|Title of Invention||
"RARE-EARTH OXIDE DISPERSED SINTERED STAINLESS STEELS"
|Abstract||The present invention relates to process of producing rare-earth oxide dispersed stainless steel, by mixing stainless steel powder with a controlled amount of rare-earth oxide additives to form a homogeneous mixture. Further, pressuri/ing the homogenous mixture to produce a compact stainless steel powder and sintering the compact stainless steel powder to produce the rare-earth oxide dispersed stainless steel.|
|Full Text||RARE-EARTH OXIDE DISPERSED SINTERED STAINLESS STEELS
FIELD OF INVENTION
The present invention relates to a powder metallurgical process of producing a improved class of stainless steels with addition of controlled quantities of rare-earth oxides additives.
BACKGROUND AND PRIOR ART
For the past many years, continuous efforts to improve the properties of powder metallurgy (P/M) stainless steels by changing its composition and modifying using sintering process are being carried out. Such efforts are aimed at attaining improvements in property at lower processing costs of P/M stainless steel and make them more competitive to other forms of stainless steels. Powder metallurgy used for manufacturing ferrous and non-ferrous products is a continuously developing technology. For the past few decades, the powder metallurgy methods have been cost effective compared to other methods like forging, casting etc. The powder metallurgical process involves mixing of alloy powders or powder of elements and compacting in a die, the powder compacts then formed are heated or sintered in a furnace having controlled atmosphere to bond the powder particles metallurgically by using variety of sintering processes. Products produced by powder metallurgical process like P/M stainless steels are used in variety of fields like automotive industries, hydraulics, household appliances etc.
P/M stainless steels are usually produced by solid state sintering process from stainless powder compacts prepared by pressurizing stainless steel powder. However, solid-state sintered stainless steel do not attain full density and usually consist of up to as high as 15% porosity in the sintered state. Thus, this not only limits the mechanical properties of the P/M stainless steels but also results in lower corrosion resistance as compared to the wrought steels. One approach for enhancing the densification involves liquid phase sintering. However, stainless steels powders are typically prealloyed. The use of prealloyed powders has led to the new process in liquid phase sintering called as supersolidus liquid phase sintering (SLPS). SLPS involves heating a prealloyed powder between the solidus and liquidus temperature to form liquid phase [2, 3]. During SLPS, the liquid phase forms within the particles, causing each particle to fragment into individual grains. These fragmented
particles undergo repacking, and thereby cause compact densification. The resulting sintering rate is rapid once the liquid is formed due to capillary action.
In stainless steel powder compacts developed by pressurizing method, initial porosity is at the interparticle/ powder boundaries. Sintering process involves chemical bonding of the interparticle boundaries resulting in grain boundaries. As a result of fast grain growth kinetics, the intergranular porosity becomes intragranular. The removal of intragranular porosity during sintering process is difficult as it involves bulk diffusion. Accordingly, in the prior arts some alloying additives were used in restricting grain growths thereby resulting in improvement in sintered density by promoting grain boundary diffusion and high densification. However, such additives involve ceramic particulates which are either incompatible with stainless steel or act as barriers to diffusion.
SUMMARY OF INVENTION
The object of the invention is to provide a process of producing rare-earth oxide dispersed stainless steel having improved properties.
Accordingly, the invention provides a process of producing the rare-earth oxide dispersed stainless steel using control addition of rare earth oxides. In the present process, stainless steel powder is mixed with controlled quantities of the rare-earth oxides to form a homogeneous mixture. The homogenous mixture is then compacted to produce a compact stainless steel powder. The compact stainless steel powder is sintered to produce the rare-earth oxide dispersed stainless steel.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the variation of sintered densities for the 316L stainless steel sample with compaction pressure and sintering temperature.
Fig. 2 shows the variation of sintered density with increase in the addition of Y^Oi for 316L stainless steel sample.
Fig. 3(a) and 3(b) shows the microstructure of 316L P/M stainless steel sample sintered at 1250l)C and 1400°C, respectively.
Fig. 4(a) and 4(b), shows the micrograph of the 316L + 10% Y2O? stainless steel composites sintered by solid state sintering at 1250°C in both secondary electron (SE) mode and back scattered electron (BSE) mode.
Fig. 5(a) and 5(b) shows the SEM micrograph of the 316L + 10% Y2O3 stainless steel composite sintered by SLPS at 1400°C in both secondary electron (SE) mode and back scattered electron (BSE) mode.
Fig. 6, represents the variation in sintered densities of 434L stainless steel samples with compaction pressure.
Fig. 7, depicts the comparison of variation of density with addition of YiO^ for 434L stainless steel composites.
Fig. 8(a) and 8(b) shows the microstructure of 434L stainless steel samples sintered at 1250°Cand 1400°C.
Fig. 9 (a) and 9 (b) shows the comparison of the SEM micrographs of 434L + 10% YiO.1 composites sintered at 1250°C and 1400°C respectively.
Fig. 10 (a) and 10 (b) shows the SEM micrographs of 434L + 10% YaO.1 composites sintered at 1250°C and 1400°C, respectively.
DETAILED DESCRIPTION OF INVENTION
The present invention discloses a process for producing improved stainless steels using sintering through controlled addition of rare-earth oxides. Such process, offers improvement in the properties of the stainless steel for example, density, corrosion resistance, wear resistance, hardness etc. Rare earth oxides are oxides of rare earth elements. Rare earth oxides are the most stable from various rare earth compounds, the fundamental raw material of other compounds and are basic products of raw material in rare earth oxide industry. Rare earth oxides are generally prepared by ignition of corresponding hydroxide, oxalate, carbonate salt, nitrate salt, soleplate salt in the air. Rare earth elements from which rare earth oxides form are typical metallic elements with chemical activity only next to alkaline and alkaline earth metals. Rare earth elements are typically used for glass polishing, petroleum cracking, control roads in nuclear reactors, cryogenic refrigerants, camera lenses, optical glasses etc.
Specifically, the method for forming improved sintered stainless steels by controlled addition of rare-earth oxides pertaining to the present invention comprises the processes of mixing stainless steel powders like austenitic or ferritic stainless steel powders, prepared by methods already known in the art for example atomization, with the controlled amount of rare-earth oxides resulting in a homogeneous mixture. In one embodiment of the present
method, the amount of rare earth oxides varies between 2.5 to 25wt%. In yet another
embodiment of the present method, the size of particles of the stainless steel powder and the
rare earth oxides varies from 10-100um and 0.5-10 urn, respectively. The homogeneous
mixture produced is then compressed at high pressure uniaxially using a pressurizing
machine to produce stainless steel compacts. The pressurizing machine may be for example
a hydraulic press machine etc.. During the pressurizing process, inter particle/powder
boundaries are developed in the stainless steel powder compacts or stainless steel
composites. The stainless steel compacts or stainless steel composites are then sintered using
either a solid state sintering or a supersolidus liquid phase sintering process in a furnace at
varying temperature, preferably 1250°C and 1400°C, respectively to develop the improved
stainless steels. In an exemplary embodiment, the sintering atmosphere comprises hydrogen
which reduces the oxidation of the stainless steel compacts or stainless steel composites.
However, during sintering the interparticle/powder boundaries become grain boundaries by
developing chemical bonds. To restrict the grain growth, the rare-earth oxides are added in a
controlled rate. These rare-earth oxides segregate at the intergranular boundaries and restrict
grain growth thereby increasing the sintered density. The rare-earth oxides have some
solubility for Fe, Cr, Ni etc resulting in short-circuit diffusion pathways and modification in
the sintered microstructure. Further, the usage of rare-earth oxides in stainless steels for
example ferritic stainless steels, austenitic stainless steels etc results in developments of
alloys with improved properties like high hardness, wear resistance and corrosion resistance.
In one embodiment, the rare-earth oxide used is yttria powder.
Although the process has been described as using a particular rare earth oxide namely yttria powder, in other embodiments, the process may employ one or more other rare earth oxides to obtain the same product.
The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and the description of how to make and use the present invention, and arc not intended to limit the scope of what the inventors regard as their invention nor are they intended to represent that the experiments below are all and only experiments performed
EXAMPLE 1: Improved class of stainless steels produced form austenitic stainless steel 3 161 having improved properties.
A stainless steel powder namely austenitic stainless steel powder 316L produced by gas atomization or any other process is mixed with ¥203 (yttria) in a mixer for example a tubular mixer to obtain a homogeneous mixture. The homogenous mixture is compressed at pressures from 200 to oOO MPa using a press machine to develop compact powder with the use of a die. The press machine is for example a hydraulic press machine or any other machine. The compact powder produced is sintered using a solid state sintering process and supersolidous liquid phase sintering process at temperatures 1250°C and 1400"C in an atmosphere containing hydrogen. The hydrogen presence in the atmosphere reduces the rate of surface oxide formation i.e. oxidation of the compact powder during sintering, 'fable 1. shows the various experimental variables used for the preparation of the rare-earth oxide dispersed sintered stainless steels.
Table 1: Experimental Variables used for the preparation of P/M stainless steel using
austenitic stainless steel
Fig. 1, shows the variation of sintered densities for the 316L stainless steel sample with compaction pressure and sintering temperature. The figure indicates that the sintered density of the compacts increases with the increase in pressure and the maximum sintered density was obtained for the sample compacted at 600MPa. The experiment indicated that the samples sintered at 1400T (SLPS) were having higher sintered densities than the
samples sintered at 1250°C (solid state sintering). The higher sintered density due to SLPS can be attributed to the liquid formation that increases the sintering rate. From the observations, the analysis was that the sintered densities were maximum when the sample was compacted at 600MPa. Such results were due to the higher green density of the sample compacted at 600 MPa. Accordingly, future compaction experiments with Y2O3 composite stainless steels were prepared at 600 MPa pressure. Fig 2, shows the variation of sintered density with increase in the addition of ¥203 far 316L stainless steel sample. The graph indicates that there is a decrease in the sintered density with the addition of ¥203 at both the temperatures. The addition of such particles reduces the interparticle bonding leading to decreased density in the sample.
The micrographs capture the microstructure of the sample as a function of sintering temperature and effect of Y2O3. The sample of 316L stainless steel and the composites with 10% ¥20^ is considered. Fig 3(a) and 3(b) shows the microstructure of 316L P/M stainless steel sample sintered at 1250°C and 1400°C, respectively. The figure clearly indicates that the sample sintered at 1400°C has higher grain growth and lower porosity. The higher grain growth is due to higher sintering temperature. Moreover, the pores tend to become more rounded because of higher driving force to attain minimum energy at the interface
The SEM micrographs were considered to explain the effect of ¥203 addition on the microstructure. In the present case, the composite with 10% ¥263 is considered. Fig 4(a) and 4(b) relates to a micrograph of the 316L + 10% Y:>O3 stainless steel composites sintered by solid state sintering at 1250°C in both secondary electron (SE) mode and back scattered electron (BSE) mode, respectively. In the BSE mode the ¥203 dispersoids were not revealed as the density of the dispersoids are similar to that of surface. The BSE mode depicts the surface morphology of the sample more clearly and reveals the pores network on the surface. Pores visible on the surface of the sample were irregular in shape. Fig 5(a) and 5(b). shows the SEM micrograph of the 316L + 10% ¥263 stainless steel composite sintered by SLPS at 1400"C in both secondary electron (SE) mode and back scattered electron (BSE) mode, respectively. Pores were regular on the grain boundaries and the grain growth was higher than the solid state sintered sample, due to the higher temperature sintering.
HXAMPLE 2: Improved class of stainless steels produced form ferritic stainless steel 434L having improved properties.
A ferritic stainless steel powder 434L produced by gas atomization or any other process is mixed with YiO^ (yttria) in a mixer for example a tubular mixer to obtain a homogeneous mixture. The homogenous mixture is compressed at pressures from 200 to 600 MPa using a press machine to develop compact powder with the use of a die. The press machine is for example a hydraulic press machine or any other machine. The compact powder produced is sintered using a solid state sintering process and supersolidous liquid phase sintering process at temperatures 1250"C and 1400°C in an atmosphere containing hydrogen. The hydrogen presence in the atmosphere reduces the rate of surface oxide formation i.e. oxidation of the compact powder during sintering. Table 2, shows the various experimental variables used for the preparation of the rare-earth oxide dispersed sintered stainless steels.
Table 2: Experimental Variables used for the preparation of P/M stainless steel using erritric stainless steel
Fig 6, represents the variation in sintered densities of 434L stainless steel samples with compaction pressure. The figure shows that the sintered densities were maximum for the samples compacted at 60()MPa due to the higher green density of the sample. The samples sintered by SLPS were having higher densities than the samples sintered by solid state sintering similar to that of 316L stainless steel samples. Fig 7, depicts the comparison of variation of density with addition of YjC^ for 434L stainless steel composites. The graph
shows that the densities of the composites decrease with addition of Y;>()i when sintered at 14()0°C. However, when sintered at 1250°C, the density ofthe composite containing 10% Y^Oj increases drastically. The density of the sample reaches that of the straight 434L stainless steel sample. In the case ofthe sample sintered at 1250°C, the oxides are distributed at the grain boundaries, when compared to samples sintered at HOOT, where the YiOi particles get distributed uniformly in the sample.
The micrographs capture the microstructural evolution trajectory ofthe sample as a function of sintering temperature and effect of YiOj. The sample of 4341, stainless steel and the composites with 10% YiO.i is considered. Fig 8(a) and 8(b) shows the microstructure of 4341, stainless steel samples sintered at 1250°C and 1400°C, respectively. The micrograph in Fig 8(b) shows that the sample sintered at 1400°C has higher grain growth. The grain boundaries are clearly distinguished and the pores are rounded. In the sample sintered at I250"C, pores are preferentially segregated at the grain boundaries and there ar5e no clear grain boundaries. The pores were also irregular in shape and the porosity was higher.
The effect of YiOi addition on the microstructure is described with help of SEM micrographs. Here, the case in which the composite with 10%i YiO.i is considered. The micrographs elucidate that the grain growth was more in the sample sintered at HOOT. Moreover, in the sample sintered at 1400T, the oxides are distributed uniformly throughout the grain surface. In the case ofthe sample sintered at 1250T, the Y2O3 agglomerates are seen at the grain boundaries. Figure 9 (a) and 9 (b) compare the SEM micrographs of 434L 1 10% YiO.i composites sintered at 1250T and 1400T respectively. In the sample sintered at 1250T, the YiO.1 are distributed along the grain boundaries. Moreover, the pores are also concentrated at the grain boundaries. In the sample sintered at HOOT, the YiCh are distributed uniformly on the surface ofthe sample. The same set of samples was observed in BSE mode and SE mode, the results are shown in Fig 10(a) and l()(b), respectively. The left side ofthe Figure shows the SE mode and the right side ofthe figure shows the BSE mode. The pores are more rounded during SLPS as shown in Fig 10 (b).
1. From Web hUp://'www.mpif.org/
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1. A process of producing rare-earth oxide dispersed stainless steel, the method comprising:
mixing stainless steel powder with rare-earth oxide additives to form a homogeneous mixture wherein quantity of the rare-earth oxide additives varies within the range of 2.5 to 25 wt%;
pressurizing the homogenous mixture to produce a compact stainless steel powder; sintering the compact stainless steel powder to produce the rare-earth oxide dispersed stainless steel.
2. The process as claimed in claim 1, wherein the stainless steel powder is an austenitic
stainless steel powder.
3. The process as claimed in claim 1, wherein the stainless steel powder is a ferritic
stainless steel powder.
4. The process as claimed in claim 1, wherein size of stainless steel powder particles
range from 10 to 10()u.m.
5. The process as claimed in claim 1, wherein the compact powder is sintered using
solid state sintering.
6. The process as claimed in claim 1, wherein the compact powder is sintered using
supersolidus liquid phase sintering.
7. The process as claimed in claim 1, wherein the rare-earth oxide additives is an yttria
8. The process as claimed in claim 1, wherein the size of the rare-earth oxide particles
ranges from 0.5 to lOum.
atmosphere of hydrogen.
10. The process as claimed in claim 1, wherein the homogenous mixture is pressurized
within a range of 200 to 600 MPa.
|Indian Patent Application Number||1224/DEL/2006|
|PG Journal Number||13/2013|
|Date of Filing||18-May-2006|
|Name of Patentee||INDIAN INSTITUTE OF TECHNOLOGY KANPUR|
|Applicant Address||KANPUR -208016|
|PCT International Classification Number||C22C38/06|
|PCT International Application Number||N/A|
|PCT International Filing date|