Effect of Shot Peening Intensity on Residual Stress Distribution of Typical Q345 Steel Cruciform Welded Joints on Ship
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摘要:
对船舶典型Q345钢十字型焊接接头进行了不同喷丸强度(0.167,0.183,0.198 mm)下的喷丸处理,研究了喷丸后接头残余应力表面和沿深度方向的分布以及疲劳寿命,确定了最优喷丸强度。结果表明:喷丸处理消除了焊接接头表面残余拉应力,引入了较高水平残余压应力,改善了表面残余应力分布均匀性,增加了残余应力影响层深度,延长了疲劳寿命;随着喷丸强度增加,引入的残余压应力增大,分布均匀性先提升后减小,残余应力影响层深度增加,疲劳寿命先延长再缩短;最优喷丸强度为0.183 mm,此时焊接接头表面残余压应力较大,分布均匀性最好,残余应力影响层深度较大,疲劳寿命最长。
Abstract:The typical Q345 steel cruciform welded joint on ship was treated by shot peening under different shot peening intensities (0.167, 0.183, 0.198 mmA). The surface and depth distribution of residual stress and fatigue life of joint after shot peening were studied, and the optimal shot peening process was obtained. The results show that after shot peening, the residual tensile stress on the surface of the welded joint was eliminated, the high level of residual compressive stress was introduced, the distribution uniformity of the surface residual stress was improved, the depth of the residual stress affected layer was increased, and the fatigue life was prolonged. With the increase of shot peening intensity, the introduced residual compressive stress increased, the distribution uniformity first increased and then decreased, the depth of the residual stress affected layer increased, and the fatigue life first prolonged and then was shortened. The optimal shot peening intensity was 0.183 mmA. Under this intensity the residual compressive stress on the welded joint surface was large, the distribution uniformity was the best, the depth of the residual stress affected layer was large, and the fatigue life was the longest.
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Keywords:
- Q345 steel /
- cruciform welded joint /
- shot peening /
- residual stress /
- fatigue life
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0. 引言
大型船舶在长距离、高速航行过程中受长期循环载荷以及短期极端载荷的共同作用,服役工况苛刻,因此对船体结构的承载能力和使用寿命提出了更高的要求[1-2]。船体多采用成形钢板焊接而成,其焊接接头处易因焊接过程中的高度集中瞬时热输入和非均匀加热冷却而产生非均匀膨胀和收缩,并引起塑性变形,从而产生残余应力[3-5]。焊接残余拉应力的存在会给焊接结构的应力腐蚀性能和疲劳性能等带来不利影响[6-7]。因此,控制或消除焊接残余拉应力对于保障船舶结构安全具有重要意义。
焊接结构失效主要源于表面,喷丸、滚压、激光喷丸、超声冲击等表面强化技术[8-10]可以使工件表面产生一定的塑性变形,从而消除焊接残余拉应力并引入残余压应力。引入的残余压应力能够抵消服役过程中由工作载荷引起的部分拉应力,减少应力集中,从而增强材料的抗疲劳性能;并且,残余压应力层的存在有助于抵抗磨损和破坏,从而延长疲劳寿命。相对而言,喷丸因强化效果好、操作简单、不受工件形状和尺寸限制、成本相对较低等优点在工业领域获得广泛应用[11-16]。喷丸处理可以在焊接结构件不同区域引入不同水平的残余压应力[14-16],应力演变规律复杂[17-20]。在喷丸过程中,喷丸强度作为控制参数,表征了喷丸束流引入残余压应力层深的能力,通过调整喷丸强度,可以确保喷丸达到预期的表面强化效果。目前,关于喷丸处理对十字型焊接接头残余应力分布影响的研究并不多见。作者对船舶典型Q345钢十字型焊接接头进行了不同喷丸强度下的喷丸处理,研究了喷丸强度对焊接接头残余应力分布的影响,确定了最优喷丸强度,拟为船体典型十字型焊接接头残余应力调控提供参考。
1. 试样制备与试验方法
母材为尺寸分别为400 mm×200 mm×8 mm和400 mm×30 mm×8 mm的Q345钢板,化学成分(质量分数/%,下同)为0.16C,0.31Si,0.15Mn,0.005P,0.004S;焊丝为JQ.CE71T-1氧化钛型CO2气体保护药芯焊丝,直径为1.2 mm,化学成分为0.44C,0.33Si,1.15Mn,0.010P,0.005S;母材与焊丝熔敷金属的屈服强度分别为391,474 MPa,抗拉强度分别为510,550 MPa,断后伸长率分别为22%,27%。采用CO2气体保护多层多道焊焊接十字型接头,坡口形式和焊道布置见图1,焊接工艺参数见表1,较窄钢板竖直放置。焊接完成后,采用超声波探伤检查焊缝质量,以无裂纹、未熔合、夹渣、咬边和未填满弧坑等缺陷为标准,检查合格后采用线切割制取如图2所示的试样。
表 1 焊接工艺参数Table 1. Welding process parameters焊道 焊道编号 电流/A 电压/V 焊接速度/(cm·min−1) 打底焊 1~4 200~240 30~32 30~40 填充焊 5~8 240~280 32~34 40~50 盖面焊 9~11 240~280 32~34 40~50 采用XN-9065P型气动式喷丸机对试样进行喷丸处理,弹丸使用平均直径为0.6 mm的S230铸钢丸,流量为25 L·min−1,喷嘴沿焊接方向前后移动,入射方向垂直喷丸区域表面,喷丸时间为150 s,喷丸覆盖率为100%,喷丸强度分别为0.167,0.183,0.198 mm(A式试片)。
采用MTS型动态疲劳试验系统进行室温循环疲劳试验,应力峰值为380 MPa,应力比恒定为0.1,频率为15 Hz,加载波形为正弦波形,定义试样宏观断裂循环次数为疲劳寿命。采用μ-X360s型便携式X射线残余应力分析仪测试表面和不同深度处的残余应力,电压为30 kV,电流为1 mA,铬靶,Kα射线。表面残余应力测试前先将待测表面进行砂纸打磨、电解抛光(电解液为饱和NaCl溶液)和无水乙醇+丙酮清洗;在垂直于焊缝方向距试样边缘4,8,12,16,20 mm的5条线上各取16个测试点,测试点以焊缝为中心呈对称分布,距试样中心线距离分别为5,7,9,11,16,21,26,31 mm,近焊缝区测试点较密集,远离焊缝区测试点较稀疏。测试不同深度残余应力时,采用线切割切取焊缝区和热影响区试样,对其进行电化学腐蚀剥层处理,腐蚀介质为饱和NaCl溶液,电压为15 V,每腐蚀10 s测试剥层深度和残余应力。
2. 试验结果与讨论
2.1 表面残余应力分布
由图3可见:喷丸处理前后试样表面横向与纵向残余应力分布均相近。未喷丸焊接接头的焊缝区、热影响区和母材区均存在残余拉应力,横向平均残余拉应力分别为214,129,102 MPa,纵向平均残余拉应力分别为145,93,65 MPa;喷丸后焊接接头表面残余应力均为较高水平的压应力;随着喷丸强度由0.167 mm增至0.198 mm,焊缝区、热影响区、母材区横向平均残余压应力分别增加了10.1%,29.6%,6.5%,纵向平均残余压应力分别增加了28.1%,3.2%,9.4%。
表面残余应力的均匀分布有助于降低应力集中,减小局部应力峰值,从而降低裂纹萌生概率,提高焊接结构整体抗疲劳性能,延长使用寿命。可以用残余应力平均值与标准方差比值的绝对值R来表征残余应力分布均匀性,R越大则残余应力分布均匀性越好。由图4和表2可知:相比未喷丸接头,喷丸接头焊缝区、热影响区和母材区的表面残余应力R值均增加,表面残余应力分布均匀性提升;随着喷丸强度增加,表面残余应力分布均匀性先提升后减小,当喷丸强度为0.183 mm时,均匀性最好;相比焊缝区,增加喷丸强度对热影响区、母材区残余应力分布均匀性改善效果更好。
表 2 不同喷丸强度下焊接接头表面残余应力相关计算结果Table 2. Calculation results related to surface residual stress of welded joints under different shot peening intensities区域 喷丸强度/mm 残余应力/MPa 残余应力平均值/MPa 残余应力标准方差/MPa R 焊缝 0 −63~278 164 46.73 3.51 0.167 −161~−447 −308 83.88 3.67 0.183 −296~−428 −349 46.12 7.58 0.198 −227~−473 −360 53.65 6.73 热影响区 0 −61~129 47 55.19 0.86 0.167 −245~−350 −272 24.64 11.06 0.183 −305~−382 −313 25.58 13.01 0.198 −268~−395 −335 52.31 6.21 母材 0 −89~169 50 66.25 0.77 0.167 −347~−436 −403 24.18 16.68 0.183 −363~−453 −419 18.64 22.49 0.198 −380~−482 −429 22.79 18.83 在喷丸过程中弹丸对材料表面的剧烈冲击作用下,材料表层发生剧烈塑性变形,大量位错激活、增殖,导致晶粒细化。随着喷丸强度增加,材料表层组织细化显著,发生塑性变形的晶粒增多,且晶粒间互相协调变形,所以材料的残余应力分布均匀性提升[21-22];然而,过大的喷丸强度会导致材料表面粗糙度增加,产生缺口效应,从而形成应力集中,应力分布均匀性降低[23]。
2.2 残余应力沿深度方向分布
由图5可见:在焊缝区,喷丸强度为0.167,0.183 mm时横向残余压应力最大值出现在表面,分别为396,407 MPa,喷丸强度为0.198 mm时最大值(430 MPa)出现在次表层(深度约为32 μm),不同喷丸强度下纵向残余压应力最大值均出现在次表层(深度小于100 μm),分别为367,372,423 MPa;在热影响区,不同喷丸强度下横向、纵向残余压应力最大值均出现在表面,分别为373,382,471 MPa。根据赫兹压力模型[24]可知,残余应力深度方向的分布主要由表层的塑性延伸和表层由赫兹压力引起的塑性变形2个过程相互竞争决定,当表层塑性延伸过程占主导时,最大残余压应力出现在表面,当赫兹压力足够大时,塑性变形过程占主导,最大残余压应力出现在次表层。此外,随着喷丸强度增加,残余压应力最大值增大,残余应力影响层深度增加,焊缝区分别为249,287,446 μm,热影响区分别为320,338,475 μm。
2.3 疲劳性能
由图6可见:随着喷丸强度增加,焊接接头疲劳寿命先延长再缩短,喷丸强度为0.183 mm时疲劳寿命最长。由此可见,喷丸强度为0.183 mm时,焊接接头表面残余压应力较大,分布均匀性最好,残余应力影响层深度较大,疲劳寿命最长,为最优喷丸工艺参数。
3. 结论
(1)喷丸处理后Q345钢十字型焊接接头表面残余拉应力消除,形成了较高水平的残余压应力,随着喷丸强度增加,残余压应力增大。
(2)喷丸处理后焊接接头表面残余应力分布均匀性得到改善,残余应力影响层深度增大。随着喷丸强度增加,表面残余应力分布均匀性先提升后减小,当喷丸强度为0.183 mm时分布均匀性最好,残余应力影响层深度则持续增大;相比焊缝区,增加喷丸强度对热影响区和母材区表面残余应力分布均匀性改善效果更好。
(3)喷丸处理后焊接接头疲劳寿命延长,且随着喷丸强度增加呈先延长再缩短的变化趋势,当喷丸强度为0.183 mm时疲劳寿命最长。
(4)最优喷丸工艺为喷丸强度0.183 mm,此时焊接接头表面残余压应力较大,分布均匀性最好,残余应力影响层深度较大,疲劳寿命最长。
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表 1 焊接工艺参数
Table 1 Welding process parameters
焊道 焊道编号 电流/A 电压/V 焊接速度/(cm·min−1) 打底焊 1~4 200~240 30~32 30~40 填充焊 5~8 240~280 32~34 40~50 盖面焊 9~11 240~280 32~34 40~50 表 2 不同喷丸强度下焊接接头表面残余应力相关计算结果
Table 2 Calculation results related to surface residual stress of welded joints under different shot peening intensities
区域 喷丸强度/mm 残余应力/MPa 残余应力平均值/MPa 残余应力标准方差/MPa R 焊缝 0 −63~278 164 46.73 3.51 0.167 −161~−447 −308 83.88 3.67 0.183 −296~−428 −349 46.12 7.58 0.198 −227~−473 −360 53.65 6.73 热影响区 0 −61~129 47 55.19 0.86 0.167 −245~−350 −272 24.64 11.06 0.183 −305~−382 −313 25.58 13.01 0.198 −268~−395 −335 52.31 6.21 母材 0 −89~169 50 66.25 0.77 0.167 −347~−436 −403 24.18 16.68 0.183 −363~−453 −419 18.64 22.49 0.198 −380~−482 −429 22.79 18.83 -
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