Steel is basically an alloy of iron and carbon. These plain carbon steels are relatively cheap, but have a number of limitations with respect to their properties. These limitations are as follows.
- Plain carbon steels cannot be strengthened above 690 N/ sq m without loss of ductility and impact resistance.
- It is not very hardenable i.e. the depth of hardening is limited.
- Plain carbon steels have low corrosion and oxidation resistance.
- These steels must be quenched very rapidly to obtain a fully martensitic structure, leading to the possibility of quench distortion and cracking.
- The steels have poor impact resistance at low temperatures.
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 1.0 % and 50 % by weight.
Alloy steels are usually of three types. They are microalloyed steels, low alloy steels and high alloyed steels. Microalloyed steels are a type of alloy steels that contains small amounts of alloying elements (usually 0.05 % to 0.15 %). These steels are also sometimes called high strength low alloy (HSLA) 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 up to 4 %, while in second definition low alloy steels contain alloying elements up to 8 %. Steels having alloying elements higher than this amount come under the category of high alloy steels. 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.
Effects of 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 may need heat treatment to achieve some of these improved properties. The effect of alloying elements on the yield stress of steel is shown in Fig 1.
Fig. 1 Effect of alloying elements on the yield stress
Chromium, vanadium, molybdenum, and tungsten when added to steels improve strength by forming second phase carbides. Manganese, silicon, nickel, and copper are added to increase strength of the steels by forming solid solutions in ferrite. Addition of small quantities of nickel and copper improve corrosion resistance. Molybdenum addition in steel helps to resist embrittlement. Zirconium, cerium, and calcium increase toughness in alloy steels by controlling the shape of inclusions. Manganese sulphide, lead, bismuth, selenium, and tellurium increase machinability.
The alloying elements tend to form either compounds or carbides. Aluminum dissolves in the ferrite and forms the compounds Al2O3 and AlN. Silicon is very soluble and usually forms the compound SiO2*FexOy. Nickel is also very soluble in ferrite and hence it forms compounds, normally Ni3Al. Manganese mostly dissolves in ferrite forming the compounds MnS, MnO*SiO2, but will also form carbides in the form of (Fe, Mn)3C. Chromium forms partitions between the ferrite and carbide phases in steel, forming (Fe,Cr3)C, Cr7C3, and Cr23C5. The type of carbide that chromium forms depends on the amount of carbon and other types of alloying elements present. Tungsten and molybdenum form carbides if there is enough carbon and an absence of stronger carbide forming elements (like titanium and niobium). These elements form the carbides Mo2C and W2C, respectively. Vanadium, titanium, and niobium are strong carbide forming elements, forming vanadium carbide, titanium carbide, and niobium carbide, respectively.
Alloying elements also have an effect on the eutectoid temperature of the steel. Manganese and nickel lower the eutectoid temperature. These alloying elements are known as austenite stabilizing elements. With enough of these elements the austenitic structure in alloy steels can be obtained even at room temperature. Carbide forming elements raise the eutectoid temperature of alloy steels. These alloying elements are known as ferrite stabilizing elements.
Alloying elements (except cobalt) reduce the critical cooling velocity by making the transformation to the equilibrium phase slower. Alloy steels may therefore be hardened by an oil or even air quench, reducing the risk of cracking or distortion that can result from a rapid water quench. Most elements also lower the Ms and Mf temperatures to below room temperature, leading to some ‘retained austenite’ in the quenched structure.
Classes of alloy steels
There are several categories of alloy steels. Some of them are described below.
- Constructional alloy steels – These alloy steels have relatively low content of alloying elements as compared to alloy tool steels. 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.
- Alloy tool and die steels – These steels are used in making cutting and forming tools. The total content of alloying elements in alloy tool steels ranges from 0.25 % to over 38 %. There are several categories of tool steels. These categories can be further classified according to their basic properties. Each category has a large number of grades. These steels are used for high quality drills, reamers, milling cutters, threading tools, punches, plastic moulds, punch press tooling, and wrenches. Most of the alloy tool steels are hardened in oil and air. Hence they are often referred as oil hardened or air hardened tool steels. When compared with plain carbon tool steels, alloy tool steels harden more deeply and are more shock resistant.
- High speed steel – This alloy steel is also known as high speed tool steel and has carbon content in the range of 0.7 % to 1.5 %. This steel generally has one or more elements such as chromium, vanadium, molybdenum, tungsten, and cobalt. The first four are carbide formers and combine with carbon to form carbides. These carbides are very hard and wear resistant and hence make good cutting tools.
- Special alloy steels – Special alloy steels are designed for extreme service requirements. They include steels with very high heat resistance, corrosion resistance, or wear resistance etc.
- Marageing Steels – These steels are iron nickel alloy steels. A typical example of such steels is 18 % Ni, 8 % Co, 4 % Mo and up to 0.8 % Ti, with less than 0.05 % carbon. Heat treatment involves solution treatment at 800 deg C to 850 deg C followed by quenching of the austenite to give a BCC martensitic structure. This is less brittle than the BCT martensite found in plain carbon steels because of the low carbon. Ageing at 450 deg C to 500 deg C for 2 hours produces finely dispersed precipitates of complex intermetallics such as TiNi3 resulting in tensile strengths around 2000 N/Sq mm. They are soft enough to machine cheaply, before ageing, which can compensate for higher materials cost. They are relatively tough with good corrosion resistance and good weldability since they do not – harden so rapidly as some steels. Uses include aircraft undercarriage components, dies, tools, engine parts etc.
- Hadfield manganese steel – This is a high alloy steel that contains 12 %-14 % Mn and 1 % C. It is austenitic at all temperatures and therefore non magnetic. It has a unique property in that when the surface is abraded or deformed, it greatly increases surface hardness while retaining a tough core. For this reason it is used in pneumatic drill bits, excavator bucket teeth, rock crusher jaws, ball mill linings and railway points and switches. Water quenched from 1050 deg C to retain carbon in solution, the soft core has strength of 849 N/Sq mm, ductility of 40 % and a Brinell hardness of 200, but after abrasion this rises to 550 BHN. The reason for the rapid rise in surface hardening is uncertain, though martensite formation or, more likely, work hardening have been proposed.
- Stainless steels – These steels generally contain between 10 % to 20 % chromium as the main alloying element and are valued for high corrosion resistance. With over 11 % chromium, steel is about 200 times more resistant to corrosion than mild steel. These steels can be divided into three groups based on their crystalline structure. The first group of stainless steels consists of austenitic steels which are non magnetic and non heat treatable, and generally contain 18 % chromium, 8 % nickel and less than 0.8 % carbon. Austenitic steels form the largest portion of the global stainless steel market and are often used in food processing equipment, kitchen utensils and piping. The second group consists of ferritic stainless steels. Ferritic steels contain trace amounts of nickel, 12 % to17 % chromium, less than 0.1 % carbon, along with other alloying elements, such as molybdenum, aluminum or titanium. These magnetic steels cannot be hardened with heat treatment, but can be strengthened by cold work. The third group consists of martensitic stainless steels. Martensitic steels contain 11 % to 17 % chromium, less than 0.4 % nickel and up to 1.2 % carbon. These magnetic and heat treatable steels are used in knives, cutting tools, as well as dental and surgical equipment.