Metallurgical Principles in the Heat Treatment of Steels Nov04

Metallurgical Principles in the Heat Treatment of Steels...

Metallurgical Principles in the Heat Treatment of Steels Heat treatment of steels is carried out for achieving the desired changes in the metallurgical structure properties of the steels. By heat treatment, steels undergo intense changes in the properties. Normally very stable steel structures are obtained when steel is heated to the high temperature austenitic state and then slowly cooled under near equilibrium conditions. This type of heat treatment, normally known as annealing or normalizing, produces a structure which has a low level of the residual stresses locked within the steel, and the structures can be predicted from the Fe (iron)- C (carbon) equilibrium diagram. However, the properties which are mostly required in the steels are high strength and hardness and these are generally accompanied by high levels of residual stresses. These are due to the metastable structures produced by non-equilibrium cooling or quenching from the austenitic state. Crystal structure and phases The crystal structure of pure Fe in the solid state is known to exist in two allotropic states. From the ambient temperature and up to 910 deg C, Fe possesses a body centered cubic (bcc) lattice and is called alpha-Fe.  At 910 deg C, alpha-Fe crystals turn into gamma-Fe crystals possessing a face-centered cubic (fcc) lattice. The gamma crystals retain stability up to temperature of 1400 deg C.  Above this temperature they again acquire a bcc lattice which is known as delta crystals. The delta crystals differ from alpha crystals only in the temperature region of their existence. Fe has two lattice constants namely (i) 0.286 nm for bcc lattices (alpha-Fe, delta-Fe), and (ii) 0.364 nm for fcc lattices (gamma- Fe). At low temperatures, alpha-Fe shows strong ferromagnetic characteristic. This disappears when it is heated to around 770 deg C, since the lattice...

Martensitic Stainless Steels...

Martensitic Stainless Steels Martensitic grades of stainless steel were developed in order to provide a group of stainless steels which are corrosion resistant and hardenable by heat treatment. Martensitic stainless steels are essentially Fe-Cr-C alloys and are similar to carbon or low alloy steels with a structure similar to the ferritic steels. However, due the addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to carbon steels. The main alloying elements are chromium (10.5 % to 18 %), molybdenum (0.2 % to 1 %), no nickel (except for two grades), and carbon (0.1 % to 1.2 %). Major grades in the family of martensitic group of stainless steels are given in Fig 1. Fig 1 Major grades of martensitic stainless steels History The characteristic body centered tetragonal martensitic microstructure was first observed by German microscopist Adolf Martens around 1890. In 1912, Elwood Haynes applied for a U.S. patent on a martensitic stainless steel alloy. This patent was not granted until 1919. Also in 1912, Harry Brearley of the Brown – Firth research laboratory in Sheffield, England, while seeking a corrosion resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times. Brearley applied for a US patent during 1915. Properties The structures of martensitic stainless steels are body centered tetragonal (bct) and they are classified as a hard ferro magnetic group. In the annealed condition, these steels have tensile yield strengths of around 275 N/sq mm and hence they can be machined, cold formed, or cold worked in this condition. These stainless steels have good ductility and toughness properties, which decrease as strength increases. Martensitic stainless steels can be moderately hardened by...

Steel Hardening by Quenching and Tempering...

 Steel Hardening by Quenching and Tempering Hardening is carried out by quenching steel, which consists of cooling it rapidly from a temperature above the transformation temperature (A?).  The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite (pearlite), and to cause a partial decomposition at such a low temperature to produce the new phase called martensite. To achieve this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements. In such case hardening of steel occurs with mild quenching. Martensite is a supersaturated metastable phase and has body centered tetragonal lattice (bct) instead of bcc. After steel is quenched, it is usually very hard and strong but brittle. Martensite looks needle like under microscope due to its fine lamellar structure. Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the work piece. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidize and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water. To minimize distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated. Steel can be hardened by the simple expedient of heating to above the A? transformation temperature, holding long enough to insure the attainment of uniform temperature and solution of carbon in the austenite, and then cooling rapidly (quenching). Complete hardening depends...