E. Ahmad

Effect of microvoid formation on the tensile properties of dual-phase steel

A steel containing 0.32 wt.% C, 0.88 wt.% Mn, 0.99 wt.% Si, 0.9 wt.% Ni, and 0.9 wt.% Cr was intercritically annealed at different temperatures from 775 to 870 °C and quenched in oil to produce dual-phase steel microstructure. Tensile testing of these samples gave a series of strengths and ductilities. The tensile strength increased with the increased annealing temperatures and the martensite percentage increased with a reduction in ductility. Microvoids were formed near the fracture surfaces. The morphology of the microvoids changed with the martensite percentage from decohesion of the martensite particles to the intergranular and transgranular cracks, which defined the ultimate fracture mode of the specimens. The change in the morphology of microvoids may be due to a high percentage of carbon in the steel, which produced stresses in the matrix (ferrite) during phase transformation.

Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel

Three morphologies of martensite in dual-phase microstructure of 0.2% C steel were obtained by different heat treatment cycles. These morphologies consisting of grain boundary growth, scattered laths, and bulk form of martensite have their distinct patterns of distribution in the matrix (ferrite). In tensile testing martensite particles with these distributions behaved differently. A reasonable work hardening was gained initially during plastic deformation of the specimens. The control on ductility was found to depend on the alignment of martensite particles along the tensile axes. The increased surface area contact of martensite particles with ferrite, in grain boundary growth and scattered lath morphologies, facilitated stress transfer from ductile to hard phase. The ductility in the later part of deformation was dependent on the density of microvoids in the necked region. The microvoids are formed mostly by de-cohesion of martensite particles at the interface. The fracture of martensite particles is less prominent in the process of microvoid formation which predicts high strength of martensite.

Effect of rolling in the intercritical region on the tensile properties of dual-phase steel

A steel containing 0.088 wt% C, 1.2 wt% Mn, and 0.78 wt% Cr was rolled at intercritical temperature (790 °C) and quenched to produce dual-phase microstructure. Rolling caused anisotropic increase in tensile strength and little change in ductility. The results suggest that rolling increased strength by a combination of strengthening of the ferrite and an increase in the stress transferred to the martensite. Up to 20% rolling reduction strengthened the ferrite by work hardening, larger reductions then reduced the strength of ferrite, anisotropically, due to increased recovery. Subgrains in ferrite were observed after rolling in the intercritical region which can contribute to the ultimate strength of the rolled material.

Ethylene glycol assisted low-temperature synthesis of boron carbide powder from borate citrate precursors

Abstract B4C powders were synthesized by carbothermal reduction of ethylene glycol (EG) added borate citrate precursors, and effects of EG additions (0–50 mol% based on citric acid) on the morphologies and yields of synthesized B4C powders were investigated. The conditions most suitable for the preparation of precursor were optimized and optimum temperature for precursor formation was 650 °C. EG additions facilitated low-temperature synthesis of B4C at 1350 °C, which was around 100–300 °C lower temperature compared to that without EG additions. The lowering of synthesis temperature was ascribed to the enlargement of interfacial area caused by superior homogeneity and dispersibility of precursors enabling the diffusion of reacting species facile. The 20% EG addition was optimal with free residual carbon lowered to 4%. For smaller EG additions, the polyhedral and rod-like particles of synthesized product co-existed. With higher EG additions, the morphology of synthesized product was transformed into needle and blade-like structure.

Influence of martensite volume fraction on fatigue limit of a dual-phase steel

Different dual-phase microstructures were produced in a steel containing 0.16 C (wt.%) by careful design of the heat treatment schedule in order to study the effect of microstructures, volume fraction of martensite, and epitaxial ferrite on fatigue limit. It was observed that the fatigue limit was raised by an increasing martensite content and aspect ratio, and reduced by the presence of epitaxial ferrite (new ferrite). However, these changes were very clearly and directly related to simultaneous changes in strength.

Crack path morphology in dual-phase steel

Intercritical heat treatment (ICHT) and thermomechanical processing (TMP) were used on steel having 0.16% C to vary the morphology, distribution of ferrite, and martensite phases, in order to study the resistance to fatigue crack propagation and crack path morphology in dual-phase steel. A crack growth rate has been determined at ∼10−10 to 10−3 m per cycle in ICHT and TMP samples. The tortuous morphology of the crack path was observed in unrolled materials, which resulted in reduction of the crack driving force from crack deflection and increased the ΔKth. In thermomechanical processed materials, the crack tended to cross the martensite and the crack path become less circuitous, resulting in decrease a threshold stress intensity factor (ΔKth) as compared with unrolled material.

Effect of degassing parameters on sinterability of Al/B4C powder mixture

Abstract The 6061-Al powder was degassed under vacuum, argon and hydrogen before mixing with 5 mass-%B4C particles. After compaction, the supersolidus liquid phase sintering was performed at 630°C under high purity argon and nitrogen atmosphere. The purpose of this experimental plan was to study the effect of different degassing and sintering environments on the compressibility and sinterability of Al/B4C composites. The efficacy of degassing condition in-terms of sintered density is in the order of hydrogen, argon and vacuum (highest). Al/B4C composite sintered under argon atmosphere after vacuum degassing showed ∼8% higher sintered density as compared to the sample with similar condition except degassing. Al/B4C composite sintered under N2 atmosphere without degassing showed ∼89% density and 65 HRB while the composite sintered under N2 atmosphere after degassing in vacuum showed ∼98% density and 89 HRB with faded boundaries between 6061-Al particles in the microstructure.

Effect of cold rolling and annealing on the grain refinement of low alloy steel

A low alloy steel containing 0.06 wt% C, 1.5 wt% Mn and 0.1 wt% V was given 85% cold rolling reduction. The aim of the rolling reduction was to induce energy due to stresses and distribution of carbides, used for grain refinement in subsequent annealing. The rolled specimens were heat treated at various temperatures from 590°C to 650°C for different lengths of soaking times ranging from five minutes to two hours, to promote the process of re-crystallization. At temperatures with long soaking times the re-crystallization process is expected to be completed with minimum of grain coarsening due to carbide distribution, especially vanadium carbides. A smooth drop in hardness with increase in annealing times was observed which may be due to recovery from stressed conditions during process of re-crystallization. Texture observations supported the re-crystallization process as the preferred orientation of (200) plane in rolled condition was successively reduced with annealing temperatures. Tensile properties observations of two hour annealing times at 590°C to 650°C clearly demonstrated that ductility increased at all annealing temperatures with maximum gain at 625°C and strength is decreased.

Effect of thermomechanical processing in the intercritical region on hardenability of austenite of a dual-phase steel

Steels slabs containing different percentages of C, Mn, and Cr were intercritically heat treated and rolled at 780 and 790 °C; they were then quenched to produce dual-phase microstructure in order to study the martensitic hardenability of austenite present in them. It was found that rolling of the two-phase (α+γ) microstructure elongated austenite particles and also reduced the martensitic hardenability of austenite particles, probably because the rolling increased the α/γ interfacial area, thus promoting the formation of ferrite during cooling. The martensite particles obtained in the rolled material were also elongated or “fibered” in the rolling direction. It was observed that the thermomechanical processing of a two-phase (α+γ) mixture has the detrimental effect of increasing the quenching power needed to yield a specific amount of martensite.

Inhibitor effects of sodium benzoate on corrosion resistance of Al6061-B4C composites in NaCl and H3BO3 solutions

Sodium benzoate (SB) is used for the first time to inhibit the corrosion of Al6061-B4C composites in H3BO3 and NaCl solutions. Al6061100−x –x wt% B4C (x = 0, 5, and 10) composites are manufactured by a powder metallurgy route. The corrosion inhibition efficiency of SB is investigated as a function of the volume fractions of B4C particles by using potentiodynamic polarization and electrochemical impedance techniques. Without the use of an inhibitor, an increase of the B4C particles in the composite decreases the corrosion resistance of Al6061-B4C composites. It is found that SB is an efficient corrosion inhibitor for Al6061-B4C composites in both investigated solutions. The corrosion inhibition efficiency of SB increases with an increase in B4C content. Since SB is an adsorption type inhibitor, it is envisaged that an extremely thin layer of molecules adsorbs onto the surface and suppresses the oxidation and reduction. It is found that the inhibitor effect of SB is more pronounced in a H3BO3 environment than in NaCl solution. Further, the mechanism of corrosion inhibition by SB is illustrated by using optical and scanning electron microscopy of corroded samples. It is found that the adsorption of benzoate ions on the Al surface and its bonding with Al3+ ions forms a hydrophobic layer on top of the exposed Al surface, which enhances the protection against dissolved boride ions.