Ramen Datta

Weldability and toughness evaluation of pressure vessel quality steel using the shielded metal arc welding (SMAW) process

The present study was carried out to assess the weldability properties of ASTM A 537 Cl. 1 pressure-vessel quality steel using the shielded metal arc welding (SMAW) process. Implant and elastic restraint cracking (ERC) tests were conducted under different welding conditions to determine the cold cracking susceptibility of the steel. The static fatigue limit values determined for the implant test indicate adequate resistance to cold cracking even with unbaked electrodes. The ERC test, however, established the necessity to rebake the electrodes before use. Lamellar tearing tests carried out using full-thickness plates under three welding conditions showed no incidence of lamellar tearing upon visual examination, ultrasonic inspection, and four-section macroexamination. Lamellar tearing tests were repeated using machined plates, such that the central segregated band located at the midthickness of the plate corresponded to the heat-affected zone (HAZ) of the weld. Only in one (no rebake, heat input: 14.2 kj cm-1, weld restraint load: 42 kg mm-2) of the eight samples tested was lamellar tearing observed. This was probably accentuated due to the combined effects of the presence of localized pockets of a hard phase (bainite) and a high hydrogen level (unbaked electrodes) in the weld joint. Optimal welding conditions were formulated based on the above tests. The weld joint was subjected to extensive tests and found to exhibit excellent strength (tensile strength: 56.8 kg mm-2, or 557 MPa), and low temperature impact toughness (7.4 and 4.5 kg-m at-20 °C for weld metal, WM, and HAZ) properties. Crack tip opening displacement tests carried out for the WM and HAZ resulted in δm values 0.36 and 0.27 mm, respectively, which indicates adequate resistance to brittle fracture.

Development of API X-70 Grade Plates and Strips in SAIL

API X-70 grade plates and hot strips microalloyed with Nb, V, and Ti have been developed simultaneously from two units of SAIL at Bhilai and Bokaro Steel Plants, respectively. Suitably designed steel (C: 0.1% max., Mn: 1.7% max., Nb: 0.06% max., V: 0.04% max., and Ti: 0.02% max.) was processed through vacuum arc degassing (VAD), and cast into 240 mm thick slabs and hot rolled into 20 mm thick plates, deploying an appropriate thermomechanical controlled processing (TMCP) technology. The tensile properties (YS: 473–531 MPa, YS/UTS: 0.74–0.88, %El: 32–48%) and Charpy impact toughness properties (CIE: 113–159 J at 0°C) were found to be quite attractive, owing to their fine microstructure of ferrite (∼10 µm) and low pearlite (volume fraction of ∼12%). In case of hot strips, the chemistry was suitably modified with leaner alloy chemistry (C: 0.08% max., Mn: 1.5% max., Nb: 0.06% max., V: 0.05% max., and Ti: 0.02% max.) and slabs were control rolled into 10/12 mm thick strips followed by accelerated cooling prior to coiling. The mechanical properties of hot strips varied within a narrow range of YS: 535–557 MPa, YS/UTS: 0.86–0.88, %El: 33–50%, and Charpy impact toughness property: 150–180 J at 0°C showing appearance of finer ferrite grains: ∼8 µm. Examination of precipitates in both plates and hot strips revealed presence of fine NbV(CN) and complex M4C3 type coarse carbides. The mechanical properties of Electrical Resistance Welding (ERW) pipes manufactured using hot strips satisfied all specified requirements of American Petroleum Institute (API) norms. Both the products are now ready for their commercial exploitation.

Hydrogen-Induced Cracking in GMA Welds of Vanadium–Titanium Microalloyed High Strength Steel

High strength low alloy (HSLA) steels are developed to provide better strength, toughness, and improved weldability. These steels are often used as better alternatives for quenched and tempered steels due to the excellent combination of these properties. The beneficial properties in these steels are achieved by careful control of composition and by adopting suitably controlled thermomechanical processing (TMCP). Improved weldability is achieved by maintaining carbon at lower level. To compensate the effect of carbon, small amounts of alloying elements, such as niobium, vanadium, titanium, aluminium, and nitrogen are added. Though these steels have good weldability, they suffer from hydrogen induced cracking especially at high strength levels. In the present study, hydrogen induced cracking susceptibility of a newly developed indigenous vanadium–titanium microalloyed high strength steel was evaluated by means of implant cracking test. Welding was carried out using the GMAW process at different heat input levels. It was found that the coarse-grained heat affected zone (CGHAZ) of the steel is prone for hydrogen induced cracking at lower heat inputs. The implant test results indicated that the critical stress level increases with increasing heat inputs. The results were correlated with the microstructural modifications that occur in the CGHAZ at different welding conditions. The microhardness variations in that region were in good agreement with the observed results.

Weldability characteristics of torr and corrosion-resistant TMT bars using SMAW process

Torr steel rebars, also known as cold twisted deformed (CTD) rebars, are used extensively for the construction of reinforced cement concrete (RCC) structures. These steels, which are characterized by a high carbon content and are subjected to a cold twisting operation to attain the desired strength level and bond strength, suffer from low ductility and poor bendability properties. Furthermore, these rebars are not suitable for coastal, humid, and industrial conditions where corrosion rates are very high. To combat these problems, recent efforts at the Steel Authority of India Limited (SAIL) have led to the successful development of corrosion-resistant thermomechanically treated (TMT) rebars with a minimum yield strength of 500 MPa. These rebars are characterized by a low carbon content, exhibit excellent strength-ductility-corrosion properties, and are rapidly replacing traditional torr rebars in corrosion-prone areas for a wide range of applications, namely, concrete reinforcement structures, bridges, flyovers on dams, etc. A comprehensive evaluation of the weldability properties of corrosion-resistant Cu-TMT rebars was carried out, and they were compared with those made of torr steel in order to assess their suitability for various structural applications. Implant and restraint cracking (RC) tests were carried out to assess the cold-cracking resistance of the weld joint under different welding conditions. The static fatigue limit (SFL) values were found to be similar, namely, 640 MPa (torr steel) and 625 MPa (Cu-TMT steel) under condition of no preheating and no rebaking using a heat input of 7.5 KJ/cm, indicating adequate cold-cracking resistance for both the steels. Restraint cracking tests yielded critical restraint intensities (Kcr) in excess of 16,800 MPa for both of the steels. Based on the weldability tests, the optimized conditions for welding were formulated and extensive tests were carried out on the welded joints. Both of the steels exhibited adequate strength levels (tensile strength (TS): torr rebars, 524 Mpa; Cu-TMT rebars, 630 MPa) and adequate low-temperature impact toughness properties, ensuring a high integrity of the fabricated products.

Integrated management of the uzi fly, Exorista bombycis (Louis) (Diptera: Tachinidae), a parasitoid of the silkworm, Bombyx mori L.

Abstract Exorista bombycis (Louis), known as uzi fly, causes considerable damage to silkworm rearings in several silkproducing countries including India. Various control measures have been adopted by farmers to control uzi fly. Although these measures help to reduce uzi infestation, the economic loss is not completely avoided. We therefore undertook an IPM programme involving spraying of an insecticide (a liquid formulation of 1% benzoic acid) against eggs and augmentative release of the indigenous gregarious parasitoid Nesolynx thymus (Girault) (Hymenoptera: Eulophidae) against puparia. In all, 129 silkworm rearings consisting of 16575 layings were subjected to integrated management between September 1990 and August 1992. All the silkworms raised were sprayed with a recommended dosage of insecticide and a total of c. 17.9 million adults of N. thymus were released during rearing of silkworms. The results indicate that the IPM approach is highly successful as a 77.0% decline in the losses due to uzi infest...

Weldability evaluation of high tensile plates using GMAW process

High tensile plates, SAILMA-450 high impact (HI) (yield strength, 45 kg/mm2 minimum; ultimate tensile strength, 57 kg/mm2 minimum; elongation, 19% minimum; Charpy impact energy 2.0 kg.m at −20 °C minimum) were successfully developed at the Steel Authority of India Ltd., up to 32 mm plate thickness. Since then the steel has been extensively used for the fabrication of impellers, bridges, excavators, and mining machineries, where welding is an important processing step.The present study deals with the weldability properties of SAILMA-450 HI plates employing the gas metal arc welding process and carbon dioxide gas. Implant and elastic restraint cracking tests were conducted to assess the cold cracking resistance of the weld joint under different welding conditions. The static fatigue limit values were found to be in excess of minimum specified yield strength at higher heat input levels (9.4 and 13.0 kJ/cm), indicating adequate cold cracking resistance. The critical restraint intensities, Kcr, were found to vary between 720 and 1280 kg/mm2, indicating that the process can be utilized for fabrication of structures involving moderate to low restraint intensities (200 to 1000 kg/mm2). Lamellar tear tests conducted using full thickness plates at heat input levels ranging from 10 to 27 kJ/cm showed no incidence of lamellar tear upon visual, ultrasonic, and four-section macroexamination. These tests were repeated using machined plates, such that the midthickness of the plates (segregated zone) corresponded to the heat affected zone of the weld. No cracks were observed, indicating good lamellar tear resistance of the weld joint. Optimized welding conditions were formulated based on these tests. The weld joint was subjected to extensive tests to assess the physical properties and soundness of the weld joint. The weld joint exhibited good strength (64.7 kg/mm2) and impact toughness (5.7 and 3.5 kg.m at −20 °C for weld metal and heat affected zone properties. Crack tip opening displacement (CTOD) tests carried out for parent metal, heat-affected zone, and weld metal resulted in δm values of 0.41, 0.40, and 0.34 mm, respectively, which indicates adequate resistance to cleavage fracture. It was concluded that the weld joint conforms to the requirements of SAILMA-450 HI specification and ensures a high integrity of the fabricated products.

Effects of tempering in a ferrite-martensite steel

In this investigation we studied the effects not only of tempering, but also of cold rolling before tempering, on the microstructure and mechanical properties of a plain C-Mn steel initially having a ferrite-martensite structure. The tempering temperatures used varied from 600 to 700 o C. Two distinct types of structures, namely the normal tempered microstructure and the dual-phase microstructure, were observed. The associated mechanical properties and structure-property correlations are discussed

Influence of Metallurgical Factors on the Performance of IS:7887 Gr. 3 Billets for Fastener Applications

The quality advantages of continuous casting (concast) have enabled the production of a wide range of billets for various end applications, including IS:7887 Gr. 3 billets for the fastener industry. This paper discusses the influence of the internal quality of these concast billets on the processing of a wide range of products for the fastener industry. Internal soundness, inclusion volume fraction, and cleanliness were found to have a strong influence on the cracking susceptibility of self-tapping screws and nuts. A high incidence of hairline cracks on nut surfaces was found to be due to a combination of a high volume fraction of inclusions (0.54%) and the presence of complex manganese (Mn) (aluminate-silicate)-type inclusions.

Some new results in thermomechanical processing of microalloyed steels

In recent times, efforts have been directed towards a better understanding of the mechanisms associated with deformation and restoration of austenite during continuous multipass hot rolling of microalloyed steels. The correspondence between the condition of austenite before transformation and the resultant microstructure upon cooling holds the key to the attainment of interesting properties.In the present paper, some results obtained on the deformation of austenite in a microalloyed steel, using a hot compression machine, are presented. Here, the idea was to simulate actual plate rolling or hot strip rolling conditions and study the evolution of microstructure at different stages of the hot deformation process i.e. after precise reductions at given strain rate and temperature of deformation.The paper further discusses recent results obtained by us on the influence of hot deformation parameters (strain, finish rolling temperature, temperature of deformation) and cooling rates (air cooling, spray water cooling) on the microstructure of microalloyed steel. The precise conditions leading to the evolution of acicular ferrite and bainitic microstructures have been identified. New information on the influence of short tempering treatments (15 min at 550, 600 and 650°C) on the microstructure and properties of a microalloyed steel are also outlined.