THERMO-MECHANICAL PROCESSING AND MECHANICAL BEHAVIOUR OF HIGH PHOSPHOROUS STEELS

Abstract

Goods and services are produced to satisfy the needs of society. The useful life of the goods is shortened by corrosion. This fuels the quest for corrosion resistant products. One such product is archaeological iron like the Delhi iron pillar. The Delhi iron pillar has withstood corrosion for about 1600 years. Other ancient iron artefacts like Konark temple beams have withstood marine environment for long periods of time. These were made by iron produced by a sinter-forge technique using charcoal. Ancient iron relied on high phosphorous content for strength and corrosion resistance. The artefact developed a passive film in wet dry conditions, which protected it from further environmental damage. If high phosphorous steel is produced through the melt route using coke, it exhibits cold shortness. This brittle behaviour is caused due to grain boundary segregation of phosphorous. Therefore, if ductile and tough high phosphorous steels could be produced using the modern iron and steel making methods, the cost of corrosion to society could be reduced to a great extent. The present work is an attempt towards the achievement of the aforesaid goal. The thesis is divided into six chapters. The first chapter contains the introduction. The second chapter contains the details of the literature survey. The third chapter contains the experimental details of the work done. The fourth chapter contains the results of the experiments and discussions related to the findings. The fifth chapter contains the summary of the work done and the conclusions. The sixth chapter deliberates upon the suggestions for future work. In order to improve the ductility and toughness of high phosphorous a combination of alloying elements, heat treatment and thermo-mechanical processing can be used. The alloying elements can be chosen out of carbon, silicon, nickel, nitrogen, boron, and molybdenum. The heat treatment temperatures can be chosen from the (α+γ) region of the equilibrium phase diagram. Thermo-mechanical processing includes rolling and forging in the present context. The alloying elements of carbon and nitrogen improve grain boundary cohesion by displacing phosphorous from the grain boundaries. Silicon reduces the ductile brittle transition temperature of high phosphorus steels. Heat treatment in the intercritical region of ferrite + austenite for long periods of time causes a partitioning of alloying elements between austenite and ferrite at high temperatures. The partitioning is retained at lower temperatures. Phosphorous is concentrated in the ferrite grain interiors and carbon and nitrogen are retained iii near the grain boundaries. The partitioning of the alloying elements is more effective when the material is subjected to thermo-mechanical processing like forging or rolling. Grain refinement is an added advantage when the material is thermo-mechanically processed. A set of high phosphorous steel compositions using the aforesaid alloying elements has been designed, melted and cast in the present endeavour. These compositions were subjected to dilatometry tests to ascertain the transformation temperatures. The knowledge of the transformation temperatures has been used to design heat treatments and thermo-mechanical processing schedules. The metallographic studies of the steels have been conducted in order to understand the partitioning of alloying elements like phosphorous and carbon, etc., between the ferrite and austenite phases during the holding of the high phosphorous steels in the intercritical phase (α+γ) field. This understanding of the diffusion behaviour of phosphorous in the (α+γ) phase field has been used to improve the toughness of the high phosphorous steels by removing phosphorous from the grain boundaries. The results of the metallographic studies were used to design heat treatments for preparing samples for studies of mechanical properties and corrosion behaviour. The mechanical properties of the high phosphorous steels have been studied in five sets. As cast, hot forged at 1150°C, hot forged at 1150°C and subsequently heat treated at 900°C for one hour, hot forged at 1150°C and subsequently heat treated at 900°C for six hours, and hot forged at 900°C and subsequently hot rolled at 900°C for one hour. Tensile, Charpy impact and Vicker's hardness tests have been conducted. The results were compared with the mechanical properties of plain carbon steel and the results obtained by other researchers who studied iron and steel compositions containing phosphorous contents higher than 0.04 wt.%. The best toughness was observed in the samples hot forged at 1150°C and subsequently heat treated at 900°C for six hours. This sample was also evaluated for corrosion behaviour amongst others. The corrosion behaviour was studied using potentiodynamic polarization and electrochemical impedance spectroscopy. The samples chosen were annealed samples and that which was hot forged at 1150°C and subsequently heat treated at 900°C for six hours. The potentiodynamic polarization and electrochemical impedance spectroscopy studies revealed that the high phosphorous steels were able to withstand 0.1 wt. % chloride concentration in saturated iv Ca(OH)2 solution. Plain carbon steel could withstand only upto 0.06 wt. % chloride concentration in saturated Ca(OH)2 solution. The Gleeble 3800 thermo-mechanical simulator was used to subject high phosphorous steel samples to hot compression at temperatures of 750, 800, 850, 900, 950, 1000, and 1050°C and strain rates of 10, 1, 0.5, 0.1, 0.01, 0.001 s−1. Grain size of high phosphorous steel samples subjected to hot compression at temperatures of 800, 850, 900, 950, and 1050°C and strain rates of 10, 1, 0.1, and 0.01 s−1 was found out. The results indicated that the grain size decreased with decrease in deformation temperature and increase in strain rates. Adiabatic rise in temperatures disturbed the trend. Processing map (Murty and Rao) has been constructed for high phosphorous steel composition S1, using samples subjected to hot compression at temperatures of 750, 800, 850, 900, 950, 1000, and 1050°C and strain rates of 10, 1, 0.5, 0.1, 0.01, and 0.001 s−1. The results have been validated by comparing them with electron microscopic studies. The safe processing windows for the steel have been found out.