Deterioration Of Toughness Research Articles

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.

Continuous Cooling Transformations in Nuclear Pressure Vessel Steels

A class of low-alloy steels often referred to as SA508 represent key materials for the manufacture of nuclear reactor pressure vessels. The alloys have good properties, but the scatter in properties is of prime interest in safe design. Such scatter can arise from microstructural variations but most studies conclude that large components made from such steels are, following heat treatment, fully bainitic. In the present work, we demonstrate with the help of a variety of experimental techniques that the microstructures of three SA508 Gr.3 alloys are far from homogeneous when considered in the context of the cooling rates encountered in practice. In particular, allotriomorphic ferrite that is expected to lead to a deterioration in toughness, is found in the microstructure for realistic combinations of austenite grain size and the cooling rate combination. Parameters are established to identify the domains in which SA508 Gr.3 steels transform only into the fine bainitic microstructures.

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.

The Mechanism of Martensite-Austenite Microconstituents Formation during Thermomechanical Controlling Processing in Low Carbon Bainitic Steel

Bainite or the mixture of bainite and martensite is required to reach high strength levels in low carbon high strength steel. However, the bainite reaction rarely goes to completion, resulting in mixed structures of predominately bainitic ferrite and minor amounts of retained austenite, cementite or martensite mainly located at the ferrite grain boundaries. The exact nature of this minor transformation product depends on several factors including bulk composition, segregation and cooling rate. When the minor phase is largely martensite, the non-bainitic microstructure is called martensite-austenite microconstituent or MA. Interestingly, MA is believed to be one of the main factors causing the deterioration of toughness of steels. MA is also often associated with hydrogen-related cracking. In this current study, the formation of martensite-austenite constituents was studied experimentally and the results analyzed theoretically.

Effect of microstructure evolution on strength and impact toughness of G18CrMo2-6 heat-resistant steel during tempering

G18CrMo2-6 steel is one kind of low alloy CrMo steel with ferrite/bainite phase constituent. It is widely adopted in the large casting parts of pressure vessels. This paper aims at investigating evolution of G18CrMo2-6 steel microstructure, tensile strength and impact toughness during the tempering (680°C). The tempered microstructure characterization shows that the tempering time mainly affects the precipitation of three kinds of carbides, naming MC, M3C and M23C6. Both tensile strength and impact toughness do not obey the monotonic evolution with the tempering time but the complex changes. The shift of mechanical properties results from the interaction among matrix phases and the evolution of three different kinds of carbides. The softening of matrix due to the dislocation recovery and the decrease of the amout of carbon and other alloy elements in matrix lattice due to the precipitation of M23C6 cause the decreasing tensile strength for the whole tempering except for that an increase during the short-term tempering controlled by the precipitation of MC and the spheroidization and refinement of M3C carbide. On the other hand, both the softening of matrix and the spheroidization and refinement of M3C carbide are responsible for the increasing impact toughness. However, the precipitation and coarsening of M23C6 at ferrite grain boundaries during long-term tempering results in the sharp deterioration of impact toughness. It attributes to the larger grain boundary carbides result in the lower critical fracture stress of a carbide-ferrite interface.

Evaluation of Microstructure and Mechanical Properties in a Thick Plate of G91 Steel and its Weld for High Temperature Nuclear System

The purpose of this study is to investigate the microstructure and mechanical properties in 9Cr-1Mo-V steel and its weld. A 220 mm thick forged plate with a typical composition of G91based on ASME A336 was used. Narrow gap welds were produced by submerged arc welding (SAW) with two different welding speeds. The microstructures in the base metal were typical tempered martensite at all locations, and the sizes of prior austenite grains were increased with the depth from the surface. The yield and tensile strengths tend to decrease with an increase in test temperature, especially at temperatures higher than 500oC. The upper shelf energy of the specimen from center was lower than that from the surface, and it also showed a higher index temperature. The toughness deterioration at the center might be caused by the larger size of the prior austenite grains and the existence of the delta ferrite. In the case of the weld, the larger sizes of the weld beads were observed in the upper region with a lower welding pass speed. There was no significant difference in the strength of the upper and lower welds, but the elongations of the upper weld were slightly larger than those of the lower weld. In the Charpy impact test, the lower weld showed a better impact toughness than the upper weld caused by the relatively smaller size of the weld beads and finer microstructures owing to a lower heat input by the increased welding speed.

Evolution of M-A Constituents in HAZ and Its Effect on Toughness of MAG Joint of Granular Bainitic Steel Q690C

To study the evolution of M-A constituents and their effects on toughness of each HAZ(heat affected zone) sub-zone, a low carbon granular bainitic steel Q690 is welded using automatic metal active-gas welding(MAG) welding and Charpy V-notch impact test is carried out. The microstructures, fractograph and statistical results of M-A constituents is analysed. The results show that the microstructure of weld metal(WM) is acicular ferrite and the dimples on fracture surface of the WM are small and dense, showing ductile fracture feature. Although large amounts of M-A constituents existed in grain boundary in the WM, the toughness is not deteriorated owing to the blocky shape and small size. The microstructure of FZ(fusion zone) and CGHAZ(coarse grain heat affected zone) are upper bainite with string M-A constituents between them; the toughness of FZ is the worst because of the cleavage fracture of coarse grains; the toughness of other sub-zones, in which M-A constituents exist in grain boundary with small blocky shape, are close to base metal(BM). So, the evolution of distribution, size, and shape of M-A constituents is controlled by matrix microstructure; fine bainite and acicular ferrite could remedy the deterioration of toughness derived from M-A constituents; the microstructure control should focus on the matrix microstructure.

Continuous Cooling Transformation, Microstructure and Mechanical Properties of High-Strength Low-Alloy Steels Containing B and Cu

This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

Physical Simulation on Microstructure and Properties for Weld HAZ of X100 Pipeline Steel

In this paper, the simulation heat affected zone (HAZ) continuous cooling transformation (SH-CCT) diagram of X100 pipeline steel was drew by using Gleeble 3500-HS thermal/mechanics simulation test machine, and the microstructure and properties of the HAZ of single-pass/double-pass welding were simulated and studied. The results show that the hardening trend of X100 pipeline steel weld coarse grain HAZ (CGHAZ) is significant when using the small welding heat input. To prevent the hardening of weld CGHAZ and reduce the cold cracking sensitivity, moderate heat input welding specification (between 12 and 18 kJ/cm) and weld preheating process (between 100 and 150°C) should be adopted. When appearing obvious embrittlement, the microstructure of double-pass weld intercritically reheated coarse grain HAZ (ICCGHAZ) of X100 pipeline steel is mainly upper bainite, the precipitation and the chain-like distribution of quantity more martensite-austenite (M-A) constituents in austenite grain boundaries is one of the main reasons leading to toughness deterioration. In conclusion, optimization control measures on the microstructure and properties of X100 pipeline steel weld HAZ are presented by above comprehensive analysis.

Effects of chemical composition on the microstructure and mechanical properties of FCAW-S weld metal containing 2% Ni

In this study the effects of alloying elements on the microstructure and mechanical properties of 600MPa grade FCAW-S weld metals containing 2% Ni were examined. Carbon, manganese and aluminum contents were varied in the ranges of 0.075%–0.101%, 1.19%–1.69%, and 0.66%–1.49% respectively. Regardless of the Al content, all of the weld metals showed a bainite dominant microstructure with no δ-ferrite. This indicates that when a weld metal contains 2% Ni, the Al content can be increased up to around 1.5% without concern about the deterioration of impact toughness due to the presence of δ-ferrite. The tensile strength of the weld metals varied from 595 MPa to 702 MPa dependent upon the chemical composition. Multiple regression analysis showed that while C and Mn have strong influences on the tensile strength, Al has little influence. Therefore, the Pcm index of weld metals could be used as an indication of their tensile strength. Impact toughness of the weld metals was influenced most by tensile strength and showed that the 50J transition temperature increased by 36 °C when the tensile strength was increased by around 100 MPa. Therefore, an excessive increase of the tensile strength should be avoided to attain higher impact toughness. Even when inclusion mean diameters were increased from 0.588 μm to 0.708 μm with an increase of the Al content from 0.66% to 1.49%, the size difference showed little influence on the impact toughness of the weld metals in this experiment.

Effects of Normalizing Processes on Microstructure and Impact Toughness in Ti-bearing Weld Metal of Multilayer MAG Welded HSLA Steel

The influences of different normalizing heat treatments on microstructure, non-metallic inclusions and impact toughness in MAG Ti-bearing weld metal of HSLA steel has been studied. It has been shown that for the Ti-bearing weld metal the impact toughness after normalizing treatment decreases significantly against prolonging holding time and increasing normalizing temperature. The Mn-depleted zone forms around the Ti-bearing phase (MnTiO3) precipitated on Mn-Si oxide. Proeutectoid ferrite preferentially nucleates at the Mn-depleted zone and the interface between austenite and proeutectoid ferrite becomes the nucleation sites for pearlite thereafter. Mn-depleted zone formation increases the pearlite nucleation sites, and makes the pearlite fine. The dissolve of Ti-bearing precipitate causes disappear of Mn-depleted zone at strong normalizing processes (longer normalizing time and higher normalizing temperature), and the number of ferrite nucleation sites decreases, then the pearlite become coarser, which causes the deterioration of impact toughness.

Variation in microstructures and mechanical properties in the coarse-grained heat-affected zone of low-alloy steel with boron content

The correlation between mechanical properties and boron content in the coarse-grained heat-affected zone (CGHAZ) of low-alloy steel was investigated using 10ppm and30 ppm boron added low-alloy steels. A Gleeble system was used to simulate various CGHAZs as a function of heat input. The segregation behavior of boron in the CGHAZ was estimated through secondary ion mass spectrometry (SIMS) and particle tracking autoradiography (PTA). Vickers hardness and Charpy impact tests were performed in order to assess the mechanical properties of the CGHAZs. In the steel containing 30ppm of boron, boron segregation was relatively high even with low heat input, and increased compared to 10ppm boron added steel. Furthermore, the boron segregation level maintained a maximum at intermediate heat input conditions from 60 to 500kJ/cm. This is believed that the boron segregation can be reached a maximum level for relatively low heat input despite the faster cooling rate because of an increase in the boron segregation rate. Also, impact toughness decreased with increased boron content at identical heat input conditions. It is believed that the effect of segregation and precipitation of boron on deterioration of impact toughness is insignificant. However, deterioration of impact toughness may be due to the remarkable increase in the fraction of a second phase, such as the martesite–austenite (M–A) constituent, and the hardening of the matrix with increased boron content.

Sliding wear behavior of Fe50−xCr15Mo14C15B6Erx (x=0, 1, 2 at%) bulk metallic glass

The sliding wear behavior was investigated for Fe50−xCr15Mo14C15B6Erx (x=0, 1, 2at%) bulk metallic glasses (BMG). Minor alloying with 1–2at% erbium resulted in an increase in hardness and a deterioration of the indentation fracture toughness, but the wear resistance increased with increasing Er content. The Fe48Cr15Mo14C15B6Er2 bulk metallic glass was then annealed and the wear resistance increased in the following order: crystallized, as-cast, and structurally relaxed condition. Transmission electron microscopy studies did not reveal sliding wear-induced crystallization, but instead shear band formation. The results demonstrate that micro-alloying is a useful approach for improving the wear resistance of bulk metallic glasses and that the combination of micro-alloying and annealing can substantially improve the wear resistance.

Effect of Heat Input on Toughness of Coarse-Grained Heat-Affected Zone of an Ultra Low Carbon Acicular Ferrite Steel

The effect of heat input on toughness of coarse-grained heat-affected zone of an ultra low carbon acicular ferrite steel were investigated when the welding was conducted with high heat input. Microstructural observations, energy dispersive X-ray spectroscopy analyses were conducted using optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The microstructures of coarse-grained heat-affected zone consist of predominantly bainitic microstructure and a small proportion of acicular ferrite grains. The bainitic microstructures become coarsened with increasing heat input. The impact toughness of coarse-grained heat-affected zone remained at a higher level when the heat input ranged from 42 to 55 kJ/cm. It became not stable and dropped to a lower level when the heat input increased to 110150 kJ/cm. The enhancement in impact toughness was attributable to the MnS precipitation on the pre-formed Ti oxides as well as the formation of intragranular ferrite. When specimens were welded with higher heat input, the deterioration of impact toughness was caused by the coarsening of austenite grains.

Effect of Heat Input on Impact Toughness of Coarse-Grained Heat-Affected Zone of a Nb-Ti Microalloyed Pipeline Steel

The effect of varying heat inputs (20, 100, 200 kJ/cm) on the microstructures and toughness of the simulated coarse-grained heat-affected zone of a Nb-Ti microalloyed pipeline steel were investigated utilizing optical and electron microscope. Results showed that the impact toughness of the coarse-grained heat-affected zone maintained a higher level at the heat input of 20 and 100 kJ/cm, whereas it dropped to a much lower level at the heat input of 200 kJ/cm. The good toughness was attributed to the grain refinement and the homogenous distribution of fine and elongated martensite/austenite constituents. The deterioration of toughness for high heat input simulated welding was mainly caused by the coarse bainitic microstructure and massive martensite/austenite constituents.

Sigma phase precipitation and properties of super-duplex stainless steel UNS S32750 aged at the nose temperature

The nose temperature for σ-phase precipitation in super-duplex stainless steel (SDSS) UNS S32750 was evaluated by hardness method. Color-optical microscopy, scanning electron microscopy, energy spectrum analysis, impact and corrosion testing were carried out to investigate characteristics of microstructure and properties of the SDSS aged at the nose temperature. The experimental results indicate that the nose temperature of precipitation is 920 °C and aging at this temperature tiny σ phases can precipitate at phase interfaces or ferrite grain boundaries within 2 min. Prolonging aging duration the amount of σ-phase increases and a dual structure with σ and γ is obtained when aging for 120 min. The precipitation of σ-phase leads to severe deterioration in impact toughness (longitudinal/transverse direction) and corrosion resistance of SDSS.

Twin-wire submerged arc welding process of a high-strength low-alloy steel

The measurement of thermal cycle curves of a high-strength low-alloy steel (HSLA) subjected twin-wire submerged arc welding (SAW) was introduced. The thermal simulation test was performed by using the obtained curves. The impact toughness at −50 °C temperature of the simulated samples was also tested. OM, SEM and TEM of the heat-affected zone (HAZ) of some simulation specimens were investigated. The results showed that the HSLA endured the twin-wire welding thermal cycle, generally, the low-temperature toughness values of each part of HAZ was lower than that of the parent materials, and the microstructure of coarse-grained zone(CGHAZ) mainly made up of granular bainite is the reason of the toughness serious deterioration. Coarse grain, grain boundary carbide extract and M-A island with large size and irregular polygon, along the grain boundary distribution, are the reasons for the toughness deterioration of CGHAZ. The research also showed that selected parameters of twin-wire SAW can meet the requirements to weld the test steel.

Recent developments on WC-based bulk composites

In order to achieve improved properties and performance with WC-based cemented carbides, research efforts have been directed towards the development of nanostructured cemented carbides. With the recent development of ‘spray conversion process’ for synthesizing nanosized powders and the advent of spark plasma sintering technique, it has been possible to successfully develop bulk nanostructured cemented carbides, possessing improved hardness and wear resistance. On a different note, realisation of the fact that the presence of metallic binder phase is deleterious towards certain applications of WC-based cermets has led to a recent surge of interest towards the development of novel ‘binderless’ WC-based ceramics by replacing the metallic binder phase with ceramic sinter-additives. More recently, it has been possible to develop dense WC-based ceramic composites without considerable deterioration of fracture toughness in the absence of the metallic binder phase. In the above perspective, the present review focuses mainly on the recent research results concerned with the processing and characterisation of nanostructured WC-based cermets and binderless WC-based ceramic composites.

Degradation of impact fracture during accelerated ageing of weld metal on microalloyed steel

The effect of accelerated ageing on the toughness and fracture of the longitudinal weld metal of an API5L-X52 linepipe steel was evaluated by a Charpy V-notch impact test, fracture analysis and transmission electron microscopy. Ageing was performed at 250°C for 100–1000 h. The impact results indicated a significant reduction in the fracture energy and impact toughness as a function of ageing time, which were achieved by the scanning electron microscope fractographs that showed a decrease in the volume fraction of microvoids by Charpy ductile failure with the ageing time, which promoted brittle fracture due to transgranular cleavage. The minimum volume fraction of microvoids was reached at 500 h due to the peak age. The microstructural analysis indicated the precipitation of transgranular iron nanocarbides in the aged specimens, which was related to the deterioration of toughness and change in the ductile to brittle behaviour.

Toughness of Advanced High Strength Bainitic Steels

Carbide free bainite has achieved the highest strength and toughness combinations to date for bainitic steels in as-rolled conditions. By alloying designing and with the help of phase transformation theory, it was possible to improve simultaneously the strength and toughness because of the ultra-fine grain size of the bainitic ferrite plates. Ultimate tensile strengths ranging from 1600 MPa to 1800 MPa were achieved while keeping a total elongation higher than 10 %. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. However, it has been observed that the presence of coalesced bainite leads to a dramatic deterioration in toughness in these novel high strength bainitic steels.