Structural study of Mild Steel It will help you to understand the experiment.

Metallography or microscopy consists of the microscopic study of the structural characteristics of a metal or an alloy. Microscopic study depends largely upon the care taken in the preparation of the Mild Steel. The ultimate object is to produce a flat, scratch free, mirror like surface.

2010

SUST 5/22/2010 Structural study of Mild Steel

K.M. Mostafizur Rahman

Name of the Experiment: Structural study of Mild Steel.

Objectives:

(1) To learn about the micro structure of Mild Steel.

(2) To learn the structural characteristics of steel.

(3) To view what kind of structure mild steel has.

Introduction: Metallography or microscopy consists of the microscopic study of

the structural characteristics of a metal or an alloy. Microscopic study depends largely upon

the care taken in the preparation of the Mild Steel. The ultimate object is to produce a flat,

scratch free, mirror like surface.

Mild Steel can be prepared either by hand polishing or machine polishing prior to

microscopic examination. Every substance in the world or universe is consists of smallest

particle, which is called atom. In metal also numerous particles are exist in particular shape

and size. We can find anything with substance is a magnified form. We can see the internal

structure, shape, size, arrangements of several considered by its whole characteristics of

internal structure. The microstructure will reveal the mechanical and thermal treatment of the

metal, and it may be possible to predict its expected behavior under a given set of conditions.

Experience has indicated that success in microscopic study depends largely upon the care

taken in the preparation of the mild steel. The most expensive microscope will not reveal the

structure of mild steel that has been poorly prepared. The procedure to be followed in the

preparation of mild steel is comparatively simple and involves a technique which is

developed only after constant practice. The ultimate objective is to produce a flat, scratch-

free, mirror- like surface.

Description: To achieve the expected shape of the specimen there are several

process which should be done very carefully and perfectly.

Preparation of specimen by hand can be done by the following successive steps.

(1) Cutting of the specimen or sampling.

(2) Mounting the specimen.

(3) Filling or rough grinding.

(4) Intermediate polishing.

(5) Etching etc.

Now I’m going to describe the whole process that we’ve performed

in our workshop during the operation.

1. Sampling: The choice of a sample for microscopic study may be very important.

If a failure is to be investigated, the sample should be chosen as close as possible to the

area of failure and should b compared with on taken from the normal section.

As we are going to study about mild steel so we’ve taken a cylindrical piece of mild steel

then we cut it as 2 cm. long piece by a hacksaw blade.

Fig: sampling operation

2. Mounting the specimen: Specimens are small or awkwardly

shaped so they must be appropriately mounted in a suitable material or rigidly clamped in

a mechanical vice.

Figure: Sample

Figure: Mounting operation

Vice

Workpiece

Sample to be cut

workpiece into a vice. Filing is done in one surface where the structure of mild steel is to be

inspected. Filing ensures the smoothness of the workpiece and the better the smoothness the

better the result.

3. Intermediate polishing: After sufficient filling we stopped filling and

then start polishing by a series of emery papers containing successively fine abrasives.

The first paper is usually No. 2.5 then 2, 1.5, 1 & 0. This type of polishing helps to make

mirror image type smoothness on the test part. The important thing is to maintain the

serial of the emery papers.

4. Fine polishing: In this stage, the specimens polished on a polishing machine

which contain velvet clothe on its rotator disc. In time of polishing operation we hold the

smooth side of our specimen on the rotator disc and give NH4Cl salt and water as a

smoothing agent. This fine polishing helps the leftover roughness from the specimen.

Figure: Filing operation

Figure: Intermediate polishing (serially)

Figure: Fine polishing Machine

Filing or rough grinding: Whenever possible, the specimen should be of a

size that is convenient to handle. A soft sample may be made flat by slowly moving it up and back across the surface of a flat smooth file. Before filing operation we’ve attached our

Figure: After polishing

highest energy, leading to surface relief which allows different crystal orientations, grain

boundaries, precipitates, phases and defects to be distinguished in reflected light microscopy.

There are many tried and tested etchants available but there are mandatory safety issues

associated with the preparation and use of all of these. Some etching reagents are listed in the

table.

Etching reagent Composition Uses Remarks

Nitric acid (Nital) White nitric acid 1-5ml

Ethyl or methyl

alcohol (95% or

absolute) also amyl

alcohol

In carbon steels:

1. To darken

pearlite and give

contrast between

pearlite colonies

2. To reveal ferrite

boundaries.

3. To differentiate

ferrite from

martensite.

Etching rate is

increased selectivity

decreased with

increasing

percentages of

HNO3. Reagent 2

(picric acid) usually

superior.

Picric acid (picral) Picric acid 4g

Ethyl or methyl

alcohol (95% or

absolute)

For all grades of

carbon steels:

annealed.

Normalized,

quenched, and

tempered,

spheroidized,

austempered. For

all low-alloy steels

attacked by this

reagent.

More dilute solutions

occasionally useful.

Does not reveal

ferrite grain

boundaries as readily

as Nital etching time

a few seconds to

1 min or more.

Ferric chloride and

hydrochloric acid.

Ferric chloride 5g

Hydrochloric acid

Water

Structure of

austenitic nickel

and stainless steels

Ammonium

hydroxide and

hydrogen peroxide

Ammonium hydroxide

Water

Hydrogen peroxide

Generally used for

copper and many of

its alloys.

Peroxide content varies

directly with copper content

of alloy to be etched

immersion or swabbing for

about 1min. fresh peroxide

for good results.

100ml

100ml

50ml

100ml

5 parts

5 parts

2 -5 parts

Etching: The purpose of etching is two-fold. Grinding and polishing operations produce a

highly deformed, thin layer on the surface which is removed chemically during etching. Secondly, the etchant attacks the surface with preference for those sites with the

Among them we’ve used Nital as the etching reagent for our inspection.

Inspection of microstructure of specimen: After etching operation

our workpiece is ready for final inspection. This time we used metallurgical microscope to

view the micro structure of mild steel.

Metallurgical Microscopes: At this point it is appropriate to discuss briefly the

principles of the metallurgical microscope. In comparison with a biological type, the

metallurgical microscope differs in the manner by which the specimen is illustrated. Since a

metallographic sample is opaque to light, the sample must be illustrated by reflected light.

A horizontal beam of light from some light source is reflected, by means of plane-glass

reflector, downward through the microscopes objective onto the surface of the specimen.

Some of this incident light reflected from the specimen surface will be magnified in passing

through the lower lens system, the objectives, and will continue upward through the plane-

grass reflector and be magnified again by the upper lens system. The total magnification is

equal to the product of the magnifications of the objectives and the eyepiece. It is possible to

mount a camera bellows above the eyepiece and the use table-type microscope for

photomicrography. However, the bench-type met allograph illustrated, which is specifically

designed for both visual examination and permanent recoding of metallographic structures by

photographic methods, will give superior photomicrographs.

Retinal image

Human eye

Eye lens

Field lens

Primary real image of object

formed by objective & field lens of

principal focal point of eye lens or

within focus distance as illustrated

(image inverted and reversed)

Form light source

Plane glass reflector

Back focal point of

objective

Figure: illustrating the principle of the metallurgical compound

microscope and the trace of rays through the optical system from

the object field to the final virtual image

Objective

Inspection process: This time we mount the smooth side of workpiece on the

metallographic microscope. After putting the specimen on the microscope and setting at

proper zoom we found a very clear view of the microstructure on the monitor of metallurgical

microscope.

Figure: Metallurgical microscope

Place to put the workpiece

Figure: Microstructure of mild steel

Characteristic of mild steel:

1. From the monitor we’ve found that the microstructure of mild steel contains ferrite

and pearlite.

2. As both ferrite and pearlite have very poor hardness so we can surely say mild steel

have poor hardness.

3. As both ferrite and pearlite can be heat treated so we can say mild steel can be heat

treated to change its properties.

4. As it possess poor hardness that’s why it can’t be used in engineering application

without heat treatment.

Discussion: Study of microstructure of mild steel is very important because we can make

change of its properties by adding some other ingredients on it. But before we’ve to know its

internal properties and this practical experiment explore a new window to learn about its

properties. We’re very thankful to our respective teachers who’ve given this great scope to

us. But we’ve also fallen some great problem in time of performing this operation which are

slow down our experiment greatly like one is, the rotator belt of polishing machine was tore

apart but the lab assistance didn’t know that before starting our operation so we suspend the

operation after 2 week and started another one although the 1st was not completed at all! And

after setting the belt we started our first one when we are performing our 2nd experiment it

means we are running 2 experiments simultaneously! We hope our respective teacher will

give our lab assistance such instruction that we’ll never fall such misshape.

Conclusion: It is very important an engineer to determine the internal properties of

any structural material that’s why the Study of microstructure of mild steel is very essential

for us. It is possible to determine grain size and the size, shape and distribution of various

phases and inclusions which have a great effect on the mechanical properties of the metal.

The macrostructure will reveal the mechanical and thermal treatment of the metal and it may

be possible to predict its expected behavior under a given set of conditions. This will help us

to choice right types of steel for various purposes also to develop new types of steel in future.

This experiment also explores a new concept about steel.

Figure: Ferrite crystals Figure: Pearlite crystals.

Edited by Sobuj

SUST

Dept of Industrial & Production

Engineering

Registration No. 2008334053

k.sobuz@gmail.com