Molybdenum in Steels

Molybdenum in Steels Molybdenum (Mo) (atomic number 42 and atomic weight 95.95) has a density of 10.22 gm/cc. Melting point of Mo is 2610 deg C and boiling point is 5560 deg C. The phase diagram of the iron molybdenum (Fe-Mo) binary system is at Fig 1. Fig 1 Iron molybdenum binary system Mo is normally referred in short as ‘moly’.  It has many important uses in alloy steels, stainless steels, alloy cast irons and super alloys. It is a powerful hardenability agent and is a constituent of many heat treatable alloy steels. Mo retards softening at higher temperatures. Hence it is used in boiler and pressure vessel steels, as well as several grades of high speed and other tool steels. Mo improves the corrosion resistance of stainless steels. In HSLA (high speed low alloy) steels, it produces acicular ferrite structures. Mo is the basis for many of the as-rolled DP (dual phase) steels used in automotive applications. While Mo may often be used interchangeably with chromium (Cr) and vanadium (V), in many cases the properties it imparts are unique. Due to it, the use of Mo has increased considerably over the past several decades. Available forms Mo is supplied as ferro-molybdenum (Fe-Mo) and as molybdic oxide (MoO3). Fe – Mo contains a minimum of 60 % Mo. Silicon (Si) and copper (Cu) may be present in quantities up to 1 % each. It is relatively expensive and is sparingly used for addition. Technical MoO3 has a minimum of 57 % Mo. SiO2 is the main impurity, but it may also contain small amounts of Cu, sulfur (S), and phosphorus (P). MoO3 is supplied either in cans or as briquettes. MoO3 briquettes may also contain some amount of carbon (C). Considerable quantity of Mo is recovered...

Heat Resistant Steels...

Heat Resistant Steels The properties of steel and its yield strength considerably decrease as the steel absorbs heat when exposed to high temperatures. Heat resistance means that the steel is resistant to scaling at temperatures higher than 500 deg C.  Heat resistant steels are meant for use at temperatures higher than 500 deg C since they have got good strength at this temperature and are particularly resistant to short and long term exposure to hot gases and combustion products at temperature higher than 500 deg C. These steels are solid solution strengthened alloy steels. As these steels are used over a certain broad temperature ranges, these steels are usually strengthened by hard mechanism of heat treatment, solid solution and precipitation. All the heat resistant steels are composed of several alloying elements for the purpose of achieving the desired properties and are used in applications where resistance to increased temperatures is critical. The level of the heat resistance of the heat resistant steels depends on the environment conditions in which they operate and cannot be characterized by a single testing method. Maximum service temperatures which can be extended to 1150 deg C depending on the alloy content can be severely reduced by the presence of some compounds such as sulphurous compounds, water vapour or ash. Resistance to molten metal and slag is also limited in these steels. In heat resistant steels, the two most important elements are chromium for oxidation resistance and nickel for strength and ductility. Other elements are added to improve these high temperature properties. The effect of various alloying elements is described below. Chromium – Chromium is the one element which is present in all the heat resistant steels. Besides imparting oxidation resistance, chromium adds to high temperature strength and carburization resistance. Chromium...