淬火和回火工艺对3.5Ni钢组织和力学性能的影响

通过拉伸和冲击试验以及SEM、TEM和EBSD组织观察,研究了不同热处理工艺参数对3.5Ni钢显微组织和力学性能的影响。结果表明:3.5Ni钢在860℃保温1 h水淬后得到细小的板条马氏体(LM)加粒状贝氏体(GB)组织;570℃回火后,LM的板条变粗,GB中的M/A岛也溶解消失,基体上有大量渗碳体析出。随着回火温度的升高,板条继续合并长大,并且开始出现多边形铁素体,渗碳体也不断长大。回火温度为570~600℃时,低温韧性随回火温度的升高而增加,但是继续升高回火温度会使得低温韧性下降。研究表明:3.5Ni低温钢经860℃水淬+600℃回火的热处理可以获得最佳的力学性能。     

Plastic behaviours during tempering by crystal plasticity analyses using fast Fourier transform

ABSTRACT

Heat treatment, such as quenching and carburising, often involves volume change caused by phenomena such as thermal expansion, phase transformation, and carbide precipitation during tempering. During the tempering process, an external force induces additional plastic deformation. The authors termed this phenomenon ‘tempering plasticity’. In this study, we performed crystal plasticity analysis using fast Fourier transform considering the volume change in carbides to assess the mechanism of tempering plasticity. As a result, tempering plastic strain occurred as the volume fraction of carbide increased, and the tempering plastic phenomenon could be reproduced based on the transformation model proposed by Greenwood–Johnson. The result supports the idea that the volume change accompanied by carbide precipitation is an important mechanism that invokes tempering plasticity.     

A Study of Structure Evolution in Pearlitic Steel Wire at Increasing Plastic Deformation

In this work was studied the influence of the plastic deformation's intensity under deep drawing on microstructures of a pearlitic steel containing 0.8% C. Varying the levels of deformation causes diverse dislocation movements as well as modified structural states in the individual phases of the pearlitic eutectoid steel. It is shown that in the course of plastic deformation there is a reduction of interlamellar distance in a pearlite and increase in dislocation density. In some parts partial spheroidisation cementite plates is observed. The bands formed in dislocation structure are found out. The analysis of failure mechanisms of steel with pearlite structure after plastic deformation is carried out. During the deformation of pearlite, the increase in stress at the phase boundary, owing to the elastic strain incompatibility between the ferrite and cementite phases, resembles the stress concentration at grain boundaries. Pores form at the interface between the surface of the ferritic matrix and the spheroidised carbide particles. Such micro pores occur by means of plastic deformation of the ferrite's interstices around the stronger cementite owing to the reduction of the ferritic interstices and their subsequent cracking.     

球墨铸铁加热过程中渗碳体石墨化的原位观察

采用激光共聚焦扫描显微镜直接观察了球墨铸铁在加热过程中渗碳体的分解和石墨形态的演化。结果表明：球墨铸铁在加热过程中渗碳体逐渐溶解,加热速率越快,渗碳体的分解速率越快。加热过程中石墨形态的演变行为与加热速率有关。加热速率较慢,组织中出现了小石墨,原有石墨长大。加热速率较快,渗碳体周围的小石墨逐渐溶解变小。     

20CrNi2Mo钢齿轮磨削裂纹成因分析

采用宏观分析、化学成分分析、金相检验以及硬度测试等方法对某20CrNi2Mo钢制齿轮磨削裂纹产生原因进行了分析。结果表明:该齿轮磨削裂纹产生的主要原因是其渗碳区域存在严重的内氧化缺陷和魏氏渗碳体组织,降低了齿轮表层磨削面的残余压应力,增加了齿轮的磨削开裂敏感性,当磨削应力超过材料断裂强度时,便会产生磨削裂纹。     

帘线钢盘条晶界渗碳体腐蚀方法研究

显示帘线钢盘条晶界渗碳体有多种方法。采用显微硬度计对晶界渗碳体区域进行定位,4种方法腐蚀帘线钢盘条晶界渗碳体,经苦味酸钠溶液在60℃,腐蚀15 min的组织晶界渗碳体呈黑色,级别高,其他组织不出现;经体积分数为4%硝酸酒精溶液腐蚀的组织晶界渗碳体呈白色,级别很低,铁素体呈白色;经苦味酸酒精溶液在室温腐蚀15 s,250℃保温8 min,组织晶界渗碳体呈白色,级别较低,铁素体呈白色;经体积分数为4%苦味酸酒精溶液腐蚀的组织晶界渗碳体呈白色,级别较高,铁素体呈白色。结果表明,晶界渗碳体清晰度由大到小的顺序为:苦味酸钠溶液腐蚀,苦味酸酒精溶液腐蚀,苦味酸酒精溶液腐蚀加热染,硝酸酒精溶液腐蚀。     

温轧低碳钢渗碳体形态对终轧组织性能的影响

通过温轧铁素体+珠光体组织获得两种渗碳体分布形态,并在300℃变形获得不同的位错密度,结合随后的快速升温至奥氏体低温区以及快冷后单道次轧制变形获得了铁素体晶粒尺寸为200～500nm的亚微米晶钢,拉伸性能测试表明利用该工艺获得的样品除了具有很高的抗拉强度(1000MPa)之外,其屈强比相对较低,断后总伸长率≥20%。TEM观察发现,原始组织中渗碳体弥散分布的试样更容易在上述工艺下获得等轴亚微米晶铁素体以及适量板条马氏体和孪晶马氏体组织,而原始组织中渗碳体分布集中的试样其终态组织中等轴晶铁素体的比例显著减少,且铁素体晶内位错密度要低于前者。     

渗碳体的结构和一次渗碳体形态的研究

对钢铁中的基本相渗碳体的晶格结构作了进一步的描述:一个碳原子位于由6个铁原子组成的三棱柱体中,每个铁原子为2个三棱柱体所共有;每个渗碳体晶胞含4个这种上下分层、反向、错位排列的三棱柱体,相应的化学计量比为12:4(即3:1)。应用计算机可以形象地揭示渗碳体的结构。根据成分过冷原理对从过共晶铸铁液相中结晶出树枝状一次渗碳体进行了分析。