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...

Niobium in Steels

Niobium in Steels  Niobium (Nb) (atomic number 41 and atomic weight 92.91) has density of 8.57 gm/cc. The element is also known as Columbium (Cb). Melting point of Nb is 2467 deg C and boiling point is 4740 deg C. Cb was discovered in 1801 by Charles Hatchett, who named the element to signify the American origin. This name was used in USA while Europe used the name niobium for the same element. To end this confusion, the name niobium was chosen for element 41 at the 15th Conference of the Union of Chemistry in Amsterdam in 1949. A year later this name was officially adopted by the International Union of Pure and Applied Chemistry (IUPAC) after 100 years of controversy, despite the chronological precedence of the name Columbium. Columbium name is still used in many places in USA. The phase diagram of the Fe-Nb binary system is at Fig 1. Fig 1 Fe-Nb phase diagram  The use of Nb dates back to 1925 when it was used to replace tungsten (W) in tool steel production. By the 1930s, Nb was being used to prevent corrosion in stainless steels. The ability of Nb to maintain fine grain sizes in steels at higher temperatures has been known since 1940s and steels that take advantage of this effect have been commercially produced for many years. In recent years, however, Nb is being known more as one of the most important element for the micro alloying. Nb plays an important role in HSLA (high strength low alloy) steels. Nb also has important applications in tool steels, wear and abrasion resistant steels, steels for high temperature service, stainless steels and super alloys. Many of these uses depend on the strong affinity of Nb for carbon (C) and/or nitrogen (N). Addition agents...

Tool Steels

Tool Steels The term tool steel is a generic description for those steels which have been developed specifically for tooling applications. These steels are used for making tools, punches and dies etc. Tools used for working steels and other metals must be stronger and harder than the steels or the materials they cut or form. Normally tool steels are known for their distinctive toughness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red hardness). Some of the operations that tool steels are used for include drawing, blanking, mould inserts, stamping, metal slitting, forming and embossing, although their use is not limited to just these areas. The metallurgical characteristics of various compositions of tool steels are extremely complex. There are hundreds of different types of tool steels available and each may have a specific composition and end use. Tool steels are mainly medium to high carbon steels with specific alloying elements added in different amounts to provide it special characteristics. The carbon in the tool steel is provided to help harden the steel to greater hardness for cutting and wear resistance while alloying elements are added to the tool steel for providing it greater toughness or strength. In some cases, alloying elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness to it so that the tool retains its hardness and strength when it becomes extremely hot. Various alloying elements in addition to carbon are chromium (Cr), cobalt (Co), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and vanadium (V). The effect of the alloying elements on the properties of tool steels is as follows. Chromium –...

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...