The ISO workpiece material classification group P includes mild and general steels.
The workpiece material classifications were decided based on material machinability.
However, the machinability of mild steel and general steel varies greatly. A common factor shared between the two materials is that they both generate continuous type chips that can easily damage the surface finish.
Machinability of workpiece materials
As the name explains, mild steel is soft, and welding can easily occur. However, the cutting resistance when machining mild steels is low, and this factor makes it necessary to choose tools with a priority on chip control and welding resistance.
In the meantime, chip control of general steel is not so troublesome, allowing for tool choice to be based mainly on tool life and machining efficiency.
When steel contains higher levels of carbon, it increases its heat treatable properties and is therefore harder and more adhesive. This type of steel generally, referred to as high carbon steel generates high temperatures during machining, necessitating heat resistant cutting tool grades. On the contrary, low carbon steel has the same inherent problems as mild steel.
Steel gears is an example of group P workpiece materials.
Tool selection for steel machining
Steel is the most commonly used metal on earth. Due to this fact steels are finely classified according to its use, therefore a wide range of tools are available for machining them.
As already mentioned, there is a very wide range of steels with many different properties. This makes it difficult to set a standard for choosing tools for steel machining. Detailed consideration is required for each type of steel and indepth knowledge of the features of the cutting tools to be used. However, during the machining of any steel, the common problem is that of the heat generation, making it essential to use heat-resistant tools.
When machining carbon steel with cemented carbide tools, welding and crater wear is a common occurrence.
This is because of the high affinity between the carbon particles, within the carbon steel, and the WC (tungsten carbide) in the cemented carbide. Generally, as the carbon content of a steel decreases, the steel (low carbon steels) will become more prone to welding problems. On the other hand as the carbon content increases the steel (medium to high carbon steels) will suffer more easily from crater wear. This is because when the carbon content increases the cutting edge temperatures will increase.
Properties of alloy steel depend on the elements within the alloy.
Alloy elements, that are often used include Ni (nickel), Cr (chromium), Mn (manganese) and Mo (molybdenum), have the following features:
Nickel (Ni) - Becomes adhesive
Chrome (Cr) - Becomes hard and forms hard particles
Manganese (Mn) - Becomes hard and adhesive
Molybdenum (Mo) - Becomes hard and forms hard particles
Each alloy, if included in small amounts, can provide better machinability, such as improved chip control and better welding resistance. However, as the amount of alloy increases, the steel becomes harder and stronger, eventually making it a difficult-to-cut material.
As the name explains tool steel is a hard, strong type of material.
The range of tool steels includes carbon tool steels, alloy tool steels and high-speed steels. Tool steel contains more than 0.6％ carbon, and as the carbon content increases, the amount of carbide precipitation grows, making tool steel harder and less vulnerable to wear.
Carbon tool steel mainly contains carbon, while alloy tool steel is made by adding different alloying elements to the carbon tool steel to strengthen the precipitating carbides and steel base metal.
High-speed steel is heat-treated at higher temperatures to further increase its hardness properties, additionally it has enough hardness to endure the heat generated during high speed cutting.
Although any tool steel is very hard due to the heat treatment process, care should be taken as these hardening effects could be lost because of excessive cutting temperatures.
When machining mild steel, problems such as welding and poor chip control are predominant and measures to prevent them should be taken.
As mild steel is relatively soft compared to other steels and has not been influenced by heat treatment, little damage to tools occurs during machining. But, it does have machining problems such as poor surface finishes caused by chip welding and lower machinability due to long, continuous chips wrapping around tools and workpieces.
Therefore, when cutting mild steel, it is important to select tool geometries that enable good chip control as well as choosing weld-resistant tool materials.
Cold forged materials are materials forged at room temperatures.
Only soft materials such as low carbon steel are capable of being cold forged.
As cold forged materials are already work hardened by the forging process, it is usual to normalize the material to enable machining to be carried out. The normalizing process and the small machining allowance of the forging tends to make the chips continuous.
Cold forged materials with these features such as thin walls are seen as difficult-to-cut materials because they are very prone to vibration, chip control is difficult, and the surface finish is easily damaged during machining.
Hot forgings are produced at high temperatures. This process is used for materials that are hard and therefore difficult to forge at room temperature.
Hot forged materials are relatively easy-to-cut materials such as high carbon steel and alloy steel. So, with the exception of the hard surface scale, hot forged materials present few problems for machining.
Free cutting steels are, as the name explains, steels whose alloy components are added to facilitate easy machining.
However, “free cutting properties” given from a materials engineers’ viewpoint do not always have good effects on cutting tools.
In the past, leaded carbon steel had a lower cutting resistance. This was achieved by ensuring a slippery interface between the tool face and the workpiece; which was created by the lead within the steel whose liquid phase could be formed at relatively low temperatures. The increased lubrication therefore extended tool life and improved chip control.
After the use of lead was restricted because of environmental concerns, virtually lead free, easy cutting steel was developed and is now available in the market as re-sulphurized carbon steel. It is becoming widely used and gains its free cutting properties by dispersing fine sulphide throughout its structure, this lowers cutting resistance and improves chip control. However, hard sulphide can rub against the cutting tool and can cause a comb-like wear on the edges.
However, free cutting steels do not always deliver free cutting properties. Therefore, it is necessary to understand the effects of the alloy elements added to each free cutting steel.
Pre-hardened steels are materials often used as a mould material.
Generally, heat treatment distorts or deforms materials, or produces a rough surface finish because of the influence of the heat. This means that heat-treating of finished components after machining is not suitable to maintain dimensional accuracy and a uniform depth of heat treatment. As a result, pre-hardened steels that save time and costs of post machining heat-treatment have become common place.
However, heat treatment before machining makes materials hard and adhesive, making machining more difficult.
Pre-hardened steels have a uniform level of hardness and toughness throughout their structure. Steel manufacturers emphasise these points in their various sales material. Hardness and other properties are adjusted to an easy-to-machine level as much as possible. Pre-hardened steels used as mould materials have the same life as post machining heat-treated material, thereby rendering heat treatment unnecessary and only requires the supplied material to be machined to the desired shape.
Despite the efforts to make these materials easily machinable, some pre-hardened steels have a hardness equivalent to that of hardened steel, and can therefore be considered
as difficult-to-cut materials.