1896 1920 1987 2006

Metallographic Specimens

Observation

Group members：

Yuxin Chen, Muyang Xu,

Yifan Wu, Xieyu Tu,

Yuqing Wang, Chenyangzi Lin

Instructors：

Prof. Weimin Zhou and Prof. Xueyan Wu

Specimens

•Steel T10 annealing, carbon content 1%, annealing and slow cooling

•Steel T10 water quenching, carbon content 1%, quenching in water

•Steel T10 normalizing, carbon content 1%, normalizing, cooling in air

•Steel T10 oil quenching, carbon content 1%, quenching in oil

•Steel 45 annealing, carbon content 0.45%, annealing and slow cooling

•Steel 45 water quenching, carbon content 0.45%, annealing and slow cooling

•Steel 45 oil quenching, carbon content 0.45%, annealing and slow cooling

Introduction

Carbon steel

Table1 The microstructure of different types of carbon steel

type Carbon content microstructure

Pure iron <0.02 Ferrite

Carbon steel hyposteel 0.02~0.8 Ferrite+pearlite

eutectoid steel 0.8 Pearlite

Hypereutectoid steel 0.8~2.06 Pearlite+Secondary

cementite

White cast iron Hypoeutectic white

iron

2.06~4.3 Pearlite+Secondary

cementite+ledeburite

Eutectic white iron 4.3 ledeburite

Hypereutectic white

iron

4.3~6.67 Ledeburite+cementite

Critical temperature

Steel grade Ac1 Ac3 Ar1 Ar3

45 724 780 682 760

T10 730 800 700 -

Table2 The critical temperature of the carbon steel

Introduction

Annealing

Increase its ductility and reduce its hardness

Heat a material to above its recrystallization temperature

Temperature preservation

Slowly cooling.

Introduction

Quenching

Heat to the temperature above Ac3 or Ac1

Heat preservation

Great cooling rate to cooling the material rapidly below

Ms

Salt water, water, mineral oil, air, etc.

Make martensite or bainite transformation from

supercooled austenite

The strength, hardness, wear resistance, fatigue strength

and toughness can be significantly improved

Satisfy ferromagnetism, corrosion resistance

Introduction

Difference between normalizing and annealing is that the

cooling rate of the former is slightly larger than the latter.

T10 annealing

100x 500x

Results and analysis

T10 annealingThe general

appearance of the

surface of T10

steel after

annealing can be

seen, in which the

white and bright

area shows ferrite,

the layer

structure shows

pearlite, and the

dark area in

brown color

shows pearlite

with denser layers.

Most of the

microstructures

photographed

through the objective

lens magnifying 50

times are pearlite. In

the structure of

pearlite, both ferrite

and cementite are

white, black and

brown thin area

shows their

boundaries, because

the interface can be

easily corrupted by

etchants.

T10 water quenching

Its grain sizes are

much smaller than

that of a usual

sample. The

relatively whiter and

brighter areas are

ferrite, while the

denser and darker

areas can be deduced

to be martensite and

retained austenite.

The curve noted V4

represents the

cooling situation

when quenching in

the water. Due to

the rapid cooling

rate, other phases

have not enough

time to form, and

eventually the

martensite and

retained austenite

can appear.

The microstructure of

plate martensite in

dark color can be

observed distinctly.

Compared with the

microstructure of the

sample quenching in

the oil whose cooling

rate is relatively lower,

the grain sizes of the

sample quenching in

the water are much

smaller resulting in

higher hardness.

T10 oil quenching

100x 500x

The white part is

the residual

austenite and the

dark part is the

martensite.

Some pearlite

microstructure

which is

composed of

ferrite and

cemenite can also

be seen clearly.

The white

part is ferrite

The dark

area is

cementite

and pearlite,

evenly

distributing

at the ferrite.

T10 normalizing

part A is the network cementite, Fe3C, containing 6.69% carbon, which is hard and

brittle, the hardness is about 800HB. Part B is pearlite, which is a two-phased

lamellar (or layered) structure composed of alternating layers of ferrite (88 wt%)

and cementite (12 wt%). And C is denser layers of pearlite, which is shown like

massive structure.

Compared with metallography of T10 by annealing, the grains of the specimen

under normalizing are much finer, and the distribution of the phases are much

more uniform.

A

C

B

45 steel after water quenching

100x 500x

Method of water quenching

1. Heat the steel up to a very high temperature. At

this temperature, it's easy to shape the steel.

2.Rapidly cool it down by water.

Have you ever seen water quenching?

Here I show you a picture of quenching.

According to our

knowledge,

during quenching

process, austenite

will become

martensite.

Here in this

picture, we can

see some needle

like structure.

That is martensite

structure.

Because

martensite is very

hard, so the steel

becomes harder.

45steel after oil quenching

100x 500x

If we use oil to replace

water, then the cooling

rate is lower than

water. So the structure

will have some

differences with water

quenching.

In this figure, we can

see that its grain size is

smaller than grain size

after water quenching.

Besides, there are some

other structures in this

figure.

45steel after annealing

100x 500x

The method of annealing

1. Slowly heat the steel up. For the 45steel that we

observe, usually it's heated up to the point that all

the microstructures become austenite.

Ferrite+pearlite→Austenite

2. After maintaining at that temperature for enough

time, then gradually cool it down.

Austenite→Ferrite+pearlite

3. Seems nothing has changed? Actually, during the

annealing process, the grain size becomes smaller

and defects become less.

As we can see in

this figure, the

grain size after

annealing is the

smallest.

And we can see

some plate like

structures, it's

formed during

cooling process.

Conclusion

1. Firstly, Annealing can decrease grain size and reduce

defects.

2. Compared to annealing, normalizing can show finer

grains and more uniform distribution of phases.

3. As to quenching, water quenching and oil quenching

show different microstructures according to different

materials.

Thank you！