Stainless steels

Question 1

Characteristics

Answer 1

> contain minimum of 11.5 wt% Cr in solid soln
thin film of chromium oxide on free surfaces, passivating the steel
same density and young’s modulus as plain carbon steels, high electrical resistivity, low thermal conductivity

Question 2

Effects of alloying elements on phase stability

Answer 2

> C, Ni, and Mn are austenite stabilizers and expand gamma loop
Cr is ferrite stabilizer
depending on amounts of alloying elements, stainless steel can be austenitic, ferritic, martensitic or duplex

Question 3

Types of stainless steels

Answer 3

1. austenitic: 3XX or 2XX
2. martensitic: 4XX or 5XX
3. ferritic: 4XX
4. precipitation hardenable: 6XX
5. duplex

Question 4

Precipitation hardenable SS

Answer 4

> high strength and corrosion resistance
strengthening is not from carbon or martensite, but from gamma prime precip like Ni3Ti and Ni3Al
Ni3Ti, Ni3Al, Ti and Mo contribute to strength

Question 5

Duplex SS

Answer 5

> equal amounts of ferrite and austenite
17-30wt% Cr, 3-13wt% Ni and Mo for pitting corrosion resistance
great ductility, toughness and strength
as temp v, strength ^, but ductility doesn’t ^
high rate of work hardening prevents necking and gives higher ductility
comp: C: 0.02 %; Cr: 26%; Ni: 4.7%; Mo: 1.4%; N: 0.2%