Alloy steels and heat resistant steels

Question 1

when do we use unalloyed steels

Answer 1

for general engineering applications such as mechanical, automotive, plant and structural frames.
in these applications cost has to be the lowest possible

Question 2

how can we strengthen pure iron (which is soft)

Answer 2

✓ C and N as interstitial atoms
✓ Cold- or warm-working (=workhardening) (increases dislocation density)
✓ decrease grain size
✓ Non-equilibrium constituents such as martensite can be created by suitable cooling cycles
✓ Precipitation hardening (with dispersion of hard particles)
✓ incorporating another phase (multi-phase steels)

Question 3

Typical microstructure of C-steels

Answer 3

• soft ferrite and harder pearlite
• An increase in the cooling rate increases the proportion of pearlite as well as hardness (reason: higher amount of harder pearlite, but also to the fact that the lamellar structure becomes finer)
• Ideally, the pearlite colonies in a low C steel should be homogeneously dispersed in the ferrite. As a matter of fact, low C steels contain rolling-induced “banding”
• ferrite and pearlite bands lie side-by-side in the rolling direction

Question 5

how can we estimate microstrucuture in the welding haz

Answer 5

with welding CCT diagrams

Question 6

effect of formed martensite in the haz after welding

Answer 6

multiaxial shrinkage stresses, which can lead to brittle fracture, particularly if the welded joint is rigidly constrained and if there are undercuts

Question 7

how can we avoid martensite formation in the HAZ

Answer 7

-we can keep the material to be welded at higher temperature to cool down slower, we preheat the material to a temperature higher than room temperature

-we can use a low carbon steel which are more hard to transform in martensite

Question 8

how can we estimate the tendency to produce cold cracking in HAZ

Answer 8

with the carbon equivalent value (CEV)

Question 9

how can we reduce tool wear

Answer 9

-lowering the workpiece C content and by the removal of oxide inclusions from the melt
-adding non-abrasive phases that can act as “chip breakers” (most used is manganese sulphides MnS)

Question 10

how can we increase strenght of structural steels (metodi principali non specifici)

Answer 10

alloying, heat treatment and thermo-mechanical rolling

Question 11

thermo-mechanical processing:
-how to poduce fine grain bainite
-microalloying
-quench at exit of rolling line

Answer 11

✓ Rapid cooling into bainite temperature range after rolling produces fine-grain bainite
✓ Small amounts of V, Nb and Ti used to control microstructure during hot rolling and further cooling
✓ possible to partially quench the steel with a more intense water flow onto the product at exit from the rolling line: only the surface transforms into martensite, which is then tempered by the heat released from the core of the bar

Question 12

what is a multi phase steel and when are they used

Answer 12

• Multi-phase steels feature strengthening by coarse two-phases. (mixture of two or more different types of constituents)
• frequently used as sheet materials in car body manufacturing, where they have to withstand deep drawing, stretch-forming, hydroforming and welding

Question 13

when do we need a soft-annealed microstructure

Answer 13

when extensive machining or other shaping operations are required

Question 14

how can we improve actual service properties

Answer 14

by the Quench & Tempering treatment of near-net-shape components

Question 15

which is the goal of Structural steels for full heat treatments (Q&T)

Answer 15

produce tough components that do not readily fracture when subjected to severe dynamic or cyclic loads.

Question 16

applications of steel with 0.25 to 0.50 % C

Answer 16

• for forged parts and machined components
• used in highly dynamically and cyclically loaded parts

Question 17

applications of Steels with 0.5 to 0.6% C

Answer 17

• a high hardening capacity due to carbon in combination with low tempering temperatures
• suitable for leaf- and coil-springs, torsion-bars and cup springs

Question 18

def of maraging steel

Answer 18

If precipitation-hardening (aging) of martensite (maraging=aging of martensite => precipitation-hardening of martensite) is not due (only) to alloy carbides but (mainly) to intermetallic phases, then steels are referred to as maraging steels

Question 19

how can we obtain maraging steels

Answer 19

For specific steel compositions after a solution-annealing treatment at 820°C, austenite is present; it can transform into a soft nickel-rich martensite by quenching.
Then, precipitation-hardening of the martensite (maraging) can be achieved on ageing at about 480°C: After artificial ageing the strength rises from approx.1000 up to 2100 MPa

Question 20

applications of maraging steels

Answer 20

• used in aircrafts (landing gear, helicopter undercarriages, rocket motor cases) in motorsport and other
• applications which require high strength-to-weight materials combined with high fracture toughness.

Question 21

main characteristics of materials for high temperatures

Answer 21

• Applications implying long service times at high temperature
• Heat-resistant steels and non-ferrous alloys need to have a stable microstructure (approaching equilibrium) during high-temperature service

Question 22

which steels can we use for high temperatures applications

Answer 22

✓ Martensitic steels are generally not used because tempered martensite is stable only for service temperatures lower
than tempering level itself
✓ Austenitic wrought and cast steels based on Cr, Ni (Si) have a higher intrinsic creep strength than ferritic steels.
✓ Solution-annealing and quenching for austenitic stainless steels only brings temporary benefits, C and N can
precipitate during service, depending on the temperature, and are not suitable for stabilizing austenite
✓Ferritic wrought steels for high temperature are alloyed with Cr, Al, Si to improve oxidation resistance, whereas
castable steels have only Cr and Si because Al can cause oxide skins and inclusions in the casting
✓ When only oxidation resistance has to be considered, austenitic steels are less favourable owing to larger thermal
expansion coefficient that accelerates scale damage during temperature fluctuations
✓ Ferritic steels are however prone to grain coarsening and to σ-embrittlement.

Question 23

Long term exposure to hot environments under (constant) load also leads to….

Answer 23

creep

Question 24

Creep behaviour

Answer 24

✓ after the load application at a constant temperature, the creep rate (𝜀) continuously decreases down to a minimum value, then increases, eventually leading to final fracture
✓ diffusion of interstitial elements and then of substitutional elements starts above room temperature
✓ dislocations are then able to leave their slip planes, which is known as “climbing”

Question 25

stages of a creep curve

Answer 25

✓ primary stage: the steel starts to creep after application of a constant force or stress
✓ secondary stage: resulting strengthening lowers the creep rate to a minimum value, which remains constant because of virtually balanced deformation-related strengthening (increase in the dislocation density) and thermally related softening (recovery, decrease in the dislocation density)
✓ tertiary stage: creep rate increases up tu rupture

Question 26

tell some creep resistant materials

Answer 26

creep resistance increases going down the list:

• Low-alloy structural steels used in the normalized or Q&T state. They are strengthened by carbides and carbonitrides of Cr, Mo, V, W, Nb
• quenched & tempered chromium steels. Their chromium content of 9 to 12% imparts a greater scaling resistance
• austenitic Cr-Ni steels (slower diffusion rate in the FCC lattice), some of which are used in the semi-hot worked or precipitation-hardened state
• High-alloy cast iron
• Nickel-based alloys

Question 27

Microstructural development in steel alloys, in addition to (thermo-mechanical) rolling, is controlled by means of

Answer 27

✓ Controlled holding at high temperature at the end of the rolling process
✓ Controlled cooling after rolling, leading to complex-phase steels