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激光冲击强化对Cr5钢表面粗糙度保持能力的影响规律研究
英文题名Study on surface roughness retention of Cr5 Steel under laser shock peening
吴学星
导师魏延鹏
2018-05-22
学位授予单位中国科学院大学
学位授予地点北京
学位类别硕士
学位专业工程力学
关键词激光冲击强化 表面粗糙度保持能力 冷轧轧辊 硬化层 残余应力
摘要

在轧制高质量型材的过程中,轧辊表面的粗糙度保持能力对于轧辊的寿命以及轧制工件的质量非常重要。激光冲击强化(LSP)是一种先进的金属材料表面改性技术,广泛用于提高金属材料的抗疲劳、抗腐蚀和耐磨性能等。激光冲击强化过程所利用的是激光诱导等离子体产生的冲击波效应,是一个高压力幅值、短脉宽、高应变率的加载过程,在这种极端加载条件下高强材料的强化效果是目前研究所欠缺的。关于激光冲击强化对于轧辊表面质量改善的研究已经有初步的探索,但是LSP对于轧辊表面粗糙度保持能力的影响以及激光参数优化和强化机理研究还有待进一步的探索。

本文以激光冲击强化对于Cr5钢表面粗糙度保持能力的影响规律为研究目的,通过光学显微镜、XRD物相分析、硬度测试和残余应力测试等测试手段对于Cr5钢在不同激光参数冲击强化后的力学特性进行表征,并进行激光参数的优化。而后,运用摩擦磨损测试手段和粗糙度测量的方法对于轧辊表面粗糙度在摩擦磨损过程中的演变过程进行记录,探索激光冲击强化对于轧辊表面粗糙度保持能力的影响作用。最后,给出了LSP对于轧辊表面粗糙度保持能力的强化机理。得到了以下结果:

1.激光冲击强化后Cr5钢表面会产生明显的凹坑,无论是方形光斑还是圆形光斑,其中心会因较高的激光功率密度而出现 “残余应力洞”现象。

2.关于光斑形状的影响规律:当激光功率密度相同时,方形光斑冲击后的硬度提升小于圆形光斑,这是由于由圆形光斑转换为方形光斑过程中会有能量损失。

3. 关于激光功率密度对强化效果的影响规律:在功率密度较小时,功率密度的增加会使LSP效果增强,临界功率密度大约在23 GW/ 附近,此时Cr5钢的表面硬度最高提升31%,硬化层厚度可以达到900~1000μm,残余应力最高提升600MPa左右;而超过该临界功率密度之后硬度、硬化深度和表面残余应力不会继续增加。

4.关于冲击次数的影响:多次冲击可以增强LSP的效果,但是经过3次冲击后,Cr5钢的表面强化效果会达到饱和,继续增加冲击次数反而降低表面硬度和残余应力。

5.摩擦磨损试验中,轧辊表面发生的磨损经历了两个阶段:磨合阶段和稳定磨损阶段。在磨合阶段,LSP可以显著减缓轧辊表面粗糙度的下降速度;在稳定磨损阶段,经LSP处理后的轧辊的稳定磨损能保持在能够在更大的表面粗糙度Ra下。总之,对于轧辊的磨合阶段和稳定磨损阶段,激光冲击强化都具有能够提高表面粗糙度保持能力的效果。激光功率密度对于轧辊的粗糙度保持能力的临界值同样在23 GW/ 左右。

6. LSP作用于Cr5钢提高其表面粗糙度保持能力的机制分为两个方面的原因共同作用的结果:一是由于基体硬化及晶粒细化导致轧辊表面质量的提升,二是强冲击波诱导的残余应力使Cr5钢表面粗糙度保持能力得到提高。

 

英文摘要

The roughness retention ability of the roll surface during cold rolling process is of great significance for the life time of cold rolls and the quality of the rolling workpieces, especially in the produce of the high-quality rolling section. Laser shock peening (LSP) is an advanced and effective surface treatment technique to improve the fatigue life, wear resistance, corrosion resistance and other mechanical characteristics of metal parts. During an LSP process, a shock wave with high-amplitude pressure is generated and propagates into the target material, resulting in plastic deformation and deep compressive residual stress in the surface layers of metal materials. Some preliminary explorations have been made on the study of LSP for the improvement of roll surface quality, yet few work has been carried out to investigate the effect of LSP on high-strength metal, and little attention has been devoted to the roughness remaining ability of cold rolling rolls and the laser parameter optimization.

The aim of this paper is to examine the effect of laser shock processing on the surface roughness retention in abrasive wear behavior of Cr5 steel. The mechanical properties of Cr5 steel after LSP with different laser parameters were characterized by optical microscopy, XRD phase analysis, hardness test and residual stress test. Furthermore, the parameters of LSP were optimized. Then, the abrasion wear test and roughness measurement were taken to record the evolution of the roll surface roughness, and the effects of laser shock strengthening on the roll surface roughness retention ability was explored. Finally, the strengthening mechanism of the LSP on surface roughness retention of the roll surface is given. The result we got is as follows:

1.After LSP treatment, a round dent was produced in Cr5 steel surface and a residual stress hole will appear in the center of the spot due to the high laser power density, regardless of whether it is a square spot or a round spot.

2.We found the influence of the spot shape during the LSP for Cr5 steel. Under the same laser power density, the hardness increasement after the LSP with square spot is smaller than that of the round spot, which is due to the energy loss during the conversion from a circular spot to a square spot.

3. The effect of laser power density is as follows. When the power density is small, the increase of power density will enhance the LSP effect, and the critical power density is about 23 GW/ . Under the critical power density, the maximum value of surface hardness of Cr5 steel will increase 31%, and the thickness of the hardened layer can reach 900-1000μm. The residual stress can be increased by about 600MPa at most. However, beyond this critical power density, the hardness, hardening depth, and surface residual stress will not increase.

4.About the influence of the shot times, we find that multiple impacts can enhance the effect of LSP. But after three impacts, the surface hardening effect of Cr5 steel will be basically saturated and more impacts will cause the decrease of surface hardness and residual stress instead.

5.During the whole abrasion process, the evolution of the roller surface can be divided into two stages: the running-in stage and the steady wear stage. In the running-in stage, LSP can significantly slow down the roll surface roughness deterioration. In the steady wear stage, the stable wear of the LSP-treated roll can be maintained at a higher surface roughness Ra. Above all, LSP is an effective surface treatment technique to improve the surfaces roughness retention of Cr5 steel. The critical value of the laser power density for the roller surface roughness retention ability is also about 23 GW/ .

6.The reason for better surface roughness retention of the LSP samples is interpreted as the combined effect of grain refinement, increased microhardness and compressive residual stress induced by LSP.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73137
专题流固耦合系统力学重点实验室
作者单位中国科学院力学研究所
推荐引用方式
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吴学星. 激光冲击强化对Cr5钢表面粗糙度保持能力的影响规律研究[D]. 北京. 中国科学院大学,2018.
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