Proportion Of Acicular Ferrite Research Articles

Effect of Arc Length on Oxygen Content and Mechanical Properties of Weld Metal during Pulsed GMAW

Pulsed gas metal arc weld (GMAW) was widely used for the advantages of controllable heat input, all-position welding, and no spatter. In order to obtain an ideal welding process, the stability of the arc length was studied in many researches, but the influence of arc length on the properties of weld metal was ignored. In this paper, the effect of arc length on oxygen content and mechanical properties of weld metal during pulsed GMAW was studied. Q690 high strength steel was selected as the base metal, and ER69-G solid wire, with a diameter of 1.2 mm, was used as the electrode wire. Additionally, the shielding gas and the wire feed rate were 82% Ar + 18% CO2 and 4 m/min, respectively. The results showed that as the arc length raised from 2.9 mm to 9.2 mm, the oxidation reacted more completely in the droplet transfer zone, and the oxygen content of the weld metal increased significantly. The tensile strength of the weld metal reduced but the −40 °C impact energy heightened. Due to the longer arc, the proportion of acicular ferrite (AF) in the microstructure decreased, but the proportion of lath bainite (LB) and granular bainite (GB) decreased. The higher oxygen content of weld metal was useful for the formation of inclusions, which promoted the nucleation of acicular ferrite and dimples, contributing to the growth of plasticity and toughness of weld metal.

Microstructure Evolution and Mechanical Properties of Underwater Dry Welded Metal of High Strength Steel Q690E Under Different Water Depths

Abstract Q690E high strength low alloy (HSLA) steel has been intensively applied in maritime engineering. Also, the underwater dry welding (UDW) technique has been widely used to repair important offshore facilities. In this paper, joints of Q690E steel were fabricated through single-pass underwater dry welding at three pressures (0, 0.2, and 0.4 MPa). To study the effect of the pressure on the microstructure and mechanical properties of the UDW joint, an optical microscope (OM) and scanning electron microscope (SEM) were used to observe the microstructure and fracture morphology of the welded joints. The electron backscattered diffraction (EBSD) technique was used to analyse the crystallographic features and the crystallographic grain size of the ferrites. The proportion of acicular ferrite (AF) in the UDW joints and the density of low-angle boundaries increase dramatically with the increasing depth of water. The weld metal of UDW-40 shows higher strength because more fine ferrites and low-angle boundaries within UDW-40 impede the dislocation movement.

Study of the Effect of Microalloying on Microstructure and Mechanical Property Formation for Rolled Product of Strength Class K52 Produced Under CRU Conditions

Results are provided for a study of the effect of different microalloying systems (Nb, Nb–V, V–N) on structure formation and mechanical properties of rolled product of strength class K52 produced by combined technology of melting, casting of thin slabs 90 mm thick, and direct rolling with subsequent accelerated cooling and winding on a coil. It is shown that all of the micro-alloying versions selected provide the required strength properties, while metal microalloyed solely with niobium demonstrates the best brittle fracture resistance at low temperature. It is shown that addition of niobium to steel not only refines grain size in the metal matrix, but also changes ferrite morphology increasing the proportion of acicular ferrite, which improves steel cold resistance.

Effect of Mg on the Evolution of Inclusions and Formation of Acicular Ferrite in La–Ti‐Treated Steels

The size distribution and compositions of inclusions and microstructures in steels with different Mg contents are systematically investigated. The results show that Mg treatment modified and refined the inclusions. With Mg addition, the dominant inclusions in the samples evolve from LaAlO3 with MnS to a Mg‐enriched core enwrapped by a La‐enriched shell with MnS. While the Mg content increases from 11 to 66 ppm, the core changes from MgO·Al2O3 to MgO, and the shell develops from LaAlO3 to La2O2S. With the increase in Mg content from 0 to 44 ppm, the number density of effective inclusions is improved, and the proportion of acicular ferrite (AF) increases from 15% to 39%. However, the AF fraction decreases to 16% due to the relatively low amount of effective inclusions in the sample Mg3 with 66 ppm Mg.

Effects of Zr-Ti on the microstructure and properties of flux aided backing submerged arc weld metals

Zr-Ti alloy powders were added into the weld groove to join D36 steel by using flux aided backing submerged arc welding (FAB-SAW). The inclusions, microstructure and mechanical properties of Zr-Ti miroalloyed weld metals were investigated. The results showed that the core of inclusions was composed of Zr, Al, Ti, Si, Mn oxides, while the surface layer of inclusions was composed of TiO (or Ti2O3), ZrO2 and MnS. The first precipitated ZrO2 can act as nucleation sites for Al, Ti, Si, Mn oxides and increases the fraction of inclusions with size of smaller than 1.0 μm. Due to the refined size and increased density of inclusions, the proportion of acicular ferrite (AF) was significantly increased while the proportion of grain boundary ferrite (GBF) and ferrite side-plate (FSP) were remarkably decreased in the microstructure. The tensile properties and impact toughness at −20 °C of weld metal were obviously promoted. The nucleation of AF was ascribed to the low lattice misfit between TiO (and ZrO2) and α-Fe as well as a high inert interfacial energy of MnS. When the content of Zr was 0.09 wt %, the percentage of AF in FAB-SAW weld metal was reached to 82%, resulting in the weld metal with elongation of 25%, ultimate strength of 615 MPa and impact energy at −20 °C of 145 J.

Addition of MgO Nanoparticles to Carbon Structural Steel and the Effect on Inclusion Characteristics and Microstructure

An innovative approach for pre-dispersing MgO nanoparticles with AlSi alloy nanoparticles was established, and the nanoparticles were dispersed well in carbon structural steel. After adding different mass fractions of MgO nanoparticles in steel, the majority of inclusions contained MgO·Al2O3 spinel and MgO-Al2O3-bearing hybrid inclusion, and these inclusions promoted acicular ferrite (AF) formation. With increasing amount of added nanoparticles, the average inclusion size increased from 0.90 to 1.50 μm and the inclusion size was considerably refined, but the ability of inclusions to induce AF was greatly declined. It was revealed that the inclusion size was the decisive factor influencing the inducing ability of inclusions for AF, which also got a solid support from the nucleation thermomechanical and dynamic analyses. When the mass fraction of MgO nanoparticles reached 0.05 pct, the proportion of AF in microstructure is relatively larger and the degree of interlocking of the AF within the microstructure was optimized. The ferrite grains also got refined and the average grain size decreased by more than 94 pct compared with that of the original steel.

Effect of submerged arc welding parameters on microstructure of SA516 steel weld metal

The influence of submerged arc welding (SAW) process parameters on the microstructure of SA516 grade 70 steel weld metal (WM) was investigated in the present work. Steel plates of 17 mm thickness were submerged arc welded using welding currents of 700–850 A and welding speeds of 5·3–15·3 mm s−1. The morphologies and volume fractions of the various ferrites in the WM were studied using optical microscopy and the morphologies and chemical compositions of the WM inclusions were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. The results showed that the WM grain structure coarsened but the grain width of prior austenite grains decreased with increasing heat input. Also, the proportion of acicular ferrite (AF) in the WM increased initially, while the volume fractions of grain boundary ferrite and Widmanstätten ferrite decreased with increasing welding current. However, with further increasing the welding current above 800 A, less AF was produced. The weld nugget area decreased with increasing welding speed at all currents, but did not affect the amount of AF produced.Au cours de ce travail, on a examiné l’influence des paramètres du procédé de soudage à l’arc sous flux en poudre (SAW) sur la microstructure du métal soudé (WM) de l’acier SA516, Gr. 70. On a soudé à l’arc sous flux en poudre des tôles d’acier de 17 mm d’épaisseur en utilisant des courants de soudage de 700 à 850 A et des vitesses de soudage de 5·3 à 15·3 mm s−1. On a étudié la morphologie ainsi que la fraction volumique des diverses ferrites du WM en utilisant la microscopie optique et l’on a examiné la morphologie ainsi que la composition chimique des inclusions du WM en utilisant la microscopie électronique à balayage (SEM) et la spectroscopie à dispersion d’énergie (EDS). Les résultats ont montré que la structure de grain du WM grossissait mais que la largeur de grain des grains antérieurs d’austénite diminuait avec une augmentation de l’apport de chaleur. Également, la proportion de ferrite aciculaire (AF) dans le WM augmentait initialement, alors que les fractions volumiques de ferrite des joints de grain (GBF) et de ferrite de Widmanstatten (WF) diminuaient avec l’augmentation du courant de soudage. Cependant, avec une augmentation supplémentaire du courant de soudage au-dessus de 800 A, moins de ferrite aciculaire était produite. La région du noyau de soudure diminuait avec une augmentation de la vitesse de soudage à toutes les valeurs de courant, mais la quantité d’AF produite n’était pas affectée.

Effects of Mn and Ti on microstructure and inclusions in weld metal of high strength low alloy steel

The influences of alloying elements on chemical composition of non-metallic inclusions, impact toughness and microstructure in weld metals of high strength low alloy steels have been studied. Results indicated that microstructure had changed from a mixture of acicular ferrite, proeutectoid ferrite, ferrite side plates and microphases to a mixture of acicular ferrite, bainite and microphases due to the addition of Mn and Ti. The impact toughness of weld metal was improved correspondingly. The volume fraction and composition of inclusions both influenced the proportion of acicular ferrite. Mn and Si based oxide globular inclusions located at the boundary of acicular ferrite plates in the weld metal produced using C–Mn–Si–Cu wire. When Mn and Ti were added to welding wires, the inclusions within acicular ferrite plates permitted fewer primary acicular ferrite plates to grow into relatively larger dimensions. Secondary acicular ferrites nucleating on pre-existing ferrite plates refined microstructure effectively.

Weldability of a Low Carbon High Strength Ti-Containing Bainitic Steel Produced by CSP

The weldability of a 700 MPa grade low carbon Ti-containing microalloyed bainitic steel produced by compact strip production (CSP) has been investigated by gas metal arc welding. Microstructural features of the welded joint of the investigated steel have been investigated utilizing optical and scanning electron microscopy (SEM). The microstructures in the heat affected zone (HAZ) consist of a predominantly bainitic ferrite and a proportion of acicular ferrite which formed on Ti-oxide and/or nitride particles. The acicular ferrite formed earlier effectively partitions prior austenite grains into smaller separate regions. The bainite transformed at lower temperatures is thus restricted in the smaller regions so that fine-grained mixed microstructures are obtained. The superior toughness of the weld joint of the investigated steel is attributed to the prior formation of acicular ferrite in the heat-affected zone.

Dynamics of austenite to ferrite phase transformation in ferrous weld metals

The dynamics of the austenite to ferrite phase transformation inferrous weld metals have been investigated in order to define the nucleation and growth processes leading to the intragranular microstructure commonly known as acicular ferrite. Weld metals of increasing hardenability have been thermally cycled, partially transformed, and the nucleation and growth reactions frozen at temperatures in the reconstructive and displacive regimes by quenching. Information has also been obtained about the rates of transformation at different temperatures in continuously cooled, fully transformed specimens. The present work has shown that acicular ferrite is not a single transformation product, but forms as a result of a succession of competing nucleation and growth processes at austenite grain boundary and intragranular nucleation sites during undercooling. The sequence of transformations is consistent with the theoretical activation energy barriers against nucleation of the different sites. The present work demonstrates that large intragranular inclusions (greater than ∼ 3 μm) nucleate acicular ferrite at reconstructive transformation temperatures but the amount of acicular ferrite is limited by low inclusion numbers and slow rates of reaction. Small intragranular inclusions (∼ 0.5 μm) nucleate large proportions of acicular ferrite at displacive reaction temperatures because of their substantial numbers and the rapid rates of reaction. Boron retards reconstructive ferrite transformation at austenite grain boundary and large intragranular inclusion sites so that the temperature at which they become operative is significantly lowered. A greater proportion of acicular ferrite is then formed displacively. Boron has little effect on the onset of displacive transformation, which occurs at ∼ 630°C in all the weld metals investigated. The maximum rate of transformation occurs at temperatures in the range 597–610°C, depending on the alloying. These temperatures are 40–70 K above the calculated and experimentally observed bainite start temperatures, indicating that the greater proportion of acicular ferrite formed displacively is Widmanstätten ferrite rather than bainite.

Formation of acicular ferrite at oxide particles in steels

Experimental steels similar in composition to structural grades were prepared from weld metal deposits to study the formation of acicular ferrite under conditions experienced in the heat affected zone for a range of welding processes. The formation of acicular ferrite under these conditions is found to be dependent on the presence of a suitable distribution of oxide inclusions > 0·4 μm in size. The characteristics and proportion of acicular ferrite in the microstructure also depend on the prior austenite grain size and cooling rate. The relationship between these factors is presented in a simplified quantitative model, which is supported by data from limited welding trials. Metallographic observations suggest that acicular ferrite forms in two stages. The first involves the formation of relatively large primary acicular ferrite plates by multiple nucleation at intragranular inclusion sites, and the second involves the formation of many smaller acicular ferrite grains that grow sympathetically from the primary plates.MST/1027

Formation of acicular ferrite at oxide particles in steels

Experimental steels similar in composition to structural grades were prepared from weld metal deposits to study the formation of acicular ferrite under conditions experienced in the heat affected zone for a range of welding processes. The formation of acicular ferrite under these conditions is found to be dependent on the presence of a suitable distribution of oxide inclusions > 0·4 μm in size. The characteristics and proportion of acicular ferrite in the microstructure also depend on the prior austenite grain size and cooling rate. The relationship between these factors is presented in a simplified quantitative model, which is supported by data from limited welding trials. Metallographic observations suggest that acicular ferrite forms in two stages. The first involves the formation of relatively large primary acicular ferrite plates by multiple nucleation at intragranular inclusion sites, and the second involves the formation of many smaller acicular ferrite grains that grow sympathetically from the primary plates.MST/1027