Oxygen and Steels

Oxygen and Steels  Oxygen (O) (atomic number 8 and atomic weight 15.999) has density of 1.429 gm/litre at standard temperature and pressure. Melting point of oxygen is -218.79 deg C and boiling point is – 182.96 deg C. It is a colourless gas but the colour of liquid oxygen is pale blue. The phase diagram of the Fe-O binary system is at Fig 1. Fig 1 Fe- O binary phase diagram  There is a very strong relationship between oxygen  and steel. Oxygen is first used in the steel making process which is a controlled oxidation process. Excess oxygen going to steel during steel  making process,  if not properly taken care of, is source of many steel defects like porosity, inclusions etc. Oxygen is used in the processes of cutting, lancing, scarfing and welding of steels. Oxygen is also the cause of steel destruction by the processes of rusting, scaling and corrosion. Further transport of oxygen takes place in cylinders, tanks and pipelines made of steel. Oxygen is also used (oxy-fuel process) for heating of the steel. During steel making process, the main sources of oxygen in steel are as follows. Oxygen used for blowing in the steel making process Use of oxidizing slags and oxidizing materials ( ores, sinter etc.) during steel making processes Atmospheric oxygen dissolves in the liquid steel during steel tapping and casting operations Oxidizing refractories used in various vessels for holding liquid steel in the process of steel making Rusted and wet scrap Solubility of oxygen in liquid steel is 0.23 % at the steel making temperatures ( 1600 – 1700 deg C). However it decreases during cooling down and then drops sharply during solidification of steel reaching a level of 0.003 % in solid steel. Solubility of oxygen in steel...

Lead in Steels

Lead in Steels  Lead (Pb) (atomic number 82 and atomic weight 207.21) has density of 11.34 gm/cc. Melting point of Pb is 327.5 deg C and boiling point is 1749 deg C. The phase diagram of the Fe-Pb binary system is at Fig 1. Pb has a face centered cubic (f.c.c.) crystal structure.  Fig 1 Fe-Pb binary system Pb is only slightly soluble in liquid or solid steel. When added , Pb does not remain in the metal but it remains separate from and mechanically dispersed in the steel  as submicroscopic metallic inclusions when it solidifies. It does not change the mechanical properties of the steel appreciably. The sole function of Pb is to improve the machinability of steel. This accounts for the wide use of Pb for high strength steel for which machinability is required. Also, Pb only slightly influences mechanical properties of steel. However, with the growing consciousness of environmental issues in recent years, there is a tendency toward eliminating Pb because it is seen to be one of the pollutants of the environment. Addition agents and addition practice Elemental Pb is available as cored wire (Pb micro shots), pellets and fine shots suitable for addition in the continuous casting tundish, ladle metallurgical furnace, or degassing furnace. Alloys containing bismuth (bi) are also available for addition. Addition of Pb in liquid steel is carefully controlled to ensure its even dispersion in the solidifying steel. Segregation of Pb can be an issue. Pb segregation in steel  can lead to problems during cold working and machining of steel. Due to this Pb bearing steels are normally checked for segregation. The steel sample is usually heated at 700 deg C and is examined for uniform distribution of Pb. This test is known as sweat test. Pb does not...

Nitrogen and Steels

Nitrogen and Steels Nitrogen (N) (atomic number 7 and atomic weight 14.008) has density of 1.25 gm/litre at standard temperature and pressure. Melting point of N is -210 deg C and boiling point is -195.8 deg C. The phase diagram of the Fe-N binary system is at Fig 1. Fig 1 Fe-N phase diagram N is present in all commercial steels. Since the of concerns of presence of N in steels are normally small and its analysis being complex and expensive, its existence is generally ignored even in steel specifications in various standards. However, whether present as a residual element or added deliberately as an alloying element, the effects of N in steel are significant.  N is an important and inexpensive alloying addition to steels. In recent years there has been an increasing demand to reduce and control the amount of dissolved gases in steel. N is one of the important gas which when dissolved in liquid steel affect its properties significantly. Hence control of N content of steels during steelmaking is important. N in steel can be in its uncombined form as free N or in the form of a compound or nitride. Steel from an electric arc furnace (EAF) normally has higher N levels (70-110 ppm) compared to that produced in a basic oxygen furnace (BOF) where N varies between 30 and 70 ppm. Hence, N is of particular importance in an EAF plant. In certain stainless steel grades the amount of N can be at the level of 3000 ppm. N levels in degassed steels can be below 10 ppm.  N exists in steel as an interstitial quite similar to, but much more soluble than, carbon (C) and as nitrides of iron (Fe), aluminum (Al), vanadium (V), niobium (Nb), titanium (Ti),...

Nitrogen in Steels May23

Nitrogen in Steels

Nitrogen in Steels All steels contain some nitrogen which can enter the steel as an impurity or as an intentional alloying addition. The quantity of nitrogen in steels normally depends on the residual level arising from the steelmaking processes or the amount aimed in case of deliberate addition. There are significant differences in residual levels of nitrogen in steels produced from the two main steelmaking processes. Basic oxygen furnace (BOF) process generally results into lower residual nitrogen in steels, typically in the range of 30 to 70 ppm while electric arc furnace (EAF) process results into higher residual nitrogen, typically in the range of 70 to 110 ppm. Nitrogen is added to some steels (e.g. steels containing vanadium) to provide sufficient nitrogen for formation of nitride to achieve higher strength. In such steels nitrogen levels can increase to 200 ppm or higher. Nitrogen in the liquid steel is present in the form of solution. During the solidification of steel in continuous casting, three nitrogen related phenomena can happen. These are Formation of blow holes Precipitation of one or more nitride compounds Solidification of nitrogen in interstitial solid solution. The maximum solubility of nitrogen in liquid iron is around 450 ppm, and less than 10 ppm at ambient temperature (Fig 1). The presence of significant quantities of other elements in liquid iron affects the solubility of nitrogen. Mainly the presence of dissolved sulfur and oxygen limit the absorption of nitrogen because they are surface active elements. Fig 1 Solubility of nitrogen in iron Nitrogen is generally considered as undesirable impurity which causes embrittlement in steels and affects strain aging. However nitrogen produces a marked (intersititial solid solution) strengthening when diffused into the surface of the steel, similar to the strengthening observed during case hardening (Nitriding)....