STUDY ON COERCIVE FORCE CHARACTERISTICS OF CARBONSTEEL DURING TENSILE LOADING & EVALUATION OF HEAT TREATMENT EFFECTIVENESS IN BOILER MATERIALS THROUGH COERCIVE FORCE MEASUREMENTS

摘要

Structural members made of carbon steel are subjected to various types of loads during service. The magnetic characteristic "Coercive Force" in ferromagnetic materials is sensitive to the microscopic and submicroscopic structural changes in the material due to localized plastic deformation of the material before the initiation of cracks. Hence this parameter can be measured in test specimens in lab with known loads by creating plastic deformation up to the point of fracture. These values can be compared to the values measured in the in-service components made of the same material and the extent of damage if any can be ascertained. This Non-destructive method is very useful in predicting the residual life of components. In this paper the details of the lab experiments conducted in IS2062 Carbon steel material are explained. Further, Heat treatment is a very important process employed to improve the mechanical properties of Heat Affected Zone & weld and to relieve the residual stresses in fabricated parts. This process is carried out by using either an oil/gas fired or electrical furnaces in production shops where the entire weldment is kept under uniform temperature condition throughout the heat treatment operation. However at erection sites, local post weld heat treatment is often applied by wrapping heating coils around the circumference of weld joints covering the weld and certain region close to the weld joint. In this context the effectiveness of heat treatment becomes a matter of interest, especially in temper sensitive materials. At present there are many methods for determining the effectiveness of Heat treatment such as Hardness measurements and Microstructure evaluation in weld and HAZ of weld joints. In addition residual stress measurements using X ray diffraction method or Hole drilling strain gage method are also carried out as a laboratory tool, to establish the Heat treatment procedure. Non-destructive method of evaluating the effectiveness of Heat treatment by measurement of coercive force is possible. A patent for the usage of this method has also been applied. This non-destructive testing method is based on the change in coercive force due to changes in submicroscopic imperfections in the material. As high temperature heat treatment process brings about changes in the micro and submicroscopic levels due to creep and plastic deformation, the application of the coercive force to evaluate the change in structure becomes a possibility. In this study the feasibility of using the change in Magnetic coercive force parameter as a measure of effectiveness of Heat treatment has also been examined. Trials have been carried out in pipe joints and plate joints in carbon steel and low alloy steel materials and Magnetic coercive force measurements have been correlated. The results reveal that there exists a strong relation between effectiveness of Heat treatment and Magnetic coercive force in the case of thermal stress relieving heat treatment.