INTRODUCTION TO ENGINEERING MATERIALS - FERROUS METALS - PART 2

Audio: Introduction to engineering materials – ferrous metals – part 2

In my previous article, we discussed the classification of engineering materials and why it is important to choose the proper material. We dived deeper into the topic of plain carbon and low alloy steels. In this article, we will look into stainless, tool, and specialty steels. So, let us start!

Stainless steel

Stainless steel is used due to its high corrosion resistance in various environments. It is highly alloyed with a minimum of 11% Chromium that permits a thin, protective surface layer. In addition, nickel and molybdenum can be used to further enhance corrosion protection.

To increase the strength and hardness of stainless steel, carbon is used. Increased carbon content can reduce corrosion resistance. Stainless steel, in addition to the high corrosion resistance, also possesses’ high strength and ductility, but it is significantly harder to work in manufacturing and more expensive than plain carbon or low alloy steel.

The three most important groups of stainless steel are:

Austenitic stainless steels

Austenitic stainless steels contain around 18% chromium and 8% nickel and are a group with the highest corrosion resistance. They are not magnetic and heat treatable, but they are very ductile. Austenitic stainless steels are usually used for chemical and food processing equipment, machinery parts requiring high corrosion resistance, medical use, etc.

Let us look at some austenitic stainless steels:

EN	Material number	AISI
X 12 CrNi 17 7	1.4310	301

X 12 CrNi 17 7 is an austenitic stainless steel with maximal 0,15% carbon content, 16% – 18% chromium and 6% – 8% nickel. It is used for automotive trim, kitchen equipment (spoons, forks, pots, etc.), hose clamps, springs, wheel covers, aircraft components, and various industrial applications. Machinability is fairly good and can be welded by all of the standard welding techniques.

EN	Material number	AISI
GX 3 CrNiMo 17 12 2 KD	1.4404	316L

GX 3 CrNiMo 17 12 2 KD is an austenitic stainless steel with maximal 0,03% carbon content, 16% – 18% chromium and 10% – 14% nickel. It is used in marine environments and chemical, petrochemical equipment, etc. Machinability is fairly good. It can be welded by all common fusion and resistance methods except oxyacetylene welding.

Ferritic stainless steels

Ferritic stainless steels contain less than 0.12% carbon and up to 30% chromium. They are magnetic and have good strength, moderate ductility, excellent corrosion resistance, moderate formability, and are relatively inexpensive. Ferritic stainless steels are less ductile and corrosion resistant the austenitic. They are not heat-treatable. Ferritic stainless steels are used for making parts ranging from kitchen utensils to jet engine components.

Let us look at some ferritic stainless steels:

EN	Material number	AISI
X 5 CrTi 12	1.4512	409

X 5 CrTi 12 is a ferritic stainless steel with maximal 0,08% carbon content and 10% – 12% chromium. It is used for automobile mufflers, catalytic converters, tailpipes, farm equipment, structural supports and hangers, transformer cases, diamond tread plate, shipping containers, etc. Machinability is fairly poor, and it can be welded by shielded metal arc, gas metal arc, and gas tungsten arc welding.

EN	Material number	AISI
X 10 CrAl 24	1.4762	446

X 10 CrAl 24 is a ferritic stainless steel with maximal 0,20% carbon content and 23% – 27% chromium. It is used for boiler baffles, furnace parts, X-ray tube bases, oil burner components, glass molds, annealing boxes, industrial mufflers, etc. Machinability is fairly poor and can be welded by the most common welding techniques.

Martensitic stainless steels

Martensitic stainless steels have a higher carbon content than ferritic and up to 18% chromium but no nickel. They are magnetic and have high strength, hardness, and fatigue resistance but are less corrosion resistant than the other two groups. Martensitic stainless steels can be heat treated. They are usually used for cutlery and surgical instruments.

Let us look at some martensitic stainless steels:

EN	Material number	AISI
X 12 Cr 13 KD	1.4006	410

X 12 Cr 13 KD is a martensitic stainless steel with maximal 0,15% carbon content, 11% – 14% chromium and maximum 1% manganese. It is used for cutlery, steam and gas turbine blades and buckets, bushings, valve components, fasteners, screens, kitchen utensils, etc. Machinability is fairly poor and can be welded by the most common welding techniques.

EN	Material number	AISI
X 70 CrMo 15	1.4109	440A

X 70 CrMo 15 is a martensitic stainless steel with 0,60% – 0,75% carbon content, 16% – 18% chromium and maximum 1% manganese. It is used for ball bearings and races, gage blocks, molds and dies, cutlery, valve components, knives and measuring instruments, etc. Machinability is fairly poor and can be welded by the most common welding techniques.

We can divide stainless steel into two additional groups:

Precipitation hardening (PH) stainless steels

PH stainless steels have a typical composition of 17% chromium and 7% nickel, with the addition of a small amount of aluminum, copper, titanium, and molybdenum as alloying elements. These steels can be strengthened by precipitation hardening. Due to the fact that PH stainless steels maintain high strength and corrosion resistance at high temperatures, they are suitable for use in the aerospace industry.

Let us look at one of the most widely used PH stainless steels:

EN	Material number	AISI
X 5 CrNiCuNb 16-4	1.4548	17 - 4 PH

X 5 CrNiCuNb 16-4 is a PH stainless steel with maximal 0,70% carbon content, 15% – 17% chromium, 3% – 5% nickel, 3% – 5% copper, and maximum 1% manganese. It is used in aircraft and gas turbines, nuclear reactors, paper mills, oil fields, chemical process components, etc. Machinability is fairly poor and can be welded by common fusion and resistance methods.

Duplex stainless steels

Duplex stainless steels contain approximately 50% ferrite and 50% austenite structure. Their corrosion resistance is similar to the austenitic grades. The combination of mechanical properties, corrosion resistance, formability, and weldability is not obtained by any of the usual stainless steels. They are widely used in harsh environments (chloride-rich, naval environments, etc.).

Let us look into one duplex stainless steel:

EN	Material number	AISI
X 2 CrNiMoCuN 25-6-3	1.4507	2507

X 2 CrNiMoCuN 25-6-3 is duplex stainless steel with maximal 0,03% carbon content, 25% chromium, 7% nickel, and 4% molybdenum. Applications include components exposed to strong chloride-rich environments. Among them are a variety of components in desalination plants, heat exchanger tubes in seawater cooling plants, drive shafts for ocean-going vessels, containers for the pulp and paper industry, tube and pipe systems at petrochemical refineries, and human implant components. Machinability is fairly poor and can be welded by the most common welding techniques.

Tool steel

Tool steels are usually high-carbon, high-alloys steels that obtain high hardness by a quench and temper heat treatment. In addition, they possess high strength, hardness, hot hardness, wear resistance, toughness, toughness under impact, and temperature resistance. Tool steels are usually used for cutting tools in machining operations, dies for die casting, forming dies, etc.

High-speed tool steels

High-speed tool steels were designed for cutting tools in the machining process. They are formulated with high wear and hot hardness.

Example of high-speed tool steel:

EN	Material number	AISI
-	-	T4

T4 is high-speed tool steel with carbon content between 0,70% – 0,80%, 3,75% – 4,50% chromium, and 17,50% – 19% tungsten. It is usually used for cutting tools, broaches, and cold extrusion punches. Machinability is fair, and it can be welded.

Hot-working tool steels

Hot-working tool steels are intended for hot-working dies in forging, extrusion, and die-casting.

Example of hot-working tool steel:

EN	Material number	AISI
X 40 CrMoV 5 1	1.2344	H13

X 40 CrMoV 5 1 is hot-working tool steel with carbon content between 0,32% – 0,45%; 4,75% – 5,50% chromium, 0,80% – 1,20% vanadium, and 1,10% – 1,75% molybdenum. It is usually used for hot die work, dies casting, and extrusion dies. Machinability is fairly good, and it can be welded.

Cold-work tool steels

Cold-work tool steels are die steels used for cold working operations such as sheet metal presswork, cold extrusion, and certain forging operations. They all provide good wear resistance and low distortion.

Example of cold-work tool steel:

EN	Material number	AISI
X 153 CrMoV 12	1.2379	D2

X 153 CrMoV 12 is cold-work tool steel with carbon content between 1,40% – 1,60%; 11% – 13% chromium, max 0,60% manganese, and 0,70% – 1,20% molybdenum. It has high hardness, wear resistance, and mild corrosion resistance. X 153 CrMoV 12 is usually used for punches, stamping and forming dies, forming rolls, slitters, shear blades, tools, scrap choppers, thread rolling dies, etc. Machinability is poor, and it can be weldable.

Water-hardening tool steels

Water-hardening tool steels have a high carbon content without alloying elements (or with a small amount). They are low-cost and are limited to low-temperature applications. They can only be hardened by fast quenching in water.

Example of water hardening tool steel:

EN	Material number	AISI
C 105 U	1.1545	W110

C 105 U is water-hardening tool steel with carbon content between 0,95% – 1,10%; 0,10% – 0,40% silicon, and 0,10% – 0,40% manganese. It has high hardness and wear resistance. C 105 U is usually used for punches, machine knives, chisels, scissor blades, cutting plates, milling cutters, etc. Machinability is very good and can be welded with common fusion and resistance methods.

Shock-resistant tool steels

Shock-resistant tool steels are intended for use in applications where high toughness is required, as in many sheet metal shearing, punching, and bending operations.

Example of shock-resistant tools steel:

EN	Material number	AISI
-	-	S2

S2 is shock-resistant tool steel with carbon content between 0,40% – 0,55%; 0,90% – 1,20% silicon; 0,30% – 0,60% molybdenum, and 0,30% – 0,50% manganese. It retains ductility even in the hardened condition, which enables it to perform in shock impact applications. S2 is usually used for chisels, hammers, punches, and similar repetitive, hard-impact applications. Machinability is fairly good and can be welded with common fusion and resistance methods.

Mold tool steels

As the name suggests mold tool steels are used to make mold tools.

Example of mold tool steel:

EN	Material number	AISI
40 CrMnMo 7	1.2311	P20

40 CrMnMo 7 is mold tool steel with carbon content between 0,28% – 0,40%; 1,4% – 2,0% chromium; 0,6% – 1,0% manganese, and 0,30% – 0,55% molybdenum. It is usually used for low-temperature applications, such as die-casting dies and injection molds. Machinability is relatively good and can be welded by the most common welding techniques.

Low-alloy tool steels

Low-alloy tool steels are generally reserved for special applications.

Example of low-alloy tool steel:

EN	Material number	AISI
-	-	L6

L6 is low-alloy tool steel with carbon content between 0,65% – 0,75%; 0,60% – 1,20% chromium; 0,25% – 0,80% manganese; 1,25% – 2,00% nickel, and max. 0,50% molybdenum. It is usually used in machine tool applications such as bearings, springs, rollers, or chuck parts. Machinability is very good, and it is weldable.

Specialty steels

Maraging steels

Maraging steels are ultra-high-strength steels with good fracture toughness but at a cost greatly exceeding conventional steels. They are low-carbon alloys containing high amounts of nickel and fewer proportions of cobalt, molybdenum, titanium, and sometimes chromium. They are pretty processable by forming/machining and can be welded. Applications include parts for missiles, machinery, dies, and other situations where these properties are required and justify the high cost of the alloy.

Example of maraging steel:

EN	Material number	Manufacturer reference
S162	1.6359	ATI C-250

ATI C-250^TMis maraging steel with max. carbon content 0,03%; 18,50% nickel, 8,50% cobalt, and 3,25% molybdenum. It is usually used for missile and rocket motor cases, wind tunnel models, recoil springs, flexures, actuators, landing gear components, high-performance shafting, gears, and fasteners. ATI C-250^TM is easily machined, and weldability is good.

Free-machining steels

Free-machining steels are developed to improve machinability. Alloying elements include sulfur, tin, bismuth, selenium, tellurium, and/or phosphorous. These elements lubricate the cutting operation, reduce friction, and break up chips for easier disposal. Free-machining steels are more expensive than common steels, but higher production rates and longer tool life quickly return invested.

Example of free-machining steel:

EN	Material number	AISI
11 SMnPb 28	1.0718	12L13

11 SMnPb 28 is free-machining steel with 0,08% carbon content, 1,10% manganese, 0,30% silicon, and 0,25% lead. Applications include high-speed screw machine products and other bulk applications of joining elements. Machinability is excellent, and it can be welded.

Source: McMASTER-CARR

Closing words

Reading this article, you might realize that you have already encountered many of mentioned steels in your life. In mechanical engineering, steels are extremely important, and you are more like to encounter them than not. Depending on the industry that you are in, some of the steels you will encounter and use more than others. However, it is useful for you to know what different types of steels, or materials in general for that matter so that you can always keep striving for better solutions.

This article, together with part 1, covers the whole group of ferrous metals called steels. In part 3, we will cover the second group of ferrous metals called cast irons.

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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