What is steel?

Iron is made by removing oxygen and other impurities from iron ore. When iron is combined with carbon, recycled steel, and small amounts of other elements it becomes steel.

What is carbon steel?

Carbon steels are a series of alloys of carbon and iron containing up to about 1% carbon and up to 1.65% Mn, with elements added in specific quantities for deoxidization and residual quantities of other elements and it is the most simple and cheapest form of steel

It is usually defined as steel made of Iron (Fe) with small Carbon (C) addition but without any other alloying elements. This statement is not accurate since most Carbon Steels also contain small amounts of Manganese (Mn), Phosphorus (P), and Sulfur (S). The SAE designation of Carbon steel is in the format of 10xx. For example: 1020contains 0.2% of carbon and 1045contains 0.45% of carbon.

where: xx indicates the amount of carbon.

The higher the carbon content in the general carbon steel, the greater the hardness, the higher the strength, but the lower the plasticity, as well as responds to heat treatment (hardenability).

• Low carbon steel (C: ≤ 0.25%), also known as mild steel, low carbon steel is easy to accept a variety of processing such as forging, welding and cutting.
• Medium carbon steel (0.25% < C≤ 0.6%), there are killed steel, semi-killed steel, boiling steel, and other products.
• High carbon steel (C: > 0.6%), often used as a tool steel, with carbon content from more than 60 to 1.70%, can be hardened and tempered.
• Steel with less than 15% Carbon is very soft; thus, it is almost impossible to break the chips, and BUEis easily formed on the cutting edge.

Above 0.3%, the material becomes gradually harder, and therefore wear is formed faster on the cutting edge. 

The machinability is very sensitive to the amount of carbon since carbon directly influences the hardness of steel. The “Sweet Spot” that yields the highest machinability rating is around 0.2%.

Carbon Steel – Machinability Chart

Free-Cutting Steels

The steels for high-speed machining, commonly called free-cutting steels, have been specially designed to be machined by chip removal with high productivity.

Free-cutting steels is a nickname for carbon steel with an increased amount of alloying elements for the sole purpose of improving their machinability which are commonly used in many masses production fields such as the automobile industry and household appliances – contain sulfur and some other elements, including tellurium, bismuth, and lead, which promote machinability.

• 11xx series: the amount of Sulfur (S) is increased from 0.05% in plain carbon steels to 0.1%. It adds about 20% to the machinability when compared with the equivalent materials in the 10xx series. On the other hand, the tensile strength decreases by about 10%, and the material is more brittle, the sulfur ensures the fragmentation of the chip
• 12xx series: Sulfur (S) content is further increased to 0.25%, and Phosphorus (P) content is increased from 0.04% in the 10xx series to 0.5%. As a result, the machinability increases by another 40% at the price of further decline in mechanical properties.

(The sulfur ensures the fragmentation of the chip)

• SAE 12L14is a free-cutting steel where the Phosphorus is replaced by 0.25% of Lead (Pb), which boosts the machinability by another 35%. This improvement happens because the Lead melts locally at the point of cut, lead reduces the friction between the tool and piece, extending the life of the tools. However, many material manufacturers and machine shops try to avoid lead supplements due to environmental damage and health risks.
• Tellurium and Bismuth further accentuate these characteristics.

Free-cutting steels are usually supplied in bars or rolls without heat treatment; some of these can however be tempered, normalized, or annealed before finishing.

Range of production and execution

Machinability rate comparison between plain carbon steel and free cut steels
with the same carbon content

Low Alloy Steels:

A low-alloy steel is a type of metal mixture composed of steel and other metals that possess desirable properties (on top of the carbon and Manganese). Low-alloy steel contains about 1%-5% of alloying elements. Therefore, it possesses precise chemical compositions that provide better mechanical properties that are intended to prevent corrosion. These elements are added to improve the strength, toughness, corrosion resistance, wear resistance, hardenability, and the steel’s hot hardness.

Common alloying elements:

Alloying elements are used to alter the mechanical and chemical properties of steel to give them advantages over standard carbon steel. While there are many alloying elements used to achieve various enhanced properties, certain elements are much more common than others. These are 5 common alloying elements:

• Chromium
• Molybdenum
• Vanadium
• Manganese
• Nickel

1) Chromium

Chromium added to carbon steel in percentages usually greater than 11% creates stainless steel. At this percentage and greater, the corrosion resistance of a steel vastly increases, and oxidation of the iron is prevented in many conditions. The iron does not oxidize because the chromium will oxidize first and form a protective layer over the steel. Chromium also helps to improve mechanical properties, even in smaller amounts. It will increase the steel’s strength, hardness, and ability to be heat treated.

Most stainless steels have 12-20% Chromium content, and at these levels, it also decreases the machinability. Chromium is Present in most low alloy steels with a weight content of 0. 5-1.5%. However, this is achieved only at higher amounts, above 5%. However, at lower content, as in low alloy steels, it improves hardenability without a negative effect on machinability.

- Common steels with high amounts of chromium include grades 439, 309, 2205 stainless steels. Grade D2 tool steel also has significant amounts of chromium.

2) Molybdenum

Molybdenum is present in weight content of 0.15-0.35%, it’s like chromium, has an effect on the corrosion resistance of steel. Molybdenum can also increase the hardenability, toughness, and tensile strength of steel, has no adverse effect on machinability. It increases the hardenability by lowering the required quench rate during the heat-treating process to make a strong and hard steel. Molybdenum can also reduce the risk of pitting in steel as it improves resistance to chloride induced corrosion. The Pitting Resistance Equivalent, or PRE, is calculated by multiplying the amount of molybdenum, chromium, and nickel by coefficients and then adding the values together.

- Common steel grades with high amounts of molybdenum are the “A” group of tool steels, maraging steels such as Grade 250, and many stainless steels.

3) Vanadium

Vanadium is used to help control the grain size of the steel, keeping it small. The grain size is kept small because the vanadium carbides that form when vanadium is added to a steel block the formation of grains. This blockage prevents the grains from growing to be as large as what they would be without the added vanadium. This finer grain structure helps to increase ductility. In some steels, carbides formed by vanadium can increase the hardness and strength of steel.

- Steel alloys with high amounts of vanadium include A3, A9, O1, and D2 tool steels. Blade steels such as M390 and VG 10 also have relatively high amounts of vanadium.

4) Manganese

Manganese is frequently used in steels to help with the heat-treating process. When steels are heated and quenched to increase hardness and strength, the quench must be done a fast rate. The faster this rate, the more unstable the process becomes. Manganese allows hardness and strength to increase the same amount but at a slower quench rate. This helps to reduce the risk of defects forming during the heating and quenching process.

Steels with high amounts of manganese include A10, A4, and O2 tool steels. 201 stainless steels also have a relatively high amount of manganese.

5) Nickel

Nickel is used to manufacture austenitic stainless steels because it is an austenite promoter. When amounts of chromium around 18% or greater are used and nickel composition is greater than 8%, austenitic stainless steel is created. This combination is extremely corrosion resistant, and austenitic grades are some of the most widely used stainless steels. Nickel is also used to improve the mechanical properties of steel. It is used to increase toughness and impact strength but at the expense of decreasing the machinability, even at lower temperatures.

- Steels with high amounts of nickel include all of the austenitic stainless steels. Alloy steels such as 23XX and 25XX groups also have high amounts of nickel.

Low Alloy Steel – Machinability Chart

High Alloy Steel:

High-alloy steels are defined by a high percentage of alloying elements, high alloy steels can contain 5-20% content of alloying elements. The most common high-alloy steel is stainless steel, which contains at least 12 percent chromium. Stainless steel is generally split into three basic types: martensitic, ferritic, and austenitic. Martensitic steels contain the least amount of chromium, have a high hardenability, and are typically used for cutlery. Ferritic steels contain 12 to 27 percent chromium and are often used in automobiles and industrial equipment. Austenitic steels contain high levels of nickel, carbon, manganese, or nitrogen and are often used to store corrosive liquids and mining, chemical, or pharmacy equipment.

They are called tool steels since they are mostly used to manufacture tools for cutting, pressing, extruding, and coining metals and other materials.

Tool Steels Classification (Main Groups)

Tool Steel – Machinability Chart