Steels- Stainless Steels Flashcards
What are stainless steels?
A family of corrosion resistant steels based on a minimum of 12 wt% Cr in solid solution
What does the addition of Cr enable the steel to do?
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.
Ferritic stainless steels
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
Martensitic stainless steel
C up to 1.5wt%, Cr up to 30wt%. Good for cutlery as produces a hard cutting edge, structural components, tools, cutting devices
What is Cr (stabiliser)
A ferrite stabiliser
Austenitic stainless steel
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
Duplex stainless steel
23-30% Cr, 2.5-7% Ni+Ti or Mo.
Cheaper than austenitic
Precipitation hardened stainless steel
Allowed with V, Al, Ti and/or Nb. Used for structural members, springs
What does stainless mean?
Resistant to attack by strong acids (HNO3).
Resistant to atmospheric corrosion.
Resistant to aqueous corrosion.
Resistant to high T oxidation.
What does L mean in the name of a stainless steel?
The grade is a low carbon variant (half of normal)
Corrosion rate vs %Cr for atmospheric corrosion
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
Corrosion rate vs %Cr for high temperature oxidation
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)
What conditions are stainless steels not effective in?
Basic or reducing environments.
In the presence of all- (salt water).
Problems in stainless steels
GBs are susceptible to corrosion.
Sensitisation
What does sensitising do and give examples of sensitising heat treatments?
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
C curve for sensitisation and what it means for cooling rate
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.
%Cr profile for region around a Cr23C6 particle
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
Solutions to sensitisation
- Heat treatment at 1050-1100C followed by water quench.
- Reduction in C concentration.
- Use of stabilising elements (Ti and/or Nb) for stabilisation
How does stabilisation work?
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.
Traditional and latest stabilisation treatments
Traditional: Cr, 0.1C, 0.4Ti. Makes α(Cr)+TiC.
Latest: Cr, 0.02C, 0.1Ti, 0.3Nb. Makes α(Cr)+(Nb, Ti)C
Problem with using Ti alone for stabilising treatment
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)
Advantage of stabilising with Nb
Very few surface defects form so Nb recovery in steelmaking is around 98%.
