Papers by Keyword: Acicular Ferrite

Abstract: Non-metallic inclusions in weld metals often have a role inducing the formation of acicular ferrite, which is known to improve the toughness and other properties of the weld metal. The ability of the inclusions to promote the acicular ferrite formation depends on various factors such as chemical composition, morphology and size of the inclusions. In multipass welding, additional thermal cycles affect the inclusions in the pre-existing weld passes, potentially causing compositional and morphological changes in the inclusions. These changes may influence the inclusions’ ability to promote the formation of acicular ferrite. In the current study, the thermal cycles of multipass welding were produced on a single pass weld by physical simulation. Coarse-grained heat-affected zones (CGHAZ-W) in weld metal were simulated using three different cooling times from 800 °C to 500 °C (t_8/5). Inclusions in the heat-affected zones of the weld were analyzed using field emission scanning electron microscope equipped with energy dispersive spectroscope (FESEM-EDS), after which they were classified according to their chemical composition. The results showed that the inclusion content in the weld metal was affected by the thermal cycles. In the CGHAZ-W simulation the number of Mn-bearing inclusions increased compared to the unaffected single-pass weld metal. Increasing cooling time was observed to increase the area fraction of MnS in CGHAZ-W. The increase of these inclusions was expected to affect the microstructure by increasing the acicular ferrite fraction.

Abstract: Nickel alloying element has been known to play a role in controlling the formation of microstructures in the weld metal (WM). This experiment aims to study the relationship between nickel addition to the microstructure evolution and toughness of shielded metal arc welded SS400 low carbon steel plates. Three welded samples, Ni-01, Ni-03 and Ni-09 were fabricated using electrodes containing different nickel: <0.1% Ni, 0.3% Ni, and 0.95% Ni, respectively. Microstructure of WM was examined using an optical microscope and scanning electron microscopy, and mechanical properties were measured in strength and toughness. The results showed the impact toughness increased when the nickel content of electrodes increased. From the three experimental welded samples, WM of Ni-09 which using electrodes containing 0.95% Ni demonstrated the best result for the impact toughness. It may caused Ni-09 has more AF and finer grains size compared to Ni-01 and Ni-03. It seems with nickel addition in the WM, finer grain size and acicular ferrite (AF) formation were developed.

Abstract: Acicular ferrite (AFα) formed on oxide particles in steel weld metals has a positive effect on toughness at low temperature. Good lattice coherency between AFα and oxide is one of the proposed reasons for promotion of AFα formation. Lattice coherency is affected by crystal structure of oxide and crystal orientation relationship (OR) between oxide and AFα. In the present study, ORs among AFα, oxide and γFe are investigated in a low carbon steel weld metal. Cube-cube (C-C) OR is observed between γFe and the oxide. It is probable that the oxide liquefied at high temperature, and then crystalized having the C-C OR with the surrounding γFe during cooling in welding process. Near Kurdjumov-Sachs (K-S) and near Baker-Nutting (B-N) ORs are observed between γFe/AFα and oxide/AFα, respectively. The misorientation from the B-N OR is larger than that from the K-S OR just after nucleation of AFα. This implies that AFα forms satisfying a near K-S OR with γFe essentially. It is supposed that formation of both the C-C (γFe/oxide) and near K-S (AFα/γ) ORs results in apparent formation of the near B-N OR between oxide and AFα.

Abstract: This research aimed to investigate the microstructure and hardness properties of hardfacing surface on SCM440 alloy steel by using metal active gas and flux cored arc welding processes. Due to the difficulty of welding the high strength steel, the changes in base metals’ microstructures were found after welding. Preheating the specimens at 350°C and post weld heat treatment the specimens at 550°C were performed for 1 hour, to reduce the residual stresses and avoid the undesired formation of microstructures. The weld metals’ microstructures that were found from both welding processes are acicular ferrite, polygonal ferrite and side plate ferrite. The hardness value of weld metal resulted from flux cored arc welding process is higher than that of the metal active gas welding process. Each welding process produced different quantities of weld metals’ microstructures, causing the difference in hardness values. The data will be used for investigating and improving parameters of shaft repairing, in order to use it more effectively.

Abstract: This paper focuses on the application of miniaturized fracture tests to evaluate the fracture and hydrogen assisted cracking (HAC) resistance of a selected microstructural constituent (acicular ferrite, AF) which only occurs in microscopic material volumes. Site-specific Focused Ion Beam (FIB) micro-machining was used to fabricate sharply notched micro-cantilevers into a region fully constituting of AF. The micro-cantilevers were subsequently tested under uncharged and hydrogen charged conditions with a nanoindenter. The load displacement curves were recorded and analysed with a simplified plastic hinge model for the uncharged specimen, as AF demonstrated an essentially ductile behaviour. The simplified model assisted with FE simulations provided values of the critical plastic crack tip opening displacement (CTOD). A value of the conditional fracture toughness was thereby determined as 12.1 MPa m^1/2. With LEFM, a threshold stress intensity factor, Kth, to initiate hydrogen crack propagation in AF was found to range between 1.56 MPa m^1/2 and 4.36 MPa m^1/2. All these values were significantly below the corresponding values reported for various ferrous alloys in standard macro-tests. This finding indicates that the fracture and HAC resistance at the micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.

Abstract: Using the oxide of high melting-point and high stability to pin the grain boundaries is an effective method to improve the welding performance of the HSLA steel in this study. A kind of HSLA steel is designed in this experiment. The thermal stability second phase particles which would not be dissolved or aggregated at high temperature will be expected by means of adding calcium into the steel in the form of Si-Ca alloy. The effect of calcium addition on the cast microstructure of HSLA steel was analysed. The results show that the cast microstructure is mainly consist of lamellar and acicular ferrite, a small amount of pearlite and bainite. Compared with the original steel, there are acicular ferrites presenting in the experimental steel after adding 5 wt% Ca, which are the microstructure that we hope to get. The acicular ferrite will have a positive impact on the mechanical properties of the subsequent rolled steel.

Abstract: New technology of micro-jet welding could be regarded as a new way to improve plastic properties of welds. The main purpose of that paper was analysing of plastic properties of welds made by MIG welding method with micro-jet cooling. The main reason of it was investigate possibilities of getting better plastic properties of welds made by MIG welding method with micro-jet cooling than plastic properties of welds made by ordinary welding method. It is possible for steel because higher amount of acicular ferrite (AF) in weld metal deposit (WMD) is obtained in MIG welding method with micro-jet cooling in relation to ordinary welding method (example: MIG welding method without micro-jet cooling). Moreover, it is possible to steering of weld structure and properties of the weld. During research Erichsen cupping tests and bending tests were carried out for welds made by MIG welding method with micro-jet cooling and ordinary welding method (MIG). Different kind of cooling gases were used to weld cooling. In this case comparison of plastic properties for different cooling gases was done. High amount of acicular ferrite influences positively on plastic properties. Higher values of plastic parameters were observed for welds made by MIG method with micro-jet cooling than for ordinary welding method. Different plastic properties were obtained for different cooling gases. In this research welds made by new method of welding (welding with micro-jet cooling) were compared witch welds made by ordinary welding method. New method of welding is very promising and capable of industrial application, mainly due to the significant improvement of weld properties and quality. That research was made only for steel welding with using MIG welding method. Another method of welding and another material were not tested. Other methods of welding have not been tested, but it is suspected that similar phenomena are taking place. Practical implications MIG method with micro-jet cooling it is way to get better plastic properties of welds in relation to welds made by ordinary welding method. It is very important because it could be used to steering of mechanical properties of welded constructions. This may have a positive impact on the parameters of the welding process (example: welding speed) and the quality of welded joints.

Abstract: A type of X80 grade high strength and toughness pipeline steel was designed and researched. The strengthening mechanism of the steel was analyzed by SEM, TEM and XRD, and the CO₂ corrosion behavior of the steel was simulated by high-temperature and high-pressure autoclave. The result shows that the microstructure of the base metal is mainly acicular ferrite with a small amount of granular bainite. Acicular ferrite consists of laths which occlude and interweave with each other, and there are many dislocation and carbonitrides distributing in acicular ferrite, which made the pipeline steel have good strength and toughness. Under the simulation of the actual working conditions, the activity of reactants is low at 30°C, so the corrosion rate is smaller at this temperature; the maximum of corrosion rate occurs at 60°C; when the temperature increases to 90°C, the corrosion rate is lower than that of 60°C, that is because hindering corrosion effect which take by the acceleration deposit of corrosion product is better than the acceleration corrosion reactions.

Abstract: Experimental methods, such as OM, SEM and X-EDS, were used to study the microstructure of X80 pipeline steel. It mainly consists of fine acicular ferrite (AF). X80 pipeline steel possesses high strength and impact energy at-30°C approaches to 400J. Grain refinement and precipitation hardening are the main reasons for high strength, and toughness improvement can be attributed to grain refinement and particular microstructural characteristics of AF.

Abstract: Specific types of non-metallic inclusions are known to act as heterogeneous nuclei for the formation of acicular ferrite, which provides excellent toughness. By increasing the amount of acicular ferrite in the microstructure, the properties of HSLA steels can be optimized significantly.Although the formation of acicular ferrite caused by heat treatments (thermomechanical treatments or welding) is quite well described in literature, there is less information to find about the formation of acicular ferrite immediately out of the liquid melt. Within the present study experiments on laboratory scale are carried out simulating the influence of cooling conditions and Ti-content on size, chemical composition and morphology of non-metallic inclusions and consequently on the amount of acicular ferrite. All experiments were carried out with a dipping test simulator enabling very well controllable cooling conditions. Optical microscopy in combination with special etching methods as well as SEM/EDS-analysis was used for microstructure and inclusion characterization.