Low Alloy Steels
Low Alloy Steels
Alloying elements are added in the steels for various reasons which include improved corrosion resistance and / or improved mechanical properties at low or elevated temperatures. Alloying elements are also used to improve the hardenability of quenched and tempered steel.
The term ‘alloy steel’ is used for those steels which have got in addition to carbon other alloying elements in their composition. Alloy steels are made by combining steels with one or more other alloying elements. These elements are normally metals. They are intentionally added to incorporate certain properties in steel which are not found in the plain carbon steels.
There are a large numbers of alloying elements which can be added to steel. Total amount of alloying elements in alloy steels (other than micro alloyed steels) can vary between 1 % and 50 %. Common alloying elements include manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloying elements include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, antimony and zirconium.
Any steel which contains more than 1.65 % manganese, 0.60 % silicon, and 0.60 % copper, or a definite minimum amount of any other element, is termed an ‘alloy’ steel. The alloy steels are normally divided into two classes namely (i) low alloy steels, and (ii) high alloy steels. They are divided according to composition based on the content of the alloying elements.
Alloying elements addition to the steels improves a range of properties in alloy steels as compared to carbon steels. These properties are strength, hardness, toughness, corrosion resistance, wear resistance, hardenability, machinability, heat resistance, fire resistance, and hot hardness etc. Alloy steels can need heat treatment to achieve some of these improved properties.
Alloy steels are normally of three types. They are micro-alloyed steels, low alloy steels, and high alloyed steels. Micro-alloyed steels are a type of alloy steels which contains small amounts of alloying elements (normally in the range of 0.05 % to 0.15 %). These steels are also sometimes called high strength low alloy steels. The difference between the low alloy steels and high alloy steels is somewhat arbitrary.
Some people define low alloy steels as those steels which contain alloying elements upto 4 % while as per other second definition low alloy steels contain alloying elements upto 8 %. Steels having alloying elements higher than this amount come under the category of high alloy steels.
Steel is considered to be low-alloy steel when any definite range or definite minimum quantity of any of the following elements is specified or required within the limits of the recognized field of constructional alloy steels: aluminum, boron, chromium (upto 3.99 %), cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, or any other alloying element added to obtain the desired alloying effect. Low alloy steels have normally ferritic-pearlitic, pearlitic, or bainitic structure.
The term low alloy steel is being used to differentiate the steels from high alloy steels. Low alloy steels constitute a category of ferrous materials which show mechanical properties superior to plain carbon steels as the result of additions of alloying elements. High alloy steels include steels with a high degree of fracture toughness, maraging steels, austenitic manganese steels, tool steels, and stainless steels.
Low alloy steel is an important material in economic and defence contexts. The mechanical properties of low alloy steels depend on the internal organization of the microstructure, and the internal organization depends on the influence of important factors, such as alloying elements and the process parameters. The low alloy steels contain nickel, molybdenum, and chromium, which add to the weldability, notch toughness, and yield strength of the material.
The chemical composition requirements of low alloy steel are as per various standards. The low alloy steels can be suitable for some structural applications. The residual alloying elements can have effect on the mechanical properties of hot finished steel. The alloying elements also have their effects on the hardenability and mechanical properties of quenched and tempered steels.
Despite the addition of the alloying elements, low alloy steels are not necessarily difficult to weld. Still, knowing exactly what type of low alloy steel is critical for achieving good weld integrity, as is proper filler metal selection. When welding the low alloys steels, preheat and post heat treatments are typically not needed.
For many people, the primary function of the alloying elements in the low alloy steels is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, additions of alloying elements are used to reduce environmental degradation under certain specified service conditions.
The application of low alloy steels varies greatly across many industries. Applications for these steels range from military vehicles, earth moving and construction equipment, cross-country pipelines, pressure vessels and piping, oil drilling platforms, and so on. Several common low alloy steels are used for building ship hulls, submarines, bridges, and off-highway vehicles.
Through the addition of particular alloying elements, low alloy steels possess precise chemical compositions and provide better mechanical properties than many conventional mild steel or plain carbon steels. The alloying element in low alloy steel typically comprises 1 % to 4 % and is added based on its ability to provide a very specific attribute. For example, the addition of molybdenum improves material strength, nickel adds toughness, and chromium increases high temperature strength, hardness, and corrosion resistance. Manganese and silicon, the other common alloying elements, provide excellent deoxidizing capabilities.
As with the plain carbon steels, there is an established classification system of the designations for the low alloy steels. The classification is based on the principal alloying element(s) in the steel. These principal elements include carbon, manganese, silicon, nickel, chromium, molybdenum, and vanadium. Each element, either singly or in combination with other elements, imparts certain properties and characteristics to the steel.
Low alloy steels can be classified according to (i) chemical composition such as nickel steels, nickel-chromium steels, molybdenum steels, chromium molybdenum steels et, and so on, (ii) heat treatment such as quenched and tempered, normalized and tempered, annealed, and so on, and (iii) as per the weldability.
Since there are wide variety of chemical compositions possible in low alloy steels and also due to the fact that some steels can be used in more than one heat treated condition, there exist certain overlap in the classification of low alloy steels. Hence, it is rather difficult to classify low alloy steels. There are four major groups of low alloy steels which are normally used. These groups are (i) low carbon quenched and tempered steels, (ii) medium carbon ultra high strength steels, (iii) heat resistant chromium-molybdenum steels, and (iv) bearing steels.
Low carbon quenched and tempered steels combine high yield strength (from 350 MPa to 1,035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The different types of low carbon quenched and tempered steels have various combinations of these characteristics based on their intended uses. Some of these steels are used in military for armour purpose mostly as plates. Also, some of these steel are also produced as forgings or castings.
Low alloy steels are normally not used without appropriate heat treatment. By far the largest tonnage of the low alloy steels is of the type which has nominal carbon contents in the range of around 0.25 % and 0.55 %. If low alloy steel is to be used for carburized parts, the nominal carbon content normally does not exceed 0.2 %. Various combinations and amounts of manganese, silicon, nickel, chromium, molybdenum, vanadium, and boron are normally present in these steels to enhance the properties of the quenched and tempered alloy.
The microstructure (tempered martensite or bainite) produced by quenching and tempering low alloy steels is characterized by good toughness, that is, the capacity to deform without rupture, at a given strength level. The basic phenomenon of obtaining a favourable microstructure by heat treatment is also possible in plain carbon steels, although primarily in thin sections. Thus, the most important effect of alloying elements in low alloy steel is to induce the formation of martensite or bainite, and accompanying superior properties, in large sections. The level of hardness or strength of these structures is a function of the carbon content of the transformation products rather than of the alloying elements present.
The general effect of alloying elements dissolved in austenite is to decrease the rate of austenite transformation at sub-critical temperatures. Since the desirable products of transformation in these steels (martensite and lower bainite) are formed at low temperatures, the decreased transformation rate is essential. Thus, work pieces can be cooled more slowly, or larger work pieces can be quenched in a given medium, without transformation of austenite to the undesirable high temperature products (pearlite or upper bainite). This function, decreasing the rate of transformation and thereby facilitating ultimate transformation to martensite or lower bainite, is known as hardenability, the most important effect of alloying elements in hardenable steels. By increasing hardenability, alloying elements greatly extend the potential for enhanced properties to the large sections required for many applications.
Many of the hardenability curves serve to illustrate the principle that, regardless of composition, tempered steels of the same hardness have approximately the same tensile strength. The maximum (as-quenched) hardness of heat-treated steel depends primarily on the carbon content. Alloying elements have little effect on the maximum hardness which can be developed in steel, but they profoundly affect the depth to which this maximum hardness can be developed in a part of specific size and shape.
Thus, for a specific application, one of the first decisions to be made is what carbon level is needed to obtain the desired hardness. The next step is to determine what alloy content gives the proper hardening response in the section size involved. This is not to imply that tempered martensitic steels are alike in every respect, regardless of composition, because the alloy content is responsible for differences in the preservation of strength at elevated temperatures, in abrasion resistance, in resistance to corrosion, and even, to a certain extent, in toughness. However, the similarities are sufficiently marked to permit reasonably accurate predictions of mechanical properties from hardness rather than from composition, thereby justifying the emphasis on hardenability as the most important function of the alloying elements.
Medium carbon ultra high strength steels are structural steels which are having yield strengths which can exceed 1,380 MPa. Some of these steels are covered by designations given in various standards while some other steels are having proprietary compositions. Product forms for these steels include billets, bars, rods, forgings, sheets, pipes and welding wires.
Chromium-molybdenum low alloy steels are heat resistant steels which contain 0.5 % to 9 % chromium and 0.5 % to 1 % molybdenum. The carbon content is normally below 0.2 %. The chromium provides improved oxidation and corrosion resistance while the molybdenum increases strength at elevated temperatures. These steels are normally produced in the normalized and tempered, quenched, and tempered or annealed conditions. Chromium-molybdenum steels are extensively used in the oil and gas industries and in fossil fuel and nuclear power plants.
Bearing steels are used for ball and roller bearing applications. The most popular bearing steel has got around 1 % carbon and 1.5 % of chromium. Ball bearing tests conducted on such steel in 1901 by Stribeck indicated its suitability for the application and was apparently adopted some 120 years ago for bearings by Fichtel & Sachs of Schweinfurt in 1905, and has persisted to this day as a key steel in the production of bearings, with progressive improvements in fatigue performance achieved primarily by improvements in cleanliness with respect to non–metallic inclusions. It represents the majority of the bearing steel produced annually. Tab 1 gives nominal compositions of some of the bearing steels.
|Tab 1 Nominal compositions of some bearing steels|
|Sl. No.||Grade||Nominal compositions, %|
|High carbon bearing steels|
|2||ASTM A 485-1||0.97||1.10||0.60||1.05|
|3||ASTM A 485-3||1.02||0.78||0.22||1.30||0.25|
|5||SUJ 1||1.02||Less than 0.50||0.25||1.05||Less than 0.25||Less than 0.08|
|Carburizing bearing steels|
As seen from the table, there are two categories of steels which find application in the majority of bearings. The first category consists of those steels which have high carbon content (around 1 % carbon) and are hardened throughout their sections into a martensitic or bainitic condition. The second category of steels consist of are carburizing steels which have low carbon (0.1 % C to 0.2 % carbon) and are case hardened. These steels have soft cores but tenacious surface layers introduced using processes such as case or induction hardening. Bearing steels can also be divided in a broad sense into classes such as intended for normal service, high temperature service, or service under corrosive conditions. Fig 1 shows micro-structure of high carbon and carburizing bearing steels.
Fig 1 Micro-structure of high carbon and carburizing bearing steels
Low alloy steels normally need additional care throughout their manufacture. They are more sensitive to thermal and mechanical operations, the control of which is complicated by the varying effects of different chemical compositions. For securing the most satisfactory results, consumers normally consult with steel producers regarding the working, machining, heat treating, or other operations to be used in fabricating the steel, mechanical operations to be employed in fabricating the steel, mechanical properties to be obtained, and the conditions of service for which the finished articles are intended.
There are several low alloy steels which are not designed for just their room-temperature strength properties. These steels have additional properties which are important, such as corrosion, heat resistance, and formability. Some of the popular low alloy steels are described below.
Constructional alloy steels – These alloy steels have low content of alloying elements. Total content of alloying elements in these steels ranges from 0.25 % to around 6 %. This class of alloy steels is used in the construction of bridges, buildings, ships, auto frames, and railroads etc. Construction alloy steels are used for such machine parts as shafts, gears, levers, bolts, springs, piston pins, and connecting rods etc.
Nickel steel – The average steel in low temperature environment has higher strength but low elongation and toughness, thus increases the chance for brittle fracture. If the steel is needed in a low temperature environment then it is required to have superior low temperature toughness which is essential. Nickel steel is suitable steel for the low temperature service. Low temperature service steel is formed by adding 2.5 % to 3.5 % of nickel in the carbon steel to enhance its low temperature toughness. Nickel can strengthen ferrite matrix while lowering Ar3 (third transformation temperature) which helps with fine grain formation. In addition to the normalizing treatment during the production process of low alloy low temperature service steel, quenching and tempering are also parts of the mechanical properties improvement treatment.
Weathering steel – All low alloy steels have a tendency to rust in the presence of moisture and air. This rust is a porous oxide layer which can hold moisture and oxygen and promote further corrosion. The rate of rust formation depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. Weathering steels are low alloy steels with improved corrosion resistance. These steels work by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal.
Weathering steels are with a carbon content of less than 0.2 % to which mainly copper, chromium, nickel, phosphorus, silicon, and manganese are added as alloying elements to a total of no more than 3 % to 5 %. The enhanced corrosion resistance of weathering steel in relation to mild steel or plain carbon steel is due to the formation in low aggressive atmospheres of a compact and well‐adhering corrosion product layer known as patina. This definition of weathering steel, however, has not remained unchanged but has evolved as new weathering steel compositions have been developed to achieve improved mechanical properties and / or withstand increasingly aggressive atmospheric conditions from the corrosion point of view, especially in marine environments.
In 1968 ASTM standard A‐242 presented two specifications for weathering steels, one with high P content ( less than 0.15 %) and the other with a lower phosphorus content (0.04 % maximum). The latter was ultimately replaced by ASTM standard A‐588 weathering steel (Tab 2). This steel possesses less resistance to atmospheric corrosion due to its lower phosphorus content, but for this same reason it has better weldability.
|Tab 1 Chemical composition of commonly used weathering steels|
|Elements / Steel type||Unit||ASTM A-242 (COR-TEN A)||ASTM A-588 Gr. A (COR-TEN B)|
|Typical concentration||Typical concentration|
|Carbon (C)||%||0.15 maximum||0.019 maximum|
|Manganese (Mn)||%||1.0 maximum||0.8-1.25|
|Phosphorus (P)||%||0.015 maximum||0.15 maximum||0.04 maximum||0.04 maximum|
|Sulphur (S)||%||Less than 0.05||Less than 0.05|
|Copper (Cu)||%||0.2 minimum||0.25-0.4||0.25-0.4||0.3-0.4|
|Nickel (Ni)||%||0.5-0.65||0.4 maximum||0.02-0.3|
High tensile and high yield strength steel – In this series of low alloy steels manganese, nickel, chromium, and molybdenum etc. are added. These elements can increase strength of ferrite matrix, improve the hardening tendency, and allow better control of grain size. This type of steel under as welded condition can meet the requirement of high strength, corrosion resistance, or improved notch toughness and other mechanical properties. This steel type has good weldability with the yield strength ranging from 480 MPa to 830 MPa, and tensile strength ranging from 620 MPa to 1,030 MPa.
Low alloy steels with high temperature properties. An example of low alloy steel which is used for its high-temperature properties is ASTM A 470 turbine rotor steel. These steels are used in steam turbines for electric power generation and normally contain combinations of nickel, chromium, molybdenum, and / or vanadium. An example of the microstructure of ASTM A 470 rotor steel is shown in Fig 2.
Fig 2 Micro-structures of steel with high temperature properties and dual phase steel
Low alloy steels with formability – There are some steels which are designed for optimal formability in sheet-forming applications. The common steel which is specified as drawing quality is special killed steel. This cold-rolled, low-carbon sheet steel has specified aluminum content. The aluminum combines with nitrogen in the steel to form aluminum nitride precipitates during the annealing process. These aluminum nitride precipitates are instrumental in the development of a specific crystallographic texture in the sheet which favours deep drawing. Another type of steel used for applications requiring optimal formability is interstitial-free steel. In this very-low-carbon sheet steel, the interstitial elements, carbon and nitrogen, are combined with carbide- and nitride-forming elements, such as titanium and niobium. The steel is rendered ‘free’ from these interstitial elements which degrade formability.
Dual phase low alloy steels – Dual phase low alloy steels refer to a class of high strength steels which are used in applications where the yield strength of the steel is increased during the forming process itself. The dual phase steels are composed of two phases namely a soft ferrite matrix and a dispersed second phase of martensite (5 % to 30 %). In addition to martensite, small amounts of bainite and residual austenite can exist (Fig 2). The soft ferrite phase is normally continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated in the lower strength ferrite phase surrounding the islands of martensite, creating the unique high work hardening rate exhibited by these steels.
The dual phase steel behaves like composite materials where the ferrite matrix assures high cold formability, and the martensite is the strengthening element. The correct proportion between the two phases allows a continuous yield point, low yielding stress, a high elongation value, a smooth flow stress curve with a high strain hardening coefficient, and better plasticity and formability. The microstructure of steel gives a good combination of high tensile strength, low yield-to-tensile strength ratio and very high initial work hardening rate with good elongation values. Dual phase steel is very formable, providing more flexibility in part design. The strength of the formed part is much higher than high strength low alloy steel, especially at very low strain. The high initial work hardening rate and high tensile strength give dual phase steel a very high capacity to absorb energy, making these steels suitable for use in structural and reinforcement applications.
Bake hardenable low alloy steels – A bake hardenable steel is the steel which shows a capacity for a significant increase in strength through the combination of work hardening during part formation and strain aging during a subsequent thermal cycle such as a paint baking operation. The bake hardenable steels are designed to increase strength during the paint-baking cycle of automobile production. These steels contain elements which develop compounds that precipitate at the paint baking temperatures. These precipitates harden the steel.
Bake hardenable steel have been designed to increase strength during the paint-baking cycle of automobile production. These steels contain elements which develop compounds which precipitate at the paint-baking temperatures. These precipitates harden the steel. These steels have adequate carbon and / or nitrogen in solution to cause strain-aging. In general, bake hardenable steels are aluminum-killed steels with an adequate amount of aluminum to combine with the nitrogen as aluminum nitride.
Bake hardenable steels take advantage of the low solute carbon to produce controlled carbon strain aging to augment the yield strength of formed automotive panels, thus improving dent resistance or permitting some thickness reduction. The strain comes from press forming and the aging is accelerated by the paint baking treatment. Bake hardenable steels contain enough super-saturated solute carbon so that the aging reaction typically adds 27 MPa to 55 MPa to stamped panel yield strength.
Medium carbon ultra high strength low alloy steels – The medium carbon low alloy family of ultra high strength steels include grade 4130, the higher strength grade 4140, and the deeper hardening, higher strength grade 4340. Several modifications of the basic 4340 grade steel have been developed. In one modification, silicon content is increased to prevent embrittlement when the steel is tempered at the low temperatures for the required very high strength. In certain steel grades, vanadium is added as a grain refiner to increase toughness, and the carbon is slightly reduced to promote weldability. One steel grade contains vanadium, slightly higher carbon, chromium, and molybdenum than 4340, and slightly lower nickel. Other less widely used steels which can be included in this family are 6150 and 8640 grades of steels.
Medium carbon low alloy ultra high strength steels are readily hot forged, normally at temperatures ranging from 1,065 deg C to 1,230 deg C. For avoiding stress cracks resulting from air hardening, the forged parts are to be slowly cooled in a furnace or embedded in lime, ashes, or other insulating material. Prior to machining, the normal practice is to normalize at 870 deg C to 925 deg C and to temper at 650 deg C to 675 deg C, or to anneal by furnace cooling from 815 deg C to 845 deg C to around 540 deg C if the steel is a deep air-hardening grade. These treatments impart reasonably hard structures consisting of medium-to-fine pearlite. In this condition, the steel has a machinability rating of around 50 % of that of the free cutting steel.
Alloy spring steels – These spring steels are used for conditions of high stress and shock or impact loadings. These steels can withstand a wider temperature variation than high carbon spring steels and are used in either the annealed or pre-tempered conditions. Silicon is the key element in most of the alloy spring steels. A typical example of alloy spring steel contains 1.5 % – 1.8 % silicon, 0.7 % – 1 % manganese, and 0.52 % – 0.6 % carbon.
Electrical steels – Electrical steel is a kind of special steel which is tailored to show certain specific magnetic properties such as small hysteresis area (small energy dissipation per cycle or low core loss) and high permeability. It is also called lamination steel, silicon steel, Silicon-electrical steel, or transformer steel. The steel contains specific percentage of silicon (around 3 %) in it which is responsible for its unique property. Electrical steels have special physical properties which make them suitable for application in the production of electric equipments and appliances with rotating magnetic fields.