High Strength Low Alloy Research Articles

High strength low alloy steel joint fabricated by laser welding with real-time high-frequency resistance heating

The balance between softening and hardening is still a great challenge for the laser welding of the high strength low alloy steel. For example, welding with low heat input was commonly used to avoid softening, but it would inversely harden the joint and introduce the undesired welding defects such as hydrogen-induced cracks. This work proposed a simple and robust processing technique, denominated High Frequency electric cooperated Laser Welding. Laser heat source was combined with high-frequency electric heat source to balance heat input distribution and reconcile the contradiction between softening and hardening in the welding of high strength low alloy steel. As a structural material sensitive to welding softening and hardening, S690QL steel was adopted to show the fundamental advancement of the new process. Benefitted from the skin effect and proximity effect of high-frequency current that improve the heat input distribution and the regulation of the high-frequency heat input proportion, the martensite content in the heat affected zone was greatly reduced, whilst the grain size was limited to a low value. Under the high-frequency heat input proportion of 47.1 %, lower bainite distributed throughout the heat affected zone. The softening and hardening issues of the joint therefore have been improved simultaneously: the tensile strength of the joint was almost equal to that of the base material, the impact energy of weld seam and the heat affected zone reached 77.6 J and 66.2 J respectively.

Influence of Ce on the corrosion resistance of high-strength low-alloy steel in simulated marine environments

Steel with enhanced corrosion resistance in both simulated deep sea and shallow sea environments was developed by adding cerium (Ce) to high-strength low-alloy (HSLA) steel. Through conventional characterization techniques, the electrochemical performance, rust layer composition, and protective properties of the steel were analyzed. The study found that the steel exhibited superior corrosion resistance when the Ce content was 0.2 wt%. This improvement is largely attributed to Ce's role in promoting the formation of α-FeOOH, which results in a dense and protective rust layer. Additionally, Ce consumes oxygen and inhibit the cathodic depolarization reaction. Furthermore, due to the low oxygen solubility in the simulated deep sea environment, the cathode reaction is further inhibited, leading to a lower corrosion degree in the deep sea environment compared to the shallow sea environment.

Influence of microalloying on precipitation behavior and notch impact toughness of welded high-strength structural steels

Microalloying elements such as Nb and Ti are essential to increase the strength of quenched and tempered high-strength low alloy (HSLA) structural steels with nominal yield strength ≥ 690 MPa and their welded joints. Standards such as EN 10025–6 only specify limits or ranges for chemical composition, which leads to variations in specific compositions between steel manufacturers. These standards do not address the mechanical properties of the material, and even small variations in alloy content can significantly affect these properties. This makes it difficult to predict the weldability and integrity of welded joints, with potential problems such as softening or excessive hardening of the heat-affected zone (HAZ). To understand these metallurgical effects, previous studies have investigated different microalloying routes with varying Ti and Nb contents using test alloys. The high-strength quenched and tempered fine-grained structural steel S690QL is the basic grade regarding chemical composition and heat treatment. To evaluate weldability, three-layer welds were made using high-performance MAG welding. HAZ formation was investigated, and critical microstructural areas were identified, focusing on phase transformations during cooling and metallurgical precipitation behavior. Isothermal thermodynamic calculations for different precipitations were also carried out. Mechanical properties, especially Charpy notch impact toughness, were evaluated to understand the influence of different microalloys on the microstructure of the HAZ and mechanical properties.

Strong and ductile low carbon low alloy steels with multiphase bimodal microstructure

Restrained by the strength-ductility tradeoff, it is still challenging to develop advanced high-strength low carbon low alloy (LCLA) steels with superior strength-ductility combinations and cost-effectiveness to satisfy industry demands. In this study, an innovative 2-cyclic quenching and partitioning (Q&P) heat treatment was developed to produce a novel LCLA steel with the optimized microstructure, in which a bimodal grain size distribution across various constituent phases was achieved. Tensile test results show that the 2-cyclic Q&P LCLA steel exhibits excellent mechanical properties with a uniform elongation, close to 18 %, nearly triple that of conventional Q&P LCLA steel while maintaining a tensile strength above 1 GPa. To reveal the underlying mechanisms of such exceptional strength-elongation synergy, the detailed deformation behaviors of the developed LCLA steel were characterized while the evolution of hetero-deformation-induced (HDI) stress and effective stress was investigated from the perspective of the dislocation model. It is indicated that, with increasing strain, the heterogeneous structures promote strong strain partitioning which leads to extensive geometrically necessary dislocations (GNDs) pile-ups at hetero-interface and persistently strong HDI strengthening effect, and produce the coordinated deformation among constituent phases to realize dislocation forest strengthening, collectively contributing to the enhanced work hardening capacity and hence overcoming the strength-ductility tradeoff. This study provides a new processing strategy for developing strong and ductile LCLA steels.

Effects of Partially Replacing Mo with Nb on the Microstructure and Properties of High-Strength Low-Alloy Steel during Reverse Austenization

This study investigated the effects of partially replacing expensive Mo with cheaper Nb on the microstructure and properties of high-strength low-alloy (HSLA) steel during reverse austenisation. The mechanical properties of the steel in the hot-rolled state were lower with a partial replacement of Mo by Nb. However, after pre-tempering and reheating and quenching, the strength increased greatly while the ductility and toughness did not decrease much. Thus, the negative effects of replacing Mo with Nb were mostly alleviated, and a good balance between strength, ductility and toughness was achieved. After heat treatment, the mass percentage of precipitates increased substantially, which helped to pin grain boundaries during austenisation. The percent of high-angle grain boundaries greatly increased while the average effective grain size decreased, which improved grain refinement. The results showed that combining a partial replacement of Mo by Nb with heat treatment allows the microstructure and mechanical properties of HSLA steel to be effectively controlled while improving the balance between cost and performance. These findings provide valuable insights into the preparation and design of steels with similar microstructures.

Effect of charging temperature of continuous casting slab on the surface cracks of HSLA steel thick plate

In order to investigate the formation mechanism of the surface cracks on high-strength low-alloy (HSLA) steel thick plates, this paper simulated the temperature history of different hot charging processes, and deduced the microstructural transformation behavior and recrystallization austenitization process under different charging temperatures. This paper proposed four hot charging modes and ultimately elucidated the effect of charging temperature on the surface cracks of HSLA steel thick plates. Experimental results showed significant differences in the microstructure and recrystallized austenite grains under different charging temperatures. Analysis revealed that the recrystallized austenite grains were determined by the microstructure. The presence of austenite in the microstructure significantly increased the size of recrystallized austenite grains, thereby disrupting the uniformity of grains. Based on the relationship between charging temperature, microstructure, and recrystallized austenite grain state, the hot charging process of HSLA steel was divided into four temperature zones: high-temperature single-phase zone, high-temperature two-phase zone, medium-temperature three-phase zone, and low-temperature two-phase stable zone. The occurrence of the surface cracks in HSLA steel was caused by the recrystallized austenite mixcrystal structure generated during charging in the medium-temperature three-phase zone. The uneven recrystallized austenite grains caused by undercooled austenite were the direct cause of the surface cracks, while high charging temperature was the root cause. When the charging temperature entered the medium-temperature three-phase temperature zone, recrystallized austenite grains would exhibit mixcrystal structure. These mixcrystal structure deteriorated the surface quality, promoting the formation, deformation, and propagation of surface cracks. To reduce the occurrence rate of surface cracks, for steel grades sensitive to surface quality, the charging temperature should be controlled in the low-temperature two-phase stable zone.

Investigation on Surface Roughness and Power Consumption for Sustainability Assessment in Hard Turning of HSLA Steel With SPPP‐AlTiSiN–Coated Carbide Tool Under Various Cooling‐Lubrications

ABSTRACTThe present research analyses the power consumption (Pc) and surface roughness (Ra) in hard turning of high‐strength low‐alloy (HSLA) grade AISI 4140 steel using a recently developed AlTiSiN‐coated carbide tool under different cooling‐lubrication conditions (dry, flooded, nanofluid‐MQL). The nanofluid was prepared by mixing the MWCNT nanoparticles with an eco‐friendly automotive radiator coolant (base fluid). The cooling‐lubrication performance is investigated briefly by comparing the machining responses like machined surface morphology, tool wear, cutting force and temperature. The experiments associated with 46 trials were performed by considering various machining variables, namely cutting speed, nose radius, depth of cut, feed and cooling‐lubrication methods. From the perspective of predictive modelling and multi‐response optimisation, response surface methodology has been employed to minimise power consumption and surface roughness. Thereafter, the predictive modelling and optimisation results are implemented for economic analysis and energy‐saving carbon footprint evaluation. This innovative research also addresses comparative environmental sustainability evaluation in hard turning under different cooling‐lubrication conditions using a life cycle assessment methodology for cleaner and safer production. Results indicate that cutting speed was the most influential item in power consumption enhancement. Furthermore, compared with dry and flooded turning, lower cutting force, reduced cutting temperature, shorter width of flank wear and better surface morphology were obtained under nanofluid‐MQL machining. It has been observed that nanofluid‐MQL machining outperformed sustainability improvement concerning techno‐economically viable societal acceptable and environmental friendliness.

Corrosion study of glass coated high strength low alloy steel

In the present study, the glass slurry coating to a high-strength low-alloy steel (HSLA) substrate was investigated for corrosion resistance material. The crystallographic phases, degree of crystallinity, lattice parameters, and potential changes due to the coating process, and microstructural changes such as visual inspection of the surface, revealing thickness, uniformity, adhesion, potential defects, and elemental composition of the bare and coated steel were investigated by using the X-ray diffraction (XRD) and scanning electron microscopy and electron dispersive spectroscopy (SEM/EDS). The electrochemical behavior of the bare and coated steel samples was compared using potentiodynamic polarization and electrochemical impedance spectroscopy at various immersion periods in a 3.5 wt.% of sodium chloride (NaCl) solution. The higher corrosion resistance and lower corrosion rates of the coated steel surfaces were observed. The protection effectiveness of coatings is evaluated by examining their impedance and polarization characteristics. The effectiveness of the glass coatings in inhibiting corrosion is attributed to their role as a physical barrier and their ability to release corrosion inhibitors into the surrounding electrolyte. This information can be crucial for real-world applications requiring corrosion resistance.

Flux enhancement with titanium or vanadium oxides addition for superior submerged arc welding of HSLA steel plates

A high-strength low-alloy (HSLA) steel plate of 10 mm thickness underwent submerged arc welding with enhanced fluxes containing additional titanium oxide (TiO2) or vanadium oxide (V2O5). The addition of TiO2 led to the development of a finer acicular ferrite structure but coarsening the carbide and martensite/austenite (M/A) constituents, which marginally improved the hardness, tensile strength, and ductility of weld metal. Conversely, incorporating V2O5 facilitated a substantial vanadium absorption (0.7 wt. %) in the weld metal, giving rise to a distinctive acicular microstructure less reliant on ferrite nucleation at non-metallic inclusions than conventional acicular ferrite. The distinctive microstructure, unique to vanadium steels, combined lath bainite with irregularly shaped granular bainite. The resultant dual-mode bainitic structure, coupled with a more uniform distribution of refined microphase constituents, outperformed the conventional acicular ferrite, delivering more than 20% and 13% improvements in yield and tensile strengths respectively, as evidenced by transverse tensile tests on the weld metals.

Role of M-A constituents in bainitic microstructure on crack propagation behavior in the ICCGHAZ of HSLA steels for offshore applications

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.

Effect of Post-Deformation Cooling on Microstructure and Mechanical Properties of a Cu-Containing High-Strength Low-Alloy (HSLA) Steel

Effect of Post-Deformation Cooling on Microstructure and Mechanical Properties of a Cu-Containing High-Strength Low-Alloy (HSLA) Steel

Tailoring the strength and low-temperature toughness of HSLA structural steel by adding trace Ce

In this study, the trade-off between strength and low-temperature toughness of high-strength low-alloy (HSLA) structural steel was solved by adding trace Ce, which increased yield strength by 5.4 % and Charpy V-notch (CVN) impact energy at −40 °C by 57.9 %. The strengthening and toughening mechanism was revealed by multi-scale characterization. The results show that the strain hardening ability is improved after adding Ce. The grain size is refined and the geometrically necessary dislocation (GND) density increases to ensure the strength improvement. The CVN impact specimens change from cleavage fracture to quasi-cleavage fracture. The grain size, inclusions and high angle grain boundaries (HAGBs) are refined and optimized, which enhances the resistance to impact crack propagation and improves the low-temperature impact toughness.

Cluster mediated high strength and large ductility in a strip casting micro-alloyed steel

Exhibiting exceptional mechanical properties and formability, high strength low alloy steels characterized by a single ferrite microstructure with finely dispersed nano-precipitates (ferritic HSLA steel) have garnered notable attention in the automotive industry. Nevertheless, to maximally utilize the precipitation hardening effect, these steels necessitate substantial additions of carbide-forming elements, unavoidably narrowing the process window and escalating the cost. Strip casting, featuring a streamlined process chain and high energy efficiency, has emerged as a promising technique for developing ferritic HSLA steels. In this work, leveraging the process characteristics of strip casting, we report that a novel single ferrite microstructure with multi-atomic layered clusters distributed in both interphase-precipitation and random fashions was engineered in a low Nb micro-alloyed ferritic HSLAs via raising the coiling temperature to 650 ℃. The multi-atomic layered clusters play a pivotal role in tailoring dislocation behaviors, facilitating local double cross-slips, contributing to dislocation multiplication and homogeneous distribution. These mechanisms collectively sustain mild work hardening to higher strains, leading to combined strength and ductility increments. In comparison to their cluster-free bainitic counterparts coiled at 480 ℃, the results demonstrate significant mechanical improvements with an increase in ultimate strength (630 MPa to 670 MPa) and a 90 % rise in plasticity (10.3 % to 19.1 %), signifying an alternative pathway for advancing the utilization of strip casting technology in designing and processing novel low-cost, high-performance HSLAs.

Correlation between microstructure and fracture mechanism of 960 MPa Grade ultra-high toughness steel

This study investigated the correlation between microstructure and fracture mechanisms in a 960 MPa grade ultra-high toughness high strength low alloy steel produced through controlled rolling and off-line heat treatment. The steel features a low-carbon composition (≤0.1 wt%) and a fine-grained(≤10 μm) bainitic microstructure. Through microstructural characterization and fractographic analysis, extensive quantitative analysis of grain, packet, dimple, and cleavage facet sizes was conducted. Fracture surfaces were examined to identify crack initiation sites. The findings indicate that grain refinement significantly enhances toughness. In fine-grained conditions, bainitic packets that approximate the size of the grains modulate crack propagation pathways. The near-oriented bainite across adjacent grains forms structures comparable to abnormally large grains, crucial for crack nucleation. Moreover, martensite/austenite (M/A) islands, while crucial to the initiation of cracks, require the synergistic interaction with grain boundaries and inclusions to significantly influence fracture initiation. This investigation elucidates the complex interplay between microstructural refinement and fracture mechanisms.

Formation cause and oxidation behaviour of surface crack on the hot-rolled thick plate of high strength low alloy steel

The formation causes and oxidation behaviour of two types of small-sized and irregular morphological surface cracks on S355 hot-rolled thick plate were investigated. The study employed surface pickling, microstructure analysis and inclusion observation to characterise the cracks. It was observed that the surface morphology of two types of cracks exhibited similarities, while their subsurface morphology differed. The Type-A cracks displayed a bulge shape with a rounded tail, while the Type-B cracks had a V-shaped morphology with a slender tail. The Type-A cracks predominantly consisted of FeO. The surface part of the Type-B cracks contained unusual large-sized carbon particles and Al2O3 inclusions, and the iron oxides within the subsurface part followed the sequence of FeO–Fe2O3–Fe from the centre of the cracks to the steel matrix. It was observed that the Type-A cracks originated from subsurface bubbles in the continuous casting slab, which deformed and developed into surface cracks during the rolling process. The iron oxide inside these cracks was formed through a single oxidation process, and the bulge shape in the subsurface part was a distinguishing characteristic. The Type-B cracks were attributed to surface microcracks on the continuous casting slab, which were linked to the presence of unusual large-sized inclusions. The iron oxides within these cracks underwent two oxidation processes. The subsurface morphology and oxide characteristics are the significant features to identify these small-sized and irregular morphological cracks.

Annealing response and yielding behavior of cold rolled advanced HSLA steels

High-strength low-alloy (HSLA) steel is well established in car body manufacturing. They comprise a lean low-carbon alloy concept and can be produced by a wide variety of processing routes. However, cold-rolled HSLA steels are typically limited to a yield strength of 460 MPa by current standards. This research work will present a concept of extending the yield strength range of cold-rolled HSLA steels to above 500 MPa. A key aspect is to find the right balance between recovery and recrystallization of the cold rolled matrix during the annealing stage thereby mitigating the influence of solute and precipitated niobium. Additional solute draging power was introduced by alloying molybdenum to one of the investigated steels. A detailed microstructural analysis using transmission electron microscopy revealed that under recovery annealing conditions an ultrafine-grained microstructure with ferrite grain sizes between 0.5 and 1 μm can be established leading to large strength contribution via the Hall-Petch relationship. Strength losses related to partial recrystallization can be recovered to an extent of around 200 MPa by the precipitation of nano-sized niobium carbide particles. It is demonstrated that the presence of niobium and molybdenum in solute condition have the most prominent influence on obstructing recrystallization via solute drag. It is furthermore shown that the magnitude of precipitation strengthening correlates with the Lüders strain. The analysis of the work hardening behavior indicated that the n-value in ultrafine-grained condition approaches a lower limit value of 0.03 and raises to above 0.2 with progressing recrystallization as well as precipitation strengthening. The detailed evaluation of the tensile characteristics indicates that the strength limit for these HSLA steels ranging between 830 and 880 MPa is reached when the average grain size is reduced to 0.46 μm. The results of this study used to define robust processing conditions to securely produce steel grades in the 600–800 MPa yield strength range.

Development of 485 MPa Class High Strength Low Alloy Steel for Power Plant Infrastructures

Development of 485 MPa Class High Strength Low Alloy Steel for Power Plant Infrastructures

The influence of the experimental methodology on evaluating the hydrogen embrittlement susceptibility of AISI 4340 steel manufactured by different routes

High strength low alloy steels are widely used in the offshore and marine industries for the manufacturing of fasteners. In this context, the occurrence of hydrogen embrittlement is a risk to the service life of these components. The scope of this work is to evaluate the impact of the steelmaking process on the hydrogen embrittlement susceptibility of AISI 4340 steel through different experimental methodologies. The effects of the presence of segregation bands and inclusions on the hydrogen interaction with the microstructure of the steel were studied through hydrogen electrochemical permeation tests and mechanical characterization. The hydrogen embrittlement susceptibility was investigated by performing uniaxial standard tensile tests with prior hydrogen charging, slow strain rate tests with in-situ hydrogen charging, and incremental step loading tests with notched specimens and in-situ hydrogen charging. Fractographic analyzes were carried out to support the interpretation and discussion of the results. It was observed that continuous casting route results in a microstructure more susceptible to hydrogen embrittlement, due to the presence of coarser segregation bands, with a greater density of impurities and inclusions.

Understanding the influence of environmental conditions on the low cycle fatigue behaviour of high strength low alloy (HSLA) steel under air and corrosive (3.5% NaCl) conditions

Offshore structures which are made up of high-strength low alloy (HSLA) steel experience fatigue load due to sea waves and corrosion. In the present work, the influence of environmental conditions (air and 3.5 % NaCl solution) on the strain-controlled fatigue life of High Strength Low Alloy (HSLA) steel has been determined. Low cycle fatigue experiments were conducted on HSLA steel specimens of 12 mm diameter which were prepared parallel to the rolling direction. The strain amplitude was varied in the range of 0.4 to 0.7 % for a constant test frequency of 0.3 Hz. For all the total strain amplitudes (Δεt/2), a significant reduction in fatigue life was observed while testing the specimens under the presence of 3.5 % NaCl solution compared to the air environment. Microscopic investigation indicates that rigorous grain boundary oxidation has happened, and it has created the grain boundary cracking and several sites for fatigue crack initiation and propagation. The presence of corrosive compounds (Fe2O3, FeOOH) was also confirmed by the X-ray diffraction patterns. In addition, mixed modes (transannular and intergranular) of fractures were observed due to the presence of 3.5 % NaCl solution.

Essential role of Si in enhancing corrosion resistance of high strength low alloy steels in marine environment

High strength low alloy steels are extensively applied in marine engineering structures (cross-sea bridge, ocean platform, etc), which are usually subjected to severe corrosion in marine atmosphere. In this work, the corrosion behaviors of 550 MPa-grade high strength low alloy marine steels with varied Si content were studied in simulated marine atmosphere. Results show the increase of Si could significantly facilitate the formation of α-FeOOH and enrichment of Cr and Cu in inner rust layer, making the rust layer compact and cation-selective. Si and Cr can be co-enriched in inner rust layer to form mixed or complex oxides with amorphous network structure, which could greatly improve the compactness and stability of rust layer and thereby enhance the corrosion resistance of low alloy steel.