Thermomechanical Processing of Metals and Alloys

For commercial products in any industry, their external shapes are the result of hot deformation i.e. hot rolling. The necessary mechanical properties are deduced from the alloy design and through heat treatment after hot-deforma-tion. Thermomechanical processing (TMP) is a technique designed to improve the mechanical properties of materials by controlling the hot-deformation processes, which origi-nally were designed to produce the required external final shape of the product.The first introduction of TMP for commercial production was controlled rolling of C-Mn steel plates of 40kgf/mm2 grade for ship-building in the 1950s. During the World War II, a number of transport ships, so-called 'Liberty' ships suffered from the occurrence of brittle fractures initiated at welded joints. This inci-dent stimulated the concept of toughness, which is diff-erent from the concept of ductility, and notch toughness became a requirement for ship-building and other structu-ral steel plates. At that time, the concept of ductile-brittle transition temperature through grain refinement was introduced. It was reported that an improvement of about 10-15°C in the 20J transition temperature could be possible through controlled low temperature hot-rolling process

High Strength Aluminium Alloys with Emphasis on Scandium Addition

Aluminium is perhaps the youngest among the large group of metallic elements. It was first commercially produced in the year 1886 and rose in prominence meteorically to occupy today the second position in the total weight pro-duced, next only to steel. Aluminium is a silvery white ductile metal with a FCC structure. It has excellent electrical and thermal conductivity (next only to copper and silver). It has low density (2.7 g/cc) which gives it a very high specific modulus and strength. Pure aluminium is the soft metal with the modulus value of 70 GPa and the yield strength of about 40 MPa. One of the most important properties of aluminium is to form a thin protective oxide layer on the surface. This layer is tenacious, adherent, deformable and impervious to most corroding elements

Effect of thermomechanical processing schedule on the texture and microstructure of pipeline grade API X80 microalloyed steel

The presence of micro alloying constituents like Ti, Nb in microalloyed steel enhances its mechanical properties through grain size control and precipitation strengthening. The strengthening capability of microalloying additions can be fully utilized by choosing proper thermomechanical processing (TMP) schedule. The TMP schedule needs to be designed based on determination of critical temperatures of transformation in steel including no recrystallisation temperature (i.e ., Tnr) of microalloyed steels. The TMP was carried out on API X80 grade pipeline steel using Gleeble® 3800 Thermal mechanical simulator. The samples were deformed in plain strain condition at three different temperatures (860, 950 and 1050°C) by keeping other deformation parameters constant. The deformation temperature was chosen based on CCT and Tnr determination studies. The deformed samples were examined in EBSD and TEM to obtain the texture and microstructural information. It is also observed that formation of acicular ferrite and bainite microstructures in these steel is very much dependent on the deformation strain levels in the austenite matrix prior to transformation. The grain boundary misorientation angle distribution obtained from EBSD analysis can be a very important parameter to distinguish the different microstructural constituents apart from grain shape and image quality. Effect of thermomechanical processing schedule on the texture and microstructure of pipeline grade API X80 microalloyed steel (PDF Download Available). Available from: https://www.researchgate.net/publication/303524095_Effect_of_thermomechanical_processing_schedule_on_the_texture_and_microstructure_of_pipeline_grade_API_X80_microalloyed_steel [accessed Nov 15 2017]

An investigation of sliding wear behaviour of WC–Co coating

Dry sliding wear tests on specimens of mild steel (MS) and WC coated mild steel (MSC) specimens were performed against a hardened EN32 steel (EN32) and a WC coated EN32 steel (EN32C) discs. Four different combinations of specimen and counter surface were tested under dry sliding conditions. Results suggest that wear mechanisms differ depending on the combination of materials under sliding contact. Expectedly the MS specimen suffered high wear loss, but the MSC specimen showed interesting results. When slid against EN32, MSC specimens showed negative wear results whereas positive wear results occurred against EN32C. Steady wear rate was attained after a critical sliding distance

Liquation cracking susceptibility of partially melted zone in 304B4 SS multipass weldments

AISI 304B4 stainless steel (SS) containing 1.3 wt% boron is reported to be weldable under moderate restraint forces provided welding parameters are chosen carefully. However, during fabrication of components, extensive cracking issues are being reported. Hence, a study involving assessment of base metal, unmixed zone and partially melted zone (PMZ) of 304B4 steel and its welds made using E309 and boron containing consumables on hot cracking is carried out. In this article, results obtained on cracking susceptibility of PMZ formed in this steel are presented and discussed. In order to evaluate hot ductility behavior of PMZ formed in unmixed zone in a multipass weld, PMZ was first simulated using “Gleeble™” thermo-mechanical simulator. Subsequently, hot ductility tests were conducted on this simulated PMZ. Results of the study showed that liquation cracking susceptibility of PMZ of this steel subjected to repeated thermal cycle is higher than that of 304B4 base material. Microstructural analysis of tested specimens showed that formation of Fe-rich ((Fe,Cr)2B + Cr2B) borides in PMZ lowers ductility recovery temperature by 50 °C which results in an increase in cracking factor by 84% of that of base metal. Reasons for high cracking susceptibility of PMZ formed on fused 304B4 have been deduced

Effect of repetitive corrugation and straightening on Al and Al–0.25Sc alloy

The improvement in strength of Al and Al–0.2Sc alloys through repetitive corrugation and straightening (RCS) was studied. The RCS was carried out using both single teeth as well as multiple teeth corrugative setup attached to a universal testing machine. The improvement in strength was obtained by measuring the hardness after processing. The rotation of sample by 90° between successive passes resulted in higher hardness values as compared to the simple bending and straightening. The increment in hardness obtained with same number of passes in the case of Al–0.2Sc alloy is more as compared to Al and is possibly due to blocking of dislocations that are introduced during deformation by Al3Sc precipitates

Difffusion bonding of non weldable high alloy steel

Diffusion bonding is a viable joining process which could minimize the weldability related problems of high Mn steel. The steel used for the present investigation is a twinning induced plasticity (TWIP) high alloy steel containing 23 Mn, 4 Si and 3% Al. The diffusion bonding of TWIP steel was carried out after Ni coating of joining surface. The diffusion bonding process was carried out in high vacuum hot press at 700, 800 and 900°C for 120 minutes under uniaxial load of ~25MPa. The detailed microstructural analysis and elemental distribution across the joint was carried out using optical microscopy and electron probe microanalysis (EPMA). The EPMA analysis showed that the diffusion bonding was favored by the large scale inter-diffusion of Ni and Mn through the ferrite phase in TWIP steel. The elemental mapping of the joint processed at 900°C revealed that maximum wt. % of Ni at any point is ≤ 60% which is responsible for the quality of sound joint with no line interface

Failure Analysis of SA213-T22 Re-heater Rear Tube of Thermal Power Plant

SA213-T22 steel had been widely used in petrochemical and power generation plants. But due to prolonged service exposure in various conditions, these tubes have been ageing continuously and deteriorations of the materials cannot be avoided. This paper presents a case study of SA213 type-T22 alloy steel reheater tube that failed in a boiler of a power plant. The failed tube was investigated through visual examination, mechanical properties evaluation, and microstructural analysis to find the root cause of failure. The tube was found to fail by long term over-heating due to formation of thick oxide scale formation leading to creep

Study of microstructural degradation of a failed pinion gear at a cement plant

The failed pinion gear assembly of a cement plant was analyzed by visual, residual stress, microstructure and mechanical property examination to determine the root cause of its failure. It is observed that the worn out region contains several surface micro cracks, especially closer to the tip (addendum) as compared to the root (dedendum) of the gear tooth profile. The surface analysis of the worn out regions showed depletion of protective chromium oxide layer at the surface and formation of iron oxide scales due to corrosion attack by lubricants, contaminants resulted in micro pitting and subsequent wear. Residual stress measurements showed tensile residual stresses at the tip (face) of the gear teeth and large compressive stresses at the root (flank) of the gear teeth. The cross-sectional microstructure examination revealed the damages due to rolling contact surface fatigue in the form of subsurface dark etched region bands and coarsened carbides in the surface. Apart from this, the difference in the tempered core microstructure below the case hardened layer between the flank and face regions lead to compressive residual stress in case of soft core and tensile stress in case of harder core respectively. The concentration of tensile residual stresses due to untempered core at the tip aided the micro-pitting and micro-cracking due to the rolling contact surface fatigue was responsible for the initiation of surface cracks and final failure of the gear

Light metals research - Legacy and current standing

The ever increasing emphasis on the specific strength and the stringent material characteristics sought in various industries, catering to aerospace, defence, automobiles and load bearing structures have led to an unprecedented worldwide thrust on the development and processing of light metals and alloy products. These include Al-, Mg-, and Ti-alloys, metallic foams and metal matrix composites. CSIR-NML not only has an illustrious legacy but has kept pace with international research on light metals and alloys till date. Research in this area has culminated in several new materials and processes, which have seen commercial exploitation and/or transfer of technology to the industries. This paper highlights the accomplishments of CSIR-NML vis-à-vis contemporary national and international research trends