Steels are found everywhere in our daily lives. The utensils, tools, equipment, etc are all made up of steel. The superior properties such as strength, malleability, ductility, weldability, etc have made it really useful in multiple applications. We can say that we can not think of our life in the absence of steel. If you are wondering what is the real meaning of steel, then you can say that it is an alloy of iron and carbon.
Thus, here we will be discussing the types of steel. Besides that, we will also discuss the role of major components used in it. Also, we will be looking at one of the most popular stainless steels (316L), used in the medical field.
Role of Carbon
The role of carbon is to provide strength to iron. An increase in the carbon content increases the strength and hardness of the alloy. On the other hand, this will reduce the ductility, weldability, and melting point. Carbon is a poor conductor of heat and electricity. So, adding carbon will reduce the electrical and thermal properties of the alloy. This decrease in the thermal property will make heating and cooling a slow process.
Types of Steels
Based on the composition of the elements steel are of four types
1. Carbon Steel
Types of Carbon Steel
(Carbon Steel Properties)
- Low Carbon: – It consists of up to 0.25 percent of carbon. It is quite weak and soft but more ductile and wieldable. Thus, it is used in making grills, railings, ladders, etc.
- Medium Carbon:- It consists of 0.25 to 0.6 percent of carbon content.
- High Carbon:- It consists of 0.6 to 1.5 percent of carbon. It is hard, tough, and non-ductile. So, it is used in making bolts, cutters, etc.
- Most abundantly found in the market
- Found in different shapes, sizes, and applications
- Can rust quickly
Carbon Steel Vs Cast Iron
Cast iron also consists of iron and carbon. It is not pure iron. But they do have some minor differences which are stated below.
|Carbon Steel||Cast Iron|
|Carbon is low (less than 2 percent)||Carbon content is high (2-3.5 percent)|
|Not as hard as cast iron||Harder and stronger in comparison to carbon steel|
|Less brittle||More brittle and can break quickly|
|Can be malleable and ductile||Neither malleable nor ductile|
2. Stainless Steels
Stainless Steel is formed when at least 10.5% of chromium (with low carbon content) is mixed in the steel. Some of them are also known as medical steels.
The main feature of stainless steel over the other forms is that it has higher corrosion resistance. This feature is provided by the chromium element. Chromium lies on the upper layer of the metal to protect it from corrosion. When chromium gets in contact with the atmosphere it forms chromium oxide (Cr2O3). This prevents the inner metals to get in contact with the atmosphere; thus prevents from undergoing further corrosion. This thin oxide layer lies at the outer layer of the steel.
Role of Nickel
The addition of nickel makes the steel austenitic. We know that chromium oxide, molybdenum, and silicon tend to stabilize in the ferritic state. In other words, they like to be in the Body-Centered Cubic (BCC) structure. This will make the steel weaker. So, to overcome this problem nickel is used, which stabilizes the atoms in Face-Centered Cubic (FCC) stage. Nickel also makes it non-magnetic. This non-magnetic property is one of the major properties for medical implants such as bolts and screws used in orthopedics.
Types of Stainless Steel
- Precipitation Hardening
- Major elements present in these steels are iron, chromium, nickel, manganese, molybdenum, etc., at various compositions.
- Ferritic and Austenitic are excellent corrosion resistant while others are moderate corrosion resistant.
- Austenitic is non-magnetic while the remaining four are magnetic.
- Austenitic has a face-centered cubic structure of atoms. Martensitic and Ferritic have a body-centered cubic structure. Duplex is the combination of both Ferritic and Austenitic.
- Most stainless steels are brittle at low temperatures.
- Strength and cost increases from Martensitic to Precipitation Hardening.
- Ductility and weldability decrease from Martensitic to Precipitation Hardening.
Stainless Steel Vs Carbon Steel
|Stainless Steel||Carbon Steel|
|It must consist of at least 10.5% of chromium in the alloy of iron and carbon.||It is mainly an alloy of iron and carbon.|
|Chromium protects it from rusting.||Since there is no chromium in the carbon steel, it is highly susceptible to rusting.|
|It is not as tough and durable as carbon steel||A high concentration of carbon makes it much more durable and tough|
|It is expensive||It is cheaper|
Stainless Steel in Medical Devices
Medical steels are also known as surgical steels. Some of the most common medical grades of steels are Austenitic (316, 316L, etc.) and Martnesitic (410, 420, 440, etc.). The most common applications of these steels are as follows.
- Medical tools such as scissors, forceps, etc.
- Implants such as rods, screws, etc.
- Medical equipment such as a blood bag refrigerator, incubator, oven, etc.
The general criteria for being medical steel are as follows.
- Should be ductile and non-magnetic
- Should be corrosion resistive
Stainless Steel 316L
316L is a type of 300 series austenitic steel where ‘L’ indicates a low level of carbon in the alloy. A decrease in the carbon content makes it more ductile. A ductile material can be easily changed into a thin layer without breaking. This is one of the reasons, why surgical tools such as cautery pencils, forceps, scissors, etc., are generally made up of 316L steel.
The addition of Molybdenum (around 2%), in the austenitic steel, increases the non-corrosive nature of the material. So, the implants will be more biocompatible to the human body.
Similarly, we know that most stainless steel is brittle at low temperatures. So, they cannot be operated at a low temperature. The only machine I have got to work at a very low temperature is the Ultra-Low Deep Freezer of REMI Company. It can operate at -860C which is useful for storing RBC. I really do not think there is any machine in the health sector which works below this temperature. For those kinds of machines, the best stainless steel will be austenitic steel which can be operated at -150OC to 8700C.
3. Alloy Steels
It is a kind of carbon steel that has been alloyed with a small amount of chromium and vanadium. Unlike stainless steel, the concentration of chromium is less than 10.5%. Similarly, other elements such as nickel and molybdenum are not included.
4. Tools Steels
It is a type of carbon and alloy steel that is designed to make tools such as a hammer, axe, knife, slide wrench, etc.