【作者】 严春妍；

【导师】 李午申； 薛振奎；

【作者基本信息】 天津大学 ， 材料加工工程， 2008， 博士

【摘要】 伴随着全球液化天然气(Liquefied natural gas,简称LNG)消耗量持续增长,国内外建造新的大型LNG设施的需求不断增加,目前,在大型低温储罐和压力容器中,低温韧性优良的9%Ni钢基本上取代了Ni-Cr不锈钢,成为建造LNG低温储罐的主要用材。本课题以9Ni钢为研究对象,应用相变热力学、动力学、金属物理学和弹塑性力学等基本理论,采用物理模拟和数值模拟相结合的手段,研究了这种低温钢在不同焊接工艺参数下焊接接头的强韧化规律以及冷裂纹倾向,并计算了9Ni钢的焊接温度场、应力场和组织转变场,得出保证9Ni钢焊接接头低温强韧化的焊接工艺参数,实现了对焊接接头的组织和性能的预测。采用一次回归正交试验方法,并利用焊接热模拟技术,考察焊接热输入和层间温度两个因素对9Ni钢焊接热影响区的组织和性能的影响。结果表明,9Ni钢焊接热影响区的组织主要为板条马氏体,并可能存在少量的残余奥氏体。在较大冷却速度范围内,9Ni钢的焊接粗晶区的硬度没有出现太大的下降,在试验的焊接工艺条件下没有出现明显的软化现象;在粗晶热影响区中,热输入是影响过临界粗晶热影响区(SRCGHAZ)和临界粗晶热影响区(IRCGHAZ)低温韧性的主要工艺参数;临界粗晶热影响区(IRCGHAZ)是9Ni钢焊接热影响区中韧性最差的部分。对焊接热影响区的硬度和韧性分别建立了预测模型,预测值与实测值吻合较好。基于焊接传热学、热弹塑性理论以及相变热力学、动力学理论,考虑了材料的物理性能与温度之间的非线性关系,以及温度场、组织转变场和应力应变场之间的作用,利用SYSWELD工程有限元计算软件对9Ni钢焊接温度场、组织转变场和应力应变场进行了计算。采用斜Y型坡口焊接裂纹试验,并结合焊接数值模拟技术,对9Ni钢焊接冷裂纹敏感性进行了研究。结果表明,9Ni钢焊接冷裂敏感性不是很大,在不同的预热温度、环境温度和环境湿度下,采用Ni基焊条进行焊接时,断面裂纹率都较低,焊接条件较苛刻时,可以采用75℃左右的预热温度进行焊前预热。更多还原

【Abstract】 As global liquefied natural gas (LNG) consumpution incereases fast in recent years, demands to build more large LNG equipments are becoming strong in many countries. Presently, 9% Ni steel has replaced Ni-Cr stainless steel in building large storage tanks and pressure vessels for its excellent cryogenic toughness, becoming the mostly-used material for large LNG storage tanks construction. In this paper, 9% Ni steel was studied based on fundamental theories like phase transformation kinetics and thermodynamics theory, metal physics theory, elasto-plasto mechanics theory, and so on. Using physical simulation and numerical simulation, mechanical properties and cold crack susceptibility of 9% Ni steel for different welding conditions were investigated. Welding temperature field, stress-strain field and phase transformation field were simulated using SYSWELD simulation software to optimize welding parameters and predict microstructures and mechanical properties.Orthogonal regression and thermal simulation technology were used to investigate the influences of heat input and interlayer temperature on the microstructure and mechanical properties of heat affected zone (HAZ) of 9% Ni steel. Results showed that 9%Ni steel HAZ is mainly constructed of martensite, with very little retained austenite at low cooling rate. No considerable decrease in HAZ hardness was observed for a large range of cooling rates. No softening happened in HAZ for field welding. The predominant factor influencing cryogenic toughness of super-critically reheated coarse-grained HAZ (SRCGHAZ) and inter-critically reheated coarse-grained HAZ (IRCGHAZ) is heat input, which should be kept low with a little higher interlayer temperature to achieve high cryogenic toughness of the welding joint. IRCGHAZ is the primary local brittle zone within the HAZs. HAZ mechanical property (mainly HAZ hardness and toughness prediction) models which were coincident with actual experimental values were presented.Based on welding heat transfer theory, heat elastoplastticity theory and phase transformation theory, the three fields (temperature field, microstructure field and stress-strain field) were simulated using SYSWELD software taking into mutual influences and nonlinear relationships between temperature and mechanical properties.Assisted by numerical simulation, slit-Y cracking test was conducted to investigate the cold cracking susceptibility of 9% Ni steel. Results showed that cold cracking susceptibility of 9%Ni steel HAZ was generally low. Using nickel-based welding material, low section fracture ratio can be achieved for different preheat temperatures, ambient temperatures and ambient humidities. A preheat temperature of 75℃is recommended for welding in severe environment.更多还原