真空燒結及熱處理對添加TaC微粉之440C不銹鋼其顯微組織與機械性質之強化

Strengthening of Microstructures and Mechanical Properties on 440C Stainless Steel by Adding TaC Powders in Vacuum Sintering and Heat Treatments

王致凱(Chih-Kai Wang)

指導教授 ： 張世賢

國立臺北科技大學/工程學院/材料及資源工程系研究所/碩士(2014年)

摘要

AISI 440C為高含碳量之麻田散鐵系不銹鋼，具有優異的硬度，耐蝕性以及機械強度所見長，因此廣泛的應用在工具鋼方面。其強化機制主要是固溶強化以及由鉻和鉬組成的碳化物之析出硬化，其中位於晶界之M7C3和析出於Fe基地的M23C6為主要的析出強化相；固碳化物之分佈、大小皆對AISI 440C 之機械性質有決定性的影響。此外，金屬基複合材料(MMC)已知具有相當優越的耐磨耗性質，例如許多相關研究會在鐵基材料上添加MC型碳化物，來作為鐵基金屬基材之強化介質以提升其性質；碳化鉭(TaC)因具有高硬度、高熔點以及和鐵基合金的相對化學穩定性，故本研究以碳化鉭加入AISI 440C中改善其性質與碳化物之析出。
本研究分別將不同比例的碳化鉭粉末(10, 20 and 30 wt%)及兩種不同粒徑(25-45 μm與25 μm以下)之AISI 440C粉末，以球磨法將AISI 440C與TaC之粉末進行均勻混合，經過加壓成型後，以1270、1280、1290°C進行真空燒結，接著進行淬火與回火熱處理，淬火之參數為1100°C持溫40分鐘，並以0.5 MPa之氮氣進行淬火；回火之參數則以480°C持溫150分鐘，並重覆三次。最後進行各項機械性質分析與微結構之觀察，分析項目包含：視孔隙率、硬度、橫向抗彎強度(TRS)以及腐蝕試驗等材料性質測試；並以XRD、OM、SEM與EDS等對微觀組織及析出物進行判別與分析。
實驗結果顯示，以液相燒結之440C-TaC視孔隙率可達到2%以下，但T30複合材料，因過多的TaC團聚阻礙Fe液相之擴散，使得燒結體仍有6.56%之孔隙。在機械性質方面，T10(S)於1270°C燒結後TRS最高，為2260 MPa，硬度方面則是T20(S) TaC於1290°C燒結後，具有最高硬度HRA 85.2。對照其微觀組織可發現，原先位於晶界之M7C3碳化物在添加TaC後逐漸被取代甚至消失，並轉變為M23C6碳化物析出於基地成為強化相，耐腐蝕方面則是T20(S)擁有對低的極化第流密度(1.14×10-3 A•cm-2)。經過熱處理之後，T10(S) 之抗彎強度更進一步提升(2260 MPa → 2458 MPa)，且硬度達到HRA 83.8，主要是因為經熱處理後之440C-TaC，其位於基地之M23C6進行分解與再出析出細小顆粒之碳化物，進一步提升了基地相之強度，但耐蝕性也略為下降。整體來說，淬火與回火熱處理能夠有效改善碳化物大小，並具有強化440C-TaC基地相之效果。

關鍵字

AISI 440C ； 金屬基複合材料 ； 碳化鉭 ； 真空燒結 ； 硬度 ； 熱處理

並列摘要

AISI 440C, which possesses the high carbon of martensitic stainless steel, is widely used in applications in the tool industry which require good hardness, corrosion resistance and mechanical strengthening. The primary strengthening mechanisms include solid-solution strengthening and precipitation-hardening effects of chromium and molybdenum carbides. As the main strengthening phases of M23C6 and M7C3 are precipitated in the matrix and grain boundary, respectively, the distribution and particle size of the carbides are important for AISI 440C steel. In addition, the metal matrix composites (MMC) are well known for its excellent wear resistance. For instance, many investigations have depended on the characteristic of carbide as a reinforcing medium in a ferrous metal matrix in order to enhance performance. Tantalum carbide (TaC) has proven efficacy due to its high hardness, high melting temperature and chemical stability with Fe-based alloys; research has shown that adding various mounts of TaC into AISI 440C steel also improves these properties, as well as the carbide precipitation.
In this study, we added and mixed different ratios of TaC powders (10, 20 and 30 wt%) to AISI 440C alloy powders (25-45 μm and below 25 μm) by the mechanical alloying process. After the compaction and forming processes, 440C-TaC alloy powder was sintered at 1270, 1280 and 1290°C by vacuum sintering, respectively. Furthermore, a series heat treatment process (quenching followed by tempering) was performed, by which the samples were heated to 1100°C and maintained at that temperature for 40 min for quenching, and 0.5 MPa of N2 for quenching media. Meanwhile, the tempering temperature was 480°C for 150 min, and repeated three times. Various material characterization techniques were used to evaluate the specimens’ properties, including porosity, hardness and transverse rupture strength (TRS), as well as corrosion tests. Moreover, the microstructures were analyzed using XRD, OM, SEM and EDS techniques.
The experimental results showed that the porosity of 440C-TaC was decreased by less than 2% by the liquid-phase sintering process. Excess TaC hindered the liquid diffusion of the Fe elements, which resulted in the T30 still having 6.56% porosity. In addition, the 1270°C-sintered T10(S) specimens possessed the highest TRS value (2260 MPa), while the 1290°C-sintered T20(S) specimens obtained the highest hardness value (HRA 85.2) and lowest corrosion current density（1.14×10-3 A•cm-2）. The microstructural evaluation revealed that the M7C3 carbides located on the grain boundary were gradually replaced and even disappeared after TaC particles were added. Meanwhile, all the rod-shaped M7C3 carbides transformed to M23C6 carbides, and precipitated on the matrix as a strengthening phase. Therefore, the TRS value of the T10(S) was obviously increased(2260 MPa → 2458 MPa) after heat treatment, while retaining its higher hardness (HRA 83.8). The chief concerns and factors were the hexagonal-shaped M23C6 carbides which decomposed and re-precipitated, resulting in the enhanced strength. The results showed that heat treatment effectively improved the particle size of the carbides and the strengthening of the matrix phase of 440C-TaC steel.

並列關鍵字

AISI 440C ； MMC ； TaC ； Sintering ； Vacuum Sintering ； Hardness ； Heat- Treatment

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