17-4PH不銹鋼之乾壓燒結製程研究

Abstract

一般的17-4PH粉末硬度高 (~320HV)，在加壓時不易塑性變形，粉末壓縮性差，不易達到理想之生胚密度及燒結密度。若採用粒徑較小的17-4PH粉末雖能得到高密度的燒結工件，但粉末間的摩擦力過高，導致其流動性差，粉末充填困難。由於上述的問題使17-4PH不銹鋼在傳統乾壓燒結製程上的應用受限。本實驗選用粉末硬度較低的430L混合系統(430L+Ni+Cu+Nb)和母合金混合系統(Fe+17-4PH母合金)，嘗試提高其壓縮性，再利用噴霧造粒製程，改善粉末的流動性，此造粒粉還會用於乾壓和溫壓製程。本文之第一部份在瞭解成形壓力、溫度和黏結劑對造粒粉生胚密度的影響，第二部份則在瞭解生胚燒結後的性質並與使用17-4PH預合金粉之試片作比較。 研究發現以430L為基礎粉，添加適量之鎳、銅、鈮元素粉混合成17-4PH之成分，燒結後試片的孔隙率較17-4PH預合金高的多。若使用316L作為鎳元素之來源後，能有效改善此情況，燒結後之性質與採用17-4PH預合金粉者相似，相對密度達97%以上；在母合金混合系中因母合金粉含有大量的鉻元素，在含氮氣氛下燒結容易生成氧化鉻或氮化鉻，阻礙元素之擴散，產生因鎳聚集而生成的奧斯田鐵區域，若使用真空燒結則能使組織均勻，相對密度達97%。此外，使用430L和母合金混合系統均具有較高的粉末壓縮性，使用800MPa之成形壓力時生胚密度最高可達6.44g/cm3和6.38g/cm3。 430L和母合金混合系統經過噴霧造粒製程後，造粒粉之粒徑為預合金的6至7倍，粉末流動性改善至11sec/50g，乾壓後生胚密度可進一步提高到6.51g/cm3和6.50g/cm3，燒結後試片可達97%相對密度，且工件收縮率在6%以下;若使用溫壓成形，在黏結劑Tg以上之溫度加壓均能改善生胚密度，可分別達6.64g/cm3和6.57g/cm3，燒結後收縮率小於5%，試片性質均與使用17-4PH預合金粉者相似。所以利用430L混合系統與母合金混合系統之造粒粉適合取代17-4PH預合金粉，能改善17-4PH預合金之壓縮性、流動性和尺寸穩定性等問題。並能應用在傳統乾壓燒結製程上。

17-4PH is seldom produced using conventionally press-and-sinter process because the high hardness and poor compressibility of the powders cannot obtain high green density and thus cannot obtain high sintered density. Although fine powders have high sinterability and can solve this problem, they are not easy to flow because of the interparticle friction. That’s why 17-4PH is limited in conventionally press-and-sinter route. In this study, 17-4PH was prepared by using soft fine base powders, 430L system (430L+Ni+Cu+Nb) and master alloy system (Fe+17-4PH master alloy), to improve the compressibility. Spray drying process was also applied to improve the flow rate. The effects of compaction pressure, temperature and binder on green density and sintered density were examined. All the results were compared to those of using 17-4PH pre-alloyed samples. It was observed that using 430L base powder with elemental powder additions of nickel, copper and niobium had low sintered density. After using 316L as the source of nickel, the densificaition problem was solved. Relative density reaches 97% or greater. It was observed that the master alloy system was not suitable for sintering in atmospheres containing nitrogen because chromium oxide / nitride easily formed and inhibited diffusion. Vacuum or hydrogen sintering can solve this problem. Both 430L and master alloy systems provide high compressibility and the green density reaches 6.44g/cm3 and 6.38g/cm3 respectively. After spray drying process, the green density of spray dried powder increased to about 6.50g/cm3. After sintering, the density reached 97% and the shrinkage was below 6%. Warm compaction at a temperature above the Tg of the PVA, further increased the green density of spray dried powder to about 6.57g/cm3 and the flow rate was not affected. After sintering, shrinkage was below 5% and the mechanical properties were similar to those of using 17-4PH pre-alloyed powders. Therefore, spray dried powders made of 430L system and master alloy system solve the problems of using 17-4PH pre-alloyed powders and could open up a new market for conventional press-and-sinter industry.