Engineering Materials – NOTES – Chapter 10 – Phase Transformations in Metals

 

Heat treatment produces phase transformations in metals.  The temperatures and times associated with these phase transformations can dramatically alter material properties.  (for example: heat treatments of eutectoid carbon steel (0.76% C) can vary the strength between 100 and 300 ksi!).  Our goal is to develop an understanding of phase transformations so that we can apply proper heat treatment to produce a desired set of properties.

 

Phase Transformations

 

Basic Concepts

Phase Transformation – An alteration in the number and/or character of phases. 

 

Classifications of Phase Transformations (that we will look at):

·        Simple Diffusion dependent transformations – no change in the number or composition of the phases present.  (examples: solidification of pure metal, recrystallization, grain growth)

·        More complex diffusion dependent transformations – Some alteration in phase composition and the number of phases present (example: eutectic reactions).

·        Diffusionless transformation – A metastable phase is produced.

 

The Kinetics of Solid State Reactions

The kinetics of phase transformations include:

·        Nucleation – formation of small particles of the new phase which are capable of growing.

·        Growth – The nuclei increase in size

The transformation from one phase to another depends upon both time and temperature.  We will examine the impact of these in conjunction with iron-carbon alloys.

 

Microstructural and Property Changes in Iron-Carbon Alloys

Time and temperature are HUGE factors in the way we are able to manipulate the properties of steels.  We will look at some of the phase transformations that happen within steels.

 

Isothermal Transformation Diagrams

(What is “isothermal”?)

Phase changes take time to occur.  The time required is a function of the cooling temperature.  We will first consider a set of systems that are rapidly brought to a fixed temperature and then the temperature is held constant while the phase transformation occurs (isothermal!).  (How do we do this in practice?)

 

Pearlite – Eutectic Reaction

Consider a eutectic steel composition (0.76%C) as it cools from unstable austenite to pearlite. 

Fig 10.4 shows the time for phase transformation as a function of temperature.  The curve on the left indicates the start of the transformation with the curve on the right being the completion of the transformation. (Note time is plotted on a log scale).  Fig 10.5 shows the process at 625C.  Transformation begins after about 3s and is complete after about 15s.

 

Why is cooling rate important?  Don’t we just end up with pearlite in any case?

Lower cooling temperatures limit diffusion rates.  This creates fine pearlite with thin alternating layers of ferrite and cementite.  Longer cooling times at higher temperatures creates more coarse pearlite.  Fig 10.6.  The layer thickness in the pearlite will affect the mechanical properties of the material.

 

Bainite

If the eutectic steel is cooled below 540C, we will not get pearlite!  Instead, bainite is produced.  The cooling diagram is shown in Fig. 10.9.  Note the top half (above the neck at point, N) is the region where pearlite (P) is formed.  Below point N, bainite (B) is formed instead of pearlite.

 

Spheroidite

Heating a pearlite or bainite microstructure at a temperature BELOW the eutectic temperature (say 700C) for 18-24 hr produces spheroidite.  This is a set of Fe3C spherical particles embedded within a continuous a phase matrix (Fig. 10.10, Also, the Figure on page 298 shows mixed pearlite and spherodite)

 

Martensite

Rapid cooling at a relatively low temperature (quenching near room temperature) produces neither pearlite or bainite, but a structure known as martensite. 

 

Martensite is the strongest and hardest phase in steel and is very important in the heat treatment and hardenability of steels. 

 

Martinsite formation is athermal (not time dependent).  It is a combination of martinsite and austenite, the ratios of which are constant at a given temperature. Fig. 10.13

 

Note that rapid quenching to form martensite may crack a metal due to the internal stresses that are produced by the different transformation rates within the metal (the surface cools fast, producing martensite, while the interior cools more slowly).

 

Martensite is a Body-Centered Tetragonal (BCT) structure (similar to BCC, except one side is longer than the other two sides). 

 


Cooling to Produce Specified Microstructures – Work through example problem 10.1 on pages 312-314.

 

Continuous Cooling Transformation Diagrams

Much heat treatment finds it impractical (or impossible) to cool a sample instantaneously to a fixed temperature and then hold that temperature.  Slower cooling materials produce delays in the transformation reactions (because the temperature is slowly changing).  The result is a shift in the cooling curves (Fig. 10.16).  Fig. 10.17 shows a continuous cooling transformation (CCT) diagram.  Some important points about the CCT:

·        Bainite will not form in continuous cooling of most steels.  Thus, the CCT is cut off at the nose, as shown in Fig. 10.17.  Any curve passing through the line, AB, stops the transformation to pearlite and the remaining austenite begins to transform to martinsite when the M line is crossed.

·        How does cooling rate affect the phases formed?  The CCT contains 3 regions divided by different cooling rates Fig. 10.18.

o       At cooling rates faster than the critical cooling rate, pure martensite is formed.

o       Rates passing through the AB line of Fig. 10.17 form a combination of pearlite and martensite.

o       Slower rates form pure pearlite.

·        What is the effect of alloys on the CCT? – Alloys and carbon shift the nose of the diagram to the right (more time so the critical cooling rate can be slower).  This improves the hardenability of steels Have you seen this happen anywhere??? (as in the Jominy bar experiment!)

·        It is difficult to cool steels with less than about 0.25% carbon (and no alloys) fast enough to form any martensite.  These mild steels are typically not heat treated to produce martensite. (Review: Can we do anything else to make them harder?  Stronger?)

 

Mechanical Behavior of Iron-Carbon Alloys

 

We have been talking about pearlite, bainite, spherodite and martensite and how they form.  The next (and hopefully obvious) question is: Why do we want to form any of these?  What properties do each of these possess?

 

Pearlite

Recall, pearlite is a layered combination of cementite (Fe3C) and ferrite (Fe with some C dissolved in it). 

Cementite is harder and more brittle than ferrite.  The more cementite, the harder the steel.  (increasing the amount of C in the steel produces more cementite and not more ferrite because the ferrite is saturated with C at 0.022%)

 

What is the difference between fine pearlite (thin layers) and coarse pearlite (thick layers)?

What would you expect and why?

 

Fine pearlite is stronger and harder because:

·        Strong adherence at the boundary between cementite and ferrite.  The ferrite at the cementite boundary is restricted in its movement.  If the layer of ferrite is thin, a higher percentage of the ferrite is near a boundary.

·        Phase barriers form a barrier to dislocation movement.

 

Spheroidite

Spheroidite (Fig. 10.10) has less boundary area between cementite and ferrite.  The ferrite matrix makes this phase very ductile, but with low strength and hardness.  Notably tough

 

Bainite

A finer structure – generally stronger and harder than pearlite.

 

Martensite

·        Hardest and strongest of the microstructures for steel alloy.

·        Very brittle (with almost no ductility)  BCT has relatively few slip systems.

·        The different densities of austenite and martensite may cause cracking during quenching for carbon steels with more than 0.5% carbon.

 

Tempered Martensite

The brittleness of martensite and the internal stresses that arise from quenching make it impractical for most applications.  However, we can take some advantage of the strength of martensite by subjecting it to a heat treatment called tempering.

 

How is it done?

Takes place at temperatures below the eutectoid (typically 250-650C)

 

What is happening?

·        Diffusion is the key process in tempering

·        The single phase BCT martensite transforms to two phase (a + Fe3C) tempered martensite (that actually does not look much like the original martensite at all!) Fig. 10.24 compared to Fig. 10.12

 

What does tempered martensite look like?

Small cementite particles distributed within a ferrite matrix (Fig. 10.24).  Similar to spheroidite except the cementite particles in martensite are much smaller.

 

What are the properties of tempered martensite?

Tempered martensite is strong with some ductility restored.  The smaller the cementite particles, the greater the strength due to the larger boundary area of the particles (similar to fine pearlite).  The particles grow larger as the tempering temperature is increased, making the material more ductile, but less strong.  If tempered for a long time, the structure becomes spheroiditic (cementite particles are large).

 

Fig. 10.27 – A summary of the different structures and the cooling rates with which they are associated.

 

Homework Problems: 10.1, 10.9, 10.10, 10.14(a)(d)(g), 10.18(b), 10.21, 10.22, 10.23, 10.26, 10.28, 10.29, 10.30, 10.31, 10.33