Cr-Mn grades, Cr-Ni grades, Cr-Ni-Mo grades, high performance austenitic grades and high temperature austenitic grades. The austenitic grades have good to excellent corrosion resistance, good formability and weldability. Their impact strength at low temperatures is often exploited in cryogenic applications. Due to their austenitic microstructure, the grades are non-magnetic in the solution annealed condition. Cold working increases their strength and certain grades are therefore supplied in the temper rolled condition, which may lead to them becoming magnetic due to the presence of some martensite. Read more about the five subclasses:
The Cr-Ni grades are general purpose grades , which are mainly alloyed with chromium and nickel, with the exclusion of molybdenum. These grades are sometimes referred to as 18-8 type of stainless steels, indicating the approximate chromium and nickel content respectively.
In order to improve the strength and machinability, some of the grades are alloyed with either nitrogen or sulphur. There are also stabilized grades where titanium or niobium is added to increase the mechanical properties at high temperatures by the formation of hardening carbides. Contrary to earlier practice where titanium and niobium stabilisation was also used to avoid detrimental carbide precipitates on welding, for modern low-carbon Cr-Ni steels this is not necessary.
In the Cr-Mn grades are also referred to as “200-series” grades. Following the AISI/ASTM nomenclature, their nickel content is decreased and the austenitic microstructure is maintained by replacing some of the nickel with manganese and nitrogen. The chemical composition of grade Core 201/4372 is around 17 % Cr, 4 % Niand 7 % Mn. This grade has almost the same formability, corrosion resistance and weldability as grade 4301, but with higher strength.
These general purpose grades, have an increased corrosion resistance owing to alloying with molybdenum (2–3 %), and are sometimes referred to as “acid-proof” type of stainless steels. The chromium content is around 17 % and the nickel content 10–13 %.
Our grades are also available to be alloyed with nitrogen to improve the strength, or with sulphur to improve machinability. Our offering also includes stabilized grades where titanium or niobium is added to increases the mechanical properties at high temperatures by the formation of hardening carbides. Contrary to earlier practice where titanium and niobium stabilisation was also used to avoid detrimental carbide precipitates on welding, in modern, low carbon Cr-Ni-Mo steels this is not necessary.
High performance austenitics
The high performance austenitic stainless steels were developed for use in very demanding environments and have even higher alloying content. The chromium content varies between 17 and 25 %, nickel between 14 and 25 % and molybdenum between 3 and 7 %. Our grades are also available to be alloyed with nitrogen to further increase the corrosion resistance and strength. Some grades are alloyed with copper to increase the resistance to certain acids.
The austenitic grades Ultra 254 SMO and Ultra 6XN are sometime referred to as 6Mo superaustenitic grades, and Ultra 654 SMO as a 7Mo superaustenitic grade.
High temperature austenitic grades
The high temperature austenitic stainless steels are designed primarily for use at temperatures exceeding 550 °C, i.e. in the temper- ature range where creep strength is the dimensioning factor. The compositions of these steels are designed to provide a long service life in dry gases at high temperatures (800–1150 °C), i.e. good oxidation resistance rather than resistance to aqueous corrosion.
The high temperature austenitic grades are characterised by high chromium (17–25 %) and high nickel (8–20 %) content with the exclusion of molybdenum. Silicon is added in some grades to increase the oxidation resistance. The Outokumpu Therma MA grades are alloyed with silicon and also with cerium to further increase the resistance to oxidation, and with nitrogen to improve the creep strength.
What is austenitic stainless steel?
Austenitic stainless steels benefit from very high ductility and toughness. When delivered in the quench-annealed state they are very soft and formable, and when delivered as cold stretched or temper rolled they have increased hardness and strength. The strength of austenitic steel grades is reinforced with increased levels of carbon, nitrogen and, to a certain extent, molybdenum.
In general, austenitic stainless-steel grades have excellent weldability. They do not require post-weld treatment and can be readily welded by a full range of conventional welding methods, including:
- Shielded metal arc welding (SMAW)
- Gas tungsten arc welding, TIG (GTAW)
- Gas metal arc welding, MIG (GMAW)
- Flux-cored arc welding (FCAW)
- Plasma arc welding (PAW)
- Submerged arc welding (SAW)
Good corrosion resistance
Standard austenitic stainless steels are general purpose steels with good corrosion resistance to atmospheric corrosion and to many organic and inorganic chemicals. This makes them well suited for processing, storing and transporting foodstuffs and beverages and many other applications.
Outokumpu austenitic steels
Outokumpu austenitic stainless steels include the workhorses grades Core 304/4301 EN 1.4301 (ASTM 304) and Core 304L/4307 EN 1.4307 (ASTM 304L) as well as enhanced grades for improved corrosion resistance.
The latest addition to Outokumpu austenitic stainless steel grades is the Supra 316plus.
Austenitic Outokumpu Supra 316plus (EN 1.4420, ASTM UNS S31655) is designed for highly corrosive environments. Its high chromium and nitrogen contents deliver improved corrosion resistance. The grade has high strength, good formability and excellent weldability. Supra 316plus works extremely well for challenging conditions.
Supra 316plus enables lighter structures, which allows for increased transport load volumes resulting in a decreased total number of transports needed. These factors can even reduce the fuel consumption, thus Supra 316plus leaves a lower carbon footprint.
The high chromium content makes the Supra 316plus appropriate for replacing standard grades in many applications. Supra 316plus is ideal for a variety of applications including LNG, tank containers, heat exchangers, water treatment and tubular projects. The grade also works well in architectural applications.
Low-Ni and low-Mo are cost efficient alternative for traditional molybdenium alloyed stainless grades. Outokumpu Supra 316plus austenitic stainless steel with its high chromium content delivers high strength, good formability and excellent weldability. These qualities allow for use in a variety of applications including heat exchangers, water treatment and piping as well as in architectural applications such as indoor and outdoor facades. Outokumpu Supra 316plus is an austenitic CrNiMo stainless steel with 21 wt.-% chromium and relatively low nickel and molybdenum content. Due to its high nitrogen content, Supra 316plus has increased mechanical strength and shows a high degree of work hardening on mechanical deformation. Due to its high chromium content, Supra 316plus offers similar or better corrosion resistance than austenitic CrNiMo standard grades in many corrosive environments. Supra 316plus is used in applications where increased corrosion resistance and a combination of high mechanical strength and good formability are needed. Due to its tendency to work hardening, Supra 316plus can absorb an increased amount of energy during deformation. It can be delivered in temper rolled condition with different strength levels.
The high-chromium austenitic Supra 316plus is acid-resistant and suitable for example in:
- Tanks and containers
- Road tankers
- pulp and paper
- water treatment applications