Analysis of the Strength and toughness of welded joints in ultra high strength steel with a step-cooling laser welding
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摘要:
为解决超高强钢焊后接头强韧性失配的问题，文中研究借鉴淬火配分工艺(quenching and partitioning，Q&P)原理提出一种分步冷却工艺，研究分步冷却焊接工艺对焊接接头力学性能的影响. 基于焊接数值模拟的方法设计了分步冷却系统，并与现有激光焊系统集成，开展了超高强钢分步冷却焊接试验，对比分析了分步冷却焊接工艺对焊接接头力学性能的影响，探讨了分步冷却激光焊接头强韧化机理. 结果表明，相较于激光焊，分步冷却激光焊使得焊接接头各区域的宽度均有所下降，焊接接头的屈服强度提高5.8%，抗拉强度提高4.9%，韧性值提高约6.8%. 这是由于分步冷却导致冷却速率较高，原奥晶粒尺寸减小，从而形成更细、更窄的板条马氏体，提升了接头的强度；同时由于自配分的作用，残余奥氏体含量增加，引发相变诱导塑性效应（transformation-induced plasticity，TRIP）导致韧性提高.
Abstract:To address the strength-toughness mismatch in ultra-high strength steel welded joints, a step-cooling process inspired by the Quenching and Partitioning (Q&P) principle was proposed and its impact on the mechanical properties of welded joints was investigated. The step-cooling system was designed using welding numerical simulation methods and integrated with existing laser welding equipment. The conventional laser welding tests and the step-cooling laser welding tests were conducted on ultra-high strength steel. The influence of step-cooling laser welding process on the mechanical properties of welded joints was compared and analyzed. Additionally, the strengthening-toughening mechanism of step-cooling laser welded joints was elucidated. Results demonstrate that compared to conventional laser welding, the step-cooling laser welding reduces the width of various joint zones while enhancing yield strength by 5.8%, increasing tensile strength by 4.9%, and improving toughness by approximately 6.8%. This improvement primarily stems from accelerated cooling rates during the step-cooling process, which refine prior austenite grains and produce finer/narrower lath martensite, thereby enhancing joint strength. Moreover, the self-partitioning effect increases retained austenite content, inducing transformation-induced plasticity (TRIP) effect that contributes to toughness enhancement.
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Keywords:
- ultra high strength steel /
- step cooling /
- strength and toughness /
- laser welding
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0. 序言
核反应堆包层结构经受着极其恶劣的服役环境，要求其在长期服役过程中保持结构和冶金的完整性. 9Cr-1.5W-0.15Ta耐热钢具有较低的辐照肿胀系数和热膨胀系数、较高的热导率等优异的热物理性能和良好的力学性能，被认为是核聚变/裂变发堆包层结构的理想候选材料之一[1-4].
为了减小热影响区宽度，保持接头良好的组织性能，多采用低热输入、高能量密度的特种焊接技术对9Cr-1.5W-0.15Ta耐热钢进行焊接[5-7]. 搅拌摩擦焊(Friction stir welding, FSW)是一种新型固态塑性连接技术，焊接热输入较低，可以保持焊缝性能与母材相近，焊接变形和残余应力较小等的特点[8-9]. 与搅拌摩擦焊相比，电子束焊(electron beam welding，EBW)是一种高效率、高能量密度的熔化焊接方法，具有焊接冶金质量好、焊接熔深大和焊接热影响区窄的特点，具有适用性强、操作简便等优势[10-11]. 因此，对比研究9Cr-1.5W-0.15Ta耐热钢搅拌摩擦焊缝和电子束焊缝组织和力学性能的差异具有重要意义.
文中对9Cr-1.5W-0.15Ta耐热钢电子束焊缝和搅拌摩擦焊缝的微观组织、硬度和冲击性能进行比较，分析微观组织与力学性能之间的关联性，并论述接头的断裂机制.
1. 试验方法
试验采用的母材为9Cr-1.5W-0.15Ta耐热钢，其化学成分如表1所示. 母材热处理工艺如下：1000 ℃下正火保温60 min，水淬之后在700 ℃下回火60 min. 搅拌摩擦焊机为北京赛福斯特技术有限公司生产的FSW-3LM-020型设备，搅拌头的材料为W-25%Re合金. 焊接工艺参数为焊接速度60 mm/min，焊接转速300 r/min和焊接压力10 kN. 电子束焊机选择KL110型真空电子束焊机设备. 焊接加速电压60 kV，焊接电流30 mA，焊接速度600 mm/min，在全聚焦状态下以束流垂直于板面的方式进行焊接.
沿垂直于焊接方向切取尺寸为25 mm × 10 mm的试样，经过粗磨、细磨和抛光后，在5 g FeCl3, 20 mL盐酸和100 mL蒸馏水的腐蚀液中侵蚀90 s制备金相试样. 采用光学显微镜(OLYMPUS GX51)和电子扫描显微镜(SEM，TDCLSU 1510)对接头区域的微观组织进行观察. 采用型号为Tecnai G2F30透射电子显微镜在300 kV加速电压下观测析出相.
低温冲击试验试样尺寸如图1所示，冲击试验后，采用电子扫描显微镜观测冲击试样断口形貌. 硬度试验是在金相试样上焊缝区域测试，载荷为9.8 N，加载时间为15 s.
表 1 9Cr-1.5W-0.15Ta耐热钢化学成分(质量分数，%)Table 1. Chemical composition of the 9Cr-1.5W-0.15Ta heat resistant steelC Cr Mn V W Ta Si Zr N S P Fe 0.1 9 0.5 0.2 1.5 0.15 0.05 0.005 0.007 0.002 0.002 余量 ![]()
图 1 焊缝低温冲击试样取样位置和几何尺寸(mm)Figure 1. Dimension and position of impact toughness testing sample in the weld2. 试验结果与分析
2.1 微观组织
9Cr-1.5W-0.15Ta耐热钢母材、电子束焊缝和搅拌摩擦焊缝的微观组织如图2所示. 图2a, b是9Cr-1.5W-0.15Ta耐热钢母材的微观组织，由于经过正火、淬火和回火等热处理后，具有完全的回火的组织特征，晶粒尺寸大约为20 μm，并在原奥氏体晶界和晶内形成均匀分布的析出相(M23C6和MX). 电子束焊缝的微观组织如图2c, d所示，其特点为晶粒粗大，组织不均匀，且晶界处的M23C6析出相和晶内MX析出相均发生完全溶解. 虽然电子束焊能量密度较大，熔池中心温度高，但其高温停留时间短，焊后冷却速度较大，因此在熔合线形成较大的温度梯度，促进粗大的树枝状组织的形成. 从焊缝两边生长的晶粒在焊缝中心处相遇，形成了垂直于母材原始晶粒取向的组织结构.
图2e, f为搅拌摩擦焊缝的微观组织. 在焊接过程中，焊缝由于受到搅拌针剧烈的搅拌而引起严重的塑性变形和摩擦，产生的局部高温作用使得组织发生动态再结晶，加之焊后冷却速率较大，发生马氏体转变[12]. 因此，搅拌摩擦焊缝的组织由回火组织转变为板条马氏体. 焊缝区域晶粒发生明显细化，这是由于该区域受到搅拌针的机械作用，动态再结晶的晶粒发生破碎而细化. 此外，在搅拌摩擦焊缝中晶界上的M23C6析出相发生完全溶解，而晶内依然存在球状MX析出相，这表明焊缝区域经历的焊接热循环峰值温度高于M23C6相的熔点(860 ℃)、但低于MX相熔点(1310 ℃)[13-14].
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图 2 母材、电子束焊缝和搅拌摩擦焊缝的微观组织特征Figure 2. Microstructure of base metal, EBW weld and FSW weld. (a) metallographic of base metal; (b) SEM microstructure of base metal; (c) metallographic of EBW weld; (d) SEM microstructure of EBW weld; (e) metallographic of FSW weld; (f) SEM microstructure of FSW weld图3为母材和搅拌摩擦焊缝中析出相特征. 母材中M23C6碳化物和球状MX相分别均匀地分布在原奥氏体晶界和晶内(图3a, b). 焊后晶界处M23C6碳化物发生完全溶解，球状MX碳氮化物无明显变化，但对位错产生强烈的钉扎作用，同时在板条马氏体内生成大量的针状M3C相，主要由W，Cr，Fe和C组成(图3c ~ 3f). 这主要是由于M23C6碳化物的溶解在晶界和晶内之间产生C和Cr原子的浓度梯度，同时焊接过程中的奥氏体化再结晶和马氏体转变诱导位错和空位等晶格缺陷增殖，为针状M3C碳化物析出提供了形核质点和原子扩散通道，促进了M3C碳化物的析出[15].
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图 3 母材和搅拌摩擦焊缝析出相的特征Figure 3. Characteristics of precipitates for base metal and FSW weld. (a) M23C6 phase in base metal; (b) MX phase in base metal; (c) M23C6 phase in FSW weld; (d) MX phase in FSW weld; (e) M3C phase in FSW weld; (f) energy spectrum of M3C phase2.2 力学性能
2.2.1 显微硬度
表2为母材、电子束焊缝和搅拌摩擦焊缝硬度结果. 相比于9Cr-1.5W-0.15Ta耐热钢的硬度(272 HV)，两种焊缝的硬度明显增大，电子束焊缝硬度值为475 HV，搅拌摩擦焊缝硬度值为425 HV. 焊缝区明显硬化，这是由于在焊接过程中焊接热循环峰值温度高于母材的相变温度，在焊后快速冷却导致焊缝中形成大量的板条状马氏体组织，使得焊缝的硬度增大[16-17].
表 2 母材、电子束焊缝和搅拌摩擦焊缝显微硬度(HV)Table 2. Microhardness of the base metal, EB and FSW welds母材 EBW焊缝 FSW焊缝 272 475 425 2.2.2 冲击韧性
图4是9Cr-1.5W-0.15Ta耐热钢母材、电子束焊缝和搅拌摩擦焊缝在−20 ℃下的冲击吸收能量. 由图可知，母材的冲击吸收能量为34.35 J，搅拌摩擦焊焊缝冲击吸收能量为31.1 J，而电子束焊焊缝的冲击吸收能量为4.2 J，仅为母材的12.2%和搅拌摩擦焊缝的13.5%.
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图 4 母材、电子束焊缝和搅拌摩擦焊缝在−20 ℃下的冲击韧性Figure 4. Impact toughness of base materials, EBW and FSW welds at −20 ℃接头的力学性能主要取决于其微观组织特征. 与母材相比，搅拌摩擦焊接头韧性稍有降低，这是焊缝区晶粒细化、高角度晶界增加阻碍裂纹扩展而改善接头韧性和位错密度增加而恶化冲击性能的共同结果[18-20]. 除此之外，残余奥氏体的存在也对改善接头韧性具有重要影响[21]. 相比于搅拌摩擦焊接头，电子束焊接头韧性显著降低，这主要是由于在焊缝中树枝状组织的形成，使焊缝韧性明显降低. 另外，电子束焊接过程中热输入较大，引起晶粒粗化和析出相溶解等组织变化，对接头的冲击韧性产生重要影响.
2.2.3 冲击断口形貌
图5为母材、电子束焊缝和搅拌摩擦焊缝试样冲击后的断口形貌. 由于微观组织特征的差异，导致焊接接头力学性能的不同，同时也在冲击断口形貌上表现明显的不同. 母材的冲击断口形貌表现为典型的韧窝特征，并且韧窝大小和形状存在明显差别，发现小尺寸韧窝密度远多于大尺寸韧窝(图5a). 电子束焊缝冲击断口则表现为典型的解理断裂，同时局部还可以发现较深的裂纹(图5b). 搅拌摩擦焊缝冲击断口形貌均表现为大小和形状均匀的韧窝特征，在部分韧窝底部存在第二相粒子脱落的现象，并且由于冲击变形而形成少量的撕裂痕，断裂方式属于微孔聚集型断裂(图5c). 综上，母材和搅拌摩擦焊缝的冲击断裂方式属于典型的延性断裂，而电子束焊缝的冲击断裂方式属于脆性断裂.
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图 5 冲击试样的断口形貌Figure 5. Fracture morphology of impact specimens. (a) base metal; (b) EBW weld; (c) FSW weld电子束焊缝和搅拌摩擦焊缝的冲击断口形貌与母材有不同程度的差异. 在搅拌摩擦焊缝中，由于发生动态再结晶，晶粒尺寸明显细化，同时仅部分低熔点析出相溶解，冲击断口表现尺寸较大的韧窝特征，导致冲击韧性发生稍稍降低. 然而，对于电子束焊缝，微观组织为粗大的树枝状晶，并且析出相均发生溶解，导致在冲击过程中协调变形能力变弱，断口表现为典型的解理断裂特征. 因此，电子束焊缝的冲击韧性显著降低.
3. 结论
(1) 9Cr-1.5W-0.15Ta耐热钢电子束焊缝呈树枝状晶微观组织，晶粒粗大，组织不均匀，且析出相均发生溶解；搅拌摩擦焊缝则由细小、均匀的板条马氏体微观组织组成，部分晶界析出相发生溶解.
(2) 由于在焊缝中有大量板条马氏体生成，9Cr-1.5W-0.15Ta耐热钢电子束焊缝和搅拌摩擦焊缝的硬度值均发生了显著增大，电子束焊缝的硬度值最高可达到475 HV.
(3) 两种焊缝的冲击韧性均低于母材，但由于电子束焊缝和搅拌摩擦焊缝中晶粒尺寸、析出相的差异，不同焊缝表现不同的力学性能. 电子束焊缝的冲击吸收能量仅为母材的12.2%；搅拌摩擦焊缝的力学性能较好，其冲击吸收能量为母材的90%.
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图 3 温度场模拟结果
Figure 3. Temperature field simulation results. (a) simulated weld shape; (b) thermal cycle curve
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图 5 分步冷却激光焊系统
Figure 5. Step-cooling laser welding system. (a) welding system; (b) step-cooling system
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图 6 焊缝宏观形貌
Figure 6. Macromorphology of weld. (a) front of laser welding; (b) front of step cooling laser welding; (c) back of laser welding; (d) back of step cooling laser welding
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图 7 焊接接头横截面宏观形貌
Figure 7. Macromorphology of welded joints cross section. (a) laser welding; (b) step cooling laser welding
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图 8 焊接接头力学对比
Figure 8. Mechanical properties of welded joints. (a) micro hardness distributions; (b) representative tensile strain-stress curves under different welding process; (c) toughness of welded joint
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图 9 拉伸断口形貌
Figure 9. Tensile fracture surfaces. (a) laser welding; (b) step cooling laser welding
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图 10 峰值温度为906 ℃的实测热循环曲线
Figure 10. Measured thermal cycle curve with peak temperature of 906 ℃
表 1 试验材料的热物理性能参数
Table 1 Thermophysical properties of experimental material
温度
T/℃比热
C/(J·kg−1·℃−1)热导率
λ/(W·m−1·℃−1)25 440 68.1 125 485 65.9 325 568 56.3 525 677 45.1 725 970 39.8 925 602 36.2 1225 643 36.0 
下载: 导出CSV
表 2 试验材料的化学成分(质量分数，%)
Table 2 Chemical compositions of the material
C Si Al Mn Nb Fe 0.20 1.00 ~ 1.20 0.60 ~ 0.80 0.60 ~ 0.80 0.025 余量
下载: 导出CSV
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