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HeatTreatmentTechnology
Dr.SantoshS.Hosmani
DEPT. METALLURGY & MATERIALS SCIENCE,
COLLEGE OF ENGINEERING, PUNE
The role of alloying elements in Steels
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Classification of iron alloy phase diagrams:
(Ref.: Wever, Archiv fr Eisenhttenwesen 2, 193, 19281929)
Fe-Ni system
Ni Mn Co
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5C, N, Cu, Zn, Au
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, , , ,
Ti, V, Mo, Cr
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B,
Ta, Nb, Zr8Ref.: Book by H.K.D.H. Bhadeshia & R.W.K. Honeycombe
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9Ref.: Book by H.K.D.H. Bhadeshia & R.W.K. Honeycombe
Effect on Fe-C phase diagram
Austenitesta i izinge ements:Ni,Mn,Co,C,N
Ferritestabilizingelements:Cr,Si,Mo,W,V
A1temperature is lowered by the austenite-formers (e.g. Ni,
Mn, Co) and raised by the ferrite-formers (e.g. Cr, Si, Mo,
W, V).
A chrome steel containin 12% Cr and 0.4% C re uires a
higher austenitizing temperature than a eutectoid carbon
steel, whereas a 3% Ni steel will already begin to
.
This state of affairs is clearly of great practical importance
w en ese s ee s are e ng use a empera ures aroun
A1.10
Effect on Fe-C phase diagram
Influence of allo in element addition on eutectoid tem erature and
eutectoid carbon content is displayed in following figure:
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What are our observations from the followin hase-dia rams ?
The amount of Ti required to close the -phase loop is much less thanthe re uired for Cr in binar Fe-allo s
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Hardening effects of alloying elements in solid solution in fully
annealed ferrite
Here, factors affecting hardness are:
and solvent atoms,
Concentration of solute atoms
Elastic modulus of solute
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Classification of alloying additions
A useful grouping is based upon the effect of the element on:
(a) the stability of the carbides and
(b) the stability of the austenite/ferrite.
1. Elements which tend to form carbides:Cr, W, Ti, Nb, V, Mo and Mn.
2. Elements which tend to ra hitise the carbide:Si Co Al and Ni.
Only a small proportion of these elements can be added to the steel
before graphite forms during processing.
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Classification of alloying additions
. , , .
These elements alter the critical points of iron in a similar way to
carbon by lowering the A3point, thus increasing the range in which
austenite is stable and the also tend to retard the se aration of,
carbides.
They have a crystal lattice (f.c.c.) similar to that of -iron in- .
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Classification of alloying additions
4. Elements which tend to stabilise ferrite: Cr W Mo V Si and Ti.. , , , , .
These elements are more soluble in -iron than in -iron. They
diminish the amount of carbon soluble in the austenite and thus tend
to increase the volume of free carbide in the steel for a given carbon
content. On the binary equilibrium diagram of these elements withpure iron the A3point raised (although it may be lowered initially), until
the two points merge to form a closed gamma loop.
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re
peratu
CrTe
18Fe CAnimation: S.S. Hosmani
One convenient way of illustrating quantitatively the effect of an alloying element
-
Classification of alloying additions
ternary system the -phase field boundaries for increasing concentration of a
particular alloying element. This is illustrated in Figure below for titanium andchromium, from which it can be seen thatjust over 1wt% Ti will eliminate the-
oop, w e w r s requ re o reac s po n. er ernary sys ems
can be followed in the same way, e.g. in FeVC, vanadium has an effect
intermediate between that of titanium and of chromium.
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Classification of alloying additions
,
amount of each of these elements
the austenite phase disappears
-
point down to room temperature.
No critical points exist and such
steels e. . 18% chromium irons
are not amenable to normal heat
treatment, except recrystallisation
after cold work.
This effect, however, can be
counteracted by adding certain
elements like 2% of nickel is
FeCrsystem
Here, in Fe-Cr phase diagram, we
added to the 18% chromium
stainless steel to enable it to berefined by normal heat-treatment;
can no e a , or r . , e
ferrite phase becomes stable over
the entire temperature range up to
the melting point. If C is present
carbon has the same effect. ,
is required to nullify carbons
austenite stabilizing effect.20
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Stainless steels are categorized into four groups on the basis of
composition and crystal structure:
ferritic,
austenitic,
martensitic and
duplex stainless steels.
-
addition.
The effect of other elements present in Cr-Ni steels can be
expressed as Ni equivalent (if they stabilize the austenite) and as Cr
equivalent (if they stabilize ferrite):
. u. noequ va ent+++++=
W0.751.5Ti1.75Nb5.5Al5V1.5Mo2SiCrequivalentCr +++++++=
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TheSchaeffler dia ramde icts the hases resent in the allo as
a function of Ni and Cr equivalents:
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- - ,
effect of ferritic and austenitic stabilizing alloying elements should
be taken into account.
mani
S.S.Hos
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Formationofduplexmicrostructureduringsolidificationofthemelt
Du lexstainlesssteelssolidif 100%ferrite ferrite .
Uponcoolingtheferritestartstopartiallydecomposeintoaus en e a nuc ea esan grows rs a egra n
boundariesofferrite,followingfavorablecrystallographic
orientationsinsideofthegrains.
Asthetemperaturelowerstheferritecontentdecreases
asausteniteincreases(andcarbidesandseveralintermetallicphasesma also form durin coolin :de endin u on coolin rate
Man literature refers to denote ferrite in DSSs asferrite rather than ferritebecause DSSs
2424
contain ferrite which is resulted (transformed) from liquid andnotfrom austenite (refresh memory for
FeC diagram).
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Duplex stainless steels (DSSs) are in between the austenitic and the ferritic grades,
SomeoftheimportantaspectsofDSSs
combining the best mechanical and corrosion resistance (especially, pitting, stress
corrosion cracking) properties of both.
s a resu o e r g mec an ca s reng , goo erma c on uc v y an exce en
corrosion resistance DDSs are extensively used both in pulp and paper industries, inchemical and petrochemical plants. They also find some applications in food and
biomedical fields as well.
The wide use of DSSs is closely connected to their specific microstructure, formed by
roughly 50%50% austeniteferrite ratio: higher yield and ultimate tensile strength than
the austenitic grades, with good ductility and toughness. But, on the other hand, the
microstructural anisotropy of the hot rolled materials can result in variability of
mechanical properties, such as tensile strength and fracture toughness.
Theaustenite+ferrite matrixisattainablebycombiningvariousphasestabilizingelements.
CrandMo:ferritestabilizers.Ni(andN):austenitestabilizers.
Some economical advantages as a result of lower nickel content than the austenitic
grades.25
Pittingresistanceequivalentnumber(PREN):
PREN=%Cr+3.3x%Mo+17x%N
Figure:NominalPRENvaluesfordifferentstainlesssteels
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[$$]
[$$]
Ref.[$$]:D.S.Bergstrom,J.J.Dunn,J.F.Grubb,W.A.Pratt:PatentNumberUS20036551420B1(2003).
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Nickelfreeduplexstainlesssteels
s we nown a n r ogena oy ng n s ee pro uces a var e y o excep ona proper es
such as high strength, high ductility and resistance to stress corrosion cracking (SCC).
Nickel could be completely replaced by nitrogen in order to enhance SCC resistance and
reduce the alloying element cost.
A much lower nitrogen content is needed to maintain a 50% austenite phase compared
with the necessary addition of nitrogen to reach a 100% austenitic microstructure.
Except for nonmagnetic applications, nickelfree DSS are definitely promising in regard to
their properties and cost, and thus are suitable, for example, as reinforcing steels or
.
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Nickelfreeduplexstainlesssteels
Table: Chemical Composition, Austenite Content and Calculated PREN Values of the DSSs.
, ,
10%Mn and 0.35%N.
SteelD103contains additionally about 3%Mo together with the further elevated nitrogen
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concentration of 0.45% in oder to improve the corrosion resistance.
Nickelfreeduplexstainlesssteels
Figure: The calculated nitrogen section ofthe phase diagram of alloyFe-22Cr-10Mn-N Figure: The calculated nitrogen section ofthe phase diagram of alloy Fe20Cr10Mn
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y ermo- ac. 3MoNby ThermoCalc.
D 10 D 10-3
Nickelfreeduplexstainlesssteels
Figure: The dependence of the ferrite
content on annealing temperature.
In comparison with commercial Duplex 2304 and Duplex 2205, the D10 and D103 alloys
show amore stable austenite content at high temperatures:
Due to its potent austenitic stabilizationand rapid diffusion ratein FeCr alloys, nitrogen insuch nickelfree DSS increases the microstructural stability at high temperatures.
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Alsocompare phase diagrams on previous slide with the phase diagram on Slide # 48 or 49 .
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Nickelfreeduplexstainlesssteels
Table: T icalRoom-tem erature Mechanical Pro ertiesof the Investi atedAllo s. .
In comparison with commercial Duplex 2304 and Duplex 2205, the D10 and D103 alloys
.
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Nickelfreeduplexstainlesssteels
Table: Results of Pitting and Crevice Corrosion Tests of the Experimental DSS.
Tccc: critical crevice corrosion temperature
In comparison with commercial Duplex 2304 and Duplex 2205, the D10 and D103 alloys
havepoorCrevice Corrosion Resistance.
With respect to corrosion properties, D103 alloy seems to be better choice than D10
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alloy; and properties are somewhat comparable to Duplex 2205.
Nickelfreeduplexstainlesssteels
Table: Typical Composition and Costs of DSS and Austenitic Stainless Steels.
D10 and D103 Steels possess the highest ratio of PRE value to alloying cost due to a high
nitrogen content and the absence of nickel.
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ProblemsassociatedwithDSSs(Warnings)
, , ,
temperature processing and can degrade the mechanical and corrosion properties of the
DSS.
During welding of DSS parts, such undesirable phases can form in heat affected zone (HAZ).
Therefore, welding of DSS parts is challenging task.
DSSs are characterized bytwo embrittling temperature ranges(Cshaped curves) which exhibitseveral secondary phases, carbides and nitridesprecipitation at different holding times:
em r tt ement
a spinodal decomposition of the a ferrite in two
phases: an Crrich phase and anFerich phase. nucleation and growing of NiSiMo rich f.c.c. G
36Figure:TypicalTTTdiagramforDSSs
phase, characterised by a very slow precipitation
kinetic (the overall concentration in Gforming
elements increases from 40 to 60% between 1000
and 30000 h at 350C).
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The stability of a given secondary phase usually depends on Fe, Cr, Mo and Ni contents in
.
The effect of alloying elements on the formation of those phases is indicated in the
following Figure:
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BSE and X EDS analysis
Howtoidentifysecondaryphasesinmicrographs?
can help in identification.
Figure: BSE micrograph of CD3MWCuN Figure: BSE micrograph of CD3MN
a oy annea e at . ontrast
difference can be seen between and .
EDS scanning assured the brighter phase
asenveloped by.
ta en at a ter ay eat
treatment.
38Forverysmallsize(tiny)particles:TEMcan,ofcourse,helpinidentification
IdentifythefollowingtypesofStainlesssteelsusingtheirmicrostructures:
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