Nodular cast iron is produced by adding magnesium to the molten grey cast iron. When Mg (magnesium) added to graphite cast iron, it forms nodules instead of flakes. In this article, you’ll learn about Nodular cast iron.

Nodular cast iron is also known as ductile cast iron, spheroidal cast iron or SG iron.

Problem with Grey Cast Iron

Gray cast iron consists of graphite flakes. These flakes make the cast iron brittle and thus a problem for steelmakers.

If you consider the flakes in the cast iron, each flake has very fine ends. Due to the small area at the end of graphite flakes, once the load is applied to the material it can cause heavy stress concentration at the tip. This stress concentration can cause cracks at the tip and cause the material to fail.

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Annealing and tempering are both heat treatment processes. It improves some properties of metals and alloys. But there is a fundamental difference between them. Here we will try to find the answer to a very common question that which heat treatment process used for softening hardened steel?

Process of Heat Treatment

Before understanding the basic difference between annealing and tempering, one has to understand the general process of heat treatment. The heat treatment can complete in the following 3 steps.

1. Heating a metal (or alloy) at a specific temperature.
2. Holding a metal (or alloy) at an increased temperature for a specified period.
3. The cooling of the metal (or alloy) according to the specified process and medium.

These three phases will determine the heat treatment process and improve some mechanical properties in the metal or alloy.

Annealing Vs Tempering

Annealing and Tempering, both are used mostly for softening the steel. But both are the two different methods of heat treatment. Let’s have a look at their objectives.

The main objectives of Annealing are:

1. Soften the metal, so that metal can be cold worked or easily machined.
2. Refine the grain structure and its size in order to increase strength and ductility.
3. Relief the internal stress which may be caused by hot or cold work.
4. Altering the magnetic, electrical, and physical properties.
5. Remove the trapped gas in metal during casting.

The main objectives of Tempering are:

1. Reduce the brittleness of already hardened steel.
2. Increase the ductility and toughness of hardened steel.
3. Reduce the hardness of hardened steel and make it soft.
4. Relief the internal stress which may be caused by the rapid cooling of steel during quenching.

Annealing

In the annealing process, steel is heated above the recrystallization temperature. Hold the steel for some time and then slowly cooled. Annealing can also be categorized as Full Annealing and Process Annealing.

Full Annealing Heat treatment process:

The main objective of the full annealing process is to:

1. soften the metal
2. refine its grain to increase ductility
3. relieve internal stresses.
4. remove trapped gases in the metal

The process consists of heating the steel at about 50°C above the upper critical temperature for hypo-eutectoid steel and 50°C above the low critical point for hyper-eutectoid steel. Then keep the steel at this temperature for sufficient time, So that all the steel transforms into austenite or austenite-cementite. It is then allowed to cool in the furnace for some time. The cooling rate can range from 20°C to 200°C per hour depending on the steel and desired properties.

Process Annealing Heat Treatment process

The main objective of the process annealing is to:

1. relieve internal stresses.
2. Increase the Machinability of steel.
3. Softening of low carbon steel for cold work

Process annealing generally for low carbon steel(< 0.25% Carbon) which is work hardened. For this process, low carbon steel is heated close to the lower critical temperature (below the ferrite-austenite region).

This is held long enough to permit recrystallization of the ferrite phase, then cooled in still air. Since the material stays in the same phase throughout the process, the only change that occurs is the shape,
size, and distribution of grain structure.

Tempering

Tempering is a re-heating process subsequent to quench hardening. Quenched hardened steel is very brittle to work. This brittleness can reduce by tempering method.

When the medium carbon steel is heated above the upper critical temperature and sudden (rapidly) cooled in a suitable medium, austenite transforms into martensite. This martensite is very hard and brittle in nature. Also, it is a somewhat unstable structure. That is why tempering is done immediately after quenching (when steel cools down about 40°C).

In the tempering process, quenched hardened steel is heated below the lower critical temperature. At 200°C to 250°C, the internal stress will be removed and the ductility will increase without changing the structure of the martensite and without compromising the hardness of the steel.

At 300°C to 400°C, a softer and tougher structure troostite will form. This troostite is responsible for reducing the hardness. Above 400°C martensite begins to change into sorbite and completely changes at 700°C. Sorbite is much softer and less strength than troostite but it increases ductility.

Up to 350°C, mineral oil is best for tempering. Above 350°C, the liquid salt bath (a mixture of nitrate and nitrites) are preferred. It is very important to heat the steel gradually to avoid cracking. After heating to the desired temperature, the steel has to hold at this temperature for some time (depending on the dimensions and structure of the steel) and then cooled in still air.

So, tempering is performed on hardened steel for the desired combination of hardness, strength, ductility, toughness, and structural stability. Now, decide yourself, which heat treatment process used for softening hardened steel? Comment Below.

Read the book Material Science by R.S Khurmi for more knowledge.

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Mostly, iron ore is found in the form of oxides. But it is also present in the form of carbonate and sulfide.

What is Iron Ore?

Iron ore is a mineral found in nature from which ferrous metal can reasonably be extracted. Iron ore is actually rich in ferrous metal. Such as magnetite, hematite, goethite, and limonite are iron oxides while siderite is iron carbonate. Iron pyrite is also a mineral of iron sulfide.

How is Iron Ore formed?

Over billions of years ago, the oceans contained an abundance of iron but no dissolved oxygen(free oxygen in water). The iron ore started to deposit when the organisms became capable of photosynthesis and began releasing oxygen into the water. As free oxygen is available in water, dissolved iron begins to form hematite and magnetite.

Read The Effect of Tungsten on Steel and Different material.

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The effect of tungsten on steel has always been an important question for materials scientists and engineers. Hence in this article, we are going to study tungsten and its effect on steel and on other materials.

Background information

Tungsten is one of the rare metals on earth. Its chemical symbol is ‘W’ that stands for Wolfram. Tungsten is actually a Swedish word which means “heavy stone”.

Pure Tungsten metal was first found in 1783 by two Spanish chemists the de Elhujar brothers. Due to its highest melting point and lowest vapor pressure of all metal, it always became an interesting metal to study. As the steel industry began to grow and began to be used on a large scale, engineers began to need some more qualities and properties that steel alone could not achieve. Such as corrosion resistance, strength, hardness, red hardness, rising critical point, etc. Tungsten was then introduced into steel for these additional properties

Tungsten in Steel Alloy

Steel itself is an alloy of iron(Fe) and carbon(C). We can get a variety of steel with different properties by mixing different metals. Tungsten is one of the first metals used to improve steel.

Important Properties of Tungsten (W) for steel

1. The highest melting point of all metal. i.e. 3,422°C (6,192°F ).
2. The highest tensile strength at temperatures over 1650°C.
3. Expands less than any other pure metal from heat.
4. High resistance to corrosion.

Effect of tungsten

1. Tungsten when added to steel, will raise the critical temperature.
2. It will improve red hardness of steel.
3. Tungsten when added to steel, will improve resistance to plastic deformation.
4. Tungsten when added to steel, improves wear resistance at high temperatures.

Application of Tungsten steel

1. Tungsten steel as tool steel for cutting other material.
2. Tungsten steel as duplex steel for corrosion resistance applications such as offshore oil and gas production.

Click here for Material Science Quiz.

References

• https://www.tungsten.com

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Vantablack was the famed as blackest material until this anonymous material did not invent. Vantablack can absorb 99.96% of visible light. However, this anonymous material can swallow 99.995% of visible light. This is a super black material on the earth.

Carbon Nanotubes (CNT) Technology

This material can absorb 99.995% of light shining on it, darker than Vantablack. This Unnamed super black material is made of vertically aligned carbon nanotubes (CNT).

It can reflect 10 times lesser light than other super black material including Vantablack. Actually, like many other inventions this super-black material invented by accident. The researchers from the Jiao Tong University, China were seeking to develop a CNT on conductive material such as aluminium. In this experiment, they noticed the black of the material on specially treated aluminium foil.

“Before testing, I remember recognizing its black colour. After that, it looks even darker. Therefore, I think it is advisable to measure the optical reflection of the sample, ”said mechanical engineer Kehang Cui from Jiao Tong University, China.

Despite the invention of the planet’s darkest material, Scientists have nevertheless to clarify. This black material will be on display at an art exhibition “The Redemption of Vanity” in London.

Experiment by Massachusetts Institute of Technology

The Massachusetts Institute of Technology (MIT) artist-in-residence Diemut Strebe worked with scientists and coated a 16.78-carat yellow diamond with this super-black material. Because of the extraordinary quality of this CNT, it makes the lightless void like a black-hole.

It is very obvious that in future we can use this material where we need to remove the light such as in optical devices. In the Space telescope, we could use it for space exploring in a much better way.

The research has also published in ACS Applied Materials & Interfaces. The details by the scientists are also public for everyone.

Vantablack Car

Previously, BMW came with the Vantablack car X6 SUV, and literally it is very difficult also for LiDAT to catch it. Now, Vantablack paints are also available in the market in the can.

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Answer: Modulus of elasticity.

Stiffness: Stiffness is a structural property of a material that offers resistance to deformation when an external load is applied.

The unit of stiffness is N/m (Newron/meter). But Young’s modulus or modulus of elasticity determines the stiffness of a material. To understand stiffness in a simple way,

For example, consider two different materials. Material 1 and material 2. We have to increase its length by 1 mm. Material 1 needed 50 kN force and material 2 needed 100 kN force. So we can say material 2 is stiffer than material 1. Because material 1 deformed easily with less force but material 2 resist more against deformation or required more force.

On the other hand, Young’s modulus is the mechanical property of the material which shows the relationship between stress and strain. By Hook’s law, within elastic limit stress is directly proportional to strain. Which means

E is Young’s modulus or modulus of elasticity.

In brief, It means stiffness depends on the modulus of elasticity or Young’s modulus of the material. And of course on the geometrical shape of the object. The value of E (Young’s modulus) is higher means of stiffer material.

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