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Machining carbon steel

Carbon steel 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, where xx indicates the amount of carbon. For example; 1020 contains 0.2% of carbon and 1045 contains 0.45% of carbon.

Introduction

Graph for machiability of steel vs carbon content

Carbon (C) is the primary alloying element in steel. It increases hardness and tensile strength, as well as response to heat treatment (hardenability). Carbon steels usually have up to 1% carbon content. 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%. Steel with less than 0.15% Carbon is very soft; thus, it is almost impossible to break the chips, and BUE is easily formed on the cutting edge. Above 0.3%, the material becomes gradually harder, and therefore wear is formed faster on the cutting edge.

Classification of Carbon Steels

Class Carbon Content Ductility Strength and Hardness Chip Control Machinability Rating
Low Carbon Steel Below 0.2% High Low Poor - Very long chips 55-75%
Carbon Steel 0.2% - 0.5% Medium Medium Good 65-75%
High Carbon Steel More than 0.5% Low High Good 45-65%


Low Carbon Steel – Machining Tips

Due to its high ductility, it is hard to achieve chip control when machining low carbon steel. The chips will be soft on the higher end of carbon content (0.12-0.2%), but using a tight chip-former will usually achieve decent chip control. As the carbon amount gets smaller, it will be impossible to accomplish any chip control, and you should expect soft and long chips. Use a sharp cutting edge with a high rake angle to reduce the risk of build-up edge.

Medium Carbon Steel – Machining Tips

The mid-range of 0.2-0.4% carbon is the most popular range of carbon steel on the shop floor, and for a good reason. This section yields a good balance between machinability, strength, and chip control. And general use cutting tool and grade that is designed to machine steel will work well.

High Carbon Steel – Machining Tips

High carbon steels are less popular. When better mechanical properties are required, the more common choice is low alloy steels since they have better machinability for the same strength level.
Due to the low ductility, chips are short and break easily; therefore, you should opt for a stronger cutting edge. Strength and hardness increase as the carbon amount increases; Therefore, opt for a hard carbide grade, especially when the carbon gets about 0.7%.

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