Effect of different precipication hardening temperature on microstructural and mechanical properties of Hadfield steels

High manganese steels, which were founded by Sir Robert Hadfield in England in 1882 and named as Hadfield steel since its patent was taken in 1883, are frequently used in industries such as excavation, petrochemical, railway, cement and mining due to their unique properties such as high strength, high toughness and high work hardening. Especially, when these steels work under load or impact, their hardness which is around 220 HB can increase to 500-600 HB, and this unique situation is seen as one of the biggest advantages of manganese steels. This is especially important for the production of impact and jaw crushers in mining applications where toughness and a certain amount of hardness and wear resistance are expected. In general, high manganese steels are preferred in engineering applications, whose content is soft and whose surface gains a certain hardness and wear resistance properties when impacted. With the increasing interest in high manganese steels, the usage areas of these materials have become more diverse, and accordingly there have been serious increases in heat treatment or alloying studies within the scope of performance improvement. Manganese steels are generally produced with carbon and manganese contents designed in accordance with the ratio of 1:10. In high manganese steels, carbon is usually kept at 1%-1.4% content, and the amount of carbide in the steel increases with increasing carbon. In addition to this, carbon content up to 1.2% increases the strength and toughness values and wear resistance of these steels due to solid precipitation hardening mechanisms. Another important alloying element is manganese, which prevents the ferrite transformation of the steel and causes it to remain stable at 100% austenite under suitable cooling conditions. In manganese steels, the tensile strength and elongation amount of the steel increases with manganese additions between 12% and 14%, while the yield strength remains almost the same. In cases where high manganese steels operate under low loads or when better wear resistance is expected, chromium can be added to these steels between 1% and 2.5%. This addition increases the yield strength, wear resistance and hardness of manganese steel and decreases its toughness. Generally, in Hadfield steels with thick sections where rapid cooling cannot be fully achieved, molybdenum can be added up to a maximum of 2% to keep carbide precipitation under control. Apart from these mentioned alloying elements, the phosphorus content in manganese steels is also has great importance in terms of their damage. Phosphorus causes the formation of phosphorous eutectic forms at the grain boundaries of manganese steels which significantly reducing the mechanical properties of these steels. For this reason, the phosphorus ratio in manganese steels should not exceed 0.04%. High manganese steels called Hadfield steels have been alloyed with additions of molybdenum, chromium, vanadium, niobium, aluminum and cerium, and it is aimed to improve the mechanical properties of the materials with these alloys. In addition to alloying studies in high manganese or Hadfield steels, another important factor which is the subject of many researchers is applied or designed heat treatment process. High manganese steels become stable in the austenite structure under ideal cooling conditions as a result of the time required for their solidification, and also contain carbide and pearlite colonies within the austenite main structure. These formed carbides placed in the grain boundaries of the steel causes a serious brittleness by reducing the mechanical properties. For this reason, these materials cannot be used in their casting form in the applications where high toughness is required. This situation requires that high manganese steels must be heat treatmented after casting in order to achieve high toughness values. The purpose of the designed heat treatment is to provide instant cooling of the manganese steel after heating and to obtain a carbide free austenitic structure by preventing carbide formation. For this reason, solution annealing heat treatment is applied to manganese steels after heating to a temperature between 1050°C and 1150°C, depending on the section thickness. The biggest disadvantage for high manganese steels called Hadfield steels is their low yield strength. Low yield strength can cause the parts to be broken or become unusable before the work-hardening mechanism is formed. Due to the sectors in which Hadfield steels are used and the size of the machines in these sectors, the costs of part replacement or renewal are high. For this reason, it has become important to extend the service life of Hadfield steels with welding application. However, there are important points that need to be considered during the welding processes of these steels. First of all, in a welding application, the temperature should be kept below 300°C in order to eliminate the risk of carbide precipitation. Otherwise, carbide precipitation will occur in the region heated by welding and this will cause the part to gain brittleness from this region. In order to prevent overheating during welding, it is recommended to weld at short intervals. Welding applications of high manganese steels are generally made by electric arc welding method. The machinability of high manganese steels, called Hadfield steels, has many difficulties and these materials are characterized as difficult to machine due to their high work-hardening capabilities. The most used method in the machining of these materials is turning. However, disadvantages such as microhardness increase of the steel during turning, rapid tool wear, excessive chip accumulation on the cutting edge, hardening of the steel at high machining speeds occur, all of which create serious problems for the machining process of manganese steels. In order to avoid such problems, it is recommended that the machining processes of manganese steels be carried out using the hot machining method with the installation of appropriate fixtures and machining systems. In the hot machininh method, the manganese steel to be machiend will be superficially heated with a heat source and the machining process will be significantly facilitated as the work hardening ability of the steel will decrease with this heating. If these steels operate under low, medium and high loads, different microstructure and mechanical properties are expected. For example, while high toughness properties are expected under high loads, high wear resistance, high hardness or a strong combination between these two mechanical properties and toughness value are expected under low loads. In this thesis, it is aimed to subject the high manganese steels operating under low loads to a precipitation hardening heat treatment after solution annealing heat treatment and to examine the changes in their mechanical properties with the carbide phases to be formed. The aim of the precipitation hardening heat treatment of manganese steels is to improve the hardness and wear properties of the materials to a certain ratio and to increase their service life under appropriate conditions. The high manganese steel used in this thesis contains 1,16% carbon, 13,21% manganese and 2,13% chromium. In line with the researches, the addition of chromium, especially up to 2%, increases the yield strength and hardness values of manganese steels. Microstructural and mechanical tests were applied to determine and cpmmnet the results obtained within the scope of the study. In order to understand the microstructural situation, first of all, optical microscope examinations were carried out at two different magnification levels. The reason why optical microscope examination is also important in high manganese steels is that the carbides formed in the structure are defined as thin or thick and the thick carbides, which have a reducing effect on the mechanical properties, can be easily observed under the optical microscope. Afterwards of the microstructural analysis, scanning electron microscopy was used to observe the fine carbides and to take EDS analyzes from grain boundaries and grain boundaries. In the structural examination, the X-ray diffraction test method was used to finally understand the composition of the material under test. For the determination of mechanical properties, hardness and charpy impact tests were applied to the samples. Thus, it is aimed to establish a relationship between the hardness change of the materials and the toughness values. The high manganese steels casted within the scope of this study were first subjected to solution annealing heat treatment at 1080°C. Thus, the carbides formed in the structure and caused by the slow cooling after casting were dissolved. To understand whether the designed solution annealing heat treatment temperature and time were chosen correctly at this stage, the conditions of the samples after casting and after solution annealing were subjected to microstructural and mechanical tests. Randomly dispersed carbides were detected at the grain boundaries and in the grain as expected in the microstructural observation of the cast samples. As a result of this, the charpy impact energy obtained from the sample was 4.5 J, and the hardness result was 263 HV. In the microstructure tests performed after solution annealing, it was determined that the carbides in the sample were dissolved. In the mechanical tests, it was found to be 206 J with an increase of 4577% as a result of chapy impact test compared to the casting state, and the hardness value decreased from 263 HV to 228 HV with a decrease of 13%. It was observed that the solution annealing heat treatment performed according to the obtained toughness and hardness values was successful and a significant increase in the toughness property of the material was observed, and the obtained findings corresponds to the literature. Afterwards, the samples, which were subjected to solution annealing heat treatment, were subjected to precipitation hardening heat treatment at 450°C, 550°C and 650°C separately. First, the sample which was subjected to precipitation hardening heat treatment at 450°C was examined. As a result of microstructural investigations, it was determined that the precipitation hardening heat treatment performed at this temperature was sufficient for the formation of fine grain boundary carbides. Due to the low energies and good austenite matrix lattice compatibility of these fine carbides, it was expected not to cause embrittlement and not seriously affect the mechanical properties. According to the mechanical tests performed, it was determined that the charpy impact test result decreased from 206 J to 194 J, and the hardness decreased from 228 HV to 211 HV. After examining the results of precipitation hardening at 550°C; it was determined that the carbide formation mechanism was strengthened with the increasing precipitation hardening temperature and the carbides deposited on the grain boundary became large enough to be seen even under an optical microscope with this temperature. In parallel with this, the mechanical properties started to decrease at the precipitation hardening temperature of 550°C. As a result of the charpy impact test, it decreased from 206 J to 37 J with a decrease of 82%, however, the hardness increased by 2% to 233 HV. Finally, in precipitation hardening at 650°C, a significant amount of carbide precipitation was detected in the structure, and the austenite matrix lost its stability. Accordingly, the charpy impact test result decreased by 98% to 4 J, and the steel became completely brittle. The hardness result increased by 32% to 300 HV. In this study, the effect of selected precipitation hardening temperatures on the microstructure and mechanical properties of manganese steel was investigated. As a result of the heat treatments at different temperatures, it was determined that the toughness value decreased, while the hardness value increased depending on the amount of carbide precipitation in manganese steel.