Processing-Microstructure-Property Relationships in Materials - Coursework Example

TalalAlrashidi MATE 580 Christopher M. Weyant 03/02/201 Processing-Microstructure-Property Relationships in Materials Introduction Bain and Davenport discovered a novel steel microstructure in the year 1920 which they temporarily referred to as martensite-troostite. Subsequently, troostite was named, the fine pearlite which was then called Bainite by Steel Corporation of the U.S and Bain’s colleagues. Bainite refers to a microstructure in acicular form. Bainite is found in steels at temperatures ranging between 250oF to 550oF in relation to the steel alloy content. Bainitic microstructures can be created in a multiplicity of steel deliberately to achieve a particular toughness and strength combination and are resistant to fatigue or creep deformation such as hydrogen embrittlement susceptibility. According to Mahnken & Schineidt (1826), Bainitic structures formed as a result of welding have detrimental effects on welded joint’s fracture toughness. The composition of bainitic steel includes carbon, iron and another component from aluminum or silicon group. The component added amounts to at least 1.5% of the bainitic steel total weight. This paper will highlight alloy development that has solved prior processing problems with processing high fracture toughness bainitic steel. The development of bainitic steel microstructures is a technological interest to replace the older ones. Processing of bainitic steel removes fine carbides giving rise to high resistance to void structures. This is an attribute in bainitic steel that engineers draw attention to and analyze in detail. This paper will review relationship between microstructure, processing and properties of bainitic steel. 2. Processing techniques A number of processing techniques for bainitic steel fabrication are under exploration. Bainite transformation is a proven elusive technique which involves the addition of silicon components to suppress the formation of cementite which leads to the resultant displacive reaction i.e. the transformation causes a stress-relieving outcome due to orientation relationships found in bainitic microstructures. Silicon addition results in a microstructure of retained austenite that make up wear-resistant steel. According to Ershov&Nekrasova (109), there are two sets of transformation that provide a processing route and selecting the optimum steel to achieve a specific set of properties. These transformations are: Time-Temperature Transformation (TTT) and CCT (Continuous Cooling Transformation) (See figure 1 & 2) Figure 1: A TTT diagram estimated to investigate properties of steel Temperature (oC) increase is represented on the Y-axis while increasing time (S) is represented on the X-axis. The line dashed represents a path for cooling to obtain ferrite-bainite microstructures. The pearlitic region boundary is schematically drawn. In this case, a sample is austenitised and rapidly cooled afterwards to a lower temperature which is held whilst the transformation rate is measured. Figure 2: A CCT diagram estimated to investigate properties of steel Increasing temperature (oC) is on the Y- axis while increasing time(S) is represented on the X-axis. From figure 2, a sample is either furnace cooled, quenched or air cooled. In the recent past the work has produced a microstructure that resists hydrogen embrittlement. However, the importance of these products of transformation lies in the mechanical properties effects and notably how steel commercial alloys can be designed through silicon embodiment. As a result of fabrication, suitable processing, and appropriate alloy composition design bainitic steel microstructures arise in commercialized steel. This method improves the mechanical properties of bainitic steel. Commercialized steel is used in welding and fabrication, examining their features of transformation would be depicted in the heat affected zone (HAZ). Bainitic structures arise in those zones thus fundamental in scheming HAZ stoutness (Ershov&Nekrasova 123) Another novel technique is the partitioning and quenching. It is a high temperature treatment of steel with austenite and martensite combinations. A customary tempering and quenching of such combinations generates attractive properties. Because of a less Carbon Equivalent Value (CEV), this method improves the weldability of steel. Quenching technique provides an extensive spectrum of decomposition structures of austenite. Various specimen of austenite are heated to a given temperature then placed in the equipment for hardening. Figure 3 below provides an illustration of the model device used. Figure 3: Equipment for quenching technique According to Nicholas (58), the cooling end of the specimen has a martensite structure while the side that cools slowly has the pearlite structure. Other structures formed as a result of austenite decomposition are located between the pearlite and martensite structures. The sheet feed rollers in figure 3 above provide the high energy for heating the specimens. As the water flows in the quench, it results into accelerated cooling (See figure 3). 3. Microstructural results During the processing of bainitic steel, the material is austempered at below noise temperatures i.e. the standardized room temperature of 290oK. A structure which is non lamellar in the form of iron carbide and ferrite is produced. As discussed earlier, Bainite is a needle like microstructure. Pearlite is iron carbide nucleated and accompanied by succeeding ferrite configuration. Bainite is ferrite nucleated, consequently, iron carbide precipitation follows forming a matrix of ferrite. As a result of low transformation temperature, ferrite needles become thinner while carbide distribution is finer. The resultant is termed as lower Bainite. High temperature transformations have units of a plate shaped ferrite or a lath arranged in interlath and packets of carbide precipitates. The resultant material is known as upper Bainite. Lower Bainite gives rise to an elevated strength but an inferior toughness. Normally, austempering reaction is a two step process but for the case of steel processing, it is one step. During the austempering process in steel, decomposition of austenite forms Bainite. Steel austempering offers an increased ductility, a short time cycle to become firm, and reduced distortion. Temperature of above 600oF is required for upper Bainite while between 450OF to 600OF is the temperature range required for lower bainitic steel (Maximilien&Ludovic 2196). Processing of bainitic steel involves variations of other components like chromium, molybdenum, copper, carbon, and silicon. Silicon inclusion prevents cementite formation during upper Bainite transformation. Chromium addition improves its ability to be hardened by reducing graphitization. An M5 temperature of 565oF is depressed by chromium and a selected sufficient carbon of 0.5 %. Carbon in this amount also inhibits formation of pearlite throughout the austempering process. It also provides depression of the lower from the upper Bainite boundary line of transition. Refinement of ferrite-austenite structure is achieved at the initial quenching due to this Bainite boundary line of transition (Mahnken&Schineidt, 1829) Addition of copper to the composition also attains this refinement. While copper is undesirable to be included in the composition due to development of pearlite phase, it improves resistance to corrosion. After austempering the samples at 500OF, microstructures appeared as a tempered martensitic composition. A mixture of austenite and lower bainitic microstructures appears when a given sample is austempered at 650, 700 and 750OF with the presence of chromium in the sample produce steel with segregated regions. Upon cooling, retained austenite regions transform into martensite. Such samples contain martensite in some amounts after being austempered at 600, 650, and 750 0 F. Although some of these samples are austempered in the higher bainitic region, no typical microstructures of bainitic structures are obtained. This appears as a result of segregated regions present in steel that hindered the transformation of upper bainitic. According to Nicholas (68), Steel is composed of pearlite, martensite, and Bainite. The microstructures include; Bainite, martensite, and ferrite. These compositions are illustrated in figure 4 and 5 below. Figure 4: Steel composition (Pearlite, Martensite, and Bainite) Figure 5: Microstructures composition (Bainite, Martensite, and Ferrite) 4. Property Evaluation and results Bainitic steel has several properties which are investigated through experimentation. These properties are: mechanical properties, fracture toughness, magnetic, electrical, and thermal. Mechanical properties are tensile i.e. bainitic steel has the ability to oppose forces that try to pull apart after an investigation of austempering in quite a number of temperatures. Austempering samples at a temperature of 500o F results in a remarkably high strength with considerable ductility amount. As-cast microstructures of relatively little strength but with a superior ductility are developed compared to austempered structures. Tensile strength decreases as austempering temperature increases. This is an indirect proportion of variables. Increasing austempering temperatures increases the material ductility. In this case, a direct proportionality of variables is depicted. Generally, an increased quantity of austenite increases the material’s ductility (Reed &Abbaschian, 102). After tempering and quenching, it is reported that fracture toughness of this material is greater than low steel alloys. Austempering takes place at different temperatures to determine the fracture toughness of a material. Austempering at 600o F yields a recommendable fracture toughness and strength. One would expect an increase in fracture toughness of steel as the content of austenite increases. This may sometimes not be the case since fracture toughness depends on the total carbon in the component as well as strength yielded (Abreu2 24). Saturation magnetization is given at a range between 170emu per gram to over 250emu per gram. Through a simple model, this saturation differs from ferritic fraction because austenite structure is weakly magnetic. A sample with the smallest austenite or largest ferrite austempered at 650o F has saturation magnetization of about 195 emu per gram. Austempering a sample containing ferrite in large amounts at a temperature slightly above 650o F results in saturation magnetization of about 205 emu per gram. Coercive fields for most used samples range between 6Oe to 25Oe. Scattering at the boarders between ferrite and austenite structures facilitates minimal resistivity. For instance, austempered samples of 750o F and 720o F have low resistivity as compared to those with small austenite volume fractions. Such samples of small austenite volume fractions having thermal conductivity for a temperature of 650oF have increased values above thermal conductivity compared to those with larger amounts of austenite (Maximilien&Ludovic 2199). A number of techniques are used to measure these properties: mechanical testing, heat treatment, fractography and metallography, and X-Ray Diffraction. For example, X-Ray Diffraction examination is carried out to establish the proportion of carbon present in austenite and austenite content. Fractions of austenite and ferrite are also determined by use of integrated intensities that provide a direct comparison. Some of these intensity plane values are: 110, 220 and 311 of austenite and 110 and 211 of ferrite. The results are analyzed and it appears that austenite is formed as slivers between needles of ferrite (Mahnken&Schineidt 1829). 5. Summary Bainite and martensite appear similar in the first place due to the transformation mechanism aspects they share. However, they are different in terms of their morphological structures. As reported by an electron microscope test, Bainite microstructures seem darker than those of martensite because of their low reflexivity. Bainite is martensite and pearlite intermediary in terms of stiffness or hardness. For that matter, bainitic microstructures are useful in case that no heat treatments are considered necessary after cooling initially to attain hardness. Works cited: Abreu, Herrera. “Influence of Temperature and Time of Austempering Treatment on Mechanical of SAE 9254 Commercial Steel.” Steel Research International 83.1 (2012): 22-31. Print. Ershov, Michael &Nekrasova, Smolensk.Transformation of Cementite into Austenite. India: Pune University, 1982. Print. Mahnken, Austschbach R &Schineidt, Shirley. “On the Simulation of Austenite to Bainite Phase Transformation.” Computational Materials Science 50.6 (2011): 1823-29. Print. Maximilien, Liberty & Ludovic, Vincent. “Temperature DependantPolycrystal Model Application to Bainitic Steel Behavior under Tri-axial Loading in the Ductile-Brrittle Transition.” International journal of solids and structures 48.15 (2011): 2196-208. Print. Nicholas, Robert. Quenching and Tempering of Welded Steel Tubular. Springfield, Massachusetts, USA: G & C Merriam Company, 1992. Print. Reed, Hill & Abbaschian, Reza.Physical Metallurgy Principles, 3rd Edition. Boston: PWS-Kent Publishing, 1991. Print. Read More