粉末射出成形燒結硬化合金鋼

Ultrahigh-Strength Sinter-Hardening MIM Alloy Steels

徐禎(Chen Hsu)

指導教授 ： 黃坤祥

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.10955

摘要

燒結硬化型合金鋼含有豐富的高硬化能合金元素，如錳、鉬、鉻、鎳、銅等，使其不需經過淬火即可得到良好的強度以及硬度。此型合金鋼已普遍用在傳統粉末冶金製程中，但仍未見金屬粉末射出成形工件，也未見於各國之材料標準中。本研究嘗試最佳化一粉末射出成形Fe-Ni-Cr-Mo燒結硬化型合金鋼之製程參數，其結果為：在1330⁰Ｃ燒結兩小時，在200⁰Ｃ回火兩小時，或是燒結後施以液態氮深冷處理後再以200⁰Ｃ回火兩小時。本研究之合金的機械性質已大幅超越美國金屬粉末工業聯盟(MPIF)所有材料之標準，並與鍛造之AISI 4340 合金鋼相當。
在本研究所試驗的Fe-Ni-Cr-Mo系合金之中，Fe-6Ni-0.8Cr-0.8Mo擁有最好的機械性質，其理想碳含量區間約為0.35~0.45%。該合金在燒結後具有52.5HRC之硬度，拉伸強度為2130MPa，降伏強度為1470MPa，而伸長量為7.0%。回火後其拉伸強度與降伏強度分別略降至1990MPa以及1450MPa，而伸長率增加至8.6%。若經過深冷處理並回火，拉伸強度為2110MPa，降伏強度則大幅增加至1780MPa，並仍擁有7.4%高伸長率。在各種條件下之衝擊能約為 30~60J 。另一Fe-8Ni-0.8Cr-0.8Mo合金亦具有與Fe-6Ni-0.8Cr-0.8Mo相似之性質，其拉伸強度與硬度略低而延性較佳，但降伏強度有約200MPa之差距。其原因為鎳含量過高而導致太多殘留沃斯田鐵。
本研究亦發現錳可藉由Nitronic60 預合金粉添加至合金中，有益於機械性質，並且真空燒結後無明顯重量損失。Fe-6Ni-0.8Cr-0.8Mo-0.2Mn合金在燒結後之強度可達2270MPa，降伏強度為1560MPa，伸長率為 6.6%。回火後則分別為2010MPa，1420MPa，與8.2%。深冷處理並回火後則分別為2100MPa，1830MPa，與7.1%。
與粉末冶金件相比，本研究之合金具有相當高之伸長量及衝擊能，由X光繞射儀以及穿透式電子顯微鏡分析發現本研究之合金含有約6~18%之殘留沃斯田鐵，並與麻田散鐵基地呈交錯層狀結構。故殘留沃斯田鐵可扮演麻田散鐵中間的緩衝角色，阻止微裂縫行進與成長。

關鍵字

金屬射出成形 ； 燒結硬化型合金鋼 ； 深冷處理 ； 硬化能

並列摘要

Sinter-hardening alloy steels contain abundant high-hardenability alloying elements, such as Mn, Mo, Cr, Ni, and Cu, and can obtain high strength and hardness without quenching. These alloys have been adopted in conventional press-and-sintered (P/S) parts. However, there is no such alloy employed in the metal injection molding (MIM) industry. This study modified the sintering temperature and heat treatments of Fe-Ni-Cr-Mo MIM sinter-hardening steels and the optimized parameters are: sintering at 1330⁰C for 2 hours, tempering at 200⁰ C for 2 hours, and cryogenic treatment can be added before tempering to achieve higher yield strength. Using the optimized process, the mechanical properties of the steels developed in this study are much better than those of the P/S and MIM standard alloys and are similar to those of wrought AISI 4340.
Among the Fe-Ni-Cr-Mo alloys examined, Fe-6Ni-0.8Cr-0.8Mo has the best mechanical properties when its carbon content is in the range of 0.35~0.45%. It has a hardness of 52.5HRC, an UTS of 2130MPa, a yield strength of 1470MPa, and an elongation of 7.0% after sintering. After tempering, the UTS and yield strength decrease to 1990MPa and 1450MPa, respectively, and the elongation increases to 8.6%. When cryogenically treated and tempered, the yield strength increases to 1780MPa, and the elongation remains at about 7.4%. The impact energies of all these alloys are about 30~60J.　The Fe-8Ni-0.8Cr-0.8Mo has similar mechanical properties to those with 6% Ni, except that the yield strength is about 200MPa lower.
Manganese can be added to the Fe-Ni-Cr-Mo steel using Nitronic60 powders to improve the mechanical properties and without obvious weight loss after vacuum sintering. The Fe-6Ni-0.8Cr-0.8Mo-0.2Mn steel has 2270MPa in UTS, 1560MPa in yield strength, and 6.6% in elongation after sintering. After cryogenic treatment and tempering, the properties improve to 2100MPa, 1830MPa, and 8.2%.
The XRD and TEM analyses indicate that the steels developed in this study contain about 6~18% retained austenite, which forms a layered structure with the martensite. This ductile austenite could inhibit the crack propagation and thus improves the elongation and impact energy.

並列關鍵字

metal injection molding ； sinter-hardening steel ； cryogenic treatment ； hardenability

參考文獻

[1] 黃坤祥，粉末冶金學，中華民國粉末冶金協會，第二版，2003，第246-255，271，274-275頁。

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[4] H. E. McGannon, The Making, Shaping, and Treating of Steel, United States Steel Corporation, 1971, pp. 1132.

[5] D. R. Askeland, The Science and Engineering of Materials, PWS Publishing Company, 4th ed., Boston, MA, 1989, pp. 378.