Abstract—Based on the results of multiple impact bending and static tension tests obtained using a unique technique, we determine the possible causes of the scatter of impact toughness values and their bimodal distribution for low-carbon microalloyed pipeline steel with a simulated microstructure of the heat-affected zone of a welded joint. The influence of the second phases, namely, complex nonmetallic inclusions based on titanium–niobium nitride and containing aluminum oxide and calcium sulfide, is shown.

Impact bending tests of rolled products, base metal and welded joints of pipes manufactured from low-carbon microalloyed steels are performed. The structure of different parts of welded joints is studied. Fractures of impact samples and compositions of nonmetallic inclusions on fracture surfaces are analyzed. The embrittling mechanisms and the causes of scattering of the impact toughness are considered. These may include coarse bainite within a large (deformed) austenite grain, a region near the fusion line with an unfavorable orientation of crystallographic cleavage planes 001 in a HFC welded joint, and large grains of grain-boundary ferrite in the weld and in the HAZ under arc welding. The critical grain size of the α-phase (the maximum fraction), at which the embrittling effect of the nonmetallic inclusions begins to be observed in the steels is 50 – 80 μm.

The impact toughness, microstructure and break features of low-alloy steel weld samples produced by automatic submerged arc welding have been compared. It has been established that when a notch is applied along the weld axis, fracture occurs along large grains of grain-boundary ferrite 80–140 μm in size with an unfavorable cleavage planes orientation 001. The combination of such grains with non-metallic inclusions 3–5 μm in size facilitates the transcrystalline cleavage initiation. The effect of austenite grain size, crystallographic texture, and intergranular fracture on the impact toughness value is shown. Continuous chains of grain-boundary ferrite at the front of the main crack line up along the notch-parallel boundaries of columnar grains of axial orientation, which increases the tendency to transcrystalline cleavage. At the same time, with the lateral orientation of the columnar grains of the former austenite, the front line of the main crack intersects only certain separate areas of the grain-boundary ferrite. Offset notch results in an increase in both impact toughness and plasticity of weld metal.

In this work control welded joints of plates made of pipeline steel type 07KhG2B of strength class K60 are studied, obtained in a laboratory multi-arc welding stand and samples processed according to welding thermal cycles in a Gleeble 3180 test complex. It is shown that modeling and simulation techniques used make it possible to calculate the thermal cycle and form a structure in a steel sample that is as close as possible to the structure of the HAZ of a real welded joint. Studies carried out show general features of the effect of microstructure on specimen impact strength with a simulated HAZ microstructure and specimens from an actual welded joint in terms of the effect of matrix structure, large particles of titanium and niobium carbonitride phases, and MA-constituents. It is shown that the toughness of specimens with a simulated HAZ overheating area microstructure correspond to the worst test results of a real welded joint in the zone (GOST 6996) and the fusion line (DNV-OS-F101). This result confirms the validity of applying the simulated test technique for evaluating steel weldability.

The influence of high-frequency current welding by a pressure method on cold resistance of a low-carbon steel welded pipe joint with edge convergence angles of 4° and 7° in regimes without and with welding bridge formation is studied, as well as the main microstructural factors that determine welded joint cold resistance. As a result of fractographic analysis, a favorable effect of increasing the edge convergence angle of from 4° to 7° on welded joint contamination with oxide non-metallic inclusions, formed during metal high-temperature oxidation with welding heating of pipe edges before upsetting, is established. Cold resistance of the base metal is significantly better than a welded joint in a condition after local heat treatment depending upon quenching-tempering regime, despite the fact that metallographic fineness of the welded joint structure is either greater or is at the same level with the optimum local heat treatment regime. This difference may be due to welded joint metal crystallographic texture (including the unfavorable location of ferrite cleavage planes along a fracture plane) and/or “crystallographic” rather than “metallographic” grains (accumulations of grains having similar orientation). The combination of welded joint local heat treatment and pipe heat treatment makes it possible to minimize the difference in base metal pipe and the welded joint cold resistance.

The paper presents the results of studies of the structural features of plate rolled products of low-carbon pipe steels 04KhNDB, 05KhGB and 06GNFB differing in the basic composition and content of microalloying elements. In the production of industrial rolled products for each steel under study, the choice of rolling temperatures was made individually, but in the same way from the point of view of structure formation. The completion of the finishing stage of rolling was carried out in the austenite region near the temperature of the beginning of the phase transformation, the beginning of cooling - from the austenite region, in order to form a homogeneous structure and exclude the formation of structural banding. The steels have a similar microstructure after rolling, but the structure of the axial zone, which is basically the weak point in tests in hydrogen sulfide, is different. The microstructure of steel 04KhNDB is almost uniform in thickness and a slight segregation of manganese has little effect on the hardness of the axial zone. In steels 05KhGB and 06GNFB, segregation of manganese and an increase in the proportion of intermediate transformation products in the axial zone leads to the formation of elongated regions with increased hardness. This is most pronounced in steel 06GNFB, in which cracks were revealed after testing for hydrogen cracking in accordance with the NACE TM0284 method. It has been established that with a homogeneous microstructure in the axial zone of rolled products without areas of increased hardness and in the absence of manganese sulfides in it, large inclusions of titanium and niobium carbonitrides do not adversely affect the resistance to hydrogen cracking.

The transience of heating and cooling processes during high-frequency welding (HFW) of pipes leads to the formation in the heat-affected zone (HAZ) of a coarse-grained strained bainitic and/or martensitic structure with high hardness. Local or volumetric heat treatment is used to correct the microstructure of the HAZ. The paper presents the results of a study of the structure and properties of welded pipe joints produced at the Vyksa Steel Works. It has been established that the microstructure of the welded joint of pipes made of low-carbon steels (not more than 0.07% C) is mainly bainite of lath morphology, and for steel with a high content of carbon (~0.3%) and manganese ‒ martensite. To estimate the temperature field in terms of the angle and thickness of the pipe wall during local heat treatment a finite element model was created. The paper shows that the use of the model made it possible to optimize the heating parameters during the local heat treatment of pipes of various sizes. The experiments on local heat treatment of pipes with a diameter of 325 mm, made from rolled products 8 mm thick of steel 09Г2С, showed a significant refinement of the initial microstructure of the HAZ, an increase in uniformity and the disappearanc of structural heat-affected zones. In medium carbon steels (~0.3% C) with a high manganese content, the thermal cycle of HFC welding leads to formation of martensite. The local heat treatment in the form of normalization does not allow one to exclude the formation of martensite in the segregation zones due to the high cooling rate. The local heat treatment of welded joints in pipes made of steel 30Г (Russian Standard) in the high tempering mode (650–700 °C), with preliminary cooling of the welded joint to a temperature below the completion of the martensitic transformation (in segregation zones ~50 °C), makes it possible to release the segregation martensite and form the microstructure of sorbitol there.

Features of thermo-mechanical rolling in various types of rolling mills are studied: plate mill (continuous, semicontinuous), Steckel mill, and a casting and rolling complex with a continuous broad strip mill. Limitations are considered for each type of mill due to the dimensional range, composition and location of equipment, size and composition of an original workpiece, and the possibility of manufacturing product with a good set of high strength, ductility, and cold resistance. It is shown that implementation of thermomechanical rolling taking into account features of the production scheme and understanding structure formation processes manufacture of cold-resistant rolled product is possible in all of types of the mills considered. Use of contemporary and precise structure formation models may reduce the effect of limitations with the exception of physical features, and expand the possibilities of rolled product structure and property formation

Results are presented for a study of the weldability of a wide range of pipeline steels by a method of simulating welding thermal cycles. It is shown that the method of simulating the microstructure by exposing base metal to a welding thermal cycle using a Gleeble test complex can be considered as a universal method for assessing steel weldability. The method makes it possible to evaluate steel reaction to thermal action for various welding methods and regimes, and in particular to establish the microstructure and mechanical properties (hardness, toughness, crack resistance). Alongside precise reproduction of the welding thermal cycle for a given range of pipes and welding modes, Gleeble makes it possible to separate the effect of individual thermal cycle parameters on the microstructure and to establish a number of important features. Microstructural mechanisms that determine formation of the heat-affected zone metal properties are discussed. It is shown that during simulation it is important to provide an austenite grain size corresponding to an actual welded joint and the type (section) of the impact specimen, which will allow comparison of material and welding technology.

The aim of the work was to study the effect of cooling rate on the formation of the structure and impact strength of a welded joint of 0,07C-1,6Mn-0,2Cr-Nb pipe steel. The cooling rate w8/5 was varied from 0.9 to 12.4 °/s in the temperature range of 800–500 °/s by changing the heat input of welding, the groove width, and the temperature of 22 mm thick plates welded on a laboratory welding stand. The conducted studies confirm the previously obtained results on the weldability of pipe steels, that a decrease in the heat input of welding and, as a result, an increase in the cooling rate of the w8/5 welded joint is accompanied by an increase in impact toughness. For the studied steel, a significant increase in the impact toughness of KCV-40 (more than 2.5 times) occurs in the range of cooling rates w8/5 from 0.9 to 7.3 °/s and is typical not only for tests along the fusion line, but also in the center of the seam weld. It is shown that with an increase in the cooling rate of the welded joint in the structure of the weld metal, the maximum length and width decrease, and, at the same time, the proportion of grain boundary ferrite crystallites. At high cooling rates, the matrix of the weld structure consists entirely of acicular ferrite, which is favorable in terms of increasing the density of high-angle grain boundaries, toughness, and cold resistance. In the heat-affected zone, with an increase in the cooling rate, the maximum and average grain sizes decrease; grain-boundary and Widmanstatt ferrite is eliminated; a completely bainite structure is formed and the proportion of lath bainite increases, which also leads to an increase in the density of high-angle boundaries and contributes to an increase in impact strength. With an increase in the cooling rate, the proportion of the MA-constituents in the structure of the seam and the HAZ are increases. It is assumed that an increase in the cooling rate, simultaneously with a favorable effect on the structure of the weld and the weld pool, reduces the time for the processes occurring in the weld pool, which can be an unfavorable factor in terms of the toughness of the weld.

The degree of microstructural factors influences on the impact strength of four K60 strength class steels after simulating a thermal welding cycle in the coarse-grained area of the weld-affected zone is investigated. It has been shown that the differences between steels with different impact strength KCV–20 are mainly caused by the differences in the content and structure of titanium nitrites. In steels prone to brittle fracture, titanium nitrides act as sources of cleavage cracks origin. In the case of blocking the transition of the cleavage crack from nitride to the matrix, other less significant factors affect the impact strength. It is shown that high-angle grain boundaries stop cleavage cracks by limiting their size, which leads to a decrease in the probability of a crack crossing the grain boundary. The correlation of the impact strength with the volume fraction of the MA component particles and the residual austenite in the studied microstructures is weakly expressed. Such particles cannot be considered as localized sources of cleavage cracks. Differences in the level of Cr + Ni + Cu alloying in the studied steels, which are not characterized by the cleavage cracks origin on titanium nitride inclusions, can be considered as an important factor of impact strength. The mechanism of this influence may be associated with a change in the tendency to cleavage of the ferritic matrix.

Increased resistance to brittle fracture in any zone of the welded joint is one of the main requirements for quality of steel pipes for transportation of the hydrogen sulphide gas, which determines the relevance of identification of the reasons of the impact toughness decrease of the welded joint area. The purpose of the study was to investigate the microstructure formation features of the seam zone of pipes made of low-carbon steels of different chemical composition. The effect of microstructure of the coarse-grained area of the heat affected zone on toughness of welded joint has been estimated by the laboratory thermal cycling testing complex Gleeble 3180. The samples with cross-section of 10×10×110 mm made of commercially available steel grades 04ХНДБ, 06ГНФБ and 05ХГБ (Russian Standard) with different contents of niobium and manganese were used as material for the research. To eliminate the effect of heating rate and holding time in austenitic region on the microstructure, the simulation was carried out in a cyclic mode with the same heating rate up to 1350 °С and subsequent cooling to 800 °С. The cooling rate in the interval 800–500 °C varied from 2 to 64 °C/s. The microstructure formation of the seam zone was studied by scanning electron microscopy and reflected electron diffraction. Despite the differences in chemical composition, the studied steels showed similar microstructure after simulation of the coarse-grained area of the heat-affected zone. For all the steels an increase in the share of rack bainite and the density of high angle boundaries with increasing cooling rate was found. It was established that at close values of average grain size of ferrite in the samples from steel 04ХНДБ the grain homogeneity is higher due to decrease in size and quantity of the largest grains. In specimens made of 05ХГБ and 06ГНФБ steel grades, the impact toughness of simulated coarse-grained area of the heat affected zone stabilizes at high level due to formation of more favorable dispersed structure when cooling rate increases more than 8 °C/s.

A simulated coarse-grained heat affected zone microstructure formation mechanism is established in high strength low alloy steels using electron backscatter diffraction (EBSD). The governing effect of dispersion and ratios between different types of ferritic structural constituents on variation in impact strength is demonstrated. It is assumed from results of simulating a heat affected zone coarse-grained area that a reduction in welding energy input leads to a shift in ductile-brittle transition temperature towards a lower temperature.

In this paper, we study the morphology of the bainite component of the microstructure of rolled products and the mimic coarse-grained region of the thermal effect zone of K60–K65, 09G2S, and 08KhN2MFB low-carbon steels by methods of optical microscopy and electron backscatter diffraction. In the homogeneous microstructure of the former austenite—both in the coarse-grained heat-affected region and in rolled products—the density of high-angle grain boundaries is shown to be higher in lath bainite than that in granular bainite. In the case of a substantially inhomogeneous microstructure of the former austenite, the size effect of the initial grain structure on the effective grain size (high-angle boundaries) prevails over the change in the bainite morphology. The lower-temperature component, lath bainite, turns out to be coarser-grained (in the case of its formation from large-sized austenite grains) than granular bainite. The impact viscosity and cold resistance of a metal, the microstructure basis of which is a mixture of granular and lath bainite, increased at an increase in the density of high-angle boundaries, which in turn is determined by both the grain size of the initial austenite and the morphology of bainite.

The influence of finish rolling and accelerated cooling stages on structure and property formation for low-manganese steel under laboratory conditions is studied. Rolling ends at temperatures in both the γ-region and at various temperatures in the γ + α-region. Accelerated cooling completion temperature is varied in the range of 630–350°C. Comprehensive analysis of the microstructure, crystallographic texture, and fine structure parameters of laboratory rolled specimen is performed. Specific features of structure and property formation are studied and compared with known pipe steels. The range of rolling parameters providing an optimum combination of strength, toughness, and cold resistance is determined.

The problem of achieving a higher level of ductility of pipe steels of strength grades K60 (X70) and K65 (X80) is described. Results of laboratory and industrial experiments are shown, and requirements for pipe microstructure with improved deformation capacity are formulated. Technology is described providing the specified type of structure and steel target mechanical properties, and different production schemes for LTCC are applied making it possible to achieve the prescribed strength and ductility properties over a wide variation range. Results are provided for mass industrial production of rolled product and pipe with high deformation capacity.

We describe a study of fractures and metal structure in the fusion zone (FZ) and weld center (WC) for longitudinal welds and base metal (BM) in large-diameter pipe with strength class K60 (Grade X70) tested for crack resistance including a determination of the crack tip opening displacement (CTOD) on through-cut SENB samples (N-P per BS EN ISO 15653). Tests indicated that CTOD–20 (at temperature–20 °C) is generally more stable for BM and WC metal than for FZ metal. The effect of crack front displacement from the specified location on measured CTOD–20 values was studied for FZ samples. The CTOD–20 values for the FZ metal were found to depend on the failure mechanism caused by initiation of the fatigue crack, including maximum fatigue-crack length and maximum ductile-failure segment length. The structure of the FZ metal along the fatigue crack front was studied at various crack resistance levels using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The failure mechanism and resultant CTOD values for FZs in double-sided welds depend not only on the structural characteristics of the metal in the heat-affected zone (HAZ) (ratio of lath-like bainite to granular bainite, the martensitic-austenitic (MA) component and cementite morphology), but also on whether or not a fatigue crack is present in the section of pipe base metal where the deformation is largest. This differentiates the CTOD samples from the KCV samples. The fact that the BM area is located at the crack front has a substantial effect on the fracture mechanism in a FZ CTOD.