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Plain carbon steels are further subdivided into four groups:

Low

Medium

High

Very high

Low. Often called mild steels, low-carbon steels have less than 0.30 percent carbon and are the most commonly used grades. They machine and weld nicely and are more ductile than higher-carbon steels.

Medium. Medium-carbon steels have from 0.30 to 0.45 percent carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.

High. With 0.45 to 0.75 percent carbon, these steels can be challenging to weld. Preheating, postheating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.

Very High. With up to 1.50 percent carbon content, very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.

Low-alloy Steels

When these steels are designed for welded applications, their carbon content is usually below 0.25 percent and often below 0.15 percent. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.

These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them. Preheating might also prove necessary. This can be determined by using the carbon equivalent formula, which we'll cover in a later issue.

High-alloy Steels

For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:

Austenitic

Ferritic

Martensitic

Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone (HAZ).

Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.

Austenitic stainless steels offer excellent weldability, but austenite isn't stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.

Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It's important to note that austenitic alloys can't be hardened by heat treatment. That means they don't harden in the welding HAZ.

Steel Classification Systems

Before we look at a couple of common steel classification systems, let's consider one more high-carbon metal, cast iron. The carbon content of cast iron is 2.1 percent or more. There are four basic types of cast iron:

Gray cast iron , which is relatively soft. It's easily machined and welded, and you'll find it used for engine cylinder blocks, pipe, and machine tool structures.

White cast iron, which is hard, brittle, and not weldable. It has a compressive strength of more than 200,000 pounds per square inch (PSI), and when it's annealed, it becomes malleable cast iron.

Malleable cast iron, which is annealed white cast iron. It can be welded, machined, is ductile, and offers good strength and shock resistance.

Ductile cast iron, which is sometimes called nodular or spheroidal graphite cast iron. It gets this name because its carbon is in the shape of small spheres, not flakes. This makes it both ductile and malleable. It's also weldable.

Classification of Carbon and Low-Alloy Steels

Abstract:

The American Iron and Steel Institute (AISI) defines carbon steel as follows:Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Steels can be classified by a variety of different systems depending on:

The composition, such as carbon, low-alloy or stainless steel.

The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods.

The finishing method, such as hot rolling or cold rolling

The product form, such as bar plate, sheet, strip, tubing or structural shape

The deoxidation practice, such as killed, semi-killed, capped or rimmed steel

The microstructure, such as ferritic, pearlitic and martensitic

The required strength level, as specified in ASTM standards

The heat treatment, such as annealing, quenching and tempering, and thermomechanical processing

Quality descriptors, such as forging quality and commercial quality.

Carbon Steels

The American Iron and Steel Institute (AISI) defines carbon steel as follows:

Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Carbon steel can be classified, according to various deoxidation practices, as rimmed, capped, semi-killed, or killed steel. Deoxidation practice and the steelmaking process will have an effect on the properties of the steel. However, variations in carbon have the greatest effect on mechanical properties, with increasing carbon content leading to increased hardness and strength. As such, carbon steels are generally categorized according to their carbon content. Generally speaking, carbon steels contain up to 2% total alloying elements and can be subdivided into low-carbon steels, medium-carbon steels, high-carbon steels, and ultrahigh-carbon steels; each of these designations is discussed below.

As a group, carbon steels are by far the most frequently used steels. More than 85% of the steel produced and shipped in the United States is carbon steel.

Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.

For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.

High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon steels are used for spring materials and high-strength wires.

ltrahigh-carbon steels are experimental alloys containing 1.25 to 2.0% C. These steels are thermomechanically processed to produce microstructures that consist of ultrafine, equiaxed grains of spherical, discontinuous proeutectoid carbide particles.

High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition.

The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:

Weathering steels, designated to exhibit superior atmospheric corrosion resistance

Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on cooling

Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation hardening but with low carbon content and therefore little or no pearlite in the microstructure

Microalloyed steels, with very small additions of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardening

Acicular ferrite steel, very low carbon steels with sufficient hardenability to transform on cooling to a very fine high-strength acicular ferrite structure rather than the usual polygonal ferrite structure

Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, thus providing a high-strength steel of superior formability.

The various types of HSLA steels may also have small additions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

Low-alloy Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

As with steels in general, low-alloy steels can be classified according to:

Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels

Heat treatment, such as quenched and tempered, normalized and tempered, annealed.

Because of the wide variety of chemical compositions possible and the fact that some steels are used in more than one heat-treated, condition, some overlap exists among the alloy steel classifications. In this article, four major groups of alloy steels are addressed: (1) low-carbon quenched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) heat-resistant chromium-molybdenum steels.

Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are used primarily as plate. Some of these steels, as well as other, similar steels, are produced as forgin

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Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.These steels usually are iron with less than 1 percent carbon, plus small amounts of manganese, phosphorus, sulfur, and silicon. The weldability and other characteristics of these steels are primarily a product of carbon content, although the alloying and residual elements do have a minor influence.
Plain carbon steels are further subdivided into four groups:
1.Low
2.Medium
3.High

Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.
For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.

High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon steels are used for spring materials and high-strength wires.For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:

1. Austenitic

2. Ferritic

3. Martensitic

Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone (HAZ).

Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.

Austenitic stainless steels offer excellent weldability, but austenite isn't stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.

Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It's important to note that austenitic alloys can't be hardened by heat treatment. That means they don't harden in the welding HAZ.

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Types of Steel can also be classified by a variety of different factors:

1. Composition: Carbon range, Alloy, Stainless.

2. The production method: Continuous cast, Electric furnace, Etc.

3. Finishing method used: Cold Rolled, Hot Rolled, Cold Drawn (Cold Finished), Etc.

4. Form or shape: Bar, Rod, Tube, Pipe, Plate, Sheet, Structural, Etc.

5. De-oxidation process (oxygen removed from steelmaking process): Killed & Semi-Killed Steel, Etc.

6. Microstructure: Ferritic, Pearlitic, Martensitic, Etc.

7. Physical Strength (Per ASTM Standards).

8. Heat Treatment: Annealed, Quenched & Tempered, Etc.

Carbon Steel

Carbon Steel can be segregated into three main categories: Low carbon steel (sometimes known as mild steel); Medium carbon steel; and High carbon steel.

Low Carbon Steel (Mild Steel):  Typically contain 0.04% to 0.30% carbon content. This is one of the largest groups of Carbon Steel. It covers a great diversity of shapes; from Flat Sheet to Structural Beam. Depending on the desired properties needed, other elements are added or increased. For example: Drawing Quality (DQ) – The carbon level is kept low and Aluminum is added, and for Structural Steel the carbon level is higher and the manganese content is increased.

Medium Carbon Steel: Typically has a carbon range of 0.31% to 0.60%, and a manganese content ranging from .060% to 1.65%. This product is stronger than low carbon steel, and it is more difficult to form, weld and cut. Medium carbon steels are quite often hardened and tempered using heat treatment.

High Carbon Steel: Commonly known as “carbon tool steel” it typically has a carbon range between 0.61% and 1.50%. High carbon steel is very difficult to cut, bend and weld. Once heat treated it becomes extremely hard and brittle.

Steel Classification Systems :-

Before we look at a couple of common steel classification systems, let's consider one more high-carbon metal, cast iron. The carbon content of cast iron is 2.1 percent or more. There are four basic types of cast iron:

1. Gray cast iron , which is relatively soft. It's easily machined and welded, and you'll find it used for engine cylinder blocks, pipe, and machine tool structures.

2. White cast iron, which is hard, brittle, and not weldable. It has a compressive strength of more than 200,000 pounds per square inch (PSI), and when it's annealed, it becomes malleable cast iron.

3. Malleable cast iron, which is annealed white cast iron. It can be welded, machined, is ductile, and offers good strength and shock resistance.

4. Ductile cast iron, which is sometimes called nodular or spheroidal graphite cast iron. It gets this name because its carbon is in the shape of small spheres, not flakes. This makes it both ductile and malleable. It's also weldable.

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1) Steel Alloy
- Steels are iron-carbon alloys that may contain appreciable concentrations of other alloying elements.
A) Low Alloy
i) Low Carbon Steel
- Contain less than 0.25 wt% C
- Formed from annealing and strengthening by cold work process.
- Microstructure => Ferrite & Pearlie
- Outstanding ductility & toughness
- Application => Automobile body components, structural shapes and sheet for pipelines, building, bridge & tin can.
ii) Medium Carbon Steel
- Contain between 0.25 & 0.60 wt% C
- Formed from austenitizing, quenching & tempering process.
- Microstructure => Tempered Martensite
- Low hardenabilities
- Application => Railway wheels & tracks, gears, crankshaft & machine parts.
iii) High Carbon Steel
- Contain between 0.60 & 1.40 wt% C
- Formed from quenching and tempering process.
- Microstructure => Fully Pearlite
- Hardest, strongest & yet least ductile
- Application => Drills, cutting tools, punches, drawing dies, blacksmith tools lathe & planner tools.

B) High Alloy
- High alloy steel is the steel that having more than 5% of alloy addition.
i) Tool
- Not resistance to corrosion (rusting)
- Have low of chromium alloy elements
ii) Stainless Steels
- Resistance to corrosion (rusting) in a variety of environment, especially the ambient temperature
- Alloying element chromium with at least 11wt% Cr or nickel & molybdenum additions.
- There are 4 types of stainless steels:-
a) Martensitic Stainless Steels
- Formed from process of annealing and with quenching & tempering.
- Microstructure => Martensite
- Application => Jet engine parts, bearings, surgical tools & cutlery.
- Magnetic
b) Austenitic Stainless Steel
- Formed from process of annealing.
- Hardened by cold work.
- Microstructure => Austenite
- Application => Chemical and food processing equipment, cryogenic vessels & welding construction.
- High chromium contents & nickel additions.
c) Ferritic Stainless Steels
- Formed from process of annealing.
- Hardened by cold work.
- Microstructure => Ferrite
- Application => Automotive exhaust component, tanks for agricultural sprays, valves, glass molds & combustion chamber.
- Magnetic
d) Precipitation Hardenable Stainless Steels
- Formed from process of precipitation hardened.
- Microstructure => Martensite
- Application => Springs, knives & pressure vessels.

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Carbon Steel Bar
When carbon is the alloying element, the steel is called carbon steel. Carbon steels are classified by the percentage of carbon in points or hundredths of one percent they contain. There are relatively few alloying elements in carbon steel; carbon is the dominant alloy. Most carbon steel is considerably less expensive than alloy steel. The three basic types of carbon steel are low-carbon steel, medium-carbon steel, and high-carbon steel.

Low-Carbon Steel
The largest percentage of all carbon steel is low-carbon steel. It contains between 0.05% and 0.35% carbon. Low-carbon steel lacks the ability to become as hard and strong as other steel. However, because it does not become very hard, it is easier to machine and work with in the manufacturing plant.
Low-carbon steel is the least expensive type of steel. For this reason, it has many uses. Applications include fence wire, auto bodies, galvanized sheets, storage tanks, large pipe, and various parts in buildings, bridges, and ships.
Low-carbon steel is not as strong and hard as some of the more expensive grades of alloy steel, but it is not weak or low in quality. All steel, even low-carbon steel, is very strong and can be trusted to support a great deal of force. These steels are easy to work, can be welded, but cannot be hardened. Low carbon steel is available as band iron, black iron sheet, and in many different shapes and sizes of bars and rods.

Medium-Carbon Steel
Medium-carbon steel contains 0.35% to 0.50% carbon. It can be heat treated. If it is heat treated properly, medium-carbon steel can become quite hard and strong. It is frequently used in forgings and high-strength castings.
Applications of medium-carbon steel include wheels, axles, crankshafts, and gears.

High-Carbon Steel
The carbon content in high-carbon steel is over 0.50%, and it may be over 1%. This type of steel can be readily heat treated to obtain high strength and high hardness.
The disadvantage of high hardness in steel is a relatively high rate of distortion and the potential of cracking or becoming very brittle during the hardening process. Nevertheless, high-carbon steel can be safely used for making tools, dies, knives, railroad wheels, and for many other applications requiring high strength.

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Carbon steel are classified based on the amount of carbon content in the steel.

Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels contain varying amounts of different metals and materials to specialize their properties. Some of the most common materials added include chromium, molybdenum, nickel, and silicon. ----

Chromium is added in smaller amounts (0.5-2%) to increase harden ability and larger amounts (4-18%) to increase corrosion resistance. Molybdenum is added in amounts of 0.25-0.40% to increase the toughness of the steel. Nickel is added in smaller amounts (2-5%) to increase toughness and in larger amounts (12-20%) to increase corrosion resistance. Silicon is added to steel in smaller amounts (0.2-0.7%) to increase strength and in larger amounts (>2%) to improve its magnetic properties.

Low Carbon Steel
Composition of 0.05%-0.25% carbon and up to 0.4% manganese. Also known as mild steel, it is a low-cost material that is easy to shape. While not as hard as higher-carbon steels, carburizing can increase its surface hardness.

Advantage
Outstanding ductility and toughness. there are machinable, weldable, and of all steel.

Application
automobile body components, structural shapes and pipelines, bulidings, bridges, and tin cans.

Medium Carbon Steel
Composition of 0.29 between 0.60% carbon. Medium carbon steel is ductile and strong, with long-wearing properties.
Advantage
They are more often utilized in tempered condition, having microstructures of tempered martensite. Addition of alloy giving rise to a variety of strength and ductility combinations.

Disadvantage
It low harden abilities. These heat treated alloy are stronger than low carbon steel but sacrifice of ductility and toughness.

Application
Railway wheel and track, gear, crankshaft, and machine part.

Higher carbon
High carbon steel will be any type of steel that contains over 0.6% carbon but less than 1.4% carbon in its composition. High carbon steel has a reputation for being especially hard, but the extra carbon also makes it more brittle than other types of steel. This type of steel is the most likely to fracture when misused.

Advantage
This type of steel is excellent for making cutting tools or masonry nails. The hardness levels and metal wear resistance of high carbon steel is also rated very highly.

Disadvantage
This type of steel is not recommended for any type of welding or welding work. Out of the commonly used types of steel, this one is the most likely to fracture or break because of extra brittleness

Application
This type of steel is preferred in the manufacturing of many tools such as drill bits, knives, masonry nails, saws, metal cutting tools, and woodcutting tools

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Low  carbon -sometimes known as mild steel generally is 0.04% to 0.30% carbon content. Soft and weak but high ductility and toughness. apply at automobile panels.

Medium carbon - contain carbon  0.25 and 0.60 wt% and a manganese content ranging from .060% to 1.65%. . May be heat treated by austenitizing, quenching and tempering to improve mechanical properties. Applications such as railways wheel and tracks.

High carbon  - Commonly known as “carbon tool steel” and normally contain carbon between 0.60 and 1.4 wt% and the hardest, strongest yet least ductile. Always used in a tempered and  hardened condition. Some application is drills, saws

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=>steel alloy
-contain appreciable concentrations of other alloying elements
-mechanical properties are sensitive to the content of carbon <10 wt%
-plain carbon steels contain only residual concentrations of impurities other than carbon and little manganese
=>low carbon steel
-contain less than about 0.25 wt% C and are unresponsive to heat treatment to form martensite
-microstructures consist of ferrite and pearlite constituents
-relatively soft and weak but have outstanding ductility and thoughness
-yield strength of 275 MPa, tensile strengths between 415 and 550 MPa , and ductility of 25% EL
-applications include automobile body components, structural shapes, and sheets that are used in pipelines, buildings, bridges, and tin cans.
=>medium carbon steel
-have carbon concentrations between about 0.25 and 0.60 wt%
-may be heat-treated by austenitizing, quenching, and then tempering to improve their mechanical properties
-have low hardenabilities and can be successfully heat-treated only in very thin sections and with very rapid quenching rates
-stronger than the low carbon steels, but at a sacrifice of ductility and toughness
-applications include railway wheels and tracks, gears, crankshafts, and other machine parts and high-strength structural components calling for a combination of high strength, wear resistance, and toughness
->high carbon steel
- carbon contents between 0.60 and 1.4 wt%
the hardest, strongest, and yet least ductile of the carbon steels
-the tool and die steels are high carbon alloys, containing chromium, vanadium, tungsten, and molybdenum
-combine with carbon to form very hard and wear resistant carbide compounds

-applications include utilized as cutting tools and dies for forming and shaping materials, as well as in knives, razors, hacksaw, blades, springs, and high-strength wire

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Low Carbon Alloy
-less than about 0.25%C
-unresponsive to heat treatments
-consists of ferrite n pearlite.(soft and weak but outstanding ductility and toughness)

APPLICATION
-tin can

Yield Strength
-275 MPa

Tensile Strength
-415 ~ 550 MPa

Ductile
-25% EL

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=>steel alloy
-contain appreciable concentrations of other alloying elements
-mechanical properties are sensitive to the content of carbon <10 wt%
-plain carbon steels contain only residual concentrations of impurities other than carbon and little manganese
=>low carbon steel
-contain less than about 0.25 wt% C and are unresponsive to heat treatment to form martensite

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Alloy steel

-steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties.

- for alloy steel, more alloying element are intentionally
1)steel alloy
-contain appreciable concentrations of other alloying elements
-mechanical properties are sensitive to the content of carbon <10 wt%
-plain carbon steels contain only residual concentrations of impurities other than carbon and little manganese
2)Low carbon

- contain less than about 0.25% wt C
-  unresponsive to heat treatment
-intended to form martensite strengthening is accomplished by cold work
-microstructure consists ferrite and pearlite constituents
-outstanding ductility and toughness
-APPLICATION: Automobile body component, structural shaped and tin can.
-yeild strength 275 MPa, tensile Strength 415-550 MPa and ductility 25% EL.
3) Medium-Carbon Steel
-carbon concentrations between 0.25 and 0.60wt%.
-heat-treated by austenitizing,quenching and then tempering to improve mechanical properties.
-low hardenabilities

-APPLICATION:gear, railway wheel and crank shaft
4) High-Carbon Steel
-carbon content in high-carbon steel is over 0.50%, and it may be over 1%.
-steel can be readily heat treated to obtain high strength and high hardness.
-high hardness in steel is a relatively high rate of distortion and the potential of cracking.
-very brittle during the hardening process.
-APPLICATION: knives, saw and

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Mild steel (steel containing a small percentage of carbon, strong and tough but not readily tempered), also known as plain-carbon steel, is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications. Low-carbon steel contains approximately 0.05–0.25% carbon[1] making it malleable and ductile. Mild steel has a relatively low tensile strength, but it is cheap and easy to form; surface hardness can be increased through carburizing.[3]

It is often used when large quantities of steel are needed, for example as structural steel. The density of mild steel is approximately 7.85 g/cm3 (7850 kg/m3 or 0.284 lb/in3)[4] and the Young's modulus is 200 GPa (29,000,000 psi).[5]

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-Low carbon
>steel contains up to 0.30% C
>generally less than 0.25wt% of carbon
>carbon content is very low less than 0.10% with up to 0.4%Mn.
>microstructures consist of ferrite and pearlite.
>these alloys are relatively soft and weak but have outstanding ductility and toughness
>machinable, weldable and least expensive
>yield strength of 275Mpa, tensile strenght between 415 and 550Mpa.
>ductility of 25%EL
>used for stamping,forging and boiler plate

-medium
>carbon concentrations between 0.25 and 0.60wt%.
>heat-treated by austenitizing,quenching and then tempering to improve mechanical properties.
>low hardenabilities

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Low
- generally less than 0.25wt% Of               Carbon
-  use for automotive industry, beam of building, and bridges

Medium
- between 0.25wt% and 0.6wt% of carbon
- use for heavy machinery, crankshaft, gears

High
- between 0.6wt% and 1.4wt% of carbon
- use for cutting tools, high strength wires, nails,

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Low-alloy Steels

When these steels are designed for welded applications, their carbon content is usually below 0.25 percent and often below 0.15 percent. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.

These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them. Preheating might also prove necessary. This can be determined by using the carbon equivalent formula, which we'll cover in a later issue.

High-alloy Steels

For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:

1. Austenitic

2. Ferritic

3. Martensitic

Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone (HAZ).

Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.

Austenitic stainless steels offer excellent weldability, but austenite isn't stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.

Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It's important to note that austenitic alloys can't be hardened by heat treatment. That means they don't harden in the welding HAZ.

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Low. Often called mild steels, low-carbon steels have less than 0.30 percent carbon and are the most commonly used grades. They machine and weld nicely and are more ductile than higher-carbon steels.

Medium. Medium-carbon steels have from 0.30 to 0.45 percent carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.

High. With 0.45 to 0.75 percent carbon, these steels can be challenging to weld. Preheating, postheating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.

Very High. With up to 1.50 percent carbon content, very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.

High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition.

The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:

Weathering steels, designated to exhibit superior atmospheric corrosion resistance

Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on cooling

Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation hardening but with low carbon content and therefore little or no pearlite in the microstructure

Microalloyed steels, with very small additions of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardening

Acicular ferrite steel, very low carbon steels with sufficient hardenability to transform on cooling to a very fine high-strength acicular ferrite structure rather than the usual polygonal ferrite structure

Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, thus providing a high-strength steel of superior formability.

The various types of HSLA steels may also have small additions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

Low-alloy Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

As with steels in general, low-alloy steels can be classified according to:

Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels

Heat treatment, such as quenched and tempered, normalized and tempered, annealed.

Because of the wide variety of chemical compositions possible and the fact that some steels are used in more than one heat-treated, condition, some overlap exists among the alloy steel classifications. In this article, four major groups of alloy steels are addressed: (1) low-carbon quenched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) heat-resistant chromium-molybdenum steels.

Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are used primarily as plate. Some of these steels, as well as other, similar steels, are produced as forgings or castings.

Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.

Bearing steels used for ball and roller bearing applications are comprised of low carbon (0.10 to 0.20% C) case-hardened steels and high carbon (-1.0% C) through-hardened steels. Many of these steels are covered by SAE/AISI designations.

Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.

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2 Types of steels :
1) Low Alloy
2)High Alloy

Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:

1)Austenitic
2)Ferritic
3)Martensitic

Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:

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ShiraziDaud

Low carbon aloy -
-Contain less than about 0.25 wt% C
-unresponsive to heat treatments intended to form martensite.
-Microstructure consist (FERRITE & PEARLITE)
-Outstanding ductility and thoughness. Addition :-
-machinable
-weldable
-off all steels
Yield strength:275MPa
Tensile strength: 415-550MPa
Ductility : 25%EL

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Steel alloys :-
1)low carbon alloy
2)medium carbon alloy
3)high carbon alloy

Composition :
1) Consist of 0.05%-0.25% carbon and up to 0.4% manganese.
-known as mild steel
-low-cost material that is easy to shape.
-Carburizing can increase its surface hardness

2) Composition of 0.29%-0.54% carbon, with 0.60%-1.65% manganese.
-Ductile and strong, with long-wearing properties.

3)Composition of 0.55%-0.95% carbon, with 0.30%-0.90% manganese.
-very strong and holds shape memory very well.
-it is ideal for springs and wire

Application :-
1) cold headed fasteners and bolts.
-high volume screw machine parts applications, such as shafts, spindles, pins, rods, sprocket assemblies.
2)railway wheels and tracks
-gears
-crankshafts
3)tool and die steels owing to the high hardness and wear resistance