Title - Microstructure Examination of Steel

To observe the constituents and structure of metals and their alloys by means of an optical microscope.

Introduction

This is about to examine the microstructure of steel and observe the constituents and structure of it. Microstructure means the structure that requires high magnification to be visible. To examine the microstructure of steel, microscopes are required. Conventional light microscope is used commonly as well as optical microscope may be used to look through specimens. However we are used the optical microscope, not the conventional light microscope to look at the surface of the specimens as many materials do not transmit light.

Microscopes can provide information about the particular features of material by concerning the composition, processing and properties of the material. We examine the microstructure of steel by using the optical microscope after grinding, polishing, and applying necessary chemicals to remove impurities to the specimen of steel. It is very important to prepare the sample correctly.

Theory

There are some procedures to follow before examine the microstructure of steel. They are,

Grinding, polishing, etching and mounting.

The specimen of steel should be ground first by using rotating discs which is covered with silicon carbide paper and water. It is necessary to apply light pressure at the center of the sample. We should continue grinding until the blemishes are removed.

Specimen of steel should be clear and reflective. To polish the specimen we use alumina powder or diamond paste. The specimen should be put on the rotating disk after pasting diamond paste or alumina powder.

To reveal the microstructure of steel we use dilute acid (2% Nital for steel) to etch. After etching specimen must be washed with alcohol. The specimen should me dry before examine the microstructure using microscope.

Five straight lines with same length are drawn on the image of microstructure that was printed to a A4 paper and then count the grains that are intersected by each line. After that total length of lines are divided by the total number of grains. Then we can calculate the average grain size.

Material and Apparatus

• A steel specimen
• Grinding and polishing machine
• Optical microscope
• Alumina powder or diamond paste
• Alcohol
• Dilute acid (2% Nital for steel)
• Mounting resins Water

Procedure

• A specimen of steel to be examined was sawn
• The specimen was grinded
• It was polished using alumina powder or diamond paste, until it projected a mirror image
• Then the specimen was etched in dilute acid (2% Nital for steel)
• After that the etched section was washed with alcohol and kept to dry
• Next, the specimen was put to the focus of an optical microscope’s lens and microstructure was observed
• The image on the microscope was printed to a A4 paper
• Five straight lines with same length were drawn on the microstructure
• Then the grains covered by the lines were counted and added them all
• Finally the total length of lines was divided by the total number of grains and average grain size was calculated.

Discussion

The phases present in the observed microstructures are grain and the grain boundaries.

Above experiment observed the microstructure of the steel. When describing the structure of the material we have to consider about two main things. They are crystal structure and the microstructure.

To describe the appearance of the material we use the microstructure and to describe the average position of atoms in a unit cell we use the crystal srutcure.So metals have a crystalline structure, which is normally not visible to naked eye. When the metal are melting and heating millions of crystals are starting to grow. When the melting is over, crystals become cool and become large crystals. This large crystal form of the material is called grains. In a crystalline structure, there are millions of grains that have their own orientation and crystal structure. The areas between grains are called as grain boundaries.

Grain boundaries are the areas between grains. Simply we can say that the interface between two grains is called grain boundary. In the crystal structure, grain boundaries are the 2D defects. Low angle and high angle grain boundaries are the different types of grain boundaries. If the disorientation between two grains is lower than 15 degrees is called low angle grain boundary. And also when the disorientation becomes more than 15 degrees is called high angle grain boundary.

Studying the structure of metal is known as metallography. Preparation of specimen for examination is an important part of metallography. To get correct observation and interpretation of the microstructure, the specimen should be prepared appropriately. Sample selection, sectioning, grinding, polishing and etching are consisting in specimen preparation. There are some important parameters in sample selection. They are the number, location and the orientation of the specimen examined.

The specimen surface should be smooth and mirror like finish. So we have to do sectioning, grinding, and polishing for that. We have to be care during sample preparation as artifacts may lead to invalid microstructure interpretation. To reveal the microstructure most materials are etched. After that using optical microscope, the specimens are examined.

Metallography is a very important part in examine the microstructure of steel. Quality controlling is the most important thing in the metallography. Cutting metals causes some side effects. It will change the properties of the surface and the deformed layers will be create at the surface. Those deformed layers are different from the underlying metal.Metallographers apply techniques to remove these shattered layers.

Microstructural testing techniques are used in some areas. Some of them are routing quality control, analysis of failure and research. To determine whether structural parameters are within specific specification, microstructure analysis is used. And also it is used in quality control. As a result of various parameters such as heat treatment, composition or processing steps there will be some changes in microstructures. To determine those changes, microstructural analysis is used in research studies. Using metallography we are able to observe steel samples. As well as determine the causes of failure of massive steel structures.

By means of metallography we can identify the causes of failure of massive steel structures.

Possible Erreors

We have to do the polishing process according to the required standards. If not, we will not be able to obtain the correct image easily and correctly.

It is necessary to use new specimen of steel for the experiment .A piece of steel that was previously used for the same experiment or and old piece of steel will cause errors. Because it will form rust again.

When we calculate the average grain size, there will be lot of inconsistencies .Because this is not always an absolute value. It will always depend on the position of lines and the length of them.

Human errors can be cause during counting the intersecting grains.

Possible Improvements

To get the correct observations, the specimen of the steel should be polished to the right amount.

When repeating this experiment, a new specimen should be observed by the microscope.

We can see the observations very clearly, so the microscope should be a good quality and higher resolution.

To obtain correct calculations, the intercept method of grain calculating should be done more than two times.

Microstructural examination techniques are used in some areas such as research studies, routine quality control and failure analysis. We can use microstructure analysis to determine if the structural parameters are within certain specification, in quality control. The amount of microstructural features can be measured and quantified by microstructure analyzing. Some of those microstructural features are grain size, amount of impurities, second phases and defects present.

To determine the cause of failure in failure analysis we can use microstructure analysis. When we do not select proper material and when the quality control is poor, failures may be occurred. To identify the material and the condition of it, failed component is used from a microstructural examination.

To find if the component was made from specified material, microstructure analysis can be used. Not only that, but also to find if the material received the proper processing treatments it can be used. To examine the fracture surface of the failed component, failure analysis is used with help of microstructural examination techniques. It will be provided information about cause of failure.

As a result of varying parameters, there will be microstructural changes. So in research studies, microstructural analysis is used to determine those microstructural changes .The parameters that occur those changes are, composition, heat treatment or processing steps.

Conclusion

The examination of microstructure provides many information about the structure of the material. It clarifies whether structural parameters are within certain specifications. Also it provides many information about properties and configuration of the microstructure.

For the evaluation of the material, the results that observed are useful. For the magnification process of the microstructure examination usually we use the optical microscope. Microstructure examination is used in industry to identify failure of materials.