Fracture Behaviour of Copper Strengthened HSLA Steel

High strength, high toughness and good weldability are the major criteria for engineering structural materials. HSLA-100 steel is essentially low carbon microalloyed and copper-strengthened high strength steel. It is one of the important materials in naval, and others structural applications in which high strength (UTS >1000 MPa) and good toughness can be obtained without impairing weldability by judiciously engineering the microstructure. This steel can provide various strength–toughness combinations over a wide range of plate thickness by different heat treatments. Available literatures concerning the influence of heat treatment on microstructure and mechanical properties are confined to the characterisation of microstructure and the variation in mechanical properties such as tensile strength and impact toughness. Such mechanical property evaluation is insufficient to characterise the mechanical behaviour of the materials. Modern engineering design approach demands a fracture mechanics based characterisation of material behaviour

Failure Analysis of Welded Reformer Tubes of a Fertilizer Unit

A fertiliser unit experienced cracking of the circumferential weld of H39WM heater tubes at the hottest region along the vertical direction and particularly those closer to the wall. The cracks occurred in the internal side and observed to grow along the circumference. Longitudinal cracks were also seen in the inner wall. A few tube samples were examined to understand the cracking problem. The service exposed tubes show changes in carbide morphology and isolated creep voids are seen in the base metal and regions closer to the weld fusion line (heat affected zone). The longitudinal cracks are very close to the weld and creep void alignment away from the weld is very less. The circumferential crack show creep voids ahead of the crack and in regions close and parallel to the main crack. Isolated creep voids were observed mostly in the inner wall region and on the outer wall creep voids are absent. The creep cracking occurred in the weld of the hotter section as weld is the weakest link in the tube column

Failure of evaporator tubes initiated by lamellar tearing during the commissioning of a waste heat recovery boiler

Successive failures of several new evaporator tubes during commissioning and trial run of a waste heat recovery boiler has been analyzed. The evaporator tubes are cold bent into U-shape and subsequently TIG welded to a 24-mm thick wear plate on either sides of the tube immediately adjacent to the bend. The failed samples showed stepped longitudinal cracks along the rolling direction and parallel to the weld fusion boundary at the fireside surface of the inner bend of the tube,prominently at the center of the bend. It was found that the failure of the evaporator tubes at the tube bends was initiated by lamellar tearing because of inherent defects in the material (segregation leading to banded structure), improper processing of material (inclusions with high aspect ratios along the rolling direction) and accelerated by high transverse shrinkage stresses over the entire bend portion of the tube introduced by the weld. The pre-existing fine cracks in the welded evaporator tube initiated by lamellar tearing have subsequently opened up during service when the total strain in the region increased because of steam pressure resulting in catastrophic failure of the tubes. Contrary to expectation, many of the elongated inclusions were found to be iron oxide scales and only few were manganese sulfide stringers enveloped by iron oxide scales. The iron oxide inclusions were resolved from the manganese sulfide by scanning electron microscopy and EDS spot analysis of the inclusions

Investigation of caving-in incident of MIG 21 aircraft

In an accident, the starboard (STBD) side of front fuselage was ‘caved in’ during ground run of the Mig-21. A post-incident inspection revealed vertical and horizontal cracking occurred at the edges of the additional air intake shutter (AAITS). The guard of the AAITS was sucked in causing extensive damage to the aeroengine. However, port side fuselage remained un-affected. The chemical composition and evaluation of the mechanical properties of the alloy match with the Russian specification D16AT, D16ATB. It means that the duralumin alloy is in hardened and aged condition. Characterization of microstructure in optical, SEM and TEM illustrate more or less negligible variation for fresh and service exposed material. From the investigation it can be inferred, on the starboard side near some of the rivet holes at the edges of AAITS opening, primary cracks were formed owing to fretting fatigue. The structural non-uniformity of annular area on the starboard side with respect to port side raised the air pressure around the frame #5 and the skin ‘caved-in’. As engine demanded more air, under severe suction pressure, the skin containing micro-cracks became unable to withstand the load and the guard was sucked in from outside towards the engine causing overload failure to the areas away from rivet hole and rest of the zones near the AAITS

Analysis of damage in high strength steels

In continuum damage mechanics (CDM) approach, damage accumulation takes place through void initiation, growth and coalescence. In this study, a Bayesian Neural Network based model has been developed to calculate the complex relationship between the extent of damage accumulation and its influencing parameters for a variety of high strength low alloy steels. The model has been applied to confirm that the predictions are reasonable

Turbine blade failure in a thermal power plant

The failure of a LP (low pressure) turbine blade of a 220 MW thermal power plant is presented. The blade was made of martensitic stainless steel and the structure was tempered martensite. There was no evidence of degradation of blade material. The fracture took place at the aerofoil region, 113-mm from the root. Throughout the blade surface Si rich phases were detected. Several pits/grooves were found on the edges of the blades and chloride was detected in these pits. These were responsible for the crevice type corrosion. The probable carriers of Cl− were Ca and K, which were found on the blade. The failure mode was intergranular type. Possibly the ultimate failure was due to corrosion-fatigue

Bamboo—A functionally graded composite-correlation between microstructure and mechanical strength

Bamboo is supposed to be one of the best functionally gradient composite materials available. In a piece of bamboo, not only the number of fibres (‘vascular bundles’) but also the fibre quality varies from outer to inner-most periphery. It has been observed that near the outer periphery, within 1 mm2 area, the number of fibres is approximately 8 whereas the same at the inner-most periphery is approximately 2. Again the cross-sectional shape of fibre at outer periphery is almost circular (diameter 0.14 mm) and compacted but at the inner-most periphery, a fibre (diameter of major axis 0.93 × diameter of minor axis 0.78 mm) has been sprayed, and contain matrix in it. This structural behaviour causes the variation of tensile strength, e.g., the strength of a fibre at the outer periphery is about 160 kg/mm2 and the same at the inner-most periphery is only 45 kg/mm2. It has also been observed that the matrix of bamboo can preferentially be removed from the fibre by alkali treatment. 10% NaOH can remove adhered matrix with little effect on fibres while 20% or stronger alkali reduces the strength of fibre

Some studies on olivine fines and clay mixtures

Olivine is a mineral composed of forsterite (2 MgO.SiO2) as major and fayalite (2 FeO.SiO,) as minor phases. During pro¬cessing of olivine minerals for foundry and other uses, lot of fines (around -100 mesh) are generated. The present study has been carried out on the olivine fines collected from the Salem district of Tamil Nadu to explore the possibilities of converting these fines into value added ceramic products by reaction sintering with clan Different combinations of olivine fines and clay have been formulated and the mixtures are heated at various tempera¬tures in the form of pellets. The sintering kinetics and physical properties have been studied and discussed in this paper. XRD & SEM and studies are also carried out to confirm the phases formed