Categories of Stainless Steel
Stainless Steels are broadly categorized in five categories namely Austenitic, Ferritic, Martensitic, Duplex and Precipitation hardening stainless steel grades. These categories are defined based on the atomic structure and alloying elements resulting in range of properties required for various end use. Please see the pictorial representation of various grades in the range of increasing chromium and nickel content.
Austenitic Stainless Steels
Austenitic is the most widely used type of stainless steel. It has excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. These are popularly known as 300 and 200 series grades which are non-magnetic in nature. 200 Series grades are alloyed with chromium, nickel, and manganese where 300 Series are alloyed with chromium and nickel.
When nickel (Ni) is added to stainless steel in sufficient quantities (also manganese and nitrogen in case of 200 series grades) the crystal structure is changed from ferrite to austenite, hence the term austenitic stainless steels. The basic composition of the austenitic stainless steels is 18% Cr and 8% Ni. This is Grade 304, sometimes loosely referred to as 18/8 or 18/10. If additional corrosion resistance is required 2-3% molybdenum (Mo) is added, termed Grade 316. The carbon content of almost all stainless steels is low (often 0.08% C max). There are also 'low carbon' or 'L' grades (0.03% C max) and stabilised grades alloyed with titanium (Ti) or niobium (Nb) to prevent a form of corrosion occurring in the region next to the weld in welded structures.
Grade 304 is easy to form and weld and is readily brake or roll formed into a variety of components for applications in the industrial, architectural, and transportation fields. Apart for 304 there are various other grades in 300 and 200 series which are selected based on corrosion resistance and high temperature requirements. A tree of popular 300 series grades are depicted below.
Some of the basic properties of austenitic stainless steels are:
- Excellent corrosion resistance.
- Excellent cleanability and hygiene factor.
- Fabricated and formed with ease.
- Excellent weldability.
- Hardened by cold work, not by heat treatment.
- Usually used in the fully annealed condition in which they are essentially non-magnetic.
- Ability to handle both extremely low (cryogenic) temperatures and, depending on the load and permissible distortion, higher service temperatures of around 600°C – or even higher if scaling resistance is the only consideration.
Ferritic Stainless Steel
These are nickel-free stainless steels. They have a varying chromium (Cr) content of 12- 18%, but a lower carbon (C) content than the martensitic. These types of stainless steels such as 409, 430, 439, 441 are popular grades and suitable for a wide range of application in sugar industry. Some of these grades offer high wear resistance whereas some of them offers higher corrosion resistance.
Ferritic steel is generally easy to form and machine for thinner gauges and is commonly used in automotive exhaust systems, appliances, boilers, cooking utensils, and indoor architecture. The most commonly used ferritic steel is type 430 and in some applications, this grade can be used as a replacement for austenitic grade 304. Type 430 is often found in washing machine drums, kitchen sinks, cutlery, indoor panels, dishwashers and other cooking utensils.
Basic properties of ferritic stainless steels:
- Moderate to good corrosion resistance which increases with the chromium content.
- Magnetic, non-hardenable and always used in the annealed condition.
- Weldability is poor which generally limits their applications as welded components to thin gauge material.
- More difficult to form (i.e. bend, draw, etc) than the austenitics.
There are many grades of stainless steels in all these categories which are suitable for various equipment's in sugar industry based on their alloying elements and required properties.
Martensitic Stainless Steels
Martensitic stainless steels are very hard and strong, though it is not as resistant to corrosion as austenitic or ferritic grades. It also classified as the 400 Series and contains approximately 13% chromium. Martensitic steel was developed to be hardened by heat treating for applications where hardness, strength, and wear resistance are required. Martensitic steel is used to make knives, fasteners, surgical equipment, and turbine blades.
The common specifications are 410, 420, 431 and CA-6NM.
Basic properties of the martensitic stainless steels include:
- Moderate corrosion resistance.
- Hardenable by heat treatment and therefore high strength and hardness levels can be developed.
- Very limited weldability Common uses include applications which need strength and hardness such as knife blades, surgical instruments, fasteners, spindles, nozzles, shafts, impellers and springs.
Generally, martensitic stainless steels are available in forms such as bar, strip and castings.
Duplex Stainless Steels
Duplex stainless steels have a structure of approximately equal amounts of ferrite and austenite and, therefore, may be referred to as ferritic-austenitic stainless steels. The chromium content varies from 18-28%. The nickel content of 4.5-8% is insufficient to develop a fully austenitic crystal structure. Most grades contain molybdenum in the range 2.5-4% plus a small nitrogen addition which enhances both strength and pitting resistance.
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.
Basic properties of the duplex stainless steel include:
- A mixed ferritic-austenitic, i.e. duplex, crystal structure which results in a high resistance to stress corrosion cracking.
- An increased level of passivity due to higher Cr, Mo and N
- Good weldability and formability.
- Higher tensile and yield strengths (compared with austenitic and ferritic stainless steels).
Common uses include applications such as heat exchanger panels and tubes, materials handling equipment, tanks and vessels where high chloride concentrations are present, e.g. sea water cooling, desalination, foodpickling plants and aggressive mine waters.
Precipitation hardening Stainless Steel
Martensitic or semi-austenitic steels can also be classified as precipitation hardening stainless steels. These steels are made to achieve enhanced properties with the addition of elements such as aluminium, copper and niobium.
Precipitation hardening stainless steels have their compositions formulated so that they can be supplied in the solution treated condition, in which they are machinable. Following fabrication operations, these steels can be hardened by a single ‘ageing’ treatment. This is at a fairly low temperature and can, therefore, be done without distorting the product. These grades are therefore ideal for the production of long, straight high strength shafts for pumps and also valve spindles.
The most common grade is 17-4PH (S17400or '630' or 1.4542) – a martensitic precipitation hardening grade. High strength wire can be produced in grade 17-7PH(S17700 or '631' or 1.4568).
The family of precipitation hardening stainless steels includes martensitic, semi austeniticand austenitic types. The 17-4 type, referred to above is a martensitic type.
These steels can be welded but this, necessarily, requires attention to the heat treatment because of possible changes that can occur during welding.