管线钢在役焊接热影响区显微组织的热模拟研究

为探讨在役焊接这种严酷的焊接条件下管线钢焊接热影响区显微组织的变化,采用焊接热模拟技术、金相分析及透射电镜对比研究了X70管线钢在役焊接热影响区和常规焊接热影响区的金相组织和精细结构.结果表明,在役焊接的快速冷却只对粗晶区的金相组织产生了较大影响,而对过渡区、细晶区和类母材区的金相组织几乎没有影响.金相显微镜下两者粗晶区的组织均为贝氏体铁素体和粒状贝氏体,但各组织的形态和数量不同.在透射电镜下观察,两者粗晶区的精细结构有较大差异,在役焊接粗晶区生成了少量细小的横穿贝氏体铁素体板条的板条马氏体,常规焊接粗晶区生成了少量的块状铁素体组织.     

Influence of peak temperature during in-service welding of API X70 pipeline steels on microstructure and fracture energy of the reheated coarse grain heat-affected zones

In this investigation, thermal simulated specimens were used to investigate the effect of second peak temperature during in-service welding on characteristic fracture energy and microstructure feature of the subcritically (SC), intercritically (IC), supercritically (SCR), and unaltered (UA) reheated coarse grain heat-affected zones (CGHAZs). The API X70 high-strength pipeline micro-alloyed steel was subjected to processing during in-service welding by applying double thermal cycle shielded metal arc welding process with heat input of 9.3 kJ/cm and thermal cycles to simulate microstructure of reheated CGHAZs. This consisted of first thermal cycle with a peak temperature of 1350 °C, then reheating to different second peak temperatures of 600, 800, 1000, and 1200 °C with a constant cooling rate of 60 °C/s. Toughness of the simulated reheated CGHAZs were assessed using Charpy impact testing at −20 °C, and the corresponding fractographs, optical micrographs, and electron micrographs have been examined. It is found that accelerating cooling rate during in-service welding has an improving effect on the microstructure of CGHAZs. Owing to small heat-input and accelerating cooling, the grain size in reheated CGHAZs is relatively small and the brittle microphases are eliminated or minimized. The Charpy impact results show that the CGHAZ fracture energy is improved after the second thermal cycle. The SC CGHAZ showed higher absorbed impact energy and the IR CGHAZ had less absorbed energy, but the phenomenon of embrittlement in IR CGHAZ is not serious. Therefore, it can be concluded that the fracture energy of CGHAZ and IR CGHAZ can be improved by accelerating cooling with appropriate cooling rate.     

Microstructure and grain boundary microanalysis of X70 pipeline steel

Grain boundaries (GBs), particularly ferrite: ferrite GBs, of X70 pipeline steel were characterized using analytical electron microscopy (AEM) in order to understand its intergranular stress corrosion cracking (IGSCC) mechanism(s). The microstructure consisted of ferrite (), carbides at ferrite GBs, some pearlite and some small precipitates inside the ferrite grains. The precipitates containing Ti, Nb, V and N were identified as complex carbo-nitrides and designated as (Ti, Nb, V)(C, N). The GB carbides occurred (1) as carbides along ferrite GBs, (2) at triple points, and (3) at triple points and extending along the three ferrite GBs. The GB carbides were Mn rich, were sometimes also Si rich, contained no micro-alloying elements (Ti, Nb, V) and also contained no N. It was not possible to measure the GB carbon concentration due to surface hydrocarbon contamination despite plasma cleaning and glove bag transfer from the plasma cleaner to the electron microscope. Furthermore, there may not be enough X-ray signal from the small amount of carbon at the GBs to enable measurement using AEM. However, the microstructure does indicate that carbon does segregate to : GBs during microstructure development. This is particularly significant in relation to the strong evidence in the literature linking the segregation of carbon at GBs to IGSCC. It was possible to measure all other elements of interest. There was no segregation at : GBs, in particular no S, P and N, and also no segregation of the micro-alloying elements, Ti, Nb and V.     

Microstructure model for the heat-affected zone of X80 linepipe steel

An important aspect of the integrity of oil and gas pipelines is the heat-affected zone (HAZ) of girth welds where the microstructure of the as-hot rolled steel is altered with potentially adverse effects on the HAZ properties. Therefore, it is critical to evaluate the HAZ microstructure for different welding scenarios. Here, an integrated microstructure evolution model is proposed and applied to the HAZ of an X80 linepipe steel. The model considers dissolution of Nb-rich precipitates, austenite grain growth and austenite decomposition into ferrite and bainite. Microstructure maps showing the fraction of transformation products as a function of distance from the fusion line are obtained and used to compare the effect of different welding procedures on the HAZ microstructure.     

Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

The dynamic fracture toughness K_1d and J_1d, arrest toughness K_1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from to . At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been derived:

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. It can describe the fracture process at different temperatures and loading rates. At a loading rate of , the relationship can predict arrest toughness well. It provides the possibility of measuring arrest toughness with small size specimen. An empirical equation has been derived: CVN=4.84×10⁶T^−2.8K_1d(K_1a), which correlates K_1d and K_1a with CVN in one equation. This means that we can calculate K_1d and K_1a when we get CVN.     

Effects of nitrogen content and weld cooling time on the simulated heat-affected zone toughness in a Ti-containing steel

An increase of nitrogen content in a 0.02 wt% Ti-containing carbon-manganese steel resulted in a low coarsening rate of TiN particles in the heat-affected zone (HAZ), which led to an accelerated ferrite transformation instead of ferrite side plates during weld cooling cycle. The mixed microstructure of ferrite side plate, acicular ferrite and grain boundary polygonal ferrite in the simulated HAZ produced higher toughness. However, the increase of nitrogen content gradually increased the free nitrogen content in the HAZ and deteriorated HAZ toughness. Impact energy of the simulated HAZ (with Δt_8/5 ∼60 s) at –20 °C deteriorated by about 97 J per 0.001 wt% free nitrogen, in the free nitrogen range from 0.0009 wt% to 0.0034 wt%, even though the HAZ has the tough mixed microstructure. Cooling time after welding influenced the HAZ microstructure and toughness as well, and maximum toughness was obtained when cooling produced the tough mixed microstructure. Therefore, for a high HAZ toughness, both nitrogen content and cooling time should be controlled to obtain the tough mixed microstructure and to keep the free nitrogen content low. The optimal nitrogen content and cooling time from 800 °C to 500 °C were 0.006 wt% and between 60 s and 100 s, respectively, in this experiment.     

Development of TiOx-bearing steels with superior heat-affected zone toughness

TiO_x steels with superior toughness of the heat-affected zone (HAZ) and applicable to a wide range of welding heat inputs have been developed using advance metallurgical techniques. A promising practice in industrial production is adopting a Ti-killing process in which ferro-titanium alloys are utilized as killing agents in an Al-free molten steel. This process creates Ti-oxides with various crystalline structures (referred to as TiO_x) in a steel matrix. These TiO_xinclusions improve the toughness of coarse-grained HAZ by promoting the formation of intragranular acicular ferrite (IAF), which can section an austenite grain into several colonies and refine the effective grain size to fracture resistance. The volume fraction of IAF is closely related to the number of TiO_x inclusions, inclusion diameter, austenite grain size and the hardenability of steels. The favourable conditions for the formation of IAF and the concept of process control are discussed.     

Microstructure of X52 and X65 pipeline steels

The microstructure of two commercial pipeline steels X52 and X65 was examined to provide a foundation for the understanding of the IGSCC mechanism of pipeline steels. Observation of the microstructure was carried out using scanning electron microscopy (SEM) and an analytical electron microscope. The microstructure of X52 and X65 pipeline steels shows banding of pearlite rich and ferrite rich areas. The ferrite grains were about 10 μm in size with curved grain boundaries. There was carbide at the ferrite grain boundaries for X52 steel, and there was circumstantial evidence to suggest carbon segregation at the boundaries. The pearlite colonies were consistent with nucleation by a number of different mechanisms. This revised version was published online in September 2006 with corrections to the Cover Date.     

Estimation of the heat-affected zone for the electron-beam welding of plates

We describe a procedure and present some experimental results of evaluation of the heat-affected zone (HAZ) for butt electron-beam welding of the plates of variable thickness with regard for the nonuniform distribution of energy. It is shown that the heat removal caused by the increase in the thicknesses of the plates leads to changes in the sizes of the HAZ in the upper part of the welded joint. It is established that the instantaneous cooling rate changes with the distance from the reference point as a result of changes in the rate of energy input, which is responsible for the inhomogeneity of the microstructure over the thickness of the joint.     

Effect of simulated welding thermal cycle on microstructure and mechanical properties of X90 pipeline steel

The effect of welding thermal cycle simulation on the microstructure and mechanical properties of X90 pipeline steel was investigated by means of microstructure analysis, tensile- and Charpy impact-tests. At the heat input of 15 kJ/cm, the microstructure of coarse-grained heat affected zone is mainly composed of lath bainite and granular bainite, resulting in excellent strength and toughness. At 25 kJ/cm with two thermal cycles, however, strength and impact toughness decrease due to the formation of more polygonal ferrite with coarser grains.     

Room temperature creep and its effect on flow stress in a X70 pipeline steel

This paper studied the phenomenon of room creep deformation and its effect on tensile property of a X70 pipeline steel under stress-control loading pattern using round tensile test specimen. Significant time-dependent deformation under constant load was observed in the steel at room temperature, and the deformation is not only dependent on loading stress rate but also dependent on the loading process. It is also found that the loading-unloading-reloading process reduces the subsequent creep strain, while the occurrence of room temperature creep obviously enhances the subsequent yielding strength and the flow stresses.     

On the relation of microstructure and impact toughness characteristics of DSAW steel of grade API X70

In this research, mechanical characteristics of double submerged arc-welded line pipe steel of grade API X70 (70 ksi yield strength) were investigated. Different experimental examinations including chemical analysis, microstructural investigation, microhardness, tensile and Charpy tests and standard fractography were carried out on different zones of test material. Impact properties of the fusion zone, the heat-affected zone (HAZ) and the base metal were measured on an instrumented Charpy rig and correlated then to their microstructural features. The experimental results showed that the base metal was the toughest, and fusion zone was the least tough region with average Charpy energy of 222 and 128 J, respectively. The latter fulfilled service requirements set by API 5L, despite its relatively low impact energy. Interestingly, the cast microstructure and presence of grain boundary phases such as proeutectoid ferrite in fusion zone confirmed its low energy characteristics. Reduction in HAZ impact energy (compared to base metal) was observed too, together with grain coarsening and the associated HAZ softening adjacent to the weld.     

Analysis of corrosion of hot-rolled X70 steel plate during storage

In this work, analysis was performed to investigate the reason resulting in corrosion, especially pitting corrosion, of a stock of hot-rolled X70 steel plates after 4 months of storage in a warehouse. Results demonstrated that corrosion and pitting corrosion of X70 steel plate during storage is attributed to the remnant and concentration of cooling water on the top surface of the plate. Pitting corrosion occurs locally at defects on surface deposit that is generated during corrosion of the steel. The small anode vs. big cathode geometry enhances the pitting corrosion process. Chloride ions enter the pits to maintain the charge-neutrality, while ferrous ions that are generated during corrosion diffuse out of the pits to result in corrosion product layer at pit mouth.     

Precipitate alterations in the heat-affected zone of a Grade 100 microalloyed steel

Microalloy precipitate alterations (particularly dissolution) in the heat-affected zone (HAZ) of a Grade 100 steel, microalloyed by titanium, niobium, and vanadium and produced in the form of a plate with a thickness of 8 mm, was examined both theoretically and experimentally. For theoretical analysis of precipitate dissolution, pairs of effective peak temperature and holding time were extracted from the thermal cycles of welding, and were superimposed on the Ashby and Easterling non-equilibrium solubility curves for different fractions of precipitate dissolution. Intersections between the effective T–t curves and the non-equilibrium solubility curves gave critical pairs of effective peak temperature and holding time for dissolution of different fractions of a precipitate, which resulted in the establishment of precipitate dissolution profiles in the HAZ. Experimental analysis of precipitate alterations was carried out using carbon extraction replicas in a transmission electron microscope. The theoretical analyses were in agreement with experimental results, showing that it is the dissolution of small Nb-rich particles that paves the way to grain growth in the coarse-grained HAZ. Reprecipitation was generally suppressed in the low heat-input weld sample. There was some reprecipitation in the higher heat-input weld samples. Coarsening of TiN did not occur in the HAZ, due to the large size of these particles in the steel examined.     

Microstructural degeneration of simulated heat-affected zone in 2.25Cr–1Mo steel during high-temperature exposure

The microstructural of simulated heat-affected zone (HAZ) in 2.25Cr–1Mo steels has been investigated. The experiments with different heat inputs were carried out in a high speed dilatometer; the thermal cycles used corresponded to the actual thermal cycles that occurred in the coarse-grained region of the real HAZ. It was found that with a heat input of 20 kJ cm⁻¹, the simulated HAZ microstructure gave larger amounts of lower bainite with significant amounts of martensite. The 20 kJ cm⁻¹ heat input specimens were tempered at 700 °C for different time intervals ranging from 1 to 50 h. A sequence for corresponding microstructural degradation has been proposed.     

Microstructural evolution of simulated heat-affected zone in modified 2.25Cr-1Mo steel during high temperature exposure

The effect of heat input with 20, 50 and 80 kJ/cm on the microstructural evolution of the simulated heat affected zone (HAZ) has been studied in a modified 2.25Cr-1Mo steel. The microstructures of simulated coarse-grained HAZ has been examined by optical metallography and transmission electron microscopy. It was found that large amounts of martensite with small quantities of bainite exist in the specimen with 20 kJ/cm. However, significant amount of bainite with a few amounts of allotriomorphic ferrite can be detected in the specimen with 50 kJ/cm heat input. In the case of heat input with 80 kJ/cm, the simulated HAZ microstructures were composed chiefly of bainite with a few amounts of martensite and allotriomorphic ferrite. In order to study the sequences of carbide transformation, the HAZ specimens were tempered at 700°C for different intervals (1, 2, 5, 10, 20, 50, 100, 200 and 500 h). The sequence of carbide transformation in the HAZ zone of this modified 2.25Cr-1Mo steel has been proposed.     

Towards a prediction of the hardness of the heat-affected zone of steel weldments

The prediction of the level of hardness developed in the heat-affected zone (HAZ) of steel welds is discussed. It is composed of a thermal model that predicts the cooling behaviour from input welding parameters and a material model for calculating the HAZ hardness as a result of weld cooling. Experimental investigations were carried out on eight different steel welds using three different electrodes under two different welding processes. Comparisons of the experimental results as well as the experimental results reported in the open literature, against the calculated values for both HAZ hardness and cooling time, were conducted. The results presented in this paper show good agreement between calculated and measured values of both cooling rate and hardness. The calculations can be carried out readily in small pocketsized computers. On Leave from Mechanical Engineering Department, Assiut University, Assiut, Egypt.