Carbon Steel: The Ultimate Guide

Carbon steel is the main material of today's industry. It's used in bridges, pipes, car axles, springs and kitchen knives. It is also the most prevalent metal of the world.

But there is no such thing as a single carbon steel. It is a family. Add a small amount of carbon and the soft flexible metal can become a hard brittle metal. This guide explains how to do this. You will learn the types, the main numbers and the most appropriate uses of these.

What is Carbon Steel?

Carbon steel is a type of alloy steel containing carbon and iron. Carbon is the main alloying element. The metal is over 98% iron.

Other elements are found in minor concentrations. AISI/SAE requirements do not specify a minimum for elements such as chromium, cobalt or nickel for carbon steel. Manganese can reach 1.65%. Silicon and copper are both kept at or less than 0.60%.

Typically, the carbon content is in the range of 0.05% to 2.1%. This is sometimes expressed as carbon wt. or wt.%. It is a term for weight percentage. A steel containing 0.20 wt. % carbon has 0.20 g of carbon atoms for each 100 g of steel.

What is it about the Plain Carbon Steel?

The most basic is the plain carbon steel. It contains high levels of iron, carbon and low levels of manganese and silicon. No attempt is made to leave out special features. No Chromium or Ni to improve its performance.

This preserves the simplicity of the process and also means the costs will not be high. Plain carbon steels are ones that are made of 1018 or 1045 steel. What is generally referred to as carbon steel is in this category.

Process and manufacturing of producing carbon steel

Carbon steel is primarily from iron ore. The ore is put into a blast furnace along with coke and limestone. Coke is the fuel. Limestone is used to remove impurities. The temperature in the furnace is about 1650°C and the ore liquefies to become molten iron.

There is a transfer of carbon to the iron from the coke. It is now being manufactured with a carbon content of about 4% by weight. This is beyond the capacity of the steel. Therefore, oxygen is blown into the melt by the makers. The unburned carbon is oxidized by the oxygen. As soon as the carbon reaches the desired level, they stop.

Instead, there are many mills that are now using electric arc furnaces. These melts the scrap steel and provide a more precise control of the final mix of steel with electricity. After the steel is melted, it can then be heat treated or annealed. The following steps are combined to give it a little more strength and ductility.

The influence of Carbon on the properties of Steel.

Carbon is a fortifying agent. As the carbon level rises the structure of iron becomes stiffer. Strength and hardness get greater. However, they are a little more expensive. Ductility drops. Weldability drops. The steel becomes brittle and at higher carbon grades, can break with sudden impact.

The compromise is at the very heart of carbon steel. It is not possible to obtain the maximum hardness and the maximum ductility in the same sample. You choose the balance which is suitable for the job.

The question "Is carbon ductile?" is commonly asked. There is no one correct answer, it depends on the meaning. Graphite or Diamond is pure carbon which is non-ductile. Does NOT become wire when stretched. But in the case of carbon steel, that is not so. Low carbon steel is very ductile. The higher the carbon content, the less ductile the product will be. The elongation of high carbon steel is generally only 5%-15%.

Types of Carbon Steel

Carbon steel falls into four groups. Each one has its own carbon range and its own strengths.

The lines are drawn a bit differently by the sources. Others set the low carbon limit at 0.30%. Specific cut-offs vary between each standard.

Carbon in Low Carbon Steel

In low carbon steel, the amount of carbon is kept below approximately 0.25% to 0.30%. It is also known as mild steel. It is the most common type of carbon steel. It is less expensive, welds easily and produces readily.

Low carbon steels contain a very small amount of carbon which will not allow them to harden through after quenching. Makers strengthen it instead using cold working. They may be hardened surface only by carburizing. Typical grades are ASTM A36, AISI 1018 and SAE 1008.

HSLA steel (high-strength, low-alloy) is a close cousin. It adds small amounts of copper, nickel, vanadium or molybdenum. That provides additional strength and increased corrosion resistance, but retains good formability.

Medium Carbon Steel

Medium carbon steel is steels that contain between 0.25% and 0.60% carbon. It also contains a typical amount of manganese, ranging from 0.60% to 1.65%. Manganese strengthens the steel and makes it harder.

The most desirable property is its response to heat treatment. You preheat it up, send it to the quenches and then temper it. The steel becomes stronger and retains needed toughness. Common grades are AISI 1045, 1050 and 1144. They are found in gears, shafts and axles.

High Carbon Steel

High carbon steel has carbon content ranging from 0.60 to 1.25%. This is the metal that is used in knives, springs, cutting tools and other high carbon applications. Highly resistant to digestion and slow in wearing down. However, it is not very ductile. More difficult to machine and less resistant to cracking when welded.

These include tool steels and die steels. They are added with chromium, vanadium, molybdenum and tungsten. These are the ones which form hard carbides which increase wear resistance. Typical grades available include AISI 1075, 1095 and D2.

Ultra-High Carbon Steel

The steels that have the highest amount of carbon content that can be achieved are called Ultra-high carbon steel. It holds about 1.25% to 2.1% carbon. Hardness can pass 60 HRC. This makes it one of the toughest of all steels to work with.

The price to pay for a payment is brittleness! Can be difficult to handle and may not be appropriate for jobs that involve flex. It glitters where a hard, crisp, long-lasting edge is needed. Consider cutting tools, knives and some industrial components.

Carbon Steel Properties

There are some properties which only move a little from one grade to another. Some people walk a considerable distance carrying carbon content.

The density of Carbon Steel is 7.85 g/cm³.

The density of carbon steel is about 7.85 g/cm³. That equals 7,850 kg/m³, or 0.284 lb/in³. This is essentially constant regardless of the amount of carbon in the steel. Thus, the density of low carbon steel and high carbon steel are almost the same. This number is used by engineers to calculate the weight of beams, plates and parts of machines.

Young's Modulus of Carbon Steel.

The Young's modulus of carbon steel is approximately 200GPa, or approximately 29,000ksi. It is also consistent in the mild, medium and high carbon grades. This number indicates the stiffness of the steel. Stiff metal is not easily bent when loaded. Hence, carbon steel is suitable for structural frames, heavy machinery and springs.

The melting point of carbon steel is 1370°C.

The melting point of carbon steel falls between about 1,425°C and 1,540°C. Steel does not melt at a single temperature in Fahrenheit, it is approximately 2,500°F to 2,800°F. It melts over a range of temperatures and becomes a liquid. There is a slight reduction in melting point with increasing carbon content. Low to medium carbon steels melt to the high end.

Thermal Conductivity for Carbon Steel Steel.

The thermal conductivities of carbon steel are approximately 45 W/m·K to 55 W/m·K at room temperature. Increases slightly with carbon increase. This is much less than copper or aluminum. However, this is better than stainless steel. The values vary with grade and temperature, refer to the data sheet for your particular material.

Magnetism, Rust and Temperature

All of the carbon steels are magnetic. This is because most of them are composed of iron. Magnets are used to separate the scrap. Magnetic chucks are used to hold parts in the hands of machinists.

Carbon steel does rust. It does not have a chromium coating like stainless steel. The surface becomes iron oxide when it is exposed to moisture and oxygen. The majority of parts require a coat. All of the following works with paint, powder coating, plating, black oxide, galvanizing and oiling.

Temperature also matters. In very cold conditions (-29°C to -45°C), carbon steel may become brittle. Low carbon grades are most at risk. The strength degrades rapidly with temperature at the high end, above approximately 500°C.

Cost

Carbon steel is inexpensive. Iron and carbon are plentiful. It's easy to do. It is produced in great quantities by mills. It is also 100% recyclable.

The yield strength and tensile strength of carbon steel.

Yield strength: that stress at which steel begins to deform for good. This is what is meant by people when they talk about carbon steel yield or carbon steel yield stress. It tends to increase with the increase of the carbon content.

The table below shows typical values for common grades.

Consider these to be regular numbers. The real numbers are modified by heat treatment, cold working and part size. For instance, A36 is given as a yield strength of approximately 250 MPa in some standards. Please use the data for your specific grade.

Common Grades at a Glance

There are 3 grades of low carbon that recur frequently. ASTM A36 holds about 0.25% to 0.29% carbon. It is used in frames and bridges, and comes in the form of plates and beams. It has a yield strength of approximately 250 MPa. AISI 1018 holds 0.15% to 0.20% carbon. Cold drawn bars are obtained with a yield of approximately 370 MPa. It is an easy machine to work with and is appropriate for shafts, pins and gears. SAE 1008 holds 0.10% carbon or less. It is soft, very high yield and forms very well. This makes it a popular choice for automotive use and applications.

Of medium carbon grades, AISI 1045 and 1050 are the classics. The 1045 is quenched and tempered to yield 450-600 MPa. 1050 is a slight increase. Both are heat treatable. AISI 1144 is a free-machining, sulfurized steel. Easy to cut and poor weld.

In the high carbon range, AISI 1075 is a tough spring steel. AISI 1095 is a popular choice of knives. D2 tool steel holds 1.40% to 1.60% carbon. It is not only harder at 58-62 HRC, but it also has a higher wear resistance. It is used in the shops for stamping dies and shredder blades.

High Carbon vs Low Carbon Steel

This is what people look for the most when comparing. The side-by-side view is below.

So which one should you select? Use for easy forming, easy welding and low cost – low carbon steel. If a combination of strength and toughness is desired, choose medium carbon steel. If hardness and edge life are most important, go with high carbon steel; if edge life is more important, sacrifice some hardness and go with high carbon steel; if some edge life is acceptable, go with plain carbon steel.

Applications of carbon steel.

Low carbon steel is used in structures such as buildings and bridges as a skeleton. It also uses for car bodies, car chassis and water and gas pipes, furniture and household appliances. It is commonly used in the energy field for pipelines and pressure vessels due to its weldability and toughness.

Medium carbon steel can be seen where parts are subjected to repeated stresses. Examples include crankshafts, axles, gears, couplings and sprockets. It is also used in railway tracks and train wheels.

Knives, saw blades, drill bits, springs, wire ropes and cables are all made from high carbon steel. Ultra-high carbon steel is the strongest against all sorts of wear. These can be found in cutting tools, heavy duty bearings and mining drills. In addition, carbon steel is used in cookware, such as chef's knives and frying pans. It's safe to use and will not harm the child if it is not rusted.

Machining Carbon Steel

In most cases carbon steel machines well. However, the grade is an important factor. Low carbon steels such as 1018 are easy to cut and dull cutting tools quickly. Which is why it's ideal for large volumes. Medium carbon steel (such as 1045) is an easy choice for mechanical components and provides a clean surface finish. High carbon steel, such as 1095, is more difficult to cut. It requires lubricious cutting speeds, carbide tools and ample cutting fluid.

The primary foe is heat. Causes wear of tools and may cause bending of thin components. It's controlled by good tool paths, coated carbide tools and strong cooling.

The advantages and disadvantages of carbon steel.

Carbon steel is a pretty good material, to say the least. It has a low cost, and is strong and flexible about its uses. A broad spectrum of properties are available through heat treatment. Takes coatings well and is 100% recyclable.

It does have its limitations. Requires protection from rust. It is less strong at temperatures above 500°C. Can become brittle in very cold temperatures. The high carbon grades are also difficult to weld and to machine.

Final Thoughts

Carbon steel is cheap, durable and easy-to-make. Carbon is the secret of its. Tune the amount of metal and you have a malleable metal that readily welds together. More yields a hard metal with an edge.

The density, Young's modulus and melting point is consistent throughout the grades. The ones that move are strength, hardness and ductility. So, begin with what your part is supposed to do. Then compare the carbon level with that job.

FAQs

1. What is the density of an iron steel?

 The density of carbon steel is about 7.85 g/cm³, or 7,850 kg/m³. It remains almost constant for low, medium and high carbon grades.

2. What is the melting point of Carbon steel?

 It melts between about 1,425°C and 1,540°C (2,500°F to 2,800°F). The lower the carbon content the lower the melting point.

3. The difference between high carbon steel and low carbon steel is:

 Low carbon steel has a lower carbon content and is less hard, ductile and easier to weld. High carbon steel is harder and stronger than low carbon steel. However, it is less malleable and difficult to weld and cut.

4. Does carbon steel rust?

 Yes. It will rust if exposed to moisture and oxygen. It can be protected with paint, galvanization, plating or black oxide.

5. What is the Young's modulus of carbon steel?

 It is about 200 GPa (29,000 ksi). This value remains fairly constant for all carbon steels.