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材料研究学报, 2024, 38(3): 197-207 DOI: 10.11901/1005.3093.2023.205

研究论文

不同温度回火低合金钢缺口拉伸性能的预测

齐恺力1,3, 胡德江2, 高崇3, 刘峰1,4, 庞建超,3, 邵琛玮3, 杨梦起2, 李守新3, 张哲峰3

1.辽宁石油化工大学机械工程学院 抚顺 113001

2.南方电网调峰调频发电有限公司检修试验分公司 广州 511400

3.中国科学院金属研究所 师昌绪先进材料创新中心 沈阳 110016

4.季华实验室 佛山 528200

Notch Tensile Properties Prediction of Low-alloy Steel Processed by Different Tempering Temperatures

QI Kaili1,3, HU Dejiang2, GAO Chong3, LIU Feng1,4, PANG Jianchao,3, SHAO Chenwei3, YANG Mengqi2, LI Shouxin3, ZHANG Zhefeng3

1.School of Mechanical Engineering, Liaoning Petrochemical University, Fushun 113001, China

2.Maintenance and Test Branch, China Southern Power Grid Power Generation Co., Ltd., Guangzhou 511400, China

3.Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

4.Ji Hua Laboratory, Foshan 528200, China

通讯作者: 庞建超，副研究员，jcpang@imr.ac.cn，研究方向为材料疲劳与断裂

责任编辑: 吴岩

收稿日期: 2023-03-30   修回日期: 2023-05-22  

基金资助: 国家重点研发计划(2022YFB3708200)
南方电网调峰调频发电有限公司检修试验分公司科技项目(022200KK52180006)

Corresponding authors: PANG Jianchao, Tel:(024)83978879, E-mail:jcpang@imr.ac.cn

Received: 2023-03-30   Revised: 2023-05-22  

Fund supported: National Key Research and Development Program of China(2022YFB3708200)
Science and Technology Project of Maintenance and Testing Branch, China Southern Power Grid Power Generation Co. Ltd(022200KK52180006)

作者简介 About authors

齐恺力，男，1995年生，硕士生

摘要

使用电子背散射衍射技术和扫描电镜观察在不同温度回火的35CrMo低合金钢的微观组织和应力集中系数不同的缺口试样拉伸断裂形貌，研究了回火温度对其拉伸性能、损伤机制和力学性能的影响。结果表明：淬火态和回火温度为150~200℃的35CrMo钢，其显微组织均为回火板条状马氏体；在400℃回火的试样其组织为均匀的屈氏体组织；缺口试样的断裂形式均为韧性和脆性混合断裂。两种应力集中系数(Kt = 3，5)的试样其缺口抗拉强度随回火温度变化的趋势相同，随着回火温度的升高缺口抗拉强度先升高后降低。Kt = 3回火温度为150℃的试样其抗拉强度最高为2626 MPa；Kt = 5回火温度为200℃的试样其抗拉强最高为2450 MPa。缺口的敏感度都> 1，即在不同温度回火后都发生缺口强化效应，且缺口敏感性随着回火温度的升高有下降的趋势。随着应力集中系数的增大在不同温度回火的试样其缺口强化效果都呈现出先上升后下降的趋势，在400℃回火的缺口试样其强化效率最显著。基于硬度与缺口抗拉强度的关系提出一种快速预测缺口抗拉强度的方法，预测结果的误差< 8%。

关键词： 金属材料; 低合金钢; 回火温度; 微观组织; 缺口拉伸预测

Abstract

The microstructure and notch tensile fracture morphologies at different stress concentration factors of the low alloy steel 35CrMo for the head cover bolts of the pump turbine of a storage power station were investigated by electron backscatter diffraction microscopy and scanning electron microscopy. The effects of tempering temperature on the relationship between tensile properties, damage mechanism and mechanical properties of 35CrMo steel were studied. The results show that the microstructures of tempered state at 150~200oC and quenched state are composed of lath martensite. After tempering at 400oC, the microstructure of tempered troostite is more uniform. The final fracture of the notched specimens is a mixture of ductile and brittle fracture. At the two stress concentration factors (Kt = 3, 5), the notch tensile strength has the same changing trend with the tempering temperature. With the tempering temperature increasing, the notch tensile strength first increases and then decreases. For Kt = 3, the highest tensile strength is 2626 MPa at tempering temperature of 150oC; when Kt = 5, the highest tensile strength is 2450 MPa at tempering temperature of 200oC. Notch sensitivity ratio (NSR) is greater than 1, that is, notch strengthening effect occurs after different tempering temperatures, and notch sensitivity tends to decrease with the increase of tempering temperature. With the increase of stress concentration factor, the notch strengthening effect of specimens treated at different tempering temperatures shows an increasing trend first and then a decreasing trend, and the notch strengthening efficiency was the most obvious when tempering at 400oC. Finally, based on the relationship between hardness and notch tensile strength, a fast prediction method of notch tensile strength was proposed, and the prediction error was less than 8%.

Keywords： metallic materials; low-alloy steel; tempering temperature; microstructure; notch tensile prediction

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本文引用格式

齐恺力, 胡德江, 高崇, 刘峰, 庞建超, 邵琛玮, 杨梦起, 李守新, 张哲峰. 不同温度回火低合金钢缺口拉伸性能的预测[J]. 材料研究学报, 2024, 38(3): 197-207 DOI:10.11901/1005.3093.2023.205

QI Kaili, HU Dejiang, GAO Chong, LIU Feng, PANG Jianchao, SHAO Chenwei, YANG Mengqi, LI Shouxin, ZHANG Zhefeng. Notch Tensile Properties Prediction of Low-alloy Steel Processed by Different Tempering Temperatures[J]. Chinese Journal of Materials Research, 2024, 38(3): 197-207 DOI:10.11901/1005.3093.2023.205

高强度螺栓的应用较为广泛[1]。随着工程机械和能源设备的发展，对螺栓的性能提出了更高的要求。抽水蓄能机组启停频繁且运行工况复杂，其顶盖与座环的连接螺栓承受多重载荷[2]，一旦损坏后果极为严重。高强度螺栓的可靠性，影响整个设备的安全服役。

用于制造高强度螺栓的材料，有低合金钢、非调质钢、硼钢和低碳马氏体钢[3]。低合金钢的综合性能优异(其强度为700~1200 MPa)且加工工艺简单，常用于制造高强度螺栓[4,5]。低合金钢的合金元素含量低于5%，根据其成分可分为Cr-Mo[6]、Cr-Mo-V[7]和Ni-Cr-Mo[8]等系列。

材料的热处理，可改变其微观结构和力学性能[9]。进行不同制度的热处理，可制备出不同性能的高强度螺栓用钢[10]。目前，关于高强钢的合金成分、轧制工艺以及热处理工艺对组织和性能的影响的研究，取得了较大进展[11~14]。为了满足服役要求，螺栓用低合金钢需要调质处理，即淬火+高温回火。但是，低合金钢的碳和合金元素的含量较高，硬度和变形抗力大，需要进行球化(软化)退火预处理。特别是，在提高材料强度的同时还需要保持一定的塑韧性[15,16]。目前，用有限元模拟，研究螺栓在拉伸载荷下的力学性能，并探索螺栓连接拉伸断裂失效模式和机理[17~19]。但是，大部分研究侧重螺栓连接的整体结构，缺少对螺栓本体材料的优化和探索。

螺栓的螺纹使其宏观几何产生不连续性，通常看作多缺口零件。构件的几何不连续引起应力集中，在应力集中处发生失效[20~22]。另外，装配螺栓时需要施加较大的预紧力，而检测其引起的螺纹早期故障极为困难。蓄能机组中的水泵水轮机顶盖螺栓其公称直径可达100 mm以上，难以直接进行全尺寸螺栓构件的拉伸试验。因此，了解螺栓在拉伸载荷下的失效行为至关重要[23]。研究蓄能机组顶盖螺栓材料不同热处理状态的缺口拉伸性能，有利于螺栓连接顶盖与座环及其整个机组的安全可靠性设计。另外，根据特定应力集中系数设计不同尺寸的缺口弧度也较为困难，而且特定非标件小尺寸缺口加工精度难以保证。因此，建立一种缺口抗拉强度的预测方法，可为优化小尺寸缺口试样的拉伸性能提供参考。鉴于此，本文研究蓄能机组顶盖螺栓钢35CrMo在不同温度回火后各应力集中系数的缺口拉伸性能，根据拉伸断裂形貌研究回火温度及应力集中系数对拉伸性能和断裂机制的影响，以探讨预测缺口抗拉强度的方法。

1 实验方法

实验用材料为低合金结构钢35CrMo，用电感耦合等离子体发射光谱法(ICP)测定的化学成分，列于表1。服役态螺栓材料为调质态(Quenched and tempered state，QT)，热处理制度为淬火 + 高温回火，即在860℃保温60 min后油淬冷却至室温，然后在650℃回火120 min后空冷至室温。

表1   35CrMo钢的化学成分

Table 1  Chemical composition of 35CrMo steel (mass fraction, %)

CSiCrMoMnPSFe
0.350.351.100.200.800.0040.0022Bal.

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用电火花线切割机在尺寸为M110 mm × 4 mm的服役态高强度螺栓上沿轴向取样，取样棒料的直径为15 mm，长度为70 mm。为获得更宽的强度范围以选取缺口拉伸性能最佳的热处理工艺，对服役态35CrMo钢进行四种制度的热处理，其工艺在图1中给出。为了得到均匀的奥氏体化组织，先将实验用钢棒料加热到860℃保温30 min，随后油淬冷却至室温。然后分别在150、200和400℃回火，保温时间为90 min，随后空冷到室温。将未回火和在不同温度回火的试样，分别标记为Q、QT150、QT200和QT400。

图1

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图1   顶盖螺栓用35CrMo钢的热处理工艺示意图

Fig.1   Schematic diagram of heat treatment process for the head cover bolt of 35CrMo steel


根据以应力集中系数作为等效参量的螺栓模拟构件模型，可将螺栓螺纹部分简化为圆环缺口模型[24]。据此将淬火、回火和未回火的棒料加工成缺口直径为5 mm的标准拉伸试样(HB5214-96)，长度为64 mm。由于不同尺寸螺栓螺纹的底角半径不同，取两种缺口底角半径为0.25 mm和0.08 mm，其拉伸试样的理论应力集中系数分别为Kt = 3和Kt =5。不同应力集中系数缺口拉伸试样的尺寸，在图2中给出。

图2

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图2   缺口拉伸试样的尺寸

Fig.2   Dimension of notched tensile specimen (a) Kt = 3; (b) Kt = 5 (unit: mm)


根据国家标准GB/T 228-2021在型号为Instron 5982万能实验机上进行常规室温拉伸实验，拉伸速率为1 × 10-3 s-1，拉伸速度为1.8 mm/min。用扫描电子显微镜(SEM)JSM-6510观察断口形貌。每种试样测量三次，取抗拉强度、断面收缩率等数据的平均值。用超景深三维体式显微镜(型号为KEYENCE VHX-1000E)测量断面收缩率。断面收缩率(Z)为

Z=F0-FF0×100%

其中F为拉伸后的横截面积；F0为拉伸前的横截面积。

用全自动显微硬度测试系统LECO AMH43测试样品截面的显微硬度，载荷为4.9 N，保载时长15 s。根据国家标准GB/T 4340.1-2009中有关金属材料的维氏硬度实验测试试样的显微硬度。选取每个样品上分散的10个位置进行测量，取其结果的平均值。

用电子背散射衍射显微技术(EBSD)ZEISS SIGMA 500型场发射扫描电子显微镜观察试样截面的微观组织。用EBSD微观组织观察时，截取尺寸为8 mm × 8 mm × 2 mm的方块试样并将其研磨和机械抛光，然后在HClO4∶CH3(CH2)3OH∶CH3OH = 1∶7∶12溶液中进行室温电解腐蚀以去除表面应力。用型号为JSM-6510的扫描电子显微镜(SEM)观察拉伸断口的形貌，加速电压为20 kV，工作距离为18~22 μm。

2 实验结果

2.1 试样的显微组织

图3给出了在不同温度回火后35CrMo钢的EBSD图像。可以看出，Q试样的组织为大量板条马氏体和部分残余奥氏体(图3a)。QT150和QT200试样的微观结构由板状回火马氏体和一些残余奥氏体组成(图3b，c)。而QT400试样的微观结构为均匀的回火屈氏体(图3d)。

图3

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图3   不同回火温度35CrMo钢的EBSD结构特征

Fig.3   EBSD microstructures of 35CrMo steel processed at different tempered temperatures (a) Q; (b) QT150; (c) QT200; (d) QT400


2.2 缺口拉伸性能

在不同温度回火的35CrMo钢，其应力集中系数不同的缺口试样的抗拉强度，列于如表2。图4给出了在不同温度回火的35CrMo钢缺口试样(Kt = 3和5)的拉伸应力-位移曲线。可以看出，应力集中系数Kt = 3的QT150其抗拉强度最高为2626 MPa，应力集中系数Kt = 5的QT200其抗拉强度最高为2450 MPa。图5给出了回火温度对材料缺口抗拉强度的影响。可以看出，随着回火温度的提高缺口抗拉强度先升高后降低，应力集中系数的增大使抗拉强度有所下降。断面收缩率在表3中给出，可见在不同温度回火的缺口拉伸试样其断面收缩率为5%~10%。在400℃回火的试样其断面收缩率最大，表明在回火400℃的试样塑性最大。图6给出了缺口抗拉强度和断面收缩率的关系，随着缺口抗拉强度的下降，断面收缩率增大。这个结果，与其它金属强度与延性的反比关系相同[25]。

表2   不同回火温度35CrMo钢的缺口抗拉强度

Table 2  Notch tensile strength of 35CrMo steel processed by different tempering temperatures (MPa)

Heat-treatment proceduresQQT150QT200QT400
Kt = 32337262625092213
Kt = 52164230524502142

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图4

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图4   不同回火温度35CrMo钢缺口试样的拉伸应力-位移曲线

Fig.4   Tensile stress-displacement curves for notched specimens of 35CrMo steel under different tempering temperatures (a) Kt = 3; (b) Kt = 5


图5

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图5   回火温度对35CrMo钢缺口抗拉强度的影响

Fig.5   Effect of tempering temperature on notch tensile strength of 35CrMo steel


表3   不同回火温度35CrMo钢的缺口拉伸断面收缩率

Table 3  Percentage reduction of area of 35CrMo steel processed by different tempering temperatures (%)

Heat-treatment proceduresQQT150QT200QT400
Kt = 35.556.918.309.61
Kt = 55.258.126.588.92

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图6

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图6   缺口抗拉强度与断面收缩率的关系

Fig.6   Relationship between notch tensile strength and contraction percentage


3 讨论

3.1 缺口拉伸性能

金属材料试样上的缺口，使其塑性降低和脆性增大。金属材料试样上的缺口产生三向应力状态和应力应变集中，从而使材料变脆，称为缺口敏感性。为了量化和比较缺口对各种材料拉伸性能的影响，用缺口试样的抗拉强度σbN与等截面尺寸光滑试样的抗拉强度σb之比表征材料的缺口敏感性[20]，记为NSR (Notch sensitivity ratio)

NSR=σbNσb

NSR < 1，表明材料对缺口敏感，即缺口使材料的抗拉强度明显降低[26]。NSR越大表明材料的缺口敏感性越低，NSR越小表明材料的缺口敏感性越高。脆性材料如铸铁、高碳钢的NSR总是< 1，表明缺口根部尚未发生明显塑性变形就已经断裂，这种材料对缺口极为敏感。高强度材料的NSR一般也< 1。NSR ≥ 1，表明材料对缺口不敏感，即缺口对材料抗拉强度的影响较小。塑性材料的NSR一般> 1。NSR > 1，表明已经发生缺口强化[27]。

Yang等[28]预测不同热处理35CrMo钢的高周疲劳行为及疲劳强度时，对在不同温度回火(其热处理制度与本文的缺口拉伸试样热处理制度相同)的光滑试样进行拉伸实验。在不同温度回火的35CrMo钢光滑试样的拉伸性能，列于表4。对在不同温度回火的两种应力集中系数缺口试样拉伸性能与光滑试样(Kt = 1)的拉伸性能进行了对比。结果表明，在不同温度回火的缺口试样其抗拉强度比光滑试样显著提高。其原因是，光滑试样的单向应力状态转变为缺口试样的多向应力状态，使抗拉强度提高。对于塑性较好的材料，缺口使其屈服强度或抗拉强度提高但是塑性降低，即所谓“缺口强化”。35CrMo钢出现了明显的“缺口强化效应”，而且缺口的存在约束了塑性变形而使拉伸位移减小。

表4   35CrMo钢在不同温度回火后的拉伸性能[28]

Table 4  Tensile properties for 35CrMo steel after tempering at different temperature[28]

Sampleσb / MPaσy / MPaZ / %A / %
Q1977138033.2010.80
QT1501952136442.7811.00
QT2001891148747.6612.05
QT4001566135251.4812.10

Note:σb—tensile strength, σy—yield strength, Z—percentage reduction of area, A—elongation after fracture

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缺口拉伸实验，主要用于比较淬火低中温回火的各种高强度钢的力学性能。屈服强度< 1200 MPa时，其缺口强度均随着材料屈服强度的提高而提高。但是，屈服强度高于1200 MPa则表现出不同的特性[29]。随着回火温度的提高，35CrMo钢缺口试样的抗拉强度先升高后降低，QT200的光滑试样其最高的屈服强度为1487 MPa，对应的缺口试样(Kt = 5)的最高抗拉强度为2450 MPa；QT150的光滑试样其屈服强度为1364 MPa，对应的缺口试样(Kt = 3)的最高抗拉强度为2626 MPa。

图7a给出了缺口试样的抗拉强度与光滑试样抗拉强度的关系。可以看出，随着光滑试样抗拉强度(横坐标代表)的提高缺口试样的抗拉强度先提高后降低。这表明，35CrMo钢的缺口抗拉强度有一个极值，抗拉强度高的试样不一定有高的缺口抗拉强度。用 式(2)计算出的NSR值列于表5，NSR与光滑抗拉强度的关系在图7b中给出。NSR > 1时，缺口对抗拉强度产生增强效果。用缺口抗拉强度增强率

R=σbN-σbσb×100%

图7

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图7   不同回火温度处理35CrMo钢缺口拉伸与光滑拉伸性能对比

Fig.7   Comparisons between notch and smooth tensile properties of 35CrMo steel after tempering at different temperature treatments (a) relationship between notch tensile strength and tensile strength of smooth specimens; (b) relationship between tensile notch sensitivity ratio and tensile strength of smooth specimens


表5   在不同温度回火后35CrMo钢的缺口NSR值

Table 5  Notch sensitivity ratio of 35CrMo steel after tempering at different temperature

Heat-treatment proceduresQQT150QT200QT400
Kt = 31.1821.3451.3271.413
Kt = 51.0951.1811.2961.368

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直观地描述缺口对抗拉强度的增强效果。根据 式(3)计算出的结果，列于表6。结合图7b可以发现，随着光滑试样抗拉强度的下降，缺口敏感性指标NSR均> 1，且逐次增高，即缺口对材料抗拉强度增强率的影响随着抗拉强度的降低而增大，其中QT400试样的缺口对其抗拉强度的增强率最大(41.3%和36.8%)。

表6   不同温度回火的35CrMo钢的缺口抗拉强度增强率

Table 6  Percentage of notch tensile strength enhancement of 35CrMo steel processed by different tempering temperatures (%)

Heat-treatment proceduresQQT150QT200QT400
Kt = 313.234.532.741.3
Kt = 59.5018.129.636.8

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在不同温度回火的缺口试样其NSR值均> 1，表明这四种热处理工艺的拉伸实验对缺口均不敏感，也表明缺口试样的抗拉强度大于光滑试样的强度。产生这一结果的原因是：缺口的存在改变了附近的应力状态，使缺口根部附近所受的应力由单向应力变为三向应力，使试样的屈服应力比单向拉伸时高，从而产生缺口强化，在强度提高的同时塑性降低。对比表3和表4的断面收缩率可以发现，缺口试样断面收缩率与光滑试样相比显著降低。其主要原因是，试样缺口处的多向应力约束了塑性变形使局部塑性降低。虽然提高了抗拉强度，但是材料的脆化倾向增大，而这并不是金属材料内在性能的变化引起的。

从图8a可见，对于在不同温度回火的35CrMo钢试样，随着应力集中系数的增大，其抗拉强度呈现出先升高后降低的趋势。为了进一步研究应力集中系数Kt对35CrMo钢缺口强化效果的影响，图8b给出了NSR与应力集中系数Kt的关系。可以看出，在Q、QT150、QT200和QT400状态下，NSR随着应力集中系数的增大先增高而后减小。

图8

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图8   应力集中系数对不同回火温度35CrMo钢拉伸性能的影响

Fig.8   Effect of stress concentration factor on tensile properties of 35CrMo Steels under different tempering temperatures treatments (a) relationship between Kt and tensile strength; (b) relationship between Kt and NSR


3.2 缺口试样的拉伸断口

在不同温度回火的35CrMo钢缺口试样的拉伸断口形貌，在图9和图10中给出。可以看出，缺口试样的拉伸宏观断口与光滑试样的拉伸宏观断口形貌明显不同。四种热处理的缺口试样断口形貌没有拉伸断口的三个典型的特征和明显的颈缩现象(图9a、d、g、j；图10a、d、g、j)，断面整体较为平坦，也没有出现光滑圆柱体拉伸后的杯壁状剪切唇。从宏观形貌可以看出，裂纹均从边缘起始沿着径向扩展直到断裂。四种试样断口的表面出现明显的凹凸不平的撕裂棱，且沿着裂纹扩展的方向撕裂棱的尺寸越来越大，是在裂纹快速扩展中形成的。另外，从断口宏观形貌还可以看出，随着回火温度的升高断口表面逐渐变的粗糙。

图9

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图9   Kt = 3的不同回火温度35CrMo钢的拉伸断口形貌

Fig.9   Notch tensile fracture morphology of 35CrMo Steel after tempering at different temperature at Kt = 3 (a~c) Q; (d~f) QT150; (g~i) QT200; (j~l) QT400


图10

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图10   Kt = 5的不同回火温度35CrMo钢的拉伸断口形貌

Fig.10   Notch tensile fracture morphology of 35CrMo Steel after tempering at different temperature at Kt = 5 (a~c) Q; (d~f) QT150; (g~i) QT200; (j~l) QT400


Kt = 3的不同热处理材料35CrMo钢缺口试样的拉伸裂纹起始位置的断口形貌，在图9b、e、h和k中给出。可以看出，四种热处理的缺口拉伸试样的裂纹均在缺口根部表面萌生。Q、QT150、QT200和QT400试样邻近表面的断裂趋于平坦，Q试样临近表面处韧窝较多但比较浅且断裂面比较平整，而QT400试样临近表面的断面较为平坦，韧窝较少，但是断裂面起伏比较大，沿裂纹扩展方向韧窝逐渐增大，数量逐渐增多，断裂面更加粗糙。Q、QT150、QT200和QT400四种热处理工艺的缺口拉伸试样内部区域的断口形貌分别在图9c、f、i和l中给出。可以看出，断口上均有大小不一的韧窝和孔洞，是典型的纤维区特征。Q、QT150和QT200试样的断口内部为韧窝和断裂小平面结合的形貌，表现出韧性和脆性的混合断裂特征。QT400试样的断口内部形貌是较少但是较大的韧窝和解理平面，断裂表面更加起伏不平并出现二次微裂纹，表现为准解理断裂特征。

Kt = 5的不同热处理顶盖螺栓材料35CrMo钢的缺口试样的拉伸裂纹起始位置的断口形貌，在图10b、e、h和k中给出。与Kt = 3缺口试样断口形貌不同的是，裂纹起始位置虽然都在缺口根部表面处，但是断口裂纹起始位置的表面粗糙，有明显的撕裂特征且在缺口一周出现数量较多高低不平的撕裂棱和大量韧窝。其原因是，应力集中系数的增大使应力更高，应力三轴度最大位置更向缺口根部移动[30]，从而使起始断裂特征发生变化。

3.3 抗拉强度的预测

测量硬度较为容易且包含了强度的信息，因此人们用硬度推测材料的抗拉强度。强度与硬度关系的经验公式为

σb=a+bH

其中a和b是与材料种类有关的系数，H代表布氏硬度(HB)、洛氏硬度(HRC)或维氏硬度(HV)[31]。研究发现，只有维氏硬度< 3922.8 N/mm2时才与抗拉强度有严格的线性关系[32~34]。Pang等[25]研究金属材料的疲劳性能时，基于材料硬度和疲劳比的关系提出了一个普适的疲劳强度预测模型。

用不同热处理制度处理的35CrMo钢，具有不同的硬度。未回火试样的维氏硬度为5512.71 N/mm2，在150℃回火的试样硬度为5415.92 N/mm2，在200℃回火的试样硬度为5081.1 N/mm2；在400℃回火的试样硬度为4574.28 N/mm2。

对比在不同温度回火的35CrMo钢试样其硬度和缺口试样的拉伸性能发现，随着硬度的增大其抗拉强度有增大的趋势，于是假设缺口试样的抗拉强度和硬度的关系为

σbN/HV=a+bHV

其中σbN为缺口抗拉强度(MPa)；a、b为与材料有关的参数；HV为维氏硬度(N/mm2)。拟合应力集中系数分别为Kt = 3和Kt = 5的缺口试样的抗拉强度和维氏硬度的比值(σbN/HV)与维氏硬度的关系，结果在图11a、b中给出。可以看出，拟合结果在8%以内，表明在两种应力集中条件下的缺口抗拉强度和维氏硬度的比值(σbN/HV)与维氏硬度(HV)之间有良好的线性关系：

σbN/HV=0.685-4.144×10-5HV, Kt=3
σbN/HV=0.826-7.464×10-5HV, Kt=5

图11

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图11   35CrMo钢的缺口抗拉强度与硬度的关系

Fig.11   Relationship between notch tensile strength and hardness of 35CrMo steel (a) σbN/HV vs. HV (Kt = 3); (b) σbN/HV vs. HV (Kt = 5); (c) σbN vs. HV (Kt = 3); (d) σbN vs. HV (Kt = 5)


由 式(5)可得缺口抗拉强度与硬度之间有抛物线关系

σbN=(a+bHV)HV

由 式(8)可分别得到两种应力集中系数缺口抗拉强度与硬度的关系为

σbN=(0.685-4.144×10-5HV)HV, Kt=3
σbN=(0.826-7.464×10-5HV)HV, Kt=5

图11c、d给出了根据 式(9)和(10)画出的曲线，误差仅为8%。

为了验证 式(5)和 式(8)的普适性，对Megahed等[35]得到的马氏体钢的缺口试样拉伸数据进行拟合，其结果为

σbN/HB=0.763-6.992×10-5HB
σbN=(0.763-6.992×10-5HB)HB

其曲线在图12中给出，可见拟合误差均< 7%。

图12

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图12   马氏体钢的缺口抗拉强度与硬度的关系

Fig.12   Relationship between notch tensile strength and hardness of martensitic steel (a) σbN/HB vs. HB; (b) σbN vs. HB


4 结论

(1) 油淬后35CrMo钢的组织由大量板条状马氏体和部分残余奥氏体组成，在150℃和200℃低温回火后得到回火马氏体，随着回火温度的提高马氏体逐渐分解，在回火400℃后得到均匀的回火屈氏体组织。

(2) 35CrMo钢的缺口拉伸敏感度均> 1，表明产生了“缺口强化”效应。随着应力集中系数的增大，缺口强化效果呈现出先升后降的趋势。随着回火温度的提高，缺口抗拉强度先上升后下降。在150℃回火Kt = 3的缺口抗拉强度最高为2626 MPa，在200℃回火Kt = 5的缺口抗拉强度最高为2450 MPa。

(3) 缺口拉伸试样均在缺口根部表面开裂，裂纹沿径向扩展，其最终的断裂均为韧性和脆性混合断裂。缺口拉伸断口较为平坦，没有拉伸断口的三个典型特征和明显的颈缩现象。断口表面出现明显凹凸不平的撕裂棱，且沿扩展方向越来越大。

(4) 根据硬度和缺口拉伸数据，提出一种根据硬度预测缺口试样抗拉强度的快速预测方法即抛物线关系，预测结果的误差均< 8%。

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The influence of heat treatment temperature and cooling rate on the microstructure, phase and tensile deformation behavior of Ti-alloy Ti6246 alloy was investigated. The results show that the α′′ martensite was observed in prior β phase after solution heat treatment followed by water quenching. While a fine transformed β microstructure produced as a result of air cooling. For the air cooling alloy, both the size and volume fraction of the secondary α grain increased with the increasing solution heat temperature in air cooling samples. A “double yield” phenomenon appeared in the engineering stress-strain curves of the water quenching alloy samples. After aging treatment, the strength of water- and air-cooled alloys samples increased but the plasticity decreased. An optimal property in strength and ductility was achieved for the alloysamples after soluted solution treated at 900-920°C and then aged at 595°C.

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[J]. 材料研究学报, 2017, 31: 352

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研究了热处理温度和冷却方式对Ti6246合金显微组织、相组成以及室温拉伸性能的影响。结果表明：固溶热处理后合金的相组成主要与冷却方式有关。在β单相区及(α+β)两相区固溶后水冷,β相均转化为α′′马氏体和少量亚稳β相。空冷组织中的β相转变为含有少量次生α相的β转变组织,随着热处理温度的提高次生α相的含量逐渐增加,尺寸也逐渐增大。时效后组织中的亚稳相发生分解,析出细小的次生α相。固溶后水冷试样的拉伸曲线上出现“双屈服”现象,且随着固溶温度的提高合金第一屈服点逐渐升高。水淬和空冷合金试样在595℃/8 h时效后其室温拉伸强度提高,延伸率及断面收缩率降低,水淬试样室温拉伸性能的变化更大。固溶后空冷且在595℃时效处理的合金,其室温拉伸性能可达到较好的强塑性匹配。

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It is generally observed that the existence of geometrical discontinuity like notches in materials will lead to strength weakening, as a resultant of local stress concentration. By comparing the influence of notches to the strength of three typical materials, aluminum alloys with intermediate tensile ductility, metallic glasses with no tensile ductility, and brittle ceramics, we observed strengthening in aluminum alloys and metallic glasses: Tensile strength of the net section in circumferentially notched cylinders increases with the constraint quantified by the ratio of notch depth over notch root radius; in contrast, the ceramic exhibit notch weakening. The strengthening in the former two is due to resultant deformation transition: Shear failure occurs in intact samples while samples with deep notches break in normal mode fracture. No such deformation transition was observed in the ceramic, and stress concentration leads to its notch weakening. The experimental results are confirmed by theoretical analyses and numerical simulation. The results reported here suggest that the conventional criterion to use brittleness and/or ductility to differentiate notch strengthening or weakening is not physically sound. Notch strengthening or weakening relies on the existence of failure mode transition and materials exhibiting shear failure while subjected to tension will notch strengthen.

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Notches with different types (V, U), different root curvature radii (0.10, 0.13, 0.25, 0.85 mm) and different length (0.25, 0.50, 0.75, 1.00 mm) were pre-made in tensile samples from HR3C steel tube before and after service for 50 000 h, and then the notch sensitivity of the steel tube was studied by tensile tests. The results show that the unserviced HR3C steel tube had no notch sensitivity. After service, the steel tube was embrittled, and therefore became sensitive to notches. The embrittled steel tube had higher sensitivity to the V-notch than to the U-notch, especially being sensitive to the V-notches with a relatively large angle, relatively small root curvature radii (0.10-0.25 mm) and relatively long length (0.50-1.00 mm). When the stress concentration factor was greater than 3.5, the notch sensitivity of embrittled HR3C steel tube was high.

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