High Strength Low Alloy Steel Research Articles

Comparison of microstructural characteristics and mechanical properties of the high-strength low-alloy steels fabricated by wire arc additive manufacturing versus conventional casting

A comparison study on the microstructure and mechanical property between the wire arc additive manufacture (WAAM) high-strength low-alloy (HSLA) steel and the as-cast one was carried out. A more optimized CMT-based WAAM parameter was selected to fabricate a nearly defect-free single-passed thin-wall part. It was found that the main body of the deposit at the middle zone exhibited a typical granular bainite (GB) duplex microstructure consisting of bainite ferrite (BF) and martensite/austenite (M/A) island. Also, the middle zone exhibited a band segregation between the two adjacent columnar prior-γ grains. The band segregation distance is apparently shorter compared with the cellular segregation distance in the as-cast steel. In addition, the as-cast steel has the bainitic microstructure consisting of larger grain-sized BF and nano-twinned martensite island. On a basis of the microstructure characteristics, the nonequilibrium solidification and solid-solid phase transition accompanying with the segregation behavior during the WAAM deposition of the molten pools were studied. Such the above structured as-deposited steel, in comparison to the as-cast one, was decreased in σy from 594.9 MPa to 426.2 MPa, but increased in σuts from 778.6 MPa to 858.4 MPa, and also greatly increased in εtotal from 10.9% to 20.7%. The apparently enhanced εtotal of the as-deposited steel was interpreted in connection with the fabrication processing-induced microstructural variation of the two steels.

Influence of tool plunging rate on mechanical properties and microstructure of friction stir welded DMR249A high strength low alloy (HSLA) steel butt joints

Abstract The main aim of this study is to analyse the influence of tool plunging rate on tool wear and stir zone characteristics of DMR249A high strength low alloy (HSLA) steel joints developed using friction stir welding (FSW). The HSLA steel plates were welded at five levels of tool plunging rates varying from 2.0 mm min−1 to 3.0 mm min−1. The tool rotational speed of 600 rpm and welding speed of 30 mm min−1 were kept constant during welding. The wear of FSW tool was analysed by microscopic observations of tool condition after welding, stir zone characteristics, weight loss measurement and pin profile analysis. The characterization techniques of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to analyse the microstructural characteristics of stir zone of welded joints. Results disclosed that the severe degradation of FSW tool occurred due to the sudden dynamic impact load on tool pin while plunging and more diffusion of work piece material into the tool pin which leads to insufficient plasticized material flow. The tool plunging rate of 2.5 mm min−1 showed minimum tool wear and produced defect free sound joint.

A comparative assessment of the effects of laser shock peening and ultrasonic shot peening on surface integrity and ratcheting fatigue performance of HSLA steel

In this work, a comparative assessment of the effects of laser shock peening (LSP) and ultrasonic shot peening (USP) on surface integrity and ratcheting fatigue performance of ASTM A588 Grade D high strength low alloy steel was done. Surface roughness, microhardness, microstructure, residual stress, and micro-texture analyses were used to describe the unpeened and peened surfaces. Three sets of tension-compression asymmetric cyclic loading tests were carried out at room temperature on unpeened and peened specimens to evaluate the effects of the applied treatment on strain accumulation and fatigue life. The quantitative dislocation densities of selected specimens were calculated in order to examine the mechanism underlying the softening behavior of USP-treated specimens during fatigue testing. The results are presented in light of the substantial surface roughness, lower compressive residual stress, and weakening of texture intensity revealing an unexpectedly large drop in fatigue strength following USP treatment compared to the unpeened and laser-peened specimens.

Microstructure, Texture, and Anisotropic Properties of High-Strength Low-Alloy Steel

The effects of cold rolling reduction rates and recrystallization annealing temperature on the microstructure, texture, and anisotropic properties of high-strength low-alloy (HSLA) steel were investigated using scanning electron microscopy and electron backscatter diffraction. The results revealed that the constituents of recrystallized, substructured, and deformed structures were strongly affected by cold rolling reduction rates ranging from 33.3% to 66.7% and recrystallization temperatures ranging from 780 to 840 °C. At an annealing temperature of 820 °C, when the cold rolling reduction rate was 33.3%, HSLA steel exhibited a low percentage of recrystallization, with cubic, γ-linear, rolled, and Z-texture (the texture at Euler angles φ1 = 30° and Φ = 20°–30°) structures. The rolled texture and Z-texture increased the strength anisotropy and disappeared at high cold rolling reduction rates. When the annealing temperature was increased from 780 °C to 820 °C, the proportion of recrystallized grains increased, the rolling texture disappeared, and grain orientation gradually gathered in the cubic texture and γ line texture, resulting in low anisotropy of strength. At an annealing temperature of 840 °C, the deformation of the grain disappeared; however, the anisotropy increased compared to annealing at 820 °C because of the formation of a new texture of {001}<−1–20>.

Microstructure-crystallographic texture and substructure evolution in unpeened and laser shock peened HSLA steel upon ratcheting deformation

ABSTRACT Uniaxial ratcheting behaviours associated with microstructural development of unpeened and laser shock peened ASTM A 588 Grade D High Strength Low Alloy (HSLA) steel were studied in this investigation. The mechanism of plastic deformation and crystallographic texture evolution during ratcheting was also studied. For this, the primary experimental works involved were stress-controlled ratcheting fatigue tests (on unpeened/laser-peened specimens) at room temperature. Followed by this, microstructure and texture evolution on the surfaces and/or cross sections of the deformed specimens, using electron back scattered diffraction (EBSD) were studied in detail. Additionally, the substructural evolution of some selected samples was also studied using transmission electron microscopy (TEM). The results indicated that the average grain size of all the unpeened ratcheted specimens was marginally reduced after fatigue tests. The laser-peened specimen, however, showed a marginal increase in average grain size after ratcheting. Continuous dynamic recovery and recrystallisation (CDRR) during ratcheting was thought to be the cause of the observed reduction in average grain size in unpeened specimens, whereas continuous dynamic recrystallisation (CDRX) was believed to be the controlling factor for marginal accretion of grain size in laser-peened specimens. The plastic deformation of investigated steel was qualitatively explained by the observed dislocation patterns and their evolution.

Strong resistance to Zn-assisted liquid metal embrittlement of austenitic-TWIP/martensitic-HSLA multi-layered steel sheets additively manufactured by laser cladding

As a novel design idea to overcome a shortcoming of liquid metal embrittlement (LME) occurred frequently in twinning-induced plasticity (TWIP) steel, multi-layered steel (MLS) sheets consisting of austenitic TWIP and high-strength low-alloy (HSLA) steel grades have been suggested. Two- and four-time clad MLS sheets (C2 and C4) were fabricated by a powder-fed laser additive manufacturing (AM) process using HSLA powders on the TWIP surface, and the LME was investigated in various resistance-spot-welding-time stages. The C2 and C4 sheets consisted of the austenitic TWIP substrate and tempered-martensitic HSLA-clad layer, and the HSLA layer had two or four sublayers having different C and Mn contents reduced step-wise from the TWIP/HSLA interface. LME cracks formed in the C2 spot-welded for 240 or 380 ms, whereas they did not form at all in the C4 spot-welded for 380 ms. This different LME susceptibility depended mainly on the HSLA-sublayer microstructures beneath the Zn coating after their high-temperature exposure during the spot welding. Since LME cracks propagated mainly along prior austenite grain boundaries, the whole austenite existed in the austenite range played a key role in determining whether the LME cracking occurred or not. The total volume fraction of reverted austenite (appeared as fresh martensite at room temperature) and retained austenite was very high at about 68% in the C2 welded for 240 ms, thereby leading to the LME cracking. It was lower than 20% in the C4 even under the severe welding condition of 380 ms, which provided the reason for no LME cracking.

Revealing the precipitation kinetics and strengthening mechanisms of a 450 MPa grade Nb-bearing HSLA steel

This study investigates the precipitation kinetics and strengthening mechanisms of a 450 MPa grade Nb-bearing high strength low alloyed steel. The results reveal that Nb(C, N) and NbC particles with a mean size of 8.2 nm, are distributed in grain interior and grain boundaries (GBs). There are two predominant precipitation morphologies in the grain interior, namely disc-like and spherical shapes. The former precipitated in ferrite and the latter precipitated in high-temperature austenite, relying on the Baker-Nutting and Cube-Cube orientation relationships (ORs), respectively. The particle ratio of disc-like NbC precipitates varied from 1.75 to 2.36, which was consistent with the theoretical particle ratio of about 2.43. Besides, NbC or Nb(C, N) particles with a nearly disc-like morphology were found along GBs, also exhibiting the Baker-Nutting OR with ferrite. Thermodynamic calculations indicated that 39 ppm nitrogen increased the precipitation temperature by about 44 °C. Moreover, under heavy hot-rolling reduction, a significant fraction of Nb(C, N) and NbC precipitated in austenite. Precipitation kinetics analysis reveals that both the resultant heightened deformation stored energy (DSE) and trace nitrogen effectively accelerated the nucleation and growth transformation. The high DSE facilitated the strain induced precipitation in austenite, which presented a linear precipitation distribution. With the increased DSE, both the precipitation-temperature-time curve and nucleation rate time curve shifted towards higher temperature and shorter time. Eventually, NbC and Nb(C, N) particles induced strengthening contribution was estimated to be approximately 84 MPa. The main strengthening contributions in the material include grain boundary and solution strengthening.

Online Grain Size Measurement by Laser Ultrasonics in a Hot Rolling Mill

Laser ultrasonics (LUS) has shown through numerous studies and in-situ laboratory experiments, its suitability to track microstructural features evolution as well as metallurgical transformation in steels during thermal cycles with temperatures reaching 1250°C. This prior work has motivated the use of this technology to be in applied in an industrial hot strip mill (HSM) in Eisenhüttenstadt, Germany. For this purpose, a transportable trolley which embedded LUS components (Generation and Detection Lasers, Two-Wave Mixing Interferometer, Dedicated Fibered Optical Head) was designed. The industrial investigation was carried on moving strips during normal production at several locations of the HSM, resulting in signals for high-strength lowalloy (HSLA) and advanced high-strength steel (AHSS) grades. The development of a specific data processing tool allowed calculating the grain size from the analysis of the online recorded signals. Such results were then compared to predictions from metallurgical models, finding reasonably close values. LUS appears as a suitable solution to monitor microstructural features and metallurgical transformations occurring in steel in a Hot Strip Mill (HSM) line. This approach could enable in the future the development of more reliable models and control loop of the process. This action was supported by the European commission and led by a consortium composed by ArcelorMittal, Imagine Optic and IMI-NRC.

Effect of nitrogen on the corrosion behavior of high-strength low-alloy steel in simulated marine atmosphere

Effect of nitrogen on the corrosion behavior of high-strength low-alloy steel in simulated marine atmosphere

Fabrication of Bimetallic High-Strength Low-Alloy Steel/Si-Bronze Functionally Graded Materials Using Wire Arc Additive Manufacturing

In this paper, bimetallic functionally graded structures were fabricated using wire and arc additive manufacturing (WAAM). The bimetallic walls were built by depositing Si-Bronze and high-strength low-alloy (HSLA) steel, successively. The microstructural evolution of the built structures, especially within the fusion zone between the dissimilar alloys, was investigated in relation to their mechanical properties. The built bimetallic walls showed a high level of integrity. An overall interface length of 9 mm was investigated for microstructural evolution, elemental mapping and microhardness measurements along the building direction. Microhardness profiles showed a gradual transition in hardness passing through the diffusion zone with no evidence for intermetallic compounds. Failure of the tensile specimens occurred at the Si-Bronze region, as expected. Bending tests confirmed good ductility of the joint between the dissimilar alloys. Direct shear test results proved a shear strength comparable to that of HSLA steel. The obtained results confirm that it is appropriate to fabricate HSLA steel/Si-Bronze FGMs using WAAM technology.

Correlation between welding structure and corrosion behavior of high strength low alloy (HSLA) ship steel in Cl--containing solutions

The high-strength low-alloy (HSLA) steel industry is in great demand due to the continued growth of the ship and marine sectors. The microstructure of HSLA steel is greatly influenced by both the heat input during welding and the variations in salinity resulting from changes in climate. This paper presents the development of HSLA ship steel using a vacuum electric arc furnace and heat treatment techniques. Electrochemical corrosion tests were conducted on the substrate, heat-affected zone, and fused zone of HSLA ship steel. The results showed that the heat treatment process can improve the microstructure homogeneity of the HSLA ship steel, eliminate chemical composition segregation, refine the grain, and promote the formation of the microstructure of pearlite and ferrite. The welded interface structure was uniform and free from defects or inclusions. The metallographic microstructure of the fused zone showed a massive distribution of grain boundary ferrite, martensite, and granular bainite, and that of the heat-affected zone displayed superheated characteristics consisting of pearlite and acicular ferrite. The interlocking microstructure of the base material enhanced performance in a 4.2 % NaCl solution. The corrosion resistance of the heat-affected zone and fused zones of HSLA ship steel deteriorated. Grain boundary ferrite may be related to the inferior corrosion resistance of the heat-affected zone, while the coarse microstructure of granular bainite led to the serious corrosion resistance in the fused zone of HSLA ship steel.

Effect of precipitation on the mechanical behavior of vanadium micro-alloyed HSLA steel investigated by microstructural evolution and strength modeling

In the current work, the relationship between the microstructural evolution and strengthening mechanisms in a vanadium micro-alloyed high strength low alloy (HSLA) martensitic steel has been systematically investigated by multi-scale characterizations and modeling. Two heat treatment routes, i.e., solution quenching plus tempering (SQT) and solution quenching plus isothermal annealing (QIA) are applied to produce a martensitic matrix combined with considerable nano-scale MC-type carbides (M = V, Mo, Cr) in the experimental steels. Theoretic calculations of the strengthening contributions are conducted based on quantitative characterizations of microstructural parameters, the overall modeling results agree quite well with the experimental results for estimating yield strength of the specimens. Precipitation and dislocation strengthening are found to be the main factors causing strength evolution with different isothermal holding time (IHT) at 600 °C. In both SQT and QIA specimens, the prolonged IHT contributes to an increased particle size and volume fraction of MC precipitates but a decreased dislocation density, and the most excellent mechanical properties are achieved with IHT of 2 h. The precipitation strengthening of SQT specimens is higher than that of QIA specimens because of its finer particle size and higher volume fraction of the MC precipitates, resulting in the higher yield strength of the SQT specimens than that of the QIA specimens. This can be explained by the hierarchy lath martensite that is highly dislocated with numerous interfaces, providing considerable nucleation points and high-speed atomic diffusion paths for precipitation, leading to the significantly refined MC particle size in the SQT specimens. Moreover, the MC precipitates exhibit no significantly coarsening during isothermal holding at 600 °C, which is a result of the presence of Mo/Cr atoms in MC precipitates that yields precipitate size stabilization due to its higher chemical potentials than that of V, thus maintaining a stable precipitation strengthening after IHT exceeding 2 h.

Influence of lanthanum on inclusions and as-cast microstructures in a low-alloy high-strength steel

Influence of lanthanum on inclusions and as-cast microstructures in a low-alloy high-strength steel

Contrasting the mechanical and metallurgical properties of laser welded and gas tungsten arc welded S500MC steel

S500MC steel is a grade of high-strength low-alloy steel (HSLA) which is widely used in the automotive industry and for agricultural machinery and equipment. Considering properties of this alloy, selection of the welding process and parameters becomes essential to ensure that HSLA assemblies meet specific service requirements. In this work, mechanical and metallurgical properties of S500MC steel produced by autogenous laser beam welding (LBW) and automatic gas tungsten arc welding (GTAW) were compared. Tensile testing, metallography, hardness testing, and fractographic analysis were performed on the welded specimens, revealing that the heat input by these welding processes caused significant microstructural changes within the joints. In LBW samples, the heat input about 10 times lower than that in GTAW produced a finer microstructure, narrower fusion zone width, and smaller heat-affected zone. All fractures of the GTAW specimens occurred in the base metal, while all fractures of the LBW specimens occurred in the weld zone, both regardless of the heat input. GTAW joints exhibited higher mechanical properties (even higher than those obtained in the base metal) as compared to LBW joints.

Experimental study on dynamic behaviour of High Strength Low Alloy Steels at cryogenic temperatures

The dynamic behaviour of the High Strength Low Alloy (HSLA) Steel before and after heat treatment was examined experimentally by performing quasi-static and high strain rate testing at room temperature and cryogenic temperatures. The high strain rate tensile testing of the material was performed on a Split Hopkinson Tensile Bar (SHTB) setup. A customized liquid nitrogen chamber equipped with UTM and SHTB was used to perform the cryogenic temperature testing. The stress–strain response of the material under different loading conditions was recorded and a modified Johnson–Cook material model was derived. It was observed that the flow stress of the material increased when tested at a high strain rate and it further increased at a cryogenic high strain rate loading environment. The microstructure of the material before and after heat treatment was also observed to identify the dominant phase change in microstructure contributing towards improvement in strength after heat treatment. The failure analysis of each category of the samples was examined under the Scanning Electron Microscope (SEM) after the testing. The formation of small and deep dimples for as-received samples and bigger and shallow dimples for heat-treated samples distinguished the type of failure between the quasi-static, high strain rate and cryogenic high strain rate material testing.

Microstructure and dynamic mechanical behavior of wire-arc additive manufactured high-strength steel

The working environment of high-strength steel components produced by wire-arc additive manufacturing (WAAM) often faced challenges of explosion and impact, but the microstructure evolution and dynamic mechanical behavior of WAAM produced high-strength steel were still unclear. In this study, low alloy high-strength steel （HSS） components were produced by WAAM. Split Hopkinson Pressure Bar (SHPB) tests at different strain rates were carried out on WAAM-HSS, and the specimens before and after dynamic compression were characterized by techniques such as SEM, EDS, XRD, EBSD, etc. The results showed that： The as-built WAAM-HSS plate mainly consists of five typical structures: lath martensite (ML), acicular ferrite (AF), polygonal ferrite (PF), martensite-austenite (M-A), and precipitated phase δ. The central area of the precipitated phase is Si and Mn, and the C element is mainly distributed in the surrounding area of the precipitated phase. Under dynamic compression, the as-built WAAM-HSS exhibited obvious yield phenomenon, and the yield stress increased with the increase of strain rate. The yield-strengthening mechanism is mainly contributed by grain refinement strengthening mechanism and dislocation density strengthening mechanism. Under dynamic compression, the martensite transformation occurred in the WAAM-HSS. The deformation mechanism is dislocation slip and twinning, which compete with each other. Dislocation slip deformation is dominant at low strain rate and twinning deformation is more dominant at high strain rate.

Dynamic recrystallization behavior and microstructure evolution of high-strength low-alloy steel during hot deformation

The dynamic recrystallization (DRX) behavior and microstructure evolution of the high-strength low-alloy (HSLA) steel were studied by isothermal compression experiments at temperatures ranging from 850 to 1150 °C and strain rates ranging from 0.01 to 10 s−1 with a true strain of 0.8. The true stress-strain curves show that the flow stress increases with increasing strain rate and decreasing deformation temperature. The constitutive equation of the HSLA steel was established based on the peak stress, and the deformation activation energy was calculated to be 414.263 kJ/mol. Processing maps corresponding to different true strains were constructed based on the dynamic material model. High strain rates and low deformation temperatures are more likely to lead to flow instability. The microstructure deformed at low temperatures and high strain rates mainly consists of elongated initial grains. The DRX nucleation mechanism of the HSLA steel was explained. Many fine grains nucleate at the initial grain boundaries, forming a necklace structure. The serrated or bulged grain boundaries are the beginning of DRX. In addition, the DRX volume fraction model was established, and the DRX grain size was analyzed. High temperatures and low strain rates promote the nucleation and growth of the DRX.

Фрактальна параметризація щільності розподілу включень в структурі металу зварних швів

One of the modern proposed methods actively used for diagnosing metal structures is the method of fractal analysis. This method seeks to find the conjunction between the fractal dimension of individual components of the metal structure and its mechanical properties. The comprehensive use of microanalysis and fractal analysis allows not only for comparing structural characteristics but also for finding a numerical parameterization of the elements of this structure. The problem of advance the mechanical properties of welded joints in metal structures remains relevant due to the constant increase in high-strength low-alloy steels used in welded structures. As known, the mechanical properties of welded joints are determined by their structural components, including non-metallic inclusions. A purely qualitative assessment based on the DSTU 8966:2019 standard does not provide an opportunity for a thorough analysis of such an important characteristic as the distribution of inclusions by size and the character of their distribution in the metal matrix. Existing knowledge is based on the peculiarities of the influence of non-metallic inclusions on the mechanical properties of steel welded joints, which requires constant expansion and deepening cognitions, as this work aims to do. The method of fractal analysis allows considering non-metallic inclusions as voids in the metal matrix, which enables parameterization of the inclusion characteristics in the metal structure. The paper describes a methodology that takes into account the distance between inclusions as well as the size of inclusions, which differently affect the mechanical characteristics of the samples. In particular, this article is a continuation of the work [1], which describes an approach to finding relationships between fractal dimensions and mechanical properties. This work focuses on the impact of proposed inoculants on mechanical properties through the distribution of inclusions throughout the metal sample. Keywords: fractal, inclusion, microstructure, inoculants, distribution

High temperature deformation behaviour and microstructure evolution in Cu containing High Strength Low Alloy (HSLA) steel

ABSTRACT The hot deformation behaviour of a Cu containing high strength low alloy (HSLA) steel has been evaluated by carrying out hot compression tests over a temperature range of 900 °C to 1100 °C and constant true strain rates of 0.0003 to 1 s−1. Although single peak flow curve is observed predominantly, a tendency to exhibit multiple peaks at low strain rates (0.0003 and/or 0.001 s−1) at temperature greater than 950 °C. The important parameters pertaining to dynamic recrystallisation (DRX), such as critical, peak, steady state stress and strain were ascertained, and the DRX state diagram depicting the variation of critical parameters governing DRX with Zener-Hollomon parameter Z was established. The volume fraction of recrystallised grains, computed from flow stress, as expected, follows a sigmoidal trend indicating higher kinetics at low strain rates for a given temperature. Reconstruction of prior austenite grain boundaries (PAGB) based on electron back scattered diffraction (EBSD) technique for a few select specimens clearly elucidated the effect of temperature and strain rate on the DRX behaviour of the steel. The DRX state diagram established here clearly demarcates the deformation space i.e. temperature-strain rate-stain space into regimes where the material exhibits full DRX, hardening/softening + DRX and only hardening (no DRX) behaviour. This would be useful for designing suitable forging sequence to achieve the desired microstructure and mechanical properties.

High-strength and high-toughness weld of S690QL HSLA steel via a high-speed high frequency electric cooperated arc welding without additional heating measures

Problems in balancing joint strength and toughness, caused by the strict requirements of welding heat, have been a long-troubling problem that attracts close attention in the High Strength Low alloy (HSLA) steel welding field. Additional heating measures have been considered as an only and necessary solution with limited effect to deal with the above issue, which not only complicate the welding process but also have some incidental impacts. In this paper, a novel method named High Frequency Electric Cooperated Arc Welding (HFAW) was employed to join typical HSLA steel S690QL without additional pre- or post-heating measures. Relationships between the welding thermal characteristics, microstructure and mechanical properties of different parts in the joint were revealed and compared. Results showed that sound HFAW joint was achieved with a welding heat input and speed of 3630.6 J/cm and 2.3 cm/s, while that in conventional GMAW was 7077.1 J/cm and 0.5 cm/s, respectively. The width of the softening zone in the optimal HFAW joint was narrow to 0.95 mm. Lath Martensite and Bainite were generated in the coarse grain zone with an average grain size of 14.9 μm, rather than coarse feathery Bainite in the GMAW joint (with doubled grain size). Benefitted from the fine grain size, better phase composition, narrow and slight softening, the HFAW joints exhibited excellent tensile strength, moreover, the impact toughness at hardened zone and softening zone reached 0.87 J/mm2 and 1.15 J/mm2, which were at least 60% higher than that in the GMAW joint (0.58 J/mm2 and 0.61 J/cm2).