Bainite Packet Research Articles

Correlation of Fatigue Limit and Crack Growth Threshold Value to the Nanobainitic Microstructure

The recently developed nanobainitic steels show high ultimate tensile strength (UTS) as well as high ductility. Although this combination seems to be desirable for fatigue design, fatigue limit of nanostructured bainite is often disappointingly low. To improve fatigue properties we tried to earn a fundamental understanding of the microstructural parameters governing fatigue behavior.Therefore our hypothesis to improve the fatigue behavior was not necessarily avoiding the initiation of a fatigue crack – which could lead to failure of the material – but to improve the ability of the present microstructure to slow down or stop growing cracks. Thus, the key to understand the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features which could act as barriers for growing cracks.We tried to correlate our results of fatigue tests and analysis of fracture surfaces to the size of microstructural features like bainitic ferrite plates, crystallographic bainite blocks and packets or prior austenite grains, as well as cracks induced at nonmetallic inclusions. Thereby we found that the crystallographic bainite block size governs fatigue behavior. Additionally, threshold values were determined from crack growth experiments and related to the characteristic microstructural features.

The Role of the Bainitic Packet in Control of Impact Toughness in a Simulated CGHAZ of X90 Pipeline Steel

X90 pipeline steel was processed with the simulated coarse grain heat affect zone (CGHAZ) thermal cycle with heat input varying from 30 kJ/cm to 60 kJ/cm, the microstructures were investigated by means of optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), and transmission electron microscope (TEM), and the impact properties were evaluated from the welding thermal cycle treated samples. The results indicate that the microstructure is primarily composed of lath bainite. When decreasing the heat input, both bainite packet and block are significantly refined, and the toughness has an increased tendency due to the grain refinement. The fracture surfaces all present cleavage fracture for the samples with different heat inputs. Moreover, the average cleavage facet size for the CGHAZ is nearly equal to the average bainite packet size, and the bainitic packet boundary can strongly impede the crack propagation, indicating that the bainitic packet is the most effective unit in control of impact toughness in the simulated CGHAZ of X90 pipeline steel.

Effect of Nb microalloying on the heat affected zone microstructure of girth welded joints

The effect of Nb content in the range 0.07–0.10% on the HAZ microstructure of large diameter pipes is reported in this paper. Results show that Nb is able to influence the size of the bainitic packet and cells in the heat affected zone, i.e. the microstructural parameters affecting impact toughness and hardness behavior. Such an effect leads to an improvement of both toughness and hardness for the steel with higher Nb content.

Microstructure and fracture toughness of multipass friction stir welded joints of API-5L-X80 steel plates

Friction stir welding with the optimum parameters offers high toughness and mechanical properties for pipeline steel applications; however, at subzero temperatures such joints have shown decreased toughness relative to room temperatures values as well as predominantly brittle behavior. This paper relates certain microstructural features to fracture toughness for the different regions of one- and two-pass friction stir welded joints, including base material, heat-affected, stir and hard zones. Toughness was assessed at 0°C, −20°C and −40°C by crack tip opening displacement testing. The microstructures in the stir and hard zones were composed mainly of granular bainite, and bainite packets with irregular and straight ferrite plates. Hardness mapping distinguished the hard zone from the base material and stir zone, and from the heat affected zone that undergoes a softening relative to base material. The effects of low temperatures on the toughness were more pronounced in notches located at the stir zone and hard zone than for other notches. Detailed SEM fractography revealed the complementary roles of martensite-austenite particles, and bainite packets in the initiation and propagation of cleavage fracture in the hard zone at −40°C.

Low-Temperature Toughness of the Austempered Offshore Steel

This study investigates low-temperature toughness of the offshore steel with two different austempering heat treatments. Toughness of upper bainite is significantly lower than that of lower bainite. Low impact toughness of the upper bainite is caused by the presence of martensite–austenite (MA) and increasing the amount of low-angle lath boundaries in upper bainite package. EBSD equipped in the SEM demonstrates an effective approach to analyze the misorientation angle of lath boundaries in upper bainite packet.

Impact toughness scattering of bainitic steel in the ductile-brittle transition temperature region

The impact toughness scattering in the ductile-brittle transition temperature (DBTT) region was experimentally examined on mixed and homogeneous grains of low alloy high strength bainitic steel under dynamic loading conditions. The results revealed that the mixed grain microstructure had larger impact toughness scattering than the homogeneous one, and the impact toughness scattering was mainly caused by the scattering in the cleavage fracture stress σf. The value of σf is related to the size of the microcrack formed in the bainitic packet. When a bainitic packet-sized microcrack propagates from one bainitic packet into the adjacent packet, cleavage fracture occurs. The cleavage fracture is controlled by the few coarse packets in the microstructures, and the σf scattering is influenced by the varied distances/relative locations between these coarse packets, and homogenizing the distribution of fine bainitic packet sizes is an effective way to reduce the impact toughness scattering in the DBTT region.

Application of local approach to fracture of an RPV steel: effect of the crystal plasticity on the critical carbide size

Avoidance of brittle fracture is a key issue of the integrity assessment of the nuclear reactor pressure vessels (RPV). The microstructural heterogeneities of these heavy components are in direct relation with the scatter of brittle fracture toughness. A microstructure informed brittle fracture (MIBF) model is used here to capture the effect of some microstructural features. This local approach type model is based on the Griffith criterion for brittle fracture applied to micro cracks nucleated on carbides. The size distribution of the cleavage initiators introduces a source of scatter for the fracture stress. The originality of the MIBF model is to introduce a second source of scatter: the stress distribution inside a representative aggregate of the bainitic microstructure. These stresses are the average for each bainitic packet of the maximum principal stresses. Their non-uniform distribution is due to the incompatibilities of plastic deformation introduced by crystallographic misorientations between packets. Finite elements simulations on aggregates representative of the steel bainitic microstructure are performed to identify the stress distributions. The size distribution of cleavage initiators is the carbide size distribution as the carbides cracking induced by plastic deformation is considered to be the main damage leading to cleavage propagation in RPV steel. The only free parameter of the model is the effective surface energy γf. Applied to the Euro fracture toughness dataset, the MIBF model predicts the specimen size effect on fracture toughness as well as the temperature dependence of the fracture toughness up to 200 MPa√m. The analysis of the numerical results shows that taking into account the incompatibility stresses widens the size range of carbides potentially implied in the fracture process. However, approximatively half of these carbides have still a size larger than the largest observed sizes underlining the importance to perform the determination of size distributions on a very large number of precipitates.

Effect of Bainitic Packet Size Distribution on Impact Toughness and its Scattering in the Ductile-Brittle Transition Temperature Region of Q&T Mn-Ni-Mo Bainitic Steels

The impact toughness of Q&T Mn-Ni-Mo bainitic steel in the ductile–brittle transition temperature (DBTT) region was measured. The fracture micromechanism associated with the microstructural features was studied, by statistically analyzing the grain sizes of various structures. The results reveal that the cleavage fracture behavior is controlled by bainitic packets. There exists a critical bainitic packet size. When a microcrack formed in the bainitic packet with a size exceeding the critical size propagates into adjacent packets, brittle cleavage fracture occurs. The probability of finding the bainitic packets larger than the critical size dominates the impact toughness and its scattering. It is suggested that refining the bainitic packets and homogenizing their sizes is an effective method of improving the impact toughness and reducing its scattering in the DBTT region.

Microstructural Unit Controlling Cleavage Crack Propagation in High Strength Bainitic Steels

The strengthening mechanisms which are operative in bainite are very well known: small bainite packet, small width of the laths, dislocation density and size and number of carbide particles (Fe3C), among others. Bainite packet size has been traditionally considered as the value measured by optical microscopy (OM), as electron back scattered diffraction (EBSD) technique is relatively recent. In a V-microalloyed steel with bainitic microstructure of C=0.38%, V=0.12% and N= 0.0214% the average length and width of ferrite laths and of cementite carbides were measured. On the other hand, the bainite packet size was measured by OM and EBSD with a misorientation of 15o. These values of the microstructural units have been taken in account to calculate the effective surface energy γp given by Griffith’s model for cleavage fracture. It was concluded that bainite packet size determined by EBSD with a misorientation angle criterion of 15o was the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainite packet size, it was concluded that the effective surface energy of cleavage fracture (γp) would be between 71.6 and 82.6 J m-2.

Charpy Impact Properties of Grain Boundary Allotriomorphic Ferrite and Granular Bainite Duplex Microstructure

A new type of high strength and low cost bainitic steel with ultra-low carbon content and high Si content has been developed on the basis of Mn-series air-cooling bainitic steels. The tensile properties of YS>690MPa and the impact toughness of AKV>60J at-40°C were obtained by controlling the processing parameters. This was attributed to the formation of the grain boundary allotriomorphic ferrite (FGBA) and the granular bainite (GB) with different shape of M/A islands. The high strength due to the inter-lath lamellar M/A islands or retained austenite companying with high dislocated bainitic ferrite laths of average 300nm width. The effect of microstructure on the impact crack initiation and propagation was studied. The results showed that crack initiation occurred in two different types of sites: at interphase boundaries of bainite ferrite (BF) and M/A islands, at grain boundaries. The FGBA and bainite ferrite (BF) both had blunting effect on microcrack tip to reduce the crack propagation path. Because of the presence of FGBA, the unit crack path was short, at less than 5μm. The blunting effect of BF could be enhanced by the M/A islands, which force the cracks change the propagation path and reduce the unit crack path to less than the size of bainite packets. The mechanism of low temperature microcrack origin of the ultra-low carbon bainitic (ULCB) steel with the microstructure of the FGBA and GB was also discussed.

Complex Microstructural Banding of Continuously Cooled Carbide-Free Bainitic Steels

Chemical segregation of alloying elements during solidification of steel grades leads to development of a banded microstructure, causing a degree of anisotropy that can be detrimental to the mechanical behavior under service conditions. It is well-known that the presence of strongly orientated martensite bands in carbide-free bainitic microstructures, associated to inhomogeneous Mn redistribution during solidification, leads to a remarkable deterioration in toughness in advanced high strength bainitic steels. In this study, while bands were clearly visible on light optical micrographs of continuously cooled carbide-free bainitic steels, scanning electron microscopy examination revealed only a gradual transition between matrix and bands, both with a granular bainitic structure. Electron backscatter diffraction was used to quantify the bainitic packet size and volume fraction of martensite/austenite constituent between and within the bands, after a process of optimization of the analysis settings in order to minimize the inherent difficulties linked to sub-micrometric and minority phase indexation. The quantitative microstructural results showed negligible morphological differences between bainitic structure bands and matrix, only solute segregation of Cr and Mo was detected by energy-dispersive X-ray spectroscopy within bands, which must be responsible for a stronger resistance against metallographic etching in those regions.

Microstructures and mechanical properties of hot-rolled Nb-microalloyed TRIP steels by different thermo-mechanical processes

The microstructural evolution and mechanical properties of hot-rolled Nb-microalloyed TRIP steel with different states of austenite (recrystallized/pancaked) prior to the transformation of austenite to ferrite were investigated under the thermo-mechanical process based on controlled cooling or based on dynamic transformation of undercooled austenite (DTUA) by hot compression tests using a Gleeble-1500 hot simulator, in combination with OM, SEM, TEM, XRD and ICP–OES. The results indicated that the influence of the states of austenite prior to the transformation of austenite to ferrite on the multi-phase microstructures and the mechanical properties of the tested steel became weak in the case of the process based on DTUA, in comparison with the case of the process based on controlled cooling. The much refined and uniform microstructures with smaller sizes of ferrite and bainitic packets and the more stable retained austenite were obtained by the process based on DTUA, resulting in the markedly improved combination of strength and ductility of the tested steel.

Morphology and crystallographic orientation relationship in isothermally transformed Fe–N austenite

The 225°C isothermal transformation of a high-nitrogen austenite with Fe–2.71wt.% N was investigated by means of electron microscopy. It was found that the transformation products were composed of ultrafine α-Fe and γ′-Fe4N plus retained austenite γ, which were in two types of morphologies, namely, (i) with the retained austenite patches dispersed among the (α-Fe+γ′-Fe4N) packets and (ii) with the ultrafine α-Fe and γ/γ′-Fe4N laths interwoven with each other within a single bainitic packet. A cube–cube orientation relationship between the γ (austenite) and γ′-Fe4N, and a near Greninger–Troiano (G–T) one between the γ (austenite) and the bainitic α-ferrite were detected. The morphology, orientation relationship and high hardness (>1000HV) of the transformation products indicated that the isothermal transformation of the high nitrogen austenite was analogous to a bainitic one.

Investigation of Microstructural Features Determining the Toughness of 980 MPa Bainitic Weld Metal

The microstructural features that control the impact toughness of weld metals of a 980 MPa 8 pct Ni high-strength steel are investigated using instrumented Charpy V tester, optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD), and finite-element method (FEM) calculation. The results show that the critical event for cleavage fracture in this high-strength steel and weld metals is the propagation of a bainite packet-sized crack across the packet boundary into contiguous packets, and the bainitic packet sizes control the impact toughness. The high-angle misorientation boundaries detected in a bainite packet by EBSD form fine tear ridges on fracture surfaces. However, they are not the decisive factors controlling the cleavage fracture. The effects of Ni content are essential factors for improving the toughness. The extra large cleavage facets seriously deteriorate the toughness, which are formed on the interfaces of large columnar crystals growing in welding pools with high heat input.

Analysis of V(C, N) nanoparticles in a medium carbon bainitic microalloyed steel and their influence on strengthening

Abstract Continuous cooling transformation diagrams were plotted for a microalloyed steel in dilatometric tests at different cooling rates, and maximum and minimum cooling rates for bainite formation were determined. Austenite grain and bainite packet sizes were measured in three bainitic microstructures obtained by continuous cooling from austenitisation temperatures of 950, 1 050 and 1 150 °C, respectively. The results show that bainitic packet growth cannot exceed the austenite grain boundaries where it has nucleated. V(C, N) particle size was measured using field emission gun scanning electron microscopy and transmission electron microscopy, respectively. Both techniques yielded very similar values, but transmission electron microscopy is preferable due to its higher resolution and the possibility to carry out electron diffraction and dispersive X-ray analyses. The presence of precipitates has contributed to raising the yield strength, which can be predicted by Orowan's expression.

Microstructure and Mechanical Properties of a Low-Carbon Mn-Si Multiphase Steel Based on Dynamic Transformation of Undercooled Austenite

The microstructure evolution of 0.20C-2.00Mn-2.00Si steel treated by the thermomechanical process to manufacture hot-rolled, transformation-induced plasticity (TRIP) steel based on dynamic transformation of undercooled austenite was investigated using a Gleeble 1500 (Dynamic Systems, Inc., Poestenkill, NY) hot simulation test machine in combination with light microscope (LM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The mechanical properties of this steel with different multiphase microstructures were also analyzed using room-temperature tensile tests. The results indicated that the multi-phase microstructures consisting of fine-grained ferrite with a size of 1–3 μm, bainite packets, and retained austenite and martensite were formed for the used steel by a thermo-mechanical process involving dynamic transformation of undercooled austenite, controlled cooling, isothermal bainite treatment and water-quenching. With the increase in the strain of hot deformation of undercooled austenite, the fraction of ferrite increased, that of bainite decreased, and that of martensite increased. At the same time, the fraction of retained austenite (RA), as well as the carbon content of RA, first increased and then decreased. For the used steel treated by such process, the tensile strength is about 1200 MPa with a total elongation of about 20 pct, and the product of tensile strength and total elongation can be up to 25,000 MPa × pct.

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size.

Measurement of bainite packet size and its influence on cleavage fracture in a medium carbon bainitic steel

For a bainitic steel of C = 0.38%, the bainitic packet size measured by OM is about 3.5 times larger than that measured by EBSD with a 15° grain boundary misorientation. This ratio is used to calculate the effective surface energy given by Griffith's model for cleavage fracture. Average ferrite lath width and cementite carbide width values are also measured. Following the general form of Griffith's equation, the effective surface area of cleavage fracture γp is determined for each microstructural unit. It is concluded that the bainite packet size determined by EBSD with a 15° misorientation angle criterion is the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainitic packet size, it is concluded that the effective surface energy of cleavage fracture (γp) is between 71.6 and 82.6 J m−2.

Effect of Process Parameters on Microstructural Evolution and Grain Refinement in a Low Carbon High Strength Microalloyed Dual Phase Steel

The effects of process parameters on microstructural evolution and grain refinement are determined in a Nb-Ti microalloyed high strength dual phase steel. With the increase of cooling rate, final microstructures change from a mixture of acicular ferrite (AF)+martensite/retained austenite (M/A) to conventional bainite (CB)+M/A. Accordingly, the morphology of M/A constituent changes from an equiaxed island in AF to an elongated interlath in CB. The length and width of bainite packets become smaller with the increase of cooling rate and the decrease of deformation temperature. The length of individual bainitic ferrite plates within the packets become smaller with the increase of cooling rate and the decrease of deformation temperature, whereas the thickness of them varies slightly with them. The optimized relaxing time on grain refinement is 60 s. The reheating temperature, reduction ratio and interrupt temperature has no obvious effect on the formation of dual phase of acicular ferrite or bainite and M/A.

Grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels

The microstructural features and grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels were investigated using optical microscopy, scanning electron microscopy, and electron backscattering diffraction. The coarse-grained region of the heat-affected zone consists of predominantly bainite and a small proportion of acicular ferrite. Bainite packets are separated by high angle boundaries. Acicular ferrite laths or plates in the coarse-grained region of the heat-affected zone formed prior to bainite packets partition austenite grains into many smaller and separate areas, resulting in fine-grained mixed microstructures. Electron backscattering diffraction analysis indicates that the average crystallographic grain size of the coarse-grained region of the heat-affected zone reaches 6–9 μm, much smaller than that of austenite grains.