One of the most important elements in the Earth's crust is iron. In our culture, that element symbolizes something hard, rigid, and permanent. Strong personalities are called 'iron' – just like the (in)famous Iron Lady (Margaret Thatcher). When the discussion comes to less solid grounds, people are looking for cast-iron proofs or arguments. Iron is also an important element in the human body – it is the main substance to create hemoglobin.
Over 2500 years ago contemporary Greek doctors recommended to patients with anemia to hammer iron nails into an apple, leave it for a few hours, and then eat such 'ironized' fruit. Of course – after removing nails.
The pure iron melting temperature is 1539 °C (approx. 2802.2 °F); boiling point – 3000 °C (approx. 5432 °F). Iron is the fourth most common element in the Earth's crust. Usually is mined as a metal ore in the form of metal oxide. Initially, iron was extracted out of found meteorites - that is why we know that element today under that name. In Sumerian and old Egyptian 'iron' meant 'the metal from the skies'.
In today's machining – especially numerically controlled – iron is used differently than years ago. On the other hand – characteristics of that element are well-known for professional CNC machine operators.
What do we understand as iron alloys?
Iron alloys are metallic materials that are the effect of casting carbon elements into the iron. Such addition can be delivered as pure carbon, for example as graphite, a solid solution in the crystal lattice of ferrite, austenite, or as iron carbide (Fe3C), often called a cementite. Such alloys with carbon addition below 2.04% are called steels (eg. austenites, used to produce stainless steel); over that, (>2.04% of carbon) such alloys are called 'cast iron'. Cast iron has a mediocre stretching resistance; on the other hand, steels are highly stretchable.
To improve steel properties such material is often subjected to additional thermal processing (eg. hardening and other types of thermal improvement). Such treatment decreases further plastic processing but increases its rigidity.
Type of steel used in industries
The main division criterion of steel types is the presence of added elements – others can be focused on the properties of a particular alloy. Based on the alloy composition we can divide the following types of steels:
- an unalloyed steel – some of them are dedicated for further thermal treatment,
- low-alloy steels – with the addition of elements below 5% – are much more susceptible to hardening, have higher creep and tempering resistance,
- high-alloy steel – with the addition of elements above 5% – with additions focused on achieving special alloy properties.
On the other hand – steel alloys can be divided by the application. It is even more important in the machining application. The most popular types of steel alloys are:
- automatic steels – the most often used to produce small parts – nuts or washers. Often delivered in the form of drawn or peeled rods and used in the lathes. In machining that type of steel produces short fragile chips and does not cause faster tool wear, thus rarely causing build-up on the cutting bits. That alloy allows to achieve good quality surface,
- carburizing steel – parts made out of that alloy have good core ductility and surface hardness after additional carbonization. In machining such alloy it is recommended to use high machining speed to decrease the possibility of build-up on the blade (especially made of cemented carbide) and low feed. Such material is the most often used to machine gears, shafts, and gears,
- heat-treatable steel – such alloys are often better after rough machining and before finishing processes. Their machinability is strongly dependent on the carbon contents. During processing it is recommended to use low machining speeds to maintain low tool degradation,
- nitriding steel – the main additives of such alloy are often aluminum, chromium, and molybdenum. Such a combination creates hard nitrides and increases surface hardness. Due to that characteristic parts made of nitrided steel have very good contact tension strength and resistance. Parts made of that alloy are machined before the nitriding process,
- tool steels – the alloys created to be used in the production of tools. Such material characterizes very high hardness, abrasion resistance, and negligible deformations of the final shape during final usage. It is possible to better such alloy in further processing to improve even more its features,
- stainless steel – a group of specialized alloys with unique physicochemical features like very good corrosion resistance (against atmospheric, chemical, or alkaline agents). Such features are strongly related to the composition of the alloy,
- acid-resistant steel – such alloys gain their features in the process of austenite stabilization (eg. thanks to high contents of chromium – 17 to 20% – or nickel – 8 to 14% – and other additives like manganese, titanium, molybdenum or brass) after finishing machining.
What are the machinability factors of steel
The machinability factor of steel is a combination of the type of machining process (milling, turning, etc.), selected tools, and machining parameters. An additional factor is of course the type of processed alloy and its mechanical properties, structure, or chemical composition. Elements are added intentionally to change the mechanical and thermal features of the final alloy or part. Among the most often used elements to create iron alloys, it is worth mentioning:
- chromium – due to modification of alloy structure increases steel susceptibility for hardening and overall machining,
- nickel – improves rigidity of alloy but also deteriorates machinability of alloy,
- silicon – creates in alloy structure hard silicon oxides, that improve overall alloy strength, but causes also faster deterioration of machining tools during processing,
- phosphorus – added to alloys changes chip structure (while machining chips are shorter), and with contents above 0.1% can increase machinability. Higher contents can impact the quality of the surface (betters it) but in cost of faster degradation of machining tools,
- sulfur – also impacts chips structure (they are more fragile) but that additive can increase the possibility of burrs in the machined surface,
- manganese – improves heat-treatment potential and overall rigidity of an alloy,
- lead – due to low melting temperature such additives can create a thin layer on the surface of machined material. That can lead to slower deterioration of machining tools and forces. Also, phosphorus has an impact on chip structure (shorter and fragile).
Steel is an important material for machining parts
Steel as an iron alloy is a challenging material for every machining operator and requires vast knowledge in areas such as metallurgy and machining processes. Combined and properly executed made that alloy efficient in machining and easy to work with.
To minimize the risk of poor quality of production every machining process should be tailored to the requirements of the selected steel alloy. A professional machining plant, equipped with high-quality CNC machines (mills, lathes) can deliver the highest quality parts manufactured within order requirements and on time.
A few words about us
At RADMOT, we can offer CNC milling services, CNC turning services as well as many additional services, including washing, aluminum anodizing, laser marking and assembly.
Contact us and tell us what you need. We have been providing CNC services for 40 years. Our quote is completely free. And if you are in doubt about which technology will work best for you, we are able to advise customers from many industries on machining and quickly determine the price of machining the parts you order - our expertise is at your disposal.