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摘要: 采用軸向應變幅控制的低周疲勞試驗研究了總應變幅對4Cr5MoSiV1熱作模具鋼700 ℃低周疲勞行為的影響，包括循環應力響應行為、循環應力應變行為、循環遲滯回線和應變疲勞壽命行為等。結果表明：隨著總應變幅從0.2%增大到0.6%，4Cr5MoSiV1鋼在700 ℃時循環應力響應均表現為先循環硬化再循環軟化的特性，并且應力幅最大值從220 MPa增大到308 MPa。同時，隨著總應變幅的增大，4Cr5MoSiV1鋼在700 ℃下的低周疲勞壽命由6750循環周次降低到210循環周次，且其過渡壽命約為1313循環周次。疲勞斷口形貌分析結果顯示，高溫低周疲勞過程中裂紋主要萌生于試樣表面處，且隨著應變幅增大，裂紋源逐漸增多，疲勞條紋間距變寬，其斷裂方式由韌性斷裂轉變為脆性斷裂。透射電鏡分析結果顯示，循環軟化可能與板條結構轉變為胞狀結構、基體發生位錯湮滅、碳化物的析出和粗化有關。
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關鍵詞:
- 4Cr5MoSiV1鋼 /
- 高溫低周疲勞 /
- 循環硬化或軟化 /
- 疲勞壽命 /
- 疲勞斷口
Abstract: 4Cr5MoSiV1 hot-die steel exhibits excellent thermal fatigue and comprehensive mechanical properties, and it is widely used in hot forging die and hot extrusion die. Under actual service conditions, mold cavity temperature reaches 700 ℃ during mold operation. Mold cavity surface produces tension and compression strain owing to acute heat and cooling-constraints of subsurface layer, thereby resulting in local plastic deformation of mold and low-cycle fatigue. Therefore, low-cycle fatigue behavior of 4Cr5MoSiV1 steel at 700 ℃ is studied to provide reference data for component design and life prediction of 4Cr5MoSiV1 steel. The effect of total strain amplitude on low-cycle fatigue behavior of 4Cr5MoSiV1 steel at 700 °C has not been studied. The influence of total strain amplitude on the low-cycle fatigue behavior of 4Cr5MoSiV1 steel at 700 ℃ was studied using the low-cycle fatigue test with an axial strain amplitude control, including cyclic stress-response behavior, cyclic stress-strain behavior, cyclic hysteresis loop, and strain-fatigue life behavior. Results show that, with the increase of the total strain amplitude from 0.2% to 0.6%, the cyclic stress responses of 4Cr5MoSiV1 steel at 700 ℃ show the characteristics of cyclic hardening and recycling softening, and the maximum stress amplitude increases from 220 MPa to 308 MPa. As the total strain amplitude increases, the low-cycle fatigue life of 4Cr5MoSiV1 steel at 700 ℃ decreases from 6750 cycles to 210 cycles, and its transition life is approximately 1313 cycles. The results of fatigue fracture morphology analysis show that the crack mainly occurs on the surface of the sample during the high-temperature low-cycle fatigue. With the increase in the strain amplitude, the crack source gradually increases, the distance between fatigue stripes widens, and the fracture mode changes from ductile to brittle fracture. The results of TEM analysis show that the cyclic softening may be related to the change of lath structure to cellular structure, dislocation annihilation of matrix, carbide precipitation, and coarsening. -
圖 4 700 ℃靜態試驗結果。（a） 拉伸曲線；（b） 左圖中所選應變范圍的局部放大圖
Figure 4. Results of static tests: (a) tension diagrams; (b) magnification of the left diagram section and selection of deformation amplitude
圖 5 4Cr5MoSiV1鋼700 ℃的循環應力響應
Figure 5. Cyclic stress response of 4Cr5MoSiV1 steel at 700 ℃
圖 6 4Cr5MoSiV1鋼在700 ℃時的循環應力幅與塑性應變幅的關系曲線
Figure 6. Cyclic stress amplitude versus plastic strain amplitude of 4Cr5MoSiV1 steel at 700 ℃
圖 7 4Cr5MoSiV1鋼在不同應變幅下半壽命時的遲滯回線
Figure 7. Hysteresis loops of 4Cr5MoSiV1 steel at half lifetime under various strain amplitudes
圖 8 4Cr5MoSiV1鋼在700 ℃時的應變幅?載荷反向周次關系曲線
Figure 8. Strain amplitudes versus reversals to failure curves of 4Cr5MoSiV1 steel at 700 ℃
圖 9 4Cr5MoSiV1鋼在不同應變幅下的源區形貌。（a）
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ；（b）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.3 \% $ ；（c）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ ；（d）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.6 \% $ Figure 9. Crack initiating source area morphology of 4Cr5MoSiV1 steel at different strain amplitudes: (a)
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ; (b)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.3 \% $ ; (c)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ ; (d)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.6 \% $ 
圖 10 4Cr5MoSiV1鋼在不同應變幅下的擴展區形貌。（a）
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ；（b）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.3 \% $ ；（c）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ ；（d）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.6 \% $ Figure 10. Cracking propagation morphology of 4Cr5MoSiV1 steel at different strain amplitudes: (a)
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ; (b)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.3 \% $ ; (c)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ ; (d)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.6 \% $ 
圖 11 4Cr5MoSiV1鋼在不同應變幅下的疲勞瞬斷區形貌。（a）
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ；（b）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.3 \% $ ；（c）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ ；（d）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.6 \% $ Figure 11. Final fracture morphology of 4Cr5MoSiV1 steel at different strain amplitudes: (a)
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ; (b)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.3 \% $ ; (c)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ ; (d)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.6 \% $ 
圖 12 4Cr5MoSiV1鋼在700 ℃保溫不同時間的微觀組織。（a） 225 min；（b） 41 min
Figure 12. Microstructure of 4Cr5MoSiV1 steel at 700 ℃ at different time: (a) 225 min；(b) 41 min
圖 13 4Cr5MoSiV1鋼在不同狀態下的微觀組織。（a） 700 ℃，225 min；（b） 700 ℃，41 min；（c）
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ , 700 ℃，225 min；（d）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ , 700 ℃，41 minFigure 13. Microstructure of 4Cr5MoSiV1 steel under different states: (a) 700 ℃，225 min; (b) 700 ℃, 41 min; (c)
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ , 700 ℃, 225 min; (d)$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ , 700 ℃, 41 min
圖 14 4Cr5MoSiV1鋼在不同狀態下組織的透射電鏡照片. （a） 調質態；（b）
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ；（c）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ Figure 14. TEM micrographs of 4Cr5MoSiV1 steel under different states: （a） quenched and tempered state；（b）
$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.2 \% $ ；（c）$ \Delta {\varepsilon }_{\mathrm{t}}/2=0.4 \% $ 表 1 4Cr5MoSiV1鋼的化學成分（質量分數）
Table 1. Chemical compositions of 4Cr5MoSiV1 steel
% C Cr Mo V Si Mn Fe 0.40 5.00 1.10 1.00 1.00 0.30 Balance 
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表 2 4Cr5MoSiV1鋼的700 ℃機械性能
Table 2. Mechanical properties of 4Cr5MoSiV1 steel at 700 °C
Yield strength，σ0.2/MPa Tensile strength，σm/MPa Elongation，A/% Reduction of area，Z/% 187 331 60 91
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表 3 4Cr5MoSiV1鋼的低周疲勞測試結果
Table 3. Low-cycle fatigue test results of 4Cr5MoSiV1 steel
(Δεt/2)/% (Δεe/2)/% (Δεp/2)/% ${N}_{\mathrm{f}}$ 0.2 0.1425 0.0575 6750 0.3 0.164 0.136 2399 0.4 0.171 0.229 618 0.6 0.164 0.436 210
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