添加石墨對17-4 PH金屬射出成形零件之SiO2夾雜物及機械性質之影響

Abstract

17-4 PH射出成形不銹鋼結合了良好的機械性質與抗腐蝕能力，因此已廣泛應用於航太與醫療市場。以往之文獻多在探討17-4 PH的時效條件與合金元素的添加對機械與抗腐蝕性質之影響，鮮少著墨於碳含量及夾雜物的關係。本實驗將焦點放在添加適當比例的石墨以還原水噴霧粉中所含SiO2，希望藉此提升17-4 PH射出成形不銹鋼之機械性質，同時探討因此所增加之碳含量對17-4 PH射出成形件抗腐蝕能力之影響。結果顯示，添加0.20%石墨即可還原大部分的SiO2，添加0.26%石墨﹙燒結後碳含量為0.031%﹚於17-4 PH不銹鋼時可獲得最佳之硬度及強度，此時肥粒鐵相比例由未添加石墨之17.1%降為12.8%。經真空燒結後硬度可達HRC 30.3，抗拉強度為1048 MPa，延性為9.8%。經固溶處理和時效後硬度可達HRC 40.4，抗拉強度為1311 MPa，同時保有優良的延性9.0%。同時，晶界上會析出不連續相的NbC，抑制了碳化鉻的生成，因此維持了17-4 PH優良的抗腐蝕能力。 本實驗並且針對316L射出成形不銹鋼和燒結硬化合金鋼中之SiO2進行研究。316L不銹鋼之SiO2來源與17-4 PH相同，皆是在水噴霧製粉時產生，而燒結硬化合金鋼之SiO2則是來自於製作羰基鐵粉步驟中所添加之分散劑。雖然兩者的SiO2來源不同，但燒結時分別可藉石墨的添加和原始粉中的固溶碳而還原。 本實驗同時為17-4 PH射出成形件找出另一適當的熱處理條件，即在真空爐中燒結後以風扇冷卻至室溫，由於冷卻速率也相當快，所以可直接以H900條件時效。此製程省略了固溶處理，但仍可獲得與一般步驟相同的機械性質。直接時效者之肥粒鐵相比例和硬度都較固溶再時效者為低，但麻田散鐵相比例相對提高。經過計算兩不同熱處理程序之試片的麻田散鐵相和肥粒鐵相之硬度總合相同，這就是射出成形件可以省略固溶處理之原因。

Metal injection molded (MIM) 17-4 PH stainless steels are wildly applied in the aircraft and medical industries due to their good combination in mechanical properties and corrosion resistance. Most previous works have been focused on the effects of aging conditions and alloying additions on the sintered properties and little studies are about the carbon content and SiO2 inclusions, which is believed to deteriorate significantly the mechanical properties. Therefore, the objective of this study was to get improved mechanical properties and corrosion resistance by controlling the carbon content and eliminating SiO2 inclusions in the 17-4 PH MIM parts. The results show that most SiO2 could be eliminated by adding 0.20% graphite. The highest hardness (HRC 30.3) and tensile strength (1048 MPa) was achieved by adding 0.26% graphite, which resulted in a carbon content of 0.031%. The relative density was 99.1%. After solution and aging treatment, the hardness and tensile strength were increased to HRC 40.4 and 1311 MPa, respectively, and the elongation only decreased slightly from 9.8 to 9.0%. These are very close to those of the wrought 17-4PH stainless steels. Since the cooling rate in the vacuum furnace, which was equipped with a fan, was relatively fast in the cooling stage of the sintering cycle, direct H900 aging, without solution treatment, was attempted and the mechanical properties are comparable to those using standard solution-aging treatment. Although graphite was added, chromium carbide was not found in sintered parts due to the formation of NbC at the grain boundaries. As a result, the good corrosion resistance of the 17-4 PH was retained. The same approach of adding graphite to eliminate SiO2 was also applied to 316L and sinter-hardening steels and similar results were obtained.