Complex Phase Steels

Complex Phase Steels  The complex phase (CP) steels belong to the group of advanced high strength steels (AHSS) grade, which gain their strength through extremely fine grain size and a micro structure containing martensite in small amounts, and pearlite embedded in the ferrite/bainite matrix. A very high grain refinement is achieved by precipitation of micro alloying elements such as niobium (Nb), or titanium (Ti), or retarded recrystallization. The advantage of the CP steels is that cold forming, without subsequent quenching and tempering, is possible, thus implying a considerable cost saving potential. CP steels are currently being produced as hot rolled steel strips as well as cold rolled advanced high strength steels, which are hot dip galvanized for corrosion protection. The chemical composition of CP steels, and also their microstructure, is very similar to that of TRIP steels, but, additionally it contains some quantities of Nb, Ti and or V (vanadium) to cause the precipitation strengthening effect. Typically, CP steels have no retained austenite in the microstructure, but contain more hard phases like martensite and bainite. The microstructure of CP steels is composed of a very fine ferrite with the high volume fraction of hard phase, For cold shaped products, a triple phase steel containing ferrite, bainite and martensite can be designed which are obviously more difficult to produce. The bainitic complex phase microstructure exhibits better strain hardening and strain capacity than that for fully bainitic micro structure. It involves a strength graded microstructure where the martensite and bainitic ferrite phases are separated by a third phase of intermediate strength. Fig 1 shows typical micro structure of CP steels. Fig 1 Typical micro structure of CP steels  Properties of CP steels The mechanical properties of CP steels are characterized by continuous yielding and high uniform...

TRIP Steels

TRIP Steels  TRIP steels are high strength steels. TRIP stands for ‘transformation induced plasticity’.  They are new generation of low alloy steels. These steels offer outstanding combination of strength and ductility as a result of their micro structure. TRIP steels rely on the transformation of austenite grains into the harder phase of martensite during deformation for achieving their mechanical properties. The locations of these grains in the microstructure are of major importance because they influence the impact of the TRIP effect, the microstructural localization and therefore the macroscopical deformability of the material. Microstructure and composition  The microstructure of these steels is composed of islands of hard residual austenite and carbide free bainite dispersed in a soft ferritic matrix.  The retained austenite is embedded in a primary matrix of ferrite. In addition to a minimum of 5 % to 15 % of retained austenite, hard phases such as martensite and bainite are present in varying amounts. Austenite is transformed into martensite during plastic deformation (TRIP effect), making it possible to achieve greater elongations and lending these steels their excellent combination of strength and ductility. Fig 1 shows the typical microstructure of TRIP steel. Fig 1 Typical micro structure of TRIP steel  TRIP steels typically require the use of an isothermal hold at an intermediate temperature, which produces some bainite. The higher silicon and carbon content of TRIP steels also result in significant volume fractions of retained austenite in the final microstructure. TRIP steels use higher quantities of carbon than dual phase steels to obtain sufficient carbon content for stabilizing the retained austenite phase to below ambient temperature. Higher contents of silicon and/or aluminum accelerate the ferrite/bainite formation. They are also added to avoid formation of carbide in the bainite region. Silicon though a key element for the formation of retained austenite, is undesirable...

Microstructures of Iron and Steels...

Microstructures of Iron and Steels The microstructures of iron and steels is complicated and diverse which is influenced by composition, homogeneity, heat treatment, processing and section size. Microstructure of castings looks different than those of the wrought products even if the composition is same and even if the same heat treatment is given. Pure iron is polymorphic. Two allotropic phases exist for pure iron in solid state depending on the temperature. One is bcc (body centered cubic) and the other is fcc (face centered cubic). The bcc crystalline form (?-iron) is stable until a temperature of 912 deg C when it is transformed to fcc (?-iron). The ?-iron remains stable until 1394 deg C, and then it reverts to bcc structure (?-iron). ?-iron is stable until the melting point of 1538 deg C. High purity iron is very weak. The ability of iron to accommodate heavy interstitials, namely carbon and nitrogen, is mostly responsible for the strength and the hardening effects. Ferrite ?- iron refers only to the bcc form of pure iron that is stable below 912 deg C. Ferrite is a solid solution of one or more elements in bcc iron. The carbon solubility of ferrite depends on the temperature: the maximum being 0.02 % at 723 deg C. Ferrite may precipitate from austenite in acicular form with certain cooling conditions. Ferrite is a very soft, ductile phase, although it loses its toughness below some critical temperature. ?-ferrite is magnetic below 768 deg C. Austenite ? – iron refers to fcc form of pure iron that is stable between 912 deg C and 1394 deg C. Austenite is a solid solution of one or more elements in fcc iron. Austenite is stable above 723 deg C depending upon C content. It can...

Rails and Rail Steel

Rails and Rail Steel Rail steel is used to make rails for railway lines and for other uses such as tracks for moving equipments like cranes, transfer cars etc. Heavier rails carry heavier and faster trains on the tracks. The rails represent a substantial fraction of the cost of a railway track. Worn, heavy rail from a mainline is often reclaimed and downgraded for re-use on a branch line, siding or yard or rerolled in rerolling mills to produce other steel products. Rail steel is hot rolled steel of a specific cross sectional profile (an asymmetrical I- beam) designed for use as the fundamental component of railway track. The rail profile is the cross sectional shape of the rail perpendicular to the length of the rail (Fig 1). Fig 1 Typical cross section of a rail The importance of the rail steel can be known from the fact that even after years of service and high stress, there is no difference between the grain structure of a used rail and a new rail. Age, traffic and weather do not change its basic properties. All stresses are relieved through heating in the used rail prior to being rerolled. This rerolling decreases the grain size of the used rail steel and hence improves its resiliency. History Earlier wooden rails were used on horse drawn wagon ways.  By 1760s strap iron rails, which consisted of thin strips of cast iron fixed onto wooden rails came into use. These were superseded by cast iron rails that were flanged (i.e. ‘L’ shaped) and with the wagon wheels flat. In 1789, the edge rails where the wheels were flanged were introduced and, over time it was realized that this combination worked better. The earliest of this in general use were...