The paper presents a research on the influence of multistage heat treatment with the assumed parameters of temperature and time on the microstructure and mechanical properties of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel. In the as-cast state GP91 cast steel was characterized by a microstructure of lath martensite with numerous precipitations of carbides of the M23C6, M3C and NbC type, with its properties higher than the required minimum. Hardening of the examined cast steel contributes to obtaining a microstructure of partly auto-tempered martensite of very high strength properties and impact strength KV on the level of 9-15 J. Quenching and tempering with subsequent stress relief annealing of GP91 cast steel contributed to obtaining the microstructure of high-tempered lath martensite with numerous precipitations of the M23C6 and MX type of diverse size. The microstructure of GP91 cast steel received after heat treatment was characterized by strength properties (yield strength, tensile strength) higher than the required minimum and a very high impact energy KV. It has been proved that GP91 cast steel subject to heat treatment No. 2 as a result of two-time heating above the Ac3 temperature is characterized by the highest impact energy.
G. Golański and J. Kępa
The Effect of Ageing Temperatures on Microstructure and Mechanical Properties of GX12CrMoVNbN9 -1 (GP91) Cast Steel
Increase in the steam parameters, the so-called supercritical parameters, resulted not only in the growth of efficiency of power units, or reduction in the emission of pollutants into the atmosphere, but also it has contributed to the development of heat-resisting steels and cast steels. One of the newly developed and implemented cast steels in the power industry is martensitic GX12CrMoVNbN9-1 (GP91) cast steel, created on the basis of chemical composition of P91 steel. The cast steel of a microstructure and properties similar to the base material replaces in the power industry the grades of steel used previously which do not meet the increasingly high requirements. A study has been made with regard to the effects of ageing at 550 and 600°C and time up to 6000 hrs on the changes in microstructure and mechanical properties of GX12CrMoVNbN9-1 (GP91) cast steel. A detailed microstructural research and the analysis of secondary phases was carried out using the electron microscopy technique, whilst the research on mechanical properties included a hardness test, static tensile test and impact strength measurement. The microstructural observations showed that the lath martensite microstructure was maintained after 6000 hrs. The coarsening of M23C6 carbides was revealed. Ageing of the investigated cast steel resulted in a slight decrease in the strength properties (Rp0.2, TS and HV30), i.e. of around 6%, which was accompanied by a significant decrease in impact strength KV. Moreover, in the microstructure of investigated cast steel there were the processes of recovery and polygonization of the matrix observed.
T. Garstka, B. Koczurkiewicz and G. Golański
Diagnostic Examination of P265GH Boiler Steel Plate Using the Barkhausen Method
The article presents the results of diagnostic examination of P265GH boiler steel plate. During blanking, cutting and cold forming of elements from this plate, their spontaneous plastic deformation and warping took place, indicating an unfavourable internal stress state of a considerable magnitude occurring in the steel plate. In order to identify the causes of this situation, examinations were carried out, which included a microstructure assessment and a Vickers hardness test. In view of the absence of clear differentiation, in terms of both structure and hardness, in different steel plate areas, diagnostic examination was performed by the Barkhausen magnetic method, which included the determination of the principal direction of residual stresses and the determination of their anisotropy based on the measurement of the effective value of Barkhausen noise. As their result, a significant anisotropy of residual stresses was revealed on the steel plate surface. Moreover, the adverse phenomenon of perpendicularity of the principal directions of stresses was found to occur on the opposite plate surfaces, being the direct cause of warping of elements cut out from the plate
G. Golański, P. Gawień and J. Słania
Examination of Coil Pipe Butt Joint Made of 7CrMoVTiB10-10(T24) Steel After Service
The paper presents the results of examination of coil pipe butt joint made of 7CrMoVTiB10-10 steel (T24). Tested joint was in service for approximately 12 000 hours at the temperature of 540°C and pressure of 5.48MPa. Tests have revealed that the structure of all regions of the homogenous welded joint are correct and without any welding imperfections. Operation of the weld contributed mainly to advantaged precipitation of carbides especially on grain boundaries - frequently in the contact area of three grains' boundaries as well as in the form of continuous network of precipitates. The measurement showed high hardness of the weld, what indicates high stability of the microstructure. It makes possible to find that post-weld heat treatment is indispensable.
A. Zieliński and G. Golański
The aim of the paper was to examine the influence of repair welded joints on the service life of steam pipelines for over 220 000 hours of service in creep conditions. The research included the study of the microstructure using scanning microscopy, the tests on mechanical properties at room and elevated temperature, determining the nil ductility transition temperature, and short-time creep tests to determine the residual life of the material. The tests allowed determining the time of further safe operation of elements of the steam pipeline with a repair welded joint, with reference to the base material, and the welded joint after service. The assessment of residual life and disposable residual life, and at the same time determining the possible time of further safe operation, has an essential meaning in the case of elements serviced considerably beyond the calculated working time.
J. Słania, G. Golański and W. Gasz
Resistance to stretching, a quality of specimen made by braze welding method were examined as well as usefulness of this method in industry was described. Technology and materials, which are used during braze welding, are depicted on the base of an inspection of joints. The results of macroscopic examinations of sample joints (base material, heat-affected zone and braze weld) are given. Furthermore, the results of the resistance, plasticity and hardness tests of sample joints are presented.
G. Golański, M. Lachowicz, J. Słania, J. Jasak and P. Marszałek
The paper presents the research on the microstructure and mechanical properties of a pipe made of 16Mo3 steel, overlaid with superalloy based on Haynes NiCro625 nickel. The overlay weld was overlaid using the MIG (131) method. The performed macro - and microscopic tests have shown the correct structure of the overlay weld without any welding unconformities. The examined overlay weld was characterized by a dendritic structure of the primary crystals accumulating towards the heat removal. It has been proved that the content of iron in the surface zone does not exceed 7%, and the steel-superalloy joint shows the highest properties in comparison with the materials joined.
G. Golanski, A. Zielinski, J. Słania and J. Jasak
The paper describes the influence of different times of ageing on mechanical properties and microstructure stability in hardened and tempered VM12 steel exposed to service temperature - 600°C. Detailed microstructural and microchemical analysis of secondary phases was carried out using scanning electron microscopy (SEM + EDX) and X-ray phase analysis of carbide isolates. Performed research has proved high stability of strength properties of the investigated steel, which is connected with the lath stability of the microstructure of tempered martensite. Slight changes in strength properties were accompanied by over 50% reduction in impact strength KV of the examined cast steel, from the level of 83 J in the as-received condition to 38 J after 30 000 hrs of ageing at the temperature of 600°C. Significant decrease in impact energy KV of VM12 steel results from the growth of the amount and size of precipitations on grain boundaries.
A. Zieliński, M. Miczka and G. Golański
The results of investigations on the microstructure of T23 and T24 low-alloy steels as well as P91 and P92 high-chromium steels in the as-received condition and after 70.000 h annealing at 550-650°C are presented. The quantitative analysis of the existing precipitates was performed for representative images of microstructure. The statistical analysis of collected data allowed the parameters of a selected theoretical statistical distribution to be estimated. A forecast of average precipitate diameter and standard deviation of such a distribution for the time of 100,000 hours at 550 and 600°C for T23 and T24 steels and at 600 and 650°C for P91 and P92 steels was calculated. The obtained results of investigations have made it possible to compare changes in the microstructure of various steel grades due to long-term impact of elevated temperature. They have also confirmed the possibility of using, in evaluating the degradation degree of materials in use, the forecasting methods that derive from mathematical statistics, in particular the theory of stochastic processes and forecast by analogy methods. The presented approach allows the development of a forecast of precipitate diameter probability density under the microstructure instability conditions for selected steel grades. The assessment of material condition that takes into consideration, but is not limited to, the precipitate diameter measurement is useful as an assessment component in estimating the time of safe service of power unit elements working under creep conditions.
G. Golański, I. Pietryka, J. Słania, S. Mroziński and J. Jasak
The paper presents the results of research on the microstructure and mechanical properties of 12HMF steel after longterm service. The investigated material was taken from a pipeline with circumferential welded joint after 419 988 hours of service at the temperature of 490°C, steam pressure 8 MPa. Performed research has shown that the 12HMF steel after service was characterized by a typical microstructure for this grade of steel, that is a ferritic-bainitic microstructure without any visible advanced processes of its degradation. The investigation of mechanical properties has shown that the examined steel after service was characterized by a very low impact energy KV, and yield strength lower than the required minimum. Whilst tensile strength and yield strength determined at elevated temperature was higher and similar to the standard requirements, respectively. It has been proved that the main cause of an increase in brittleness and a decrease in yield strength of the examined steel should be seen in the segregation of phosphorus to grain boundaries and the formation of precipitate free zones near the boundaries.