Alloy Steels

Alloy Steels 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...

Alloying elements and their influence on properties of steel...

   Alloying elements and their influence on properties of steel Steel is the most commonly used alloy of iron. The typical influence on the property of steel by the presence of alloying elements is depicted in Fig 1.         Fig 1 Effect of alloying element on steel  The following gives an overview of some of the influences of alloying elements on the properties of steel. Aluminium (Al) – It is used as a deoxidizer.  Aluminium restricts austenite grain growth in reheated steels and is normally added to control grain size.  Aluminium is very effective alloying element in controlling grain growth prior to quenching.  It is an alloying element in nitriding steels. Boron (B) – It is added to fully killed steel to improve hardenability. Boron treated steels are produced with boron in a range of 0.0005% to 0.003%. Whenever boron is substituted in part for other alloys, it should be done only with hardenability in mind because the lowered alloy content may be harmful for some applications. Boron is a potent alloying element in steel. Boron is most effective in lower carbon steels. Carbon (C) – It has a major effect on the properties of steel. Carbon is the primary hardening element in steel.  Hardness and tensile strength increases with the increase in carbon content up to about 0.85% C. Ductility and weldability of steel decrease with increasing per cent of carbon. Chromium (Cr) – It is commonly added to steel to increase corrosion resistance and oxidation resistance. It increases hardenability and improves high temperature strength.  With added carbon it gives wear and abrasion resistance properties to steels. As a hardening element, chromium is normally used with a toughening element such as nickel for superior mechanical properties. Chromium is a strong carbide former element....