Brittle crack-arrest fracture toughness in a high heat-input thick steel weld

Brittle crack-arrest fracture toughness (K_Ia) was determined as a function of the temperature in a large-scale (1 x 1 m) 50-mm thick steel base metal and its high heat-input (32 kJ/mm) weld. An impact initiates crack propagation from a notch at low temperature toward a higher temperature region where the crack stopped due to the improved fracture toughness. The relationship between the toughness and crack-arrest temperature provides the K_Ia at ?10 °C of about 100 MPa√m in the weld specimen, which is a significant decrease compared to that of the base metal (240 MPa√m). Furthermore, the path of the crack propagation was discussed in terms of the grain size, hardness, and Charpy impact energy of the localized crack region.     

Failure of weld joints between carbon steel pipe and 304 stainless steel elbows

A number of weld joints between carbon steel (CS) pipe and type 304 stainless steel (SS) elbows constituting a gas piping system of a petrochemical unit developed cracks after a relatively short period of usage, resulting in leakage. The gas flowing through the pipe, was hydrogen rich at a temperature of 45 °C and a pressure of 16 kg/cm². Light optical metallography and scanning electron microscopy, combined with energy dispersive X-ray spectroscopy, inductively coupled plasma and microhardness testing were used to determine the most probable cause of failure. Analysis showed that the cracks originated at the interface between the CS pipe and the SS root weld. A narrow band between the CS pipe and SS weld exhibited a hardness of Rockwell C 60 suggesting the formation of martensite due to C segregation at welding temperature and subsequent quenching during cooling. The ferritic region of CS adjacent to the weld was decarburized and was devoid of pearlite; corroborating C diffusion. The weld region was diluted comprising mainly of Fe with small amounts of Cr and Ni. Cracking is thought to have initiated at the hardened region. However, the failure might have been aided by hydrogen rich medium and soft C-depleted ferrite region.     

Creep resistance of similar and dissimilar weld joints of P91 steel

Abstract

Two experimental weld joints, a similar weld joint of 9Cr–1Mo steel and a dissimilar weld joint of 9Cr–1Mo and 2.25Cr–1Mo steels, were fabricated by the TIG+E method and post-weld heating was applied. Creep testing was carried out at temperatures ranging from 525 to 625°C in the stress range 40–240 MPa. Creep rupture strength was evaluated using the Larson–Miller parameter. Extended metallography including transmission electron microscopy was performed and critical zones were indicated where fractures were concentrated during the creep exposure. At high temperatures rupture of the dissimilar weldment occurred in the heat affected zone (HAZ) of the weld metal while rupture of the similar weldment was located in the HAZ of the parent material. The processes of recovery seem to be the main causes of decrease in creep rupture strength of both weld joints in comparison to the parent materials.     

高温回火热处理对9Cr-1Mo钢焊缝组织韧性的影响

以ESAB公司的三种9Cr-1Mo焊条的焊缝金属为研究对象,以Formastor-D全自动热膨胀记录仪分析了焊缝金属的连续冷却曲线,用MTS试验机分析了焊缝组织的断裂韧性.研究结果表明,Cr、Ni、C和Mn等元素导致OK1和OK3的Ms点应比OK2的低,奥氏体稳定性升高.710℃高温回火处理后,OK2焊缝组织的韧性较高.回火组织中碳化物沿晶界或相界析出使裂纹扩展更加容易从而使9Cr-1Mo焊缝组织的韧性降低.     

无钴马氏体时效钢电子束焊接缺陷的微观研究

为研究18Ni无钴马氏体时效钢电子束焊接缺陷的形成原因,对比分析了同一成分不同批次的两种无钴马氏体时效钢的基体及焊接接头,采用金相显微镜和扫描电镜观察了微观组织结构.研究表明:两种材料的晶粒大小差别较大,但基体组织均为细小板条马氏体组织,晶界分布有片状的奥氏体组织,晶体内部也有少量的奥氏体颗粒;焊缝组织为粗大柱状晶,熔合线附近热影响区组织发生了再结晶,晶粒已明显合并长大.晶粒大小和奥氏体含量的不同,是导致两试样导热性能不同的主要原因,对于导热性能差的材料,焊接时应尽量降低能量输入,减少熔池金属量,以防止焊接缺陷的出现.     

Effects of weld strength undermatch on fracture toughness of HAZ notched weldments in a HSLA steel

In the present work the effects of weld strength undermatch on fracture toughness of heat affected zone (HAZ) have been studied. In the investigation a high strength low alloyed steel (HSLA) with 800 MPa strength class was used, and the undermatched welded joints were made with two weld strength mismatch levels. Three-point bending test specimens with crack depth to specimen width ratio a/W ranging from 0.05 to 0.5 were extracted from the welded joints. The test results show that strength mismatching gives an obvious influence on the fracture toughness of coarse grained HAZ for the undermatched joints. The lower the weld strength mismatching, the higher the fracture toughness of the HAZ. In addition the tendency of fracture toughness change with crack depths is much the same as in previous studies on base metals or weld metals, that is, fracture toughness of the HAZ is increased with reduction of crack depths. From the measured results it shows that the macroscopically mechanical heterogeneity of the welds may have more important influence on the fracture toughness of the HAZ than the meso-heterogeneity in the reheated coarse grained HAZ. Furthermore, numerical verification indicates that the stress triaxiality at crack tip may be the essential reason for the change of fracture toughness of HAZ. It is also shown that the yield strength of HAZ determined by the limit load in the three-point bend test represents the combinative effects of HAZ and its surrounding materials. This revised version was published online in August 2006 with corrections to the Cover Date.     

Creep of degraded 2.25Cr-Mo steel in hydrogen

The results of creep tests of 2.25 Cr-Mo steel are used to show that the secondary creep rate in a gaseous hydrogen is higher than in air both for the metal in the intact state and after its operation in the technological processes of hydrocracking of petroleum. This means, first, that the properties of the metal guaranteeing its serviceability in the initial period of its service are exhausted in the process of operation and, second, that hydrogen affects both the degradation and creep processes. Indeed, hydrogen intensifies the diffusion processes and, hence, accelerates the degradation of the metal and facilitates creep. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 91–96, September–October, 2007.     

Fracture toughness correlation with microstructure and other mechanical properties in near-eutectoid steel

The variation of yield strength and fracture toughness was investigated for four different heat treatments attempted on specimens of a near-eutectoid steel. The aim of this study was to optimize the microstructure for simultaneous improvements in strength and toughness. Further, the fracture toughness deduced through empirical relations from tensile and charpy impact tests was compared with those measured directly according to ASTM Designation: E 399. Among the four different heat treatments attempted in this study, the plane strain condition was valid in the fracture toughness tests for (i) normalized and (ii) hardened and tempered (500°C for 1 h) treatments only. The latter of the two heat treatments resulted in simultaneous improvement of strength and plane strain fracture toughness. The finely-dispersed carbides seem to arrest the crack propagation and also increase the strength. The pearlitic microstructure of the former leads to easy crack propagation along cementite platelets and/or cementite/ferrite interfaces. The nature of variation of empirically determined toughness values from tensile tests for different heat treatments is similar to that measured directly through fracture toughness tests, although the two sets of values do not match quantitatively. On the other hand, the toughness data deduced from charpy impact test is in close agreement with that evaluated directly from fracture toughness tests.     

Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750

This paper investigates the weldability, metallurgical and mechanical properties of the UNS 32750 super-duplex stainless steels joints by Gas Tungsten Arc Welding (GTAW) employing ER2553 and ERNiCrMo-4 filler metals. Impact and tensile studies envisaged that the weldments employing ER2553 exhibited superior mechanical properties compared to ERNiCrMo-4 weldments. Microstructure studies performed using optical and SEM analysis clearly exhibited the different forms of austenite including widmanstatten austenite on the weld zone employing ER2553 filler. Also the presented results clearly reported the effect of filler metals on strength and toughness during the multi-pass welding. This research article addressed the improvement of tensile and impact strength using appropriate filler wire without obtaining any deleterious phases.     

Influence of precipitation on dislocation substructure and creep properties of P91 steel weld joints

Abstract

Two trial weld joints were prepared using the GTAW and SMAW methods and they underwent creep testing at temperatures between 525 and 625°C. The longest time to rupture was 45,811 h. Two main processes occurred during creep exposures: recovery and precipitation of secondary phases. Slight coarsenings of the M₂₃C₆ carbide, precipitation of Laves phase and Z-phase were observed after long tests at high temperatures. Some differences in microstructure and creep failure were found in the individual zones of weldments. After long exposure at temperatures up to 600°C, fractures occurred in the fine-grain heat-affected zone as a result of a low density of fine vanadium nitride and a high density of coarse particles at grain and subgrain boundaries. At 625°C, growth of Laves phase caused a softening of the ferritic matrix and crack propagation in the weld metal.     

Corrosion cracking and intergranular corrosion of 12Kh18N10T steel weld joints

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 5, pp. 51–56, September–October, 1990.     

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.     

考虑变量相关性的正交异性板细节疲劳可靠性评估

传统的疲劳可靠性评估方法忽略了功能函数中变量的相关性,致使计算得到疲劳可靠度指标精度降低。针对这一问题,基于既有悬索桥主梁细节应变和环境温度大样本监测数据,讨论疲劳功能函数变量之间的相关性关系。提出基于Copula函数的相关性变量的联合分布模型,解决联合概率分布建模中多重积分求解困难的问题。研究表明,受环境温度的影响,靠近顶板焊接细节的日等效应力幅和应力循环次数存在较强的相关性,Gaussian Copula函数可作为相关性变量的最优连接函数,对于U肋-顶板细节,桥梁结构服役100年时,考虑与不考虑变量相关性的可靠度指标值分别5.3和6.9,两者之间相差约1.3倍。当年交通增长率分别等于3%和5%时,可靠度指标在服役年限为94.6年和67.1年时间到达目标可靠度指标。对于U肋-U肋对接细节,考虑和不考虑变量的相关性对可靠性指标影响相对顶板细节较小。     

Effect of weld heat input on toughness and structure of HAZ of a new super-high strength steel

Fracture morphology and fine structure in the heat-affected zone (HAZ) of HQ130 super-high strength steel are studied by means of SEM, TEM and electron diffraction technique. Test results indicated that the structure of HAZ of HQ130 steel was mainly lath martensite (ML), in which there were a lot of dislocations in the sub-structure inside ML lath, the dislocation density was about (0.3∼ 0.9) x 10¹²/cm². No obvious twin was observed in the HAZ under the condition of normal weld heat input. By controlling weld heat input (E ≤20 kJ/cm), the impact toughness in the HAZ can be assured.