stainless steels

Question 1

main principles stailess steel

Answer 1

• High corrosive resistance
• based on a film that protect the below material, passivating effect
• During high temperature corrosion Cr also plays a role in the barrier effect

Question 2

Problems with temperature and corrosion

Answer 2

• dissolved C and N lead to reprecipitation of Cr-rich phases on heating that has an adverse effect on the chemical and mechanical resistance of the alloy.
• Most of the stainless steels have a small amount of C and N but heating in the temp. range in which chromium diffusion takes place leads to susceptibility to intergranular corrosion.

Question 3

austenitic vs ferritic

Answer 3

• ferritic phase has a higher YS but austenite has an higher UTS
• Same resitance to scaling in air
• Better resistance of adhesion of scale in ferritic
• at high temp. is better the austenite due to higher hot strength and creep resistance

Question 4

pitting and intergranular corrosion

Answer 4

Pitting is caused to mechanical or chemical damage on the surface damaging the passivating layer
Intergranular corrosion is caused by a local drop of the amount of Cr below the minimum value for the passivating effect

Question 5

Improvements against corrosion

Answer 5

The resistance is improved by increasing the percentage of Cr Mo and N

Question 6

how to avoid corrosion during welding or anneling

Answer 6

• Low C content
• Binding C with a more stable carbides
• Solution annealin
• Increasing the Cr content

Question 7

issues of stainless steel

Answer 7

• Higher thermal expansion and shrinkage coefficient of austenitic grades
• Fe-Cr alloys can rapidly form a brittle sigma phase
• Single phase ferritic steels are also very sensitive to grain growth
• In martensitic steels cold cracking are frequently during welding

Question 8

ferritic stainless steel

Answer 8

• austenite phase is preserved at high temp by C N Mn, austenite slows down ferrite grain growth

Question 9

Martensitic stainless teel

Answer 9

• Contraction of the austenite field by Cr can be overcome by adding austenite stabilizer elements as C Ni and reducing Cr.
• hardenability increases with the C content
• Owing to the deeper hardness penetration due to Cr these steels can be hardened in air
• tempering at <200°C keeps the hardness high
• if elevated tempering temp. are required must be taken into accout that chromium start participates in carbides above 400°C
• Martensitic should be tempered over 600°C beacuse the carbon content precipitates as carbide and the chromium content becomes uniform

Question 10

ferritic-austenitic stainles steels

Answer 10

• The combination of ferrite and austenite increase YS
• The heat treatment consist in a solution of annealing and accelerated cooling

Question 11

Application of stainles steels

Answer 11

• Chemical equipment
• For human enviroments
• Food handling
• Car
• Ships
• Train

Question 12

Keypoints of stainles steels

Answer 12

• Corrosion resistance
• Crashworthiness
• Lightweight design not for density but thanks to weldability and formability
• Aestethical aspect
• Good combination of stregth and ductility

Question 13

electric vehicles

Answer 13

• stainless steel is perfect for battery housing
• crash requirements is also a significant challenge
• The combination of high strength and enormous ductility made stainless steel very suitable for this application

Question 14

chemical equipment

Answer 14

• materials shall undergo passivation
• unifrom corrosion and pitting, need high PREN
• corrosion and wear need martensitic steels even with coating with hard facing alloys.

Question 15

PREN

Answer 15

• is an equation that express the resistance against corrosion
• Pitting Resistant Equivalent Number