heat treatment: lecture q&p, m3 concept
TRANSCRIPT
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1. Schaeffler diagram2. M3 Concept 3. Quenching & Partitioning Heat Treatments
Lecture Series- C
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• The Schaeffler diagram is an important tool for predicting the constitution
of austenitic Cr-Ni steel welds with carbon contents up to 0.12%. However,
it does not allow determination of the composition and volume
• of the carbide phase.
• This Schaeffler diagram is especially suited to weld metals in order to
predict the structure.
Schaeffler diagram
Schaeffler and Delong diagrams are used to predict structure on the basis of alloying elements. (Stainless Steel Weld) Plots the compositional limits at room temperature of austenite, ferrite and martensite, in terms of nickel and chromium equivalents The Cr and Ni equivalent can be empirically determined as:
Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)
Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)
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Chrome and nickel are among the most important alloying elements here. All ferrite formers have a chromium equivalent and all austenite formers a nickel equivalent.
Schaeffler diagram
Modified Schaeffler diagram
Delong modified the schaeffler diagram Ferrite No.(FN) is also plotted on schaeffler diagramWidely use in predicting phase-structure in weld metal Also include calculation of volume and composition of carbide phase
Modified Schaeffler diagram FN can be roughly determine by:
FN = 3.34 Creq – 2.46 Nieq – 28.6
--> FN between 3-7 (max.) is preferredSolidification mode of S.S. during casting or welding can be predicted roughly as under:
Creq/Nieq < 1.5 (Austenitic)
Creq/Nieq > 2.0 (Ferritic)
In b/w 1.5 and 2.0 is the mixed structure
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Schaeffler-Delong diagram
FN = Ferrite No.
Low %ferrite leads to solidification cracking in weld metal, but low %ferrite render/reduce Stainless steel more corrosion resistant
Schaeffler-Delong diagram
M3 Concept
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• The performance of steel products is closely related to the constitutes and morphology of microstructures.
• The characterization and effective control of microstructure are now from micron scale to nano scale steadily (to be in nano order).
• The properties have been raised from the order of 106 to 109 unit (to be in Giga order).
Future Perspective of Steels
Future Perspective
TensileStrength
FatigueStrength
Fatiguecycles
Creep strength
Improved from MPa to GPa order
Improved from MPa to GPa order
Improved from Mega cyles to Gega
Cycles
Improved from Mega secs to
Giga secs
Third Generation Steels
Toughness ≥ 200 J
Ductility ‘A’ ≥ 20 %
Y.S. ≥ 800-1000 MPa
Target Values
Mass production on existing mills !
M3 Concept
Fig. – The performance of steels are associated with the microstructure, characterized by Multi-phase, Meta-stability and Multi-scale
M3 Concept
Multi-phase
Meta-stability
Multi-scale
Mixture ofBainite,
Martensite,Retained austenite
Stability of austeniteStrain-induced
Martensite (SIM)
UFG, Ultra Fine GrainNano-laths martensite
Nano-Bainite
M3 ConceptSchematic of Research Targets and Future Directions
Third generation AHSS with improvednductility and reduced cost. Third generation martensitic steels for improved creep strength.
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Quenching & Partitioning Heat Treatments
Quenching & PartitioningA Novel method, proposed by G. Speer, for the development of multiphase
steels with considerable retention of austenite in the microstructure.
The Q&P process consists of a first quench (quenching step) to a
temperature below the Ms but above the Mf to form a mixture of martensite
and austenite.
Subsequent isothermal treatment (partitioning step) at the same
temperature (one-step treatment) or at a higher temperature (two step
treatment), in order to transfer the C from the supersaturated αM into the γ.
Final structure = Decarburized (carbon depleted) martensite + C enriched
austenite + Fresh martensite.
Quenching & Partitioning
Fig. – Schematic representation of One and Two step Q&P process
Quenching & PartitioningIn addition to carbon partitioning into austenite,other processes that could occur during the ‘‘partitioning’’ step are:
Tempering of Martensite Carbon trapping at dislocations and interfaces in the martensite Formation of carbides (both transition carbides and/or cementite) Decomposition of the austenite to bainite or other transformation
productsCompetition occurs b/w theses processes.*Bainite has been recognized as a potential constituent particularly at increased quench temperatures where the amount of martensite is limited and bainite transformation kinetics are more rapid than at a lower temperature.
Quenching & Partitioning
Fresh martensite is formed after quenching from partitioning temperature to
room temperature, b/c of unstable austenite.
This microstructure can lead to an interesting combination of mechanical
properties.
Good formability, as a result of the TRIP effect from the retained austenite,
and a strength higher than that of conventional TRIP steels due to martensite.
Q&P – Design Requirements
(a) Absence of ferrite and/or pearlite formation during the quenching step.
(b) Retardation or inhibition of bainite formation, in order to minimize possible overlapping of carbon partitioning and formation of bainite.
(c) Retardation or minimization of the precipitation of carbides, which consumes carbon that is then no longer available for carbon enrichment of the austenite.
(d) A sufficiently high carbon content for thermal stabilization of a considerable fraction of retained austenite at room temperature.
Q&P Steel - MicrostructuresFig. - Light optical micrograph of Q&P microstructure in AISI 9260 steel quenched to 190°C and partitioned at 400°C. Nital etch; retained austenite appears white.
Fig. – SEM micrograph showing blocky austenite within the martensite matrix formed during Q&P heat treatment.
Q&P Steel - Microstructures
Fig. - Field emission scanning electron micrographs of medium carbon low alloy steel samples quenched at 180 ◦C, partitioned at 300 ◦C for (a) 30 s (b) 120 s and (c) 900 s respectively.
Q&P Steels - Properties
Fig. - Comparison of impact toughness at room temperature between Q&P and Q&T at various partitioning (or tempering) temperatures.
Q&P Steels - Properties
Fig. - Comparison of the engineering stress-strain curves and strain hardening of Q&P and Q&T