Influence of chromium and niobium content on the mechanical properties and heat affected zone simulations of low-carbon bainitic steels

Abstract. Controlled rolling followed by directing quenching results in steels with excellent mechanical properties, as well as saving cost by passing reheating cost after normalized rolling. The effect of six different combinations of niobium and chromium on microstructures and on mechanical properties of thermomechanically rolled and direct-quenched low carbon microalloyed steel plates were investigated. The target mechanical properties were 700 MPa yield strength with good impact toughness. Bainite was the dominant microstructure of all investigated steels. The prior austenite morphology and transformed microstructure was studied and compared to tensile properties, impact toughness, hardness and hardenability. Also, Gleeble simulation of heat affected zone (HAZ) was performed using t8/5 of 5 s and 15 s to simulate weldability of steels. Four out of six compositions produced similar mechanical properties as S700 steel, which is currently produced in industry. 4 %Cr with 0.035% Nb can get better mechanical properties than with 0.06 % Nb. Addition of Cr also resulted in hardenability of austenite. Decrease of Cr content increased formation of quasi-polygonal ferrite, and therefore increased impact toughness and elongation and decreased the yield strength. Steels with 4% Cr were also observed to have negligible decrease in hardness at coarse grained heat affected zone (CGHAZ) compared to hardness level of base material.Kromi- ja niobipitoisuuden vaikutus mekaanisiin ominaisuuksiin ja hitsaussimulointeihin matalahiilisessä bainiittiteräksessä. Tiivistelmä. Kontrolloitu valssaus yhdessä suorakarkaisun kanssa mahdollistavat teräksille erinomaiset mekaaniset ominaisuudet sekä vähentävät kustannuksia, kun energiaa vievä normalisointivaihe voidaan jättää välistä. Työssä tutkittiin yhteensä kuuden erilaisella niobi- ja kromipitoisuudella olevan koostumuksen vaikutusta termomekaanisesti valssatun ja suorakarkaistun mikroseostetun matalahiilisen teräksen mikrorakenteeseen ja mekaanisiin ominaisuuksiin. Tavoitteena teräksen mekaanisille ominaisuuksille olivat 700 MPa myötölujuus sekä hyvä iskusitkeys. Bainiitti oli dominoiva mikrorakenne kyseisissä mikroseostetuissa teräksissä. Perinnäisen austeniitin morfologiaa ja siihen muodostunutta mikrorakennetta tutkittiin suhteessa mekaanisiin ominaisuuksiin, kuten lujuuteen, venymään, iskusitkeyteen ja karkenevuuteen. Lisäksi hitsattavuutta tutkittiin simuloimalla hitsauksessa syntyvää lämpovyöhykettä Gleeble-simulaattorilla kahdella eri t8/5-ajalla; 5 sekuntia ja 15 sekuntia. Neljällä kuudesta tutkimuksesta koostumuksesta saatiin teollisesti tuotetun S700-teräksen kaltaisia tuloksia mekaanisten ominaisuuksien kannalta. Lisäämällä neljä prosenttia kromia ja 0.035% niobia saatiin parempia mekaanisia ominaisuuksia kuin lisämäällä pelkästään 0.06% niobia. Kromin lisäys myös paransi austeniitin karkenenevutta, kun taas kromin vähentäminen edesauttoi kvasipolygonaalinen ferriitin muodostumista ja siten kasvatti iskusitkeyttä ja venymää myötölujuuden kustannuksella. Lisäksi neljä prosenttia kromia sisältäneiden terästen karkearakeinen lämpövyöhyke pehmeni vähäisesti perusaineeseen verratuna

On the role of Nb on the texture and microstructure of a novel as-rolled medium carbon wear resistant slurry pipeline steel

Abstract A field emission scanning electron microscope equipped with EBSD has been employed to evaluate the role of 0.013 wt% Nb on the evolution of the microstructure and texture of a novel thermomechanically processed low-alloy, medium-carbon steel. Specimens were subjected to hot-rolling with a total reduction of 80%, four passes in the recrystallization regime and four passes below the non-recrystallization temperature. Immediately after rolling, the strips were quenched to 420 °C and subsequently cooled slowly in a furnace to simulate strip coiling with transformation of austenite to bainite. The results showed that Nb microalloying results in a finer microstructure with a sharper texture when compared to an identical steel but without Nb. In addition, analysis of the retained austenite texture indicated that the main bcc texture components are the product transformed brass and copper components in the parent austenite

Infulence of chromium content of the mechanical properties and HAZ simulations of low-carbon bainitic steels

Abstract During thermomechanically controlled processing (TMCP) of carbon steels, controlled rolling in norecrystallization regime followed by directing quenching results in excellent mechanical properties, besides enabling cost-saving in respect of energy costs normally incurred during reheating and quenching procedure. The effects of three different combinations of Cr with 0.06 wt.% of Nb on the microstructures and mechanical properties of thermomechanically rolled and direct-quenched low-carbon (0.035 wt.%) microalloyed steel plates have been investigated to obtain balanced mechanical properties. Laboratory-scale ingots were cast, hot rolled and direct quenched into 12 mm thick plates as per an experimental plan. Tensile properties, impact toughness, hardness and hardenability were studied. In addition, coarse grained heat affected zone (CGHAZ) simulations were performed in a Gleeble simulator to evaluate the weldability of the investigated steels using cooling time from 800 °C to 500 °C (t8/5) of 5 s and 15 s

Direct-quenched and tempered low-C high-strength structural steel:the role of chemical composition on microstructure and mechanical properties

Abstract The direct quenching of low-carbon steels after thermomechanical processing on hot strip mills is able to produce both strong and tough coiled plate without the need for subsequent tempering. The process is energy and time efficient with relatively low emissions when compared to conventional reheating, quenching and tempering. For some applications, however, it is desirable to combine direct quenching with tempering, and, bearing in mind the form of the semi-finished product, it is of interest to study the effect of tempering whole coils in a bell furnace. Here, the effects of boron, carbon, titanium, vanadium and tempering temperature on the microstructure, crystallography and mechanical properties of direct-quenched steels has been studied with the aid of simulated bell furnace heating and cooling cycles. All steels contained (in wt.%) 0.2Si–1Mn–1Cr-0.65Mo-0.03Al, while there were two levels of C (0.095 /0.140), V (0 /0.08), Ti (0 /0.025) and B (0 /0.0015). Tempering was performed with peak temperatures at 180 and 570 °C. The paper reveals several possible alloying and processing routes to strong and tough low-C steel. Carbon controls the strength and toughness, while titanium and boron affects the grain size of coarsest grains (d90%), Vanadium has a strong effect on strength retention during tempering at 570 °C: an addition of 0.08 wt% vanadium increases yield strength by 70 MPa and ultimate tensile strength by 100 MPa. The removal of boron from the steel is shown to have a huge impact not only on the microstructure but also on the impact toughness

Hydrogen‐induced stress corrosion cracking studied by the novel tuning‐fork test method

Abstract A novel tuning‐fork test method was developed to study hydrogen‐induced stress corrosion cracking of high‐strength steels. Special tuning‐fork specimens are designed to enable accurate stress adjustment via constant displacement under cathodic hydrogen charging conditions. In this study, the testing method is further developed, making the potentiostatic hydrogen charging possible through the modifications of the corrosion cell. Different direct‐quenched, low‐ and medium‐carbon steel grades, with a hardness range of 300–550 HBW, are investigated with both galvanostatic and potentiostatic hydrogen charging techniques. For each steel grade, the lowest fracture stress and highest no‐fracture stress are determined. Both hydrogen charging techniques produce similar results, and it is observed that the fracture stress decreases with the increase in hardness. However, the potentiostatic technique produces larger differences between the lowest fracture stress results, thus having a better resolution

The effect of Pd and Ni coatings on hydrogen permeation experiments of as-quenched martensitic steel

Abstract Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient (D), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5–0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6–6.6 × 10⁻⁷ cm²/s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10⁻⁷ cm²/s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses

Poncet’s Disease in the Preclinical Phase of Rheumatoid Arthritis

We report on a patient with seropositive polyarthritis retrospectively diagnosed as Poncet’s disease in the preclinical phase of seropositive rheumatoid arthritis. Our patient developed rheumatoid arthritis more than 2 years after being successfully treated for pulmonary tuberculosis and an initial inflammatory polyarthritis consistent with the diagnosis of Poncet’s disease. This case illustrates the importance of recognizing Poncet’s disease in a patient presenting with polyarthritis in order to avoid inappropriate long-term disease modifying antirheumatic treatment. It also illustrates the need for adequate follow-up of patients with Poncet’s disease after treatment with antituberculosis treatment so that progression to a primary inflammatory arthritis such as rheumatoid arthritis may be identified timeously. Although seropositivity for rheumatoid arthritis has been reported in Poncet’s disease as well as in tuberculosis, it is rather uncommon, and long-term follow-up of patients with Poncet’s disease is essential particularly if they have positive serological tests for rheumatoid arthritis. In this case report, we describe the first reported case of Poncet’s disease in the preclinical phase of rheumatoid arthritis and review the literature related to this rare disease presentation

Influence of chromium content on the microstructure and mechanical properties of thermomechanically hot-rolled low-carbon bainitic steels containing niobium

Abstract The effect of chromium content in the range of 1 wt.%–4 wt.% on the microstructure and mechanical properties of controlled-rolled and direct-quenched 12 mm thick low-carbon (0.04 wt.%) steel plates containing 0.06 wt.% Nb has been studied. In these microalloyed 700 MPa grade steels, the aim was to achieve a robust bainitic microstructure with a yield strength of 700 MPa combined with good tensile ductility and impact toughness. Continuous cooling transformation diagrams of deformed and non-deformed austenite were recorded to study the effect of Cr and hot deformation on the transformation behavior of the investigated steels. Depending on the cooling rate, the microstructures consist of one or more of the following microstructural constituents: bainitic ferrite, granular bainite, polygonal ferrite, and pearlite. The fraction of bainitic ferrite decreases with decreasing cooling rate, giving an increasing fraction of granular bainite and polygonal ferrite and a reduction in the hardness of the transformation products. Polygonal ferrite formation depends mainly on the Cr content and the cooling rate. In both deformed and non-deformed austenite, increasing the Cr content enhances the hardenability and refines the final microstructure, shifting the ferrite start curve to lower cooling rates. Preceding austenite deformation promotes the formation of polygonal ferrite at lower cooling rates, which leads to a decrease in hardness. In hot-rolled and direct-quenched plates, decreasing the Cr content promotes the formation of polygonal ferrite leading to an increase in the impact toughness and elongation but also a loss of yield strength

Effect of prior austenite grain morphology on hydrogen embrittlement behaviour under plastic straining in as-quenched 500 HBW steels

Abstract Prior austenite grain (PAG) structure is an important factor influencing hydrogen embrittlement (HE) susceptibility of ultrahigh-strength steels. In this study, the effect of PAG shape and size on HE behaviour is investigated using a novel tuning-fork testing method and hydrogen thermal desorption spectroscopy (TDS). Different PAG structures were acquired via re-austenitization (860°C = A860, 960°C = A960) and rapid quenching of an as-received 500 HBW direct-quenched (DQ) steel, which has an auto-tempered lath-martensitic microstructure and elongated PAG morphology. Fractography reveals different crack propagation mechanisms depending on the PAG shape. With the elongated PAG structure, hydrogen-induced crack propagation transverse to elongated PAGs was transgranular quasi-cleavage. Propagation was partially intergranular with the equiaxed PAG structures, regardless of the PAG size, leading to equally faster fracture. The TDS results show that there are no significant differences between the total hydrogen contents, but re-austenitized A860 and A960 steels contain a higher fraction of weakly trapped hydrogen. This indicates that the PAG boundaries are not the dominant hydrogen traps, and the different crack propagation mechanisms are rather linked to the geometrical shape of the grain structure

Improving the yield strength of an antibacterial 304Cu austenitic stainless steel by the reversion treatment

Abstract As an implant material, Cu-bearing austenitic stainless steels can possess the antibacterial property, but their mechanical strength is low. In order to improve the yield strength of a 304Cu (17%Cr–7%Ni–3%Cu) alloy through substantial grain refinement, a research investigation has been taken up to conduct the reversion annealing treatment comprising a heavy (71%) cold rolling reduction followed by annealing at various temperatures (650–950 °C) and durations (1–5400 s). The microstructure evolution was examined by electron backscatter diffraction and further characterized by magnetic measurements, and mechanical properties were determined by tensile and hardness testing. The precipitation of Cu was confirmed by transmission electron microscopy. It was found that the reversion of deformation-induced martensite to austenite took place by the shear mechanism, followed by subgrain formation and continuous recrystallization resulting in quite non-uniform grain size distribution. The finest reversed grains were around 0.6 μm in size, but also much larger austenite grains and a small fraction of unreversed martensite existed in the final structure despite annealing at least up to 800 °C. Coherent Cu particles were observed after aging for 1.5 h at 700 and 650 °C, while the yield strength could be improved to 507 and 791 MPa, respectively, i.e. by ~2–3 times that of the annealed steel. The ductility of the steel remains still high, the fracture elongation being 36%