S Ghosh Chowdhury

An investigation of the failure of low pressure steam turbine blades

Abstract An analysis of the failure of LP turbine blades of a 210 MW thermal power plant has been presented in this paper. The blade material is of 12% Cr steel with tempered martensitic microstructure. Microstructural analysis as well as hardness and tensile tests did not indicate any degradation in terms of microstructure and mechanical properties. Physical discontinuities were observed in the braze joint which might have been formed due to improper brazing operation. Failure of the brazed joints between the blade and lacing rod was found to be due to improper brazing operations and corrosion effects during service. Fractographic evidence showed that the cracks were initiated from various points on the blade surface, which were at the interface with the lacing rod. Striations and beach marks were also observed which indicated the occurrence of high cyclic loading on the blades. Frequency data obtained from plant indicated the possibility of excessive vibration generated due to fluctuation in grid frequency during operation. Thus, the situation was aggravated due to a resonant condition of vibration, facilitating the propagation of cracks which were initiated earlier.

Mechanical property and microstructure of resistance spot welded twinning induced plasticity-dual phase steels joint

Abstract The present paper deals with the joining of twinning induced plasticity and dual phase steels, carried out by spot welding process with two different pulse durations (200 and 380 ms), by keeping other parameters constant. The microstructural and elemental analysis of the joints was characterised by optical microscopy and electron probe microanalyser. The mechanical properties of the joint were understood from microhardness measurements across the fusion zone as well as through shear and cross-tensile tests. The pulse duration of 380 ms has resulted in good macrostructure with negligible shrinkage void. It also shows smooth transition in hardness across the twinning induced plasticity steel/nugget/dual phase steel interfaces and good mechanical properties as compared to the joint prepared by pulse duration of 200 ms. Larger pulse duration results in sufficient time for the diffusion of alloying elements and yields negligible void formation due to Kirkendall effect.

Evaluation of changes in X-ray elastic constants and residual stress as a function of cold rolling of austenitic steels

Abstract Austenitic steels rapidly attain high mechanical strength when subjected to cold working. The heterogeneous plastic deformation produced in cross section of the specimen, development of preferred orientation and martensitic transformation contribute to the occurrence of residual stress in cold worked steels. AISI 304 and 316 steels were cold rolled at room temperature from 10% up to 70% deformations in steps of 10%. The formation and sigmoidal growth of martensite caused by cold rolling (CR) 304 steel was studied by X-ray diffraction. The residual stresses generated were evaluated in both the austenite and martensite phases using sin2 ψ technique. The accurate determination of residual stress by X-ray diffraction requires experimental determination of X-ray elastic constants for both the austenite and martensite phases. The changes in X-ray elastic constants as a function of CR of 304 and 316 steels were measured and their effect on residual stress values was established. The results show that tensile stress was generated initially on cold working in the austenite phase in both steels and in the dominant martensite phase in 304 steel, which decreases, passes through zero and becomes compressive at higher deformations. X-ray elastic constants were found to decrease in all cases and a maximum reduction of 15% was found.

Failure analysis of a weld repaired steam turbine casing

The present study was aimed at analysing the failure of a weld repaired turbine casing after 30 years of total service including 5 years after weld repair. The casing was weld repaired by a high Cr–Ni weld metal (24Cr–32Ni–4Mn–Fe) . The base metal, a low alloy ferritic steel (1Cr–0.5 Mo steel) with ferrite–pearlite structure did not show any abnormality to indicate significant degradation. Fracture surface showed voids and microcracks. The cracks might have initiated from the voids present in the weld region possibly by thermal fatigue mechanism. The high level of thermal stress appears to be operative due to thicker section of the casing and also due to high mismatch of thermal expansion coefficients of the base metal and the weld metal. Dilution of alloying elements took place and δ-ferrite was observed at the grain boundaries as predicted by the Schaeffler diagram. The δ-ferrite was transformed to σ-phase and alloy carbides during high temperature service (500°C) . The propagation of cracks generated by thermal fatigue was facilitated by the formation of embrittled σ-phase at the austenite grain boundary and this ultimately led to an early failure of the casing.

Premature failure of a leaf spring due to improper materials processing

Failure analysis of a leaf spring which failed prematurely during service was carried out using optical and scanning electron microscopy, hardness and tensile testing, and residual stress evaluation by X-ray diffraction. Quench cracks, which are aligned normal to the length of the leaf, have been identified in shot peened and polished surfaces. These cracks have been attributed to an improper quenching process. It is established by theoretical analysis that the leaf thickness is smaller than the critical plate thickness required for this composition of steel, and that leads to an increase in quench severity. It appears that some of these quench cracks have propagated by a fatigue mode which is confirmed by the presence of beach marks on the fractures surface. Observation of intergranular cracking and the presence of FeS inclusions at the prior austenite rain boundaries implies that some sort of grain boundary embrittlement might have facilitated crack growth and led to failure. It has been suggested that quenching should be carried out by recommended procedures guided by the thickness of the component and chemical composition of the steel. Careful inspection of the surface after quenching must also be carried out to maintain quality assurance in order to avoid premature failure.

Quantification of deformation induced α’-martensite in Fe–19Cr–3Mn–4Ni–0.15C–0.15N austenitic steel by in situ magnetic measurements

Abstract An in situ magnetic device was employed to quantify the deformation induced martensite in a Fe–19Cr–3Mn–4Ni–0.15C–0.15N (wt-%) steel during tensile testing in the temperature range of −40 to 22°C. The new device consists of an electromagnetic field which serves to magnetise the martensite phase as it forms during tensile loading and a second coil to detect the effective electrical potential difference induced by the magnetisation of tensile specimens. To implement the in situ measurement system, a correlation was necessary between the induced electrical potential difference and the deformation induced martensite fractions during uniaxial static tensile tests. The correlation procedure was found to require only the quantification of deformation induced martensite content in a tensile specimen strained until fracture using an ex situ magnetic saturation unit.

Remaining life estimation of a service exposed economiser tube

A service exposed (80,000 hr) economiser tube of a 60 MW boiler of a thermal power plant was investigated for the detailed metallurgical assessment necessary for remaining life estimation. The investigation included hot tensile tests, hardness measurement, microscopy and thickness measurement. Analysis revealed that there was no degradation of mechanical properties and microstructural features. The reduction in tube thickness was primarily due to oxidation and corrosion/erosion processes. Taking into account all the experimental data and mechanics aspects of the tube an estimated remaining life of 20,000 hrs was calculated and thus recommended for further use.

Microstructural and textural evolution during large strain hot rolling (LSR) of Mg–Al (AZ31) alloy

Abstract Grain refinement has been achieved through large strain rolling (LSR) in Mg AZ31 alloy. The evolution of microstructure and texture has been found to be dependent on the amount of reduction. After the critical amount of reduction, grain refinement proceeds through continuous dynamic recrystallisation (CDRX).

Investigation of sub-surface cracks in continuous cast billets

Sub-surface cracks in the continuous cast product of steel were studied to understand the causes of such defects and to suggest remedial measures. Even after hot rolling the cracks were not welded. The diameter of the as cast billets were 225 mm and most of the cracks were located around l cm away from the edges. The cracks were visible with the naked eye at transverse sections. The cracks were not continuous but scattered. The objective of this work was to identify the root cause of the formation of the sub-surface defects on the concast product and to ascertain whether it was due to any material related problem or due to any other cause such as processing. The existing cracks were opened through mechanical testing. The fracture surfaces were observed by SEM. Metallography and SEM observations revealed that the cracks were generated due to preferential segregation of sulphur, Cu and subsequent formation of various types of sulphide inclusions. The presence of a higher amount of Cu, particularly in the cracked region is of great concern. Cu is very harmful in steel if it exceeds the tolerable limit, particularly with the presence of high amount of S. The sulphide inclusions are found at the existing cracked regions. Inclusions at the crack is indicative of a correlation between inclusions and cracks. The inclusions act as a second phase and may influence the solidification of the matrix by restriction in directional solidification, differential contraction and even acting as a source of stress concentration—hence the appearance of cracks. The location-specific segregation of the inclusions is however surprising. Had there been dispersion of inclusions, probably the cracks would not have resulted.

Failure Investigation of Boiler Water Wall Tubes of a Thermal Power Station

AbstractFailure investigation was carried out on boiler water wall tubes of a thermal power plant through visual inspection, chemical analysis, and metallurgical analysis. Failure was in the form of thin/micro cracks along the length of the tubes which were located at the girth welding joint of tubes. Experimental results revealed that the cracking was from inward to outward of the tube thickness. Discontinuities/cavities were observed in the welded region which might have occurred due to lack of fusion of base metal and the weld metal. Cracks were initiated from the sharp corner/crack tip of the cavities/discontinuities present at the welded region under the action of hoop/thermal stress existed during the operation. Nature of the crack propagation indicates the case of typical hydrogen-induced cracking. Moreover, the presence of the cavities/discontinuities reduced the cross-sectional area of tubes resulting increased stress intensity. Increased stress beyond the flow stress of the material assisted by hydrogen-induced effect resulted the cracking of the tubes. In order to mitigate the problem, proper welding of tubes joints should be carried out followed by proper inspection after weld. Secondly, hydrogen dissolution during welding should be prevented and treatment for its removal after welding should be carried out.