现代大型电力蒸汽管道的熔焊工艺，不可避免地会改变使焊缝(Welded zone, WZ)及其附近的显微组织 [8]。尽管蒸汽管道在建造过程中均采用焊后热处理以实现强韧性改性，但是长期在高温、高压条件下服役必然使P91钢管及其焊接部位的显微组织和力学性能老化。在焊接过程中热循环已经使焊缝及热影响区(Heat affected zone, HAZ)显微组织严重劣化，严重影响整个管道的服役寿命和安全。
1 实验方法
1.1 实验材料
实验用材料取自国内某电力系统未服役的P91钢管母材和在线服役136000小时的P91钢管的焊接接头。服役钢管由直管(281 mm×56.15 mm)和弯管(272 mm×69.95 mm)焊接而成，接头区域由母材、直管热影响区(Heat affected zone of straight pipe, SHAZ)、焊缝和弯管热影响区(Heat affected zone of bent pipe, BHAZ) 四部分组成，其宏观形貌如图1所示。接头的服役温度和压力，分别为541℃和17.47 MPa。此外，未服役钢管母材(Base metal of non-service steel pipe, NBM)和服役钢管接头各区域的化学成分列于表1。
图1
图1
长期高温在线服役P91钢管接头的宏观形貌
Fig.1
Macroscopic morphology of P91 steel pipe joint in long-term online service at high temperature
表1 未服役钢管母材和服役钢管接头各区域的化学成分
Table 1
| C | Si | Mn | S | P | Cr | Mo | V | Nb | Fe | |
|---|---|---|---|---|---|---|---|---|---|---|
| NBM | 0.106 | 0.402 | 0.298 | 0.002 | 0.004 | 8.512 | 1.035 | 0.224 | 0.091 | Bal. |
| BM | 0.099 | 0.392 | 0.296 | 0.002 | 0.005 | 8.487 | 1.038 | 0.229 | 0.087 | Bal. |
| SHAZ | 0.103 | 0.403 | 0.299 | 0.003 | 0.004 | 8.504 | 1.042 | 0.235 | 0.092 | Bal. |
| WZ | 0.141 | 0.223 | 0.863 | 0.006 | 0.007 | 8.827 | 1.024 | - | 0.053 | Bal. |
| BHAZ | 0.101 | 0.385 | 0.304 | 0.002 | 0.004 | 8.497 | 1.039 | 0.231 | 0.085 | Bal. |
1.2 性能表征
在未服役钢管母材和服役钢管接头各区域切取金相试样，将其研磨和抛光后用5%硝酸酒精溶液腐蚀15 s，然后用Nova NanoSEM 50型扫描电镜观察其显微组织、碳化物数量和尺寸。
从各拉伸试样上切取厚度为0.5 mm的薄片，机械减薄至90 μm后冲成直径为3 mm的小圆片，用砂纸打磨至30~40 μm后进行电解双喷制成透射试样。双喷液为7%高氯酸酒精溶液，电解电压和电流分别为10~15 mV和50~60 mA。用JEM-2100F型场发射透射电镜和能谱仪检测碳化物类型及其分布，并观察各区域位错密度的变化。
在未服役钢管母材和服役钢管接头各区域切取测试硬度和拉伸实验的试样。用EQUOTIP BAMBINO 2型里氏硬度计测定硬度。用AG-X100kN和AG-XPLUS100kN型电子万能试验机分别进行室温和高温拉伸实验，拉伸速率均为4 mm/min，实验温度分别为25和545℃。用JXA-8530F型场发射扫描电镜观察试样的室温和高温拉伸断口的形貌。
2 实验结果
2.1 长期服役对接头各区域硬度和显微组织的影响
图2给出了服役钢管接头的硬度分布。可以看出，焊缝、直管和弯管热影响区的硬度相近(约为410~420HL)，都明显低于未服役钢管母材的硬度(约为470HL)。这表明，在高温下长期服役使接头各区域硬度发生不同程度的退化，焊缝和热影响区的退化更为严重。
图2
图2
长期高温在线服役P91钢管接头的硬度分布
Fig.2
Hardness distribution of P91 steel pipe joint in long-term online service at high temperature
图3给出了未服役钢管母材和服役钢管接头各区域的显微组织。可以看出，服役前后钢管显微组织都以回火马氏体为主。服役前的钢管晶粒细小均匀，未见明显的碳化物颗粒；服役后的钢管接头，母材、直管和弯管热影响区的晶粒尺寸未见明显变化，但是焊缝处的马氏体严重分解。同时，母材和焊缝的马氏体边界处析出的碳化物数量明显增多。这表明，经长时间高温服役后，接头各区域显微组织均出现一定程度的老化，焊缝的老化程度更高。
图3
图3
未服役钢管母材和高温长期在线服役钢管接头各区域的显微组织
Fig.3
Microstructure in base metal of non-service steel pipe and steel pipe joint in long-term online service at high temperature: (a) NBM, (b) BM, (c) SHAZ, (d) WZ, (e) BHAZ
2.2 长期服役对接头各区域拉伸性能的影响
图4
图4
未服役钢管母材和服役钢管接头各区域的室温拉伸性能
Fig.4
Room temperature tensile property in base metal of non-service steel pipe and each area of steel pipe joint in service: (a) strength, (b) elongation
图5
图5
未服役钢管母材和服役钢管接头各区域室温拉伸的应力-应变曲线
Fig.5
room temperature tensile stress-stain curve of base metal of non-service steel pipe and each area of steel pipe joint in service during
图6
图6
未服役钢管母材和服役钢管接头各区域的高温拉伸性能
Fig.6
High temperature tensile property of base metal of non-service steel pipe and each area of steel pipe joint in service (a) strength, (b) elongation
图7
图7
未服役钢管母材和服役钢管接头各区域的高温拉伸应力-应变曲线
Fig.7
High temperature tensile stress-strain curve of base metal of non-service steel pipe and each area of steel pipe joint in service during
2.3 长期服役对接头各区域断裂行为的影响
图8分别给出了未服役钢管母材和服役钢管接头各区域的室温拉伸断口形貌。可以看出，未服役钢管母材的断裂面出现大量较深的韧窝，断裂形式为韧窝断裂。母材、直管和弯管热影响区的断裂面出现浅韧窝和准解理，其形式为韧窝和准解理复合断裂。焊缝的断裂面以韧窝为主还有少量准解理，尽管与接头其它区域的韧窝尺寸相比有所增大，但是仍比未服役钢管母材的韧窝尺寸小。这表明，长期高温服役后的接头各区域的断裂特征均发生变化，断裂形式由韧窝断裂向韧窝和准解理复合断裂转变。
图8
图8
未服役钢管母材和服役钢管接头各区域室温拉伸断口的形貌
Fig.8
Room temperature tensile fracture morphology of base metal of non-service steel pipe and each area of steel pipe joint in service (a) NBM, (b) BM, (c) SHAZ, (d) WZ, (e) BHAZ
图9分别给出了未服役钢管母材和服役钢管接头各区域的高温拉伸断裂形貌。可以看出，未服役钢管母材的断裂面出现许多韧窝，为韧窝断裂。接头各区域的断裂面差异不大，与未服役钢管母材相比韧窝数量明显减少，还出现少量准解理，断裂形式为韧窝和准解理复合断裂。
图9
图9
未服役钢管母材和服役钢管接头各区域的高温拉伸断口形貌
Fig.9
High temperature tensile fracture morphology in base metal of non-service steel pipe and each area of steel pipe joint in service (a) NBM, (b) BM, (c) SHAZ, (d) WZ, (e) BHAZ
2.4 长期服役对接头各区域碳化物和位错密度的影响
图10
图10
未服役钢管母材和服役钢管接头各区域碳化物的扫描电镜形貌
Fig.10
Scanning electron microscope morphology of carbides in base metal of non-service steel pipe and each area of steel pipe joint in service (a) NBM, (b) BM, (c) SHAZ, (d) WZ, (e) BHAZ
由图11可见，未服役钢管母材的碳化物多呈椭球状，主要分布在晶内。在长期高温服役后的接头中碳化物由椭球状转变为短棒状，且沿马氏体边界析出的比例增大，焊缝的碳化物沿马氏体板条边界连续析出。此外，与未服役的钢管母材相比，服役接头各区域的位错密度均明显降低，焊缝处降低的程度更高。
图11
图11
未服役钢管母材和服役钢管接头各区域碳化物的透射电镜形貌
Fig.11
Transmission electron microscope morphology of carbides in base metal of non-service steel pipe and each area of steel pipe joint in service (a) NBM, (b) BM, (c) SHAZ, (d) WZ, (e) BHAZ
图12
图12
未服役钢管母材和服役钢管接头焊缝处碳化物的形貌和成分
Fig.12
Morphology and composition of carbides in base metal of non-service steel pipe and weld zone of steel pipe joint in service (a, b) NBM, (c, d) WZ
2.5 接头各区域力学性能的退化机制
以上结果表明，长期服役使P91钢管接头中的马氏体分解、碳化物的尺寸增大、位错密度降低，马氏体强化、位错强化、析出强化和固溶强化对硬度和强度的贡献减小，最终导致接头各区域的硬度、室温和高温拉伸性能退化。在蒸汽管道的焊接过程中焊缝处温度最高，严重的焊接热循环回火使焊缝的显微组织相比接头其它区域发生明显的老化，长时间服役使焊缝的显微组织的老化更加严重，因此焊缝的强度明显降低。
3 结论
(1) 长期高温在线服役的P91钢管接头显微组织明显老化，各区域的硬度均不同程度降低。在线硬度测试可有效反映显微组织明显老化P91接头的室温和高温拉伸性能退化行为。
(2) 与未服役的P91钢管母材相比，高温长期在线服役的P91钢管接头各区域的马氏体分解和碳化物粗化极为明显，使钢管的马氏体强化、沉淀强化和固溶强化效果明显减弱，最终使各区域的硬度、室温和高温拉伸性能明显退化。室温和高温拉伸断裂形式均由韧窝断裂转变为韧窝和准解理复合断裂。
(3) 高温长期在线服役的P91钢管焊缝区域的硬度和室温、高温拉伸性能的退化最为显著。服役前焊接产生的热循环严重回火与长期高温服役的耦合作用，是导致P91钢管焊缝力学性能显著退化的重要原因。
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