The effect of the local hard zone (LHZ) distributed in the coarse grained HAZ (CGHAZ) has been analyzed by 2-dimensional FEM on a mechanical model of the weld CGHAZ. The existence of the LHZ elevates considerably the ...The effect of the local hard zone (LHZ) distributed in the coarse grained HAZ (CGHAZ) has been analyzed by 2-dimensional FEM on a mechanical model of the weld CGHAZ. The existence of the LHZ elevates considerably the stress in LHZ and causes the discontinuity of strain at the boundary between the LHZ and the matrix. The stress distribution in the LHZ is strongly affected by the shape of the LHZ. In a slender LHZ almost the whole region in the LHZ is exposed to elevated stress, whereas in the massive LHZ only the edge region sustains high stress. The longer the LHZ becomes, the more the highly stressed area, and the peak stress in the LHZ grows even under the same volume fraction of the LHZ. These results indicate that the slender LHZ brings about unstable fracture at a lower load level than the blocky LHZ. This tendency was confirmed by CTOD test results on the weld CGHAZ of a high-strength steel. The CGHAZ with elongated M-A constituents fractures at apparently lower critical CTOD than the CGHAZ with massive M-A constituents. In conclusion, the control of the shape of the M-A constituent has a striking effect on the toughness improvement of CGHAZ.展开更多
Models have been derived for assessment and computational analysis of the hardness of the heat affected zone (HAZ) in aluminum weldment. The general model;γ = 1.2714[(αβ/α + β)] was found to predict the HAZ hardn...Models have been derived for assessment and computational analysis of the hardness of the heat affected zone (HAZ) in aluminum weldment. The general model;γ = 1.2714[(αβ/α + β)] was found to predict the HAZ hardness of aluminum weldment cooled in water as a function of the HAZ hardness of both mild steel and cast iron welded and cooled under the same conditions. The maximum deviations of the model-predicted HAZ hardness values γ, α and β from the corresponding experimental values γexp, αexp and βexp were less than 0.02% respectively.展开更多
Shielded Metal Arc Welding (SMAW) in Ductile Irons (DI) is often required by foundries for practical manufacturing reasons. The mechanical properties of the welded structures are strongly dependent on their HAZ’s wid...Shielded Metal Arc Welding (SMAW) in Ductile Irons (DI) is often required by foundries for practical manufacturing reasons. The mechanical properties of the welded structures are strongly dependent on their HAZ’s width. A model based on the behaviour of the ferritic matrix of high-Si DIs in order to make an approach in measuring their HAZ’s width is developed in this study. A series of thermal treatments on 3.35 and 3.75 wt% Si as-cast DIs and spot SMAWs is applied on these materials. The applied SMAWs are done on non-preheated and preheated samples (150℃ - 300℃). For welding we modify the amperage (100 - 140A). The micro-hardness Vickers changes in the ferrite of the as-cast samples and inside the HAZ of the welded ones can be attributed to the existence of residual stresses (RS) in the ferritic matrix and assist in estimating the HAZ’s width.展开更多
The predictability of hardness of the heat affected zone (HAZ) in aluminum weldments cooled in palm oil, based on hardness of similarly cooled mild steel and cast iron weldments has been ascertained. The general mode...The predictability of hardness of the heat affected zone (HAZ) in aluminum weldments cooled in palm oil, based on hardness of similarly cooled mild steel and cast iron weldments has been ascertained. The general model: α = 1.2769? indicates that HAZ hardness of aluminium weldment is dependant on the ratio of product to sum of HAZ hardness of mild steel and cast iron weldments cooled in palm oil under the same conditions. The maximum deviations of the model-predicted HAZ hardness values α, μ and β from the corresponding experimental values αexp, μexp and βexp were less than 0.04% indicating the reliability and validity of the model.展开更多
Submerged arc welding (SAW) has been well utilised for the production of weld joints in 304 L ASS for various industrial application. However, effective performance of the material in service has been hampered by impr...Submerged arc welding (SAW) has been well utilised for the production of weld joints in 304 L ASS for various industrial application. However, effective performance of the material in service has been hampered by improper choice of electrode. Therefore, in this study, effects of different types of electrode on the microstructure and tensile property of type 304 L austenitic stainless steel heat-affected zone (HAZ) were studied. Chemical composition of the as-received sample was determined. A number of samples were cut from the as-received sample. Afterwards, two half were joined together with 308 L, 312 L and 316 electrodes at a controlled welding speed, current and voltage of 4.6 mm/s, 160 A and 30 V to produce a constant heat input of 626.09 J/mm. An automatic SAW machine with Model Type: DX3-301, and Frequency: 50 Hz was used. And based on ASTM standard, tensile and hardness samples were prepared from the as-received and HAZs. Tensile and hardness measurements were made. Also, specimens for microscopy studies were prepared from the HAZ and as-received samples. From the results, microstructures of the HAZs revealed varied volume fraction of austenite and ferrite phases and grain sizes, and at austenite and ferrite grain boundaries, chromium carbide formation and precipitation were observed. The weld joint produced with 308 L electrode revealed optimum UTS value and YS value of 475 and 325 respectively. While weld joint produced with 316 L electrode has superior ductility of value 41%. Irrespective of the types of electrode used, the as-received sample revealed superior tensile properties over the weld joints. Also, optimum hardness value of 45.7 HRA was obtained with 308 L. Hardness value of the as-received sample was higher than that of HAZ samples.展开更多
为解决敞开式TBM穿越硬岩破碎带时卡机风险超前判识及预处理技术难题,依托某TBM隧道工程开展系统性研究。通过累计16 km TBM掘进参数及护盾监测数据统计分析,揭示硬岩破碎带TBM卡机事故的力学机制及其典型参数响应特征。基于参数特征分...为解决敞开式TBM穿越硬岩破碎带时卡机风险超前判识及预处理技术难题,依托某TBM隧道工程开展系统性研究。通过累计16 km TBM掘进参数及护盾监测数据统计分析,揭示硬岩破碎带TBM卡机事故的力学机制及其典型参数响应特征。基于参数特征分析,遴选出TBM最大总推力、刀盘最大转矩、护盾最大压力及护盾最大位移变化量作为卡机风险评价关键指标,并采用机器学习算法构建卡机风险预测模型。在此基础上,提出基于多参数权重分析的卡机类型判别方法,实现不同类型卡机事故的准确识别,为现场采取针对性处理措施提供依据。采用离散元数值模拟方法分析不同支护措施对降低卡机风险的作用效果,结合工程实践验证,提出针对不同卡机类型的优化处理方案。研究结果表明:1)当存在卡机风险时,TBM总推力和转矩会持续维持在较高水平;护盾产生较大向内位移,或主司机主动顶升产生较大向外位移;同时护盾压力急剧上升并保持高位稳定。2)所构建的神经网络预测模型准确率为80.8%,随机森林预测模型准确率达到84.6%。3)采用密集型钢拱架支护方案可使围岩压力最大降低约10%,而实施超前支护措施可使围岩压力降幅超过50%。4)当预测可能卡护盾时,需启动刀盘和皮带机脱困模式,同时加强拱架支护;当可能出现护盾刀盘双卡时,必须采用超前支护,同时加强出护盾后的支护结构。展开更多
Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a s...Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a short welding time. Expulsion has a significant impact on the final yield strength of the weld, thus the detection and characterization of expulsion events is significant for the quality assurance of resistance spot welds. In this study, hardness mapping, using a scanning hardness machine, was used as a quality assurance technique for re- sistance spot welding. Hardness tests were conducted on a resistance spot welded sample to prepare a hardness map. The test results showed good correlation between the hardness map and metallographic cross sections. The technique also provided further fundamental understand- ing of the resistance spot welding process, especially regarding the occurrence of expulsion in the nugget.展开更多
Groundwater is a finite resource which is being overexploited due to increase in demand over the years leading to decrease in its potentiality.As it is known that ground water is the only major source of drinking wate...Groundwater is a finite resource which is being overexploited due to increase in demand over the years leading to decrease in its potentiality.As it is known that ground water is the only major source of drinking water for both urban and rural India its proper management is very essential for its sustainability. In the present study,DRASTIC model is展开更多
The microstructure, hardness, and precipitate free zones（PFZ） of V55Ti30Ni15 alloys during heat treatment have been investigated in this study. The microstructure resulting from different heat treatment conditions h...The microstructure, hardness, and precipitate free zones（PFZ） of V55Ti30Ni15 alloys during heat treatment have been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine Ni Ti particles precipitate from the supersaturated V-matrix solid solution at 750 °C, increase in quantity until 800 °C, and then dissolve back into the V-matrix at 850 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 850 °C. The microstructure containing small Ni Ti precipitates resulting from the treatment of 18 h at800 °C has a good soft condition for workability. PFZ formed at the grain boundary of V-matrix during heat treatment was observed. Vacancies depletion in V-matrix maybe led to the formation of PFZ.展开更多
Using the Gleeble 3500 thermal-mechanical system to simulate thermal cycles with different peak temperatures, the hardness and microstructure in the heat-affected zones of two kinds of 100 kg class hot-rolled extra-hi...Using the Gleeble 3500 thermal-mechanical system to simulate thermal cycles with different peak temperatures, the hardness and microstructure in the heat-affected zones of two kinds of 100 kg class hot-rolled extra-high-strength steel were compared. When the peak temperature of the thermal cycle was 800℃ ,incomplete transformation occurred during quenching in both steels, and massive martensite and bainite grains were formed. The hardness was determined by the composition and distribution of the microstructure. The concentration of massive martensite was low, and hence the hardness was low,in steel #1. Conversely,the massive martensite content in steel #2 was high and uniformly distributed,resulting in a high hardness. These findings can provide a reference for improving the mechanical properties in the softened zone.展开更多
文摘The effect of the local hard zone (LHZ) distributed in the coarse grained HAZ (CGHAZ) has been analyzed by 2-dimensional FEM on a mechanical model of the weld CGHAZ. The existence of the LHZ elevates considerably the stress in LHZ and causes the discontinuity of strain at the boundary between the LHZ and the matrix. The stress distribution in the LHZ is strongly affected by the shape of the LHZ. In a slender LHZ almost the whole region in the LHZ is exposed to elevated stress, whereas in the massive LHZ only the edge region sustains high stress. The longer the LHZ becomes, the more the highly stressed area, and the peak stress in the LHZ grows even under the same volume fraction of the LHZ. These results indicate that the slender LHZ brings about unstable fracture at a lower load level than the blocky LHZ. This tendency was confirmed by CTOD test results on the weld CGHAZ of a high-strength steel. The CGHAZ with elongated M-A constituents fractures at apparently lower critical CTOD than the CGHAZ with massive M-A constituents. In conclusion, the control of the shape of the M-A constituent has a striking effect on the toughness improvement of CGHAZ.
文摘Models have been derived for assessment and computational analysis of the hardness of the heat affected zone (HAZ) in aluminum weldment. The general model;γ = 1.2714[(αβ/α + β)] was found to predict the HAZ hardness of aluminum weldment cooled in water as a function of the HAZ hardness of both mild steel and cast iron welded and cooled under the same conditions. The maximum deviations of the model-predicted HAZ hardness values γ, α and β from the corresponding experimental values γexp, αexp and βexp were less than 0.02% respectively.
文摘Shielded Metal Arc Welding (SMAW) in Ductile Irons (DI) is often required by foundries for practical manufacturing reasons. The mechanical properties of the welded structures are strongly dependent on their HAZ’s width. A model based on the behaviour of the ferritic matrix of high-Si DIs in order to make an approach in measuring their HAZ’s width is developed in this study. A series of thermal treatments on 3.35 and 3.75 wt% Si as-cast DIs and spot SMAWs is applied on these materials. The applied SMAWs are done on non-preheated and preheated samples (150℃ - 300℃). For welding we modify the amperage (100 - 140A). The micro-hardness Vickers changes in the ferrite of the as-cast samples and inside the HAZ of the welded ones can be attributed to the existence of residual stresses (RS) in the ferritic matrix and assist in estimating the HAZ’s width.
文摘The predictability of hardness of the heat affected zone (HAZ) in aluminum weldments cooled in palm oil, based on hardness of similarly cooled mild steel and cast iron weldments has been ascertained. The general model: α = 1.2769? indicates that HAZ hardness of aluminium weldment is dependant on the ratio of product to sum of HAZ hardness of mild steel and cast iron weldments cooled in palm oil under the same conditions. The maximum deviations of the model-predicted HAZ hardness values α, μ and β from the corresponding experimental values αexp, μexp and βexp were less than 0.04% indicating the reliability and validity of the model.
文摘Submerged arc welding (SAW) has been well utilised for the production of weld joints in 304 L ASS for various industrial application. However, effective performance of the material in service has been hampered by improper choice of electrode. Therefore, in this study, effects of different types of electrode on the microstructure and tensile property of type 304 L austenitic stainless steel heat-affected zone (HAZ) were studied. Chemical composition of the as-received sample was determined. A number of samples were cut from the as-received sample. Afterwards, two half were joined together with 308 L, 312 L and 316 electrodes at a controlled welding speed, current and voltage of 4.6 mm/s, 160 A and 30 V to produce a constant heat input of 626.09 J/mm. An automatic SAW machine with Model Type: DX3-301, and Frequency: 50 Hz was used. And based on ASTM standard, tensile and hardness samples were prepared from the as-received and HAZs. Tensile and hardness measurements were made. Also, specimens for microscopy studies were prepared from the HAZ and as-received samples. From the results, microstructures of the HAZs revealed varied volume fraction of austenite and ferrite phases and grain sizes, and at austenite and ferrite grain boundaries, chromium carbide formation and precipitation were observed. The weld joint produced with 308 L electrode revealed optimum UTS value and YS value of 475 and 325 respectively. While weld joint produced with 316 L electrode has superior ductility of value 41%. Irrespective of the types of electrode used, the as-received sample revealed superior tensile properties over the weld joints. Also, optimum hardness value of 45.7 HRA was obtained with 308 L. Hardness value of the as-received sample was higher than that of HAZ samples.
文摘为解决敞开式TBM穿越硬岩破碎带时卡机风险超前判识及预处理技术难题,依托某TBM隧道工程开展系统性研究。通过累计16 km TBM掘进参数及护盾监测数据统计分析,揭示硬岩破碎带TBM卡机事故的力学机制及其典型参数响应特征。基于参数特征分析,遴选出TBM最大总推力、刀盘最大转矩、护盾最大压力及护盾最大位移变化量作为卡机风险评价关键指标,并采用机器学习算法构建卡机风险预测模型。在此基础上,提出基于多参数权重分析的卡机类型判别方法,实现不同类型卡机事故的准确识别,为现场采取针对性处理措施提供依据。采用离散元数值模拟方法分析不同支护措施对降低卡机风险的作用效果,结合工程实践验证,提出针对不同卡机类型的优化处理方案。研究结果表明:1)当存在卡机风险时,TBM总推力和转矩会持续维持在较高水平;护盾产生较大向内位移,或主司机主动顶升产生较大向外位移;同时护盾压力急剧上升并保持高位稳定。2)所构建的神经网络预测模型准确率为80.8%,随机森林预测模型准确率达到84.6%。3)采用密集型钢拱架支护方案可使围岩压力最大降低约10%,而实施超前支护措施可使围岩压力降幅超过50%。4)当预测可能卡护盾时,需启动刀盘和皮带机脱困模式,同时加强拱架支护;当可能出现护盾刀盘双卡时,必须采用超前支护,同时加强出护盾后的支护结构。
文摘Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a short welding time. Expulsion has a significant impact on the final yield strength of the weld, thus the detection and characterization of expulsion events is significant for the quality assurance of resistance spot welds. In this study, hardness mapping, using a scanning hardness machine, was used as a quality assurance technique for re- sistance spot welding. Hardness tests were conducted on a resistance spot welded sample to prepare a hardness map. The test results showed good correlation between the hardness map and metallographic cross sections. The technique also provided further fundamental understand- ing of the resistance spot welding process, especially regarding the occurrence of expulsion in the nugget.
文摘Groundwater is a finite resource which is being overexploited due to increase in demand over the years leading to decrease in its potentiality.As it is known that ground water is the only major source of drinking water for both urban and rural India its proper management is very essential for its sustainability. In the present study,DRASTIC model is
基金China Scholarship Council for the financial support for Peng Jiang’s study at CSIRO
文摘The microstructure, hardness, and precipitate free zones（PFZ） of V55Ti30Ni15 alloys during heat treatment have been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine Ni Ti particles precipitate from the supersaturated V-matrix solid solution at 750 °C, increase in quantity until 800 °C, and then dissolve back into the V-matrix at 850 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 850 °C. The microstructure containing small Ni Ti precipitates resulting from the treatment of 18 h at800 °C has a good soft condition for workability. PFZ formed at the grain boundary of V-matrix during heat treatment was observed. Vacancies depletion in V-matrix maybe led to the formation of PFZ.
文摘Using the Gleeble 3500 thermal-mechanical system to simulate thermal cycles with different peak temperatures, the hardness and microstructure in the heat-affected zones of two kinds of 100 kg class hot-rolled extra-high-strength steel were compared. When the peak temperature of the thermal cycle was 800℃ ,incomplete transformation occurred during quenching in both steels, and massive martensite and bainite grains were formed. The hardness was determined by the composition and distribution of the microstructure. The concentration of massive martensite was low, and hence the hardness was low,in steel #1. Conversely,the massive martensite content in steel #2 was high and uniformly distributed,resulting in a high hardness. These findings can provide a reference for improving the mechanical properties in the softened zone.
