Steels- Stainless Steels

Question 1

What are stainless steels?

Answer 1

A family of corrosion resistant steels based on a minimum of 12 wt% Cr in solid solution

Question 2

What does the addition of Cr enable the steel to do?

Answer 2

Passivate. E.g formation of thermodynamically stable oxide relative to that of the metal. A layer of Cr2O3 (chromia) forms on the surface. This is very thin and transparent and if scratched more forms.

Question 3

Ferritic stainless steels

Answer 3

C<0.1wt%, Cr up to 30wt%. High yield strength, low work hardening rate, less expensive (no Ni), good formability. Good for automotive or anything you don’t need to be that strong, atmospheric corrosion, elevated temperatures, decorative

Question 4

Martensitic stainless steel

Answer 4

C up to 1.5wt%, Cr up to 30wt%. Good for cutlery as produces a hard cutting edge, structural components, tools, cutting devices

Question 5

What is Cr (stabiliser)

Answer 5

A ferrite stabiliser

Question 6

Austenitic stainless steel

Answer 6

Low C, Cr up to 18wt%, Ni (γ stabiliser) up to 12wt%. Strong but more expensive, chemical resistance, tanks, piping. Low yield strength, high work hardening rate, expensive due to Ni content, excellent formability

Question 7

Duplex stainless steel

Answer 7

23-30% Cr, 2.5-7% Ni+Ti or Mo.
Cheaper than austenitic

Question 8

Precipitation hardened stainless steel

Answer 8

Allowed with V, Al, Ti and/or Nb. Used for structural members, springs

Question 9

What does stainless mean?

Answer 9

Resistant to attack by strong acids (HNO3).
Resistant to atmospheric corrosion.
Resistant to aqueous corrosion.
Resistant to high T oxidation.

Question 10

What does L mean in the name of a stainless steel?

Answer 10

The grade is a low carbon variant (half of normal)

Question 11

Corrosion rate vs %Cr for atmospheric corrosion

Answer 11

Starts high for a mild plain C steel (no Cr). Convex curve (steep) down until 3% Cr then corner to horizontal until 6% Cr, then convex curve down to x axis at 10% Cr

Question 12

Corrosion rate vs %Cr for high temperature oxidation

Answer 12

Similar shape but scale of x axis changes to need higher %Cr. Starts high for mild steel. Convex curve (steep) down through 5% (Cr steel), through 10% (used for pressure vessels in nuclear power industry), then nearly horizontal for corrosion resistant steel until 15%, then curves down and inflexion to convex before meeting x-axis at over 20% Cr (oxidation resistant steel)

Question 13

What conditions are stainless steels not effective in?

Answer 13

Basic or reducing environments.
In the presence of all- (salt water).

Question 14

Problems in stainless steels

Answer 14

GBs are susceptible to corrosion.
Sensitisation

Question 15

What does sensitising do and give examples of sensitising heat treatments?

Answer 15

Results in the formation of Cr23C6 on GBs. This takes up a lot of Cr from nearby steel meaning that there are regions along GBs with less than 12wt% Cr in SS. Lowering the C content (as in L) reduces this precipitation. Temperatures and times to induce intergranular corrosion are called sensitising heat treatments.
Austenitics: 400-850C
Ferritics: >925C

Question 16

C curve for sensitisation and what it means for cooling rate

Answer 16

Is a TTT diagram (or CCT). Left of c curve is alpha (Cr, C) and right is α+Cr23C6. Cooling through the C-curve (continuous cooling) will result in Cr23C6 forming at the GBs. Faster cooling reduces this problem.

Question 17

%Cr profile for region around a Cr23C6 particle

Answer 17

The carbide particle itself has >70% Cr. Approaching it from further away is constant level of Cr (>12%) but close to it curves down steeply and meets particle at a much lower %Cr (<12%). Mirror image either side of Cr23C6 particle

Question 18

Solutions to sensitisation

Answer 18

1. Heat treatment at 1050-1100C followed by water quench.
2. Reduction in C concentration.
3. Use of stabilising elements (Ti and/or Nb) for stabilisation

Question 19

How does stabilisation work?

Answer 19

Alloy with other elements like Nb or Ti as well. These form more thermodynamically stable carbides than Cr does (higher C curves). This means they form first and use up the carbon before Cr23C6 can form.

Question 20

Traditional and latest stabilisation treatments

Answer 20

Traditional: Cr, 0.1C, 0.4Ti. Makes α(Cr)+TiC.
Latest: Cr, 0.02C, 0.1Ti, 0.3Nb. Makes α(Cr)+(Nb, Ti)C

Question 21

Problem with using Ti alone for stabilising treatment

Answer 21

Ti also combines with O2 to form TiO2 particles. Also forms Ti4C2S2 and TiN which are all surface defects and spoil the surface finish (doesn’t look as nice). Ti recovery in steelmaking is only 70-85% (rest used in defects)

Question 22

Advantage of stabilising with Nb

Answer 22

Very few surface defects form so Nb recovery in steelmaking is around 98%.