Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels

Four TRIP (Transformation Induced Plasticity) assisted steels, three TBF (TRIP Bainitic Ferrite) steels and one TPF (TRIP Polygonal Ferrite) steel, were manufactured from three different carbon contents (0.2, 0.3 and 0.4 wt.% C), to study the evolution of their microstructure and tensile mechanical properties in 15 mm thick plates. TBF steels were subjected to the same austenitization heat treatment and subsequent bainitization isothermal treatment. The TPF steel was subjected to an intercritical annealing and subsequent isothermal bainitization treatment. All were microstructurally characterized by optical, scanning electron and atomic force microscopy, as well as X-ray diffraction. Mechanically, they were characterized by the ASTM E8 tensile test and fractographies. For the TBF steels, the results showed that when the carbon content increased, there were an increase in volume fraction of retained austenite, of the microconstituent “martensite/retained austenite” and in the tensile strength; and a decrease in the volume fraction of bainitic ferrite matrix and elongation; with an improvement in TRIP behavior due to the increase in retained austenite. The TPF steel presented around 50% ductile polygonal ferrite developing better TRIP behavior than the TBF steels. The evolution of the fractographies was ductile to brittle for TBF steels with an increase in carbon content, and for TPF, the appearance of the fracture surface was ductile.

Enhancing Strain Capacity by the Introduction of Pearlite in Bainite and Polygonal Ferrite Dual-Phase Pipeline Steel

In this study the strain capacity and work-hardening behavior of bainite (B), bainite + polygonal ferrite (B + PF), and bainite + polygonal ferrite + pearlite (B + PF + P) microstructures are compared. The work hardening exponent (n), instantaneous work hardening value (ni), and differential Crussard-Jaoul (DC-J) analysis were used to analyze the deformation behavior. The best comprehensive mechanical properties were obtained by the introduction of the pearlite phase in B + PF dualphase with the tensile strength of 586 MPa and total elongation of 31.0%. The additional pearlite phase adjusted the strain distribution, which increased the initial work hardening exponent and then maintained the entire plastic deformation at a high level, thus delayed necking. The introduction of pearlite reduced the risk of micro-void initiation combined with the high frequency of high angle grain boundaries (HAGBs) in triple-phase steel, which led to a low crack propagation rate.

Influence of the Evolution of 9CrMoCoB Steel Precipitates on the Microstructure and Mechanical Properties during High-Temperature Aging

In this study, the short-term aging was carried out to reveal the evolution of precipitates and mechanical properties of heat resistant 9CrMoCoB steel during the early creep, replacing the conventional creeping. The tempered martensite lath structure (TMLS) and precipitates were observed in the as-aged 9CrMoCoB steel. TMLS in the matrix underwent a transition to the polygonal ferrite after aging only for 300 h. In comparison, the mean diameter of the precipitates increased from 183 to 267 nm after aging at 650 °C for 300 h. Also, the mean diameter of the precipitates increased from 183 to 302 nm at 700 °C. The room-temperature and high-temperature strength of 9CrMoCoB steel decreased after high-temperature aging, which may be mainly due to precipitates coarsening. Many M23C6 phases precipitate in the prior austenite grain boundary (PAGB) and lath boundary. After aging 100 h, TMLS transformed into polygonal ferrite, and the size of the precipitate at the subgrain boundary was about 100 nm, while after 300 h of high-temperature aging, large precipitates appear (400 nm) in the matrix. After 200 h of high-temperature aging, the obvious growth of precipitates on the PAGB and lath boundary weakens the pinning effect on the PAGB and martensite lath boundary and accelerates the transformation of microstructure and mechanical properties.

Numerical simulations of gradient cooling technique for controlled production of differential microstructure in steel strip or plate

<abstract><p>Numerical studies were conducted to investigate the applicability of cooling strategies for controlledly producing a microstructure in the steel strip or plate, which changes as function of the plate length. In the numerical simulations, the water spray cooling was varied as function of the plate length and as a result, the different parts of the plate were cooled at different rates. We applied the previously developed numerical code where the transformation latent heat is coupled with the heat conduction and transfer model, which has also been calibrated to correspond to experimental laboratory cooling line. The applicability of the method was investigated for controlledly creating alternating bainite and polygonal ferrite regions in plates of two different thicknesses (0.8 cm and 1.2 cm thick plates) by cooling different parts of the plate to different temperatures before switching off the water cooling so that polygonal ferrite forms in the part which has been cooled to higher temperature and bainite forms in the low temperature part. The simulation results indicate that the controlled production of such alternating regions is possible, but the resulting regions in the studied scenario cannot be very thin. The transition regions between the ferrite and bainite regions in the simulated cases are in the range of 5–15 cm. Controlled production of zones consiting of softer phase in the otherwise bainitic steel could offer a possibility for creating designed tracks in a steel bainitic strip or plate, where the mechanical working or cutting of the material is easier.</p></abstract>

Effect of different welding energy on microstructure and toughness of HAZ of low carbon bainitic steel

Low carbon bainitic steel (LCBS) with excellent combined properties is the first choice for materials of pipeline transiting geological disaster. However, welding will worsen its toughness. In this paper, three kinds of welding heat input were designed to study the relationship between the toughness and microstructure in the coarse grain zone of welding heat affect zone (CGHAZ) of LCBS. The evolution characteristics of the microstructure of LCBS and the CGHAZ, and impact fracture were investigated by optical microscopy (OM), scanning electron microscopy (SEM). The results indicated that microstructure of LCBS consists mainly of bainite ferrite (BF) and granular bainite (GB). Heat input for 22 kJ/cm, the original austenite grains become coarsening, the microstructure is a small amount of quasi polygonal ferrite (QF) and polygonal ferrite (PF), which exhibits low Charpy impact toughness. However, for heat input of 19 kJ/cm, the degree of grain coarsening is small and distribution of martensite-austenite (M-A) constituents is the chain. The statistics of image software show that with the increase of heat input (16–22 kJ/cm), the average grain size of original austenite is basically the same (25 um), which is mainly due to Nb solute drag restraint the growth of austenite grain.

Cross-Sectional Grain Size Homogeneity Effect on Structural Steel Fatigue Performance in Air and Hydrogen Environments

Structural steel mechanical properties of strength and ductility for a given microstructure are predominately driven by the average ferrite grain/packet size and by the through-thickness homogeneity of the ferrite grain/packet size in the final product. Fatigue performance, a ductility property, in air for applications of wind towers, bridges or high-rise buildings along within environments of high-pressure gaseous hydrogen for various pipeline systems is critical to the end-use design. Fracture and fatigue testing of a commercially produced low carbon 20 mm thick API X60 Sour Service steel had been completed which showed good and stable performance when compared to other commercially produced pipeline and structural steel microstructures. This commercially produced API steel was reported as “Alloy D” in prior published work. The microstructure was predominately polygonal ferrite with industrial quality of steel cleanliness, minimum of microstructural banding and a small but relatively homogenous through-thickness grain size required for a successful API X60 Sour Service specification/application. Based on the initial fatigue performance reported for the “Alloy D” through-thickness microstructure a more comprehensive study on the effect of the through-thickness grain size/homogeneity on fatigue was initiated. To isolate and study this effect of the, laboratory developed samples of a low carbon API X60 Sour Service steel with the same alloy design as “Alloy D”, which is characterized by a predominately single-phase polygonal ferrite microstructure with excellent cleanliness and no microstructural banding. Two sets of steels were made with the only difference being variations in average through-thickness and homogeneity of the final ferrite grain size.