Effect of cooling methods on the microstructure and mechanical properties of marine engineering titanium alloy ring components
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摘要: 系统探究了油相冷却(Oil Cooling, OC)、吹风冷却(Wind Cooling, WC)和埋砂冷却(Sand Cooling, SC)3种热处理工艺方法对近α钛合金大型环轧件显微组织与力学性能的影响。结果表明，经980 ℃热处理的试样在不同冷却方式下强度变化不明显，这主要是由于该温度下β相体积分数较低，冷却过程对次生α(αs)相形成的影响较小。值得注意的是，试样经OC热处理后冲击功明显下降，这与过快的冷却速率促使脆性马氏体(α’)相析出有关。在990 ℃进行WC热处理后，试样实现了抗拉强度与冲击韧性的最佳匹配，这归因于该条件下初生α(αp)相体积分数低，高温β相未析出脆相α’，且αs板条宽度适中。此外，SC热处理时在高温条件下冷速较快，在低温条件下冷速较慢，而钛合金的相体积分数与板条宽度等显微组织形貌特征在相同温度下主要由高温时冷却速率决定，因此SC的试样力学性能介于OC与WC之间。Abstract: This study systematically investigates the influence of three cooling methods—oil cooling (OC), wind cooling (WC), and sand cooling (SC)—on the microstructure and mechanical properties in large-scale ring-rolled near-α titanium alloy components. Experimental results demonstrate that the strength of specimens heat-treated at 980 ℃ shows no significant variation across different cooling methods. This is primarily attributed to the low volume fraction of the β phase at this temperature, which minimizes the effect of cooling on the formation of the secondary α phase (αs). Furthermore, the experiments revealed a marked decrease in impact energy following OC heat treatment, which is associated with the rapid cooling rate promoting the precipitation of the brittle martensite phase (α'). An optimal balance between tensile strength and impact toughness is achieved after WC heat treatment at 990 ℃. This phenomenon is mainly attributed to the low volume fraction of the primary α phase (αp), the absence of brittle α' precipitation from the prior β phase, and the moderate width of the αs laths. Additionally, SC heat treatment exhibits a faster cooling rate during the high-temperature stage and a slower rate during the low-temperature stage. Since the microstructural morphological features such as phase volume fraction and lath width in titanium alloys are mainly governed by the cooling rate at high temperature, the mechanical properties of SC-treated specimens lie between those of OC and WC.
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图 2 Ti6321合金热处理工艺示意及不同温度热处理冷却后的形貌
(a)Ti6321合金热处理工艺; (b)Ti6321合金980 ℃热处理冷却后形貌; (c)Ti6321合金990 ℃热处理冷却后形貌
Figure 2. Heat treatment processes of Ti6321 alloy and speciments of Ti6321 alloy after heat treatment and cooling at different temperature
图 3 Ti6321合金取样示意
Figure 3. Schematic illustration of the Ti6321 alloy sampling methodology
图 4 Ti6321合金980 ℃保温后不同冷却方式下不同方向显微组织
(a)～(c)OC; (d)～(f)WC; (g)～(i)SC; (a)(d)(g)径向; (b)(e)(h)轴向; (c)(f)(i)切向
Figure 4. Microstructure of Ti6321 alloy after 980 ℃ holding under different cooling methods
图 5 不同冷却方式的时间-温度曲线示意
Figure 5. Schematic diagram of time-temperature curves for different cooling methods
图 6 Ti6321合金990 ℃保温后不同冷却方式下不同方向显微组织
(a)～(c)OC; (d)～(f)WC; (g)～(i)SC; (a)(d)( g)径向; (b)(e)(h)轴向; (c)(f)(i)切向
Figure 6. Microstructure of Ti6321 alloy after 990 ℃ holding under different cooling method
图 7 Ti6321合金TD-ND面αp体积分数与αs板条宽度统计结果
Figure 7. Statistical results of αp volume fraction and αs lath width on the TD-ND plane of Ti6321 alloy
(a) 980 ℃; (b) 990 ℃
图 8 Ti6321合金980 ℃处理不同冷却方式下不同方向拉伸强度
Figure 8. Tensile strength of Ti6321 alloy in different directions at 980 ℃
(a)OC; (b)WC; (c)SC
图 9 Ti6321合金990 ℃处理后不同冷却方式下不同方向拉伸强度
Figure 9. Tensile strength of Ti6321 alloy in different directions at 990 ℃
(a)OC; (b)WC; (c)SC
图 10 Ti6321合金不同方向冲击韧性
Figure 10. Impact toughness of Ti6321 alloy in different directions
(a) 980 ℃; (b) 990 ℃
图 11 Ti6321合金990 ℃不同冷却方式下RD//ND冲击试样断口形貌
Figure 11. Fracture surface of RD//ND impact specimen for Ti6321 alloy at 990 ℃ under various cooling rates
(a) OC; (b) SC; (c) WC
图 12 Ti6321合金显微组织随冷却速率降低而演变以及试样微观结构对裂纹扩展行为的影响的示意
Figure 12. Schematic illustrations of the microstructural evolution of Ti6321 alloy with decreasing cooling rates and the effect of sample microstructure on crack propagation behavior
(a)(b)OC; (c)(d)SC; (e)(f)WC
表 1 Ti6321合金成分
Table 1. Composition of Ti6321 alloy
% Al Nb Zr Mo Fe Si Ti H C N O 6.16 3.07 2.15 1.20 0.03 <0.03 balance 0.0012 0.0011 0.0019 0.095 
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表 2 Ti6321合金980 ℃处理不同冷却方式下不同方向拉伸强度
Table 2. Tensile strength of Ti6321 alloy in different directions at 980 ℃
OC WC SC Rp0.2/MPa Rm/MPa Rp0.2/MPa Rm/MPa Rp0.2/MPa Rm/MPa ND 794±5 968±5 794.5±11.5 938.5±4.5 784±3 963.5±3.5 TD 767±2 924±2 772±1 924±2 790.5±1.5 933.5±6.5 RD 774±7 972.5±5.5 770.5±7.5 917±5 772±9 921±6
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表 3 Ti6321合金990 ℃处理后不同冷却方式下不同方向拉伸强度
Table 3. Tensile strength of Ti6321 alloy in different directions at 990 ℃
OC WC SC Rp0.2/MPa Rm/MPa Rp0.2/MPa Rm/MPa Rp0.2/MPa Rm/MPa ND 799±7 983.5±9.5 853.5±16.5 948.5±1.5 818±8 934±8 TD 785.5±1.5 974±1 834.5±25.5 937±2 798±3 931±1 RD 773±5 969.5±4.5 800.5±7.5 925.5±1.5 786±1 922.5±3.5
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