Stainless Steel

Question 1

What makes stainless steel corrosion resistant?

Answer 1

The presence of chromium (12-25%) which results in the formation of a continuous chromium oxide film.

Question 2

Apart from chromium, what are the other alloying elements in stainless steel and what do they improve?

Answer 2

Nickel and carbon which improve fabrication and strength characteristics.

Question 3

What are the most popular types of stainless steel and what are their features?

Answer 3

Austenitic AISI 304 and 316. They are nonmagnetic, tough and corrosion resistant.

Question 4

What is the most important development in stainless steel alloys?

Answer 4

The addition of modern duplex stainless steel

Question 5

What microstructure does modern duplex stainless steel have?

Answer 5

An austenitic-ferritic microstructure

Question 6

How is modern duplex stainless steel better than austenitic stainless steel?

Answer 6

It has better corrosion resistance and strength

Question 7

Why is there no such thing as an all-purpose material for corrosion resistance?

Answer 7

Because there is not just one corrosion type

Question 8

What is an example application for duplex stainless steel?

Answer 8

Industrial food processing equipment

Question 9

Why is duplex stainless steel good in food processing applications?

Answer 9

1. High salt content leads to chloride stress corrosion cracking which duplex stainless steels are better at protecting against.
2. Ease of cleaning of surfaces after possible contamination with bacteria

Question 10

What can cause SCC?

Answer 10

Mechanical stress, localised chemical attack and incorrect design of geometry of the product.

Question 11

Can you draw and describe the diagram showing Stainless steel vs 4 other materials for cleaning?

Answer 11

Draw.. After contaminating the 4 surfaces with the same population of bacteria per cm^2, abrasive cloth was used for cleaning. The figure shows that SS bacteria count was 10x less than the other 3 materials after only 5 seconds wash time, and it maintained a substantially lower count for the next 40 seconds.

Question 12

What are the reasons for using AISI 304 stainless steel in high speed trains?

Answer 12

1. High strain rate hardening capacity and ductility give a better impact performance
2. No need for anti-corrosion paint/primer
3. Structural robustness - local impact, erosion and corrosion resistance
4. No need for paint for the under-carriage locations
5. Good fire resistance due to relatively low thermal conductivity and non-combustible material
6. Material promotion reasons