雙相不銹鋼在行業內已有超過60年之歷史。其在需要高強度及高耐腐蝕性之組合之許多應用中以熱處理鑄造、鍛造及氣體霧化粉末形式廣泛使用。然而，其目前無法以水霧化粉末形式用於壓製及燒結應用中。 雙相不銹鋼之常見用途包括化學加工廠管線、石油化學工業、發電廠及汽車。其亦用於食品加工工業、醫藥製程組件、造紙及製漿工業、海水淡化廠及採礦工業中。已知雙相不銹鋼在氯化物介質中具有較高之耐晶間腐蝕(IGC)性及耐應力腐蝕開裂(SCC)性。對於基於鐵之合金，氯化物係導致介質快速腐蝕之嚴重挑戰。 據信，雙相不銹鋼之高強度及高耐腐蝕性質係由於存在等量之肥粒鐵及沃斯田鐵(austenite)相而獲得。此結構通常係藉由使用沃斯田鐵穩定劑(例如、鎳(Ni)、錳(Mn)、碳(C)、氮(N)、銅(Cu)及鈷(Co))與肥粒鐵穩定劑(例如、鉻(Cr)、矽(Si)、鉬(Mo)、鎢(W)、鈦(Ti)及鈮(Nb))之平衡來達成。 如先前所提及，據信雙相不銹鋼之高強度及高耐腐蝕性係來自微結構中肥粒鐵及沃斯田鐵之平衡。微結構不僅取決於化學性質，且亦取決於對材料實施之熱處理。目前所有雙相鋼組成在化學上均利用N，此乃因N係沃斯田鐵強穩定劑。N在與Cr一起存在於合金中時，造成形成氮化物之問題，此對諸如強度及耐腐蝕性等性質係有害的。此外，在焊接雙相不銹鋼期間，由於較慢之冷卻速率，在熱影響區(HAZ)中形成稱為「σ」之金屬間相。此σ相係含有Cr及Mo之硬質過飽和金屬間相。σ相周圍之區域缺乏Cr及Mo，且變得脆弱且耐腐蝕性較低。雙相不銹鋼常常需要退火及淬火製程以減少或消除此σ相。 在鍛造或鑄造雙相不銹鋼中，在合金冷卻期間鋼固化為肥粒鐵鋼且沃斯田鐵相自肥粒鐵中析出。在鑄造或任何熱處理之後，冷卻速率係關鍵的，此乃因冷卻速率決定結構內所析出之沃斯田鐵及任何金屬間相之百分比。 儘管鍛造雙相不銹鋼、尤其「熱軋」雙相不銹鋼自1930年代以來常見於工業應用中，但其在粉末冶金(PM)工業中很少使用。存在一些其中在熱等均壓(HIP)條件下使用氣體霧化雙相不銹鋼粉末之應用。藉由氣體霧化產生之粉末具有球形形態。此等粉末較不適於習用壓製及燒結應用。由於球形形狀，其生坯強度不足，該生坯強度係處置生坯壓製及燒結部件所需的。不規則形狀之粉末(例如利用水霧化產生之彼等)具有更高之生坯強度，此乃因粉末之不規則形狀傾向於將粉末顆粒結合在一起。目前無可用於製造經燒結雙相不銹鋼組件之水霧化不銹鋼粉末。當前在氣體霧化粉末亦及鍛造鋼中所使用之化學組成使用N作為主要合金元素來達成沃斯田鐵-肥粒鐵平衡且達成所需之機械強度。在粉末中納入N增加粉末之硬度，從而降低習用壓製及燒結應用中之可壓縮性。此可導致生坯密度降低且隨後燒結密度降低。 已進行若干次嘗試開發自水霧化粉末製得之經燒結雙相不銹鋼。Lawley等人[1]嘗試開發等效等級之AISI 329及AISI 2205，其最大抗拉強度為578 MPa。Dobrzanski等人[2]
將肥粒鐵及沃斯田鐵粉末混合以產生抗拉強度為650 MPa之雙相結構。同一小組亦利用電化學方法研究雙相不銹鋼之腐蝕性質，且得出以下結論：雙相不銹鋼顯示優於其沃斯田鐵對應體之耐腐蝕性[3]
。由於其高合金含量，該等鋼對組成亦及處理參數敏感。該等合金形成稱為σ、χ及γ’之金屬間相，其富含Mo、W、N、Ni及Cr，且降低機械性質及腐蝕性質二者。σ相在700℃至1000℃之溫度範圍內形成，而χ相在300℃至450℃範圍內形成。γ (沃斯田鐵)相可在600℃左右開始形成。 鍛造雙相不銹鋼之典型組成為Fe與21-23 wt% Cr、4.5-6.5 wt% Ni、2.5-3.5 wt% Mo及0.08-0.2 wt% N，例如SAF 2205。存在許多雙相不銹鋼組成接近此組成之專利。幾乎所有之雙相不銹鋼均依賴於N含量來增加耐腐蝕性且增加強度。迄今為止，經燒結粉末冶金(PM)雙相不銹鋼之商業用途限於使用可用於主要HIP製程之氣體霧化細粉。使用低成本水霧化粉末用於習用PM使用之主要障礙係N增加及由於在燒結期間之冷卻速率所致之金屬間化合物及碳化物析出之可能性。此外，習用燒結需要一些潤濕劑或低溫熔融成分以增加自由能並加速肥粒鐵基質內沃斯田鐵相析出之動力學。 在專利文獻中，存在一些揭示經燒結雙相不銹鋼結構之文件。 SE538577C2 (Erasteel)揭示自氣體霧化粉末製得且具有以下化學組成之經燒結雙相不銹鋼：最高0.030 wt% C、4.5-6.5 wt% Ni、0.21-0.29 wt% N、3.0-3.5 wt% Mo、21-24 wt% Cr及視情況以下中之一或多者：0-1.0 wt% Cu、0-1.0 wt% W、0-2.0 wt% Mn、0-1.0 wt% Si，其中N等於或大於0.01* wt% Cr且其餘元素為Fe及不可避免的雜質。 EP0167822A1 (Sumitomo)揭示包含基質相及分散相之經燒結不銹鋼及製造方法。分散相係沃斯田鐵冶金結構且分散於整個基質相中，該基質相包含鋼組成不同於分散相或肥粒鐵-沃斯田鐵雙相不銹鋼之沃斯田鐵冶金結構。 JP5263199A (Sumitomo)揭示包含基質相及分散相之經燒結不銹鋼之製造。該方法包括使肥粒鐵不銹鋼粉末與選自以下之粉末混合：沃斯田鐵不銹鋼粉末、沃斯田鐵-肥粒鐵雙相不銹鋼粉末、沃斯田鐵-麻田散鐵(martensitic)雙相不銹鋼粉末及沃斯田鐵-肥粒鐵-麻田散鐵不鏽 三相不銹鋼粉末。將粉末混合物壓實並燒結。 EP0534864B1 (Sumitomo)揭示N含量為0.10-0.35 wt%且自與經燒結不銹鋼具有相同化學組成之氣體霧化鋼粉末製得之經燒結不銹鋼。Duplex stainless steels have been in the industry for over 60 years. It is widely used in heat treated casting, forging and gas atomized powder forms in many applications where a combination of high strength and high corrosion resistance is required. However, it is currently not available in water-atomized powder form for pressing and sintering applications. Common uses of duplex stainless steels include chemical processing plant pipelines, petrochemical industries, power plants and automobiles. It is also used in the food processing industry, pharmaceutical process components, paper and pulp industry, desalination plants and mining industry. Duplex stainless steels are known to have high resistance to intergranular corrosion (IGC) and stress corrosion cracking (SCC) in chloride media. For iron-based alloys, chlorides are a serious challenge leading to rapid corrosion of the medium. It is believed that the high strength and high corrosion resistance properties of duplex stainless steels are due to the presence of equal amounts of ferrite and austenite phases. This structure is usually achieved by the use of iron stabilizers (eg, nickel (Ni), manganese (Mn), carbon (C), nitrogen (N), copper (Cu), and cobalt (Co)) and ferric iron. A balance of stabilizers such as chromium (Cr), silicon (Si), molybdenum (Mo), tungsten (W), titanium (Ti), and niobium (Nb) is achieved. As mentioned previously, it is believed that the high strength and high corrosion resistance of duplex stainless steels result from the balance of Ferrite and Voss in the microstructure. The microstructure depends not only on the chemical properties, but also on the heat treatment performed on the material. All current dual-phase steel compositions utilize N chemically, because N is a strong stabilizer for Vostian iron. N, when present in alloys with Cr, causes the problem of nitride formation, which is detrimental to properties such as strength and corrosion resistance. Furthermore, during welding of duplex stainless steels, an intermetallic phase called "σ" is formed in the heat affected zone (HAZ) due to the slower cooling rate. This σ phase is a hard supersaturated intermetallic phase containing Cr and Mo. The region around the σ phase lacks Cr and Mo, and becomes fragile and has low corrosion resistance. Duplex stainless steels often require annealing and quenching processes to reduce or eliminate this sigma phase. In wrought or cast duplex stainless steels, the steel solidifies into ferrite steel and the Wort phase precipitates out of the ferrite during cooling of the alloy. After casting or any heat treatment, the rate of cooling is critical as it determines the percentage of iron and any intermetallic phases precipitated within the structure. Although wrought duplex stainless steels, especially "hot rolled" duplex stainless steels have been common in industrial applications since the 1930s, they are rarely used in the powder metallurgy (PM) industry. There are some applications in which gas-atomized duplex stainless steel powders are used under hot isotropic pressure (HIP) conditions. The powder produced by gas atomization has spherical morphology. These powders are less suitable for conventional pressing and sintering applications. Due to the spherical shape, it lacks the green strength required to handle green pressed and sintered parts. Irregularly shaped powders, such as those produced using water atomization, have higher green strength because the irregular shape of the powder tends to bind the powder particles together. There is currently no water-atomized stainless steel powder available for the manufacture of sintered duplex stainless steel components. The chemistries currently used in gas atomized powders and forged steels use N as the main alloying element to achieve the Vostian iron-ferrite balance and achieve the required mechanical strength. Inclusion of N in the powder increases the hardness of the powder, thereby reducing the compressibility in conventional pressing and sintering applications. This can result in a reduction in green density and subsequently a reduction in sintered density. Several attempts have been made to develop sintered duplex stainless steels made from water atomized powders. Lawley et al. [1] attempted to develop equivalent grades of AISI 329 and AISI 2205 with a maximum tensile strength of 578 MPa. Dobrzanski et al. [2] mixed fertilizer granulated iron and Vostian iron powder to produce a dual-phase structure with a tensile strength of 650 MPa. The same group also used electrochemical methods to study the corrosion properties of duplex stainless steels and concluded that duplex stainless steels show better corrosion resistance than their Vostian iron counterparts [3] . Due to their high alloy content, these steels are sensitive to composition and processing parameters. These alloys form intermetallic phases called σ, χ, and γ', which are rich in Mo, W, N, Ni, and Cr, and degrade both mechanical and corrosive properties. The sigma phase is formed in the temperature range of 700°C to 1000°C, and the chi phase is formed in the range of 300°C to 450°C. The γ (Worthian iron) phase can start to form around 600°C. A typical composition of a wrought duplex stainless steel is Fe with 21-23 wt% Cr, 4.5-6.5 wt% Ni, 2.5-3.5 wt% Mo and 0.08-0.2 wt% N, eg SAF 2205. There are many patents for duplex stainless steel compositions close to this composition. Almost all duplex stainless steels rely on N content to increase corrosion resistance and increase strength. To date, commercial use of sintered powder metallurgy (PM) duplex stainless steels has been limited to the use of gas atomized fines that can be used in the main HIP process. The main obstacles to using low cost water atomized powders for conventional PM use are the increase in N and the potential for intermetallic and carbide precipitation due to cooling rates during sintering. In addition, conventional sintering requires some wetting agents or low temperature melting components to increase the free energy and accelerate the kinetics of precipitation of the Wostian iron phase within the ferrite iron matrix. In the patent literature, there are several documents that disclose the structure of sintered duplex stainless steels. SE538577C2 (Erasteel) discloses sintered duplex stainless steel made from gas-atomized powders with the following chemical composition: up to 0.030 wt% C, 4.5-6.5 wt% Ni, 0.21-0.29 wt% N, 3.0-3.5 wt% Mo , 21-24 wt% Cr, and optionally one or more of the following: 0-1.0 wt% Cu, 0-1.0 wt% W, 0-2.0 wt% Mn, 0-1.0 wt% Si, where N equals or More than 0.01*wt% Cr and the remaining elements are Fe and inevitable impurities. EP0167822A1 (Sumitomo) discloses a sintered stainless steel comprising a matrix phase and a dispersed phase and a method of manufacture. The disperse phase is a Vostian iron structure and is dispersed throughout a matrix phase comprising a Vostian iron metallurgical structure with a steel composition different from the disperse phase or the ferrite-Wostian iron duplex stainless steel. JP5263199A (Sumitomo) discloses the manufacture of sintered stainless steel comprising a matrix phase and a dispersed phase. The method includes mixing a ferritic stainless steel powder with a powder selected from the group consisting of: Vostian iron stainless steel powder, Vostian iron-fecal iron duplex stainless steel powder, Vostian iron-martensitic duplex Stainless steel powder and Wostian iron-fertilizer iron-Matian loose iron stainless three-phase stainless steel powder. The powder mixture is compacted and sintered. EP0534864B1 (Sumitomo) discloses sintered stainless steel with N content of 0.10-0.35 wt% and made from gas atomized steel powder having the same chemical composition as the sintered stainless steel.
本發明之某些實施例之一個目標係提供用於習用PM之合金粉末，其將在燒結循環期間產生雙相結構。 本發明之某些實施例之另一目標係提供雙相經燒結不銹鋼。 本發明之某些實施例之另一目標係獲得較肥粒鐵鋼(例如430L)高至少35%之抗拉強度及與沃斯田鐵鋼(例如316L)相比耐腐蝕性增加一倍。 本發明之某些實施例之另一目標係提供不需要燒結後熱處理來製造雙相經燒結不銹鋼之方法。 以上目標可藉由以下態樣及實施例來完成。 在本發明之第一態樣中，提供不銹鋼粉末，其包含以下各項或由以下各項組成(以重量%計)： 至多0.1%之C， 0.5-3%之Si， 至多0.5%之Mn， 20-27%之Cr， 3-8%之Ni， 1-6%之Mo， 至多3%之W， 至多0.1% N， 至多4%之Cu， 至多0.04%之P， 至多0.04%之S， 至多0.8%之不可避免的雜質， 視情況以下中之一或多者：至多0.004% B、至多1% Nb、至多0.5% Hf、至多1% Ti、至多1% Co， 其餘為Fe。 不可避免的雜質不包括C、Si、Mn、Cr、Ni、Mo、W、N、Cu、P、S、B、Nb、Hf、Ti或Co之所列示元素。不可避免的雜質可包括在鋼製造期間不能控制或難以控制之雜質。該等雜質可來自所使用之原材料且亦可來自製程。該等雜質包括、Al、O、Mg、Ca、Ta、V、Te或Sn。不可避免的雜質可至多0.8%、至多0.6%、至多0.3%。不可避免的雜質可係O。O可以至多0.6%、至多0.4%或至多0.3%存在。另一不可避免的雜質可係以至多0.2%存在之Sn，高於0.2%之Sn含量在此上下文中則不視為不可避免的雜質且由此將視為故意添加。 在第一態樣之較佳實施例中，提供由以下各項組成之不銹鋼粉末(以重量%計)： 至多0.06%之C， 1-3%之Si， 至多0.3%之Mn， 23-27%之Cr， 4-7%之Ni， 1-3%之Mo， 0.8-1.5%之W， 至多0.07%之N， 1-3%之Cu， 至多0.04%之P， 至多0.03%之S， 至多0.8%之不可避免的雜質， 視情況以下中之一或多者：至多0.004% B、至多1% Nb、至多0.5% Hf、至多1% Ti、至多1% Co， 其餘為Fe。 在第一態樣之另一較佳實施例中，提供包含以下各項之不銹鋼粉末(以重量%計)： 至多0.03%之C， 1.5-2.5%之Si， 至多0.3%之Mn， 24-26%之Cr， 5-7%之Ni， 1-1.5%之Mo， 1-1.5%之W， 至多0.06%之N， 1-3%之Cu， 至多0.02%之P， 至多0.015%之S， 至多0.8%之不可避免的雜質， 視情況以下中之一或多者：至多0.004% B、至多1% Nb、至多0.5% Hf、至多1% Ti、至多1% Co， 其餘為Fe。 在第一態樣之實施例中，粉末係肥粒鐵。舉例而言，99.5%肥粒鐵。微量之沃斯田鐵(例如，至多0.5%)係可忍受的。 在根據第一態樣之實施例中，粉末係藉由水霧化產生。 在第一態樣之實施例中，粉末係藉由氣體霧化產生。 在第一態樣之實施例中，粉末之粒徑係介於53微米與18微米之間，使得至少80 wt%之顆粒小於53微米且至多20 wt%之顆粒小於18微米。 在第一態樣之實施例中，粉末之粒徑係介於26微米與5微米之間，使得至少80 wt%之顆粒小於26微米且至多20 wt%之顆粒小於5微米。 在第一態樣之實施例中，粉末之粒徑係介於150微米與26微米之間，使得至少80 wt%之顆粒小於150微米且至多20 wt%之顆粒小於26微米。 在本發明之第二態樣中，提供製造根據第一態樣之不銹鋼粉末之方法，其包含以下步驟： - 提供熔融金屬，其具有與根據第一態樣之不銹鋼粉末之化學組成相- 對應之化學組成； - 使該熔融金屬之流經受水霧化；且 - 回收所獲得之不銹鋼粉末。 在本發明之第三態樣中，提供具有根據第一態樣之化學組成之經燒結雙相不銹鋼及其實施例。 在第三態樣之實施例中，Ni當量(Nieq
)係使得5 ＜ Nieq
＜ 11且Cr當量(Creq
)係使得27 ＜ Creq
＜38。 在第三態樣之實施例中，耐點蝕當量數(PREN)為28 ＜ PREN ＜ 33。 在第三態樣之實施例中，經燒結雙相不銹鋼之微結構之特徵在於沃斯田鐵相於肥粒鐵相內析出。 在第三態樣之實施例中，經燒結雙相不銹鋼之微結構含有30-70%之沃斯田鐵及30-70%之肥粒鐵。在第三態樣之實施例中，經燒結雙相不銹鋼之微結構含有至少99.5%之沃斯田鐵及肥粒鐵，例如至少99.8%之沃斯田鐵及肥粒鐵。沃斯田鐵及肥粒鐵之百分比可藉由ASTM E 562-11及ASTM E 1245 -03來測定。 在第三態樣之實施例中，經燒結雙相不銹鋼之微結構之特徵在於不含σ相及氮化物，例如具有少於1%之σ相及氮化物。 在本發明之第四態樣中，提供製造經燒結不銹鋼之方法，其包含以下步驟： - 提供根據第一態樣之不銹鋼粉末， - 視情況使該不銹鋼粉末與潤滑劑及視情況其他添加劑混合， - 使該不銹鋼粉末或該混合物經受固結製程以形成生坯組件， - 使該經壓實之生坯組件在惰性或還原氣氛中或在真空中在介於1150℃至1450℃之間的溫度下、較佳在介於1275℃至1400℃之間的溫度下經受燒結步驟達5分鐘至120分鐘之時期， - 使該經燒結組件經受冷卻步驟以降至環境溫度。 惰性氣氛之實例包括氮、氬及具有氬回填之真空。 還原氣氛之實例係氫氣氛、氫及氮之混合物之氣氛或解離氨之氣氛。在限定實例中，可使用二氧化碳或一氧化碳氣氛。 在第四態樣之實施例中，該固結製程包括以下步驟： - 於模具中以高達900 MPa之壓實壓力進行單軸壓實以形成生坯組件， - 自該模具頂出該所獲得之經壓實生坯組件。 在第四態樣之實施例中，該固結製程包括以下中之一者： 金屬射出模製(MIM)、熱等均壓(HIP)或積層製造技術(例如黏合劑噴射)。 根據第四態樣之方法可包括以下中之一者：雷射粉末床融合(L-PBF)、直接金屬雷射燒結(DMLS)或直接金屬沈積(DMD)。 在第四態樣之實施例中，冷卻步驟不包括強制冷卻或淬火。合金元素之效應
常見合金元素於不銹鋼中之效應眾所周知。Cr係不銹鋼中之主要元素，其在表面上形成Cr2
O3
層，該層然後防止氧進一步穿過該層，因此提供增加之耐腐蝕性。Ni係另一影響不銹鋼性質之主要元素。Ni增加鋼之強度及韌性，且當與Cr一起存在時亦增強耐腐蝕性。當與Ni一起存在時，Mo及W二者均賦予強度及韌性。Mo與Cr及Ni一起亦增強耐腐蝕性。Si作為去氧劑防止在熔融期間O結合於鋼中，Si亦係強肥粒鐵形成劑。Cu係沃斯田鐵穩定劑。Cu亦增加不銹鋼之耐腐蝕性。尤其在習用PM中，Cu藉由促進液相燒結有助於燒結。 本發明之實施例提供適於製造經燒結雙相不銹鋼之粉末以及經燒結不銹鋼。該粉末及該經燒結不銹鋼具有低或可忽略含量之N。此使在經燒結不銹鋼之製作期間形成有害氮化物之問題消除。經燒結不銹鋼較佳自經壓實及經燒結水霧化粉末製造，此乃因低N含量使得可產生具有合理可壓縮性之水霧化粉末。 由於Mo強烈地促進對均勻及局部化腐蝕二者之耐受性，因此其通常存在於不銹鋼中。Mo強烈地穩定肥粒鐵微結構。同時，Mo易於在肥粒鐵-沃斯田鐵晶粒邊界析出富含Mo之「σ」及「χ」相。該等相係有害相且不利地影響強度及耐腐蝕性。然而，由於本發明粉末之實施例中之Mo含量較低，因此在任一冷卻速率下形成σ相之可能性降低，從而消除或減少退火之後處理熱處理之需要。此亦意味著σ相將不大可能在焊接操作期間形成，焊接操作係用於雙相不銹鋼之常見製作製程。 Cr給予不銹鋼其基本耐腐蝕性且增加抵抗高溫腐蝕之耐受性。 Ni促進沃斯田鐵微結構且通常增加延展性及韌性。Ni亦由於其降低不銹鋼之腐蝕速率而具有正面效應。 Cu促進沃斯田鐵微結構。本發明之粉末中Cu之存在藉由實現液相燒結而有助於燒結製程。 預期W改良抵抗點蝕之耐受性。 Si增加強度且促進肥粒鐵微結構。其亦增加高溫下之耐氧化性及低溫下強氧化性溶液中之耐氧化性。 當存在於根據本發明之某些實施例之粉末中時，B、Nb、Hf、Ti、Co可增強該等性質。B當以小%添加時可有助於液相燒結。然而，過量B (若存在)可形成硼化物，其對機械性質及腐蝕性質二者均係有害的。當Nb及Hf存在時可藉由優先與碳組合形成微細碳化物並釋放Cr而穩定微結構以用於耐腐蝕性。不銹鋼中之Ti可增加抗拉強度及韌性。Co增加高溫機械性質。 在本發明之實施例之粉末中應使諸如C、Mn、S及P等元素之含量保持儘可能低，此乃因其可在不同程度上對粉末之可壓縮性及/或經燒結組件之機械及防腐性質具有負面效應。 以本發明之粉末之重量計，其他元素(本文指定為不可避免的雜質)之耐受含量至多為0.8%。 根據本發明實施例之粉末的組成經設計使得所產生之粉末在呈粉末形式時將具有完全(例如，至少99.5%)肥粒鐵結構，且沃斯田鐵相在燒結循環期間析出。此將容許藉由調整燒結參數來控制肥粒鐵與沃斯田鐵之比率。 Ni及Cr當量係基於以下經驗公式來計算： Creq
= Cr + 2Si + 1.5Mo + 0.75W Nieq
= Ni + 0.5Mn + 0.3Cu + 25N + 30C 其中Cr、Ni等係以重量%表示之合金中各元素之含量。 進一步之耐點蝕當量數計算為： PREN = Cr + 3.3Mo + 16N 其中Cr、Mo及N係以重量%表示之合金中各元素之含量。 組成係以使得5 ＜ Nieq
＜ 11及27 ＜ Creq
＜38為目標。此使合金處於舍夫勒(Schaeffler)圖上肥粒鐵-雙相區之邊界處。此時合金幾乎全部係肥粒鐵(例如，至少99.5%)。諸如Mo、W及Si等元素在肥粒鐵基質中過飽和。 本發明之實施例之粉末可藉由習用粉末製造製程來產生。此等製程可涵蓋原材料之熔融，之後為水或氣體霧化，從而形成所謂的預製合金粉末，其中所有元素均勻分佈在鐵基質內。與預混合粉末(其中兩種或更多種粉末混合在一起)相比，預製合金粉末之主要優點在於得以避免分離。此分離可引起機械性質、耐腐蝕性等發生變化。 當用於製造經燒結組件時，本發明之實施例之粉末可在習用單軸壓實設備中以高達900MPa之壓實壓力壓實。 習用單軸壓實欲使用之不銹鋼粉末之適宜粒徑分佈係使得粉末之粒徑介於53微米與18微米之間，從而使得至少80 wt%之顆粒小於53微米且至多20 wt%之顆粒小於18微米。在壓實之前，可將本發明之實施例之粉末與習用潤滑劑混合，該等習用潤滑劑例如(但不限於)含量為至多1 wt%之Acrawax、硬脂酸鋰、Intralube。其中混合之其他添加劑(至多0.5 wt%)可係機械加工性增強劑，例如CaF2
、白雲母、膨潤土或MnS。 可利用固結技術之其他方法，例如金屬射出模製(MIM)、熱等均壓(HIP)、擠出或積層製造技術，例如黏合劑噴射、雷射粉末床融合(L-PBF)、直接金屬雷射燒結(DMLS)或直接金屬沈積(DMD)。 在MIM製程中，欲使用之不銹鋼粉末之適宜粒徑分佈係使得粉末之粒徑介於26微米與5微米之間，從而使得至少80 wt%之顆粒小於26微米且至多20 wt%之顆粒小於5微米。 在HIP或擠出製程中，欲使用之不銹鋼粉末之適宜粒徑分佈係使得粉末之粒徑介於150微米與26微米之間，使得至少80 wt%之顆粒小於150微米且至多20 wt%之顆粒小於26微米。 粒徑分佈可藉由根據ISO 4497:1983之習用篩分操作或藉由根據ISO 13320:1999之雷射繞射(Sympatec)來量測。 在壓實或固結之後，使壓實或固結體在1150℃至1450℃範圍內之足夠高的溫度下、較佳在1275℃至1400℃範圍內之足夠高的溫度下經受燒結製程達5分鐘至120分鐘之時期。取決於欲燒結部件之形狀及大小，可應用其他燒結時期，例如10分鐘至90分鐘或15分鐘至60分鐘。燒結氣氛可係真空、惰性或還原氣氛，例如氫氣氛、氫及氮混合物之氣氛或解離氨。在燒結製程期間，肥粒鐵基質中之過飽和元素析出為沃斯田鐵相。沃斯田鐵將在晶粒邊界開始析出，將隨著進一步燒結而生長且將在晶粒自身內析出。 與其他已知之雙相不銹鋼材料相比，本發明之實施例之組成物不應在自高溫冷卻期間(不考慮冷卻速率)形成σ相或其他硬質及有害相，例如χ相及氮化物。舉例而言，σ相或其他硬質及有害相之量少於0.5%。因此，不需要施加強制冷卻或淬火。在此上下文中，強制冷卻意指使經燒結部件經受高於大氣壓壓力下之冷卻氣體。淬火意指將經燒結部件浸入液體冷卻介質中。 如圖1中所示之微結構通常將形成為含有肥粒鐵及沃斯田鐵。兩相之存在係機械及腐蝕性質升高之原因。在冷卻期間不會形成有害相(例如σ及χ)或所形成之有害相之量顯著有限，此對於目前已知之雙相不銹鋼而言係正常的。作為另一個結果，此性質將降低或消除在焊接期間(其中熱影響區(HAZ)經歷不同冷卻速率)此等相之形成。另一結果為，此組成將限制在諸如鑄造、擠出、MIM、HIP及積層製造等製程期間此等相之析出。 本發明之合金之實施例已顯示出與用已知之雙相不銹鋼合金製造之鍛造及PM產品相當或超過其之機械及腐蝕性質。 總之，本發明之實施例之某些優點可包括析出影響機械及腐蝕性質之有害σ及χ相之傾向較少。此在焊接中尤其受關注。大多數雙相不銹鋼組件在其形成後經焊接。焊接在HAZ之不同部分中產生不同之冷卻速率。由於目前已知之合金中存在氮，因此該等冷卻速率傾向於析出σ及χ相以及氮化物。不存在該等相可消除後熱處理，該等後熱處理通常涉及在高於1200℃之溫度下退火隨後快速冷卻。當部件焊接為較大結構時，此在大多數情形下將變得困難，從而限制雙相不銹鋼之使用。 實例 實例1 將粒徑低於325目之不銹鋼粉末(亦即通過45 µm篩之95 wt%顆粒)與作為潤滑劑之0.75 wt% Acrawax混合。不銹鋼粉末之化學分析為0.01 wt% C、1.52 wt% Si、0.2 wt% Mn、0.013 wt% P、0.008 wt% S、24.9 wt% Cr、2.0 wt% Cu、1.3 wt% Mo、1.0 wt% W、0.05 wt%N，其餘為Fe。 根據ASTM B528-16在750 MPa之壓實壓力下將所獲得之粉末混合物在單軸壓機中壓製且壓實成橫向斷裂強度(TRS)棒。然後將經壓製之TRS棒在1343℃下之100%氫氣氛中以7℃/分鐘之升溫速率燒結45分鐘。在此之後以5℃/分鐘之速率進行爐冷卻。然後安裝樣品並拋光以進行微結構檢查。然後利用33% NaOH將經拋光之樣品在3V下電蝕刻15 sec。利用NaOH之電蝕刻暴露出為淺棕色之肥粒鐵相，為白色之沃斯田鐵(未受影響)及肥粒鐵基質內之晶粒邊界處呈深橙色之σ相。所觀察到之微結構係如圖1中所顯示。該微結構顯示肥粒鐵(tan)及沃斯田鐵(白色)之大約50/50混合物。該微結構中無任何σ相(深橙色)之跡象。黑點係樣品中之孔隙。 實例2 根據本發明實施例及作為比較樣品之各種不銹鋼粉末係藉由水霧化來產生。不銹鋼粉末之化學組成示於表1中。將具有各種化學組成之不銹鋼熔體在感應爐中熔融，使熔融金屬經受水流以獲得鋼粉末。然後使所獲得之粉末乾燥且篩選至-325目。所篩選之粉末為-45微米，亦即95 wt%之粉末顆粒小於45微米。然後將該等粉末與0.75 wt%之潤滑劑Acrawax混合。 為測試機械性質(亦即極限抗拉強度(UTS)、屈服強度(YS)及伸長率)，以750 MPa之壓實壓力根據ASTM B925-15壓製TS樣品(狗骨狀)。然後將棒如實例1中所提及進行燒結。然後根據ASTM E8/E8M-16a測試經燒結棒之機械性質。亦進行金相檢查以確立經燒結樣品中沃斯田鐵與肥粒鐵之間的比率。測試結果示於表2中，其與來自鍛造(DSS 329鍛造)及氣體霧化及熱等均壓(hipped)條件(DSS 329 PM GA)下之已知雙相不銹鋼樣品之公開數據進行對比。 表2顯示，根據本發明之不銹鋼粉末可用於製造具有期望機械性質之經燒結雙相不銹鋼。
表1，各種不銹鋼粉末之化學組成、其產生方法及製造經燒結樣品之製程類型。
表2，自根據表1之不銹鋼粉末製造之經燒結樣品之機械性質及金相結構。 具有如實例1中之組成之本發明粉末之實施例亦在以下各種溫度及氣氛下燒結，以顯示對機械性質之效應。此等數據繪製於圖3中。 A. 在氫氣中2500°F下45分鐘 B. 在氫氣中2450°F下45分鐘 在氫氣中2450°F下60分鐘 在氫氣中2300°F下60分鐘 在氫氣中2250°F下60分鐘 在解離氨中2250°F下60分鐘 實例3 為實施腐蝕測試，製造如實例1中之TRS棒以及316L及434L之棒作為來自沃斯田鐵及肥粒鐵等級之代表。然後根據ASTM B895-16在室溫下於5% NaCl溶液中測試樣品之腐蝕。藉由比較樣品開始腐蝕所需之小時數來比較腐蝕。比較數據連同該等樣品之UTS及YS一起繪製於圖2中。圖2中之氣泡直徑代表樣品開始腐蝕所需之小時數。本發明粉末之腐蝕測試在3700小時後中斷，此乃因不存在腐蝕跡象且其已超過316L樣品之3倍。It is an object of certain embodiments of the present invention to provide alloy powders for conventional PM that will produce a dual phase structure during a sintering cycle. Another object of certain embodiments of the present invention is to provide duplex sintered stainless steel. Another goal of certain embodiments of the present invention is to obtain a tensile strength at least 35% higher than that of a fat-grained iron (eg, 430L) and double the corrosion resistance compared to a Vostian iron (eg, 316L). Another object of certain embodiments of the present invention is to provide a method of manufacturing duplex sintered stainless steel that does not require post-sintering heat treatment. The above objectives can be accomplished by the following aspects and embodiments. In a first aspect of the present invention, there is provided a stainless steel powder comprising or consisting of (in wt%): up to 0.1% C, 0.5-3% Si, up to 0.5% Mn , 20-27% Cr, 3-8% Ni, 1-6% Mo, up to 3% W, up to 0.1% N, up to 4% Cu, up to 0.04% P, up to 0.04% S , up to 0.8% unavoidable impurities, as the case may be one or more of the following: up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and the remainder Fe. The unavoidable impurities do not include the listed elements of C, Si, Mn, Cr, Ni, Mo, W, N, Cu, P, S, B, Nb, Hf, Ti or Co. Inevitable impurities may include impurities that are uncontrollable or difficult to control during steel manufacture. These impurities can come from the raw materials used and can also come from the process. Such impurities include, Al, O, Mg, Ca, Ta, V, Te or Sn. Inevitable impurities may be up to 0.8%, up to 0.6%, up to 0.3%. The inevitable impurity can be O. O may be present at most 0.6%, at most 0.4%, or at most 0.3%. Another unavoidable impurity may be Sn present up to 0.2%, Sn content above 0.2% is not considered an unavoidable impurity in this context and thus would be considered an intentional addition. In a preferred embodiment of the first aspect, there is provided a stainless steel powder (in wt%) consisting of: up to 0.06% C, 1-3% Si, up to 0.3% Mn, 23-27 % Cr, 4-7% Ni, 1-3% Mo, 0.8-1.5% W, up to 0.07% N, 1-3% Cu, up to 0.04% P, up to 0.03% S, Up to 0.8% unavoidable impurities, optionally one or more of the following: up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, the remainder being Fe. In another preferred embodiment of the first aspect, there is provided a stainless steel powder (in wt%) comprising: up to 0.03% C, 1.5-2.5% Si, up to 0.3% Mn, 24- 26% Cr, 5-7% Ni, 1-1.5% Mo, 1-1.5% W, up to 0.06% N, 1-3% Cu, up to 0.02% P, up to 0.015% S , up to 0.8% unavoidable impurities, as the case may be one or more of the following: up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and the remainder Fe. In an embodiment of the first aspect, the powder is ferric iron. For example, 99.5% Ferric Iron. Trace amounts of Vostian iron (eg, up to 0.5%) are tolerable. In an embodiment according to the first aspect, the powder is produced by water atomization. In an embodiment of the first aspect, the powder is produced by gas atomization. In an embodiment of the first aspect, the particle size of the powder is between 53 and 18 microns, such that at least 80 wt% of the particles are smaller than 53 microns and at most 20 wt% of the particles are smaller than 18 microns. In an embodiment of the first aspect, the particle size of the powder is between 26 and 5 microns such that at least 80 wt% of the particles are smaller than 26 microns and at most 20 wt% of the particles are smaller than 5 microns. In an embodiment of the first aspect, the particle size of the powder is between 150 and 26 microns, such that at least 80 wt% of the particles are smaller than 150 microns and at most 20 wt% of the particles are smaller than 26 microns. In a second aspect of the present invention, there is provided a method of producing a stainless steel powder according to the first aspect, comprising the steps of: - providing a molten metal having a chemical composition corresponding to the stainless steel powder according to the first aspect - corresponding to chemical composition; - subjecting the stream of molten metal to water atomization; and - recovering the stainless steel powder obtained. In a third aspect of the present invention, a sintered duplex stainless steel having a chemical composition according to the first aspect and embodiments thereof are provided. In the embodiment of the third aspect, the Ni equivalent (Ni eq ) is such that 5 < Ni eq < 11 and the Cr equivalent (Cr eq ) is such that 27 < Cre eq <38. In the embodiment of the third aspect, the pitting resistance equivalent number (PREN) is 28 < PREN < 33. In an embodiment of the third aspect, the microstructure of the sintered duplex stainless steel is characterized by the precipitation of the Worcester iron phase within the fertile iron phase. In a third aspect of the embodiment, the microstructure of the sintered duplex stainless steel contains 30-70% Worcester iron and 30-70% ferrite. In an embodiment of the third aspect, the microstructure of the sintered duplex stainless steel contains at least 99.5% ferrite and ferrite, eg, at least 99.8% ferrite and ferrite. The percentage of ferrite and ferrite can be determined by ASTM E 562-11 and ASTM E 1245-03. In an embodiment of the third aspect, the microstructure of the sintered duplex stainless steel is characterized by being free of sigma phase and nitrides, eg, having less than 1% sigma phase and nitrides. In a fourth aspect of the present invention there is provided a method of manufacturing sintered stainless steel comprising the steps of: - providing stainless steel powder according to the first aspect, - optionally mixing the stainless steel powder with a lubricant and optionally other additives , - subjecting the stainless steel powder or the mixture to a consolidation process to form a green component, - subjecting the compacted green component in an inert or reducing atmosphere or in a vacuum at a temperature between 1150°C and 1450°C subject to the sintering step at a temperature, preferably at a temperature between 1275°C and 1400°C, for a period of 5 minutes to 120 minutes, - subjecting the sintered component to a cooling step to reduce to ambient temperature. Examples of inert atmospheres include nitrogen, argon, and vacuum with argon backfill. Examples of reducing atmospheres are hydrogen atmospheres, atmospheres of mixtures of hydrogen and nitrogen, or atmospheres of dissociated ammonia. In limited examples, a carbon dioxide or carbon monoxide atmosphere may be used. In a fourth aspect of the embodiment, the consolidation process comprises the steps of: - uniaxial compaction in a mould with compaction pressures up to 900 MPa to form green components, - ejection of the obtained The compacted green components. In a fourth aspect of the embodiment, the consolidation process includes one of the following: metal injection molding (MIM), hot isostatic pressing (HIP), or a build-up technique (eg, adhesive injection). The method according to the fourth aspect may include one of: laser powder bed fusion (L-PBF), direct metal laser sintering (DMLS), or direct metal deposition (DMD). In an embodiment of the fourth aspect, the cooling step does not include forced cooling or quenching. Effects of Alloying Elements The effects of common alloying elements in stainless steels are well known. Cr is a major element in stainless steel, which forms a layer of Cr2O3 on the surface, which then prevents further passage of oxygen through the layer, thus providing increased corrosion resistance. Ni is another major element that affects the properties of stainless steel. Ni increases the strength and toughness of the steel and also enhances corrosion resistance when present with Cr. When present with Ni, both Mo and W impart strength and toughness. Together with Cr and Ni, Mo also enhances corrosion resistance. Si acts as an oxygen scavenger to prevent O from being incorporated into the steel during melting, and Si is also a strong ferrite former. Cu is a Vostian iron stabilizer. Cu also increases the corrosion resistance of stainless steel. Especially in conventional PM, Cu contributes to sintering by promoting liquid phase sintering. Embodiments of the present invention provide powders suitable for making sintered duplex stainless steels and sintered stainless steels. The powder and the sintered stainless steel have low or negligible levels of N. This eliminates the problem of harmful nitride formation during the fabrication of sintered stainless steel. Sintered stainless steel is preferably manufactured from compacted and sintered water-atomized powders because the low N content allows for the production of water-atomized powders with reasonable compressibility. Since Mo strongly promotes resistance to both uniform and localized corrosion, it is often present in stainless steels. Mo strongly stabilizes the iron microstructure of the fertilizer granules. At the same time, Mo is easy to precipitate Mo-rich "σ" and "χ" phases at the grain boundaries of Fe-Worthian iron. These phases are detrimental and adversely affect strength and corrosion resistance. However, due to the lower Mo content in the examples of the powders of the present invention, the likelihood of sigma phase formation is reduced at any cooling rate, thereby eliminating or reducing the need for post-annealing heat treatment. This also means that the sigma phase will be less likely to form during the welding operation, which is a common fabrication process for duplex stainless steels. Cr gives stainless steel its basic corrosion resistance and increases resistance to high temperature corrosion. Ni promotes the Worcestershire iron microstructure and generally increases ductility and toughness. Ni also has a positive effect as it reduces the corrosion rate of stainless steel. Cu promotes Vostian iron microstructure. The presence of Cu in the powders of the present invention aids the sintering process by enabling liquid phase sintering. W is expected to improve resistance to pitting corrosion. Si increases strength and promotes ferrite microstructure. It also increases oxidation resistance at high temperatures and oxidation resistance in strongly oxidizing solutions at low temperatures. B, Nb, Hf, Ti, Co can enhance these properties when present in powders according to certain embodiments of the present invention. B can aid liquid phase sintering when added in small %. However, excess B, if present, can form borides, which are detrimental to both mechanical and corrosive properties. When Nb and Hf are present, the microstructure can be stabilized for corrosion resistance by preferentially combining with carbon to form fine carbides and releasing Cr. Ti in stainless steel can increase tensile strength and toughness. Co increases high temperature mechanical properties. The content of elements such as C, Mn, S, and P should be kept as low as possible in the powders of embodiments of the present invention because they may affect the compressibility of the powders and/or the sintered components to varying degrees. Mechanical and anti-corrosion properties have negative effects. The tolerated content of other elements (designated herein as unavoidable impurities) is up to 0.8% by weight of the powder of the present invention. The compositions of powders according to embodiments of the present invention are designed such that the resulting powders, when in powder form, will have a fully (eg, at least 99.5%) ferrite structure with the Worcesterian iron phase precipitated during the sintering cycle. This will allow control of the ratio of ferrite iron to Vostian iron by adjusting the sintering parameters. Ni and Cr equivalents are calculated based on the following empirical formula: Cr eq = Cr + 2Si + 1.5Mo + 0.75W Ni eq = Ni + 0.5Mn + 0.3Cu + 25N + 30C Wherein Cr, Ni, etc. are alloys expressed in wt% the content of each element. Further, the equivalent number of pitting corrosion resistance is calculated as: PREN = Cr + 3.3Mo + 16N where Cr, Mo and N are the contents of each element in the alloy expressed in % by weight. The composition system aims to satisfy 5< Nieq <11 and 27< Creq <38. This places the alloy at the boundary of the ferric-duplex region on the Schaeffler diagram. At this point the alloy is almost entirely ferrite (eg, at least 99.5%). Elements such as Mo, W and Si are supersaturated in the ferric iron matrix. Powders of embodiments of the present invention can be produced by conventional powder manufacturing processes. These processes may involve melting of the raw materials, followed by water or gas atomization, to form so-called pre-alloyed powders, in which all elements are uniformly distributed within the iron matrix. The main advantage of pre-alloyed powders compared to pre-mixed powders, in which two or more powders are mixed together, is that segregation is avoided. This separation can cause changes in mechanical properties, corrosion resistance, and the like. When used to make sintered components, the powders of embodiments of the present invention can be compacted in conventional uniaxial compaction equipment at compaction pressures of up to 900 MPa. A suitable particle size distribution for the stainless steel powder to be used in conventional uniaxial compaction is such that the particle size of the powder is between 53 microns and 18 microns such that at least 80 wt% of the particles are smaller than 53 microns and at most 20 wt% of the particles are smaller than 53 microns. 18 microns. Prior to compaction, the powders of the embodiments of the present invention may be mixed with conventional lubricants such as, but not limited to, Acrawax, Lithium Stearate, Intralube at levels up to 1 wt%. Other additives mixed therein (up to 0.5 wt%) may be machinability enhancers such as CaF2 , muscovite, bentonite or MnS. Other methods that can utilize consolidation techniques such as metal injection molding (MIM), hot isostatic pressing (HIP), extrusion or lamination techniques such as binder jetting, laser powder bed fusion (L-PBF), direct Metal Laser Sintering (DMLS) or Direct Metal Deposition (DMD). In the MIM process, the suitable particle size distribution of the stainless steel powder to be used is such that the particle size of the powder is between 26 microns and 5 microns such that at least 80 wt% of the particles are smaller than 26 microns and at most 20 wt% of the particles are 5 microns. In the HIP or extrusion process, the suitable particle size distribution of the stainless steel powder to be used is such that the particle size of the powder is between 150 and 26 microns such that at least 80 wt% of the particles are smaller than 150 microns and at most 20 wt% Particles smaller than 26 microns. The particle size distribution can be measured by conventional sieving operations according to ISO 4497:1983 or by laser diffraction (Sympatec) according to ISO 13320:1999. After compaction or consolidation, the compacted or consolidated body is subjected to a sintering process at a sufficiently high temperature in the range of 1150°C to 1450°C, preferably at a sufficiently high temperature in the range of 1275°C to 1400°C 5 minutes to 120 minutes period. Depending on the shape and size of the part to be sintered, other sintering periods may be applied, such as 10 minutes to 90 minutes or 15 minutes to 60 minutes. The sintering atmosphere can be a vacuum, an inert or reducing atmosphere, such as a hydrogen atmosphere, an atmosphere of a mixture of hydrogen and nitrogen, or dissociated ammonia. During the sintering process, supersaturated elements in the ferrite iron matrix are precipitated as the Worcester iron phase. Wort will start to precipitate at the grain boundaries, will grow with further sintering and will precipitate within the grains themselves. In contrast to other known duplex stainless steel materials, compositions of embodiments of the present invention should not form sigma or other hard and detrimental phases such as chi and nitrides during cooling from high temperatures (regardless of cooling rate). For example, the amount of sigma phase or other hard and harmful phases is less than 0.5%. Therefore, there is no need to apply forced cooling or quenching. Forced cooling in this context means subjecting the sintered part to a cooling gas at a pressure above atmospheric pressure. Quenching means immersing the sintered part in a liquid cooling medium. The microstructure as shown in Figure 1 will typically be formed to contain ferrite and ferrite. The presence of the two phases is responsible for the elevated mechanical and corrosive properties. No or a significantly limited amount of deleterious phases formed during cooling, which is normal for duplex stainless steels known to date. As another consequence, this property will reduce or eliminate the formation of these phases during welding where the heat affected zone (HAZ) experiences different cooling rates. Another consequence is that this composition will limit the precipitation of these phases during processes such as casting, extrusion, MIM, HIP, and build-up manufacturing. Embodiments of the alloys of the present invention have shown mechanical and corrosive properties comparable to or exceeding those of wrought and PM products made with known duplex stainless steel alloys. In conclusion, certain advantages of embodiments of the present invention may include a lower tendency to precipitate detrimental σ and χ phases that affect mechanical and corrosive properties. This is of particular concern in welding. Most duplex stainless steel components are welded after they are formed. Welding produces different cooling rates in different parts of the HAZ. These cooling rates tend to precipitate σ and χ phases as well as nitrides due to the presence of nitrogen in currently known alloys. The absence of these phases eliminates post-heat treatments, which typically involve annealing at temperatures above 1200°C followed by rapid cooling. This becomes difficult in most cases when parts are welded into larger structures, limiting the use of duplex stainless steels. EXAMPLES Example 1 A stainless steel powder with a particle size below 325 mesh (ie 95 wt% particles passing through a 45 μm sieve) was mixed with 0.75 wt% Acrawax as lubricant. The chemical analysis of the stainless steel powder was 0.01 wt% C, 1.52 wt% Si, 0.2 wt% Mn, 0.013 wt% P, 0.008 wt% S, 24.9 wt% Cr, 2.0 wt% Cu, 1.3 wt% Mo, 1.0 wt% W , 0.05 wt% N, and the rest are Fe. The powder mixture obtained was pressed in a uniaxial press and compacted into Transverse Rupture Strength (TRS) bars according to ASTM B528-16 at a compaction pressure of 750 MPa. The pressed TRS rods were then sintered at 1343°C in a 100% hydrogen atmosphere at a ramp rate of 7°C/min for 45 minutes. After this, furnace cooling was performed at a rate of 5°C/min. The samples were then mounted and polished for microstructural inspection. The polished samples were then electroetched at 3V for 15 sec using 33% NaOH. Electroetching with NaOH exposes a light brown ferrite phase, white Vostian iron (unaffected) and a dark orange sigma phase at the grain boundaries within the ferrite matrix. The observed microstructure is shown in FIG. 1 . The microstructure shows an approximately 50/50 mixture of ferric iron (tan) and iron ore (white). There is no evidence of any sigma phase (dark orange) in the microstructure. The black dots are pores in the sample. Example 2 Various stainless steel powders according to the inventive examples and as comparative samples were produced by water atomization. The chemical composition of the stainless steel powder is shown in Table 1. Stainless steel melts with various chemical compositions are melted in an induction furnace, and the molten metal is subjected to water flow to obtain steel powder. The powder obtained was then dried and screened to -325 mesh. The screened powder was -45 microns, ie 95 wt% of the powder particles were smaller than 45 microns. The powders were then mixed with 0.75 wt% lubricant Acrawax. To test the mechanical properties (ie Ultimate Tensile Strength (UTS), Yield Strength (YS) and Elongation), TS samples (dog bone) were pressed according to ASTM B925-15 with a compaction pressure of 750 MPa. The rods were then sintered as mentioned in Example 1 . The mechanical properties of the sintered rods were then tested according to ASTM E8/E8M-16a. Metallographic examinations were also performed to establish the ratio between Vostian iron and ferrite iron in the sintered samples. The test results are shown in Table 2, which is compared to published data from known duplex stainless steel samples forged (DSS 329 forged) and gas atomized and hot-hipped conditions (DSS 329 PM GA). Table 2 shows that the stainless steel powders according to the present invention can be used to manufacture sintered duplex stainless steels with desired mechanical properties. Table 1. The chemical composition of various stainless steel powders, their production methods, and the type of process used to make the sintered samples. Table 2, Mechanical properties and metallographic structure of sintered samples made from stainless steel powders according to Table 1 . Examples of inventive powders having compositions as in Example 1 were also sintered at various temperatures and atmospheres below to show the effect on mechanical properties. These data are plotted in Figure 3. A. 45 minutes at 2500°F in hydrogen B. 45 minutes at 2450°F in hydrogen 60 minutes at 2450°F in hydrogen 60 minutes at 2300°F in hydrogen 60 minutes at 2250°F in hydrogen at 60 minutes at 2250°F in dissociated ammonia Example 3 To perform corrosion testing, TRS rods as in Example 1 and rods of 316L and 434L were made as representatives of the iron and ferrite grades from Voss. The samples were then tested for corrosion in a 5% NaCl solution at room temperature according to ASTM B895-16. Corrosion was compared by comparing the number of hours it took for the samples to begin corroding. The comparative data is plotted in Figure 2 along with the UTS and YS of these samples. The bubble diameter in Figure 2 represents the number of hours required for the sample to begin corroding. The corrosion test of the inventive powder was discontinued after 3700 hours because there was no sign of corrosion and it was more than 3 times longer than the 316L sample.
