Austenitic stainless steels - screw-express.com

CORROSION RESISTANCE

With the exception of certain precious metals such as gold or platinum, metals extracted from ores always tend to revert to a combined state, deteriorating on contact with the atmosphere, water or any industrial corrosive substance. So-called "stainless" steels do not have generalized and absolute corrosion resistance. They come in the form of alloys with the ability to withstand a particular environment for a given period of time. Their resistance comes from their ability to protect themselves by the spontaneous formation of a complex film of chromium oxides and hydroxides on their surface, called a "passive layer", which protects the metal substrate from generalized corrosion and from localized attacks. This extremely thin layer (of the order of 1 to 2 ų) makes the corrosion layer negligible. The most severe types of corrosion for stainless steels are pitting, crevice corrosion and intergranular corrosion. The chemical composition of stainless steels determines their behaviour with respect to these different types of corrosion. The most influential elements are chromium (Cr), nickel (Ni), molybdenum (Mo) and copper (Cu). The low carbon content is essential to preserve the mechanical properties of fasteners that can be used at high temperatures.

IRON-CARBON-CHROMIUM ALLOYS

Carbon in hot contact with chromium creates a precipitation of chromium carbide which reduces the corrosion resistance. This reduction is reduced by using stabilisers (titanium, niobium) and by reducing the carbon content. Depending on the chemical composition of the alloy, different types of steels with different behaviour are obtained. Stainless steels can be classified into four main families, each with its own characteristics: - Austenitic stainless steels - Martensitic stainless steels - Ferritic stainless steels - Austeno-ferritic stainless steels (also called "duplex")

Austenitic stainless steels (grades A1 to A5)

These are by far the best known and most widely used. In addition to a minimum chromium content of 17%, they contain nickel (7% or more) and possible additions of molybdenum, titanium, niobium, etc. In order to reduce the susceptibility to work hardening, copper may be added. Their mechanical properties in tension are generally modest but can be, for certain grades, considerably increased by work hardening. They are, however, very suitable for cryogenic use because of their lack of brittleness at low temperatures. Their resistance to corrosion increases with the chromium and molybdenum content. Their resistance to oxidation increases with their chromium content: standards with 18% chromium resist in a non-sulphurous oxidising atmosphere up to around 800°C. Beyond that, it is necessary to move towards so-called "refractory" grades, which are much more alloyed. The introduction of stabilising elements such as titanium or niobium prevents intergranular corrosion, particularly on welds, and increases the mechanical strength at high temperatures. Grade A1 steels Grade A1 steels are specially designed for machining. Due to the high sulphur content, this group of steels has a lower corrosion resistance than steels with normal sulphur content. A2 steels A2 steels are the most commonly used steels, e.g. for kitchen equipment, appliances for the chemical industry, fasteners, etc. Steels of this group are not suitable for use in non-oxidising acids and chlorine-containing agents, such as swimming pools or sea water. A3 steels A3 steels are stabilised stainless steels with the same properties as A2 steels. Grade A4 steels Grade A4 steels, alloyed with molybdenum, are "acid resistant" and give better corrosion resistance. This grade is widely used in the cellulose industry as this grade of steel is developed for boiling sulphuric acid (hence the name "acid resistant"). It is also suitable to some extent for chlorinated environments. IA4 is also frequently used in the food and shipbuilding industries. Grade A5 steels Grade A5 steels are stabilised "acid resistant" steels with the same properties as A4 steels.

Martensitic stainless steels (grades C1 to C4)

These steels typically contain 12-19% chromium, with carbon content ranging from 0.08 to 1.2%. They may contain nickel and molybdenum as well as some additional elements such as copper, titanium or vanadium. They are often supplied in the annealed condition. It is of course recommended to use them - as well as steels for mechanical engineering - in the quenched and tempered condition, representing the best compromise between corrosion resistance and mechanical properties. They are of particular interest when the service temperature does not exceed 650°C (e.g. power generation turbines). In practice, they are used: Either after quenching and stress relieving tempering at around 200°C, which enables the maximum mechanical strength to be retained, Or after quenching and tempering at between 55° and 700°C, thus ensuring a better compromise between strength / resilience / corrosion resistance. These steels combine interesting corrosion resistance with mechanical properties equivalent to those of top-of-the-range alloy steels. They can be work-hardened for improved strength and are magnetic. Grade C1 steels Grade C1 steels have limited corrosion resistance. They are used in pumps, turbines and cutlery. Grade C3 steels Grade C3 steels have limited corrosion resistance, although better than grade C1 steels. They are used in pumps and valves. Grade C4 steels Grade C4 steels have limited corrosion resistance. They are intended for machining and are otherwise similar to C1 steels.

Ferritic stainless steels (F1 grade)

These are iron-chromium or iron-molybdenum alloys with a chromium content ranging from 10.5% to 28% and a carbon content not exceeding 0.08%. These steels generally do not contain nickel. Other additive elements - such as Ti, Nb or Zr - may be introduced to improve certain properties such as weldability, corrosion resistance or cold formability. High carbon ferritic steels (>20%) are mainly used for their outstanding corrosion resistance (superferritic) and hot oxidation resistance. Some grades alloyed with molybdenum and/or titanium have a corrosion resistance comparable to that of austenitic steels. These steels do not harden and are used in the annealed condition. They are very sensitive to grain growth at high temperatures but can be used up to 800°C in an oxidising atmosphere (some above that). At high temperatures, due to the absence of nickel, they are often more resistant to sulphurous atmospheres than austenitic steels. Their low-temperature brittleness makes them unsuitable for cryogenic applications. Contrary to popular belief, the fact that this family of steels is magnetic is in no way correlated with poor corrosion resistance. Some grades have properties in this area that are comparable or even superior to those of the most common austenitic steels. F1 steels F1 steels cannot be work-hardened normally and in some cases should not be. F1 steels are magnetic. Steels in this group are normally used for simple equipment, with the exception of "superferritic" steels which have a very low C and N content. These steels can be used in highly chlorinated environments.

DESIGNATION

The system of designating stainless steel grades and quality classes for fasteners is shown in the table below. The material designation consists of two groups of characters separated by a hyphen. The first designates the steel grade, the second the quality class. The designation of the steel grades (first group) consists of a letter which designates the steel group: A for austenitic steel C for martensitic steel F for ferritic steel This letter is followed by a number which designates the variation of the chemical composition in this steel group. The designation of the quality class (second group) consists of two numbers indicating 1/10° of the tensile strength of the fastener. Example: A2-70 Designates a cold worked austenitic steel with a minimum tensile strength of 700 N/mm 2 (700 MPa). Example: C4-70 Refers to martensitic steel, quenched and tempered, with a minimum tensile strength of 700 N/mm 2 (700 MPa). The marking of stainless steels with a low carbon content not exceeding 0.03% may be completed by the letter L - Example A4L-80