Mindanao State University – Iligan Institute of Technology

College of Engineering

Department of Ceramics/Chemical/Metallurgical Engineering

Microscopic Examination of Ferrous and Non-

ferrous Alloys

Submitted to:

Dr. Mercurio I. Espinido

Submitted by:

Lagat, Carl Nikko R.

Marcera, Alexander Jethro P.

Nacario, Johannah Daphne M.

Abstract

Metallurgical examination of metals have really had a great help in determining the

existing phase in a metal, thus making it possible to determine the properties of a metal, whether

it is malleable or brittle, etc.

In this experiment, samples were obtained and their microstructures were observed under

a metallurgical microscope prior and after etching with Nital and Picral Etching Solution.

By the end of the activity, it was concluded that in order to study better the

microstructure of an alloy, etching should be done for revealing martensite, ferrite, pearlite,

cementite, etc.

Introduction

The microstructure of iron-base alloys is very complicated and diverse, being influenced

by chemical composition, material homogeneity, processing and section size. Metallographic

examinations of ferrous and nonferrous alloys have a wide range of uses especially in failure

analysis, and in research. It is very useful in the identification of the phases of an alloy and its

severity, and even its properties since it shows us a more detailed view of the alloy

microscopically; however, in order to fulfill this, an alloy must be subjected to etching, a process

in which dissolution of metal atoms at the junction of two grains, or between smaller micro

constituents and the surrounding matrix occurs.

Ferrous metallographers tend to use nital almost exclusively for etching, but nital is not

always the best reagent to use to properly reveal all microstructures. Picral is an excellent etchant

for revealing certain micro-structural constituents in steel, but the use of picral is prohibited by

some companies because picric acid can be made to explode under certain conditions. On the

other hand, the microstructures of non-ferrous metals, which could be either in pure or in alloy

state, are somehow not much complicated such as copper and aluminum since they can be

produced economically even without an alloying element.

This experiment tends to familiarize students with the typical microstructures of plain

carbon steels, cast irons and some of the non-ferrous alloys, to determine the effect of the type of

the etchant on the appearance of the phases in the microscope, and to identify the phases based

on the appropriate equilibrium diagrams (non-ferrous).

Experimental Methods

a.) Etchant Preparation

2 mL of HNO3 and 98 mL ethanol or methanol (95% or absolute; also amyl alcohol) was

obtained in a beaker. This solution is the Nital Etching Solution.

In a 250 mL beaker, 100 mL ethanol or methnol (95% or absolute; use alcohol only when

acid contains 10% or more moisture) and 4 grams of picric acid were prepared. To complete the

Picrant Etching Solution, 4 to 5 drops of 17% zephiran chloride (wetting agent) was added.

b.) Sample preparation and etching

b.1) Ferrous samples

Samples were examined prior to etching and their microstructure was observed and

sketched.

The annealed samples of hypo-, hyper- and eutectoid plain carbon steels were polished in

a polishing wheel and were etched in a 2% natal solution. The samples were examined under a

metallurgical microscope and their microstructures were sketched.

Hypoeutectoid and hypereutectoid samples were re-polished in the polishing wheel and

re-etched in 1% picral solution. Under the microscope, samples were re-examined. The

differences in the appearance of the phases using different etching solution were noted.

b.2) Non-ferrous samples

Prepared samples of non-ferrous alloys were examined and sketched to relate their

microstructures to the appropriate equilibrium diagram and to identify the phases present.

Data and Results

Figure 1. Sample, Nital etched and Picral etched

Figure 2. Sample, Nital etched and Picral etched

Figure 3. Sample, Nital etched and Picral etched

Figure 4. Sample, Nital etched and Picral etched

Figure 5. Sample, Nital etched and Picral etched

Figure 6. Sample, Nital etched and Picral etched

Figure 7. Sample, Nital etched and Picral etched

Figure 8. Sample, Nital etched and Picral etched

Figure 9. Sample, Nital etched and Picral etched

Figure 10. Sample, Nital etched and Picral etched

Figure 11. Sample, Nital etched and Picral etched

Figure 12. Sample, Nital etched and Picral etched

Figure 13. Sample, Nital etched and Picral etched

Figure 14. Sample, Nital etched and Picral etched

Figure 15. Sample, Nital etched and Picral etched

Discussions

I. Ferrous alloys: i.one hypoeutectoid steel and one high plain carbon steel

ii. four types of cast iron.

1. Compare the appearance of the phases of the plain carbon steels using nital and

picral etching solution.

Nital Picral

Nital (2%) revealed the structure of martensite, and ferrite in a martensite matrix

and to brought out ferrite grain boundaries in low-carbon steels. Picral, on the

other hand, revealed the cementite in ferritic alloys and the structure of ferrite-

cementite constituents, pearlite and bainite. If the amount and nature of the

cementite is to be examined, picral is more preferable than natal.

2. Sketch the microstructure of a hypoeutectoid steel sample and estimate its carbon

content using the lever rule.

estimate % C = .1 -.025 / .2 -0.025

figure: (top) sample number 6, (bottom) taken from the internet

(Sample number 6 displays similar microstructure with that of the photo from the

internet.)

3. Describe the microstructure of the cast iron samples prior and after etching.

One should always begin microstructural investigations by examining the as-

polished specimen before etching. Prior to etching, distorted and non-vivid

structure of its microstructure is sought. To see details of the matrix

microstructure, specimen must be etched. Etching methods based on chemical

corrosive processes have been used by metallographers for many years to reveal

structures for black-and-white imaging.

4. Sketch the four types of cast iron

White iron etched with 2% Nital Ductile iron etched with 2% Nital

Malleable iron etched with 2% nital Gray Iron etched with 2% nital

(Image for the Gray Iron is taken from internet source since sample is not available in the

experiment)

II. Non-ferrous alloys: i. one pure Al

ii. one brass sample

1. Speculate the possible composition of the sample.

Aluminium, Cobalt, Lead, Magnesium, Titanium and Zinc are the possible

compositions of the non-ferrous alloy samples.

2. Relate microstructure to the equilibrium diagram.

When etched with nital solution, the microstructure of a non-ferrous alloy

shows a little or no amount of ferrite (gray spots in the microstructure). When

observed in the phase diagrams of non-ferrous alloys, it typically don’t have a

ferrite composition.

Conclusions

It is a basic tenet of physical metallurgy that composition and processing establishes the

microstructure, and that microstructure influences most properties and service behavior. To

maintain control of the quality of steel products and to diagnose problems in processing, testing,

or service, the microstructure must be identified and, in some cases, quantified. This can only be

accomplished when the metallographer can properly distinguish the phases or constituents

present, which depends on proper specimen preparation and etching. Moreover, microstructures

of metals vary depending on their composition and on the phases existent.

References:

http://www.vacaero.com/Metallography-with-George-Vander-Voort/Metallography-with-George-

Vander-Voort/microstructure-of-ferrous-alloys.html

http://www.buehler-asia.com/brochure/apps_support-tech-notes_vol3iss7-tn37.pdf

http://www.thaithavorn.com/pdf/microstructure.pdf