FE Simulation of Center Crack Occurrence in Tube Rounds During Two-Roll Rotary

With the aid of FE (finite element) code MSC.Superform 2005, 2-D coupled thermo-mechanical simulation of center-crack occurrence in round billet during 2-roll rotary rolling process was presented using Oyane ductile fracture criteria. A simple modeling is put forward based on the spiral motion of the workpiece as an essential characteristic in movement. The influence of the feed angle and the entry cone angle of the main roll on the process was taken into account in the modeling. The soundness for simplifying the 3-D rotary rolling into a 2-D problem was discussed. By adopting the parameters of Diescher piercer in 140mm mandrel mill of Bao Steel, the distribution and development of strain / stress were analyzed, and the eigen value of ductile fracture as well. The critical percentage of diameter reduction was obtained from the simulation. The result showed a good agreement with the experimental value, and therefore was of widely guiding significance to the practical process for rationally formulating the deformation parameters of steel tube piercing.     

PARAMETERS OPTIMIZATION OF CONTINUOUS WAVELET TRANSFORM AND ITS APPLICATION IN ACOUSTIC EMISSION SIGNAL ANALYSIS OF ROLLING BEARING

Morlet wavelet is suitable to extract the impulse components of mechanical fault signals. And thus its continuous wavelet transform (CWT) has been successfully used in the field of fault diagnosis. The principle of scale selection in CWT is discussed. Based on genetic algorithm,an opti-mization strategy for the waveform parameters of the mother wavelet is proposed with wavelet en-tropy as the optimization target. Based on the optimized waveform parameters,the wavelet scalogram is used to analyze the simulated acoustic emission (AE) signal and real AE signal of rolling bearing. The results indicate that the proposed method is useful and efficient to improve the quality of CWT.     

INHIBITION MECHANISM OF Na2MoO4 FOR CARBON STEEL IN 55% LiBr SOLUTION

Electrochemical measurement and chemical immersion test are employed to investigate the inhibition mechanism of Na_2MoO_4 for carbon steel,which is studied in 55%LiBr+0.07mol/L LiOH solution at high temperature.Testing results indicate that the passive film on carbon steel surface is mainly composed of Fe_3O_4,and Mo is involved in the process of film forming,in a form of MoO_3 and MoO_2.The parametric analysis of the evolving potential and the blend potential of MoO_2 shows that Mo mainly exists as MoO_2 in passive film when the concentration of Na_2MoO_4 is lower than 150 mg/L.While its concentration is greater than 150mg/L,it mainly exists as MoO_3 inside the film and exists as MoO_2 outside the film.MoO_4~(2-) is deoxidized as MoO_2 on the surface of carbon steel,which may impede the corrosion of active-site and raise the blend potential of carbon steel,and then MoO_4~(2-) adsorbs onto the defects of the passive film and decompounds as MoO_3 during the process of film forming.The electric field caused by different valence of Mo in passive film may retard the dissolution of carbon steel and lead to an increase in the polarization impedance and a decrease in hydrogen evolution.As a result,heavy concentrated Na_2MoO_4 solution(greater than 150mg/L)has excellent inhibition effect on carbon steel in LiBr solution.     

A NOVEL APPROACH FOR THE PREDICTION OF SURFACE PROFILE OF OUTGOING WORKPIECE AND THE CALCULATION OF MEAN ROLL RADIUS IN ALLOY BAR ROLLING

In alloy bar rolling process, the component of alloyed steel influenced the spread coefficient greatly, therefore, the component influence coefficient m of different alloyed steel has been determined firstly to calculate the maximum spread. Then the curvature radius of stress free surface and the ＂critical point on the contact boundary＂ have been solved, the surface profile of outgoing workpiece has been obtained. Furthermore, the formula of the equivalent contact section area has been proposed and the mean roll radius has been calculated. The bar rolling experiment and the rigid-plastic FEM （finite element method） simulation have been carried out to verify the novel approach. Compared with experimental data and simulation results, the novel approach can be used in setting processing parameter and design of finishing groove.     

Thermal Stability of Non-Equilibrius Microstructure in Microalloyed Steel During Reheating

Cooled in water after isothermal relaxation of deformed austenite for different times, an Nb-bearing microalloyed steel always exhibits synthetic microstructures, in which bainitic ferrite dominates. Dislocation configurations and distributions of strain induced precipitates inside bainitic ferrite of samples relaxed for different times were distinct. When compared with the austenite model steel, which maintained fcc structure even at room temperature, the strain induced precipitates were not found in the sample without relaxation whereas these were distributed outside dislocations in sample relaxed for 1000s. Most of the strain induced precipitates distribute along dislocations and pin dislocations in sample relaxed for appropriate time. After bainitic transformation, the dislocations formed in deformed austenite remain to be pinned by the precipitates. When these samples were reheated to and held at 650 or 700℃, the non-equilibrious microstructures tended to evolve into equilibrious ones. The sample relaxed for 60s displayed the highest thermo-stability, whereas microstructure evolution was the quickest in the sample relaxed for 1000s even though it was the softest prior to reheating. Dislocations inside laths got rid of pinning of precipitates, and their polygonization became the precursor to the evolution of microstructures during reheated and held, followed by gradual disappearance of lath boundaries caused by dislocation climbing. Finally, recrystallization occurred and polygonal ferrite appeared. By hardness measurement, it was found that softening is not a single process occurring during reheated, in which hardness fluctuates with time. There were two peaks in the hardness-time curve of each sample having undergone relaxation, while single peak occured in the curve of the sample not being relaxed. These results indicated that the thermo-stability of microstructures was determined by their history of formation to a considerable degree.     

Cathodic Protection and Stress Corrosion Cracking of Mild Steel District Heating Distribution Systems

Abstract

Inadequate design or construction of certain district heating distribution systems has led to severe corrosion problems. These systems were then cathodically protected by means of impressed current installations, and the frequency of leaks was thereby reduced. However, following this, a number of failures due to stress corrosion cracking have been discovered. This has been traced to the alkaline environment round the pipe produced by the cathodic protecion process, combined with subsequent concentration by evaporation, the potential of the pipe then being in the range known to promote caustic cracking when residual stresses are present in the pipe. The only possible solutions to the problem for existing pipelines are either to remove the cathodic protection and revert to the likelihood of general corrosion problems, or to control the applied current so that the pipeline is kept out of the potential range known to promote stress corrosion. In practice this is difficult to achieve and may result in parts of the pipeline being under-protected.     

Erosive abrasive wear behaviour of stainless steel surface produced by plasma transferred arc hardfacing

Abstract

Erosive abrasive wear is caused by high speed impact of particles entrained in a fluid system on the surfaces of components such as boilers and furnaces. Erosive abrasive wear in boilers results from the impact of hard particles such as ash or clinker entrained in flue gases and can lead to serious damage. The life of boiler and furnace components encountering erosive abrasive wear in service, which are most commonly fabricated from carbon steels, can be improved by hardfacing with a wear resistant material. The effects of wear parameters such as particle size, flux and velocity on the erosive abrasive wear behaviour of a stainless steel surface produced by the plasma transferred arc hardfacing have been investigated using an experimental design approach. The wear resistance of the stainless steel surface was found to be twice that of the carbon steel substrate.     

Interaction energy parameters in hypostoichiometric mono-carbides,TiCx and NbCx,evaluated by statistical thermodynamics

Abstract

Interaction energy E(C-C) between nearest neighbour C atoms and E(C-M) between a C atom and M atom in hypostoichiometric monocarbide MC_x (M = Ti or Nb) were evaluated from the available a(C)-T-x relationships by statistical thermodynamic analysis (a(C): carbon activity, T: temperature, x: C/M atom ratio). Estimated E(C-C) values were positive (i.e. the C-C interaction is repulsive) for both TiC_x and NbC_x and E(C-C) varied with T showing a trend of weakening repulsion with rising T. On the other hand, the C-M interaction appeared to be strongly attractive.     

Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

Abstract

The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M₂₃C₆ carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M₂₃C₆ and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 10⁵ h extrapolated creep rupture strength at 650°C is ～100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.     

Effect of the chemical modification of the precursor of ZrO2 films on the adhesion of organic coatings

ZrO₂ films were deposited on low carbon steel by a sol-gel process as a chemical pretreatment before the application of a polyester paint. The films were obtained by a dip-coating technique using solutions of Zr(OBuⁿ)4 containing complexing agents (acetylacetone or acetic acid). These additives modified the alkoxide at the molecular level, so a new precursor was formed in solution. This new compound shows slower rates of hydrolysis and condensation than Zr(OBuⁿ)4, which allows the stability of solutions and the morphology of the gel to be controlled. Moreover, the length and the temperature of the thermal treatment influence the structure of the gel by reducing the amount of organic residues. The behavior of the films in promoting the adhesion of organic coatings was evaluated by measuring the detachment of the paint from a cross-scratch of samples which had been exposed for different times in a salt fog chamber. Tests using electrochemical impedance spectroscopy were also carried out. According to the results, most of the samples pretreated with zirconia layers showed a good performance, even better than commercial chemical treatments, when acetic acid was used as a modifier of the alkoxide precursors.