Influence of heat input on microstructure and fracture toughness property in different zones of X80 pipeline steel weldments

In this paper, microstructure observations and mechanical behaviour of fusion line and offsetting positions from fusion line by 1, 2 and 3 mm were analysed. For the welding of X80 pipeline steel plates, different magnitudes of heat inputs such as high heat input (HHI) 25 kJ/cm, medium heat input (MHI) 20 kJ/cm and low heat input (LHI) 15 kJ/cm were employed. Critical values of J‐integral (J_0.2) and crack tip opening displacement (CTOD_0.2) for predetermined regions in the X80 weldment were determined as per ASTM‐E1820a. M‐A constituents of different sizes such as small (1–2 μm), large >2 μm and slender (>4 μm) were observed in the microstructure of subzones of weldments for different heat inputs. Formation of granular bainite, M‐A constituents and inclusions of Ti, Si, Mo in the microstructure impaired fracture toughness property. In the X80 weldment, the fusion line (FL) for HHI was found weakest in terms of fracture resistance, which subsequently increases the risk of fracture.     

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb-V microalloyed steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb-V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.     

抗大变形管线钢热影响区软化问题的研究

为了研究抗大变形管线钢热影响区出现软化区的具体原因,本文采用焊接热模拟实验研究了X80级抗大变形管线钢的焊接热循环过程,结合金相显微镜、扫描电镜、EBSD、透射电镜和冲击实验,分析了热影响区软化区的组织变化、晶体学特征和冲击韧性.结果表明:当母材组织为多边形铁素体+贝氏体时,焊接热影响区的软化区出现在峰值温度600~700℃的高温回火区,此时组织转变成硬度较低的粗大铁素体+回火贝氏体,并且回复过程加快,组织中亚结构的大幅度减少和位错密度的显著降低是产生软化区的主要原因;软化区的韧性较好,但是在800℃的临界区,M/A组元发生了聚集和粗化,并且大角度晶界比例降低,导致了韧性低谷的出现.     

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.     

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.     

Effect of prior austenite grain size and pearlite interlamellar spacing on strength and fracture toughness of a eutectoid rail steel

Abstract

The influence of prior austenite grain size d_γ, and true interlamellar spacing of pearlite S_t on the strength and fracture toughness of a eutectoid rail steel has been investigated. Specimens were machined from rail sections and heat treated to produce a wide variation in d_γ and s_t. Mechanical properties studied included 0·2% proof stress σ_0·2, ultimate tensile strength σ_u, tensile ductility δ, cleavage fracture stress σ_f, and plane strain fracture toughness K_1c. All tests were performed at a temperature of ?80°C. The values of σ_0·2 and σ_u increase as s_t decreases. The proof stress is related to the mean free distance λin the pearlitic ferrite by a Hall–Petch equation. A microstructural dependence similar to that of σ_0·2 is shown by σ_f and for all but the finest pearlites σ_f is interpreted as a shear stress controlled cleavage nucleation stress. The value of K_1c first decreases with decreasing s_t and then increases for the finest spacings. This behaviour is attributed to a change in the micromechanism of cleavage nucleation as the pearlite spacing changes from coarse to fine. The value of d_y has very little effect on K_1c, but δ decreases progressively as d_γ increases. The effect of d_γ on K_1c is negligible because the fracture process zone is much smaller than the grain size and therefore the grain boundaries cannot influence the fracture processes occurring at the crack tip. The tensile ductility is interpreted as the strain necessary to develop an internal microcrack which then propagates as a quasibrittle fracture. The size of the microcrack is shown to be related to the pearlite nodule size which in turn is related to d_γ.

MST/396     

Effect of microstructure on the relationship between fracture toughness and ductility

The relationship between fracture toughness, expressed as J_IC, and ductility, given by measurements of the bulge ductility, was studied experimentally for two microstructures of 1045 steel. These microstructures consist of annealed pearlitic structure with hardness R_c = 15 and a tempered matensitic structure with R_c = 23. J_IC and bulge ductility measurements were performed in the temperature range of −100-25°C. The tensile properties at these temperatures were also obtained. The results show that the variation in flow stress with temperature is similar for both microstructures. However, the flow stress is higher, and the bulge ductility is lower, at a given temperature, for the martensitic structure. Also shifts in the transition temperature from linear elastic to elastic-plastic behavior are observed. Previously developed models by the authors describing the variation of J^IC with temperature and ductility are used to account for the behavior of the different microstructures examined.     

Effect of austenite stability on toughness,ductility, and work-hardening of medium-Mn steel

ABSTRACT

We elucidate here the stability of reversed austenite (RA) and its effect on mechanical properties in 0.05C–5.0Mn steel. With increased annealing temperature from 903 to 943?K, the volume fraction of RA was increased from ～19 to ～42%, while its stability decreased linearly because of reduced C and Mn enrichment. The tensile strength increased from 845 to 970?MPa, impact toughness at 233?K decreased from 135 to 98?J. The large volume fraction of RA with poor stability can significantly improve the work-hardening ability, but little contribution to impact toughness. The discontinuous transformation-induced plasticity effect is resulted from RA with different degree of stability because of difference in grain size and morphology.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

X120管线钢的相变组织及性能研究

为适应将来石油及天然气高效输送的需要,以及高级别管线对力学性能及焊接性能的要求,采用一种成分为0.05%C-2.01%Mn-0.30%Mo-0.48%Ni-0.35%Cu-0.076%Nb-Ti-B的试验材料,用于开发X120管线钢的试验研究.在Gleeble 1500热模拟机上,采用在奥氏体再结晶区和未再结晶区分别变...     

Effect of strain rate on fracture toughness of mild steel

Abstract

The effect of strain rate on the yield strength and fracture toughness of mild steel was studied in the strain rate range 10^-5–10² s^-1. The lower yield strength was found to be independent of strain rate in the range 10^-5–10^-3 s^-1. Beyond this strain rate, the lower yield strength was found to vary linearly with logarithm of strain rate up to a value of 10² s¹. Fracture toughness was also found to increase with strain rate up to a value of 10^-3 s^-1 and subsequently decrease gradually with increasing strain rate. However, a drastic reduction in fracture toughness was observed at a strain rate of 10² s^-1. This reduction was attributed to the change in fracture mode from ductile to cleavage.     

Influence of specimen size and microstructure on the fracture toughness of a martensitic stainless steel

The fracture toughness of alloy HT-9,² a martensitic stainless steel under consideration for fast reactor and fusion reactor applications, was determined from circular compact tension specimens using the multi-specimen R-curve method. Specimens with thicknesses of 11.94, 7.62 and 2.54mm and widths of 23.88 and 11.94 mm were tested to investigate the effects of specimen size on fracture toughness. The test results obtained from all specimens are in good agreement and thickness requirements for a valid J_1c test are satisfied. The experiment indicates that small specimens of HT-9 may be used for post-irradiation fracture toughness testing.Fractographic examination of the fracture surfaces reveals that fracture in HT-9 is significantly influenced by delta ferrite stringers present in the material. The fracture surface examination and crack opening displacement measurements for specimens tested at various temperatures are consistent with the temperature dependence of the J_1c results.     

Effect of bainitic transformation on microstructure of Si-Mn steel

Several Si-Mn steels with similar Si and Mn levels and carbon contents, ranging from 0.25 to 0.75 wt %, were studied to determine the effect of bainitic transformation on the microstructure of Si-Mn steel. The microstructure was categorized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction. The results showed the existence of an optimum transformation time to produce the maximum content of retained austenite, though the retention of a large amount of retained austenite was encouraged as a result of bainitic transformation. The microstructure consisted of carbon-free upper bainite whose individual ferrite was separated by the thin-film type of retained austenite, while the blocky type of austenite was also found. The results also showed that carbide precipitation occurred in the residual austenite after the optimum time, which decreased the retained austenite content. The retained austenite stability is discussed in relation to the carbon content and morphology of the retained austenite.     

Effect of initial notch orientation on fracture toughness in fail-safe steel

A 0.4C–2Si–1Cr–1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.     

Si对团球状共晶体奥氏体—贝氏体钢中贝氏体相变的影响

在热力学，动力学的基础上，从贝氏体铁素体在奥氏体贫碳区切变机制出发，研究了Ｓｉ对团球状共晶体奥－贝钢中贝氏体相变的影响，并提出Ｓｉ促使奥氏体奥碳区的形成，有利于贝氏体切变的新观点。     

奥氏体变形对22CrSH齿轮钢连续冷却相变的影响

为了研究奥氏体变形对22CrSH齿轮钢连续冷却相变的影响,在Gleeble 1500热模拟机上,将22CrSH钢在950℃变形0.4及未变形处理,然后连续冷却.利用膨胀曲线、光学显微镜、透射显微镜,结合各种腐蚀方法,分析了22CrSH钢相变行为及相变组织.研究表明:奥氏体变形使多边形铁素体加珠光体混合组织的临界冷速增大;当奥氏体变形及降低冷速时,大量的晶界仿晶型铁素体占据了奥氏体晶界,贝氏体相变向针状铁素体相变转变;变形使奥氏体在中温相变区稳定性增加,室温组织中M/A岛的数量增多.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.