Study on impact properties of creep-resistant steel thermally simulated heat affected zone

Radivoje Mitrovic, Dejan Momcilovic, Olivera Eric, Ivana Atanasovska, Nenad Hut
2012 Thermal Science  
The steam pipe line and steam line material, along with its welded joints, subject to damage that accumulates during operation in coal power plants. As a result of thermal fatigue, dilatation of steam pipe line at an operating temperature may lead to cracks initiation at the critical zones within heat affected zone of steam pipe line welded joints. By registration of thermal cycle during welding and subsequent heat affected zone simulation is possible to obtain target microstructure. For the
more » ... ructure. For the simulation is chosen heat resisting steel, 12H1MF (designation 13CrMo44 according to DIN standard). From the viewpoint of mechanical properties, special attention is on impact toughness mostly because very small number of available references. After simulation of single run and multi run welding test on instrumented Charpy pendulum. Metallographic and fractographic analysis is also performed, on simulated 12H1MF steel from service and new, unused steel. The results and correlation between microstructure and impact toughness is discussed, too. -primary stresses induced by the weight of the steam line itself, noted above, and the influence of hangers on secondary induced stresses [2] . As a result of thermal fatigue, dilatation of SPL at an operating temperature may lead to cracks initiation at the critical zones within heat affected zone (HAZ) of welded joints [3]. We should not forget the residual stresses due to welding that can be superimposed with operating stresses, and can also be induce initiation of cracks [4, 5] . It is necessary to mention the corrosion and stress corrosion as another possible cause of initiation of cracks in the steam line [6, 7] . However, the most significant effect on the occurrence of damage and reduction of SPL service life of materials has the appearance of creep [8] . Development of material damage during the operation is followed by the continuous creation of cavities or micropores at grain boundaries. Many authors state that the cavities were present before the process of creep, as a result of SPL steel production process. Regardless of background, the cavities grow during creep [9, 10]. Cavity size is largely dependant also on material type, however it is in the range of micron size (often also lower), therefore they are usually called "microvoids" or "micro-cavities". One of the basic acceptance tests of welds and HAZ is impact energy test by Charpy method. The classic energy impact testing, without separation of energy initiation and energy propagation of crack during the tests, does not give a complete view situation of the impact strength of the tested material. Therefore, in these studies, it is necessary to use instrumented Charpy pendulum [11, 12] . Knowledge of the microstructure and their variations with different thermal cycles of welding and impact energy dependence of the microstructure, can be obtained by improving the welding parameters during welding repair on the one hand, and making assessment of the existing welds from the point of remaining service life, energy efficiency and behavior of welded joints during accidental situations, on the other hand [13] . By registering during welding thermal cycle, during welding of full scale pipe, and then simulating obtained welding cycle on simulators, on samples of material from which the SPL is made, it is possible to obtain the desired microstructure corresponding to the selected critical microstructure within the HAZ, but in dimensions sufficient for reliable testing of destruction which is the aim this paper. Experiment Methodology and equipment Today, welding simulators are used to determine mechanical properties of the "weakest link" i. e. critical microstructure within a HAZ. Welding simulator ( fig. 1 ) is a device that achieves controlled heating and cooling similar to the thermal cycle during welding. The difference is in obtained micro structure, on a sample size of 10´10´60 mm, and at its middle part, as microstructure which corresponds to the desired critical area. The critical area of HAZ is usually coarse grain zone, and simulation enables the exact identification of fundamental mechanical properties [14, 15] . Input parameters are performed via computer, and selection of the basic parameters can be determined from the calculation of the temperature field or by the using of appropriate equations to calculate Dt 8/5 in the welding thermal cycle [16] . The origin of the parameters can be also experimental obtained, by measuring the thermal cycle with the thermocouples.
doi:10.2298/tsci111006142m fatcat:wyshd57bwfbq3b22ncrww4irxa