Microstructural Evolution of Simulated Heat-Affected Zone in Cr-Mo Alloy Steels and Phase Transformation in an AISI 410 Stainless Steel

• Main Authors: Ming-Chin Tsai, 蔡明欽
• Other Authors: Jer-Ren Yang
• Format: Others
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/30755497447113132587

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 90 === The Cr-Mo series alloy steel possesses many advantages: such as good creep resistance, easy formation, good weld-ability and good corrosion resistance. Thus this type of steel is extensively used in power generation industry for containment vessels, superheater tubes and steam pipes. Components of steam pipes are usually used in the temperature range 500-600℃, with the service stresses of about 15-30 MPa. Hence, creep rupture is the major ultimate failure mechanism during service. Considerable research has been devoted to the assessment of creep life by extrapolating the results of short-term laboratory creep tests to long-term services. However, such parametric techniques usually assume the microstructure remains unchange even during the extended service. In fact, the microstructure changes with time when exposed to high temperature and high pressures, especially in the heat-affected zone (HAZ) of a weld material. In this work, simulated HAZ experiments with heat inputs of 20, 50 and 80 kJ/cm were carried out for Cr-Mo alloy steel plates (include 2.25Cr-1Mo steel and modified 2.25Cr-1Mo steel). The thermal cycles used corresponded to the actual thermal cycles that occur in the coarse grained region of the real HAZ. The simulated HAZ samples were then heated at 700℃ for different time intervals ranging form 1 to 500h. The corresponding metallographs and transmission electron micrographs for the high temperature long-term exposed samples have been investigated. The result shows that the bainitic structure is more stable than the martensitic structure, i.e. the steel with a bainite structure is suitable to be used in high temperatures. In this study, the carbide precipitation sequence is proposed as follows: ε-carbide→ M3C→ M7C3→ M23C6. The M23C6 carbide was found to be the final stable carbide after a long period of tempering, when the recrystallization occurred. In this tempering condition during service, the safety of the Cr-Mo steel should receive special attention. Furthermore, it is very important to consider the effect of cooling rate on the martensitic transformation in the Cr-Mo alloy steels. The different cooling rate would result in the different microstructures and Ms temperature. According to the experiment data, the Ms temperature increases with cooling rate decreasing. While the Ms temperature decrease with the increase of austenization temperature. The mechanism of micro-twin and retained austenite forming had been explained using crystallographic study. The formation of micro-twin is due to mechanical accommodation; that of retained austenite results from the mechanical stabilization. These two microstructures had been investigated deeply in this thesis. At the last parts of this thesis, phase transformations in an AISI 410 steel and in a modified 9Cr-1Mo steel during continuous cooling or isothermal treatment are reported. Dilatometry and electron micrography have been employed. The results provided strong evidences on the existence of retained austenite films and micro-twins in lath martensite structure, and that the coalesced lath in tempered martensite regions depends on the local crystal orientation relationship. The reason why no bainitic structure can be obtained in these two steels has also been elucidated.