Stainless steel is divided into five major categories such as namely Austenitic, Ferritic, Martensitic, Duplex and Precipitation hardening stainless steel grades. Nature depends on types of atomic bonds, Mixtures of alloys and typical end-use of materials. Chromium and nickel content is a deciding factor in grades of stainless steel.Role of Chromium and Nickel can be easily understood by the given diagram:
Austenitic Stainless Steels:
This is the most widely used stainless steel category with excellent heat resistance and high corrosion properties. This category also has excellent mechanical properties mover wide ranges of temperatures.
Two major series that come under these categories are series 200 and 300. Both the series are non-magnetic, particularly series 200 is alloyed with chromium, Nickel and manganese whereas Series 300 is alloyed only with nickel and chromium. Now difference which makes austenitic from ferritic is Nickel. Because when Nickel has added its crystalline structure changes from ferritic to austenitic. The most widely austenitic steel used is Grade 304 with 18/8 where 18% is chromium and 8% is nickel with 0.08% carbon maximum in all range of steel in this category.
When additional corrosion resistance is required then 2-3% molybdenum is added then it becomes 316. furthermore, if less carbon is needed (Less than 0.03% Maximum carbon) then it becomes 316L and this is true for all ‘L’ categories.Properties of Austenitic Stainless steel:· High corrosion resistance.· Easy Cleaning.· Fabrication is easy.· Excellent weldability.· Usually used in the fully annealed condition under non-magnetic conditions.
Austenitic Stainless steel Grades are : 301,301L,301N,302Hq,303,303se,304,304L,304H,310,310L,310H,316,316L,321H,347,904L.
Ferritic Stainless Steel
Popularly known as Nickel-free stainelss steel. With a low carbon content, they have chromium in a varying range of 12% to 18 %. Grades comes under this categories are 409,430,439,441. High wear resistance and high corrosion resistance are properties of these grades. Most commonly used grade is stainless steel 430 which may be used as the replacement of grade 304.
Applications: automotive exhaust systems, appliances, boilers, cooking utensils, and indoor architecture, Sugar Industry, washing machine drums, kitchen sinks, cutlery, indoor panels, dishwashers, and other cooking utensils.
Properties of Ferritic Stainless steel:
· Moderate corrosion and can be increased with high chromium content.
· Non-hardenable and widely used in the annealed condition.
· Low weldability
· Bending and drawing are difficult.
Martensitic Stainless Steels:
This category belongs to very strong and Hard types of stainless steel or popularly known as series 400. This is series contains approximately 13% of chromium which increases the hardenability of a material. This material is hardened by a heat treatment process where strength and hardness are required. Grades under this category are 410,420,431 and CA-6NM
Properties of the martensitic stainless steel include:
· Low to Moderate corrosion resistance.
· Hardenable by heat treatment
· High strength and hardness levels can be developed.
· Very limited weldability Common uses include applications that need strength and hardness such as knife blades, surgical instruments, spindles, fasteners, nozzles, shafts, impellers, and springs.
Martenistic Stailness steel grades are: 410,416,420,431,440A,440B,440C.
Duplex Stainless Steels:
This category is a mixture of Austenitic and Ferritic stainless steel so it can be referred to as ferritic-austenitic stainless steel. Chromium is available in a range 18-28% and nickel from 4.5-8% which gives both hardenability and corrosion resistance property to Duplex Stainless steel.
Duplex steels are both strong and flexible and are used in the paper, pulp, shipbuilding, and petrochemical industries. Newer duplex grades are being developed for a broader range of applications
· high resistance to stress corrosion cracking.
· An increased level of passivity
· Good weldability
· Higher tensile and yield strengths
Common uses include applications such as heat exchanger panels and tubes, tanks, materials handling equipment, and vessels where high chloride concentrations are present, e.g. seawater cooling, desalination, food pickling plants, and aggressive mine waters
Duplex Stainless steels grades are : 2101,2304,2205,2507 and 2507Cu.