Steels- Hardenability Flashcards
Describe the set up for the Jominy test and how it works
Have a bar of steel fixed to a mounting fixture at the top. The bar is quenched from the bottom face (could be water spray). The cooling rate is slower as you go up the bar from the bottom. Can do hardness tests (e.g Rockwell C) at different heights up the bar (centre). Plot hardness vs distance from quenched end. A shallower hardness gradient means the steel has a greater hardenability.
Describe a continuous cooling transformation diagram for the Jominy test
The quenched end will have the fastest cooling rate so the curve decreases earliest and fastest. Likely reaches Ms before crossing any c curves to form pearlite. As go further into the bar, decrease is T is delayed and slightly shallower and c curves may be crossed. Can get α’ and pearlite, all fine pearlite, all pearlite as go further in.
Influence of austenite grain size on hardenability
For hardness vs distance from quenched end graph, get faster decrease in hardness for lower austenitising temperatures (smaller grains) and so lower hardenability
Influence of alloying elements on hardenability
Shallower hardness vs distance from quenched end curves for alloyed steels as opposed to plain carbon steels so greater hardenability. E.g is increased hardenability for 41XX (Mo, Cr) steels and further increase for 43XX (Mo, Cr, Ni) steels
How does hardness vary with percentage martensite?
Generally harder with greater proportion of martensite
Influence of carbon on hardenability
Increasing wt% C means higher and shallower curves so increased hardenability
Influence of section size on hardness
Hardness profile from the centre of the bar outwards. Symmetrical u shape with inflexion point on both sides. Lowest cooling rate in centre so lowest hardness. For same diameter bar, a steel with greater hardenability will have a much less deep u shape. For same steel, a wider bar has a much deeper u shape with lower central hardness but similar at the outsides
How does a CCT diagram look with bar diameters?
Transformation temperature vs bar diameter. Low to high bar diameters means high to low cooling rates. Have different scales for air cool (slowest), oil quench, water quench (fastest). Have top half of c curves right to left then go horizontally left for martensite. Different curves represent different proportions of each phase reached between start and finish temperatures. Right has ferrite above pearlite, going left has bainite, below horizontal is martensite.
What happens if a finish curve (like Pf) is cooled through?
Full transformation to, e.g, pearlite has occurred so no more transformations will occur as all phases present are stable at RT
How does a steel plant influence the choice of steel?
Sometimes a plant has a maximum cooling rate that can be achieved. This may not be fast enough to avoid c curves on a CCT or TTT diagram when you want to. Different steel compositions can push c curves right (longer times) so the available cooling rate can be good enough
Order of cooling rates achieved by different types of cooling
Fastest: still water
Mildly agitated coil
Still oil
Slowest: mildly agitated molten salt
What can diameter refer to?
Diameter of a cylindrical bar
Thickness of a plate
Width of square bar
Etc
What can the hardenability be predicted using?
It’s composition and grain size
Critical diameter
Dc. The diameter for a steel composition and quench that would harden to 50% martensite at the centre.
Function of ideal diameter, DI and severity of quench, H
Idea diameter
D sub I (i). The diameter that would harden to 50% martensite in an ideal quench.
A function of the base diameter and multiplying factors that take into account alloying elements
