316L不锈钢激光瞬时退火软化工艺研究
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摘要
目的解决冲压中加工硬化导致的高强度低塑性的问题。方法提出以矩形光斑的温控模式激光为热源,对工件进行选区瞬时退火,达到局部软化的目的。通过金相显微分析、显微硬度分析、力学拉伸及断口分析,分别评价激光瞬时退火软化后试样显微组织、显微硬度、抗拉强度、断后伸长率和断口形貌。结果金相组织显示,不同工艺条件下的晶粒大致呈现变形晶粒、再结晶晶粒、细小晶粒和较大等轴晶4种状态。由显微硬度可知,固溶态母材硬度为173HV0.2,加工硬化后达到341HV0.2。当激光温控温度为1400℃,扫描速度分别为5、10、15 mm/s时,软化处理后硬度分别为164、173、257HV0.2。而扫描速度一定时,激光温控温度越高,软化处理后硬度越低。对试样做室温拉伸试验发现,激光瞬时退火后强度降低,塑性提高。当温控温度为1400℃,扫描速度为5 mm/s时,抗拉强度由加工硬化后的911 MPa下降到591 MPa,接近固溶态母材的570 MPa,断后伸长率由18.2%恢复到54.7%,达到固溶态母材的95.5%。结论激光瞬时退火软化可有效降低加工硬化后的材料强度,提高材料塑性,使其恢复大变形能力。其软化程度随激光温控温度的降低、激光扫描速度的提高而降低,在较优工艺参数下,激光瞬时软化后性能甚至优于母材性能。
The work aims to solve the problem of excessive strength and low plasticity caused by hardening in stamping. The temperature-controlled mode laser with a rectangular spot was used as the heat source. The workpiece was subjected to instantaneous annealing in the selected area to achieve local softening. The metallographic microscopic analysis, microhardness analysis, mechanical tensile analysis and fracture analysis were taken to evaluate the microstructure, microhardness, tensile strength, elongation after fracture and fracture morphology of samples after laser annealing and softening. The metallographic structure analysis showed that the grains exhibited deformed grains, recrystallized grains, fine grains and large equiaxed crystalsunder different annealing parameters. From the microhardness results, the hardness of the solid solution base material was 173 HV0.2, but reached 341 HV0.2 after hardening. When the laser control temperature was 1400 ℃ and the scanning speed was 5, 10, and 15 mm/s, the hardness after softening was 164 HV0.2, 173 HV0.2, and 257 HV0.2, respectively. When the scanning speed was constant, the higher the laser control temperature was, the lower the hardness after softening was. From the tensile test, the strength of the sample was reduced after the instantaneous laser annealing but the plasticity was improved. When the temperature control temperature was 1400 ℃ and the scanning speed was 5 mm/s, the tensile strength decreased from 911 MPa to 591 MPa after hardening, which was close to 570 MPa of the solid solution parent metal, and the elongation after fracture was restored from 18.2% to 54.7% which reached 95.5% of the solid solution base metal. Laser instantaneous annealing and softening can effectively reduce the material strength after hardening, improve the plasticity of the material, and restore the strong deformation ability. The softening degree decreases with the decrease of the laser temperature control temperature and the increase of laser scanning speed. Under the optimal laser softening parameters, the performance of softened sample is even better than that of the base metal.
The work aims to solve the problem of excessive strength and low plasticity caused by hardening in stamping. The temperature-controlled mode laser with a rectangular spot was used as the heat source. The workpiece was subjected to instantaneous annealing in the selected area to achieve local softening. The metallographic microscopic analysis, microhardness analysis, mechanical tensile analysis and fracture analysis were taken to evaluate the microstructure, microhardness, tensile strength, elongation after fracture and fracture morphology of samples after laser annealing and softening. The metallographic structure analysis showed that the grains exhibited deformed grains, recrystallized grains, fine grains and large equiaxed crystalsunder different annealing parameters. From the microhardness results, the hardness of the solid solution base material was 173 HV0.2, but reached 341 HV0.2 after hardening. When the laser control temperature was 1400 ℃ and the scanning speed was 5, 10, and 15 mm/s, the hardness after softening was 164 HV0.2, 173 HV0.2, and 257 HV0.2, respectively. When the scanning speed was constant, the higher the laser control temperature was, the lower the hardness after softening was. From the tensile test, the strength of the sample was reduced after the instantaneous laser annealing but the plasticity was improved. When the temperature control temperature was 1400 ℃ and the scanning speed was 5 mm/s, the tensile strength decreased from 911 MPa to 591 MPa after hardening, which was close to 570 MPa of the solid solution parent metal, and the elongation after fracture was restored from 18.2% to 54.7% which reached 95.5% of the solid solution base metal. Laser instantaneous annealing and softening can effectively reduce the material strength after hardening, improve the plasticity of the material, and restore the strong deformation ability. The softening degree decreases with the decrease of the laser temperature control temperature and the increase of laser scanning speed. Under the optimal laser softening parameters, the performance of softened sample is even better than that of the base metal.
引文
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[17]唐洋洋,袁守谦,卫琛浩,等.热处理对不同氮含量316L不锈钢组织及力学性能的影响[J].热加工工艺,2014,43(12):212-215.TANG Yang-yang,YUAN Shou-qian,WEI Chen-hao,et al.Effect of heat treatment on microstructure and mechanical properties of different nitrogen levels in 316Lstainless steel[J].Hot working technology,2014,43(12):212-215.
[2]吕伟强,卢广玺,关绍康,等.中间退火对HTCR5052铝合金再结晶过程的影响[J].材料热处理学报,2017,38(6):91-96.LYU Wei-qiang,LU Guang-xi,GUAN Shao-kang,et al.Effect of intermediate annealing on recrystallization of HTCR5052 aluminum alloy[J].Transactions of materials and heat treatment,2017,38(6):91-96.
[3]欧阳德来,鲁世强,郑海忠,等.冷拔+中间退火2169N奥氏体不锈钢管材的织构与扩口性能[J].塑性工程学报,2014,21(1):117-121.OUYANG De-lai,LU Shi-qiang,ZHENG Hai-zhong,et al.Study on texture and flaring properties of cold drawn+intermediate annealed 2169 austenitic stainless steel tube[J].Journal of plasticity engineering,2014,21(1):117-121.
[4]YANG S,ZHAN J W,KOKAWA H,et al.Grain boundary engineering of 304 austenitic stainless steel by laser surface melting and annealing[J].Journal of materials science,2007,42(3):847-853.
[5]TSAY L W,YANG T Y.Reduction of hydrogen embrittlement in an ultra-high-strength steel by laser surface annealing[J].Fatigue&fracture of engineering materials&structures,2000,23(4):325-333.
[6]张坤,杨高林,胡勇,等.激光选区软化对304不锈钢加工硬化区域的影响[J].应用激光,2017,37(2):161-168.ZHANG Kun,YANG Gao-lin,HU Yong,et al.Effect of selective laser softening process on work hardening area of304 stainless steel[J].Applied laser,2017,37(2):161-168.
[7]杨立军,张宏志,吴雪峰,等.应用加热软化和应力效应的激光加工技术[J].红外与激光工程,2011,40(6):1038-1043.YANG Li-jun,ZHANG Hong-zhi,WU Xue-feng,et al.Laser machining technology with the effect of thermal soften and thermal stress[J].Infrared and laser engineering,2011,40(6):1038-1043.
[8]沈治.激光熔覆加工中加工温度自适应控制研究[J].机床与液压,2011,39(22):118-120.SHEN Zhi.Research on adaptive control to processing temperature in laser cladding process[J].Machine tool&hydraulics,2011,39(22):118-120.
[9]SALEHI D,BRANDT M.Melt pool temperature control using LabVIEW in Nd:YAG laser blown powder cladding process[J].International journal of advanced manufacturing technology,2006,29(3-4):273-278.
[10]BI G J,GASSER A,WISSENBACH K,et al.Identification and qualification of temperature signal for monitoring and control in laser cladding[J].Optics&lasers in engineering,2006,44(12):1348-1359.
[11]胡晓冬,徐元飞,胡勇,等.温度控制模式下激光相变硬化层深度与仿真模型研究[J].材料热处理学报,2016,37(6):235-240.HU Xiao-dong,XU Yuan-fei,HU Yong,et al.Research on laser transformation hardening and simulation model in temperature control mode[J].Transactions of materials and heat treatment,2016,37(6):235-240.
[12]王云山,王娟娟,朱福栋.激光热处理光束优化系统[J].中国激光,2008,35(11):1730-1734.WANG Yun-shan,WANG Juan-juan,ZHU Fu-dong.Beam optimum system for laser heat treatment[J].Chinese journal of lasers,2008,35(11):1730-1734.
[13]郭海滨,左秀荣,张新理,等.奥氏体化温度对奥氏体晶粒度及第二相固溶的影响[J].钢铁研究学报,2016,28(2):63-68.GUO Hai-bin,ZUO Xiu-rong,ZHANG Xin-li,et al.Effect of austenitizing temperature on size of austenite grain and solid solution of second phase particles[J].Journal of iron and steel research,2016,28(2):63-68.
[14]张建新,陈昊,高爱华,等.镁合金强化理论的种类及研究现状分析[J].热加工工艺,2016,45(1):16-18.ZHANG Jian-xin,CHEN Hao,GAO Ai-hua,et al.Categories and research status of reinforcement theory of magnesium alloy[J].Hot working technology,2016,45(1):16-18.
[15]马显锋,施惠基.表面再结晶晶粒对涡轮叶片DZ4合金疲劳性能的影响[J].表面技术,2016,45(4):111-115.MA Xian-feng,SHI Hui-ji.Effect of surface recrystallized grains on the fatigue performance of turbine blade DZ4 alloy[J].Surface technology,2016,45(4):111-115.
[16]胡水平,王哲.织构与晶粒尺寸对AZ31镁合金薄板成形性能的影响[J].塑性工程学报,2011,18(5):54-58.HU Shui-ping,WANG Zhe.Effect of texture and grain size on the formability of AZ31 magnesium alloy sheets[J].Journal of plasticity engineering,2011,18(5):54-58.
[17]唐洋洋,袁守谦,卫琛浩,等.热处理对不同氮含量316L不锈钢组织及力学性能的影响[J].热加工工艺,2014,43(12):212-215.TANG Yang-yang,YUAN Shou-qian,WEI Chen-hao,et al.Effect of heat treatment on microstructure and mechanical properties of different nitrogen levels in 316Lstainless steel[J].Hot working technology,2014,43(12):212-215.