Metallurgical investigation of the aging process on tensile fracture welded joints in pipeline steel

The effects of aging in the tensile fracture behavior of welded joints of API5L-X52 pipe steel were studied by accelerating aging at 250 °C for different periods of time. The weld metal, heat affected zone and base metal, showed an increase in yield strength while the strain-hardening exponent decreased at early stages of aging. A maximum strength and minimum hardening exponent was found at 500 h due to peak-aging. Subsequently, both properties exhibited an opposite behavior due to over-aging. Tensile fractured specimens for the three different zones exhibited ductile failure, presenting microvoid morphology associated with the coalescence of microcavities. An increase in void density and a reduction in diameter during short periods in the fractured specimens were observed. The maximum density and minimum diameter of voids were obtained at 500 h and were linked to the improvement of strength and precipitation of nanoparticles. Afterward, the fractured surfaces exhibited a reduction in density and the diameters of voids were larger, having been induced by the deterioration of strength and coarsening of particles.     

The fracture toughness of CO2 corrosion scale in pipeline steel

The fracture toughness of CO₂ corrosion scale in X65 pipeline steel has been measured using Vicker's indentation on a polished cross-section of the scale and the variation of the fracture toughness with scale forming temperature has been investigated. The results show that CO₂ corrosion scale formed at 65 °C to 90 °C is (Fe,Ca)CO₃ and that at 115 °C is (Fe,Ca,Mg)CO₃, respectively. The scale thickness decreases and the amount of Ca in the scale increases with increasing scale forming temperature. The fracture toughnesses of the outer and inner layers of the scale formed at 65 °C are 0.68 MPam^1/2 and 1.46 MPam^1/2, respectively. The fracture toughness of the CO₂ corrosion scale decreases with increasing scale forming temperature.     

铁素体管线钢的分层裂纹及其对断裂的影响

通过对针状铁素体管线钢缺口根部三维应力状态的有限元分析和不同形式的断裂实验,研究了管线钢分层裂纹产生的条件及其对断裂性能的影响.结果表明裂纹或缺口根部的三维应力状态是产生分层裂纹的必要条件,材料的强度分布影响分层裂纹的形式和方向.分层裂纹均为主裂纹扩展前材料中的弱界面在垂直该弱界面的拉应力作用下产生的,其数量和方向受裂纹端部三维应力场和材料的强度分布状态控制.分层裂纹面上的应力为零,分层裂纹有一定的间距.在断裂过程中产生的分层裂纹使裂纹或缺口根部的构形发生改变,从而对裂尖的应力状态和材料的断裂性能产生巨大的影响.穿透裂纹体的分层裂纹使其有效厚度减小,表面裂纹体的分层裂纹与裂纹扩展方向垂直.在断裂过程中产生分层裂纹需要消耗更多的能量、降低裂端三维应力约束、有效厚度降低或裂尖钝化.这些因素均使断裂扩展更加困难,而使材料韧性得到提高.     

A model for the dynamic fracture toughness of ductile structural steel

We assume in this paper that the dynamic fracture toughness K_Id of ductile structural steels is dependent on void nucleation and void growth. The void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g were obtained by modifying the void nucleation-induced and void growth-induced static fracture toughness models, respectively, considering the effect of strain rate and local temperature. By the relationship between the void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g((K_Id)²=(K_Id·n)²+(K_Id·g)²) dynamic fracture toughness K_Id could be quantitatively evaluated. With this model the dynamic fracture toughness of two structural steels (X65 and SA440) was assessed, and the causes for the differences between the static and dynamic fracture toughness were also discussed.     

A model for the static fracture toughness of ductile structural steel

Fracture of ductile structural steels generally occurs after void initiation, void growth and void coalescence. In order for ductile fracture of structural steels to occur, energy must be spent to induce void initiation and void growth. Therefore, fracture toughness for ductile fracture should be contributed from void initiation and void growth. On the basis of this suggestion static fracture toughness (K_IC) of ductile structural steels is decomposed into two parts: void nucleation-induced fracture toughness (denoted as K_IC.n) and void growth-induced fracture toughness (K_IC.g). K_IC.n, defined as the stress intensity factor at which voids ahead of a crack begins to form, is calculated from crack tip strain distribution and void nucleation strain distribution. In contrast, K_IC.g is determined by the void growth from the beginning of void nucleation to void coalescence. Therefore, K_IC.g relates to the void sizes and void distribution. In this paper, the expression for K_IC.g is given from the void sizes directly from fracture surfaces. The relationship between K_IC.n, K_IC.g and K_IC is expressed in the form (K_IC)²=(K_IC.n)²+(K_IC.g)². The newly developed model was applied to the fracture toughness evaluation of three structural steels (SN490, X65 and SA440), and the theoretical calculation agrees with the experimental results.     

A model for the static fracture toughness of ductile structural steel

Fracture of ductile structural steels generally occurs after void initiation, void growth and void coalescence. In order for ductile fracture of structural steels to occur, energy must be spent to induce void initiation and void growth. Therefore, fracture toughness for ductile fracture should be contributed from void initiation and void growth. On the basis of this suggestion static fracture toughness (K_IC) of ductile structural steels is decomposed into two parts: void nucleation-induced fracture toughness (denoted as K_IC.n) and void growth-induced fracture toughness (K_IC.g). K_IC.n, defined as the stress intensity factor at which voids ahead of a crack begins to form, is calculated from crack tip strain distribution and void nucleation strain distribution. In contrast, K_IC.g is determined by the void growth from the beginning of void nucleation to void coalescence. Therefore, K_IC.g relates to the void sizes and void distribution. In this paper, the expression for K_IC.g is given from the void sizes directly from fracture surfaces. The relationship between K_IC.n, K_IC.g and K_IC is expressed in the form (K_IC)²=(K_IC.n)²+(K_IC.g)². The newly developed model was applied to the fracture toughness evaluation of three structural steels (SN490, X65 and SA440), and the theoretical calculation agrees with the experimental results.     

A model for the dynamic fracture toughness of ductile structural steel

We assume in this paper that the dynamic fracture toughness K_Id of ductile structural steels is dependent on void nucleation and void growth. The void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g were obtained by modifying the void nucleation-induced and void growth-induced static fracture toughness models, respectively, considering the effect of strain rate and local temperature. By the relationship between the void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g((K_Id)²=(K_Id·n)²+(K_Id·g)²) dynamic fracture toughness K_Id could be quantitatively evaluated. With this model the dynamic fracture toughness of two structural steels (X65 and SA440) was assessed, and the causes for the differences between the static and dynamic fracture toughness were also discussed.     

A mathematical model to approach the fracture process of overconsolidated clay

Some soil and most rock masses contained defects (joints, cracks, bedding planes, fissures in clay, etc.). Beyond the peak shear strength of a mass of soil or rock, the stress falls as the strain increases. In this paper, the plane strain problem with double edge-cracks under shear loads is analysed based on linear elastic fracture mechanics but with finite stress concentration concept. We assumed that the fissured clay is a kind of strain-softening material and the growth of slip surface is along the maximum shear stress plane. The growth of slip surfaces in the progressive failure is expressed by the shear loads and slip displacement curves, which are obtained under the crack tip opening displacement criteria (CTOD_c) or the energy release rate criteria (J_c). The residual slip displacements are also taken into account by cumulating the elastic ones.     

On the physical process in the deformation and fracture of ultra-strength steel

This thesis summarizes the author's research into the uniaxial stretch process of ultra-strength steel, which was made by means of sound emission and fracture surface observation. In the article, the author suggests a new idea for the sectionalized power hardening and the division of the specimen fracture surface. To confirm this new idea, a theoretical explanation is given. Also included is a physical graph of the deformation and fracture during the uniaxial stretch.     

Effect of austenite transformation degree on microstructure and fracture toughness of high-strain pipeline steel

In order to clarify the relationships among austenite transformation degree, microstructure and fracture toughness, the simulated inter-critical heat-affected zones (ICHAZ) of high-strain pipeline steel were prepared with five different austenitizing degrees (0%, 20%, 50%, 80% and 100%). The microstructure and the fracture toughness of simulated ICHAZ specimens were investigated. It was found that the microstructure evolution and fracture toughness of ICHAZ were closely related to the austenite transformation degree. In partially austenitized ICHAZ, the fresh bainite and ferrite could break the original structure and decreased effective grain size. The fine grains with disorderly arranged M–A constituents could improve fracture toughness of partially austenitized ICHAZ. In contrast, the linearly aligned M–A constituents in 0%-austenitized region and the chain-distributed ones in 100%-austenitized region could lead to deterioration of fracture toughness. Furthermore, the excellent fracture toughness of partially austenitized ICHAZ was related to the high density of high-angle grain boundaries (HAGBs), homogeneous distribution of local strain as well as the high percentage of small-deformed grains (SDGs).

     

13MnNiMoNbR钢板在卷制过程中的断裂分析研究

通过试验室运用检测手段,对材料断裂分析作出的试验结果,进行真实全面的科学认证。     

13MnNiMoNbR钢板在卷制过程中的断裂分析研究

通过试验室运用检测手段，对材料断裂分析作出的试验结果，进行真实全面的科学认证。     

A statistical model for cleavage fracture in notched specimens of C–Mn steel

A statistical model for cleavage fracture in notched specimens of C-Mn steel has been proposed. This model is based on a recently suggested physical model. This statistical model satisfactorily describes the distributions of the cumulative failure probability and failure probability density of 36 notched specimens fractured at various loads at test temperature of −196 °C. The minimum notch toughness has also been discussed.     

Hydrogen effect on local fracture emanating from notches in pipeline from steel API X52

The assessment of local strength at notches in pipeline steel API X52 has been done for conditions of cathodic hydrogen charging. The relationship between hydrogen concentration and critical (failure) loading has been found. The existence of some critical hydrogen concentration which causes the significant loss of local fracture resistance of material was also shown.     

Effect of hydrogenation on mechanical properties and tensile fracture mechanism of a high-nitrogen austenitic steel

Austenitic stainless steels are frequently used for hydrogen applications due to their high ductility at low temperatures and lower hydrogen environment embrittlement compared to ferritic steels. We study the effect of electrochemical hydrogen saturation up to 40 h on tensile behavior and fracture mechanisms in high-nitrogen austenitic 17Cr–24Mn–1.3V–0.2C–1.3N steel. Hydrogen saturation weakly influences the characteristic of stress–strain curves, but decreases steel ductility, yield stress, and ultimate tensile stress. Hydrogenation provides a change in steel fracture peculiarities—a hydrogen-assisted thin brittle surface layer of ≈5 μm and ductile subsurface layer of 50–150 μm in width in hydrogen-saturated specimens. The subsurface layer shows ductile transgranular fracture with elongated dimples and flat facets. The central parts of fracture surfaces for hydrogenated specimens show ductile fracture mode similar to hydrogen-free state, but they include numerous secondary cracks both for central part and for transition zone between ductile central part and subsurface layer associated with highest hydrogen saturation. The possible reasons of decrease in hydrogen-associated ductility and change in fracture character are discussed.     

A model material approach to the study of fracture process zone of quasi-brittle materials

A new approach to study the fracture of quasi-brittle materials is introduced: the design and testing of model materials. By model material is understood a material with enlarged microstructure and which material parameters, such as stacking and mechanical properties of particles and cohesion force, can be fully controlled. In this paper a first example to the model materials approach is presented, consisting in 5 mm steel particles bonded in a precise stacking with an epoxy-based glue. It is shown how it is possible to correlate the different fracture mechanisms and ultimate peak load of the model material to the particle pair force and to the fracture process zone size. It is also seen how a quasi-brittle behaviour is produced in the presence of mechanisms that induced the crack to shift fracture planes, that is, in presence of energy dissipative mechanisms.