فهرست مطالب ابوالفضل توتونچی

Quenching-partitioning heat treatment steels (Q-P) are among the third generation of advanced high strength steels (AHSS) that have been expanded due to having a significant set of mechanical properties including high strength along with appropriate flexibility. The principles of segmentation thermal operations are based on carbon infiltration from martensite to Return austenite (􀀃􀀄) and stabilization of 􀀃􀀄 In the present study, a high strength steel containing Ti microalloy was subjected to Q-P and Q-P-T heat treatment and compared in terms of microstructure and mechanical properties. After quenching-partitioning (Q-P) and quenching-partitioningtempering (Q-P-T) heat treatments were applied to the sheet samples, their properties were determined using scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness measurement. , tensile test and Erickson's test were evaluated. The results showed that the strength and failure strain of Q-P samples were measured as 1062 MPa and 24% and Q-P-T samples as 894 MPa and 27% respectively. Q-P samples showed more strength and less flexibility than Q-P-T samples.

Quenching-partitioning (Q-P) is one of the methods of producing advanced high strength steels (AHSS). The principles of the mentioned thermal process are based on carbon penetration from martensite to residual austenite (γR) and stabilization of γR. In this research, the alloy with the chemical composition of 0.054 wt% Al, 1.5 wt% Si, 2.2 wt% Mn, 2.2 wt% Mn, 0.21 wt% Fe, containing 0.08% Ti as a micro-alloy in an induction melting furnace in VIM was cast as an ingot. The said ingot was homogenized for 3 hours at a temperature of 1200 °C. The homogenized ingot was rolled into a sheet with a thickness of 1.5 mm by hot and cold rolling processes. The prepared sheet was subjected to Q-P heat treatment and direct quenching (D-Q) and was compared in terms of microstructure and mechanical properties. The comparison of the effect of the mentioned operations on the microstructure and mechanical properties of steel was done by scanning electron microscope (SEM), X-ray analysis (XRD), micro-hardness measurement, and metallography. The obtained results showed that the sample subjected to Q-P heat treatment has better formability and strength than the D-Q sample. The strength and formability of the sample obtained from Q-P were reported as 1062 MPa and 24.52% respectively, and the strength and formability of the sample obtained from D-Q were reported as 1484 MPa and 15.21% respectively.

In the present study, the effect of Ti and Nb microalloying elements on the microstructure and mechanical properties of the quench-partitioned (Q-P) steel was investigated. The samples containing Ti and Nb micro-alloying elements were subjected to Q-P heat treatment and quench-Tempering (Q-T). After Q-P and Q-T heat treatment, the properties of the samples were examined by scanning electron microscopy (SEM), X-ray analysis (XRD), microhardness, tensile tests and. The results showed that Q-P heat treatment of both sample has better mechanical properties compared to Q-T heat treatment. In the Q-P heat treatment, the amount of retained austenite (γR) for the sample containing Ti is higher than that of the sample containing Nb. Fracture strength and strain of the sample containing Ti were recorded 1070 MPa and 24%, respectively. The fracture strength and strain of the sample containing Nb were also recorded 980 MPa and 19%, respectively. In Q-P heat treatment, the plasticity of the sample containing Ti microalloy is higher than that of the Nb microalloy sample.

Steels with quenching-partitioning (Q-P) heat treatments are the third generation of advanced high strength steels (AHSS) which have been developed due to their interesting combination of mechanical properties including high strength with good flexibility. Partitioning is based on carbon diffusion from martensite (M) to residual austenite (γ_R). In the present investigation, an alloy with a weight composition of % Al 0.054- % Si 1.5 -% Mn 2.2 -% C - 0.21 - Fe containing 0.08% Ti as a micro-alloy was prepared using a vacuum induction melting (VIM) furnace. Purification was performed using an eletro-slag refining (ESR) furnace to reduce sulfur and oxygen and then the molten alloy was cast as an ingot. The ingot was then homogenized at 1200 ° C for 3 hours. The homogenized ingot was then rolled to 1.5 mm thickness sheet by hot and cold rolling processes. After quenching-tempering (Q-T) and quenching-partitioning-tempering (Q-P-T) treatments, the properties of the samples were evaluated by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), micro-hardness, tensile and Erichsen tests. The Q-P-T samples showed more favorable combination of strength and flexibility in comparison to Q-T samples. The strength and fracture strain of Q-P-T and Q-T samples were 1050 MPa, 24%, and 1185 MPa, 18%, respectively. The Q-P-T samples with remarkably higher strength, showed significantly higher flexibility.

Fitness for Service (FFS) methodology assessment of defective pipes and pressure vessels in oil and gas industries is offered in API-579 standard. The defective pipes operation are allowed just if they have been evaluated based on FFS method and their operating pressure has been confirmed whether the defects caused by cracks or different corrosion types. Through the current study FFS evaluation procedure was benefited to find the safe operating pressure for a 16 inches defective pipe consisting a local metal loss defect (Max. depth of 8mm) with an internal pressure of 33 MPa. Furthermore, the time needed for the next FFS examination had been presented. The analyzed pipe was about 4 years in service and is located in an injection line of a natural gas reservoir. The required experimental data related to the thickness of this pipe were collected using phased array ultrasonic test on the operating site. The results indicated that the defective pipe could operate well in service for next two years without any demands for repair or replacement.

Incidence of breaks and leakages in fluid transportation pipes is a common issue in Iran. Depending on the type of pipes and environmental conditions, the breaks in the pipes may be caused by different factors, including mechanical damages, internal or external corrosions, failures, or applied stresses. In the repair of damaged pipes, there are several strategies for rebuilding and implementing the pipeline, most of which are replacing the entire exhausted pipe, using weld clamps and using composite patches. In recent years, the use of composite patches has been accepted as a low-cost, permanent, and standard method for different pipe sections with the least interruption in transportation. In the present study, the boding strength of glass fibers-reinforced epoxy composite patches on a structural steel substrate were investigated and optimal conditions of achieving enhanced adhesion strength of composite patches on the steel substrate were determined, using the Tagochi method at various curing temperatures and times. In this regard, the tensile and shear strength of epoxy, cyanoacrylate, and methacrylate-based glues as three kinds of appropriate polymers for bonding the epoxy composite on the steel substrates were tested. The mechanical strength measurements and fractured interfaces evaluations using a scanning electron microscopy (SEM) revealed that the methacrylate-based glue has the better adhesion strength to the steel substrate.

In the present study، central composite algorithm was used in order to model and optimize the mechanical behavior of “glass fiber reinforced epoxy composite - structural steel “connections. Initial tests showed that the polymer curing variables play a significant role as key process parameters in producing strong and reliable connections. After conducting Thermal Gravimeteric Analysis on polymer، by selecting curing time and curing temperature as input variables، the parameters were coded and each of them was studied in five levels. In order to estimate the desirable response and provide appropriate models، thirteen tests were conducted systematically. In order to assess the accuracy and to validate the proposed model، analysis of variance was performed successfully. The effect of curing time and curing temperature on the connection’s strength quality was studied utilizing two-dimensional graphs. Utilizing this approach the optimal bonding process variables was achieved at 40°C and 180 min for curing temperature and curing time respectively. Finally، the results obtained from micro structural characterization and fractography analyses of joints by Optical and Scanning Electron Microscope were in good agreement with the results achieved by the developed model.