Lecture 18 -- Steels I

Mat E 272Lecture 18: Steels I

November 6, 2001

Introduction:

We previously discussed fundamentals of the interpretation of binary phasediagrams, and applied these concepts toward an understanding of the variousphases and microstructures possible in iron and steel alloys. By learning how topredict microstructures as a function of composition, thermal history, and coolingrates, we can make judicious selection of materials for various applications. In ourprevious discussion, we began examining some of the possible microstructuresresulting from cooling austenite. It is precisely because of such widely differentmicrostructures that the various grades of steel serve a diverse range ofapplications. Today we expand our examination of microstructures in steel andalso in cast irons.

Lecture 18 -- Steels I

Alloy classification scheme

Lecture 18 -- Steels I

Fe-C equilibrium phase diagram

Lecture 18 -- Steels I

Phase Transformations in Steels - Spheroidite

Pearlite:

Bainite:

A two-phase eutectoid (lamellar)microstructure resulting fromequilibrium coolingtransformation of γ to (α + Fe3C).

Another two-phasemicrostructure resulting fromrapid cooling transformation of γto (α + Fe3C).

Spheroidite:Two-phase microstructure of α +Fe3C resulting from heattreatment of pearlite or bainitejust below the eutectoidtemperature (A1). Reduction inFe3C - α surface (or interfacial)area causes cementite bands orneedles to become spherical.

The problem with machining highcarbon steels is often the hard barriersto cutting caused by the layers ofcementite.

A spheroidizing heat treatmentmodifies the structure, transformingthe elongated bands or needles ofcementite into globules.

Machinability can be significantlyimproved.

Typical Fe-C microstructure following a spheroidizing

heat treatment.

Heat treat just below the eutectoid (A1)

Lecture 18 -- Steels I

Phase Transformations in Steels - Martensite

Description:The martensitic transformationis not unique to steels.

It is fundamentally differentthan other transformationsbecause NO DIFFUSION ISINVOLVED (called“athermal”).

Martensitic transformationoccur as a result of rapidquenching via local(cooperative) rearrangement ofFe atoms. This rearrangementof atoms produces a newcrystal structure (BCT). Sinceno diffusion is involved, thistransformation takes placevery fast.(i.e., at the speed ofsound in the material)

Typical martensitic microstructure(dark grains are BCT-martensite,lighter regions correspond to retainedaustenite (γ).

Bain correspondence(relating the γ FCC structure to BCT)

Relationship between carbon content andlattice asymmetry in FCC (γ) to BCT.

Lecture 18 -- Steels I

Martensitic transformation

Types of martensite:

Characteristics:

I. Lath martensiteforms in alloys with < 0.6 wt. %C(low to medium carbon steels)grows as long, thin plates alignedparallel to each other

• Martensite IS a different phase!• Extremely hard• Results in dramatic increase in

strength & decrease in ductility• Usually co-exists with retained

austeniteBody-centered Tetragonal(BCT) crystal structure

II. Plate martensitealso called “Lenticular”found in alloys with > 0.6 wt. % C(medium to high carbon steels)needle-like or plate morphology

Lecture 18 -- Steels I

Complete TTT curves (eutectoid composition)

By including the martensitictransformation, we now have acomplete set of isothermaltransformation curves for steelof a eutectoid composition.

Note that in order to obtainmartensite, you must quenchthe steel at a sufficiently fastrate so as to avoid thepearlite/bainite “nose” on thediagram

(slower cooling results innucleation and growth ofbainite or pearlite)

Lecture 18 -- Steels I

Complete TTT curves (other compositions)

Every composition has a unique TTTdiagram.

Certain elements are added to steels forthe purpose of shifting the TTT nose tolonger times (to the right).

This makes it somewhat easier to engineerspecific microstructures throughout amassive workpiece (wherein cooling ratescan differ significantly from the outside tothe center).

This is an isothermal transformationdiagram for 4340 alloy steel (containsNi, Cr, and Mo)

(note the presence of a separate “nose”for the austenite-pearlite and austenite-bainite transformation.

The martensite transformationtemperatures are not significantlychanged.)

Lecture 18 -- Steels I

Continuous cooling curves - steels

Problem:

Solution:

The isothermal transformation(TTT) curves representmicrostructures resulting fromcooling at a constanttemperature. In most situations,it is not practical to coolisothermally.

We need to modify theisothermal diagrams to accountfor uniform cooling.

The TTT curves areappropriately modified toaccount for the effects of coolingwith time. In general, this hasthe effect of shifting the austeniteto pearlite transformation curvesdownward and to the right:

Lecture 18 -- Steels I

Continuous cooling - steels

Cooling rates:

Difference:

It should be clear that differentmicrostructures will result fromdifferent cooling rates. Forexample, we know that rapidcooling gives fine pearlitewhereas slow cooling gives coarsepearlite.

How can we represent differentcooling rates on the CCTdiagram?

During continuous cooling, allavailable austenite transforms topearlite by the time the bainitetransformation becomes possible.Therefore, bainite is notrepresented on the CCTdiagrams. Any remaining γsimply transforms to martensite

The volume fraction ofmartensite depends on cooling rate:

Lecture 18 -- Steels I

Fe-C alloys: mechanical properties

As carbon content in steels increases, strength also increases but at the expense of ductility

Lecture 18 -- Steels I

Fe-C alloys: mechanical properties

Increasing carbon (i.e., Fe3C) results in increased hardness. Note how hardness depends on eutectoid lamellar spacing (fine pearlite vs. coarse pearlite)

Note thedifference inhardness

could you explain thereason forthis?

Lecture 18 -- Steels I

Fe-C alloys: mechanical propertiesHow does the hardness of martensite compare with fine pearlite?

The extreme hardness of martensite is thought to be more of a result of its crystal structure(BCT - limited # of slip systems) than its microstructure

What isthis?

Lecture 18 -- Steels I

Tempered Martensite

As-quenched:

Tempering:

Martensite is extremely brittle;so much so that it is notappropriate for most structuralapplications.

Suppose a martensiticmicrostructure is heat treated forsome time below the eutectoidtemperature (250 to 650oC).

Martensite, being metastable,transforms into ferrite +cementite. However, the size ofthe Fe3C is microscopicallysmall; much smaller than inspherodite.

These serve as very effectivebarriers to dislocation motion.

Yield and tensile strengthboth decrease but ductility is increased. Overall, thismaterial is more usable thanquenched martensite.

Note the fine particlesof cementite in a ferritematrix. This image wasmagnified nearly10,000X

Lecture 18 -- Steels I

To summarize...

.

Diffusionless transformation

Lecture 18 -- Steels I

Prediction of equilibrium microstructures - I

Given a steel ofcomposition Co, canyou predict what theroom temperaturemicrostructure wouldlook like, assumingequilibrium coolingfrom the austenitephase?

Co

Lecture 18 -- Steels I

Prediction of equilibrium microstructures - II

Now, given a steel ofcomposition C1, canyou predict what theroom temperaturemicrostructure wouldlook like, assumingequilibrium coolingfrom the austenitephase?

Based on yourpredictions, how wouldyou expect themechanical propertiesof the two metals todiffer?

C1