HSLA Steel Research Articles

Strengthening and toughening mechanism of coarse-grained heat-affected zone for the Cr-/Mo-free Cu-bearing HSLA steel under high heat input welding conditions

Strengthening and toughening mechanism of coarse-grained heat-affected zone for the Cr-/Mo-free Cu-bearing HSLA steel under high heat input welding conditions

Improvement of strength and toughness of 1 GPa Cu-bearing HSLA steel by direct quenching

A novel process based on recrystallization controlled rolled (RCR) + on-line direct quenching (DQ) and tempering (DQT) was developed to improve both yield strength and low-temperature toughness for 1 GPa Cu-bearing HSLA steel. Reheating quenching (RQ) and tempering (RQT) processes were compared to examine the effects of DQ and DQT on co-precipitation behavior of Cu-rich particles and Mo, V carbides. The evolution of microstructures under various heat treatments was characterized using optical microscope (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD). Tensile properties and low-temperature impact toughness were also tested. Differential scanning calorimetry (DSC) shows that lower activation energy of Cu precipitation in DQ steel than that in RQ steel. The lower activation energy can be attributed to higher density dislocations and finer martensitic laths, which results in high-density and finer Cu-rich particles formed in DQT steel. Meanwhile, the coarsening of Cu-rich particles in DQT steel is also inhibited resulting in the Cu-rich precipitates with ∼5.8 nm in DQ steel aging at 550 °C for 240 min. Subsequently, the precipitation of Mo, V carbides is promoted at the nucleation sites provided by the pre-precipitated high-density Cu-rich particles, with the rapid dissolution of cementite. Synergistic enhancement of yield strength and low temperature toughness has been achieved with a yield strength of 1056 MPa as well as impact energies of 204 J and 74 J at −40 °C and −85 °C, respectively.

Mechanical characterization and fatigue assessment of wire and arc additively manufactured HSLA steel parts

Wire and arc additive manufacturing (WAAM) has been attracting industrial attention due to its high productivity and flexibility. However, fatigue life data of WAAM HSLA steel is scarce. In this study, the mechanical properties and fatigue life of thin-walled HSLA steel parts fabricated by WAAM were assessed via non-destructive and destructive testing, providing a comprehensive analysis on this material’s behaviour. The manufacturing conditions have a more pronounced effect on mechanical properties than the part’s height, which seems not to influence the fatigue life of WAAM HSLA steels.

Effect of crystallographic orientation on crack growth behaviour of HSLA steel

Effect of crystallographic orientation on crack growth behaviour of HSLA steel

Development of an Alternative Alloying Concept for Additive Manufacturing Using PVD Coating

New alloys are needed to adapt the material properties and to improve the weldability of arc-based additive manufacturing processes. The classic development of welding filler materials is time-consuming and cost-intensive. For this reason, an alternative alloy concept is investigated and qualified here. This is based on the thin-film coating of welding filler materials by means of PVD coating. An HSLA steel DIN EN ISO 14341-A G 50 7 M21 is used as the base material. This is alloyed with the elements Al, Cr, Nb, Ni and Ti by means of PVD thin-film coating. This procedure represents an alternative alloy concept. In the scope of the qualification, the influence of the process and material properties is investigated, and the alternative alloying concept is compared with the classical alloying concept of secondary metallurgy. The investigations have shown that the thin film coating on the surface of the welding filler metal affects the process properties in the form of a changed arc length. Furthermore, the mechanical properties and the effect on the microstructure morphology were investigated. These were compared in the same chemical composition with a Mn4Ni2CrMo produced by secondary metallurgy. The results are in agreement with regard to the mechanical properties and the effect on the microstructure morphology.

Determination of the degree of recrystallization of a microalloyed HSLA steel by using metallographic methods

Abstract On the one hand, this work aims at describing the grain elongation of compressed samples of a microalloyed high-strength steel using appropriate parameters. On the other hand, the degree of recrystallization is to be determined based on the samples’ grain elongation. For this purpose, single-hit compression tests were performed with varying test parameter settings using a deformation dilatometer to generate samples characterized by various degrees of recrystallization. Three parameters were chosen to describe the grain elongation. It was found that the parameter defining the ratio length/ width of the original grain provides the highest sensitivity regarding changes in the grain elongation. Aiming at determining the degree of recrystallization, a critical grain elongation threshold value was determined below which it is assumed that a grain is recrystallized. The results show that choosing the right threshold value allows an automated determination of the degree of recrystallization of a microalloyed high-strength steel in the light microscope.

Review of Current Developments on High Strength Pipeline Steels for HIC Inducing Service

Nowadays, an increasing number of oil and gas transmission pipes are constructed with high-strength low alloy steels (HSLA); however, many of these pipelines suffer from different types of hydrogen damages, such as hydrogen-induced cracking (HIC). So many research efforts are being carried out to reduce the detrimental effects of hydrogen damage in HSLA steel pipes. The thermomechanical control process (TMCP) is a microstructural control technique that is able to eliminate the conventional heat treatment after hot rolling. Recent research demonstrated that TMCP provides high HIC resistance without adding high amounts of alloying elements or expensive heat treatments. However, once these HSLA steel pipes are put into service, they experience HIC damage, and the prediction of its kinetics is a necessary condition to perform Fitness-For-Service assessments. To develop a reliable predictive model for the kinetics of HIC, the relations among the microstructural features, environmental parameters, and mechanical properties have to be fully understood. This paper presents a review of the key metallurgical and processing factors to develop HSLA steel pipes, as well as a review of the phenomenological and empirical models of HIC kinetics in order to identify specific research directions for further investigations aimed to establish a reliable and sound model of HIC kinetics.

Determining of the Fatigue Crack Growth Rate of HSLA Steel at Room Temperature

Welded joint is a critical region of a welded structure and fracture mechanics analysis is inevitable in the structural integrity assessment of all welded structures. This paper shows the determining of parameters of the fatigue crack for constituents of welded joints produced of high strength low alloyed steel. The applied methodology refers to the Paris relation where the link was established between the variable load quantity or the corresponding stress intensity factor range and crack growth per cycle. Results have shown that the position of the notch and crack initiation affect the values of the stress intensity range of fatigue threshold ΔKth and parameters in the Paris’ equation. This is mostly expressed when determining growth parameters of the fatigue crack in heat affected zone of HSLA steel, where different changes of growth speed of the fatigue crack clearly express differences in structure of the crack pass.

Control of post-weld fracture toughness in friction stir processed X80 HSLA steel

The present study investigates the fracture toughness of FSW X80 HSLA steel welds. Weld cooling rate and peak temperature were varied among welds; indirectly manipulated through FSW travel speed, rpm, and weld preheat. Fracture toughness was tested according to ASTM 1820 standard along the weld centerline using surface-notched SEB specimen cooled to −40 °C. Results demonstrate that CTOD of FSW X80 has a strong inverse linear correlation with post-weld cooling rate. Low cooling rate welds exhibited CTOD values consistently above those of base metal. Microstructure similar to the base metal can be recreated throughout the weld. Weld CTOD can be selected by controlling weld CR.

A Systematical Evaluation of the Crystallographic Orientation Relationship between MC Precipitates and Ferrite Matrix in HSLA Steels.

Here we systematically investigate the crystallographic orientation relationship (OR) between MC-type precipitates (M, metal; C, carbon) and ferrite matrix in the Ti-Mo microalloyed steel with different processing. In the specimens without austenite deformation, the interphase precipitation can be obtained, and the precipitates obey Baker–Nutting (BN) OR with ferrite matrix. By contrast, in the specimens with austenite deformation, the supersaturated precipitates were formed in ferrite grains, which can obey BN, Nishiyama–Wasserman (NW), Kurdjumov–Sachs (KS) and Pitsch (P) ORs simultaneously. The cooling rate after austenite deformation can influence the OR between carbides and ferrite in the MC/ferrite system. At the cooling rate of 80 °C/s, carbides and ferrite can roughly satisfy these OR with the deviation ≥ 10°, while at the cooling rate of 20 °C/s, carbides and ferrite can strictly obey the specific OR. The energy accumulated in the deformation process and maintained in the fast-cooling process (80 °C/s) can offset the formation energy of the carbides. Thus, the carbides formed in the specimen with the cooling rate of 80 °C/s do not strictly satisfy the specific ORs to meet the rule of lowest energy, and then deviate by a small angle based on the specific ORs.

Effect of Heat-Input on Microstructure and Toughness of CGHAZ in a High-Nb-Content Microalloyed HSLA Steel.

The effect of various heat inputs on the microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAS) of a niobium microalloyed (0.14 wt.%) low-carbon steel was studied. The results showed that higher impact toughness was achieved at a low heat input of 20 kJ/cm, which resulted from the formation of acicular ferrite laths/plates. They sectioned large prior austenite grains into many smaller regions, resulting in smaller crystallographic grains and high-angle grain boundaries. Conversely, when specimens were simulated with larger heat-inputs (100, 200 kJ/cm), the microstructure of the CGHAZ was predominantly composed of granular bainite plus massive MA constituents, thus impairing the impact toughness.

Austenitic Stability and Strain-Induced Martensitic Transformation Behavior of Nanocrystalline FeNiCrMoC HSLA Steels

Austenitic Stability and Strain-Induced Martensitic Transformation Behavior of Nanocrystalline FeNiCrMoC HSLA Steels

Modelling stamped edges in FEM breakage analyses of high-strength steel safety components

The development process for new safety components includes break load tests, where the component undergoes very large strains. Besides the mechanical properties of the steel, which are changed by the forming process, the break load depends on the quality of the stamped edges. Target of this paper is to investigate whether a hole-tensile test with a stamped hole can demonstrate the effects of stamped-edge quality on the load bearing capacity – and secondary, how such a test procedure can be reproduced with a multi-scale FEM procedure. A series of Hole Tensile Tests (HTT) has been performed with a HSLA steel grade. Here, only minor differences in break load and total elongation were found. The local strain just prior to breakage at the edge of the stamped hole shows remarkably high values, but little difference is found when comparing a machined sample to stamped samples. The multi-scale FEM approach was demonstrated using literature data for dual-phase steel, which is known to be more sensitive to edge cracks. Indeed, a stamped edge, which includes significant hardening and pre-damage, shows earlier fracture in this FEM calculation – however, the crack propagation, which is needed to capture the full breakage of the HTT sample, is not modelled correctly – this limits the application of the method on safety components.

Microstructure Changes related Mechanical Properties of Welded HSLA Steel Plates

Strength and toughness of steel are the most desirable mechanical properties required in several engineering applications such as marine and oil industries. In this work, the influence of submerged arc welding (SAW) process on the evolution of microstructure and mechanical properties of locally produced low carbon steel plates that is used by Marine Industry and Maintenance Company (Misurata) as an alternative to the imported marine steel plates is investigated. Microstructure investigation, tensile tests, impact toughness, and hardness tests for both steels are conducted. Results show that microstructure is affected by the heat input of SAW leading to more uniform structure of ferritic-bainitic and finer grain sizes. Inherently, affecting the mechanical properties since a decrease in toughness in the heat affected zone HAZ and weld metal zone WM compared to the parent metal is observed. It is shown that the imported steel has higher toughness than local steel for pre-welded and welded plates, with imported steel offering significant reduction in toughness when subject to welding. The effect of welding can be related to the microstructural changes of the weld metal and HAZ attained by adapting proper consumable electrode and welding process strategies.

High-quality remanufacturing of HSLA-100 steel through the underwater laser directed energy deposition in an underwater hyperbaric environment

Underwater laser directed energy deposition (UDED) can be employed to repair and maintain the offshore engineering structures due to its advantages of flexible adjustment of feedstock materials and controllable heat input to the structures. For the first time, preprepared HSLA-100 steel plates were successfully remanufactured by UDED at an ambient pressure of 0.3 MPa (water depth of 30 m). The relationships between the hyperbaric underwater environment, solidification process, microstructures and mechanical properties of the HSLA-100 steel were clarified. The results show that the cooperation of surrounding water, central gases and gas curtain gas with large flow rates increased the cooling rates of the underwater melt pool. A lath martensitic microstructure with high dislocation densities and a number of inclusions was formed in the sample remanufactured by UDED. The in-situ precipitation of Cu-enriched nanoparticles was caused by the unique intrinsic heat treatment involved in the underwater deposition process. The average diameter of the Cu-enriched nanoparticles increased with increasing laser energy density. The microstructure of the sample remanufactured by UDED was harder than that of the sample remanufactured by in-air DED. The Charpy impact toughness and tensile properties of the samples remanufactured by UDED were close to those of the sample remanufactured by in-air DED. This work demonstrates the feasibility of high-quality remanufacturing of HSLA-100 steel via UDED in a hyperbaric underwater environment. The results obtained in this study could provide useful guidance for the application of UDED to offshore engineering structures.

Elimination of Primary NbC Carbides in HSLA Steels for Oil Industry Tubular Goods

Two high-strength low-alloyed steels alloyed with various Nb contents were prepared using continuous casting (CC) technology. In the round Steel 30Nb CC billet, it was the segregation of C and Nb in the central equiaxed zone that promoted the precipitation of primary NbC carbides at grain boundaries. After processing into seamless tubes, micron-sized primary NbC carbides were located in the middle of the segregation bands, surrounded by many nano-sized NbC carbides formed during the post-solidification cooling and/or hot-piercing rolling. Even when the Nb content was decreased down to 0.006 wt.%, there were still coarse primary NbC carbides in the segregation bands. However, no primary NbC carbide was found in the round Steel 30Nb billet prepared using the electroslag remelting technology. Therefore, this study revealed that a rapid solidification rate was relatively effective in eliminating primary NbC carbides compared with merely decreasing the Nb content.

Effect of rare-earth metals on the hot strength of HSLA steels

Abstract An experimental study was done on the effect of rare-earth metals on a high-strength low-alloy steel. The work was focused in deriving the influence on Ce and La on the hot-working behavior. For this purpose, uniaxial hot-compression tests were carried out in a wide range of temperatures and strain rates. The effect of the rare-earth metals was determined by comparison of the characteristic parameters, describing the constitutive equations of the high-temperature response of the steel with, and without, rare-earth metals. The results showed that rare-earth metals were playing a major significant role on hardening mechanisms rather than on softening by dynamic recovery. On the contrary, rare-earth metals were able to delay the onset of dynamic recrystallization. All the present experimental results suggested that the latter roles are played by solid solution strengthening, through a solute drag effect, and not by precipitated particles.

Influence of pre-straining on mechanical properties of HSLA steel by using ball indentation technique

Abstract Ball indentation technique has been employed to explore the possibilities for revealing the effect of pre-strain on mechanical properties of two groups of high-strength low-alloy steels. These steels were pre-strained up to 5%. Results are presented here for 1, 3, 4, and 5% of pre-strain effect on these steels. The dependence of yield strength, ultimate tensile strength, strength coefficient, strain-hardening component and true stress–strain curves on pre-straining effect are available from the ball indentation test, and they are validated with the standard mechanical test results. It is found that the increment of the pre-straining effect results in an increment of the yield strength and ultimate tensile strength values, while strength coefficient and strain-hardening component values are decreased. The existence of Lüders bands in the conventional stress–strain curves is also revealed in ball indentation test by the presence of the Lüders strain surrounding the indentation profile. In the conventional test it is found that the amount of Lüders bands is decreasing with increasing pre-strain. Similarly, Lüders strain as well as pile-up/sink-in of materials surrounding the indentation decrease as the pre-straining increases. It is found that ball indentation technique can be effectively used to determine the minute changes of mechanical properties due to introducing small amount of pre-straining.

Factors influencing the extent of hydrogen-enhanced brittle cracking in a Cu-strengthened HSLA steel during monotonic loading

Abstract The extent of brittle crack as a function of hydrogen charging conditions was studied for a HSLA steel using circumferentially notched cylindrical tensile samples. Two different notch depths were used. The effect of hydrogen could be well represented by an effective hydrogen potential which was defined using a representative hydrogen concentration and a diffusive time parameter, for relatively faster strain rates. The high triaxiality in deep-notched samples led to the initiation of ductile failure mechanisms overwhelming the brittle cracking process.

Significant improvement in CGHAZ toughness of HSLA steel via welding with trailing mechanical treatment

The Coarse-grained heat-affected zone (CGHAZ) is well known as the local brittle zone in the welding joints of HSLA steels. In the present work, compressive strain is applied to the specimen during the CGHAZ welding thermal simulation process to achieve microstructure refinement and modification for toughness improvement. The strain is applied to the specimen when the temperature of the simulated thermal cycle decreased to 1100 °C, with the purpose of achieving dynamic recrystallization in the CGHAZ. Parallel experiments without the compressive strain are also conducted as a reference. The microstructure of the CGHAZ with and without the compressive strain was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used for observation of the matrix and microstructure of martensite-austenite (M-A) constituents. Electron back-scattered diffraction (EBSD) techniques were used to get the information of boundary misorientation distribution. The toughness of the specimens under each condition was assessed through instrumented Charpy impact test at −20 °C. Results showed that the compressive strain refined prior austenite grain (PAG) size and packet size, while the block width and lath width were almost unaffected. Refinement of the PAG also modified the microstructure of M-A constituents. The microstructure refinement and modification of M-A constituents improved crack initiation energy and crack propagation energy, resulting in significant promotion of toughness from 60.20 J to 163.10 J.