EXPERIMENTAL STUDY ON DUCTILE-TO-BRITTLE TRANSITION OF RPV STEEL CONSIDERING GEOMETRIC SIZE
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摘要: 反应堆压力容器(reactor pressure vessel, RPV)是核电站的一道重要的安全屏障, 而RPV钢在韧脆转变区内的断裂韧性变化是核电站安全评估中的重要考虑因素. 在对RPV钢韧脆转变区内的断裂韧性变化规律进行研究时, ASTM标准等国际标准通常推荐以主曲线法进行研究. 采用不同尺寸的单边缺口弯曲(single edge-notched bending, SEB)试样及紧凑拉伸(compact tension, CT)试样完成了SA-508钢在常温至−100 °C的温度范围内的断裂韧性试验, 基于主曲线法研究了SA-508钢在韧脆转变区内的断裂韧性变化规律, 同时对主曲线法得到的基于不同尺寸断裂试样结果的韧脆转变温度预测精度进行了对比, 并通过断口微观形貌分析研究了断裂试样的破坏特征. 研究表明, 试样构形和几何尺寸对RPV钢的韧脆转变行为有显著影响. 主曲线法用于标准厚度试样的韧脆转变温度预测具有良好的精度, 但其预测的小尺寸试样的韧脆转变温度与实际韧脆转变温度区间相差较大. 随着温度的降低, 大、小尺寸试样的启裂点位置均不断靠近裂纹尖端且与试样断裂韧性呈非线性关系.Abstract: Reactor pressure vessel (RPV) is an important safety barrier of nuclear power plant, and the change of fracture toughness of RPV steel in ductile-to-brittle transition zone is an essential consideration in the safety evaluation of nuclear power plant. International standards such as ASTM standard usually recommend the Master Curve method to study the change of fracture toughness of RPV steel in ductile-to-brittle transition zone. The fracture toughness test of SA-508 steel was completed by using single edge-notched bending (SEB) and compact tension (CT) specimens of different sizes in the temperature range from room temperature to −100 °C. The fracture toughness transformation law of SA-508 steel in ductile-to-brittle transition zone was studied based on the Master Curve method, at the same time, the prediction accuracy of ductile-to-brittle transition temperature obtained by the Master Curve method based on the fracture specimens of different sizes is compared, and its failure characteristics were studied by analyzing the micromorphology of the fracture specimens. It is shown that the specimen configuration and geometry have significant influence on ductile-to-brittle transition behavior of RPV steel. The Master Curve method has good accuracy in predicting ductile-to-brittle transition temperature of standard thickness specimens, but the predicted ductile-to-brittle transition temperature of small size specimens is much different from the actual ductile-to-brittle transition zone. With the decrease of temperature, the crack initiation point of both large and small size specimens comes closer to the crack tip and has a nonlinear relationship with the fracture toughness of the specimens.
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Keywords:
- SA-508 steel /
- ductile to brittle transition /
- fracture toughness /
- master curve /
- small specimen
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引 言
反应堆压力容器(reactor pressure vessel, RPV)是核电站的一道重要的安全屏障, 容纳着高温、高压和强放射性的堆芯且运行时间长, 要求其全寿期内破漏零风险. RPV用钢为铁素体钢, 具有明显的韧脆转变现象和中子辐照脆化效应, 在压水堆运行环境下会出现断裂性能下降, 韧脆转变温度上升的现象, 增大了RPV失效破坏的风险. 为了防止反应堆压力容器在役期间, 尤其是在寿命终期材料受到较大剂量的中子辐照脆化后发生脆性断裂, 通常在反应堆中随堆放入辐照监督试样[1], 以定期取出检验分析, 了解材料受辐照后的性能变化, 保证反应堆的安全运行. 受辐照监督管空间的限制, RPV钢监督样品的尺寸和数量有限, 致使基于大尺寸试样的断裂性能评价面临极大困难. 小试样测试技术[2-9]可有效缓解辐照样品不足的问题. 用于辐照监督测试的小尺寸试样相比标准大尺寸试样的几何约束明显降低, 使得RPV材料趋于延性破坏. 温度、几何约束、辐照等多因素耦合作用使 RPV 材料的断裂失效行为变得更为复杂.
为了得到反应堆压力容器材料辐照前后在韧脆转变区的断裂韧性, 在断裂力学兴起之前, 通常用从低温到高温系列温度下的夏比冲击试验得到冲击功−温度曲线、纤维断面率−温度曲线和侧向膨胀量−温度曲线来表征材料的韧脆转变行为. 然而, 依据夏比冲击试验得到的相关参量与裂尖应力应变场不具直接相关性, 无法建立缺陷尺寸与断裂外载之间的关系, 用于反应堆压力容器的完整性评定的经验成分较大, 且具有一定的危险性. 为了满足RPV安全评估的需求, 自断裂力学发展起来后, 美国曾开展了大量辐照前后RPV材料在不同温度下的断裂韧性实验, 建立了基于参考无延性转变温度RTNDT的断裂失效评估曲线[10]. 该曲线是半经验性的, 用于反应堆压力容器钢的韧脆转变行为评价偏于保守, 且断裂韧性试验的平面应变要求和数据分散性使得试验所需试样尺寸较大, 数量较多, 辐照监督试样很难满足试验需求. 鉴于辐照监督管空间限制和材料性能的分散性现象, 参照脆性转变温度的断裂韧性−参考温度曲线方法在应用上有很大的限制, 采用少量试件得到韧脆转变区断裂韧性的方法受到了大量关注, 其中, Wallin提出的主曲线法得到了广泛的认可, 并被ASTM标准推荐.
Wallin[11-16]研究发现铁素体钢在韧脆转变区的临界应力强度因子KJC的概率分布满足如下式所示的Weibull模型
$$ {P_f} = 1 - \exp \left[ { - \frac{B}{{{B_0}}}{{\left( {\frac{{{K_{\text{I}}} - {K_{\min }}}}{{{K_0} - {K_{\min }}}}} \right)}^4}} \right] $$ (1) 式中, Pf表示当K≤KJC时的累积断裂失效概率, B为测试试样厚度, B0为设计参考试样厚度, KI为应力强度因子, K0为Weibull分布尺度参数, 对应于累积失效概率为63.2%的断裂韧性, Kmin是KI门槛值, 推荐取为20 MPa·m1/2. 由于铁素体钢的韧脆转变行为, 其KIC与试验温度T关系与断裂失效评估曲线[10]类似. 因此, Wallin认为K0值也符合类似的模型, 并且通过对多种铁素体钢试验结果的分析得到K0−T关系曲线
$$ {K_0} = 31 + 77\exp \left[ {0.019\left( {T - {T_0}} \right)} \right] $$ (2) 式中, T为试验温度, T0表示韧脆转变参考温度. 上述断裂韧性的分布规律均是基于2.54 cm (1 inch)标准厚度试样建立起来的. 鉴于辐照监督试样难以满足KIC测试的标准试样尺寸要求, 对于非标准试样, Wallin提出了其断裂韧性与标准厚度试样断裂韧性换算的公式
$$ {K_{{\text{I}}(1{\text{T}})}} = {K_{\min }} + \left( {{K_{{\text{I}}(t)}} - {K_{\min }}} \right){\left( {\frac{{{B_t}}}{{{B_{1{\text{T}}}}}}} \right)^{1/4}} $$ (3) 式中, KI(1T)为测试试样对应的标准1T (即2.54 cm厚度)试样断裂韧性值, B1T表示2.54 cm厚度, Bt为测试试样的实际厚度, KI(t)为测试试样的断裂韧性值. 将式(2)代入式(1)即得到不同失效概率Pf下的KI−T关系方程
$$\begin{split} & {K_{{\text{I}}({P_f})}} = {K_{\min }} + {\left[ {\ln \left( {\frac{1}{{1 - {P_f}}}} \right)} \right]^{1/4}} \cdot \\ &\qquad \left\{ {11 + 77\exp \left[ {0.019\left( {T - {T_0}} \right)} \right]} \right\} \end{split} $$ (4) 综上, Wallin[17]提出了可一定程度考虑断裂韧性数据分散性、尺寸效应和温度依赖性的主曲线法, 并对主曲线法所得结果的有效性进行了探讨. 该方法已被列入ASTM E1921标准[17]并广泛用于测定RPV钢的韧脆转变温度[18-23], 大幅减少了试验成本, 提高了断裂韧性下限值评估的准确性. 然而, 主曲线法也有其局限性, 其所用试样需满足高拘束度条件, 否则测得参考温度T0偏低. 张新平等[24]研究发现采用双边带侧槽的小尺寸R-CT试样的断裂韧性值比相同侧槽深度的夏比尺寸试样的测试值更加接近有效的断裂韧性值. 邓彩艳等[25]研究了面外拘束对韧脆转变温度区间的影响并分析了其变化规律, 发现钢材的韧脆转变温度曲线可以用Boltzmann函数来描述, 且发现试样厚度越大, 韧脆转变温度越高, 断裂性能下降的规律. 林赟等[26]基于主曲线法研究了国产反应堆压力容器钢的辐照韧性, 发现中子注量至1020 cm−2后, 其表现出较明显的脆化行为. 钟巍华等[27]总结了国内外小试样力学性能研究的进展, 并对小冲杆测试技术进行了探索. Zheng等[28]对韧脆转变温度与约束参数之间的关系进行研究, 发现两者间存在线性关系.
Beremin等[29]研究表明断裂过程区的微裂纹服从Weibull分布, 并以Weibull应力为驱动力, 通过Weibull应力可以预测材料的断裂失效概率. Margolin等[30-32]认为解理断裂失效应考虑微裂纹形核和微裂纹失稳两方面因素, 提出了Prometey局部断裂模型. Hohe等[33]则认为累积等效塑性应变、局部应力三轴度、最大主应力三者为控制断裂失效的主要参量并提出了相应的断裂韧性预测模型. 该系列断裂失效局部法模型的原理相近, 复杂程度不同, 且均需要借助复杂的有限元分析来实现, 在工程应用上有一定的难度.
本研究将以RPV钢为对象, 开展不同温度和几何尺寸条件下的断裂行为试验, 然后以单温度法及多温度主曲线法对试验结果进行分析, 并结合断口微观组织特征讨论温度及几何尺寸对RPV钢断裂韧性的影响, 为反应堆安全运行及延寿评估提供数据参考.
1. 材料与试验
采用RPV所用SA-508钢开展拉伸试验和断裂韧性试验. 拉伸试验采用标准圆棒试样, 试样工作段直径6 mm, 标距段长度为30 mm. 采用图1所示的不同尺寸CT和SEB试样完成不同温度下的断裂韧性试验. 图中B为厚度, a为机加工裂纹长度, 所有SEB试样的S与W之比设为4. 如表1所示, 本文设计了8类不同构形和尺寸试样以考察试样几何约束对SA-508钢韧脆转变温度的影响. 参照ASTM E1921标准采用1T CT试样(表1中的H类试样)完成−60 °C下的断裂试验, 采用单温度法分析得到其韧脆转变温度. 对于表1中的其他类型试样, 以试样实际断裂形式为依据开展不同温度下的断裂试验, 以获得各类试样的实际韧脆转变温度区间.
表 1 断裂试样尺寸及数量Table 1. Fracture specimen size and numberType Class Size Number W/mm B/mm a/W mini-SEB A 4 4 0.2 9 B 4 4 0.5 10 SEB C 10 10 0.2 9 D 10 10 0.5 10 E 20 10 0.2 12 F 20 10 0.5 11 CT G 25.4 12.7 0.4 9 H 50.8 25.4 0.4 16 低温试验均在低温环境箱中进行, 通过液氮空冷实现低温加载, 温度控制误差低于 ±3 °C. 由于试验空间和试验需求不同, 拉伸试样、mini-SEB试样、SEB及CT试样所用环境箱、试验机有所不同, 其中mini-SEB试样的断裂韧性试验在如图2(a)所示的DL-DFT5k电子式疲劳试验机上完成, 其余试样在如图2(b)所示的MTS809电液伺服材料试验机完成. 图3给出了低温下拉伸试验及断裂试验的试验照片.
当试样为脆性断裂时, 采用ASTM E1921[17]规定的方法确定启裂韧度. 对于韧性断裂的试样, 其启裂韧度可以通过ASTM E1820[34]规定的方法确定. 对于裂纹扩展后脆断的试样, 其临界断裂韧性JC取脆断时的J积分值. 在获得临界J积分JC后, 断裂韧性KJC可由下式得到
$$ {K_{J{\text{C}}}} = \sqrt {\frac{{{J_{\text{C}}}E}}{{1 - {\nu ^2}}}} $$ (5) 式中, E为弹性模量, ν为泊松比.
2. 结果与讨论
2.1 拉伸试验结果
SA-508钢在不同温度下拉伸应力−应变试验曲线如图4所示. 图5分别展示了弹性模量、屈服强度和抗拉强度随温度变化的情况. 从图中可以看出, 随温度降低屈服强度和抗拉强度逐渐增大, 而温度对弹性模量的影响不大.
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图 5 不同温度下拉伸力学性能变化Figure 5. Tensile mechanical properties change at different temperatures2.2 断裂试验结果
图6~图8展示了不同类别试样在不同温度下的载荷P与位移V之间的关系. 从图中可以看出, 随着温度的降低, 不同几何尺寸的试样均表现出由韧转脆的特征, 试样破坏的载荷上升. 表2~表9给出了不同构形试样在不同试验温度下的断裂韧性KJC和断裂形式. 可以看到, 随着温度的升高, 试样呈现脆性断裂向延性破坏转变的趋势. 在韧脆转变温度区间内, 脆性断裂与延性破坏相互竞争. 试验温度处于韧脆转变温度区下平台时, 断裂试样均表现为几乎无延性的脆性断裂. 当温度升到韧脆转变温度区上平台时, 断裂试样均表现为韧性断裂. 在同一温度下, 不同构形试样的断裂形式也可能不同, 大尺寸试样发生脆断时小尺寸试样可能表现为韧性断裂, 几何尺寸对于材料的断裂行为有着不可忽视的影响.
表 2 A类试样的断裂结果Table 2. Fracture results of class AClass A T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 359.27 288.74 ductile crack growth 2# 20 369.53 296.87 ductile crack growth 3# −10 358.80 288.37 ductile crack growth 4# −40 347.33 279.28 ductile crack growth 5# −40 350.69 281.95 brittle fracture with ductile crack growth 6# −50 350.26 281.61 brittle fracture with ductile crack growth 7# −60 323.94 260.76 brittle fracture with ductile crack growth 8# −70 171.24 139.80 brittle fracture with plastic deformation 9# −90 122.19 100.95 brittle fracture with plastic deformation 表 3 B类试样的断裂结果Table 3. Fracture results of class BClass B T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 312.63 204.35 ductile crack growth 2# 20 310.01 202.69 ductile crack growth 3# −10 256.44 168.94 ductile crack growth 4# −30 279.44 183.43 ductile crack growth 5# −40 293.37 192.21 ductile crack growth 6# −40 322.76 210.72 brittle fracture with ductile crack growth 7# −50 136.36 93.30 brittle fracture with ductile crack growth 8# −50 284.38 186.54 brittle fracture with plastic deformation 9# −60 194.78 130.10 brittle fracture with plastic deformation 10# −70 138.74 94.80 brittle fracture with plastic deformation 表 4 C类试样的断裂结果Table 4. Fracture results of class CClass C T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 394.63 316.75 ductile crack growth 2# 20 472.06 378.08 ductile crack growth 3# −10 486.56 389.57 ductile crack growth 4# −20 467.42 374.41 brittle fracture with ductile crack growth 5# −30 469.35 375.94 brittle fracture with ductile crack growth 6# −50 291.16 234.79 brittle fracture with plastic deformation 7# −70 100.30 83.60 brittle fracture with plastic deformation 8# −90 108.72 90.28 brittle fracture with plastic deformation 9# −100 86.00 72.28 fully brittle fracture 表 5 D类试样的断裂结果Table 5. Fracture results of class DClass D T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 353.36 284.06 ductile crack growth 2# 20 337.11 271.19 ductile crack growth 3# −10 355.62 285.85 ductile crack growth 4# −10 404.53 324.59 brittle fracture with ductile crack growth 5# −20 356.57 286.60 ductile crack growth 6# −30 355.10 285.44 brittle fracture with ductile crack growth 7# −50 307.47 247.71 brittle fracture with ductile crack growth 8# −70 89.71 75.22 brittle fracture with plastic deformation 9# −80 72.75 61.78 brittle fracture with plastic deformation 10# −90 51.92 45.29 fully brittle fracture 表 6 E类试样的断裂结果Table 6. Fracture results of class EClass E T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 436.81 350.17 ductile crack growth 2# 20 407.45 326.91 ductile crack growth 3# −10 490.18 392.44 ductile crack growth 4# −20 518.99 415.26 brittle fracture with ductile crack growth 5# −30 491.71 393.65 brittle fracture with ductile crack growth 6# −40 399.97 320.98 brittle fracture with ductile crack growth 7# −40 169.38 138.33 brittle fracture with plastic deformation 8# −50 256.22 207.11 brittle fracture with plastic deformation 9# −60 95.73 79.99 brittle fracture with plastic deformation 10# −60 153.25 125.55 brittle fracture with plastic deformation 11# −70 149.19 122.34 brittle fracture with plastic deformation 12# −80 72.75 61.78 fully brittle fracture 表 7 F类试样的断裂结果Table 7. Fracture results of class FClass F T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 362.18 291.05 ductile crack growth 2# 20 386.85 310.59 ductile crack growth 3# −10 432.62 346.85 ductile crack growth 4# −10 363.01 291.70 ductile crack growth 5# −20 424.05 340.05 brittle fracture with ductile crack growth 6# −30 312.49 251.68 brittle fracture with plastic deformation 7# −40 276.51 223.19 brittle fracture with plastic deformation 8# −50 129.70 106.90 brittle fracture with plastic deformation 9# −60 60.55 52.12 fully brittle fracture 10# −60 99.83 83.23 fully brittle fracture 11# −80 80.09 67.60 fully brittle fracture 表 8 G类试样的断裂结果Table 8. Fracture results of class GClass G T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 519.23 439.80 ductile crack growth 2# 20 490.52 415.66 ductile crack growth 3# 0 523.89 443.72 ductile crack growth 4# −10 465.84 394.91 brittle fracture with ductile crack growth 5# −30 168.15 144.58 brittle fracture with plastic deformation 6# −40 148.68 128.20 brittle fracture with plastic deformation 7# −50 198.21 169.85 brittle fracture with plastic deformation 8# −70 171.61 147.49 brittle fracture with plastic deformation 9# −90 107.76 93.80 fully brittle fracture 表 9 H类试样的断裂结果Table 9. Fracture results of class HClass H T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 589.267 ductile crack growth 2# 20 591.64 ductile crack growth 3# 10 541.44 brittle fracture with ductile crack growth 4# 0 402.85 brittle fracture with plastic deformation 5# −40 164.45 brittle fracture with plastic deformation 6# −50 134.72 brittle fracture with plastic deformation 7# −60 95.73 fully brittle fracture 8# −90 47.76 fully brittle fracture 9# −60 98.50 fully brittle fracture 10# −60 117.47 fully brittle fracture 11# −60 126.56 fully brittle fracture 12# −60 139.41 fully brittle fracture 13# −60 104.67 fully brittle fracture 14# −60 128.43 fully brittle fracture 15# −60 134.40 fully brittle fracture 16# −60 98.00 fully brittle fracture 断裂韧性KJC与试验温度T之间的关系如图9所示. 对比分析可知, 在其他条件一致的情况下, SA-508钢的断裂韧性值KJC随宽度W增大而逐渐减小, 随温度T降低而逐渐增大, 随裂纹长度比a/W的增大而逐渐减小, 即高约束度(即裂纹越长)和低温都会使SA-508钢表现出更低的断裂韧性.
参照ASTM E1921的单温度法对1T CT试样(H类) −60 °C下的断裂韧性进行分析得到主曲线图和韧脆转变参考温度, 分别如图9和表10所示. 同时采用ASTM E1921的多温度法对各类试样在不同温度下的断裂韧性进行分析可以得到相应的韧脆转变参考温度T0, 结果见表10. 进一步地, 采用多温度法对所有试样的断裂韧性进行分析得到主曲线图和韧脆转变参考温度, 分别如图9和表10所示. 可以看到, 标准厚度试样所得韧脆转变温度明显高于其他小尺寸试样结果. 标准厚度试样得到的韧脆转变温度与其实际韧脆转变温度区间较为接近, 而由其他小尺寸试样得到韧脆转变温度与其实际韧脆转变温度区间相差较大. 从表10结果可以看到, 主曲线法对于标准厚度试样的韧脆转变温度预测具有良好的精度, 但其用于小尺寸试样的韧脆转变温度预测精度非常有限.
表 10 各构型试样的韧脆转变参考温度(单位: °C)Table 10. Ductile-to-brittle transition temperature of specimens with various configuration (unit: °C)Class T0 Validity Ductile-to-brittle
transition regionA −104 no −95 ~ −85 B −76 yes −70 ~ −50 C −86 no −70 ~ −50 D −86 no −70 ~ −50 E −88 yes −80 ~ −60 F −73 yes −60 ~ −50 G −81 no −90 ~ −70 H −75 yes −70 ~ −50 all −88 yes single temperature method (H) −69 yes 2.3 试样断口宏微观组织特征
图10给出了几个典型温度下E类断裂试样的断口照片, 其中图10(a)为典型韧性断裂断口, 图10(b)在裂纹发生一定程度延性扩展后脆断, 图10(c)在产生一定的塑性变形后脆断, 而图10(d)为完全脆性断裂断口. 其余试样相同断裂类型下均呈现出此类特征. 通过宏观观察可以发现: 随着温度的降低, 试样的塑性变形不断减小, 即由韧性逐渐转变为脆性.
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图 10 部分典型E类断裂试样的断口扫描电镜图Figure 10. Sem images of some typical class E fracture specimens如图10所示, 对几个特定温度下E类断裂试样的断口进行电镜扫描后可发现试样断口和裂纹尖端存在撕裂带和较深的韧窝, 且塑性纤维带和韧窝沿钝化的裂纹尖端分布. 图10(c)可看到, 试样在−40 °C时的断口存在少量韧窝与解理断裂交界面, 图10(d)则显示了试样在−80 °C时的脆断断口上出现明显的舌花状解理断裂特征. 当试验温度处于韧脆转变温度区间下平台, 几乎没有延性扩展区, 试样表现出明显的脆性断裂, 随着温度增高, 试样断口处的延性扩展区逐渐变大, 试样逐渐表现出韧性断裂的特征, 直至完全韧性断裂.
图11给出了脆断试样的启裂点和启裂方向示意图. 通过电镜扫描试样断口可以量出不同试样在不同温度下的启裂点位置(与断口边缘的距离).
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图 11 脆断试样的启裂点和起裂方向Figure 11. Crack initiation point and direction of brittle fracture specimens通过测量部分试样启裂点与断口边缘的垂直距离, 得到了其启裂点位置y与启裂韧度KJC的关系曲线, 如图12所示. 结果表明随着启裂韧度的降低, 试样启裂点位置逐渐接近裂尖, 且呈现出一定的非线性规律.
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图 12 试样启裂点位置与启裂韧度关系Figure 12. Relationship between crack initiation point and crack initiation toughness3. 结 论
(1) SA-508钢在不同温度下的拉伸试验结果表明, 屈服强度和抗拉强度随温度降低逐渐增大, 而温度变化对弹性模量的影响较小.
(2)不同尺寸SEB和CT试样在不同温度下的断裂试验结果表明, 在其他条件一致的情况下, SA-508钢的断裂韧性KJC随尺寸增大而逐渐减小, 随温度降低而逐渐减小. 通过主曲线法得到了各尺寸试样的韧脆转变参考温度T0. 标准厚度试样所得韧脆转变温度明显高于其他小尺寸试样结果. 主曲线法对于标准厚度试样的韧脆转变温度预测具有良好的精度, 但其用于小尺寸试样的韧脆转变温度预测精度非常有限.
(3)对试验后的试样断口进行了宏微观分析, 结果表明, 随着温度的降低, 脆性断裂试样的启裂点位置逐渐靠近断口边缘, 且启裂点位置与断裂韧性KJC呈现出一定的非线性规律.
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图 5 不同温度下拉伸力学性能变化
Figure 5. Tensile mechanical properties change at different temperatures
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图 10 部分典型E类断裂试样的断口扫描电镜图
Figure 10. Sem images of some typical class E fracture specimens
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图 11 脆断试样的启裂点和起裂方向
Figure 11. Crack initiation point and direction of brittle fracture specimens
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图 12 试样启裂点位置与启裂韧度关系
Figure 12. Relationship between crack initiation point and crack initiation toughness
表 1 断裂试样尺寸及数量
Table 1 Fracture specimen size and number
Type Class Size Number W/mm B/mm a/W mini-SEB A 4 4 0.2 9 B 4 4 0.5 10 SEB C 10 10 0.2 9 D 10 10 0.5 10 E 20 10 0.2 12 F 20 10 0.5 11 CT G 25.4 12.7 0.4 9 H 50.8 25.4 0.4 16 
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表 2 A类试样的断裂结果
Table 2 Fracture results of class A
Class A T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 359.27 288.74 ductile crack growth 2# 20 369.53 296.87 ductile crack growth 3# −10 358.80 288.37 ductile crack growth 4# −40 347.33 279.28 ductile crack growth 5# −40 350.69 281.95 brittle fracture with ductile crack growth 6# −50 350.26 281.61 brittle fracture with ductile crack growth 7# −60 323.94 260.76 brittle fracture with ductile crack growth 8# −70 171.24 139.80 brittle fracture with plastic deformation 9# −90 122.19 100.95 brittle fracture with plastic deformation
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表 3 B类试样的断裂结果
Table 3 Fracture results of class B
Class B T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 312.63 204.35 ductile crack growth 2# 20 310.01 202.69 ductile crack growth 3# −10 256.44 168.94 ductile crack growth 4# −30 279.44 183.43 ductile crack growth 5# −40 293.37 192.21 ductile crack growth 6# −40 322.76 210.72 brittle fracture with ductile crack growth 7# −50 136.36 93.30 brittle fracture with ductile crack growth 8# −50 284.38 186.54 brittle fracture with plastic deformation 9# −60 194.78 130.10 brittle fracture with plastic deformation 10# −70 138.74 94.80 brittle fracture with plastic deformation
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表 4 C类试样的断裂结果
Table 4 Fracture results of class C
Class C T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 394.63 316.75 ductile crack growth 2# 20 472.06 378.08 ductile crack growth 3# −10 486.56 389.57 ductile crack growth 4# −20 467.42 374.41 brittle fracture with ductile crack growth 5# −30 469.35 375.94 brittle fracture with ductile crack growth 6# −50 291.16 234.79 brittle fracture with plastic deformation 7# −70 100.30 83.60 brittle fracture with plastic deformation 8# −90 108.72 90.28 brittle fracture with plastic deformation 9# −100 86.00 72.28 fully brittle fracture
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表 5 D类试样的断裂结果
Table 5 Fracture results of class D
Class D T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 353.36 284.06 ductile crack growth 2# 20 337.11 271.19 ductile crack growth 3# −10 355.62 285.85 ductile crack growth 4# −10 404.53 324.59 brittle fracture with ductile crack growth 5# −20 356.57 286.60 ductile crack growth 6# −30 355.10 285.44 brittle fracture with ductile crack growth 7# −50 307.47 247.71 brittle fracture with ductile crack growth 8# −70 89.71 75.22 brittle fracture with plastic deformation 9# −80 72.75 61.78 brittle fracture with plastic deformation 10# −90 51.92 45.29 fully brittle fracture
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表 6 E类试样的断裂结果
Table 6 Fracture results of class E
Class E T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 436.81 350.17 ductile crack growth 2# 20 407.45 326.91 ductile crack growth 3# −10 490.18 392.44 ductile crack growth 4# −20 518.99 415.26 brittle fracture with ductile crack growth 5# −30 491.71 393.65 brittle fracture with ductile crack growth 6# −40 399.97 320.98 brittle fracture with ductile crack growth 7# −40 169.38 138.33 brittle fracture with plastic deformation 8# −50 256.22 207.11 brittle fracture with plastic deformation 9# −60 95.73 79.99 brittle fracture with plastic deformation 10# −60 153.25 125.55 brittle fracture with plastic deformation 11# −70 149.19 122.34 brittle fracture with plastic deformation 12# −80 72.75 61.78 fully brittle fracture
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表 7 F类试样的断裂结果
Table 7 Fracture results of class F
Class F T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 362.18 291.05 ductile crack growth 2# 20 386.85 310.59 ductile crack growth 3# −10 432.62 346.85 ductile crack growth 4# −10 363.01 291.70 ductile crack growth 5# −20 424.05 340.05 brittle fracture with ductile crack growth 6# −30 312.49 251.68 brittle fracture with plastic deformation 7# −40 276.51 223.19 brittle fracture with plastic deformation 8# −50 129.70 106.90 brittle fracture with plastic deformation 9# −60 60.55 52.12 fully brittle fracture 10# −60 99.83 83.23 fully brittle fracture 11# −80 80.09 67.60 fully brittle fracture
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表 8 G类试样的断裂结果
Table 8 Fracture results of class G
Class G T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 519.23 439.80 ductile crack growth 2# 20 490.52 415.66 ductile crack growth 3# 0 523.89 443.72 ductile crack growth 4# −10 465.84 394.91 brittle fracture with ductile crack growth 5# −30 168.15 144.58 brittle fracture with plastic deformation 6# −40 148.68 128.20 brittle fracture with plastic deformation 7# −50 198.21 169.85 brittle fracture with plastic deformation 8# −70 171.61 147.49 brittle fracture with plastic deformation 9# −90 107.76 93.80 fully brittle fracture
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表 9 H类试样的断裂结果
Table 9 Fracture results of class H
Class H T/°C KJC/
(MPa·m1/2)KJC(1T)/
(MPa·m1/2)Fracture mode 1# 20 589.267 ductile crack growth 2# 20 591.64 ductile crack growth 3# 10 541.44 brittle fracture with ductile crack growth 4# 0 402.85 brittle fracture with plastic deformation 5# −40 164.45 brittle fracture with plastic deformation 6# −50 134.72 brittle fracture with plastic deformation 7# −60 95.73 fully brittle fracture 8# −90 47.76 fully brittle fracture 9# −60 98.50 fully brittle fracture 10# −60 117.47 fully brittle fracture 11# −60 126.56 fully brittle fracture 12# −60 139.41 fully brittle fracture 13# −60 104.67 fully brittle fracture 14# −60 128.43 fully brittle fracture 15# −60 134.40 fully brittle fracture 16# −60 98.00 fully brittle fracture
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表 10 各构型试样的韧脆转变参考温度(单位: °C)
Table 10 Ductile-to-brittle transition temperature of specimens with various configuration (unit: °C)
Class T0 Validity Ductile-to-brittle
transition regionA −104 no −95 ~ −85 B −76 yes −70 ~ −50 C −86 no −70 ~ −50 D −86 no −70 ~ −50 E −88 yes −80 ~ −60 F −73 yes −60 ~ −50 G −81 no −90 ~ −70 H −75 yes −70 ~ −50 all −88 yes single temperature method (H) −69 yes
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