Analysis of reheat embrittlement and softening of coarse-grained zone of Q960E welding joint
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摘要: 为了精确获得不同温度梯度条件下焊接热影响区的组织和性能,采用焊接热模拟的方式对一种低碳当量Q960E及其对比钢进行了两次模拟热循环试验,并获得了一次热模拟后的CGHAZ,以及二次热模拟后的未变粗晶热影响区(UA CGHAZ)、过临界粗晶热影响区(SCR CGHAZ)、临界粗晶热影响区(ICR CGHAZ)和亚临界粗晶热影响区(SR CGHAZ)的显微组织,开展了组织分析、冲击韧性测试和硬度表征.结果表明,低碳当量Q960E和高碳当量对比钢的ICR CGHAZ和SR CGHAZ均有再热脆化敏感性,在−40 ℃下对比钢的SR CGHAZ的冲击韧性低至9 J,其晶界点状和条状碳化物分布是再热脆化的主要原因;低碳当量Q960E的SR CGHAZ软化最为严重,其原因是细晶强化、位错强化和析出强化联合丧失造成的.Abstract: In order to accurately obtain the microstructure and properties of the welding heat affected zone under different temperature gradient conditions, two simulated thermal cycle tests were carried out on a low carbon equivalent Q960E and its comparative steel by using the welding thermal simulation method. And the microstructures of CGHAZ after the first thermal simulation, and UA CGHAZ, SCR CGHAZ, ICR CGHAZ and SR CGHAZ after the second thermal simulation were obtained. Microstructures were analyzed, impact toughness test and hardness characterization were carried out in this paper. Results showed that both ICR CGHAZ and SR CGHAZ of Q960E and comparative steel had reheat embrittlement sensitivity. The impact toughness of SR CGHAZ of comparative steel was as low as 9 J at −40 ℃. And the distribution of point-type and strip-type carbides formed at the grain boundary was the main reason for reheating embrittlement. The softening of SR CGHAZ with low carbon equivalent Q960E is the most serious, which is caused by the combined loss of fine-grain strengthening, dislocation strengthening and precipitation strengthening.
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Keywords:
- CGHAZ /
- reheat embrittlement /
- softening /
- thermal simulation
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0. 序言
NiCrMoV低合金钢作为一种超高强钢,综合性能良好,在船舶、压力容器等领域应用广泛[1-2]. 但由于此类高强钢具有较大的淬硬倾向,焊接时容易产生冷裂纹. 且随着强度的提高,其焊接冷裂敏感性就越大,还易导致焊接接头脆化[3-5].
对于NiCrMoV低合金高强钢焊接,除了采用预热措施之外,往往采用奥氏体焊材低强匹配接头抑制冷裂纹的产生,但是会大幅降低高强钢接头强度. 现有的奥氏体焊丝如316L或304不锈钢焊材,得到的焊缝接头强度仅为500 ~ 600 MPa. 因此采用高强度奥氏体焊材焊接超高强钢意义重大. 李大用等人[6]采用高铬镍奥氏体焊丝焊接低合金高强钢,其中氮含量约为0.1%,焊缝主要由树枝状奥氏体组成,无裂纹,接头强度可达800 MPa. 在抑制裂纹的同时可保证强度.
在奥氏体不锈钢体系中,高氮钢以氮部分或全部代替镍合金而形成固溶强化的奥氏体不锈钢,强度可达1 000 MPa[7]. 因此文中拟采用高氮奥氏体焊丝作为填充材料进行超高强钢熔化极气体保护焊(gas metal arc welding, GMAW). 目前对于此类焊丝GMAW工艺研究较少. 针对8 mm厚超高强钢板对接,初步探索了利用高氮钢焊丝GMAW工艺,分析在60°和90°不同的坡口角度下接头组织和力学性能,为超高强钢焊接提供了新思路.
1. 试验方法
试验使用直径1.0 mm的高氮奥氏体钢焊丝作为填充材料,超高强钢板为母材,试板尺寸300 mm × 200 mm × 8 mm. 所用焊丝和母材成分如表1所示.
表 1 焊丝与母材化学元素组成(质量分数,%)Table 1. Chemical component of wire and substrate材料 C Ni Cr Mo Mn Si N 焊丝 0.071 2.32 21.59 1.21 16.95 — 0.79 母材 0.32 1.8 1.0 0.7 1.2 0.4 — 采用CMT + P焊接模式,先正面焊一道,然后背部清根,反面焊一道. 工艺参数为正面送丝速度10 m/min,反面送丝速度9.5 m/min,焊接速度4 mm/s. 保护气体为93.5% Ar + 1.5% O2 + 5% N2,气体流量20 L/min. 坡口形式分别为60°和90°坡口,上下深度比为5∶3. 考虑到焊枪的可达性及减少侧壁未熔合,间隙为3 mm. 拉伸试件按国家标准GBT 2651—2008《焊接接头拉伸试验方法》加工.
2. 试验结果分析
2.1 焊缝宏观形貌
图1是超高强钢高氮奥氏体焊丝熔化极电弧焊接的焊缝宏观形貌和接头纵向截面宏观形貌. 接头成形良好,焊材与母材之间形成冶金结合,接头纵向截面未见气孔、裂纹等缺陷.
2.2 显微组织分析
图2~图4为不同坡口焊缝与热影响区组织形貌. 热影响区可分为明显的粗晶区和细晶区,均为马氏体组织,如图2a,2b,3a,3b所示,对硬度影响较大. 焊缝的组织主要是被奥氏体基体所包围的铁素体树枝晶,且铁素体枝晶很密集. 除此之外,在枝晶中间还有一些“粒状”铁素体散乱的分布在整个金相中,被奥氏体基体所包围,如图2c,3c所示. 两种坡口的焊缝金属和母材金属在结合处的“白亮带”区域有着明显的模糊状态,有相互渗透的现象,并观察到该区是奥氏体与马氏体的混合组织,如图4所示.
两种坡口接头熔合线附近热影响区组织和焊缝组织基本一致. 不同的是,90°坡口焊缝附近热影响区马氏体组织较60°的更细小,同时90°坡口热影响区马氏体和奥氏体的混合组织区的范围比60°的小. 在相同的热输入参数下90°坡口相比60°坡口,不仅能让电弧充分达到焊缝底部,且在相同的深度下,90°坡口与电弧的接触面积更大,熔池凝固快. 60°坡口情况则相反,熔池存在时间更长,这个过程提供了晶粒继续长大的条件,因此60°坡口热影响区马氏体和奥氏体混合组织区较90°的大,相应的马氏体组织也较为粗大. 同时,60°坡口焊缝区也受到影响,组织相对粗大.
如图5所示,在200 ~ 240 μm的热影响区到焊缝的过渡区,核心元素都有一个线性变化的过程,过渡区的存在说明焊缝金属和母材金属有着充分的冶金结合. 但如图4所示,90°坡口的渗透区域相对60°坡口的更大,且相对更均匀.
2.3 显微硬度
如图6所示1和2分别为60°坡口和90°坡口的硬度分布,硬度取样区域线见图1,两种硬度分布是一致的. 硬度按从母材-热影响区-焊缝的顺序依次测量,经过的组织依次为母材马氏体组织-马氏体细晶组织-混合组织-奥氏体组织. 根据图6各组试样的横向硬度分布可知,硬度最高区域主要集中在2.5 ~ 4 mm和7.5 ~ 9 mm的区域内,该区域的维氏硬度在450 ~ 590 HV之间,60°坡口该区硬度平均值为508 HV,90°坡口该区硬度平均值为536 HV. 在硬度显微镜下观测该硬度较高的区域主要为马氏体细晶区. 其中马氏体细晶区的硬度值均在500 HV及以上,平均可达530 HV;在热影响区到焊缝区的过渡区(3.5 ~ 4 mm),该区组织硬度相对马氏体细晶区有一个下降过程,主要是因为该区是马氏体组织和奥氏体组织的混合组织,该区组织相对不均匀,所以该区域的硬度有个下降过程. 由图6可知,硬度最低的区域主要为4.5 ~ 7 mm区域,均在300 HV以下,平均硬度仅在275 HV左右. 该区域是焊缝区,其组织主要以奥氏体为基体的树枝状铁素体晶粒组织,故硬度较低,两种坡口在该区硬度值相差不大.
2.4 拉伸试验
从图7可以看出,60°坡口试样主要是从焊缝位置断裂,部分沿熔合线附近位置断裂,90°坡口试样主要沿熔合线附近位置断裂. 每个试样均有颈缩现象发生. 如表2所示,采用高氮奥氏体钢焊丝获得的接头平均抗拉强度可达850 MPa,该强度是在90°坡口下获得的. 60°坡口接头的抗拉强度则与之相差较大,仅为690 MPa. 对于不同坡口接头的拉伸力学性能变化与微观组织的某些区域的大小以及晶粒的大小有关. 在相同的坡口深度下,60°坡口侧壁与电弧的接触面积更小,其熔池存在时间更长,该过程为焊缝组织晶粒继续长大提供了条件,因此60°坡口焊缝区组织晶粒更粗大,其拉伸性能较低. 对于90°坡口其与电弧的接触面积更大,其熔池凝固快,晶粒相对较小,因此强度更高,并且由于混合组织区较宽,该区域成分介于母材和焊丝之间,得到的性能相对较低,所以90°坡口多断在此处.
表 2 接头抗拉强度 (MPa)Table 2. Tensile strength of joint坡口角度 试样1 试样2 试样3 平均抗拉强度 60° 720 678 672 690 90° 887 834 829 850 3. 结论
(1) 采用高氮奥氏体丝材进行超高强钢GMAW工艺,可以获得成形良好的焊接接头.
(2) 高氮钢焊缝金属和超高强钢母材金属在接头熔合线处有着明显的“白亮带”存在,有相互渗透的现象,说明焊材和母材有着充分的冶金结合.
(3) 采用上述工艺中60°坡口进行焊接,接头热影响区马氏体组织晶粒较大,且马氏体、奥氏体混合组织过渡区小;而90°坡口焊接接头热影响区马氏体组织晶粒更细小,且马氏体、奥氏体混合组织过渡区较大.
(4) 采用高氮奥氏体焊丝进行GMAW焊接,90°坡口可获得接头抗拉强度(850 MPa)高于60°坡口接头抗拉强度(690 MPa). 90°坡口试样马氏体细晶区硬度平均值为536 HV,高于60°坡口试样(508 HV).
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图 1 不同tp2温度下再热CGHAZ的SEM像
Figure 1. SEM images of reheated CGHAZ under different tp2 temperatures. (a) UA CGHAZ of Q960E; (b) UA CGHAZ of comparative steel; (c) SCR CGHAZ of Q960E; (d) SCR CGHAZ of comparative steel; (e) ICR CGHAZ of Q960E; (f) ICR CGHAZ of comparative steel; (g) SR CGHAZ of Q960E; (h) SR CGHAZ of comparative steel
表 1 Q960E和对比钢的化学成分(质量分数,%)
Table 1 Chemical composition of Q960E and its comparative steel
材料 C Si Mn P S Cu Ni Cr Mo B V Ceq Q960E 0.12 0.23 1.30 ≤0.02 ≤0.002 — — 0.50 0.50 0.001 2 0.1 0.395 对比钢 0.14 0.40 1.0 ≤0.02 ≤0.002 0.1 2.2 0.50 0.50 0.001 1 — 0.546 注:Ceq= C + ( Mn + Si )/6 + ( Ni + Cu )/15 + ( Cr + Mo + V )/50 表 2 Q960E和对比钢的典型力学性能
Table 2 Typical mechanical properties of Q960E and its comparative steel
材料 屈服强度ReL/MPa 抗拉强度Rm/MPa 断后伸长率A50(%) −40 ℃冲击吸收能量AKV(%) Q960E 990 1 020 13.5 43, 40, 49 对比钢 1 000 1 080 13.0 45, 47, 43 表 3 CGHAZ热模拟工艺
Table 3 Thermal simulation process of CGHAZ
工艺
编号一次热循环 二次热循环 预热温度
T0/℃升温速率
v /(℃·s−1)峰值温度
Tc/℃保温时间
ts/s热循环时间
t8/5/ s升温速率
v /(℃·s−1)峰值温度
Tc/℃保温时间
ts/s热循环时间
t8/5/s1 25 150 1 250 2 7 — — — — 1A 25 150 1 250 2 7 150 1 250 2 7 1B 25 150 1 250 2 7 150 950 2 10 1C 25 150 1 250 2 7 150 750 2 12 1D 25 150 1 250 2 7 150 600 2 15 -
[1] Joseph C, Benedyk. Light metal in automotive applications[J]. Light Metal Age, 2000, 58(10): 34 − 35.
[2] 张楠, 田志凌, 董现春, 等. Q960E热影响粗晶区疲劳寿命与ΔKth值的关系分析[J]. 焊接学报, 2018, 39(7): 106 − 110. Zhang Nan, Tian Zhiling, Dong Xianchun, et al. Research on relationship between ΔKth and fatigue life of heat-affected coarse grain zone in Q960E[J]. Transactions of the China Welding Institution, 2018, 39(7): 106 − 110.
[3] Zhou Y L, Jia T, Zhang X J, et al. Microstructure and toughness of the CGHAZ of an offshore platform steel[J]. Journal of Materials Processing Technology, 2015, 219: 314 − 320.
[4] Lambert A, Drillet J, Gourgues A F, et al. Microstructure of mattensite-austensite constituents in heat affected zones of high strength low alloy steel welds in relation to toughness properties[J]. Science and Technology of Welding and Joining, 2000, 5(3): 168 − 173.
[5] Davis C L, King J E. Cleavage initiation in the intercritically reheated coarse-grained heat-affected zone: Part I. fractographiv evidence[J]. Metallurgical and Materials Transactions A, 1994, 25(3): 563 − 573.
[6] Wang C M, Wu X F, Liu J, et al. Transmission electron microscopy of martensite/austensite islands in pipeline steel X70[J]. Materials Science and Engineering A, 2006, 438(25): 257 − 271.
[7] 张楠, 田志凌, 张书彦, 等. 700MPa微合金高强钢焊接软化机理及解决方案[J]. 钢铁研究学报, 2019, 31(3): 318 − 326. Zhang Nan, Tian Zhiling, Zhang Shuyan, et al. Mechanism and solution of welding softening for 700MPa microalloyed high strength steel[J]. Journal of Iron and Steel Research, 2019, 31(3): 318 − 326.
[8] 董现春, 潘辉, 赵阳, 等. 仿弹钢板激光焊接接头的组织和性能[J]. 材料热处理学报, 2019, 40(9): 163 − 168. Dong Xianchun, Pan Hui, Zhao Yang, et al. Microstructure and properties of laser welded joint of bulletproof steel plant[J]. Transactions of Materials and Heat Treatment, 2019, 40(9): 163 − 168.
[9] 王学, 常建伟, 黄关政, 等. WB36钢临界再热粗晶区组织和性能[J]. 焊接学报, 2008, 29(10): 29 − 32. Wang Xue, Chang Jianwei, Huang Guanzheng, et al. Study on microstructure and properties of IRCGHAZ in WB36 steel[J]. Transactions of the China Welding Institution, 2008, 29(10): 29 − 32.
[10] 姚钦. HQ-80钢再热裂纹机理[J]. 焊接学报, 2004, 25(6): 77 − 81. Yao Qin. Mechanism of HQ-80 steel reheat crack[J]. Transactions of the China Welding Institution, 2004, 25(6): 77 − 81.
[11] Hu J, Du L X, Wang J J, et al. High toughness in the intercritically reheated coarse-grained (ICRCG) heat-affected zone (HAZ) of low carbon microalloyed steel[J]. Materials Science and Engineering A, 2014, 590: 323 − 328.
[12] 张楠, 董现春, 张熹, 等. 钛微合金化SQ700MCD高强钢粗晶热影响区软化的原因[J]. 机械工程材料, 2012, 36(4): 88 − 92. Zhang Nan, Dong Xianchun, Zhang Xi, et al. The softening analysis of CGHAZ in Ti microalloyed SQ700MCD steel[J]. Materials for Mechanical Engineering, 2012, 36(4): 88 − 92.
[13] 张楠, 董现春, 徐晓宁, 等. Ti-Nb微合金化高强钢的焊接接头组织和性能[J]. 材料热处理学报, 2014, 35(6): 115 − 120. Zhang Nan, Dong Xianchun, Xu Xiaoning, et al. Microstructure and property of welding joint with Ti-Nb microalloyed high-strength steel[J]. Transactions of Materials and Heat Treatment, 2014, 35(6): 115 − 120.
[14] 张楠, 董现春, 潘辉, 等. 高Ti-Nb系高强钢焊接接头回火前后的力学行为[J]. 焊接学报, 2015, 36(5): 93 − 98. Zhang Nan, Dong Xianchun, Pan Hui, et al. Mechanical behavior of welded joint of a high Ti-Nb content microalloyed high-strength steel before and after drawing temper treatment[J]. Transactions of the China Welding Institution, 2015, 36(5): 93 − 98.
[15] 张楠, 田志凌, 张书彦, 等. 感应回火对含钒900MPa级高强钢组织与性能的影响[J/OL]. 热加工工艺: 1-5[2020-11-11]. https://doi.org/10.14158/j.cnki.1001-3814.20193019. Zhang Nan, Tian Zhiling, Zhang Shuyan, et al. Effects of induction tempering on microstructure and properties of 900 MPa grade high strength steel containing vanadium[J/OL]. Hot Working Technology: 1-5 [2020-11-11]. https://doi.org/10.14158/j.cnki.1001-3814.20193019.
[16] 李晓林, 崔阳, 肖宝亮, 等. V-N微合金钢在线快速感应回火工艺中V(C, N)析出强化机制[J]. 金属学报, 2018, 54(10): 1368 − 1376. doi: 10.11900/0412.1961.2018.00119 Li Xiaolin, Cui Yang, Xiao Baoliang, et al. Effect of on-line rapid induction tempering on precipitation strengthening mechanism of V(C, N) in V-N microalloyed steel[J]. ACTA Metallurgica Sinica, 2018, 54(10): 1368 − 1376. doi: 10.11900/0412.1961.2018.00119
[17] Mohseni P, Solberg J K, Karlsen M, et al. Investigation of mechanism of cleavage fracture initiation in intercritically coarse grained heat affected zone of HSLA steel[J]. Materials Science and Technology, 2012, 28(11): 1261 − 1268.
[18] Moeinifar S, Kokabi A H, Hosseini H R M. Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones[J]. Materials and Design, 2010, 31(6): 2948 − 2955.
[19] 李永亮. 700 MPa级高强度汽车大梁钢成分设计与组织控制研究[D]. 北京: 北京科技大学, 2017. Li Yongliang. Study on composition design and microstrucyure control about 700 MPa grade high strength beam steel for vehicles [D].Beijing: University of Science and Technology Beijing, 2017.
[20] 雍岐龙. 钢铁材料中的第二相[M]. 北京: 冶金工业出版社, 2006. Yong Qilong. Secondary phases in steels [M]. Beijing: Metallurgical Industry Press, 2006.