Tungsten in Steels

Tungsten in Steels  Tungsten (W) (atomic number 74 and atomic weight 183.92) has density of 19.3 gm/cc. Melting point of W is 3410 deg C and boiling point is 5400 deg C. The phase diagram of the Fe-W binary system is at Fig 1. Fig 1 Fe-W binary phase diagram Application of W in steels started as early as in the mid-19th century and until the first quarter of the 20th century, its importance in steel has steadily increased, with the steel industry then the largest W consumer. W was among the very first alloying elements systematically studied and used to improve steel properties, for example hardness, cutting efficiency and cutting speeds of tool steels. The use of W in structural steels declined since 1940 because alloying with molybdenum (Mo) and chromium (Cr) as well as with vanadium (V) and nickel (Ni) has given better performance at lower cost. W is known mainly for its strong tendency to form extremely hard and stable carbides. This property, in fact, is the basis for the most important use of W in steels, which is as a constituent of high speed and other tool steels. Around 20 – 25 % of all W produced is used in these steels. Tungsten bearing addition agents Earlier W alloyed steels were manufactured by the addition of more or less pure W powder to the liquid steel, or, alternatively, by addition of an iron rich W pre-alloy, which was obtained from purified W ore. This rather expensive procedure limited the application of W alloyed steels because of the high price. In 1893 a new tungsten ferro alloy was introduced containing up to 80 % tungsten, called ferro- tungsten (Fe-W), which was much cheaper. Fe-W has lower melting temperatures, faster dissolution in...

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...