Fraction Of Acicular Ferrite Research Articles

Effect of inclusions on microstructure and mechanical behavior of multi-pass welded naval grade steel

This paper assesses the metallurgical characteristics and mechanical properties of multi-pass gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) of micro-alloyed steel DMR 249-A. The prime objective was to carry out a comparative study of the microstructure and mechanical properties of the joints produced by the two types of welding. A high volume of larger sized inclusions in GMAW contributed to inferior mechanical properties. The coarse-grained part of the heat-affected zone (CGHAZ) showed a lower microhardness. Fracture always occurred in the heat-affected zone, and it is believed that it is associated with CGHAZ. GTAW joints showed low tensile residual stress, higher hardness, and tensile strength. GTA weldment also showed superior impact toughness at sub-zero temperature (–60 °C). Mn-containing inclusions were seen in GTA weldments; it is believed that they promote the formation of acicular ferrite. This is believed to be responsible for the superior mechanical properties of GTA weldments. The microstructural analysis of the two weldments revealed the presence of a higher volume fraction of acicular ferrite in the GTAW. All in all, GTAW joint was found to perform better than GMAW joint.

Effect of Final Cooling Temperature on Microstructure and Mechanical Properties of a Cr-Ni-Mo-V Bainite Steel

The microstructural evolution and mechanical properties of a low-carbon high-strength bainitic steel under different final cooling temperatures were studied. The microstructures of the experimental steel at different final cooling temperatures were composed of acicular ferrite and granular bainite. A decrease in final cooling temperature resulted in an increase in volume fraction of granular bainite and a decrease in volume fraction of acicular ferrite. The specimen with the lowest final cooling temperature (459 °C) exhibited the highest yield strength, tensile strength, and yield ratio, and a lower total elongation than the specimens with the highest final cooling temperatures (501 and 535 °C) because of the higher volume fraction of granular bainite and finer bainitic ferrite lath. The specimen with the lowest final cooling temperature had a lower absorbed energy than that of the highest final cooling temperature, because the strip-like martensite–austenite (M-A) constituents that existed between the bainitic ferrite or in prior austenite grain boundaries reduced its toughness. At an intermediate final cooling temperature of 501 °C, the experimental steel exhibited excellent mechanical properties with a yield strength, tensile strength, and absorbed energy of 825 MPa, 1232 MPa, and 102 J, respectively.

Quantitative analysis of microstructure and impact toughness in the simulated coarse-grained heat-affected zone of Cu-bearing steels

ABSTRACTBecause of 0.32% Cu addition, the inclusion is modified from Al-Ti oxide covered with layer of MnS in Cu-free steel to Al-Ti oxide with shell of (Mn,Cu)S in Cu-bearing steel. The quantitative result reveals that the frequency of inclusion effectively for acicular ferrite nucleation, fraction of acicular ferrite and high angle grain boundary in the simulated coarse-grained heat-affected zone (CGHAZ) of Cu-bearing steel are higher than that of Cu-free steel, respectively. Therefore, the high density of acicular ferrite microstructures in CGHAZ of Cu-bearing steel with smaller crystallographic grain size has superior impact toughness than that in Cu-free steel.

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s−1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.

Effect of Cr Content on Microstructure and Impact Toughness in the Simulated Coarse-Grained Heat-Affected Zone of High-Strength Low-Alloy Steels

The relationship of Cr content to microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels subjected to varying heat input welding thermal cycles is investigated. Results show that the microstructures of the simulated coarse-grain heat-affected zone in steels with different Cr contents and welding heat inputs consist of predominantly bainite, a small proportion of acicular ferrite, and martensite–austenite constituents. With the Cr content increasing from 0.33 to 0.65% and 1.02%, the fraction of acicular ferrite is decreased and martensite–austenite constituents are increased. Meanwhile, when the heat input increased from 20 to 100 and 200 kJ cm−1, martensite–austenite constituents increase in amount and coarsen remarkably. The impact toughness of simulated coarse-grain heat-affected zone at −20 °C decreases obviously with increasing Cr content, especially subjected to high-heat input welding.

Thermomechanical processing of a Nb–Ti–V pipeline steel

In this research, thermomechanical processing of a high-strength low-alloy steel was simulated through hot tor- sion testing. Average schedule and continuous cooling torsion after the actual schedule were used to determine the critical temperatures of the tested steel. Also, a simplified actual schedule with 79 % reduction in the austenite pancaking re- gion was applied to investigate the effects of the temperature range of the finish rolling stage and the cooling rate on the microstructure and mechanical properties of the produced steels. Two different rolling conditions (with the temperature ranges of finish rolling of 950 to 840 °C and 910 to 800 °C) with two different cooling rates of 7.5 and 15 °C/s were ex- amined. In this way, four different steels were produced and characterized by metallography. It was observed that a de- crease in the temperature range of finish rolling and an in- crease in the cooling rate lead to an increase in the volume fraction of acicular ferrite and a decrease in the grain size. Repeated thermomechanical processing schedules followed by room temperature torsion testing were conducted to eval- uate the strength of the steels produced. The results demon- strated that a decrease of 40 °C in the temperature range of finish rolling leads to about a 77-MPa increase in yield strength. Also, it was observed that by increasing the cooling rate from 7.5 to 15 °C/s, the yield strength increases by about 45 MPa. Finally, the relationship between yield strength and grain size of the produced steels was determined according to the Hall-Petch relation.

Role of arc mode in laser-metal active gas arc hybrid welding of mild steel

Arc mode plays an important role in joint characterizations of arc welding, but it has been seldom considered in laser-arc hybrid welding. This paper investigated the role of arc mode on laser-metal active gas (MAG) arc hybrid welding of mild steel. Three arc modes were employed, which were cold metal transfer (CMT), pulsed spray arc and standard short circuiting arc. Microtexture of the joints were observed and measured via electron back scattering diffraction (EBSD) system to reveal the effect of arc mode on microstructure. Mechanical properties of the joints were evaluated by tensile and Charpy V-notch impact tests. It was found that both the stability and mechanical properties of laser-CMT hybrid welding (LCHW) is the best, while those of laser-standard short circuiting arc welding (LSHW) is the worst. OM and EBSD results showed that the fraction of acicular ferrite and high-angle grain boundaries in fusion zone decreases gradually in the sequence of LCHW, laser-pulsed spray arc welding and LSHW, while the mean grain size increases gradually. Finally, the microstructure formation mechanisms and the relationship between microstructure and mechanical properties were summarized by the loss of alloying element and the stirring effect in molten pool.

Effect of Ti on the Microstructures and Toughness of TMCP-600 Steel Weld Metals

The Ti addition effect on the characteristics of weld metal, such as impact energy, microstructure and nonmetallic inclusions, was investigated to develop a suitable gas metal arc welding wire for the high strength of TMCP (Thermo Mechanical Control Process)-600 steel. The fraction of acicular ferrite which was known to be a favorable weld metal microstructure for toughness was increased with Ti content from 0.002% to 0.025%, The impact energy of weld metal was increased whereas the ductile to brittle transition temperature was decreased with increasing Ti content. The size of nonmetallic inclusion was decreased while the density of inclusions was decreased with increasing Ti content. It was found that Ti content on the weld metal toughness had a plus effect by increasing the fraction of acicular ferrite in the weld metal microstructure.

Improvement of Charpy impact properties in heat affected zones of API X80 pipeline steels containing complex oxides

The present study aimed at investigating the improvement of Charpy impact properties in heat affected zones (HAZs) of API X80 pipeline steels containing complex oxides. Three kinds of steels were fabricated by adding Ti, Al and Mg to form complex oxides and their microstructures, tensile properties and Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al and Mg was included in the steels. After the HAZ simulation test, the steel having a number of oxides contained about 72 vol.-% of acicular ferrite in the simulated HAZ, together with bainitic ferrite and granular bainite, whereas the HAZ microstructures of the conventional steels consisted of bainitic ferrite and granular bainite with a small amount of acicular ferrite. The volume fraction of acicular ferrite increased as the amount of complex oxides increased and the heat input decreased, and thus, Charpy impact properties of the oxide containing steel HAZs were greatly improved over those of the conventional steel HAZs.

The metastable austenite transformation in Mo–Nb–Cu–B low carbon steel

The continuous cooling and isothermal transformation microstructure evolution in low carbon Mo–Nb–Cu–B microalloying steel were investigated. It was found that there are three kinds of mainly intermediate isothermal transformation structures: the grain boundary ferrite, acicular ferrite and lath-like bainitic ferrite sheaves. The growth of reconstructive ferrite was delayed or restrictive below 580 °C. The upper limiting temperature of the low “c”-curve is about 560 °C. The acicular ferrite can be transformed independently at the lower portion of metastable state region of overcooling austenite (550–500 °C). From the results of the continuous cooling test, the multi-phase of intermediate transformation products such as quasi-polygonal ferrite, granular bainite, acicular ferrite, lath bainite and M/A would appear. The fraction of acicular ferrite can be controlled by the interrupted cooling process. The acicular ferrite plates grew up independently in the prior-austenite grain and distributed randomly, the optimum amount of acicular ferrite can divide the prior-austenite grain and results in an ultra fine multi-phase microstructure.

Effects of Inclusions and Microstructures on Impact Energy of High Heat-Input Submerged-Arc-Weld Metals

Submerged-arc-weld metal made with variations of heat-input and consumables were examined to study the inclusions and their effects on the Charpy impact energy of the weld metal as a measure of the toughness. From the analysis, inclusions, of which diameters were smaller than 2 μm, seemed to be effective for acicular ferrite nucleation and the fraction of acicular ferrite was in proportion to the number density of inclusions <2 μm. However, as the heat input increased, the average size of inclusions increased but their number density decreased. It was also observed that the grain sizes of polygonal ferrite with increased heat input. Accordingly, the decrease of the Charpy impact energy with the heat input is inevitable due to these microstructural changes. But, this does not mean that the acicular ferrite cannot be nucleated.

Formation and Control of the Acicular Ferrite in Low Carbon Microalloying Steel

The influence of processing parameters on the acicular ferrite formation for the low carbon microalloying steel was studied. The results showed that the fraction of acicular ferrite could be controlled by the cooling process. The acicular ferrite/ bainitic ferrite dual phase structure can be formed. The multi-phase microstructure is ultra fine. The hardness is sensitively affected by the acicular ferrite fraction.

The role of titanium in the non-metallic inclusions which nucleate acicular ferrite in the Submerged ARC Weld (Saw) fusion zones of Navy HY-100 steel

The origin of acicular ferrite in the weld metal of submerged arc weldments on high strength steels is very complex and depends upon the chemical composition for the steel base plate and filler wire, the composition of the flux used during welding and the cooling rate of the weld metal during the transformation of the undercooled metastable austenite. The strength and toughness of weld metal improves as the amount of acicular ferrite increases due its fine basket weave microstructure and so it is important to understand the mechanism of its formation so that the volume fraction of acicular ferrite can be maximized in steel weld metal. The chemical composition of the filler wire mostly determines the final composition of the weld metal although the composition of the base plate is important because of dilution effects. In high strength steels the alloying elements such as carbon, nickel, chromium, copper nd niobium are present to achieve the required strength levels and a fortuitous outcome of this is a continuous cooling transformation (CCT) diagram with features that mean that bainite is the major transformation product during the arc welding of these steels provided a suitable weld power and preheat/interpass temperature is chosen duringmore » multi-run welding. Once a suitable weld-metal hardenability and cooling rate has been established the amount of acicular ferrite nucleated will depend on the size, number, distribution and chemical composition of the non-metallic inclusions. Suitable inclusions appear to be in the size range 0.2--2.0 micrometers with a mean size of 0.5 micrometers being about an optimum value. These inclusions usually contain manganese, silicon, aluminum and titanium as their major constituents and do not appear to be exactly spherical but have a faceted or slightly angular appearance.« less