回火4340钢的激光相变硬化

使用1.8KW固定激光功率的CO_2激光器,用5—10mm/s的扫描速度,硬化AISI4340钢试样的表面。分析了扫描速度和合金的回火处理对硬度分布曲线和激光硬化区显微组织的影响。硬化区的显微组织主要由板条和孪生马氏体组成。也发现了自回火马氏体,它取决于扫描速度。在激光处理试样的相变区,观察到在马氏体上分布着具有奥氏体壳层的部分溶解碳化物和(或)奥氏体小岛。激光处理期间,碳化物完全溶入奥氏体所需要的时间取决于回火条件。如果合金的回火温度较低,则硬度曲线中得到的硬化区较深,相变区较窄。以奥氏体中碳的扩散距离为基础,对硬度曲线作了简单的数学估算。计算结果与相变硬化区过程中测定的硬度曲线以及观察到的显微组织是完全相符的。     

高铬铸钢激光熔凝组织与性能研究

为提高锅炉燃烧器喷嘴的表面性能,采用5kW横流CO2激光加工系统对高铬铸钢表面进行熔凝处理,并进行显微组织分析和硬度测试.研究结果表明:铸态高铬铸钢晶粒较粗大、组织不均匀,原始组织以奥氏体为基体,还存在大量网状断续共晶碳化物、莱氏体.经激光凝熔后,显微组织明显细化,其试样剖面组织分为激光熔凝区(细小奥氏体 少量细小未熔碳化物)、激光相变区(奥氏体 少量晶界碳化物)、过渡区和母材4个区域.熔凝区和相变硬化区的淬硬深度依工艺参数不同.可达0.2～0.3mm.由于形成了奥氏体组织,高铬铸钢表面硬度增加不明显,硬度最大值出现在相变硬化区.     

激光相变硬化工艺参数对40Cr组织性能的影响

利用无氦横流CO_2激光加工机对40Cr表面进行激光相变硬化处理,并采用金相显微镜(OM)、显微硬度计、滑动摩擦磨损试验机和恒电位仪研究了不同工艺参数下硬化层的显微组织及性能.结果表明:相变硬化区的组织为细小的针状马氏体+残余奥氏体,过渡区组织为回火马氏体+残余奥氏体+铁素体+碳化物;40Cr经过激光相变硬化处理后,硬度、耐磨性和耐蚀性均显著提高.     

AerMet100超高强度钢激光相变硬化研究

对AerMet100超高强度钢进行了激光相变硬化试验,并对淬硬层的显微硬度分布及其物相变化进行了测试与分析.结果表明,激光相变硬化后淬硬层的硬度得到提高,比基体提高了约200 HV0.2;在同样的激光处理工艺条件下,经激光相变硬化后原始状态为淬火态试样的淬硬层的硬度和深度比退火态试样的要高和深;相变硬化后,随着回火时间的延长,逆转变奥氏体含量增多,硬化层硬度降低.     

多道网格扫描激光相变硬化的性能研究

采用YLS-3000型光纤激光器对40Cr钢表面进行不同间距的网格扫描激光相变硬化。研究不同网格间距对硬化层的显微组织、硬度、耐磨性和耐蚀性的影响。结果表明:激光相变硬化层横截面由表及里依次可分为相变硬化区、过渡区和基体。相变硬化区的组织为细小针状马氏体+少量残余奥氏体,过渡区的组织为马氏体+残余奥氏体+铁素体+未溶碳化物,基体的组织为铁素体+珠光体。网格扫描相变硬化层的平均硬度约为61 HRC,网格交叉点的平均硬度可达62 HRC。随着网格间距的增加,试样的相对耐磨性先增大再减小,当网格间距为12 mm时,相对耐磨性可达基体的3.25倍。同时,此间距试样的钝化区间最宽,约为1530 m V,耐蚀性最强。     

40Cr激光熔凝硬化组织形态及硬度研究

利用CO2轴流激光加工机对40Cr钢表面进行激光熔凝硬化处理.利用扫描电子显微镜、金相显微镜和显微硬度计研究了不同工艺下熔凝硬化层及基体的显微组织和硬度分布特征.实验表明:熔凝硬化层由熔化区、相变硬化区和热影响区组成;由表及里组织分别为极细隐晶马氏体 少量残余奥氏体、隐晶马氏体 碳化物 残余奥氏体、马氏体 回火屈氏体 铁素体.硬化层最高硬度约是基体的3倍;随着扫描速度的增加表层硬度先增加后减小,当扫描速度为2.5 m/min时,表层硬度最大,为1097.9 HK.     

T10钢激光相变硬化的组织与性能研究

采用激光相变硬化工艺对T10钢表面进行改性处理,并对改性后的组织与性能进行研究.结果表明,硬化区组织为针状马氏体 少量残余奥氏体;热影响区组织为少量针状马氏体 珠光体 网状渗碳体;基材组织为珠光体 网状渗碳体.淬硬层表面的洛氏硬度最高值为63.5HRC,淬硬层内的显微硬度分布均匀,从硬化IX---,热影响区-基材显微硬度呈梯度变化.激光相变硬化后淬硬层耐磨性比常规淬火后耐磨性提高10%左右.     

温度控制模式下激光相变硬化层深度与仿真模型研究

为探索温度可控的大功率半导体激光器作用下非平衡态的奥氏体转化温度和马氏体临界转化速度两个条件同时对中碳钢的相变硬化的作用机理,本文利用温度可控的大功率半导体直接输出激光加工系统对45钢进行温度控制模式下的激光相变硬化实验。实验表明:在相同激光相变硬化控制温度下,随着扫描速度的增加,相变硬化层深度先增加后降低。对试样的显微组织分析表明,在扫描速度较慢时,受冷却速度影响产生的激光相变硬化区成分、组织的差异是造成硬化层深度和硬度不同的原因。并基于非平衡态的奥氏体转化温度和马氏体临界转化速度为马氏体生成的判断依据,建立了基于COMSOL Multiphysics软件的三维激光相变硬化数值分析模型,探讨了温度控制模式下激光加工参数对硬化层深度的影响,与实验结果对比发现该模型能够较为准确预测温度可控的激光相变硬化层深度。     

激光扫描速度对45钢表面改性层组织及性能的影响

利用横流CO2激光加工机对正火态45钢表面进行激光相变处理,对改性层进行了OM实验、硬度测试以及硬化层磨料磨损实验.结果表明:相变硬化层由表及里依次为熔凝区、相变硬化区、过渡区和基体区;其中激光熔凝区晶粒最为细小;硬化层表面最高硬度为48 HRC;激光扫描速度对表面耐磨性有较大影响,其中扫描速度为8mm/s、功率700 W时改性层耐磨性最佳.     

4Cr13不锈钢非平衡组织激光加热时的组织遗传特征

对4Cr13不锈钢淬火及淬火 低温回火等非平衡组织经激光加热时奥氏体形成特点及加热后的表面层硬度的分布进行了研究分析。结果表明,4Cr13不锈钢非平衡组织激光加热时存在明显的组织遗传特征和相变硬化再结晶现象。当激光加热时,细小的奥氏体晶粒沿原奥氏体晶界优先析出,产生“晶粒边界效应”;原奥氏体晶内除形成少量细小奥氏体晶粒外,基本恢复原来的大小,产生“组织遗传”;在0．1mm-0．3mm加热表层内存在较小的奥氏体晶粒,产生“相变硬化再结晶”。非平衡组织激光加热后的表层硬度分布为5个区域,硬度曲线呈马鞍形。     

18Cr2Ni4W钢渗碳激光强化复合处理组织的研究

对１８Ｃｒ２Ｎｉ４Ｗ钢进行了渗碳激光强化复合处理研究。利用扫描电镜、透射电镜和图象分析仪,对其组织进行较详细的研究。结果表明,在复合工艺作用下,随着表面硬化工区层深的变化,组织结构发生明显的变化。由于钢中存在大量合金元素,致使表面硬化层产生大量的残留奥氏体,降低了表面硬度。     

Influence of heating rate on the laser surface hardening of a medium carbon steel

The finite difference method was applied to analyse the influence of heating rate on the transformation phenomenon in laser surface hardening of a medium carbon steel. The implicit scheme of this method was adopted to improve the accuracy of the numerical analysis since the very high heating and cooling rate and the small hardened zone, which characterize the process investigated, need a very detailed mesh generation. The calculated cooling rate was high enough for all the material that undergoes transformation into austenite to be transformed into martensite. From the calculated temperature change the heating rate of the transformation zone boundary could be estimated to be of the order of 10⁴ °C s^-1, which causes a delay in the austenite transformation and consequently a shift in the transformation temperature. Considering a heating rate of 10⁴ °C s^-1, the A_s (start of austenitic transformation) temperature of the investigated medium carbon steel with 0.45 wt.% C should be approximated to be about 830 °C and the A_f (finish of austenitic transformation) temperature about 950 °C. Experimental results obtained by irradiating the test specimen with a 1 kW CO₂ laser showed better agreement with the hardened zone sizes predicted by the modified A_s temperature than with those predicted by the equilibrium A_s temperature. From simulations it appeared that the occurrence of surface melting and the size and shape of the hardened zone are strongly dependent on process parameters such as beam spot diameter and traverse speed.     

激光表面淬火对铬钼铸铁组织和硬度的影响

采用TH-3DC3000型激光加工系统对铬钼铸铁进行了激光表面淬火处理,研究了不同激光功率和扫描速度对铬钼铸铁显微组织、表面硬度及硬化层深度的影响。结果表明,经激光表面淬火后,铬钼铸铁的组织由硬化区、过渡区和基体3个区域组成,硬化区组织为隐晶马氏体、残留奥氏体和球状石墨,过渡区组织为隐晶马氏体、珠光体和球状石墨,基体组织为铁素体、珠光体和球状石墨。在激光表面淬火未对试件产生过热影响时,激光功率的增大和扫描速度的降低均会提升铬钼铸铁的表面硬度和硬化层深度。在5 mm×20 mm的矩形激光光斑下,确定最优的参数组合为激光功率2300 W、扫描速度0.003 m/s,采用该参数组合对铬钼铸铁进行激光淬火处理时,表面硬度为760 HV0.3,硬化层平均硬度为724 HV0.3,硬化层深度可达1.4 mm以上。     

强度空间分布对脉冲激光表面强化的影响

建立了包含脉冲激光强度时空分布、温度相关的材料热物性参数以及多次相变特征在内的脉冲激光表面强化三维有限元模型。温度场和强化区深度分别得到了解析解和试验验证。针对不同脉冲能量级别，研究了强度空间分布形式和光斑几何形状对表面最高温度、强化区深度以及强化区形状等的影响。结果表明，激光强度空间分布是影响强化区的重要因素。为达到理想的强化效果，应根据不同的脉冲能量选择适当的空间分布形式和光斑几何形状。     

Recrystallization and the possibility of the realization of the α-γ diffusionless transformation during the ultrarapid laser heating of steels

Analysis is given of phase and structural transformations occurring upon ultrarapid laser heating in steels with different initial structures, namely, after annealing, after preliminary quenching, quenching and tempering, and after quenching with subsequent deformation and tempering. It is shown that a significant suppression of diffusion processes occurs during laser heating; this circumstance substantially affects the nature of the phase and structural transformations proceeding during laser processing. Special attention is given to studying the process of recrystallization and to the phenomenon of structural heredity during laser heating. The process of recrystallization during laser heating is considered as consisting of two stages, namely, an ordered lattice rearrangement (α-γ transformation) and the recrystallization of austenite that suffered phase-transformation-induced hardening (“phase naklep”). The effect of tempering and plastic deformation on the recrystallization of a preliminarily quenched steel consists in the intensification of the second stage, i.e., of the recrystallization of the transformation-hardened austenite. It is shown that the α-γ transformation during the laser heating of steels with the initial structure of lath martensite occurs by the “mechanism of recovery,” i.e., via the formation and growth of austenite nuclei. In steels with the initial structure of pearlite, the nucleation of austenite during laser heating can occur by a shear martensite-like diffusionless mechanism with the observance of characteristic orientation relationships between the initial ferrite and the newly formed austenite.     

冲压模具激光表面强化的搭接工艺研究

针对激光相变强化搭接带存在的回火软化等问题,对激光强化的球墨铸铁材料QT700-2搭接区的显微硬度、表面粗糙度、物相变化等进行了测试,并分析了强化机理。结果表明:选用合理的搭接率等工艺参数,球墨铸铁激光搭接强化后的硬度可达到50HRC以上,调节搭接宽度可以避免搭接区的回火软化问题。多道搭接强化后表层的马氏体组织呈区域性交错分布,材料表面形成非均匀、多尺度强化结构,强化后的表面具有良好的耐磨性和耐疲劳性。通过上述结果,对材料激光搭接强化的工艺参数进行了合理的优化。     

Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ

Abstract

Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ.