Tanmay Bhattacharyya

Models for Austenite to Martensite Transformation in TRIP-Aided Steels: A Comparative Study

Low alloy transformation induced plasticity (TRIP)-aided steels are of particular interest to the automotive industry as they offer an excellent combination of strength and ductility at affordable costs. These unique properties depend primarily on the deformation induced transformation behavior of retained austenite to martensite, which is therefore, the most important aspect of TRIP-aided steels. As such, it is important to develop a mathematical model for the transformation of retained austenite. This would not only help in a better understanding of the deformation induced transformation behavior of retained austenite but also aid in a better design of the microstructure of TRIP-aided steels. A large number of empirical as well as semi-empirical models have been developed over the years to describe and predict the variation of retained austenite with strain. A comparative study of the different available models will be presented in this article.

Failure Analysis of Motor Tire Bead Wires During Torsion Test

Torsion testing is used to determine the quality of steel wire used for motor tire beads in pneumatic tires. These steel wires must have good-tensile strength so that the tire bead can support the finished tire safely, and yet retain adequate ductility to deform easily around the forming wheel. The present paper highlights premature failure of bead wire which failed during torsion test. Torsion property is one of the important parameters of tire bead as it monitors both the metallurgical soundness and surface quality of a drawn wire. From the analysis, it has been concluded that probable reason for premature failure is due to strain aging (dynamic and static) caused by interstitial atoms which bounds the mobile dislocations resulting in increases yield strength and decreases bead formability. Moreover, the microstructural study indicates that failed specimen has misaligned and broken lamella of pearlite with globular cementite which creates the array of voids. These voids hinder the rotation of pearlite during torsion test thus leading to brittle fracture.

Study of grain growth during austenitisation of a pearlitic steel with two starting conditions

A pearlitic steel was subjected to isothermal austenitisation treatments at various temperatures and time lengths at each temperature. The steel was under two different starting conditions, namely, cast + forged condition and wire rod condition with 8 mm diameter. For the two different starting conditions, there was a significant difference in grain growth kinetics and activation energy values. Also, there was a significant drop of activation energy value at higher temperature range. The activation energy values were determined to be 161 and 108 kJ mol−1, respectively, for the temperature ranges 850–950 and 950–1050°C, in case of cast + forged sample and these were 225 and 170 kJ mol−1, respectively, for the fully processed rod sample. Self-diffusion and grain boundary diffusion were the most likely processes that governed the austenite grain growth.

Pinch Roller Failure Analysis and Resulting Enhancement of Rebar Mechanical Properties

Reinforcing bars, popularly termed “rebars,” are used to impart tensile strength to concrete structures. Concrete has high resistance to weathering and fire and high compressive strength but almost no tensile strength, hence rebars are used to provide the latter to concrete. Property consistency along the length of rebars is an important prerequisite. When the finished product is subjected to thermomechanical treatment (TMT), proper control of rolling and water box parameters and efficient pinch rolling are needed to achieve acceptable properties. Variation of yield strength (YS) along TMT bars from the front to back end has been observed within the same heat treatment. In the presented investigation, it was observed that pinch rolling ineffectiveness is the main reason for the poor mechanical properties at the back end. The pinch roller was unable to support the back end of the TMT bars properly to maintain the speed and tension of the bars, resulting in nonuniform cooling of the back end through the water box and subsequent mechanical property failure. Due to the substandard material of the pinch roller, it was unable to hold the back end of the bar properly. Based on analysis of the roller it was concluded that it failed due to improper microstructure, resulting in inadequate hardness and toughness for the stringent operating conditions. AISI H13 is a better material to use in such high-service-temperature conditions. Moreover, proper heat treatment is needed to achieve adequate hardness and microstructure properties. After proper heat treatment of pinch rollers, their service life was increased twofold, minimizing the YS variation along the rebars.

An Overview on Structure–Property Relationship of TRIP-Aided Steel

Retained austenite in microstructure of TRIP-aided steel plays an important role and governs its performance. In the present paper, the microstructure and mechanical properties of some experimental TRIP-aided steels have been discussed. Prediction of the amount of retained austenite in the final microstructure has been done using mathematical models based on (a) artificial neural network (ANN) technique and (b) the theory of bainite transformation. The microstructure and mechanical properties of the experimental TRIP-aided steels have been studied and an attempt has been made to establish a correlation between the two. Promising results on microstructure and strength–ductility combinations along with reasonably enhanced coatability were obtained on laboratory samples.

Effect of boron on the manufacturing process and final properties of ductile iron pipes (DI pipes)

The present work concerns an assessment of the influence of boron addition in spheroidal graphite iron (SG iron) melt on the manufacturing process and final properties of DI pipes. To elicit the effect of boron addition on the final properties of DI pipe, initially laboratory scale trials were conducted. The exercise includes studying the evolution of microstructure and mechanical properties of DI pipes with different level of boron addition. The results indicate that the presence of B in the melt does not have any perceptible deleterious effect on the performance of DI pipes in general, until the limit goes beyond 200 ppm. Later on, pilot trials were conducted in a plant facility. This trial comprises addition of boron at different levels in SG iron melts and studying effect on the overall manufacturing process of DI pipe production and corresponding microstructure and mechanical properties of the final product (i.e. DI pipes). The result of the plant-level trial is found in agreement with the laboratory scale trial, confirming the fact that presence of boron up to 200 ppm level in SG iron melt used for DI pipe manufacturing does not induce any deleterious effect on manufacturing process as well as product properties. Also, it reveals that the boron-added (up to 200 ppm) DI pipes show better machinability with a favourable combination of strength, ductility and hardness compared to the boron-free DI pipes.