metals pt 2 Flashcards
Whats a eutectic phase equilibria
liiquid» solid+solid
whats a peritectic phase equilibria
liquid + solid» solid
whats a monotectic phase equilibria
liquid» solid + liquid
eutectoid phase equilibria
solid»solid +solid
This reaction forms the basis of the heat treatment
peritectoid phase equilibria
solid + solid»> solid
whats a steel
alloys based upon the Fe-C or Fe-Fe3c binary phase diagram which can contain anywhere from trace amounts of C up to 1.2wt% C
list the solid phases in Fe-Fe3C phase diagram
Ferrite (alpha), ferrite (delta), Austenite (gamma), Cementite (Fe3C)
Whats do the last 2/3 numbers stand for
XXXX(X)
10201
2.01 wt%C
XX(X)/100
Hyper or Hypo eutectoid greater than the eutectoid
hyper is greater
compositions that are more than the eutectoid
In hypereutectoid steel what is the proeutectoid phase
cementite
in hypo its ferrite
Most phase transformations in steel develop from which phase
austenite
type of phase transformations in steel
-reactions near equilibrium conditions
(austenite to pearlite)
- reactions removed from equilibrium conditions
(austenite to martensite and bainite)
whats to TTT diagrams show
represent the precipitation kinetics for the transformation of products associated with the decomposition of austenite (gamma)
List the steel transformations on the TTT diagrams
from high to low temp formations
Coarse pearlite (slow cooling, coarse) fine pearlite fine pearlite and upper bainite lower bainite martensite (fine , rapid quench)
effects of temp on strength
lower temp, smaller intermellor spacing, more boundaries higher strength
Martensite (alpha dash) is hard and brittle in the as-quenched form, how is it utilised in this form
DUAL PHASE STEEL
-microstructure consisting of islands of martensite in a ferrite matrix gives rise to high rates of work hardening
how are Dual phase steels fabricated
theyre heat treated in the two phase region (ferrite alpha and austenite (gamma)). The temp with in the ferrite phase fixes the amount of ferrite formed and the composition of ferrite and austenite
slow cooling of austenite gives what
pearlite
moderate cooling of austenite gives
bainite
rapid quench of austenite gives
martensite, reheated gives tempered martensite
Indicate the particular heat treatment applied to asquenched
martensite, thereby making it useful for engineering applications
tempering
what does tempering martensite do, list the reactions that occur
heated to a temp below the A1 to make it softer and more ductile
reactions:
-seggregation of carbon atoms
-precipitation of carbides
-decomposition of retained austenite to bainite
-precipitation of alloy carbides in alloy steel which give rise to secondary holding
In TTT diagrams, transformation kinetics will change what
1- alloying element- more of these will retard the transformation kinetics, increasing hardenability
(produces a shallower gradient on the graph from the jominey test)
2- austenite grain size- bigger size, makes transformation kinetics slower, increases hardenability,
what is hardenability
the ability of steel to partially or completely transform from austenite to some fraction of martensite under heat treatment. (hardening it)
name and describe the test used to measure the hardness of steel
jominey test
-one end of the specimen quenched, plot hardness vs distance from quenched end
hardness decreases further from quenched end
How does the size of a specimen effect hardenability
cooling rate will be highest at the surface, and diminish as you go towards the centre
the cooling rate at the centre could subsequently be low enough to no longer allow for our condition of a minimum of 50% martensite at the surface (remember our critical diameter).
we would have exceeded the critical diameter for the steel under these conditions.
1. we could increase the cooling rate by using a more aggressive cooling medium,
2. we could use a different composition through increased alloy additions.
whats is the critical diameter and what is it a factor of
diameter for a steel composition and quench that would harden to 50% martensite at centre function of the ideal diameter and the severity of the quench
ideal diameter
the diameter that would harden to 50% martensite in an ideal quench function of base diameter and multiplying factors DI=base diameter x multipling factors
what is the base diameter a function of
grain size and carbon concentration
brittle fractore
low energy
rapid
clevaged , facetted surface
ductile fracture
high energy
slow
ruptured, dimpled surface
what is stainless steel
a class of corrosion resistant steels based on a minimum of 12wt% Cr in solid state
what is stainless steel resistant to
- attacks by strong acids
- atmospheric corrosion
- aqueous corrosion
- high temp oxidation
describe austenitic steels
- low yield strength
- high work hardening rate
- expensive
- excellent formability (undergoing plastic deformation without being damaged )
ferritic stainless steels
- high yield strength
- low work hardening rate
- less expensive
- good formability
stainless steel not effective in what conditions
- basic conditions
- presence of cl-
problems with stainless steel
corrosion along grain boundaries
- temps that induce intergranual corrosion are called sensitising heat treatment, it results in the formation of Cr23C6 on grain boundaries, cr increases corrosion rate, associated with c concs
solutions to corrosion of steel
- reduction in C concentration
