic stainless steels are 18/8 (18% Cr, 8% Ni) and 18/10 (18% Cr, 10% Ni), which belong in 300 series, according to the American standards AISI-SAE. The steels AISI 316 display higher resistance to corrosion and are characterized by the presence of molybdenum (around 2%). The types 304L and 316L contain the lowest quantity of carbon (< 0,03%) resulting in better performance during welding. Generally, the types of the 300 series present good corrosion resistance, excellent forming capability, low yield strength Mechanical properties of a metallic alloy are those that describe the material’s ability to compress, stretch, bend, scratch, dent or break. The most commonly used criteria for evaluating mechanical characteristics are:
a. Strength: the degree of resistance of a material to deformation. Two critical values are generally considered:

yield strength, or the stress the material can be subjected to before permanent plastic deformation occurs
tensile strength, or the stress it can be subjected to before rupture/failure

15. UTS (ultimate tensile strength) is measured in MPa (1Mpa=1N/mm3=145PSI=0.1 kg/mm3) and represents maximum resistance at failure. YS (yield strength) refers to the beginning of the “plastic” phase, where elongation no longer disappears when the stress is removed.

b. Hardness: the degree of resistance to indentation by an applied load.
c. Toughness: the capacity to absorb deformation energy before fracture.
d. Ductility (or plasticity): the ability to deform plastically without fracturing.

16. Rm=ultimate tensile strength, Rp02=yield strength and A5/A80=elongation to fracture. , relatively high tensile strength Mechanical properties of a metallic alloy are those that describe the material’s ability to compress, stretch, bend, scratch, dent or break. The most commonly used criteria for evaluating mechanical characteristics are:
a. Strength: the degree of resistance of a material to deformation. Two critical values are generally considered:

yield strength, or the stress the material can be subjected to before permanent plastic deformation occurs
tensile strength, or the stress it can be subjected to before rupture/failure

15. UTS (ultimate tensile strength) is measured in MPa (1Mpa=1N/mm3=145PSI=0.1 kg/mm3) and represents maximum resistance at failure. YS (yield strength) refers to the beginning of the “plastic” phase, where elongation no longer disappears when the stress is removed.

b. Hardness: the degree of resistance to indentation by an applied load.
c. Toughness: the capacity to absorb deformation energy before fracture.
d. Ductility (or plasticity): the ability to deform plastically without fracturing.

16. Rm=ultimate tensile strength, Rp02=yield strength and A5/A80=elongation to fracture. and good weldability, providing a wide range of applications.

Apart from the well-known 300 series, there exist the less resistant types of the 200 series with manganese. These new types use different chemistry which is differentiated by the lower chromium content (<15%) and the much lower nickel content. The reduction of nickel with the simultaneous addition of manganese limits the quantity of chromium that can be added, affecting negatively therefore the resistance to corrosion. It has been mentioned earlier that the addition of nickel is the principal way for protecting the austenitic structure in stainless steel. However, the addition of manganese in combination with the presence of nitrogen may bring about the same results – albeit at lower cost. The chromium–manganese types are characterized by notably lower nickel content and by the existence of manganese and often that of nitrogen and copper (both of which share the capability of promoting the austenitic structure). The addition of nitrogen results to a better stabilization of the austenitic phase, allowing therefore the addition of more chromium. Nitrogen also acts as a hardening factor. We use to refer to the types of the 200 series simply by mentioning their nickel content – like 4% Ni and 1% Ni. The most popular types are 201 (1% Ni, min 15% Cr, max 0,1% C) and 202 (4% Ni, min 16% Cr, max 0,08% C).

Apart from the fact that the chromium–manganese stainless steels have a lower cost, at the same time they present good forming capability and good corrosion resistance depending on their chemistry. Their characteristic advantage is their higher mechanical properties compared to those of the classical 300 series (i.e. 304), something that allows the reduction of the gauge of the material used and thus the reduction of the weight. However, we draw your attention to the fact that the 200 series (especially types with high nitrogen content) are more difficult to form. The addition of copper is a solution to this problem, since it allows the reduction of nitrogen with the content of nickel and chromium remaining stable.

The rise in the popularity of the 200 series was related to the instability of the nickel value (especially during periods of sharp increases in its price), as well as to the progress in the technology used for the steel’s production. At the same time, constant pressure for the reduction of cost, especially in the Asian market, has led to the development of austenitic types with low nickel and chromium content which often do not satisfy any international standards. Actually, several chromium–manganese types are distinctive of specific mills’ production and are defined simply from a title given by each producer. Therefore, it is suggested to manufactures that examine their potential use to take the advice of credible suppliers who have the right information and are capable of supplying good quality products of reliable origin. Moreover, we note that the potential user has the possibility to choose alternative solutions among the 400 and 300 series.

Concluding the 200 series presentation, we mention below some applications were the experience has proven their positive performance (mainly for the types with 4% Ni). Such applications include cutlery and cooking utensils, home sinks, catering equipment, trucks’ structural parts, bushes’ chassis and constructions in the sugar industry, as well as architectural constructions that are not near coasts.

Finally, we make a reference to the super austenitic stainless steels with very high nickel content Ni (>20%) and molybdenum Μο (>6%), for strong resistance to the corrosion caused by acids, chlorine and chloride solutions. The most popular type of this category is AISI 904L (19-23% Cr, 23-28% Ni, 4-5% Mo).