Heat Treatment of SteelDustin Chen

4/13/09Section 103

Lab Partner: Emmanuel Chao

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Abstract

This lab explores the effects of heat treatment on both the mechanical properties of steel,

and also on the microstructure. To better understand the effects, a TTT (time-temperature-

transformation) curve is analyzed in ferrous metallurgy. In this experiment, a cold-worked, steel

alloy with an inducted temperature gradient was used. It was found that smaller grains create a

harder substance, thus making martensite the hardest phase, then pearlite, and then ferrite. These

phases were obtained by cooling austenite differently, holding it at different temperatures.

Introduction

Steel, an alloy of iron and carbon, is widely used in engineering for many applications,

from transportation, to household materials. This versatility is partially due to the many variation

of its properties, which are in a large part due to the thermomechanical processes which are

imposed on steel. A very important necessity, then, is to understand how to use this heat

treatment to optimize performance in ferrous alloys.

Carbon is soluble in iron because the carbon atoms are able to fit into the interstitial sites

of the iron atom without distorting it too much. In the FCC phase of iron, austenite,

approximately 2% carbon can dissolve, whereas only around .02% of carbon can dissolve in the

BCC phase (ferrite).

By cooling austenite below the eutectoid temperature, it becomes unstable, precipitating

the Fe as ferrite, and distributing the carbon atoms into Fe3C (cementite). The nucleation rate of

ferrite and cementite is low, and transformation occurs when the specimen is held at a

temperature for a long time. As the temperature becomes increasing lower, the nucleation rate

increases, until below 540°C where it becomes slower again, as the carbon atoms lose mobility

in austenite, and must diffuse to Fe3C regions.

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The driving force is driven by diffusion at higher temperatures, and nucleation at lower

temperatures. Therefore, at higher temperatures, coarse pearlite is formed, the coarsest dispersion

of Fe3C and ferrite. At lower temperatures, nucleation is the driving force and finer dispersions

are produced.

Hardness of steel increases with the fineness of dispersion, and is the hardest when

martensite is formed, as the carbon atoms are distributed at random. Martensite is formed when

the alloy is cooled so quickly, it skips the diffusion process altogether.

Procedure

In this lab experiment, the alloy used is 1045 cold-rolled steel. A heat gradient was

produced by first heating the entire sample to 1050°C, and then submerging an eighth of an inch

in water. It was polished to get rid of the oxide, and then electropolished to provide a etch to

delineate grain boundaries, dislocations that intersect the surface.

Rockwell hardness

measurements were then taken along the length of the sample to observe the effect of heat

treatment on the mechanical properties (namely hardness) of steel. The sample was then

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examined under a microscope to observe differences in grain sizes, density of dislocations, and

the morthology of the microconstituents.

Results and Discussion

Figure 1

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Figure 2 Figure 3At the very top of the sample, a martensitic region is found. This region was held at

900°C, and then quickly quenched in water. From the curve (Figure 2), we can see that at 900°C,

the sample is pure austenite. By quenching the sample in water, the driving force for the

austenite to convert into ferrite is very large, and it undergoes a diffusionless phase

transformation, to martensite (Figure 3), a supersaturated ferrite with excess carbon.

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Past this, a region with martensite and

ferrite is found (Figure 5). When the pure austenite is cooled below 800 degrees, some of the

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austenite starts to precipitate as ferrite (Figure 4). When quenched into the water, the austenite

becomes martensite, and the result is a martensite and ferrite region.

Figure 4 Figure 5

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But when austenite is cooled below the

eutectoid temperature, 727°C (Figure 6), the alloy becomes unstable. The Fe precipitates as

ferrite, and the carbon diffuses to form cementite (Figure 7). However, right below the eutectoid

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temperature, not all of the austenite has transformed into ferrite and cementite. Therefore, the

remaining austenite will be transformed into martensite when quenched in water.

Figure 6 Figure 7

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As the alloy further cools, all of the austenite

will be transformed to cementite and ferrite. When quenched in water, they are not transformed

into martensite as they are already at equilibrium.

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Figure 8 Figure 9

The last region will be all martensite again. This is because the sample was held at

900°C, and then quenched in the water immediately (Figures 2 & 3).

After that alloy was heat-treated, it was subjected to a series of Rockwell hardness points

to see how the heat-treatment affected the mechanical properties. The plot of hardness vs length

along the sample are shown in Figure 10 below.

Figure 10

With this table, one can see that martensite is the hardest, then pearlite, then ferrite. This

also makes sense by examining the microstructures (martensite: Figure 3, ferrite: Figure 5,

pearlite: Figure 9). In comparing pearlite and ferrite, one can see that the grains are much smaller

in the pearlite microstructure. This causes the material to be harder, as the smaller grains create

more obstacles per unit area of slip plane, making it more resistant to being indented. The

martensite’s grain size is even smaller (The large grains viewed are ferrite forming). The small

grains are formed as a result of skipping the diffusion process. This allows the carbon to become

complete intermixed with the austenite, creating smaller grains, and thus, a harder material.

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The transformation of austenite into

ferrite begins at the austenite grain boundaries and propagates inwards. When the austenite

begins to precipitate, the grains are nucleated from the boundary of the heat affected zone,

because the effective surface energy of the nuclei will be lower there.

Figure 5

As stated above, a structure containing ferrite and martensite can be produced in a 1045

steel specimen by cooling pure austenite slightly, above the eutectoid temperature. At this point,

the carbon does not begin to diffuse into cementite, but some of the austenite will begin to

precipitate into ferrite. When this mixture of ferrite and austenite is quenched into water to cool

it, the austenite will turn into martensite, producing a structure of only ferrite and martensite

(Figures 4 and 5).

However, the 1045 steel specimen cannot be treated so that it contains only ferrite. Above

the eutectoid temperature, the specimen will exist as either pure austenite, or a combination of

austenite and ferrite, resulting in either a martensitic structure, or a structure of martensite and

ferrite when cooled. As the sample is cooled past the eutectic temper ature, some of the carbon

atoms start to diffuse into cementite. At this region, three distinct phases, austenite, ferrite, and

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cementite will co-exist. The austenite continues to precipitate into ferrite and the carbon

continues to diffuse into cementite until all of the austenite becomes ferrite and cementite. Thus,

it is not possible to have a steel specimen that contains only ferrite.

The martensitic structure is formed only by rapidly cooling austenite, so that it skips the

diffusion process. Thus, it is not on the phase diagram for the Fe-Fe3C, as the phase diagram

shows only the normal carbon-iron solution at different temperatures, not for processes induced

to create a different phase. It is a metastable phase, the product of rapidly cooling steel

containing sufficient carbon.

However, martensite is still a very important engineering material, because tempering it

allows it to be very versatile. Though it is a very strong phase, it is normally very brittle. When

the martensitic alloy is heated, the carbon trapped in the martensite diffuses to produce a

chemical composition that can either become pearlite, or bainite. However, it is not possible to

create a structure of pure pearlite or bainite, except by slowly cooling from the austenite phase.

Therefore, the result of tempering will allow the alloy to become more ductile, but the traces of

martensite within will still harden the material.

Conclusion

Steel is a very versatile alloy, in part to the many variation of its properties, which can be affected

by heat treatment.

The mechanical properties of steel is affected by the carbon dissolved in it, with the hardest being

when the particles are the most finely dispersed, as the carbon atoms are dissolved randomly in the

solution. This is the martensitic structure, formed by quenching austenite rapidly in water.

If the austenite is allowed to cool, the austenite will precipitate to ferrite. Below the eutectoid

temperature, the carbon will also diffuse to cementite. The grains get increasingly smaller with more

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cooling, thus making the structure harder. This is being there are more obstacles per unit area of slip

plane.

Some of the hardness measurements could have been slightly off. When submerging the steel into

water after heating it to 900°C, more than an eighth of an inch was initially submerged. After realizing

this, the alloy was pulled up slightly to make sure only an eighth of an inch was submerged.

For future research, analyzing a steel with a greater weight percent carbon could help explore the

large role carbon plays in strengthening iron.

References

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“Martensite” 21, Feb 2009. Wikipedia. 14, April 2009. <http://en.wikipedia.org/wiki/Martensite>

Lancaster, John. “Handbook of Structural Welding” Abington Publishing.

“The Tempering of Martensite: Part One” Key To Steel. 1999. 14, April 2009. http://steel.keytometals.com/Articles/Art127.htm

Appendices

Table 1: (length in inches)

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All with major load 60 lb, minor load 10 lb.

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