Papers by Keyword: Low Alloy Steel

Abstract: The present study was undertaken to investigate the effect of thermo-mechanical treatment (TMT) on the microstructure and mechanical behaviour of low alloy steel. Hot forging is carried out at 1200°C using mechanical press of 500 and 800 ton. The effect of hot forging reduction ratios (1.11 and 1.29) on the hardness and mechanical properties are studied. TMT samples are given different heat treatment i.e. annealing (A), normalizing (N), hardening (H), hardening and tempering (H/T) and their corresponding impact toughness are obtained. Selected heat treatment (normalizing and annealing) are given to tensile test samples and their corresponding strength and ductility are obtained. Ultimate tensile, 0.2% offset yield strength and percent elongation are measured. Hardness and impact toughness measurements were carried out for all alloy conditions. Hardness (HV), ultimate tensile stress (UTS-MPa) and 0.2% offset yield stress (MPa) increases with increasing reduction ratio. TMT leads to a sharp rise in alloy hardness and strength. Normalizing and annealing following TMT revealed a low hardness values compared to those observed in the TMT condition. Annealing reduces hardness and strength but increases ductility and impact toughness. This could be attributed to the recovery and coarsening effect. Pro-eutectoid ferrite phase are observed along the grain boundaries of low alloy steel in the TMT conditions regardless of the reduction ratios. Normalized samples show a refined pearlitic microstructure while coarse pearlite is observed in the annealed one. Good mechanical properties can be obtained by a combination of plastic deformation and thermal treatment. Heat treatment is one of the major factors used to enhance the mechanical properties of low alloy steel. An understanding of the combined effect of TMT and subsequent heat treatment on the structure and mechanical properties of low alloy steel would help in selecting conditions required to achieve the optimum mechanical properties and alloy high strength to weight ratio. This may be achieved by measuring hardness, impact toughness, strength and ductility resulting from different heat treatment following TMT.

Abstract: The corrosion behavior of low carbon bainitic steel with Cu-P alloying in the environment simulating coastal industrial atmosphere was investigated by using dry-wet cycling corrosion test. 09CuPCrNi steel and low carbon bainitic steel without Cu-P alloying were used as comparative steels. The corrosion kinetics and electrochemical impedance spectra of the steels were measured, respectively. The morphologies of rust layers were observed by SEM and the phase constitutes of the rust layers were analyzed by XRD. Low carbon bainitic steel with Cu-P alloying behaves the lowest corrosion rate and the highest resistance of rust layer. Bainite microstructure is responsible for the uniform corrosion and the formation of dense rust layer. Cu-P alloying accelerates the transformation of gamma-FeOOH and Fe₃O₄ to thermodynamic stable phase alpha-FeOOH, which improves the protective effect of the rust layer.

Abstract: The influence of the oxide inclusions on the microstucture, properties and the Heat-affected-zone (HAZ) toughness of low alloy steels has been investigated. The low alloying steels were smelted with special oxide introduction technique and the properties of HAZ have been studied with thermo-simulation. The type, size and distribution of the non-metal inclusions in steels has been studied by optical microscope and scanning electronic microscope (SEM). The microstructure and mechanical properties of base metal and samples undergoing welding thermo-simulation were also analyzed. The results show that the inclusions in experimental steel are mainly complex inclusions composed of oxide containing Ti and MnS, these complex inclusions are distributed homogenously and their size is between 1 and 3 micron. The microstructure of HAZ consists of intragranular acicular ferrite (IAF), intergranular ferrite and small amount of lath bainite while the cooling time during the phase transformation between 800°C and 500°C (t_8/5) is short. After the thermo simulation with t_8/5 increasing the toughness of HAZ decreased and the size of prior austenite grain increased. If the size of the austenite grain decreased the amount of IAF also decreases.

Abstract: Surface cladding utilizes a high energy input to deposit a layer on substrate surfaces providing protection against wear and corrosion. In this work, TiC particulates were incorporated by melting single tracks in powder preplaced onto AISI 4340 low alloy steel surfaces using a Tungsten Inert Gas (TIG) torch with a range of processing conditions. The effects of energy input and powder content on the melt geometry, microstructure and hardness were investigated. The highest energy input (1680 J/mm) under the TIG torch produced deeper (1.0 mm) and wider melt pools, associated with increased dilution, compared to that processed at the lowest energy (1008 J/mm). The melt microstructure contained partially melted TiC particulates associated with dendritic, cubic and globular type carbides precipitated upon solidification of TiC dissolved in the melt; TiC accumulated more near to the melt-matrix interface and at the track edges. Addition of 0.4, 0.5 and 1.0 mg/mm² TiC gave hardness values in the resolidified melt pools between 750 to over 1100Hv, against a base hardness of 300 Hv; hardness values are higher in tracks processed with a greater TiC addition and reduced energy input.

Abstract: Missile and rocket motor cases are often fabricated by welding process. The replacement of welded hardware with the flow formed construction eliminates many problems associated with welded joints. Data on the cold workability of the material in different heat treatment conditions is essential for the flow forming of the material. The cold workability of a 0.3C-CrMoV grain refined steel processed by air melting and electroslag refining was studied in annealed and Q&T conditions through cold rolling process. The effect of heat treatment on the strength properties of the rolled materials was also studied. The observed behaviour is explained in the light of the results of the microstructural characterization.

Abstract: Hot deformation of a continuously cast low alloyed steel is studied by means of hot compression and tensile tests carried out after austenitization between 700–790 °C at 3x10^-4 – 0.3 s^-1 of strain rate. The ferrite transformation at the applied cooling rate was determined at 710°C by means of dilatometry. The compressive flow data obtained by using a Gleeble®1500 machine are evaluated to obtain the strain rate sensitivity and the processing maps using different models. The tensile data are used to determine the ductility of the material with different deformation parameters. A new calculation method is used for the instability parameter derived from the dynamic materials model. The strain rate sensitivity does not predict any instability but all the others instability parameters do, including the new one. Pores are formed at the prior austenitic grain boundaries at low strain rates, causing a decay of ductility in the tensile samples. A minimum in the ductility was observed for low strain rates at 750°C. Low strain rates and low temperatures increase the formation of more ferrite than without deformation at the corresponding heat treatments without deformation. In these conditions, the deformation is concentrated in the softer ferrite phase. Low power efficiency was calculated at high strain rates, where no dynamic recrystallization takes place. The domains with similar efficiency of power dissipation are correlated to deformation induced ferrite formation and ferrite recovery. These domains vary with the increasing strain.

Abstract: Innovative treatments like quenching and partitioning (Q&P) have been recently proposed to improve the combination of strength and ductility of high strength steels by stabilization of significant fractions of retained austenite in a microstructure of tempered martensite. The decomposition of austenite into bainite and carbides precipitation are the two main competitive processes that reduce the content of retained austenite achievable at room temperature. A medium carbon low-silicon steel (0.46% C and 0.25% Si) has been studied to identify in which limits the austenite can be enriched in C and stabilized by Q&P, although a silicon content well below 1.5%, commonly used to retard cementite precipitation, is adopted; indeed, high Si contents are detrimental to the surface quality of the product due to the formation of adherent scale in high temperature manufacturing cycles. The heat treatments have been carried out with a quenching dilatometer, investigating the carbon partitioning process mainly below Ms, where cementite precipitation is not activated. The dilatometric curves show the progressive enrichment of carbon in the untransformed austenite and the occurrence of austenite phase transformation during the isothermal holding below Ms. A range of temperatures and times has been found where a content of about 10% of retained austenite can be stabilized at room temperature, a percentage much lower than the theoretical maximum achievable with the carbon content of this steel.

Abstract: Two low alloy steels were selected. Inclusions in them were analyzed by electronic probe micro-analyzer. The Influence of inclusions in low alloy-steels were discussed by polarization tests and simulated occluded corrosion cell combined with micro-analyse. The results showed that the inclusions in low alloy-steels were the main pitting initiation and the sulfide inclusions were easier to cause pitting corrosion than other inclusions. The inclusions also accelerated pitting corrosion propagating.

Abstract: In temper bead welding, toughness is one of the key criteria to evaluate the tempering effect. A neural network-based method for toughness prediction in the coarse grained heat affected zone (CGHAZ) of low-alloy steel has been investigated in the present study to evaluate the tempering effect in temper bead welding. Based on the experimentally obtained toughness database, the prediction systems of the toughness of CGHAZ have been constructed using RBF-neural network. The predicted toughness of the synthetic CGHAZ subjected to arbitrary thermal cycles was in good accordance with the experimental results. It follows that our new prediction system is effective for estimating the tempering effect in CGHAZ during temper bead welding and hence enables us to assess the effectiveness of temper bead welding.

Abstract: In the present study, the rare earths element YbCl₃ was added into the Zn-epoxy coating, and the MIC behavior of the low alloy steel specimen with YbCl₃-Zn-epoxy coating in the sulfate-reducing bacteria (SRB) solution was investigated and compared with the specimen of Zn-epoxy coating. Experimental results show that both the YbCl₃-Zn-epoxy and Zn-epoxy coating may offer effective protection for the basic low alloy steel. However, the corrosion potential of the specimen with YbCl₃-Zn epoxy coating was obviously higher than the Zn-epoxy coating, suggesting that the YbCl₃-Zn-epoxy coating may offer more favorable protection. Results of X-rays diffraction (XRD) and surface micrographs of specimens show that more sulfides and corrosion products of YbCl₃-Zn-epoxy coating were produced which increased the shielding property of the coating. The mechanism about the effect of YbCl₃-Zn-epoxy coating on MIC behavior was further discussed.