Slag Inclusions Research Articles

Development and use of layered ferrous microstructure

The method by which bloomery iron was agglomerated into usable artefacts by forge welding, piling, and reforging gave a layered microstructure. This was not only associated with the directionality of residual slag inclusions, but also related to variation in solute concentration, which became directional. This variation could originate in the bloom, or it could be developed by the partitioning of solutes such as phosphorus between coexisting high temperature phases. The oxidation enrichment of solutes such as Ni and As in individual surfaces, before forging together, could also lead to a layered microstructure being produced. On a larger scale, the deliberate use of differently sourced irons to make a composite product could also generate a sandwich structure.Although many early iron artefacts contain substantial amounts of phosphorus overall, the layered microstructures that phosphorus segregation generates may have offset the embrittling effect that could otherwise be expected in the presence of some carbon. The purposeful selection of high phosphorus iron (identified from effect and source) for the backs of knives, etc., could have been intended to keep a stable, high carbon content in the steel edge to which the iron was welded. The final distribution of carbon in such a forged, banded microstructure is responsible for the pattern welding or damascening of sword blades where controlled forging, folding, and twisting is carried out, the variation in hardness and etching characteristics enabling a desired pattern to be revealed at the final surface. Banded or layered microstructures also occur in wrought steels produced from cast ingots, which have variation in concentration of solutes (e.g. phosphorus) originating from solidification microsegregation. This segregation was also significant in the development of dual phase steels, of importance for the production of car bodies, where the use of an intercritical quench results in banded martensite-ferrite microstructures, and subsequent low temperature tempering precipitates excess carbon from the ferrite. Such a sheet structure work hardens more rapidly than normal sheet of the same or somewhat greater carbon content, which is appropriate for the small amount of forming usually required.Attempts have also been made to produce a tough, more stable, layered microstructure synthetically by roll bonding layers of aluminium stabilised ferrite with high carbon steel. Although wires of martensite cored ferrite made in a similar manner gave promising results of substantially higher plastic strain to fracture, attempts to swage skeins of such wire to rod were unsuccessful. Layered austenite-martensite structures can also be produced by the choice of suitable compositions. In the historical Malayan kris dagger, layers of a high nickel content iron were developed which stabilised the austenite phase, as in recent work on a composite based on nickel coated components for rapier blades and for improved resistance to stress corrosion cracking in wider use.

Stability of weld metal subjected to cyclic static and seismic loading

Brittle fracture of the welded connections was observed in the steel framed buildings in the vicinity of Northridge, California after the 17 January 1994 earthquake. Fractographic examination of numerous beam-column moment connections revealed that the fractures initiated from a notch-like defect formed when the backing bar was left in place. The backing bar notch was enlarged when poor workmanship resulted in adjacent lack-of-fusion and slag inclusions. The problem was further exacerbated when low notch toughness weld metal was deposited that was unable to prevent initiation of unstable crack extension or brittle fracture.

Fatigue life prediction of welded structures containing non-planar flaws using Quality Category under non-constant amplitude loading

The prediction of fatigue life is important for welded structures. A new fatigue life prediction method of welded structures containing non-planar flaws was developed in this article. The new method is based on the Quality Category of PD6493 and Miner's liner damage rule. It accounts for the maximum height or width of slag inclusions, and the percentage of projected area on a radiograph of porosity and fatigue loading. Fatigue life prediction is easily implemented by the method. Further, a new modification method based on past-experienced data is presented for practical applications. The application of this method is illustrated with Chinese carbon manganese steel.

Slag Analysis as a Method for the Characterization and Provenancing of Ancient Iron Objects

About 900 slags and iron objects from the period 700 b.c. to 1850 a.d. have been examined and analyzed by classical optical microscopy and by energy dispersive analytical methods. Although ancient iron objects are extremely heterogeneous, a definite correlation between the metal phase and its slag inclusions is shown; for example, wüstite-rich slags are located in ferrite, whereas glassy slags are located in pearlite. The ratios SiO 2/Al 2O 3, Al 2O 3/CaO, and so forth, of the slag inclusions are shown to be helpful in identifying the production site or provenance of an ancient iron object. These ratios are further shown not to be the same in irons produced by the direct process and in irons produced by fining or puddling. Emphasis is on iron production methods in Sweden, Norway, and Denmark, illuminating Scandinavian iron-making practices through 2500 years. The slag analysis method developed here may, however, be extended to yield information on other European sites.

Microstructural features limiting performance of PM steels: H.Danninger et al. (Technical University of Vienna, Austria.) Int. J. Powder Metall., vol 33, no 4, 1997, 43–53

Pressed and sintered structural steels exhibit mechanical properties that are markedly inferior to those of their wrought counterparts. This is due primarily to the unique microstructure of powder metallurgy products which includes porosity as the pivotal feature. It is shown here that porosity results in an out-of-proportion decrease in the effective load-bearing cross-section and this microstructural parameter is related directly to the mechanical properties. Interconnected porosity is more detrimental than isolated porosity, resulting in isolated sintered necks that are mostly small compared to the powder particles size and are thus unable to sustain higher loads. Also the distribution of porosity is rarely homogeneous, and the high porosity regions act as crack initiation sites particularly in cyclic loading. Fatigue fracture is also caused by singular defects such as secondary pores generated via a transient liquid phase or by slag inclusions. The effect of these inhomogeneities is more pronounced, the higher the strength of the material.

Failure Analysis of the Chassis Beam of a Light Commercial Vehicle/ Schadensanalyse an einem Fahrgestellrahmen eines leichten Nutzfahrzeugs

The failure analysis of prematurely failed chassis beam of a light commercial vehicle was carried out. The metallographic examination of the material indicated high levels of slag (sulphide and silicate) inclusions and uneven distribution of ferrite and pearlite phases. The failure has essentially taken place through the bolt holes. The analysis suggested that inferior quality material led to the failure, whereas the bolt holes might have acted only as stress concentration centres for the initiation of the cracks.

Detection of weld defects by computer aided X-ray radiography image processing : Suga, Y.; Kojima, K.; Tominaga, T. Proceedings of the 4th (1994) International Offshore and Polar Engineering Conference, Osaka (Japan), 10–15 Apr. 1994. Vol. IV, pp. 431–440. Edited by Y. Ueda, J.F. Dos Santos, I. Langen, Y. Tomita and K. Waagaard. ISOPE (1994) ISBN 1-880653-14-1.

The X-ray radiographic testing method is often used as a non- destructive testing method to detect weld defects, although it is difficult to detect small defects from the X-ray film. However, a computer visual image processing system can give an objective assessment, and has a high speed judgement. An image processing system to abstract weld defects from the X-ray radiography film including noises was constructed. This paper describes the image processing methods for detecting defects such as pores, slag inclusions and cracks, and displaying the defects using computer graphics.

Classification of Welding Defects in Austenitic Stainless Steel by Neural Pattern Recognition of Ultrasonic Signal

The research for the classification of the natural defects in welding zone is performd using the neuro-pattern recognition technology. The signal pattern recognition package including the user's defined function is developed to perform the digital signal processing, feature extraction, feature selection and classifier selection, The neural network classifier and the statistical classifiers such as the linear discriminant function classifier and the empirical Bayesian calssifier are compared and discussed. The neuro-pattern recognition technique is applied to the classificaiton of such natural defects as root crack, incomplete penetration, lack of fusion, slag inclusion, porosity, etc. If appropriately learned, the neural network classifier is concluded to be better than the statistical classifiers in the classification of the natural welding defects.

Gold jewellery casting: Technology design and defects elimination

Investment casting is widely used in the production of gold jewellery. In contrast to the state of other metal casting industry, however, their technological characteristics such as gating, melting and pouring are given little consideration. In this article, the technological characteristics of 18K (carat) gold alloys have been studied and are discussed. Some defects such as shrinkage porosity, gas porosity, slag inclusion and plaster hole have been investigated. Their typical characteristics, the factors that cause them and the methods that can be employed to avoid their occurrence are presented.

Defect characterization in pipe-to-pipe welds in large-diameter stainless-steel piping

Metallurgical evaluation of pipe-to-pipe welds in large-diameter, Type 304 stainless-steel piping used to construct the moderator/coolant water systems for Savannah River Site reactors has demonstrated that small weld defects found in this 1950-vintage system do not compromise the integrity of the system. The weld defects were too small for detection by the preservice standard radiographic inspection, but were found through systematic ultrasonic testing and penetrant testing evaluations of piping that had been removed during upgrades to the piping system. The defects include lack of weld penetration, slag inclusions, and other weld metal discontinuities. These discontinuities typically did not propagate during more than 35 years of service. The defects examined were too small and isolated to degrade the mechanical properties of the pipe-to-pipe weldments and, therefore, did not compromise the integrity of the piping system.

Influence of Weld Discontinuities on Strain Controlled Fatigue Behavior of 308 Stainless Steel Weld Metal

Detailed investigations have been performed for assessing the importance of weld discontinuities in strain controlled low cycle fatigue (LCF) behavior of 308 stainless steel (SS) welds. The LCF behavior of 308 SS welds containing defects was compared with that of type 304 SS base material and 308 SS sound weld metal. Weld pads were prepared by shielded metal arc welding process. Porosity and slag inclusions were introduced deliberately into the weld metal by grossly exaggerating the conditions normally causing such defects. Total axial strain controlled LCF tests have been conducted in air at 823 K on type 304 SS base and 308 SS sound weld metal employing strain amplitudes in the range from ±0.25 to ±0.8 percent. A single strain amplitude of ±0.25 percent was used for all the tests conducted on weld samples containing defects. The results indicated that the base material undergoes cyclic hardening whereas sound and defective welds experience cyclic softening. Base metal showed higher fatigue life than sound weld metal at all strain amplitudes. The presence of porosity and slag inclusions in the weld metal led to significant reduction in life. Porosity on the specimen surface has been found to be particularly harmful and caused a reduction in life by a factor of seven relative to sound weld metal. Defect combination of porosity and slag inclusions was found to be more deleterious than the case when either the slag inclusions or porosity was present alone. Discontinuties acted as crack initiation sites and also enhanced crack propagation. The LCF properties of weld samples containing discontinuities have been correlated with the damage and fracture behavior.

Recognition and Display of Weld Defects in X-ray Inspection Film by Image Processing.

A new computer-aided system to detect weld defects in an X-ray inspection film, such as blowholes, slag inclusions and cracks, and to display them on the CRT is constructed. That is, a method involving the input of a high-contrast image to a computer, followed by the subtraction of the background image from the original image to enhance defect images by means of a two-dimensional filter, is proposed. Moreover, the effectiveness of the systems proposed is confirmed by applying them to X-ray inspection films of welds obtained from the submerged arc welding process.

Early Iron Age Metal Working at the Tsodilo Hills, Northwestern Botswana

This paper describes Early Iron Age metal working at the sites of Divuyu and Nqoma, excavated by James Denbow and Edwin Wilmsen, in northwestern Botswana. This research represents the first metallographic analysis of a large collection of Iron Age metal artefacts from southern Africa. The artefacts studied were small articles of copper and iron, mostly items of jewellery and tools possibly used in their manufacture. They were classified and sampled representatively for metallographic and chemical analysis. Fifty-four artefacts were studied in detail and the fabrication technology reconstructed. The forging, and possibly also smelting, of iron took place at both sites during the Early Iron Age. The primary smelted product was inhomogeneous iron and steel, with typical fayalitic slag, characteristic of indigenous bloomery iron production. Forging was carried out in an oxidizing hearth. The technique used was poor, with no evident control over carbon content or the mechanical properties of the steel, and with no heat treatment other than annealing. The fabrication of artefacts involved forging square wire and flat sheets, which were cut into strips for beads, clips, chains, and fibre-cored wound ornaments. Crude round iron wire was used to make numerous finger rings. Copper was worked similarly, usually leaving the metal in its annealed state. Significant chemical variation in the copper artefacts and iron slag inclusions showed that diverse ore sources were exploited. There were stylistic similarities between individual artefacts from these sites in the Tsodilo Hills and Early Iron Age material from Zaı̈re and the northern Transvaal. Comparison of the fabrication technology with Late Iron Age material suggests that local indigenous iron and copper working technology in southern Africa has changed little since its inception.

FATIGUE BEHAVIOUR OF STEEL CASTINGS CONTAINING NEAR‐SURFACE DEFECTS

AbstractManufacturing defects in the near‐surface region of 2¼% Cr 1% Mo castings were investigated in a joint research programme. After ultrasonic testing and X‐ray inspection, large push‐pull specimens (cross section 1000 mm2) were taken from castings in regions with indications of defects and tested under fully reversed loading in the elastic and elastic plastic region up to predetermined levels of defect growth. The defects are micropores, pin holes, slag and oxide layers or inclusions, hot tears and small micro cracks in welds.The initiation of crack growth was analysed by two methods, namely (a) the local strain concept with idealization of defects as three‐dimensional elliptical notches and (b) by the fracture mechanics concept with idealization of defects as two‐dimensional elliptical cracks. The main results can be summarized as follows:1. The local strain concept describes the crack initiation potential of the defects more accurately than the fracture mechanics concept.2. Interpretation of casting defects as cracks produces very conservative estimates in many cases.3. Analytical evaluation of crack growth behaviour using the fracture mechanics approach for defects experiencing crack growth of at least 1 mm is much more accurate than when defects having a smaller extent of crack growth are included in the analysis.Additional evaluations of crack growth behaviour of hot tears by fracture mechanics show that crack growth starting from open and partially closed hot tears must be subjected to a larger number of cycles than normal fatigue cracks of sound material. Partially closed hot tears in the vicinity of the crack front additionally delay crack propagation.

Quasi pulse-echo ultrasonic technique for flaw classification

One of the important flaw characterization tasks in the field of ultrasonic non-destructive evaluation (NDE) is to provide flaw type information by analysing the flaw responses acquired during an inspection. Here a new quasi pulse-echo ultrasonic classification technique is presented which utilizes the time separation and amplitude difference of mode-converted diffracted signals to distinguish between smooth and sharp-edged flaw geometries. Experiments with cylindrical cavities, surface-breaking fatigue cracks and slag inclusions have been used to test the practicality of this approach. All results of these tests show good consistency in the separation of smooth and sharp-edged flaws, provided that the signal-to-noise ratio is sufficient. Furthermore, the scattering feature used for classification in this method is also verified by detailed elastodynamic scattering calculations.

Fatigue crack initiation from defects at weld toes in steel

The mechanisms of fatigue initiation and microcrack growth at the toes of good quality manual and automatic T-plate welded joints in steel were investigated, using a compliance method to measure crack depth and closure levels, and three-stage ink staining, replica techniques and SEM microscopy to monitor crack growth. Minor undercuts, slag inclusions and surface spatter at the weld toes were detected as fatigue initiation sites in manual welds. Fatigue initiation in automatic welds was mainly associated with undercut-type defects. Details of their propagation and coalescence early in the fatigue process are documented. Delayed microcrack coalescence in the case of manual welds appears to be related to the more irregular toes of the manual welds, when compared to their automatic counterparts.

The significance of weld toe undercuts in the fatigue of steel plate T-joints

The weld toe undercut is one of the defects which can initiate fatigue cracks in welded offshore structures. However, its significance as compared with other stress raisers, such as slag inclusions and local weld toe geometry is not known. This paper investigates the effect of weld toe undercut depth on the fatigue life of welded plate T-joints loaded in three-point bending. The development of the fatigue cracks is monitored using ACPD measurement techniques.

裏当金を有する溶接継手の疲労強度に及ぼすルート形状の影響の定量化

It is known that stress concentration is occured at root of welded joint with backing strip by misalignment, as the misalignment causes eccentricity to load. However, it is hard to avoid misalignment in field welded joints of steel deck plate.In this paper, fatigue behaviors were investigated on welded joints with backing strip which were fabricated by overhead position.The stress concentration factors of various root shapes were analyzed by the finite-element method. Estimating equation from root radius and root angle was derived based upon FEM results.The fatigue strengths for 2 million cycles were calculated from the fatigue test results, then the stress concentration factor for required fatigue strength was obtained.The results obtained in this reports are as follows;1. The fatigue lives of specimens with misalignment 2.0 and 3.0 mm were inferior to that of specimens without misalignment, and the fatigue life was largely decreased at misalignment 4.0 mm for manual weled and CO2 welded joints.2. Even slag inclusion and blow hole were existed in weld metal, they did not cause crack initiation. The fatigue cracks were mainly initiated at the weld root of specimen.3. The stress concentration factor was mostly influenced by the misalignment.4. Whether the root shape was convex or concave, the stress concentration factor was n ainly influenced by shape of toe at base metal side.5. The stress concentration factor can be obtained by the estimating equation, which consists of eccentricity to load and shape of toe at the base metal side.6. The stress concentration factor to satisfy required fatigue strength was given in relation to toe radius and flank angle.

Linguistic assessment of welded structures with fatigue damage

Frequently in fatigue damage assessments of welded structures, the available information includes linguistic descriptions of the damage and numerical results from tests and analyses. The precision of the numerical results and the precision of the linguistic descriptions may be inconsistent. However, the incorporation of both types of information is important for a meaningful assessment of structural damage. A linguistic damage assessment technique is presented to incorporate both forms of information into the assessment procedure. The variables used in the assessment are described with linguistic terms that are represented with fuzzy membership functions. A damage assessment function is then determined using fuzzy quantification theory and is used to determine the damaged state. A criterion function is proposed to determine the effects of procedural variables on the assessment function. Finally, the technique is demonstrated using experimental data on the effects of slag inclusions in welded joints.

Weld Investigation of Nuclear Power Plant Pressure Vessels

Welds in nuclear power plant pressure vessels, which were in accordance with the ASME Boiler and Pressure Vessel Code, though not with some other codes, were investigated nondestructively over a long period of service. The results indicate that the vessels have been operating for many years, with large slag inclusions that do not extend to the weld surfaces, causing no damage to the vessels. Hence, the practical relaxation of excessively conservative acceptance standards is discussed.