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The objective of the study is to understand the effect of martensite-austenite (MA) constituents along with the neighbouring bainitic matrix and its effect on low-temperature impact toughness in the thermo-mechanically simulated inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of offshore High Strength Low Alloy (HSLA) steels. ICCGHAZ was simulated using a Gleeble 3500 thermomechanical simulator between the inter-critical temperature near Ac1 (lower ICT) and at a temperature 50 °C higher (upper ICT) for two different steels (steel A and steel B) with predominant microstructures composed of granular bainite (GB) and bainitic ferrite (BF), respectively. MA constituents were observed in the form of necklaces along prior austenitic grain boundaries (PAGB) of simulated ICCGHAZ for both steel A and steel B. Steel A combrised of discrete island MA constituents inside the GB grains and steel B consists of lath shaped MA constituents inside BF grains. Fractographical analysis showed that secondary cracks were initiated at the MA constituents along PAGB and propagated linearly into the GB/BF grains in the ICCGHAZ. The cracks initiated at the bulky MA at grain boundaries of steel A moved towards the pre-cracked or de-bonded discrete MA inside the grains and propagated through the interlinking of microcracks. In contrast, for steel B a notable percentage of cracks were initiated through the necklace-like MA at the grain boundaries and propagated through the BF lath. Pre-cracking of the slender MA present at the lath boundaries facilitated crack propagation in steel B. Furthermore, increasing the inter-critical temperature improved the impact toughness of steel A with a predominantly GB microstructure, whereas impact toughness improvement was limited in steel B with a BF microstructure.

Precipitation in a high-strength low-alloy steel containing V and Nb additions during coiling at 500 °C, was investigated by advanced characterization techniques including electron backscattered diffraction, focused ion beam, transmission electron microscopy and atom probe tomography. During coiling, nano precipitates were preferentially formed around dislocations and grain/sub-grain boundaries in the high Kernel Average Misorientation (KAM) areas associated with bainite. Precipitates were frequently observed around cementite in the low KAM areas with cementite which were identified as granular bainite. Interphase clusters were found in the low KAM areas associated with ferrite. The site-specific methods reveal the co-existence of different kinds of precipitates. Analysis of the results indicates that the precipitation of microalloy particles close to cementite could lower the precipitation hardening contribution that could be achieved through microalloying.

A model has been developed for describing the competition between the formation of microalloyed carbonitrides by strain-induced precipitation (SIP) and by epitaxial growth on pre-existing TiN particles in γ-Fe. It is assumed that Nb and C/N atoms in the matrix will either feed the growth of pre-existing TiN precipitates or precipitate as new strain-induced particles. The model is successfully applied to the data set obtained by transmission electron microscopy (TEM) in a model alloy. Using this model, it is possible to estimate the effects of process parameters (T, applied strain), the number density of pre-existing TiN, and the Nb, C, N and Mn content on precipitate evolution and precipitation hardening. Notably, it is shown that the presence of a high number density of pre-existing TiN precipitates can suppress SIP. The conditions where epitaxial growth dominates and those in which strain-induced precipitation dominates are predicted and summarized in the form of processing maps.

Hydrogen-Induced Cracking (HIC) is a primary failure mechanism of pipeline-welded joints in the absence of external loading in the oil and gas exploration industries. Three different cooling rates after austenitization were used to simulate in the laboratory different regions of the heat-affected zone (HAZ) formed when welding an API X80 pipeline steel specially designed to enhance the HIC resistance. The samples were characterized with regard to microstructure and crystallography as well as HIC resistance. The HIC resistance test used NACE TM0284-2011 methodology. The microstructure and its homogeneity varied as a function of cooling rates. Samples containing inclusions and segregation zone from the segregation bands of specimens showed reduced HIC resistance, while specimens containing only acicular ferrite and granular bainite coupled with the absence of segregation zone showed significant improvement in HIC resistance. The best HIC resistance results came from samples presenting fine acicular ferrite consisting of fine interlocking plates, with divergent crystallographic orientations, preventing the formation of localized strain distribution inside the grain and at grain boundaries. It was also found that a large proportion of medium-angle boundaries prevent microcrack initiation and the transgranular mode of crack propagation.

The paper is based on laboratory experiments of heat treatments applied to samples of A537/A537M steel. The work continues other previous works aimed at modifying structures and properties of this steel, including through surface treatments. The experiments were performed using Taguchi methods from Quality Engineering. A number of four factors were selected as influencing the structure after heat treatment: heating temperature for hardening, cooling rate on hardening, time and tempering temperature. A number of nine experiments were performed using an L9 orthogonal matrix. Objective function was changed to maximum hardness after the heat treatment regime. The results show that the tempering temperature has the greatest influence on the final hardness of the A537 steel samples.

Additive manufacturing with steel offers new opportunities for the construction sector. In particular, direct energy deposition (DED) processes such as Wire Arc Additive Manufacturing (WAAM or DED-Arc), are able to create large structures with a high degree of geometric freedom, like force-flow-optimized steel nodes and frameworks, as well as truss structures. By using high-strength steel, manufacturing times can be shortened because less material has to be applied. In order to enable the usage of WAAM components in the construction industry, profound knowledge of the material behavior is necessary. Based on reliable process parameters, extensive experimental and numerical investigations are carried out to characterize the influence of layer orientation and overhang angle on the mechanical parameters of WAAM high-strength low-alloy steel (HSLA) walls. The results have been compared to HSLA steel sheet material. It is shown that comparable characteristics exist for Young’s modulus E, yield strength Rp,0.2 and tensile strength Rm with regard to civil engineering applications. The influence of the loading direction on the material level is similar. Only the yield strength shows a slight dependence on the layer orientation for WAAM walls (difference 4.5%). The overhang angle has no influence on the material parameters.

Cu-containing steels can suffer from hot shortness unless Ni is added to protect them but whether this problem also influences the straightening operation is not clear. Previous work on hot ductility has found that only when the tensile samples from Cu-containing steels are oxidised can any deleterious influence of copper be found. However, this paper shows that oxidation is not essential. It is more complex than that and, as Cu has been suggested for TRIP steels at levels up to 2.5% Cu to increase their strength and ductility, a greater understanding is required, both of hot shortness and cracking during straightening. The present paper explores the hot ductility behaviour of steels alloyed with Cu and Ni in the straightening temperature range, 700–1000 °C, when tested in air and in an argon atmosphere. Segregation of Cu to the sulphides and grain boundaries occurred allowing the formation of fine Cu2S particles at the austenite grain boundaries favouring intergranular failure and this was more pronounced under oxidising conditions and required strain. It was concluded that a Cu addition, as well as causing hot shortness at higher temperatures will also cause cracking problems in the straightening temperature range in the more sensitive to cracking grades of steel and although the problems are different they are nevertheless interrelated and provided there is sufficient Ni, both may be avoided.

Flash butt welding is a high-efficiency welding technology that is widely used in industrial development. However, the inclusions and defects generated during the process are unacceptable. The presence of inclusions is one of the main factors affecting the quality of flash butt welding. Suitable flash butt welding parameters such as the preheating temperature and upset distance are essential to eliminate inclusions. In this study, because the number of inclusions on the end face is greatly affected by the flash welding time and upset distance, the impact of different upset distances on the number of inclusions was studied by fixing the flash welding time. Further observations were conducted using a scanning electron microscope. Image analysis software was used on the obtained photos to quantitatively analyze the inclusions on the welding surface. A statistical analysis of the experimental data showed that the upset distance was related to the number of inclusions, and the total number of inclusions on the welding surface had a negative impact on the strength of the product.

The paper presents the findings of the strain rate effect, described in constitutive material models by strain rate sensitivity index m, to the strength and deformation material properties. These were evaluated from stress-strain diagrams recorded at tensile test using PC controlled testing machine TiraTEST 2300 according to STN EN ISO 6892-1. The high strength low alloyed steel H220PD, dual phase steel DP 600 and Trip steel RAK 40/70 were investigated at strain rates  = 0.0021, 0.083 and 0.125 s-1. The results indicate major influence of the strain rate to the strength properties while only minor influence have been found when evaluate the deformation properties. The results also shown single phase materials are more sensitive to the strain rate than dual phase materials.

Se han estudiado dos aceros HSLA (high strength low alloy) que presentan composiciones similares, a excepción del contenido en elementos microaleantes. El propósito del estudio es determinar el efecto del tratamiento térmico y la influencia del contenido en vanadio (V) sobre la microestructura y las propiedades mecánicas de la bainita en cada uno de los aceros. Para ello, se han realizado ensayos de dureza, de impacto y de tracción para cada una de las estructuras bainíticas-ferrita acicular obtenidas por medio de los diferentes tratamientos térmicos realizados. Los resultados muestran como el contenido en V promueve la formación de ferrita acicular, presentando valores bajos de dureza y resistencia mecánica pero mejorando la tenacidad.

In order to ensure performance reliability of a welded product, its quality has to be ensured by proper setting of welding parameters and welding cycle. A quality weld – a weld with no manufacturing, structural or geometric flaws, i.e. with necessary mechanical properties - is achieved only by correct parameter definition and adherence. The knowledge of various effects and relations between welding parameters and their repetition enable an optimal choice of welding parameters.

This paper presents impact energy results of specimens made from high strength fine grained steel TStE 420 after thermal cycle simulation. These results are obtained by examining Charpy specimens. Metallographic analysis is performed, hardness is measured and total impact energy is divided into ductile and brittle components.

In the automotive industry considerable attention is paid to innovations oriented especially to meeting safety and environmental requirements. Concepts of vehicle safety focus on resistance of deformation zone components to impact and resistance to intrusion of undesirable fragments into the passengers´ space. In terms of these aspects the samples of the base materials from micro-alloyed steels HSLA - H220PD, DP 600 and TRIP - RAK 40/70 and also welded by solid-state fibre laser YLS-5000 were analysed. Multi-phase structure materials TRIP - RAK 40/70 and DP 600 showed considerably higher values of absorption capacity, strength, and stiffness than in case of single-phase structure material HSLA - H220PD. The greatest values of absorption capacity, strength, and stiffness were recorded in material TRIP - RAK 40/70 with ferritic matrix and a certain fraction of residual austenite, bainite, and martensite. Absorption capacity, strength, and stiffness have strong correlation with material constant K and strain hardening exponent.