An Introduction To Classification of steels And Applications

An Introduction To Classification of steels And Applications


Introduction
According to the iron–carbon phase diagram , all binary Fe–C alloys containing less than about 2.11 wt% carbon* are classified as steels, and all those containing higher carbon content are termed cast iron. When alloying elements are added to obtain the desired properties, the carbon content used to distinguish steels from cast iron would vary from 2.11 wt%.
Steels are the most complex and widely used engineering materials because of
 (1) the abundance of iron in the Earth’s crust,
 (2) the high melting temperature of iron (1534⁰C),
 (3) a range of mechanical properties, such as moderate (200–300 MPa) yield strength with excellent ductility to in excess of 1400 MPa yield stress with fracture toughness up to 100 MPa m⁻²,
 (4) associated microstructures produced by solid-state phase transformations by varying the cooling rate from the austenitic condition

CLASSIFICATION OF STEELS
Steel s can be classified by different systems depending on :
1. Compositions, such as carbon (or nonalloy ), low -alloy, and alloy steels
2. Manufacturing methods , such as convert er, electric furnace , or electroslag remelting methods
3. Application or main characteristic, such as structural , tool, stainless steel, or heat resistant steels
4. Finishing methods, such as hot rolling, cold rolling, casting, or controlled rolling and
controlled cooling
5. Product shape, such as bar, plate, strip, tubing, or structural shape
6. Oxidation practice employed, such as rimmed, killed, semikilled, and capped steels
7. Microstructure, such as ferritic, pearlitic, martensitic, and austenitic (Figure 1)
8. Required strength level, as specified in the American Society for Testing and Materials (ASTM) standards
9. Heat treatment, such as annealing, quenching and tempering, air cooling (normalization),
and thermomechanical processing
10. Quality descriptors and classifications, such as forging quality and commercial quality
Among the above classification systems, chemical composition is the most widely used basis for designation . Classification systems based on oxidation practice, application, and quality descriptors are also briefly

FIG 1

1- TYPES OF STEELS BASED ON DEOXIDATION PRACTICE
Steels, when cast into ingots, can be classified into four types according to the deoxidation
practice or, alternatively, by the amount of gas evolved during solidification. These four types
are called killed, semikilled, capped, and rimmed steels
Killed Steels
Killed steel is a type of steel from which there is practically no evolution of gas during solidification
of the ingot after pouring, because of the complete deoxidation, and formation of pipe in
the upper central portion of the ingot, which is later cut off and discarded. All alloy steels, most
low-alloy steels, and many carbon steels are usually killed. The continuous casting billets are also
killed. . Killed steel is characterized by a homogeneous structure and even distribution of chemical compositions and properties. Killed steel is produced by the use of a deoxidizer such as Al and a ferroalloy of Mn or Si; However, calcium silicide and other special deoxidizers are sometimes used.

Semikilled Steels
Gas evolution is not completely suppressed by deoxidizing additions in semikilled steel, because it is partially deoxidized. There is a greater degree of gas evolution than in killed steel, but less than in capped or rimmed steel. An ingot skin of considerable thickness is formed before the beginning of gas evolution.
 A correctly deoxidized semikilled steel ingot does not have a pipe but does have well-scattered large blow holes in the top-center half of the ingot; however, the blow holes weld shut during rolling of the ingot. Semikilled steels generally have a carbon content in the range of 0.15–0.30%. They find a wide range of uses in structural shapes, skelp, and pipe applications. The main features of semikilled steels are:
(1) variable degrees of uniformity in composition, which are intermediate between those of killed and rimmed steels and less segregation than rimmed steel,
(2) a pronounced tendency for positive chemical segregation at the top center of the ingot (Figure 2)

FIG 2

Rimmed Steels
Rimmed steel is characterized by a great degree of gas evolution during solidification in the mold and a marked difference in chemical composition across the section and from the top to the bottom of the ingot (Figure 2). These result in the formation of an outer ingot skin or rim of relatively pure iron and an inner liquid (core) portion of the ingot with higher concentrations of alloying and residual elements, especially C, N, S, and P, having lower melting temperature. The higher purity zone at the surface is preserved during rolling .
Rimmed ingots are best suited for the manufacture of many products, such as plates, sheets, wires, tubes, and shapes, where good surface or ductility is required .
The technology of producing rimmed steels limits the maximum content of C and Mn, and the steel does not retain any significant amount of highly oxidizable elements such as Al, Si, or Ti.
Rimmed steels are cheaper than killed or semikilled steels for only a small addition of deoxidizer is required and is formed without top scrap

FIG 3

Capped Steels
Capped steel is a type of steel with characteristic s similar to those of a rimmed steel but to a degree intermediate between that of rim med and semikilled steels. Less deoxidizer is used to produce a capped ingot than to produce a semikilled ingot .
This induces a controlled rimming action when the ingot is cast. The gas entrapped during solidification is excess of that required to counteract normal shrinkage, resulting in a tendency for the steel to rise in the mold.
Capping is a variation of rimmed steel practice. The capping operation confine s the time of gas evolution and prevents the formation of an excessive number of gas voids within the ingot. The capped ingot process is usually applied to steels with carbon con tents greater than 0.15% that are used for sheet, strip, tin plate , skelp, wire, and bars.
Mechanically capped steel is pour ed into bottle-t op molds using a heavy cast iron cap to seal the top of the ingot and to stop the rimming action  .
Chemically capped steel is cast in open -top molds . The cap ping is accomplished by the addition of Al or ferrosilicon to the top of the ingot, causing the steel at the top surface to solidify rapidly. The top portion of the ingot is cropped and discarded.

FIG 4

2- CLASSIFICATION OF STEEL BASED ON CHEMICAL COMPOSITION
The iron is associated with carbon, either as a solid solution or as the chemical compound iron carbide (cementite). In the case of cast irons, some amount of carbon may be uncombined (free) in the form of flake graphite. In addition to carbon, other elements may also be present. These may be impurities such as sulphur and phosphorus which weaken the metal and are kept to a minimum. Alloying elements are added to enhance the performance of the metal
Plain carbon steels consist mainly of iron and carbon, and are the simplest of the ferrous metals. Some manganese will also be present to neutralize the deleterious effects of the sulphur and to enhance the grain structure. It is not present in sufficient quantity to be considered as an alloying element.
The amount of carbon present affects the properties of the steel. The maximum amount of carbon which can remain combined with the iron at all temperatures is 1.7%. In practice an upper limit of 1.2–1.4% is set to ensure a margin of safety. A steel, by definition, must contain no free carbon.


Low-carbon steels
These have a carbon content 0.1–0.3% plus impurities, plus some manganese to neutralize the effect of any sulphur content left over from the extraction process. Such steels cannot be directly hardened by heat treatment, but they can be readily carburized and case hardened.
The lower-carbon steels in this category are used for steel sheets for pressing out such components as motorcar body panels as they have a high ductility. The lower-carbon steels in this category are also made into drawn wire rod and tube. The higher-carbon steels in this category are stiffer and less ductile, and are used for general workshop bars, plates and girders.
Low-carbon steels are substantially stronger than wrought iron which is no longer considered to be a structural material

FIG 5
LOW CARBON STEEL; WIRE

Medium-carbon steels
(a) Carbon content 0.3–0.5%: Such steels can be toughened by heat treatment (heating to red heat and quenching – rapid cooling – in water). 
They are used for crankshaft and axle forgings where cost is important and the service requirements do not warrant stronger but more expensive alloy steels.
(b) Carbon content 0.5–0.8%: These are used for vehicle leaf springs and garden tools. Such steels can be quench hardened by heat treatment as above.

FIG 6 
MEDIUM CARBON STEEL THREADED RODS

High-carbon steels
All high carbon steels can be hardened to a high degree of hardness by heating to a dull red heat and quenching. The hardness and application depend on the carbon content, the rate of cooling from the hardening temperature and the degree of tempering after hardening:
(a) Carbon content 0.8–1.0%; used for coil springs and wood chisels.
(b) Carbon content 1.0–1.2%; used for files, drills, taps and dies.
(c) Carbon content 1.2–1.4%; used for fine-edge tools (knives, etc.).

FIG 7

Effect of carbon content on the composition,
properties and uses of plain carbon steels

FIG 8

Low-Alloy Steels
Low-alloy steels constitute a group of steels that exhibit superior mechanical properties compared to plain carbon steels as the result of addition of such alloying elements as Ni, Cr, and Mo. For many low-alloy steels, the main function of the alloying elements is to increase the hardenability in order to optimize the strength and toughness after he at treatment. In some instances, however, alloying elements are used to reduce environmental degradation under certain specified conditions.
Low-alloy steel s can be classified according to:
(1) chemical composition such as nickel steels, nickel –chromium steels, molybdenum steels, chromium–molybdenum steels, and so forth, based on the principal  alloying elements present
(2) heat treatment such as quenched and tempered, normalized and tempered, annealed and so on,
(3) weld ability.
Because of the large variety of chemical compositions possible an d the fact that some steels are employed in more than one heat-treated conditions some overlap exist s among the low -alloy steel classifications. However, these grades c an be divide d into four major groups such as
(1) low -carbon quenched and tempered (QT) steels,
(2) medium -carbon ultrahigh strength steel s,
(3) bearing steels, an d
(4) heat-resistant Cr–Mo steels
These steel s are used for gears, aircraft landing gear, airframe parts, pressure vessels , bolts , springs, screws, axles, studs, fasteners, machinery parts, connecting rods, crankshaft s, piston rods, oil well drilling bits, high-pressure tubing, flanges, wrenches, sprockets,

FIG 9
LOW ALLOY STEEL PLATES

High-Strength Low-Alloy Steels
A general description of HSLA steel is as that containing:
 (1) low carbon (0.03–0.25%) content to obtain good toughness, formability, and weldability,
 (2) one or more of the strong carbide-forming microalloying elements (MAEs) (e.g., V, Nb, or Ti),   (3) a group of solid solution strengthening elements (e.g., Mn up to 2.0% and Si),
 (4) one or more of the additional MAEs (e.g., Ca, Zr) and the rare earth elements, particularly Ce and La, for sulfide inclusion shape control and increasing toughness
In many other HSLA steels, small amounts of Ni, Cr, Cu, and particularly Mo are also present, which increase atmospheric corrosion resistance and hardenability.
HSLA steels are successfull y used as ship, plate , bar, structural sections, and forged bar products, and find applications in several divers e fields such as oil and g as pipelines; in the automotive, agricultural, and pressure vessel industries, in offshore structures and platform s and in the constructions of crane, bridges , buildings, shipbuildings, railroad , tank cars, and power transmission and TV towers

FIG 10
High Alloy Steels and Schäffler Diagram

Classification of HSLA Steels
Several special terms are used to describe various types of HSLA steels
1. Weathering steels : Steel s contain ing ~0.1% C, 0.2–0.5% Cu, 0.5–1. 0% Mn, 0.05–0.15% P, 0.15–0 .90% Si, and sometimes containing Cr and Ni, exhibiting superior atmospheric corrosion resistance . Typical applications include railroad cars, bridges , and unpainted buildings.
2. Control -rolled steels : Steel s designated to develop a highly deformed austenite structure
by hot rolling (according to a predetermined rolling schedule) that will trans form to a very fine equiaxed ferrite structure on cooling .
3. Pearlite-reduced steels : Steels strengthened by very fine-grained ferrite and precipitation
hardening but with low carbon content , an d therefore exhibiting little or no pearlite in the microstructure.
4. Micro alloyed steels : Conventional HS LA steels containing V, Ti, or Nb, as defined above. They exhibit discontinuous yielding behavior .
5. Acicular ferrite steels : Very low -carbon (typically 0 .03–0.06% ) steels with enough hardenability (by Mn, Mo , Nb , an d B addition s) to trans form on cooling to a very fine, high strength acicular ferrite structure rather than the usual polygonal ferrite structure. In addition to high strength and goo d toughness , these steels have continuous yielding behavior.
6. Low-carbon bainite steels : Steel s are strengthen ed by bainite, with very fine grains and precipitations. They contain 0.02–0 .2% C, 0.6–1. 6% M n, 0.3–0. 6% Mo, and MAEs (such as V, Nb, Ti, and B), usually containing 0.4–0.7% Cr. The yield strength of these steels is higher than 490 MPa, with good toughness
7. Low-carbon martensite steels : Steel s are strengthened by martensite with high hardenability (by addition of Mo, Mn , Cr, Nb, and B) and fine grains (by Nb addition ).
These steel s contain 0.05–0 .25% C, 1.5–2.0% Mn , 0.20–0 .50 Mo , an d MAEs (such as
Nb, Ti, V, a nd B). Some steels containing small amounts of Ni, Cr, and Cu, after rolling or forging, and directly quenching an d tempering attain a low -carbon marten site structure with high yield strength (760–1 100 MPa ), high toughness (CVN 50–130 J), and superior fatigue strength
8. Dual-phase steels : Steels comprising essentially fine dispersion of ha rd strong martensite
but sometimes also retained austenite or even bainite in a soft and fine-grained ferrite matrix.

Tool Steels
A tool steel is any steel used to shape other meta ls by cutting, forming, machining, battering, or die casting or to shape and cut wood, pap er, rock, or concrete. Hence tool steels are designed to have high hardness an d durability under severe service conditions. They comprise a wide range from plain carbon steel s with up to 1.2% C without appreciable amounts of alloying elements to the highly alloyed steel s in which alloying additions reach 5 0%. Although some carbon tool steel s an d low -alloy tool steel s have a wide range of carbon con tent, most of the higher alloy tool steel s have a comparatively narrow carbon range. A mixed classification system is used to classify too l steels based on the use, composition, special mechanical properties, or method of he at treatment .
there a re nine main groups of wrought tool steels :

FIG 11

High-speed steels : are used for applications requiring long life at relatively high operating
tempratures such as for heavy cuts or high -speed machining. High-speed steels are the most
important alloy tool steel s because of their very high hardness and goo d wear assistance in the
heat-treated condition and their ability to retain high hardness and the elevated temperatures
often encountered during the operation of the tool at high cutting speeds.
High-speed steels are grouped into molybdenum type M and tungsten type T.
Hot-work tool steels: (AISI series) fall into three major groups:
(1) chromium-base, types H1–H19,
(2) tungsten-base, types H20–H39,
(3) molybdenum-base, types H40–H59.
The distinction is based on the principal alloying additions; however, all classes have medium carbon content and Cr content varying from 1.75 to 12.75%. Among these steels, H11, H12, H13 are produced in large quantities.
Cold-work tool steels: comprise three categories:
(1) air-hardening, medium-alloy tool steels (AISI A series),
(2) high-chromium tool steels (AISI D series),
(3) oil-hardening tool steels (AISI O series).
Shock-resisting tool steels: (AISI S series) are used where repetitive impact stresses are encountered such as in hammers, chipping and cold chisels, rivet sets, punches, driver bits, stamps, and shear blades in quenched and tempered conditions. In these steels, high toughness is the major concern and hardness the secondary concern. Among these grades, S5 and S7 are perhaps the most widely used.
Low-alloy special-purpose tool steels (AISI series) are similar in composition to the W type tool steels, except that the addition of Cr and other elements render greater hardenability and wear-resistance properties, type L6 and the low-carbon version of L2 are commonly used for a large number of machine parts.
Mold steels : (AISI P series) are mostly used in low-temperature die casting dies and in molds for the injection or compression molding of plastics
Water-hardening tool steels: (AISI W series): Among the three compositions listed, W1 is the most widely used as cutting tools, punches, dies, files, reamers, taps, drills, razors, woodworking tools, and surgical instruments in the quenched and tempered condition

Stainless Steels

Stainless steels may be define d as complex alloy steels contain ing a mini mum of 10.5% Cr with or without other elements to produce austenitic, ferritic, duplex (ferritic–austenitic), martensitic, and precipitation-hardening grades.

FIG 12

austenitic stainless steels : are Fe–Cr–Ni–C and Fe–Cr–Mn–Ni–N alloys containing 16–26% Cr, 0.75–19.0% Mn, 1–40% Ni, 0.03–0.35% C, and sufficient N to stabilize austenite at room and elevated temperatures.

Ferrite stainless steels: contain essentially 10.5–30% Cr with additions of Mn and Si and occasionally Mo, Ni, Al, Ti, or Nb to confer particular characteristics. As they remain ferritic at room and elevated temperatures, they cannot be hardened by heat treatment.

Duplex stainless steels: contain 18–29% Cr, 2.5–8.5% Ni, and 1–4% Mo, up to 2.5% Mn, up

to 2% Si, and up to 0.35% N.Duplex stain less steels find applications as welded pip e products for handling wet an d dry CO2 and sour gas and oil products in the petrochemical industry,

Martensitic stainless steels:  contain 11.5–1 8% Cr, 0.08–1 .20% C, an d other alloying elements

less than 2 to 3%. They can be hardened and tempered to yield strength in the range of 550–1900 MPa (80–27 5 ksi).The standard martensitic grades are types 403, 410, 414, 416, 41 6Se, 420 , 422, 431, 440A, 440B , and 440C . They are used in manifold stud bolts , heat control shaft s, steam

valves , Bourdon tubes, gun mounts, water pump parts, carburet or parts , wire cutter blades,

garden shears, cutlery, pa int spray nozzles, glass and plastic molds, bomb shackle parts , drive

screws, aircraft bolting, cable terminal s, diesel engine pump parts, instrument parts, crankshaft

counterweight pins, valve trim, ball bearing s, and races.

PH stainless steels: are high-strength alloy s with appreciable ductility and good corrosion resistance that are developed by a simple heat treatment comprising martensite formation and low-temperature aging (or tempering) treatment; the latter heat treatment step may be applied after fabrication. PH stainless steels can have a matrix structure of either austenite or marten site. Alloy elements add ed to form precipitates are Mo , Cu, Al, Ti, Nb, and N. 

PH stainless steels may be divided into three broad groups: 

(1) martensitic type, 

(2) semiaustenitic type, 

(3) austenitic type 

A majority of these steels are classified by a three- digit number in the AISI 400 series or by a five-digit UNS designation. However, most of them are better known by their trade names or their manufacturer. All steels are available in sheet, strip, plate, bar, and wire.

Maraging Steels

Maraging steels are a specific class of carbon-free (or small amounts) ultrahigh-strength steels that derive their strength not from carbon but from precipitation of intermetallic compounds and martensitic transformation

FIG 13
A crankshaft made from maraging steel

The commonly available maraging steels contain 10–19% Ni, 0–18% Co, 3–14% Mo, 0.2–1.6% Ti, 0.1–0.2% Al, and some intermetallic compounds are Ni3Ti, Ni3Mo, Fe2Mo, etc. Since these steels develop very high strength by martensitic transformation and subsequent age-hardening, they are termed maraging steels

Maraging steels have found applications where lightweight structures with ultrahigh strength and high toughness are essential and cost is not a major concern. Maraging steels have been extensively used in two general types of applications:

1. Aerospace and aircraft industry for critical components such as missile cases, load cells, helicopter flexible drive shafts, jet engine drive shafts, and landing gear

2. Tool manufacturing industries for stub shafts, flexible drive shafts, splined shafts, springs, plastic molds, hot-forging dies, aluminum and zinc die casting dies, cold heading dies and cases, diesel fuel pump pins, router bits, clutch disks, gears in the machine tools, carbide die holders, auto frettage equipment, etc.

DESIGNATIONS FOR STEELS

A designation is the specific identification of each grade, type, or class of steel by a number, letter, symbol, name, or suitable combination thereof unique to a certain steel. It is used in a specific document as well as in a particular country. In the steel industries , these terms have very specific uses: grade is used to describe chemical composition; type is used to denote deoxidation practice; and class is used to indicate some other attributes such as tensile strength level or surface quality

In ASTM specifications, however, these terms are used somewhat inter changeably.

For example, in ASTM A 434, grade identifies chemical composition and class indicates tensile properties. In ASTM A 51 5, grade describes strength level; the maximum carbon content allow ed by the specification is dependent on both the plate thickness and the strength level . In ASTM A 533, type indicates chemical analysis, while class denotes strength level . In ASTM A 302, grade identifies requirements for both chemical composition and tensile properties. ASTM A 514 and A 517 are specifications for high-strength quenched and tempered alloy steel plate for structural and pressure vessel applications , respectively; each has a number of grades for identifying the chemical composition that is capable of developing the required mechanical properties. How ever, all grades of both designations have the same composition limits.

By far the most widely used basis for classification and designation of steels is the chemical

composition. The most commonly used system of designating carbon and alloy steels in the

United State s is that of the AISI and SAE numerical designations. The UNS is also increasingly

employed . Other designations used in the specialized fields include Aerospace Materials

Specification (AM S) and American Petroleum Institute (API ) designation.

FIG 14