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摘要: 通過熱處理制備出具有回火馬氏體組織、下貝氏體組織以及粒狀貝氏體組織的718鋼，利用光學顯微鏡、掃描電子顯微鏡、X射線衍射儀、萬能拉伸實驗機比較其顯微組織及力學性能。同時借助高速銑削實驗及光學輪廓儀，研究力學性能以及組織結構對切削性能的影響。結果表明，當切削速度低于145 m·min?1時，貝氏體組織類型比回火馬氏體組織更易切削，切削貝氏體組織比切削回火馬氏體組織的刀具使用壽命高30%～40%。當切削速度高于165 m·min?1時，馬氏體組織發生了加工軟化現象，刀具使用壽命提高，切削性能上升。粒狀貝氏體組織加工表面因為嚴重的刀具黏附而出現背脊紋路，馬氏體組織具有最佳的切削表面粗糙度。綜合考慮之下，三種組織的綜合切削性能從高到低排序為：下貝氏體組織、馬氏體組織、粒狀貝氏體組織，采用300 ℃等溫淬火工藝可以有效提升718塑料模具鋼的綜合切削性能。Abstract: Owing to strict dimension accuracy demands, pre-hardening treatment has been widely used in the mold for production of large plastic parts. However, the large volume of mold leads to the existence of tempered martensite and bainite structure on the cross section by pre-hardened heat treatment, and the uneven structure makes great influences on the cutting performance of the pre-hardening plastic mold steel. For service materials, machinability is affected by strength, work temperature, cutting conditions, plastic deformation, phase. Pioneering researchers tended to focus on the influences of temperature, cutting conditions and little is known about the effect of different microstructures in same materials. In this work, 718 steels with tempered martensite, lower bainite and grain bainite structures were prepared by heat treatment. The microstructures and mechanical properties were characterized by optical microscopy, scanning electron microscopy, X-ray diffractometer and universal tensile testing machine. Meanwhile, the effects of mechanical properties and structure on processing properties were studied by high-speed milling experiments and optical profilometer. The results show that when the cutting speed was lower than 145 m·min?1, the bainite was easier to cut than tempered martensite, and the life of tool cutting for bainite was 30%?40% higher than life of tool cutting for tempered martensite. When the cutting speed was higher than 165 m·min?1, tempered martensite microstructure worked softening and the life of tool cutting for it increased, moreover, its workability advanced. The ridges were observed on the milling surface of grain bainite because of severe tool adhesion and tempered martensite structure has the best milling surface roughness. Under consideration, the comprehensive machinability of the three kinds of microstructure are ranked from high to low: lower bainite structure, martensite structure and granular bainite structure. The adoption of 300 ℃ austempering process can effectively improve the synthesis cutting performance of 718 plastic die steel.
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Key words:
- microstructure /
- mechanical properties /
- work hardening /
- tool life /
- surface roughness
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圖 1 三種組織的熱處理工藝曲線。LB—下貝氏體；GB—粒狀貝氏體；M—回火馬氏體
Figure 1. Process curve of three microstructures: LB—Low bainite; GB—granular bainite; M—tempered mastensite
圖 2 三種試樣的組織圖。（a）下貝氏體試樣；（b）下貝氏體板條；（c）粒狀貝氏體試樣；（d） M/A島；（e）馬氏體組織；（f）馬氏體板條及板條間碳化物
Figure 2. Three different microstructures: (a) low bainite; (b) low bainite lath; (c) granular bainite; (d) M/A island; (e) martensite; (f) martensite lath and interlath carbides
圖 3 不同轉速時三種試樣的加工硬化率
Figure 3. Work hardening ratio of three specimens at different cutting speeds
圖 4 不同轉速時三種試樣的總切削力
Figure 4. Cutting forces of three specimens at different cutting speeds
圖 5 不同切削速度時三種試樣的后刀面磨損寬度曲線。（a） 125 m·min–1；（b） 145 m·min–1；（c） 165 m·min–1
Figure 5. Maximum flank wear of three specimens at different cutting speeds: (a) 125 m·min–1; (b) 145 m·min–1; (c) 165 m·min–1
圖 6 下貝氏體組織在切削一定時間后的刀具磨損狀態。（a,b）前刀面磨損圖；（c）后刀面磨損圖；（d） L1線的能譜圖
Figure 6. Tool wear of low bainite specimens after cutting for a certain time: (a,b) wear of rake face; (c) wear of flank face; (d) EDS analysis in L1 line
圖 7 不同試樣在轉速為165 m·min–1時的刀具后刀面磨損圖。（a）回火馬氏體；（b）粒狀貝氏體
Figure 7. Flank wear of different samples in cutting speed of 165 m·min–1: (a) tempered martensite; (b) grain bainite
圖 8 三種試樣在轉速為145 m·min–1下切削20 min后的刀具后刀面EDS元素分布圖。（a）回火馬氏體；（b）粒狀貝氏體；（c）下貝氏體
Figure 8. EDS element mapping of cutting tool uesd for milling of three specimens for 20 min at the cutting speed of 145 m·min–1: (a) tempered martensite; (b) grain bainite; (c) low bainite
圖 9 三種試樣切削20 min后表面宏觀形貌。（a）回火馬氏體；（b）粒狀貝氏體；（c）下貝氏體；（d）粒狀貝氏體3D形貌
Figure 9. Milling surface of three specimens for 20 min at the cutting speed of 145 m·min–1: (a) tempered martensite; (b) grain bainite; (c) low bainite; (d) 3D morphology of grain bainite
表 1 718鋼的化學成分（質量分數）
Table 1. Chemical composition of 718 steels
% C Si Mn P S Cr Ni Mo V Fe 0.40 0.28 1.28 0.015 0.003 1.82 0.99 0.33 0.09 Bal. 
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表 2 機加工參數
Table 2. Cutting conditions
Cutting speed, Vc / (m·min–1) 125,145,165 Feed per tooth, fz / (mm·z–1) 0.075 Cutting width, ae / mm 1.0 Cutting depth, ap / mm 1.0 Cooling method Air cooling Note: z represents a tooth on the milling cutter.
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表 3 三種試樣的力學性能及殘余奧氏體含量
Table 3. Mechanical properties and retained austenite of three samples
Samples Yield strength/ MPa Tensile strength/ MPa Elongation /% Section shrinkage / % Hardness, HRC Residual austenite content / % Low bainite 1145 1192 12.6 52.0 36 5.8 Grain bainite 986 1010 14.3 57.6 35 7.1 Tempered martensite 1210 1280 13.2 54.1 35 3.3
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