Large Non-metallic Inclusions Research Articles

Identification of the nature of defects arising in the production of continuously cast pipe billets for the oil and gas industry

The paper presents the results of studies of defects that occur in the production of continuously cast pipe blanks for the oil and gas industry. It is shown that the use of the technology of continuous casting of pipe billets in comparison with the traditional methods of pouring liquid steel into a mold has a number of advantages, among which is the high productivity of the technological process. However, it was noted that in many cases, an abnormally high degree of contamination with non-metallic inclusions is observed in the structure of the steel of continuously cast pipe billets. It was revealed that the largest inclusions are oxides of silicon and manganese. On the surface of the pipes, there are defects in the form of oxide spots, near which there are large non-metallic inclusions. On the surface of the oxide spot, the presence of iron oxides in the form of scale is found, which is a consequence of the secondary oxidation of the metal by air oxygen. A mechanism for the development of oxide spots and decarburized strips in pipe billets has been determined, which consists in the formation of defects in the form of scratches and cracks on the surface before rolling. These defects during heating for rolling lead to the oxidation of the metal and the formation of scale on the surface of the pipe blanks. The results of experimental studies on the development of recommendations for improving the quality of the initial metal of seamless pipes for the oil and gas industry are presented. The study of the microstructure of steel and the assessment of its contamination with non-metallic inclusions were carried out on optical and electron microscopes, and the mechanical properties of pipe blanks were studied by standard methods

Formation and aggregation behavior of inclusions in Ni-based alloys with different Mg contents

Large non-metallic inclusions affect the mechanical properties and service life of nickel-based superalloys. This study investigates the effects of magnesium elements on non-metallic inclusions in K4169 nickel-based superalloys, exploring the formation mechanisms and aggregation relationships of the inclusions. Four gradients of the magnesium content were used to simulate the erosion of MgO refractories on inclusions in the vacuum induction melting of nickel-based superalloys. The observed inclusions mainly comprised MgO, TiN, MgAl2O4, and MgAl2O4–TiN composites. To study the formation mechanism of inclusions in Ni-based alloys with different Mg contents, the predominance diagram of Mg–Al–Ti–O inclusions in nickel-based superalloy systems was calculated using Factsage thermodynamics software and classical thermodynamics separately. The aggregation behavior of MgAl2O4 and MgO inclusions on the surface of molten nickel-based superalloys was observed in situ under a high-temperature confocal laser scan microscope. The capillary force acting on inclusions in nickel-based superalloys was calculated using the Kralchevsky–Paunov model (K–P model), the calculation was verified with experimental results, and the influencing factors were analyzed. The capillary force acting on inclusions on the surface of molten nickel-based superalloys weakened in the order of MgAl2O4 > MgO > Al2O3, which is the opposite of the order for the force acting on inclusions in molten steel. Minimizing the magnesium content in the alloy is crucial to preventing the formation of large inclusions in nickel-based superalloys, and the use of magnesium-containing materials in the smelting process should thus be avoided.

Effects of inclusions on the fatigue life of SNCM439 steel in high-pressure hydrogen gas

Four heats of commercially available JIS SNCM439 steel are prepared, and fatigue tests are conducted in air and hydrogen gas. The materials evaluated are all martensitic steel with a tensile strength of 900 MPa or less and contain nonmetallic inclusions of different sizes. A decrease in the fatigue limit is observed in the specimens with large nonmetallic inclusions, but the fatigue limit in air is approximately equal to the fatigue strength at 300,000 cycles in hydrogen. However, in the finite life region, the fatigue life in hydrogen significantly decreases owing to the presence of large nonmetallic inclusions. It was observed that hydrogen considerably affects the fatigue life even at low stress amplitudes close to the fatigue limit. This effect is considered to be dependent on the size of the initial crack originating from the nonmetallic inclusions; large nonmetallic inclusions accelerate the hydrogen-induced fatigue crack growth rate.

Evaluation of Strength and Microstructural Properties of Heat Treated High-Molybdenum Content Maraging Steel

Effect of high molybdenum content ~10% as an alloying element on the strength and microstructural properties of 11% nickel—1.25% titanium maraging steel was evaluated. To increase the homogeneity and cleanliness of produced ingot, the investigated steel sample was produced by melting the raw material in an open-air induction melting furnace followed by refining utilizing a direct current electro-slag refining machine. The produced steel samples were both forged and heat-treated in optimum condition to acquire the full capacity of mechanical properties especially the tensile properties. After Forging and heat treatment at optimum condition, steel samples were evaluated by optical microscopy (OM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) analysis, electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). The experimental data showed that this steel sample has ultimate strength ~2100 MPa and elongation around 14%. High tensile properties obtained may be attributed on one hand due to the presence of high alloying lamellar martensite phase and lamellar austenite phase which has high dislocation intensity, and on the other hand, due to the high homogeneity and cleanliness of investigated samples from large nonmetallic inclusions. The results also show that a high amount of intermetallic compounds (NiMo3 and NiTi3) which are completely round and have a very low size not more than hundred nanometers.

Nonmetallic inclusions and secondary structure of continuous casting steels

The article presents the results of a study of the non-metallic inclusions and secondary structure of continuous casting steels It is estimate that main quantity of non-metallic inclusions (»72%) it inputted in steel during the deoxidization and the secondary oxidation, therefore the casting processes need to be managed very well to decrease the quantity of non-metallic inclusions in liquid steel. Reducing the secondary dendrite arms spacing for the strip casting process, in comparison with the conventional continuous casting process of thick slab, will reduce the size of non-metallic inclusion and as result could improve the mechanical properties of steel. Content of the endogenous non-metallic inclusions in stainless steel with Ti, in carbon steel and in electrical steel grades decreases in ladle and tundish in 2.7–3.2 times in comparison with quantity of non-metallic inclusions before pouring from furnace. The increasing of the tundish width decreases in 20 times the quantity of nonmetallic inclusions by sizes from 70 to 80 mm, and in 5–6 times by sizes 220–230 mm. Increasing of the tundish height reduces of the oxygen content in continuous casting of slab It was development the dependence of the secondary dendrite arm spacing with cooling rate. Analysis shown, that the secondary dendrite arms spacing for the strip casting process decreases from 5.91 to 8.31 times in comparison with the conventional continuous casting process of thick slab of thickness 220 mm. Simultaneously non-metallic inclusions sizes to decrease, too. Rapid solidification reduces the number of large non-metallic inclusions: the inclusion number larger than 1 mm is decreased by a ratio of 5 in comparison with the conventional slabs process. It was estimated influence of main parameters on the average grain sizes and the steel microstructure for the strip and conventional casting processes. The dependence of the grain size of carbon and low alloying steels grades (C = 0.08–0.6%, Si = 0.4–0.6%, Mn = 0.4–1.4%, P < 0.03%, S < 0.03%), (C = 0.04–0.6%, Si = 0.11–0.3%, Mn = 0.3–1.12%, P = 0.01–0.035%, S = 0.005–0.035%, Nb = 0.013%, V = 0.001%) and high chromium and stainless steels of type AISI 430 and 304 (C = 0.03–0.12%, Si = 0.83–1.0%, Mn = 0.8–1.0%, Cr = =16.0–18.4%, Ni = 8.47%, N = 0.03%) from casting speed range, final thickness of slab or sheet, reduction, temperature range is estimated by a multi regression analysis. The grain size of steel obtained by the strip casting process, in range 1300 to 1400 oC, is 2.3 time smaller than for the slab casting processes with slab thickness from 50 to 220 mm.

The effect of the impurities spaces on the quality of structural steel working at variable loads

Abstract The quality of carbon steels working at variable loads mainly depend of microstructure, but also of impurities. The quantity and morphology of non-metallic inclusions and spaces between impurities are correlated with the content of admixtures in the alloy, while their phase composition and structure, in particular shape, dimensions and dispersion, are determined by the course of metallurgical processes. Non-metallic inclusions as impurities found in steel can affect its performance characteristics. Their impact depends not only on their quality, but also, among others, on their size and distribution in the steel volume. The literature mainly describes the results of tests on hard steels. The article discusses the results of a study investigating the effect of the number of large non-metallic inclusions (over 10 μm in size) on the fatigue strength of structural steel during rotary bending. The study was performed on 6 heats produced in an industrial plant. Fourteen heats were produced in 140 ton electric furnaces. All heats were desulfurized and refined with argon. The experimental variants were compared in view of the tempering the research steel. The fatigue strength of steel with an impurity spaces was determined during rotary bending: the results revealed that fatigue strength is determined by the impurity spaces and tempering temperature.

Development of Manufacturing Technology for a Continuously-Cast Wheel Billet with a Controlled Aluminum Content

The quality of railway wheels significantly affects the economy and security of railway transportation. In order to improve the properties of railway wheels steel is used currently with a controlled aluminum content. A feature of this steel is the increased risk of forming large alumina non-metallic inclusions. Nonmetallic inclusions in the defective and defect-free zones of a rejected wheel are studied. The causes of defect formation and a new comprehensive technology of smelting are determined, and ladle treatment and continuous casting of steel are developed, which provide avoidance of forming these defects.The main essence of the technology is the diffusion deoxidation of metal with slag of optimum composition, control of aluminum content during ladle treatment, and use of aluminum exclusively for steel alloying.

Effect of scrap using in charge on the microstructure and properties of ZhS6U nickel-based superalloy. Part 2. Structure analysis and mechanical properties of ZhS6U prepared with scrap

The second part of the article considers influence of the scrap amount on structure and mechanical properties of ZhS6U-VI nickelbased superalloy. As the use of scrap is associated with the possibility of alloy contamination by nonmetallic inclusions and loss of alloying elements, the influence of scrap on alloy structure and mechanical properties is in great importance. The samples with diameter of 12 mm were melted in a vacuum arc furnace and were casted into a copper mold from the virgin ZhS6U-VI alloy without scrap and from alloys with 50 % and 100 % of scrap. The alloys structures were investigated using optical microscopy on etched metallographic sections that were cut from the samples’ cross-sections. The studies were carried out on the as-cast samples and the samples after solution heat treatment for 4 hours at 1210 °C. The slightly higher nonmetallic inclusions content were observed in the structure of the alloy melted with scrap in comparison with virgin alloy melted without scrap. For the sample that was melted from 100 % of scrap the inclusions secure level is 3 (ASTM E 45-97) with an average size of inclusions of 28.4 ± 0.2 μm. Herewith the presence of single large inclusions with a size of not more than 70 microns was noted. However, it has no effect on the alloy mechanical properties. Mechanical properties after heat treatment (ultimate strength (UTS) = 1090 – 1100 МPа and elongation (El) = 9 – 11 %) were obtained on the samples melted using 50 and 100 % of scrap and fully correspond to the TU1-92-177-91 standard for ZhS6U-VI nickel-base superalloy. During solidification most of the large nonmetallic inclusions are concentrated under casting surface, which makes their machining difficult. Because of that the use of 100 % scrap without its preliminary processing is not recommended. Acceptable results were achieved when the 50 % of scrap was used.

Structure and properties of 17G1S-U low-carbon pipe steel microalloyed by boron

The results of analysis of the influence of boron microalloying on structure and properties of 17G1S-U pipe steel are given in the paper. Studies of metal structure were performed by electron microscopy and local X-ray spectral analysis. It has been established that metal containing 0.006 % of boron is characterized by an increased volume concentration to 0.029 % of oxide (OS) and oxysulfide (OSB) inclusions, whose content in metal without boron reaches 0.006 %. Separate sulphide inclusions (CB), whose concentration does not exceed 0.004 % against 0.029 % in a metal without boron, containing 0.01 % S is practically absent in the metal with boron containing 0.003 % S. The microalloying of pipe steel by boron has ensured the preferential formation of small nonmetallic inclusions, evenly distributed in the volume of metal. The proportion of nonmetallic inclusions with size less than 2 (rm is 76.1 %, whereas in steel without boron it is only 58.5 %. In this case, large nonmetallic inclusions of more than 10 rm are practically absent in the sample with boron. Their share does not exceed 0.6 %, which is 22 times less than their amount in the sample without boron. The structure of the sample without boron consists mainly of ferrite and a small amount of perlite, and the sample with boron is represented by a dispersed ferritic-bainitic structure. Increasing the microhardness of both ferrite and pearlite 80 and 100 HV10, respectively, is observed by adding boron to steel. The mechanical properties of 10 mm hot rolled metal from boron-containing 17G1S-U pipe steel are characterized by increased strength properties with preservation of plastic characteristics, due to the formation of predominantly small nonmetallic inclusions and a finely dispersed ferritic-bainitic structure. The absolute values of the yield stress and the time resistance of pipe steel containing in mass %: 0.006 B and 0.003 S are achieved without heat treatment at 585 and 685 MPa, respectively, and meet the X80 strength class, while retaining sufficiently high plastic characteristics. The pipe steel without boron containing 0.01 % of S belongs to the X70 strength class and is characterized by tensile strength lowered to 540 and 610 MPa and a temporary resistance, respectively.

Rutile Electrodes Enhanced with TiO<sub>2</sub> Nanoparticles

In this study, a new method of incorporation of TiO2 nano particles into the rutile coating of shielded metal arc welding (SMAW) is presented. In contrast to the conventional method of mixing of nano particles during coating fabrication, the infiltration by immersion method is used, enabling to modify already fabricated elecrodes. To assess the benefits from the incorporated nano particles, different immersion durations were applied: 30 s, 4 and 8 min. Chemical properties of welds obtained with unmodified and modified electrodes, microstructures, tensile properties, hardnesses, SEM of fracture surfaces and EDX analses of non-metallic inclusions were done. It was found that the highest tensile properties were obtained with medium immersion time, due to the grain refinement in the first (reheated zone) and the second pass (columnar zone). This phenomenon is caused by the presence of complex oxides formed around Ti from the dissociated TiO2. This causes the partial replacement of the Widmanstaetten ferrite with acicular ferrite and subsequent grain refinement in the reheated zone. At the highest immersion duration, the specimens exhibited the lowest mechanical properties due to the appearance of large grain allotriomorphic ferrite isles and large non-metallic inclusions that act as void nucleation sites.

Production of continuous-cast pipe blank without large nonmetallic inclusions

The quality of metal in continuous-cast pipe blanks is investigated, for different steel-production technologies. The presence of large nonmetallic inclusions is largely due to disruptions in the continuouscasting technology. Other significant factors are nonoptimal modification of the nonmetallic inclusions and problems in ladle treatment. Measures ensuring acceptable content of nonmetallic inclusions are developed and tested.

Modification of cellulose and rutile welding electrode coating by infiltrated TiO2 nanoparticles

In this paper, a novel method of infiltration of TiO2 nanoparticles into the coating of the cellulose and rutile shielded metal arc welding electrode is shown. Tensile properties and strength of weld metals were correlated to the chemical composition of the weld metals, ferrite type, and non-metallic inclusion type, size and composition. As infiltration time is increased in the cellulose electrodes, the non-metallic inclusion count increases and their size decreases. They act as inoculants and lead to the replacement of Widmanstaetten with the finegrained acicular ferrite which increases the mechanical properties of the welds. The modification of rutile electrodes with low and medium infiltration time also refines the microstructure and increases the mechanical properties. Specimens welded with rutile electrodes infiltrated at maximum duration exhibited the lowest mechanical properties due to the relatively large non-metallic inclusions that act as void nucleation sites and the appearance of large grain allotriomorphic ferrite in the weld metal.

Evolution of Non-Metallic Inclusions from FeTi70R Alloys during Alloying of Fe-40Ni-20Cr Steels

In this study, the composition, size, and number of large non-metallic inclusions (g20μm) are investigated in a commercial refined FeTi70R alloy, which is used for deoxidation and alloying o ...

Evaluation of maximum non-metallic inclusion sizes in engineering steels by fitting a generalized extreme value distribution based on vectors of largest observations

In the analysis of maximum sizes of large non-metallic inclusions in steels, the common extreme value analysis is only based on a sample of largest observations in control areas on polished planes, respectively. From a simulation study it is observed that this univariate set-up may lead to a high proportion of misspecifications of the true extreme value distribution. Due to their different tail behavior, a falsely determined extreme value distribution will lead to erroneous assertions about quantiles, which serve as characteristic quantities. A multivariate extreme value analysis incorporating data of the r largest observations in every control area will work satisfactorily and should be preferred. The effects of different choices of r are also illustrated by means of a real data set of oxide inclusions.

Effect of TurboSwirl on Inclusions during Ingot Casting of Steels

The use of TurboSwirl to obtain an improved steel cleanliness during filling of an ingot was numerically studied by VOF and DPM models. It was found that a radius-reduced TurboSwirl or a proper tapered mold entrance nozzle with an adequate developed region for steel flow can reduce the risk of mold flux entrapment in a mold. The ingot casting process can create highly turbulent conditions inside the mold during the initial stages of casting. Since the TurboSwirl generates much calmer filling conditions it can promote separation of large nonmetallic inclusions. The TurboSwirl also collects large inclusions (200 μm) towards the axis of rotation, which should promote agglomeration. In addition, the residence time for inclusions of small sizes can be prolonged, increasing chance of agglomeration, which indirectly promotes their separation from steel. Moreover, the average turbulent dissipation rate in an ingot casting swirl setup is about 40 % higher than that in a no swirl setup. This further facilitates the agglomeration of inclusions before they enter the mold. The removal of nonmetallic inclusions is thus enhanced because of an increasing inclusion collision rate due to both Stokes collisions and turbulent collisions, while maintaining a calm flow inside the mold.

Post-irradiation tensile properties of the first and second operational target modules at the Spallation Neutron Source

During neutron production the target module at the Spallation Neutron Source (SNS) is damaged by cavitation-induced erosion and the mechanical properties of the AISI 316L vessel material are altered by high-energy proton and neutron radiation. Recently the first and second operational target modules at the SNS reached the end of their useful lifetime, and disk shaped specimens were sampled from the beam entrance region of both targets. Tensile specimens ranging in dose from 3 to 7 displacements per atom (dpa) were fabricated from the disk specimens using wire electrical discharge machining and tested at room temperature. This paper presents the tensile properties of the irradiated 316L vessel material removed from the first and second operational SNS target modules. Results show an increase in tensile strength and decrease in elongation values similar to previous spallation irradiated 316L results. Abnormally large elongation, 57% total elongation, was observed in a specimen irradiated to 5.4dpa and considerable scatter was observed in the uniform and total elongation data. One possible explanation for the abnormally large elongations and scatter observed in tensile test results is the so-called deformation wave phase transformation-induced plasticity effect. Microscopy characterization revealed the presence of large nonmetallic inclusions rich in Al, S, Ca, O, and Mg on the fracture surface, which may have also contributed to the scatter in the tensile elongation results. While all specimens exhibited radiation-induced hardening and a decrease in ductility, the predominate topographical morphology on all specimen fracture surfaces examined was ductile microvoid coalescence and all specimens experienced appreciable necking prior to fracture. These findings indicate that 316L retains sufficient ductility (10–20% total elongation) and fractures in a ductile manor after irradiation to approximately 6–7dpa in the mixed proton/neutron radiation environment at the SNS.

Multivariate extreme value analysis and its relevance in a metallographical application

Motivated from extreme value (EV) analysis for large non-metallic inclusions in engineering steels and a real data set, the benefit of choosing a multivariate EV approach is discussed. An extensive simulation study shows that the common univariate setup may lead to a high proportion of mis-specifications of the true EV distribution, as well as that the statistical analysis is considerably improved when being based on the respective data of r largest observations, with r appropriately chosen. Results for several underlying distributions and various values of r are presented along with effects on estimators for the parameters of the generalized EV family of distributions.

Cleaness of the First Pipeline Steel Slab along with Casting Direction

To improve the quality controlling of pipeline steel and to avoid mis-down grade, the cleaness including content of non-metallic inclusions, total oxygen and nitrogen of the first continuous casting slags of X70 pipeline steel were investigated. Results showed that, content of total oxygen, nitrogen, large non-metallic inclusions and inclusions areal density decrease with casting length increasing gradually. But at casting length of about 5.0 m which is just during the drastic changeing period of casting speed approaching to stead casting state, the content of large non-metallic inclusions and inclusions areal density increases abruptly induced by the entrapment of mold powders. Thus, to decrease the degradation rate of pipeline slab, the increasing rate of casting speed should be adjusted more evenly.

Increasing the quality of forged ingots via thermal shielding of ingot molds

Thermal shielding of the top one-third of the ingot mold intended for 58-t ingots by a cap-thermos is tested. An increase in the ingot solidification time weakens the development of defects revealed by ultrasonic inspection in the near-sinkhead zone; favors the removal of large nonmetallic inclusions in the axial part of the ingot; weakens the sulfur, carbon, and phosphorus segregation; and excludes the rejection of roll billets in these defects.

Initiation Behavior of Crack Originated from Non-Metallic Inclusion in Rolling Contact Fatigue

Abstract Recently, it has been generally accepted that flaking life is dependent on the size of non-metallic inclusion under proper condition in bearing use. Statistics of extreme values to predict the maximum non-metallic inclusion size and ultrasonic test to assess large non-metallic inclusions in a large volume are widely used as practical methods for the evaluation of bearing steel cleanliness. Murakami’s formula is well known, which describes the relationship between non-metallic inclusion size and fatigue strength. However, the formula cannot be directly applied to rolling contact fatigue because of the difference in applied stress mode. While the equation was invented to predict the fatigue limit in principal stress mode, rolling contact fatigue is mainly caused by shear stress. It is expected that the condition of bearing use will be more critical due to the downsizing tendency in various industrial or automotive machine units. Thus the research on the flaking mechanism becomes more important from the viewpoint of global ecology because that is beneficial to the improvement in bearing life and methods for cleanliness evaluation. Due to the difficulty in experimental observation, however, the details of the flaking mechanism in rolling contact fatigue caused by non-metallic inclusion has not yet been clarified. Focusing on the relationship between non-metallic inclusion and initiation of the crack, the flaking mechanism is proposed in this paper.