Detection of nonmetallic inclusion in high-strength gear steel with high cleanliness
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摘要: 研究了一種方便可靠的夾雜物評估方法：利用合適電化學充氫后的拉伸試樣獲取夾雜物并與極值統計法相結合估算不同體積鋼中非金屬夾雜物的最大尺寸并預測疲勞強度。研究選用工業生產的高潔凈度20Cr2Ni4A齒輪鋼，將淬火+低溫回火態的標準拉伸試樣進行電化學充氫，使拉伸斷口由于氫脆現象存在一些以粗大非金屬夾雜物為中心的脆性平臺，從而可方便快捷地在掃描電子顯微鏡下對夾雜物的類型、尺寸和分布進行檢測，并利用極值統計法對鋼中的最大夾雜物尺寸進行評估。為了驗證該方法的準確性，采用傳統金相法和旋轉彎曲疲勞試驗對鋼中非金屬夾雜物進行了檢測，結果表明，使用本文所提出的夾雜物評估方法預測的鋼中最大夾雜物尺寸及疲勞強度與疲勞試驗結果相吻合。因此，該方法有望成為預測高潔凈度高強度鋼中最大夾雜物尺寸及其疲勞強度的一種有效方法。Abstract: It is well known that large-sized nonmetallic inclusion seriously affects the mechanical properties of high-strength steels, particularly the fatigue properties. Therefore, significant efforts have been made to enhance the fatigue properties of gear steels by improving the cleanliness and, thus, reducing the size and the number of inclusions in steels. However, an effective inclusion inspection method is particularly important because of the relatively low-rate emergence of large-sized inclusions in highly clean steels. Herein, a new inclusion inspection strategy was proposed using a properly hydrogen-charged tensile specimen combined with the application of the statistics of extreme value (SEV) method, which can be used to conveniently and reliably estimate the maximum inclusion size in any volume of high-strength steel and its fatigue strength. A commercial heat of 20Cr2Ni4A gear steel with high cleanliness was used to verify the proposed method. Standard tensile specimens were quenched, tempered at low-temperature, and then properly charged with electrochemical hydrogen. It is found that there were many embrittled platforms, generally with large inclusions on the fracture surfaces of the specimens after normal tensile testing because of the trapping of the charged hydrogen around inclusion and the occurrence of hydrogen embrittlement. The size, composition, and distribution of these inclusions can be analyzed using a scanning electron microscopy, thus, the maximum inclusion size can be predicted using the SEV method. To verify the accuracy of the proposed method, additional inclusion rating methods of conventional optical metallographic observation and high-cycle fatigue testing were conducted. Using the proposed method, it was confirmed that the predicted maximum inclusion size and fatigue strength are consistent with that obtained via the rotating bending fatigue test. Therefore, the proposed method is a promising, efficient, and reliable for use in high-strength steels with high cleanliness to inspect the maximum size inclusion and predict fatigue strength.
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圖 1 標準拉伸試樣形狀及尺寸（單位：mm）
Figure 1. Dimensions and shape of the tested tensile specimen (unit: mm)
圖 2 20Cr2Ni4A鋼未充氫（a，b）和不同充氫電流密度下（c，d，e，f）拉伸試樣的SEM斷口形貌。（a，c，e）低倍整體形貌；（b，d，f）斷口及圓形脆性平臺高倍形貌；（c，d） 4 mA?cm–2, 72 h；（e，f） 8 mA?cm–2, 72 h
Figure 2. SEM fractographs of uncharged (a,b) and hydrogen-charged (c,d,e,f) 20Cr2Ni4A specimens at different current densities: (a,c,e) low magnification of the fracture surfaces; (b,d,f) high magnification of the fracture surface and brittle circle platform regions; (c,d) 4 mA?cm?2, 72 h; (e,f) 8 mA?cm?2, 72 h
圖 3 20Cr2Ni4A鋼在16 mA?cm–2, 72 h的充氫制度下的拉伸試樣的典型SEM斷口形貌。（a，b）低倍形貌；（c）圓形脆性平臺及平臺中心的夾雜物形貌；（d）圖（c）中圓形平臺中心夾雜物的能譜；（e）圓形平臺區域；（f）圓形平臺外區域
Figure 3. SEM fractographs of a tensile specimen of 20Cr2Ni4A after hydrogen charging at 16 mA?cm?2 current density for 72 h: (a,b) overall view; (c) brittle circle platform regions and an inclusion in the center of a circle platform region; (d) EDX of the inclusion in (c); (e) the region within the circle platform; (f) the region outside of the circle platform
圖 4 充氫拉伸樣中檢測的夾雜物尺寸分布
Figure 4. Size distribution of inclusions detected in the hydrogen-charged tensile specimens
圖 5 氫脆拉伸法獲得的極值統計圖
Figure 5. Estimation of SEV method of hydrogen-charged tensile specimens
圖 6 金相法觀察到的典型夾雜物形貌（a）及其能譜（b）
Figure 6. Typical inclusion observed by metallographic method (a) and corresponding EDX of the inclusion (b)
圖 7 20Cr2Ni4A鋼的旋轉彎曲疲勞試驗結果。（a） S–N曲線；（b）疲勞源夾雜物尺寸及其位置
Figure 7. Results of rotating bending fatigue test of 20Cr2Ni4A: (a) S–N curves; (b) the size of inclusions at fracture origin and their distances from specimen surface
圖 8 20Cr2Ni4A鋼疲勞斷口的典型非金屬夾雜物形貌（a）及其能譜（b）
Figure 8. Typical non-metallic inclusion at fracture origin of 20Cr2Ni4A obtained using the rotating bending fatigue test (a) and corresponding EDX of the inclusion (b)
圖 9 金相法與疲勞法獲得的極值統計圖
Figure 9. Estimation of the SEV method of inclusions obtained using the metallographic and fatigue specimens
圖 10 不同充氫電流密度下樣品中氫含量（a）和工程應力–應變拉伸曲線（b）
Figure 10. Hydrogen content (a) and engineering stress–strain curves (b) of the specimens before and after hydrogen-charging at varying current densities
圖 11 20Cr2Ni4A鋼的體積與疲勞強度的關系
Figure 11. Relationship between the volume of 20Cr2Ni4A steel and the fatigue strength
表 1 實驗料20Cr2Ni4A的化學成分(質量分數)
Table 1. Chemical composition of the tested steel 20Cr2Ni4A
% C Si Mn P S Cr Ni Al O N 0.15 0.29 0.45 0.016 0.007 1.44 3.37 0.027 0.0022 0.0070 
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表 2 充氫后每個拉伸試樣中最大夾雜物尺寸(V0=589 mm3, N=10)
Table 2. Summary of the maximum inclusion size detected in each hydrogen-charged tensile specimen (V0=589 mm3, N=10)
Sample No. T-1 T-2 T-3 T-4 T-5 T-6 T-7 T-8 T-9 T-10 Maximum inclusion size /μm 11.96 20.21 23.75 14.09 14.71 20.12 15.65 18.80 16.18 17.55
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表 3 每個金相樣中最大夾雜物尺寸(V0=1.53 mm3, N=15)
Table 3. Summary of the maximum inclusion detected in each metallographic specimen (V0=1.53 mm3, N=15)
Sample No. M-1 M-2 M-3 M-4 M-5 M-6 M-7 M-8 Maximum inclusion size /μm 5.23 4.48 5.60 8.45 12.99 8.87 7.42 7.63 Sample No. M-9 M-10 M-11 M-12 M-13 M-14 M-15 Maximum inclusion size /μm 6.48 8.52 8.26 7.02 6.30 8.72 8.60
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表 4 每個疲勞失效樣品起裂源處夾雜物尺寸(V0=840 mm3, N=11)
Table 4. Summary of the inclusion size detected in fatigue failure origins (V0=840 mm3, N=11)
Sample No. F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 F-10 F-11 Inclusion size /μm 23.16 22.37 28.50 31.07 26.52 26.21 28.10 26.78 29.18 30.36 20.12
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表 5 不同體積鋼中最大夾雜物尺寸的預測
Table 5. Estimated maximum inclusion size for different volumes of the tested steel
Method Volume,V0 / mm3 Maximum size of inclusion /μm V=103 mm3 V=104 mm3 V=105 mm3 V=106 mm3 V=5.02×106 mm3 Fatigue 840 23.05 32.74 40.21 47.57 52.72 Hydrogen embrittlement-tensile 589 16.00 25.26 33.29 41.25 46.81 Metallographic 1.53 18.50 22.69 26.88 31.07 34.01
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