stainless steels

Question 1

what are stainless steels

Answer 1

chemically resistant materials that should
withstand the corrosive influence of weathering, chemical substances or hot conditions for as long as possible times

Question 2

how do we obtain a stainless steel

Answer 2

✓surface films form as a result of
chemical attack which reduce further corrosion
✓Cr contents above ≈12% promote this passivating effect on the Fe matrix
✓ During high-temperature corrosion, Cr also plays a decisive role in the barrier effect providing a thicker surface film of scale by forming spinels FeCr2O4 and mixed oxides of type (Cr,Fe)2O3.

Question 3

what does the Schaeffler diagram shows

Answer 3

Schaeffler diagram shows the alloy constituents found in the solution-annealed and quenched state of steels.
it is adopted to predict constituents in stainless steels (after welding) based on their composition

Question 4

in a stainless steel a uniform surface film can only develop if ….

Answer 4

if the effective elements are evenly distributed in the microstructure, i.e. fully dissolved to form a homogeneous solid solution

Question 5

effect of dissolved C and N in stainless steel

Answer 5

Dissolved C and N lead to reprecipitation of Cr-rich phases on heating during manufacturing or whilst in service,
which has an adverse effect on chemical resistance and toughness of the alloy

Question 6

austenitic vs ferritic steels

Answer 6

• ferrite has slightly higher yield strenght
• austenite undergoes a higher degree of workhardening and thus has a higher tensile strength
• comparable resistance to scaling in air
• austenitic have higher thermal expansion
• austenite is more closely packed and its resistance to diffusion-controlled creep is reflected by a higher hot strength and creep resistance

Question 7

3 types of corrosion

Answer 7

generalized corrosion, pitting, intergranular corrosion

Question 8

talk about pitting and intergranular corrosion

Answer 8

✓ Pitting is caused by mechanical or chemical damage of the Cr-based passivating layer
✓ Intergranular corrosion is due to a local drop of the amount of Cr below that required for passivation at GBs

Question 9

how can we improve resistance against localized corrosion

Answer 9

increasing the Cr content and by adding Mo and N

Question 10

how can we avoid intergranular corrosion due to fabrication steps

Answer 10

1. reduce C content (ELC = extra low carbon grades)
2. Binding of the C to more stable carbides. This is achieved by alloying (stabilizing) with hyper-stoichiometric amounts (Ti >7xC or Nb > 12xC) to suppress the formation of chromium-rich GB carbides
3. Increasing the chromium content
4. Subsequent solution annealing (for austenitic stainless steels: about 1100°C solution annealing and water quenching)

Question 11

issues of stainless steel (4 punti)

Answer 11

✓ The higher thermal expansion (and shrinkage) coefficient of austenitic grades makes them more prone to hot
craking during welding and casting
✓ Fe-Cr alloys (mainly ferritic and duplex grades) can rapidly form a brittle phase
✓ Single phase ferritic steels are also very sensitive to grain growth
✓ In martensitic steels cold cracking if the fresh martensite can be a matter of concern for weldings

Question 12

ferritic stainless steel

Answer 12

✓ The austenitic phase field is preserved at high temperatures by C, N and Mn, so that at least partial transformation into gamma phase takes place at high temperature.
✓ This effect is desirable because the austenite fraction slows down ferrite grain growth during processing

Question 13

martensitic stainless steels

Answer 13

✓ Cr can contract the austenite field, this effect can be overcomed adding austenite-stabilising elements, such as C or Ni and reducing Cr
✓ The hardenability increases with the C content
✓ Owing to the deeper hardness penetration due to Cr, these steels can be hardened in air if they are not too thick
✓ Low temperature tempering treatments become feasible in these steels owing to relatively low cooling rates
✓ If elevated tempering temperatures are required to improve toughness, we must take into account that chromium participates in temper carbides above 400°C. Chromium depletion starts to reverse above 600°C, and localized corrosion disappears again.
✓ Martensitic stainless steels should therefore be tempered over 600°C during Q&T treatment so that the carbon content is almost completely precipitated as carbide and the matrix chromium content becomes uniform

Question 14

austenitic stainless steels

Answer 14

✓ austenitic microstructure requires 17 to 18% Cr for the lowest possible nickel content, i.e. the lowest costs
✓ Increasing the Cr equivalent to improve the resistance to pitting requires a higher Ni equivalent to avoid excessive ferrite

Question 15

Ferritic-austenitic stainless steels (Duplex)

Answer 15

Duplex steels contain almost equal proportions of ferrite and austenite
✓ The combination of ferrite and austenite promotes a kind of “composite effect” and increases the yield strength to 450 – 550 MPa (significantly higher than that of the two individual constituents (austenite and ferrite))
✓ heat treatment consists of a solution annealing at 1000 to 1150°C and accelerated cooling, usually in water, to prevent precipitates and keep the right balance of the two constituents
✓ The pitting resistance of the basic grade X2CrNiN23-4 is increased by Mo or Cr
(similar to austenitic steels)

Question 16

why do we use stainless steel in veichles

Answer 16

✓ Corrosion resistance and fire resistance
✓ Crashworthiness (increasing strength and impact energy absorption at high strain rates: strain-rate sensitivity)
✓ Lightweight design (not for density but thanks to excellent weldability and formability: thin sheets, smaller geometric details)
✓ Aesthetical aspect (shiny surfaces, no need of painting, easy cleaning and washing)
✓ Good combination of strength and ductility suitable for a number of mechanical parts in chassis, suspensions, fuel tank, exhaust pipes, catalytic converters, …

Question 17

why do we use stainless steel in trains and for which components

Answer 17

Stainless steels are used in railway applications because they are resistant to corrosion, easily fabricated and offer good
mechanical properties.

• Carriage door skins, skins for carriages and locomotives
  (austenitic grades)
• Carriage chassis and structural components (austenitic
  grades)
• Freight wagon chassis and structural components (ferritic
  grades)
• Under-train components e.g. tubes for compressed air and
  water systems, battery boxes, etc

Question 18

why do we use stainless steels for ships

Answer 18

Stainless steel is a crucial material in the marine industry
-durability,
-resistance to corrosion
-strength.
-very durable material (can withstand the harsh conditions of the marine environment)

• Not so often considered as a candidate for ships’ hulls
• Duplex stainless steels are frequently chosen for chemical tanker linings, not only for their superior corrosionresistance but also for their high yield strength
• For ships’ deck and cabin fittings and for offshore oil and gas platforms, highly-alloyed molybdenum-containing
  grades of stainless steel are preferred
• Shore-based structures (harbour railings and gratings, catwalks) are made from a wide range of austenitic, superaustenitic, super-ferritic or super-duplex stainless steels.
• Boat propellers and related shafts as well as deck components for boats and ships such as deck eyes, brackets for
  anchor ropes, housings for equipment, shackles, handrails, are generally made with austenitic grades

Question 19

why do we use stainless steel in electric veichles

Answer 19

used especially for the battery housing: the battery modules must be protected from environmental influences such as corrosion, extreme temperatures and deformation (crashes)

• Ideal characteristics for battery-pack housing are lightweight, formability, heat resistance
• Crash requirements is also a significant challenge, especially where intrusion space is low, such as with side and underfloor intrusion.
• The combination of high strength and enormous ductility gives austenitic stainless steels outstanding deepdrawing properties. These facilitate complex forms and contribute directly to component stiffness.