New thermomechanical strategy for production of high strength low alloyed multiphase steel showing transformation induced plasticity effect

Abstract

In the past few years a lot of research was carried out on the development of transformation induced plasticity (TRIP) assisted multiphase steels. Two principal ways were proposed: (i) controlled cooling during the hot rolling process to obtain hot rolled TRIP assisted multiphase steels and (ii) the combination of intercritical annealing and isothermal holding at bainite formation temperatures during continuous annealing resulting in cold rolled TRIP assisted steel products. Unfortunately, both thermomechanical methods proposed require a high silicon level to inhibit cementite precipitation to avoid a loss of stability for the metastable retained austenite. In addition, due to high silicon levels, red scale surface defects appear and only moderate hot dip galvanisability is possible. In this paper a new thermomechanical strategy for the production of high strength low alloyed TRIP assisted multiphase steels with good hot dip galvanisability and without red scale defects will be presented.     

Behaviour of Bi-adhesive Joints

The use of relatively low modulus adhesive at the ends of overlap in a bi-adhesive bondline of a bonded joint can reduce the stress concentration significantly and, therefore, potentially lead to higher strength of the joint. This study presents the two-dimensional and three-dimensional nonlinear (geometric and material) finite element analyses of adhesively bonded single lap joints having modulus-graded bondline under monotonic loading conditions. The adhesives were modelled as an elasto-plastic multi-linear material, while the substrates were regarded as both linear elastic and bi-linear elasto-plastic material. The computational simulations have been performed to investigate the bondline behaviour by studying the stress and strain distributions both at the mid-plane as well as at the interface of the bondline. It has been observed that the static strength is higher for joints with bi-adhesive bondlines compared to those with single adhesives in bondline. Higher joint strength has also been observed for optimum bi-adhesive bondline ratio through parametric studies. Effects of load level, and bondline thickness on stress distribution in the bi-adhesive bondline have also been studied. 3D analysis results reveal the existence of complex multi-axial stress/strain state at the ends of the overlap in the bondline which cannot be observed in 2D plane strain analysis. About 1/3rd of the width of the joint from the free edge in the width direction has 3D stress state, especially in the compliant adhesive of the bondline. Magnitudes of longitudinal and lateral stress/strain components are comparable to peel stress/strain components. It has also been analytically shown that the in-plane global stiffness of the joint remains unaffected by modulus gradation of the bondline adhesive.     

Microstructure of tool steel after low temperature ion nitriding

Abstract

The microstructural development in H13 tool steel upon nitriding by an ion beam process was investigated. The nitriding experiments were performed at a relatively low temperature of ～400°C and at constant ion beam energy (400 eV) of different doses in a high vacuum preparation chamber; the ion source was fed with high purity nitrogen gas. The specimens were characterised by X-ray photoelectron spectroscopy, electron probe microanalysis, scanning and transmission electron microscopy, and grazing incidence and Bragg–Brentano X-ray diffractometry. In particular, the influence of the nitrogen surface concentration on the development of the nitrogen concentration depth profile and the possible precipitation of alloying element nitrides were discussed.     

Processing and performance of Cr2O3–Fe2O3 reference electrode powders prepared by oxide coprecipitation

Abstract

Cr₂O₃–Fe₂O₃ based oxide mixtures for reference electrode powders of oxygen sensors were processed using oxide coprecipitation route. A special method for preparing reference electrode powders has been developed by mixing coarse Cr particles with the oxide mixture in the form of Cr–Fe hydroxide. Morphology and size of the mixed oxide powders were characterised by means of scanning electron microscopy and laser diffraction method. With the coprecipitation process, chemically homogeneous and very fine powders with a mean particle size of 1.53 μm were prepared. This powder mixture adhered and loosely coated to Cr particles. The processed reference electrode powders were tested in low level oxygen concentration measurements of steelmaking process under industrial scale. The reference electrode powders showed excellent results in terms of electromotive force reproducibility, response time and accuracy in soluble aluminium predictions at the oxygen concentration measurements. Most of the particles of the reference electrode powder remained separated after dipping to molten steel.     

Theoretical study of Marangoni convection and weld penetration under influence of high oxygen content in base metal

Abstract

A 3D mathematical model is developed to simulate the weld pool development in a moving weld pool of 304 stainless steels with different oxygen concentrations. It is shown that the oxygen can cause significant changes in fluid flow patterns and temperature fields. When oxygen content is <280 ppm, the weld penetration and depth/width ratios increase sharply with increasing oxygen content; positive and negative temperature coefficients of surface tension coexist and cause shallow and wide weld pool. When oxygen content is in the range of 280–500 ppm, positive temperature coefficient of surface tension dominates the flow patterns and causes deep and narrow weld pool. The weld penetration and depth/width ratios remain nearly constant. When oxygen content is >500 ppm, the weld penetration and depth/width ratios decrease again. Vortexes that have different positions, strength and directions may be found in the pool. The vortexes with opposite directions caused by positive temperature coefficient of surface tension can efficiently transfer the thermal energy from the arc, creating a deep weld pool.     

Studies on macrosegregation and double diffusive convection during directional solidification of binary mixture

Abstract

Experimental and numerical studies on macrosegregation during directional solidification of an aqueous ammonium chloride solution in a rectangular cavity are presented. Depending on the initial concentration and boundary temperature conditions, three distinct situations in the liquid during the solidification process are studied: mass diffusion only, solutal convection and double diffusive convection. The time dependent concentration of the solution inside the cavity is measured online for the above three situations using a laser based technique. This technique measures refractive index of the solution, which indirectly gives the concentration variation at a point in the solution during solidification. The interface growth is photographed using a shadowgraph technique, and the flow is visualised using a sheet of laser light scattered by neutrally buoyant, hollow glass spheres seeded in the fluid. Corresponding numerical studies are performed using a macroscopic model based on a fixed grid, enthalpy based, control volume approach. A good agreement between the experimental and computational results is observed.     

Role of composition heterogeneity on fracture micromechanisms of nodular cast iron

Abstract

The aim of the present study is to explain the role of chemical composition and heterogeneity within the microstructure of nodular cast iron on the micromechanisms of fracture. The heterogeneity in chemical composition is revealed by a thorough study of the microstructure using scanning electron microscopy, Auger electron spectroscopy and colour etching. The ferritic phase surrounding the graphite globules is enriched by silicon and depleted of manganese in the range of micrometres. Maximum manganese concentration is found in regions distant from these particles and in pearlitic regions. Simultaneously, ferritic grain boundaries in the vicinity of the graphite particles are enriched in phosphorus in the nanometer range. This interfacial segregation, reaching nearly 15 at.-%P, is most probably responsible for an embrittlement of these regions and the appearance of intercrystalline decohesion. An elastic-plastic finite element analysis of the ferritic/pearlitic microstructure demonstrates that a heterogeneous microstructure is locally subjected to heterogeneous stresses and strains, which are also expected to affect fracture mechanisms.