Stainless Steel Compacts Research Articles

Minimizing Porosity in 17-4 PH Stainless Steel Compacts in a Modified Powder Metallurgical Process

Nowadays, powder-based manufacturing processes are recognized as cost-efficient methods frequently employed for producing parts with intricate shapes and tight tolerances in large quantities. However, like any manufacturing method, powder-based technologies also have several disadvantages. One of the most significant issues lies in the degree of porosity. By modifying the morphology of the gas-atomized spherical 17-4PH stainless steel powder via prior ball milling and then raising both the pressure of cold compaction (1.6 GPa) and sintering temperature (1275 °C), the porosity could be reduced considerably. In our novel powder metallurgical (PM) experimental process, an exceptionally high green density of 92% could be reached by employing die wall lubrication instead of internal lubrication and utilizing induction heating for rapid sintering. After sintering (at temperatures of 1200, 1250, and 1275 °C), the samples aged in the H900 condition were then mechanically tested (Charpy impact, HV hardness, and tensile tests) as a function of porosity. Sintering at 1275 °C for one hour enabled porosity reduction to below 4%, resulting in 1200 MPa yield strength and 1350 MPa ultimate tensile strength with significant (16%) fracture strain. These values are comparable to those of the same alloy products fabricated via ingot metallurgy (IM) or additive manufacturing (AM).

Optimization of microwave processing parameters on powder-metallurgical 316L stainless steels

ABSTRACT Ultra-rapid microwave sintering of powder metals is recognized for better process advantages, namely less process time, uniform volumetric heating, and fine microstructural features that will improve mechanical and corrosion properties. In the present study, austenitic (316 L) stainless steel powder compacts were sintered using a multimode microwave hybrid heating system at solidus (1200°C and 1250°C) and super-solidus (1300°C) temperature with different holding times 30, 45 and 60 min. Mechanical as well as electrochemical responses were correlated with the microstructural features and densification characteristics. The samples sintered at 1300°C for 60 min have shown improved densification and higher tensile strength of 474.3 MPa with 22.1% ductility. It is also noted that an increase in sintering temperature resulted in excellent corrosion resistance in the order of ten times. The input process parameters and the output property responses are mathematically modeled with the help of RSM. The developed equations and experimental studies are in good agreement with output responses (sintered density, yield strength, %elongation, microhardness, corrosion rate) with 0.92 to 0.95 desirability. The validation of optimized output responses is evaluated by conducting confirmation tests at two conditions (1300°C for 60 min & 1300°C for 52 min) and are in line with regression model analysis.

Microwave sintering response of different grade stainless steels and its influence on metallurgical properties

ABSTRACT Ultra-rapid microwave sintering of powder metals will provide fine microstructural features that improve mechanical properties. In present study, three grades of stainless steel powder compacts (316L, 430L, 410) were produced using a uniaxial compaction unit. These compacts were sintered by microwave hybrid heating method at 1300°C (super-solidus region). The densification response, microstructural attributes, and mechanical properties were compared at 30, 45 and 60 min holding times. The compositional analysis was performed with the help of optical emission spectroscopy (OES) and scanning electron microscopy equipped with energy dispersed spectroscopy(SEM-EDS). The results obtained from both spectroscopies are compared for the sintered samples. The correlation of mechanical properties is analysed with evolved microstructural attributes (pore volume, pore shape and pore distribution). The excellent strength of 457 ± 16 MPa, 466 ± 6 MPa and 476 ± 26 MPa with 23 ± 1.3%, 14 ± 1.5% and 11 ± 1% of ductility is observed for sintered AISI 316L, 430L and 410, respectively.

Densification kinetics of P/M austenitic (316L) stainless steels processed by rapid microwave hybrid heating method at various conditions

ABSTRACT Microwave sintering of powder metals is recognised for better process advantages, namely, less process time, rapid densification, uniform volumetric and selective heating characteristics. There are certain theoretical analyses to explain the mass transport mechanisms during microwave sintering. The current study reports on an investigation of densification kinetics in austenitic (316 L) stainless steel powder compacts processed by microwave hybrid heating method at solidus and super-solidus temperatures. The analysis of densification kinetics and sintering mechanisms were studied using the grain growth exponential and time-temperature plots. The results revealed that the major densification is achieved through bulk diffusion (viscous mass flow) at a higher temperature (1250°C–1300°C) under the super-solidus region while grain boundary retardation is the mechanism for the reduced grain growth at a lower sintering temperature (1100°C–1200°C) under solidus region. The apparent activation energies are calculated using the Arrhenius plot and classical grain growth analysis simultaneously. The activation energies of 42 ± 3 kJ/mol and 24 ± 2 kJ/mol are observed for the samples sintered at a lower temperature. The higher activation energies of 219 ± 8 kJ/mol and 254 ± 7 kJ/mol are obtained for the samples sintered at 1250°C–1300°C for both analyses.

Influence of processing parameters on the porosity, thermal expansion, and oxidation behavior of consolidated Fe22Cr stainless steel powder

The objective of this study was to determine the effects of powder metallurgy process parameters on the porosity, coefficient of thermal expansion, and oxidation behaviors of porous ferritic Fe22Cr stainless steel compacts. Optimum values of the process were also investigated. The results showed that, porosity and the coefficient of thermal expansion decrease with increasing process parameters; and changing powder shape from spherical to flaky at a certain extent. The minimum and maximum porosity values of the samples were recorded as 7% and 30%, respectively. While those for the samples produced from milled powders were noted as 3%, and 19%, respectively. The coefficient of thermal expansion values were measured at 800 °C and found to be in the range of 12–14.5 × 10−6 K−1 for spherical powders and flaky powders. From oxidation kinetics point of view, both process parameters and powder shape influenced the oxidation behavior.

Effect of heat treatment on mechanical properties and microstructure of selective laser melting 316L stainless steel

Selective Laser Melting (SLM) has been one of the preferred Additive Manufacturing process to fabricate parts due to its merits in terms of design freedom, lower material waste and faster production when compare to the conventional manufacturing processes. However, due to the thermal gradient experienced during the process, the parts are exposed to the residual stress that leads to parts distortion. This work presents the effect of heat treatments on the micro-hardness of 316L stainless steel parts. In current study, SLM has been employed to fabricate 316L stainless steel compacts. Different heat treatments of 650°C, 950°C, and 1100°C for 2 hours were applied on the compacts. Hardness test were performed on the as-built and heat-treated compacts. The relationship between the microstructures and micro-hardness were discussed in this paper. The results revealed that the micro-hardness of the as-built compacts is between 209.0 and 212.2 HV, which is much higher than the heattreated compacts.

Effect of Ni3Al Addition and Heating Mode on the Electrochemical Response on Austenitic and Ferritic Stainless Steels

The effect of intermetallic Ni3Al addition and heating mode on the electrochemical behavior of sintered austenitic (316L) and ferritic (434L) stainless steels is studied. The green compacts were sintered by conventional and microwave methods in solid state at 1350°C in H2 for a period of 60 min. The sintered samples were then subjected to several characterization techniques for evaluating their density and hardness, and finally their electrochemical response was evaluated through potentiodynamic polarization scan (1 mV/sec) in 0.1 N H2SO4. Upon addition of nickel aluminide, there was an increase in the densification of the composites, which was found more pronounced in case of 434L stainless steel with nickel-aluminide dispersoid composites. A positive response was also observed on the corrosion resistance of the composites as compared with the bare stainless steel compacts.

Harmonic structure formation and deformation behavior in a (α + γ) two phase stainless steel

In the present work the harmonic structure design has been successfully applied for achieving a combination of high strength and high ductility, simultaneously, in a two-phase steel. The compacts of two-phase stainless steels with harmonic structure were prepared by controlling mechanical milling (MM) of pre-alloyed stainless steel powders followed by spark plasma sintering. The controlled MM leads to the formation of severely deformed "shell" region, wherein the subsurface region in the immediate vicinity of the powder surface consists of a nanocrystalline structure followed by the inner region consisting of dislocation cell structure. These severely deformed regions form fine-grained network during subsequent sintering, resulting in Harmonic structure. This networked structure displayed high strength, high ductility, and better uniform plastic deformation as compared to the homogeneous fine/coarse grained structure. Such a unique combination of properties in the two-phase stainless steel powder compacts was found to be associated with the ability of the harmonic structure to evenly distribute the strain during plastic deformation.

Effects of sintering atmosphere on microstructure and mechanical property of sintered powder metallurgy 316L stainless steel

In this study, nitrogen and argon were used as the sintering atmosphere for AISI 316L stainless steel compacts. The effects of sintering atmosphere on sintered density, grain morphology and mechanical properties were investigated. The mechanical and microstructural characteristics of the sintered compacts were evaluated using optical microscopy, and scanning electron microscope. Tensile strength and hardness of the compacts sintered in nitrogen atmosphere are 290MPa and 355.2HV0.1, respectively. When argon was used as the sintering atmosphere, the tensile strength and hardness of the 316L stainless steel sintered compacts have become 226MPa and 181.8HV0.1 respectively. The results of this study indicate that the compacts sintered in nitrogen atmosphere exhibits higher strength and hardness than that of the steels sintered in argon atmosphere.

Effect of chromium and manganese nitrides on the structure and properties of Kh18N15 powder stainless steel

The paper examines the doping of Kh18N15 powder stainless steel with chromium and manganese nitrides. It is established that the base of stainless steel–10% chromium nitride material is additionally doped with chromium and nitrogen during its structurization. If steel is doped with manganese nitride, complex chromium–manganese nitride forms after quenching at 950°C. One part of the nitride reacts with the base material and the other is evaporated. To prevent nitrogen losses, stainless steel compacts must be subjected to combined nitriding, in which one part of nitrogen is added as chromium and manganese nitride powders and the other is introduced during thermal treatment of the material in nitrogen-containing environments.

Microwave Sintering – A Novel Approach to Powder Technology

The present paper focus on preliminary work carried out at INETI concerning the use of microwave radiation applied to sintering of both ceramic and metal powders. Due to the characteristics of materials-radiation interaction, microwaves can become an interesting power source in powder technology and other processing routes, since it is possible to lower the sintering temperature and shorten the sintering cycles, leading to time and energy savings. Alumina, hydroxyapatite, titanium and stainless steel powder compacts were sintered in a modified commercial oven of 2.45GHz and 1000W nominal power. Microwave susceptors were used to enable temperature rise during the initial stage of the sintering cycles. Results on densification and microstructural evaluation of microwave sintered samples are reported and compared to conventionally sintered ones, when available. For similar porosity levels upon sintering, microwave radiation generally reduces sintering times from several hours to minutes. The results obtained so far are quite encouraging since in the case of alumina and stainless steel compacts, a decrease of about 200°C in the sintering temperature was achieved. It was also found that the green density plays a key role in the densification of both metallic and ceramic powders.

金属粉末射出成形によるマルテンサイト系ステンレス鋼の組織制御

金属粉末射出成形によるマルテンサイト系ステンレス鋼の組織制御

マイクロＭＩＭプロセスに及ぼす粉末粒度の影響

Producing micro size parts by metal injection molding (μ-MIM) needs more sophisticated technique than conventional MIM. Especially, to fill the micro groove cavities which size are smaller than 10 μm with feedstock by μ-MIM technique, much smaller particle size of powders than that of conventional MIM are required. Also, various properties such as powder loading and viscosity of feedstock, debinding behavior of compacts, relative density and mechanical properties of sintered compacts would be changed by using fine powders. In this study, the effects of particle size of powders on the properties of stainless steel (SUS316L) compacts were investigated for μ-MIM processing. The sintered compacts using fine powder which mean particle size is 3 μm showed high density and tensile strength.

304 및 316L 스테인레스강 미립 분말의 소결 특성

<TEX>$5{\sim}150{\mu}m$</TEX>의 오스테나이트계 304(Fe-18%Cr-12%Ni) 및 316L(Fe-18%Cr-13%Ni-2.4%Mo)미립 분말을 사용하여 소결 특성을 평가한 결과, 다음과 같은 결론을 얻었다 (1) 3.6ks의 소결 시간으로는 어느 경우에 있어서나 소결조건에 관계없이 소결체의 상대밀도는 <TEX>$95{\sim}98%$</TEX>에서 포화하여 완전 치밀화된 소결체를 얻을 수 없었다. (2) <TEX>$5{\mu}m$</TEX>분말을 진공소결 하였을 경우, ts=57.6ks에서 거의 완전 치밀화된 소결체가 얻어졌다. (3) 소결 분위기에 상관없이 304 및 316L소결체에는 <TEX>$0.5{\sim}0.6%$</TEX>정도의 산소가 잔류하였다. (4) 진공 소결의 경우, 탄소분 첨가에 의해 소결체의 잔류 산소량은 무첨가 소결체에 비하여 0.375이상 감소하였고, 조직적으로도 산화물은 거의 관찰되지 않았다. 또한 탄소 첨가는 소결체의 밀도 향상 효과로 작용하여 목적하는 완전 치밀화된 고성능 소결체를 제조하는 것이 가능하게 되었다. The characteristics of 304 (Fe-18%Cr-12%Ni) and 316L (Fe-18%Cr-13%Ni-2.4%Mo) austenite stainless-steel compacts sintered with <TEX>$5{\sim}15{\mu}m$</TEX> powder were investigated and the results led to the following conclusions: (1) When the sintering time was 3.6ks, the relative density of sintered compacts was <TEX>$95{\sim}98%$</TEX>, regardless of any other sintering condition. (2) When a vacuum sintering was done with <TEX>$5{\mu}m$</TEX> stainless steel powders, almost fully-dense sintered compacts were obtained at is = 57.6ks. (3) The amount of residual oxygen in 304 and 316L sintered compacts was <TEX>$0.5{\sim}0.6%$</TEX>, regardless of sintering atmosphere. (4) The amount of residual oxygen in the vacuum sintered compact decreased more than 0.3 % due to addition of carbon powder, thereby reducing the formation of oxides. Furthermore, the addition of carbon improved the density of sintered compact, which enables us to make a fully-dense high performance sintered compact.

In situ investigation of dewaxing and sintering of stainless steel powder compacts by impulse excitation

The dewaxing and sintering behaviour of 316LHD (high-density) powder metallurgy stainless steel grade have been in situ characterized by means of the high-temperature impulse excitation technique. The evolution of the E modulus and internal friction (Q−1) with temperature was measured and correlated with the microstructural events during the thermal cycle. The obtained results were validated by X-ray diffraction, thermogravimetry and computational thermodynamics.

1403 粉末冶金法を利用したCu粉添加によるオーステナイト系ステンレスの緻密化(S09-1 粉末成形とその評価I,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

This study focused on the effect of Cu powder addition on the densification behavior of SUS304 type austenite stainless steel compacts. The powder mixture were compacted by one axis press at 490Mpa, then vacuum-sintered at 1473-1523K for 3.6ks. As a result, the addition of Cu powder is effective to the densification of the austenite stainless steel, and the maximum relative sintered density (91.7%) was obtained with 2mass% Cu powder addition, sintered at 1523K. Moreover, porosity of the compact was remarkably decreased sintered at 1523K. The hardness of the sintered body increased with increasing sintering temperature, same as density. In addition, remarkable grain growth was not observed with increasing temperature.

Post-sintering thermal treatment of nitrogen containing pressed and sintered and PIM stainless steels

It is known that nitrogen in the sintering atmosphere impairs the corrosion resistance but enhances the strength and hardness of sintered stainless steels. In this study, an additional post-sintering thermal treatment was applied on parts that were sintered in N2 containing atmospheres and its effects on the mechanical properties and corrosion resistance were evaluated. Both pressed and sintered (P/S) 304L and powder injection moulded (PIM) 316L stainless steel compacts were examined. The results show that the Cr2N compound was formed throughout the low density 304L compact, causing poor corrosion resistance when there was nitrogen in the sintering atmosphere. However, the high density PIM 316L compact only contained Cr2N at the surfaces owing to the high sintering temperature and the lack of interconnected pores. This Cr2N compound can be suppressed when the compact is reheated to 1100°C in argon and then cooled at a moderate rate of 57°C min−1. With this post-sintering treatment, even the compact sintered in pure nitrogen showed satisfactory corrosion resistance results, similar to those of hydrogen sintered compacts.

Effect of rare earth oxide additions on oxidation behavior of AISI 304L stainless steel

AISI 304L stainless steel powder compacts containing 2 vol% high purity rare earth oxides were prepared by mixing the different powders in a vibratory mill followed by pressing. The compacts thus obtained were sintered in a vacuum furnace and isothermal oxidation measurements were carried out in a muffle furnace, in air, up to 200 hours at 900 °C. The oxidized surfaces were examined in a scanning electron microscope and micro regions of the reaction products were studied using energy dispersive analysis. The addition of rare earth oxides decreased the oxidation rate of the stainless steel. Further evidence of predominant oxygen ion diffusion controlling the overall oxidation process in rare earth containing chromium oxide forming alloys has been observed.

粉末プロセスの制御と製品の評価 レーザー焼結によるステンレス鋼粉末のち密化

Selective laser sintering (SLS) is one of the promising processes for rapid production of complex shaped parts. The relative density of sintered parts obtained by the ordinary SLS process, however, is about 60% and the infiltration process is normally needed after sintering. This study has been performed to improve the relative density and tensile strength of SLSed 316 stainless steel powder (Rapid Steel 2.0) compacts by changing the sintering conditions and adding fine carbonyl Fe powder. By changing the sintering temperature from 1070°C to 1300°C and the sintering time from 4 hours to 8 hours, the relative density was increased from 62% to 80% and the tensile strength was improved from 64 MPa to 262 MPa. As the carbonyl Fe powder was added, the relative density and tensile strength were increased. In the case of 40 wt% carbonyl Fe powder added, the tensile strength became 463 MPa since the shape of pore became spherical and the pore size was fine.

Thermal debinding processing of 316L stainless steel powder injection molding compacts

The process of thermal debinding of injection molded water atomized 316L stainless steel powder compacts was studied in a vacuum and hydrogen atmosphere using a mercury porosimetry and the thermogravimetric analysis method. The vacuum debinding rate was investigated under the conditions of different binder compositions and temperature-increase rates, especially at the low temperature stage. The process of thermal debinding in the hydrogen atmosphere was compared with that in vacuum. It was found that the binder removal rate was much higher in vacuum than that in the hydrogen atmosphere under the same experimental conditions. The mercury porosimetry was used to measure the pore structure evolution. The results of the mercury porosimetry indicated that the pore structure evolution in injection molded compacts during thermal debinding in vacuum was greatly different from that in hydrogen atmosphere. A fine inter-connected pore structure developed as a result of the evaporation and removal of the low molecular binder components at the early stage of debinding. The reason for the difference in binder removal rate and the inter-connected pore structure in these two debinding environments lies in the difference of the vapor pressure drop, which was the vapor pressure difference between that in the molded compacts and that in the ambient environment. At the different thermal debinding stages and environments, the pore structure evolution and changes in the mean pore size in the thermally debound compacts were also different. From the low debinding temperature stage to the intermediate temperature stage of about 300 °C, the pores in the thermally debound compacts in the hydrogen atmosphere were finer and the pore size distribution was more homogeneous than that in the vacuum. At the final debinding stage, the pore size distribution was almost the same in the hydrogen atmosphere as in the vacuum.