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第1章 緖論 7

第2章 破壞力學的 背景 11

1. 彈塑性 破壞靭性 11

2. 疲勞크랙傳播擧動과 確率的特性 13

가. 時間的, 空間的 確率特性 16

나. 幾何學的, 空間的 確率特性 17

다. 疲勞壽命과 時間强度의 確率分布 17

라. 疲勞壽命의 統計的 分布 19

3. 高溫크리이프 및 크리이프 破斷曲線 20

가. 高溫크리이프 20

나. 크리이프破斷 데이터의 統計的 定理 22

第3章 試驗片 및 實驗方法 25

1. 破壞靭性實驗 25

가. 材料 및 試驗片 25

나. 實驗方法 25

2. 크리이프實驗 33

가. 材料 및 試驗片 33

나. 實驗方法 33

3. 疲勞크랙傳播實驗 37

가. 材料 및 試驗片 37

나. 實驗方法 37

4. 實驗結果에 대한 推定法 40

가. 分散分析表에서 F 檢定 40

나. 分散分析後의 推定 40

第4章 破壞靭性의 評價 42

1. 實驗結果 42

가. 機械的性質 42

나. 破壞靭性値 42

2. 考察 45

가. 試驗溫度別 各 材料의 機械的强度 特性 45

나. 下界溫度에서의 破壞靭性値 45

다. 上界溫度에서의 破壞靭性値 47

라. 破面樣相에 따른 觀察 50

第5章 疲勞크랙破壞의 信賴性 特牲 53

1. 實驗結果 53

가. m과 C의 分布 53

나. 데이터의 排列 60

2. 考察 62

第6章 크리이프 特性에 의한 壽命豫測 64

1. 實驗結果 64

가. 機械的 性質 64

나. 크리이프破斷强度 66

2. 考察 71

가. 熱處理에 따른 크리이프特性 71

나. 主破斷 曲線과 壽命 豫測 73

第7章 强度에 미치는 介在物의 影響 78

1. 試驗結果 78

가. 介在物의 分布狀態 78

나. 介在物에 의한 機械的强度의 分布狀態 78

2. 考察 81

가. FATT에 미치는 影響 81

나. 遷移溫度變化에 미치는 影響 83

다. KIC 算定 및 介在物의 影響[이미지참조] 87

라. 疲勞크랙成長과 介在物의 影響 92

마. 疲勞破面 觀察 101

第8章 結論 106

Nomenclature 109

參考文獻 112

SUMMARY 121

초록보기

 In this study, tension test, impact test and fracture toughness test were performed in range of temperatures, -150℃~+150℃, using the specimens of CrMoV steel for the turbine rotor manufactured by domestic. At each test temperature mechanical properties and fracture toughness were obtained experimentally and the availability of prediction method was reviewed by comparing the test values with the estimated values. And statistical characteristics of coefficient C and exponent m in fatigue crack propagation rate equation were reviewed and confidence intervals were constructed crearly by analysis of variance. In order to investgate the effects of nonmetallic inclusion which include residual elements such as P, S, Sn, Sb, As, H, N, etc. On mechanical strengths and fatigue crack behavior, these elements were included compulsory in samples and FATT, KIC, relation between stress intensity factor range and fatigue crack propagation rate and fracture surfaces were reviewed. Creep behavior was tested at operation temperature, 530℃ and operation life was estimated using Larson- Miller parameter and Orr-Sherby-Dorn parameter. Variation of creep property was also reviewed according to heat treatment conditions.

The results can be summarized as follows.

1. The estimated fracture toughness from yield strength of rotor steel was very close to the practical test value.

2. Test specimens were not satistied with the thickness requirement of ASTME 399 in fracture toughness tests when ductile fractured zone existed at boundary between fatigue crack zone and brittle fractured zone.

3. Calculating the confidence intervals of crack propagation coefficient C and crack propagation exponent m by analysis of variance in fatigue crack propagation test, level combination of 100(1-α)% confidence intervals maximizing population mean at level of significance α＝0.05 was 1700 kgf or 2500 kgf load range Δp＝0.2 or 0.3 stress ratio R.

4. Crack propagation coefficient C and crack propagation exponent m follow a log normal distribution and a normal distribution respectively and the relation of log C and m shows a strong negative correlation in fatigue crack propagation rate equation. And in the relation C＝CoKo-m, log Co follows a normal distribution and degree of contribution of log Co distribution to the distribution of log C shows very small as compared with degree of contribution by m distribution.

5. The best combination of quenching and tempering temperatures for good creep properties is 970℃~980℃ quenching, -660℃~680℃ tempering sequence. Estimated operation life at the maximum pressure point (10.56 kgf/ mm²) in the rotor was computed about 21.9X105 hours, if the turbine rotor shaft of thermal power plant would be continuously operated at 530℃, 3600 rpm and be influenced by the effect of creep strain only.

6. 105hr creep rupture strengths estimated by Larson-Miller parameter and by Orr-Sherby-Dorn parameter were 18.1 kgf mm² (530℃)/16.0 kgf/ mm² (540℃) and 17.3 kgf/ mm² (530℃)/15.9 kg, mm² (540℃) respectively, therefore it shows that the development of rotor material having a safty factor over 2 was possible.

7. Yield strength and tensile strength of turbine rotor shaft were 65 kgf/ mm² and 80 kgf/ mm², mechanical and creep properties showed homogeneous values through the whole of the turbine rotor shaft. Creep rupture time of 1st tempering sample was. 2~3 times longer than that of 2nd tempering sample at 500℃ and 550℃. This phenomena is caused by coarsening of precipitated carbide by release of supersaturation of composite structure and by refining of austenite grain.

8 The effects of residual elements on the Fracture Appearamce Transition Temperature(FATT) can be formulated as following representative equations.

(1) FATT ＝ 132.81 + 71.56 Si + 591.86 P + 594.76 S - 111.25 Cr - 2.16 Cu + 458.46 As + 536.83 Sn + 129.64 Sh.

(2) FATT ＝ 10.73 + 87.58 Si + 371.76 P + 688.50 S - 38.61 Cr - 72.38 Cu + 777.45 N+ 277002.40 H + 1476.71 O

The elements raising FATT such as Si, P, S, As, Sn, Sb, N, H, O are concentrated at grain boundary and increase the degree of cleavage fracture surface and raise the Ductility Transition Temperature (DTT) and show a notable drop in fracture toughness KIC.

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