INTRODUCTION TO ENGINEERING MATERIALS - FERROUS METALS - PART 1

Audio: Introduction to engineering materials – ferrous metals – part 1

As a mechanical design engineer, for you not to work with different material types is impossible. You will have to choose the material (ferrous metals, polymers, etc.) based on its properties, manufacturability, cost, and the component’s function. Furthermore, you will have to specify different surface finishes, coatings, and heat treatments.

Typically, you will have legacy designs and experience in your company, but you cannot rely only on that. In order to choose the proper material, you must have deep knowledge of engineering materials. In this article, we will dive into the fundamentals of engineering materials and build the knowledge needed to move on to more complex issues. We will need a series of articles to cover the whole topic, so let us start.

Introduction

Engineering materials are the building blocks of virtually any physical product. So it is not an overstatement when I say that as a mechanical design engineer, there is no chance for you to work this job and not encounter the material selection problem. In my career as a mechanical design engineer, I have worked with a wide range of different engineering materials, and tradeoffs between the material properties, manufacturability, and cost are always a challenge. But, as you gain more experience working on your job, you will make these choices easier.

In this case, the problem with the experience is that we get used to using certain types of materials and stop looking for alternatives. We stop thinking about the choices we make and treat each case like the previous one. We have to approach each component with a fresh perspective, always keeping in mind that we can easily overprice the final product if we are not careful on the component level.

Imagine that you have a product with a hundred different components and assemblies. We use a bit more expensive material on each of the individual components than we should, we over-define the tolerances on half of the components, and we do not optimize our design for assembly, so our assembly cost is higher than it should be. Before we knew it, our product was overpriced by about 5€.

You might argue that 5€ is not a lot of money, and I would agree with you if our company plans to sell only one product. But if our company wants to sell 100 000 devices/year, we end up with 500 000€/year lost in potential revenue for the company. Of course, we will not always make everything 100% correct and perfect, but that does not mean we should not strive for it.

In the big numbers game, every detail matters. Even if you work in a company that does not sell in high quantities, you should make a habit of thinking in big numbers. One day you may come to the company where every cent matters and 1€/piece more could get your job security in danger.

Furthermore, choosing the wrong type of material can lead to delays in production, expensive reworks, or construction failures that could lead to the deaths of innocent people. That could get you and your company into significant legal problems.

Luckily for us, there are some strategies for ensuring that every time we do the due diligence. But before we go there, I would like you to take time and learn or refresh your memory about the engineering materials. So, lets us start with the classification of engineering materials.

Classification of engineering materials

Engineering materials can be divided into four main groups: metals, ceramics, polymers, and composites.

Materials in each of these groups possess different structures and properties. In this article series, we will not focus on the structure of engineering materials, but we will emphasize their properties and use in the industry.

As you can see, there are multiple different material groups, and we will divide them into various articles:

Metals – Ferrous metals
- Introduction, plain carbon steel, and low alloy steel are covered in this article.
- Stainless, tool, and specialty steels are covered in part 2.
- Cast irons are covered in part 3.
Metals – Nonferrous metals are covered in part 4.
Ceramics are covered in part 5.
Polymers are covered in part 6.
Composites are covered in part 7.

Metals

Metallic materials are a combination of one or more metallic elements. Almost without exception, in a solid state, they have a crystalline structure. These crystal structures are almost always BCC (body-centered cubic), FCC (face-centered cubic), or HCP (hexagonal close-packed structure). These structures make the metals, in general, relatively high strength, high stiffness, ductility, and shock resistant. Furthermore, metals are good conductors of heat and electricity and are not transparent to visible light.

Pure metals are occasionally used, and in most cases, alloys are used. An alloy is created by combining more than one metal with other metal or non-metallic elements.

We can divide metals into two main groups:

ferrous metals – based on iron,
nonferrous metals – all other metals.

Ferrous metals

Ferrous metals are based on iron. Iron is a metallic element with a BCC crystal structure, denoted “Fe.” Ferrous metals are essential as engineering construction materials. We can divide ferrous metals into two main groups: steels and cast irons.

Steel

Steel is an alloy of iron that contains carbon (C) between 0,02% and 2,11% (most range between 0,05 and 1,1%C). The mechanical properties of steel largely depend on the carbon content. While steel can be alloyed with other elements, the carbon content is what turns iron into steel (steels are based on iron; hence they are part of the ferrous metals group).

Most of the time, alloying elements added to steel are:

Chromium (Cr) – improves strength, hardness, wear resistance, and in significant proportions, it improves corrosion resistance.
Manganese (Mn) – improves strength, hardness, and hardenability (ease of which steels can form martensite).
Molybdenum (Mo) – increase toughness, hot hardness, hardenability, and wear resistance.
Nickel (Ni) – improves strength, toughness, and in significant proportions, improves corrosion resistance.
Vanadium (V) – improves strength, toughness, and wear resistance.
Phosphorus (P) – improves strength and toughness but reduces the ductility and impact toughness.
Sulfur (S) – improves machinability but lowers ductility. In addition, high sulfur content reduces weldability.
Silicon (Si) – improves strength and hardness (but less than manganese). It is used as a deoxidizer.

We can divide steel into the following groups:

plain carbon steels,
low alloy steels,
stainless steels,
tool steels,
specialty steels.

Plain carbon steel

Plain carbon steel is ferrous metal with alloying elements that mainly contain carbon and small amounts of other elements (manganese, silicon, phosphorus, and sulfur). The strength of carbon steels increases with carbon content. Based on the carbon content, plain carbon steels can be divided in:

Low carbon steel

Low carbon steels contain less than 0,25% of carbon. Compared to any other steel type, they are the least expensive to produce and are produced in the highest quantities. The low carbon steels are relatively soft and weak but have high ductility and toughness. Furthermore, they are machinable and weldable.

They are usually used for automobile body components, ships, structural shapes, buildings, and structural sheets used in pipelines, buildings, bridges, and tin cans. Low carbon steels usually have a tensile strength of less than 520 MPa and hardness of less than 125 HB.

Let us now look into some low carbon steels:

EN	Material number	AISI
C10	1.0301	1010

C10 is a low carbon steel with nominal 0,10% carbon content (0,08% – 0,13%). In the normalized condition, it is used for washers, rods, screws, nuts, and other fasteners. Machinability is fairly good, especially in the cold drawn and cold worked condition. It can be welded by all of the standard welding techniques.

EN	Material number	AISI
1C22	1.0402	1020

1C22 is commonly used low carbon steel with nominal 0,20% carbon content (0,17% – 0,23%). It is used for low-load machine parts, rods, shafts, bolts, pins, bushings, rivets, keys, etc. Machinability is fairly good. It can be welded by all of the standard welding techniques.

Medium carbon steel

Medium carbon steels contain carbon in range between 0,25% and 0,60%. They are used for applications that require higher strength, toughness, and wear resistance than low carbon steels. Usually, medium carbon steels are used for machinery components and engine parts like crankshafts and connecting rods, railway wheels, and tracks, axles, gears, etc. Medium carbon steels usually have tensile strength higher than 520 MPa, and hardness less than 220 HB.

Let us now look into some low carbon steels:

EN	Material number	AISI
1C30	1.0528	1030

1C30 is a medium carbon steel with nominal 0,30% carbon content (0,28% – 0,34%). It is used for machinery parts, brackets, brakes, clips, clutches, clips, springs, etc. Machinability is fairly good. It can be welded by all of the standard welding techniques.

EN	Material number	AISI
1C40	1.0511	1040

1C40 is a medium carbon steel with nominal 0,30% carbon content (0,28% – 0,34%). It is used for shafts, stressed pins, studs, keys, crankshafts, couplings, etc. Machinability is fairly good. It can be welded by all of the standard welding techniques.

High carbon steel

High carbon steels contain carbon in a range between 0,60% and 1,40%. They are used for higher strength, hardness, and stiffness applications. High carbons steels are used for springs, high-strength wires, dies, cutting tools, hacksaws, razors, blades, and other wear resistance parts. High carbon steels usually have tensile strength higher than 520 MPa, and hardness higher than 220 HB.

Let us now look into some high carbon steels:

EN	Material number	AISI
C76D	1.0614	1075

C76D is a high carbon steel with carbon content between 0,70% – 0,80%. It is used for applications where high hardness is required. Due to the high hardness, this steel is more brittle than lower carbon steels. C76D is used for springs (vehicle coil springs, spring clamps), cutting tools (knives, machetes, blades, swords), etc. Due to the high carbon content, machinability is relatively poor. It can be welded by all of the standard welding techniques.

EN	Material number	AISI
C92D	1.0618	1095

C92D is high carbon steel with a carbon content between 0,90% – 1,03%. It is used for applications where high hardness is required. Due to the high hardness, this steel is more brittle than lower carbon steels. C92D is used for springs (vehicle coil springs, spring clamps), cutting tools (knives, machetes, blades, swords, grass or grain cutting blades), brake disks, plow beams, etc. Due to the high carbon content, machinability is relatively poor. However, it can be welded by all of the standard welding techniques.

Low alloy steel

Low alloy steels, in addition to carbon, contain other alloying elements. Usually, these alloying elements are chromium, manganese, molybdenum, nickel, and vanadium, in amounts totaling less than 5% of the weight. They have better properties for given applications than plain carbon steels, like higher strength, hardness, wear resistance, toughness, etc. Often heat treatment is required to achieve these improved properties.
Low alloy steels are not easily welded, especially at medium and high carbon levels. In order to improve weldability, low carbon steels were combined with different alloying elements like copper, vanadium, nickel, and molybdenum. These groups of low carbon alloy are called high-strength, low-alloy (HSLA) steels.
Let us now look into some low alloy steels:

EN	Material number	AISI
25CrMo4	1.7218	4130

25CrMo4 is a low alloy steel with carbon content between 0,28% – 0,33%, chromium 0,80% – 1,1%, manganese 0,70% – 0,90%, and molybdenum 0,15% – 0,25%. It is primarily used where high strength and great weldability are required. Applications include the construction of commercial and military aircraft and welded tubing applications (oil and gas industry). Due to the low carbon content, machinability is relatively good. It can be welded by all of the standard welding techniques.

EN	Material number	AISI
55Cr3	1.7176	5160

55Cr3 is a low alloy steel with carbon content between 0,56% – 0,64%, chromium 0,70% – 0,9%, and manganese 0,75% – 1,00%. It is primarily used where high toughness and ductility are required. Applications include components with damping properties in the automobile industry, like springs (leaf springs) and stabilizers. Due to the high carbon content, machinability is relatively poor. It cannot be welded by all standard welding techniques (gas or arc welding methods may be used).

Closing words

As a mechanical design engineer, you will be dealing with different types of materials all the time. With the experience and the company’s legacy knowledge, you will make choices easier, but be careful, do not get too comfortable, and stop looking for alternatives. In the design process, everything adds up to the product’s final price, especially when designing a high-quantity product. Keep this in mind when choosing the suitable material for your intended purpose.

We can classify material into four major groups: metals, ceramics, polymers, and composites. Considering that engineering materials are a large topic, we will split the introduction into a few parts. We started with ferrous metals and covered plain carbon and low alloy steels. In the next part, we will continue with other steels.

Now you have an excellent overview of some ferrous metals you could encounter as a mechanical design engineer. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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