Metals Used for Machining Workpieces: Part 1: Introduction to Steel, Cast Iron, and Aluminum
Introduction
Most workpieces in machining settomgs are made from alloys (compositions of two or more metals). Alloys that contain aluminum, iron, copper, and nickel predominate.
Steel
Steel is the most common workpiece material
Steel is a ferrous alloy--- iron is its main ingredient. Steel also contains carbon, as well as other metals, including manganese, nickel, and molybdenum.
Steel can be classified according to its carbon content:
mild steel: c .05 - .25%
medium carbon steel: c 0.25 – 0.55%
high carbon steel: c 0.55-0.8%
Steel with a carbon content of more than 2% is called cast iron
Steel with a carbon content of less than 0.05% is called wrought iron.
Carbon steel is steel that contains only carbon and iron.
Different types of steel can be created by:
varying the carbon content
varying the treating process
varying the combination of other elements
Stainless Steel
Stainless steel contains at least 12% chromium. Nickel and manganese may also be added, depending on the particular type of stainless steel.
There are three main categories of stainless steel: ferritic, martensitic, and austenitic.
Austenitic stainless steel:
Austenitic stainless steels account for over 70% of all stainless steel production
Austenitic stainless steels contain a maximum of 0.15% carbon and a minimum of 16% chromium. They also contain nickel and/or manganese A common type of austenitic stainless steel is known as 18/10 stainless. 18/10 stainless contains 18% chromium and 10% nickel.
Another type of austenitic stainless steel is 18/8 stainless, with 18% chromium and 8% nickel. 18/8 stainless is known among the austenitic group for its resistance to corrosion. 18/8 can be made even more resistant to corrosion through the addition of molybdenum.
18/8 stainless steel comes in three varieties: ultra-low carbon (0.03%), low carbon (0.08%), and stabilized. Stabilized austenitic stainless steel usually contains titanium.
Austenitic stainless steels are also available in free-machining varieties. The free machining types typically contain sulfur or selenium inclusions--- which somewhat diminishes the steel’s inherent resistance to corrosion.
Ferritic stainless steel:
Resistant to corrosion, but is less durable than austenitic stainless steel
Contains between 10% and 27% chromium
May also contain aluminium, molybdenum, or titanium
Free machining stainless steels contain a low amount of chromium. Sulfur may also be added to make the steel more machinable.
Martensitic stainless steel:
Martensitic stainless steels are less resistant corrosion than either the ferritic or the austenitic varieties.
Martensitic stainless steels are both extremely strong and very machinable.
Martensitic stainless steels contain: 0.1-1% carbon, chromium (12-14%), and molybdenum (0.2-1%). These steels contain no nickel.
Cast Iron
Cast iron is an iron-carbon alloy.
The carbon content of cast iron usually runs between 2 and 4 percent. Cast iron may also contain manganese, sulfur, or silicon .
If extra protection against corrosion is desired, nickel, molybdenum, or copper can be added to cast iron.
The hardness of cast iron is measured on the Brinell scale. Cast iron’s Brinell rating is often a good indication of its machinability. (The metal's machinability decreases as its Brinell hardness increases.)
There are four basic types of cast iron: grey cast iron, malleable cast iron, nodular cast iron (SG) and alloy cast iron. The main difference between these types is the form in which graphite is present in the material.
Grey cast iron:
Grey cast iron is relatively soft ,due to weak bonds between layers of graphite within the material. Its silicon content is around 2%. Grey cast iron is highly machinable.
Nodular cast iron (SG):
Graphite is present in this form of cast iron as round nodules. Nodular cast iron also contains high levels of silicon. Castings made from nodular cast iron are common in the automotive industry. For example, many valves are made from this material.
Malleable cast iron:
Malleable cast iron also contains nodular pockets of graphite. This variety of cast iron is ductile and extremely strong.
Alloy cast iron:
Compared to the other varieties, alloy cast iron contains larger amounts of alloy elements (as its name suggests), including nickel, molybdenum, copper, and chromium. Alloy cast iron is strong, and highly resistant to heat and corrosion. However, it may also be challenging to machine.
Aluminum
In its pure state, aluminum is a weak, ductile metal that is not well suited to machining applications. Therefore, most aluminum found in machining settings is actually an aluminum alloy. The following elements are commonly added to aluminum to make aluminum alloy:
Copper: increases strength/machinability
Manganese: makes aluminum alloys easier to cast
Silicon: improves castability and resistance to corrosion
Magnesium: improves strength/corrosion resistance
Iron: increases strength and hardness
Zinc: increases strength/castability
Aluminum alloys are highly machinable. Aluminum has good thermal conductivity properties, so machining temperatures are low. This allows for high cutting speeds.