Structural Study of Mild Steel

2010 Structural Structu ral study of Mild Steel S teel

It will help you to understand the experiment. Metallography or microscopy consists of the microscopic microsco pic study of the structural structural characteri c haracteristics stics of  of  a metal metal or a n alloy. Microscopic study study depends  largely upon the care taken in the the preparation pre paration of the Mild Steel. The ultimate object is to produce a flat, scratch free, mirror like surface.

StructuralstudyofMildSteel

K.M. Mostafizur Rahman SUST 5/22/2010

 Name of the th e Experiment Experiment: Structural study of Mild Steel.

(1) To To lea l earn rn ab about out the t he micro structure structure of Mild St St ee eell . (2) To To lea l earn rn the t he structural structural cha c haracteristics racteristics of steel. (3) To view what kind of structure mild steel has.

Introduction:

Metallography or microscopy consists of the microscopic study of 

the str stru uctura cturall c ha racter racteriist stiics of a metal or a n a lloy. Microscop ic st study udy depe depends nds largely 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, scratchfree, 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) In I nter term mediate polishing polishing.. (5) Etching etc.

 Now I’m going to describe descri be t he whole process that we’ve performed in our workshop during the opera op eration. tion.

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. Sample to be cut

Figure: Sample

Fig: Fi g: sampli sampling ng operation operation

specimen en: Specim 2. Mounting the specim Specimee ns are sma small ll or

awkwardly awkw ardly

shaped so they must be appropriately mounted in a suitable material or rigidly clamped in a mechanical vice. Workpiece

Vice

Figure: Fi gure: M oun ounting ting operation

Filingg or rough grin Filin gr inding: ding:

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 sm s moo ooth th file. Before fil filing ing operatio n we’ve attached our  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.

Figure: Filing operation

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.

Figure: Fi gure: Interm Intermee di diate ate polis hing (se rially)

4.Fine 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 NH 4 Cl salt and water as a smoothing agent. This fine polishing helps the leftover roughness from the specimen.

Figure: Fin Finee polishing Machine

Figure: After polishing

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 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.

Nitr Ni triic ac aciid (N (Nital) ital)

White nitric acid 1-5ml

In carbon steels:

Ethyl or methyl 100ml 1. To darken alcohol (95% or pearlite and give absolute) also amyl contrast contr ast bet between ween alcohol pearlite colonies 2. To reveal ferrite boundaries. 3. To diff d ifferentiate erentiate ferrite from martensite. Picr Pi criic ac aciid (picral)

Picr Pi criic ac id Ethyl or methyl alcohol (95% or absolute)

Etching rate is increased selectivity decreased with increasing percentages percen tages of  o f  HNO3. Reagent 2 (picric acid) usually superior.

4g

For all grades of  carbon steels steels::

More dilute solution solutio ns occasionally useful.

100ml

annealed. Normalized, quenched, and tempered, spheroidized, austempered. For

Does not reveal ferrite grain boundaries as readily as Nital etching time a few seconds to 1 min or more.

all low-alloy steels attacked by this reagent. Ferric chloride and hydrochloric acid.

Ferricc chloride Ferri

5g

Hydrochloric acid

50ml

Water

Ammonium hydroxide and hydrogen peroxide

100ml

Ammonium hydroxide 5 parts Water

Structure of  austenitic nickel and stainless steels

5 parts

Hy drog drogen en p erox eroxide ide 2 -5 parts

Generally used for copper and many of  its alloys.

Peroxide cont ent varies Peroxide directly with copper content of alloy to be etched immersion imm ersion or sw abbing for about 1min. fresh p erox eroxide ide for good results.

Among them we’ve 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.

Mettal Me allurgical lurgical Microsc M icroscope opes: s: 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.

Retinal image Human eye

Eye lens

Primary real image of object formed by objective & field lens of  principal focal point of eye lens or within focus distance as illustrated

Field lens

(image inverted and reversed)

Plane glass reflector Form light ligh t source source Back focal point of  objective Objective

Figure: illustr ill ustrating ating the pr principle inciple of the metallurgical compound microscopee and microscop an d the trace of rays through the optica opticall system s ystem from the object field to the final virtual image

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 planegrass 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.

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. Place to put the wor workpiece kpiece

Figure: Fi gure: Me tall tallur urgical gical micro microscope scope

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. possess poor hardness tha t hat’s t’s why it can’t be used in e ngineering application 4. As it possess without heat treatment.

Figure: Figure: Fe rrite rrite crystals crystals

Figure: Pearlite crystals.

Discussion: Study of microstructure of mild steel is very important because we can make chang chan ge of o f its properties by adding so me ot her ingredients ingred ients o n 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 We’re very thankful to our respect iv ivee teachers who’ve w ho’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 d id n’t know that before starting our operation so we suspend the operation after 2 week and started another one although the 1 st was not completed at all! And after setting the belt we started our first one when we are performing our 2 nd 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  st eel is any str stru uct ctu ural materia ateriall 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.