為了提升傳統商用耐熱不鏽鋼的高溫強度，同時降低合金成本，本研究基於鐵-鎳-鉻耐熱鋼合金系統設計出五個新耐熱合金：合金A-E。設計理念包括節鎳以節省合金成本；並添加微量元素，如鈮、鈷、鎢與鉬，加上碳與矽含量的提升，藉此形成多種碳化物如MC、M7C3與 M23C6來增加高溫強度。
本研究探討設計合金A-E和商用HP耐熱鋼的諸多性質，並進行比較。包括：鑄造態微結構觀察；機械性質測試如室溫硬度、1000℃拉伸測試、900℃/65MPa與982℃/35MPa的潛變測試；氧化性質測試包括： 900℃循環氧化測試、1000℃/1100℃/1150℃預氧化研究等。
實驗結果顯示，藉由鈮、鎢、鈷和鉬的添加，以及節鎳節鉻設計，而擁有不連續碳化物分布、緩慢的二次析出物成長速率等特性的合金E，其潛變壽命比商用HP耐熱鋼高20倍以上，室溫硬度與1000℃的拉伸強度也為商用HP耐熱鋼的1.4倍。此外，透過1000℃/50h的預氧化處理，合金E也能達到與商用HP耐熱鋼相同的抗氧化能力。因此，本研究成功開發出足以取代商用HP耐熱鋼之新型耐熱不鏽鋼。

Five heat resistant steel compositions (Alloy A-E) have been designed based on the Fe-Ni-Cr alloy system in attempts to increase cost performance of conventional heat resistant steels. With systematic additions of C, Si, Co, W, Nb, Mo, and the reduction of Ni content, further strengthening can be provided by formation of various types of carbides such as MC, M7C3, and M23C6.
To evaluate their properties, experimental work includes room temperature hardness, 1000°C tensile tests, creep tests under 900°C/65MPa and 982°C/35MPa, and high temperature oxidation of as-cast and pre-oxidized samples at 1000°C, 1100°C and 1150°C.
Comparing to the properties commercial HP alloy, the newly designed alloy can exhibit more than 10 times increase in creep rupture life by discontinuous carbide distribution and higher MC to M23C6 ratio caused by Nb, W and Mo addition and Cr decrement. Besides, tensile strength at 1000°C and room temperature hardness is 1.4 times better than that of HP alloy. Furthermore, the oxidation resistance was maintained by pre-oxidized treatment at 1000°C. Advanced heat resistant steels have been developed successfully with much improved cost-performance in this work.

Abstract ii
Acknowledgements iii
Table of Contents vi
List of Tables viii
I. Introduction 1
II. Literature review 6
2.1 Heat resistant stainless steels and their applications 6
2.2 Elemental effects in heat resistant steel (HP grade) 9
2.2.1 Nickel 9
2.2.2 Chromium 10
2.2.3 Niobium 12
2.2.4 Molybdenum 14
2.2.5 Silicon 15
2.3 Typical microstructures of HP alloy 17
2.4 Mechanical properties of HP alloy 19
2.5 High temperature oxidation of HP alloy 21
III. Experiment procedure 22
3.1 Alloy Design 22
3.2 Materials for experiments 23
3.3 Mechanical properties tests 23
3.3.1 Room temperature hardness test 23
3.3.2 High temperature tensile test 23
3.3.3 Creep test 24
3.4 Oxidation tests 26
3.4.1 Cyclic weight gain test 26
3.4.2 Pre-oxidation test 26
3.5 Microstructure observations 27
3.6 XRD analysis 27
IV. Results and discussions 28
4.1 As-cast material 28
4.1.1 CALPHAD simulation (Jmatpro database) 28
4.1.2 Microstructure of as-cast material 32
4.2 Mechanical properties and microstructure evolution 38
4.2.1 Room temperature hardness test and 1000℃ tensile test 38
4.2.2 Creep test results 39
4.2.3 Microstructures before and after creep 43
4.2.4 Observation of rupture surface cross section 52
4.3 Oxidation resistance 57
4.3.1 Cyclic weight gain test at 900℃ 57
4.3.2 One hour ageing test in different temperature (Pre-oxidation study) 61
V. Conclusion 69
Reference 71

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