Stainless steel was discovered in 1913 by Harry Brearley, who noticed that a 13% chromium steel sample had not corroded after several months of experimentation. However, the full history is more complex. Stainless steel is defined as containing at least 10.5% chromium, which forms a passive oxide layer on the surface to prevent corrosion. Common types include ferritic, austenitic, martensitic, and duplex stainless steels, which differ in their crystalline structures and alloying elements. Austenitic stainless steels like 304 and 316 are the most widely used grades.

2. DICOVERY OF STAINLESS
STEEL
 There is widely held view that stainless steel was
discovered in 1913 by Sheffield metallurgist Harry
Brearley.
 He was experimenting with different types of steel
for weapons and notice that a 13% chromium steel
had not corroded after several months. However,
the picture is much more complex that this.

3. 3
•Stainless steels are defined high alloys steel which contain at least 10.5%
chromium.
•The thin but dense chromium oxide film which forms on the surface of a
stainless steel provides corrosion resistance and prevents further oxidation.
(Chromium produces a thin layer of oxide on the surface of the steel
known as the 'passive layer'. This prevents any further corrosion of the
surface. Increasing the amount of Chromium gives an increased
resistance to corrosion.)
• Stainless steel also contains varying amounts of Carbon, Silicon and Manganese
•Varying amounts of other alloying elements like Ni, Mo, V,Ti, Ni, etc may be
added to obtain certain specific property
Introduction

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Stainless steels are also classified by their crystalline structure:
1.Ferritic stainless steel
2.Austenitic stainless steel
3.Martensitic stainless steel
4.Duplex stainless steel
5.precipitation hardening stainless steel

5. 5
Ferritic stainless steel
1. chromium generally 11-20%
2. Low carbon and nickel content
3. Good corrosion resistance
4. limited weldability and toughness
5. Magnetic
6. High ductility
7. Ferritic stainless steels have a body-centered cubic crystal system
8. Due to their lower chromium and nickel content, standard ferritic
steel grades are usually less expensive & less corrosion resistance
than their austenitic stainless steel.
Application: boiler, washing machine, Automotive trim, exhaust system,
etc

6. 6
Austenitic stainless steel
• a minimum of 16% chromium
• maximum of 0.15% carbon
• sufficient nickel and/or manganese to retain an austenitic structure at all
temperatures
• non magnetic
• 200 and 300 series steel are austenitic stainless steel
• Austenitic stainless steels have an austenitic crystalline structure, which is a face-
centered cubic crystal structure
•More than 70% stainless steel are austenitic
•They can experience stress corrosion cracking (SCC)
Applications: food and beverage processing equipments, kitchen sink, chemical
industries, Cookware etc.
Examples-:
▪ 304 :- (most widely used stainless steel, also known as A2 steel, used
as tool steel) (18/8) i.e. 18% Cr and 8% Ni
▪ 316 :- marine grade steel, (18/10)

7. 7
Martensitic stainless steel
Martensitic stainless steel contains chromium (12–14%),
molybdenum (0.2–1%), nickel (less than 2%), and carbon (about
0.1–1%) (giving it more hardness but making the material a bit
more brittle).
 Martensitic stainless steels are not as corrosion-resistant as the
other two classes but are extremely strong and tough, as well as
highly machinable, and can be hardened by heat treatment.
 It is quenched and magnetic.
 body-centred tetragonal martensite microstructure
 Applications :- blade, knife, pin, surgical instrument,

8. Duplex stainless steel
• Duplex stainless steels are called “duplex” because they have a two-phase
microstructure consisting of grains of ferritic and austenitic stainless steel.
• higher strength than either ferritic or austenitic steels.
• Greater corrosion resistance particularly stress corrosion cracking
• (15-30)% Cr, (up to 5%) Mo, small amount of Ni
• S3 series stainless steel
• magnetic
Duplex S.S.
Application :- Pressure Vessels, Storage Tanks, Structural Applications (e.g. bridges),
Lean Duplex
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Standard Duplex
Super Duplex

9. Precipitation hardening stainless steel
•These steels can develop very high strength by adding elements such as
Copper, Niobium and Aluminium to the steel.
• With a suitable “aging” heat treatment, very fine particles form in the matrix
of the steel
which imparts strength.
• non magnetic
• good corrosion resistance
• high toughness and strength
• provide an optimum combination of the properties of martensitic and
austenitic grades.
Application :- aircraft parts, shaft, spindle, pump
Example:- The most popular precipitation hardening steel is 17-4 PH. The name comes
from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3%
Niobium. (17-4 PH is also known as stainless steel grade 630), which transform to
martensite at low temperatures, typically around 250°C, and are further strengthened by
ageing at between 480 and 620°C.
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