Controlled densification of boron-containing stainless steels

Transcription

1 Controlled densification of boron-containing stainless steels L. Lozada and F. Castro CEIT, Pº de Manuel Lardizabal 15, 20018, San Sebastián, Basque Country, Spain ABSTRACT Liquid Phase Sintering based on boron additions as a sintering enhancer, has been used for successfully obtaining high density PM stainless steels with improved mechanical properties and corrosion resistance. Optimisation of sintering parameters, chemical composition and alloying method for adding boron allowed obtaining wide sintering windows to guarantee reproducible attainment of targeted density and shrinkage behaviour. Liquid generation is based on an eutectic reaction involving previously formed mixed borides and the Fe-based solid solution. Thermodynamic calculations supported by experimental work show that Cr/B ratio is one of the critical parameters for controlling densification. A detailed characterisation of microstructural development, which depends on sintering conditions, chemical composition of the powder blend and the type of borides formed, is presented. Tensile and potentiodynamic tests were carried out to evaluate the mechanical properties and corrosion resistance of the materials. The influence of sintering conditions on the characteristics of industrially processed components is discussed. INTRODUCTION PM stainless steels currently used in several applications [1-3] are in increasing use associated to demands of high corrosion resistance [1, 4-7]. Nevertheless, they exhibit lower corrosion resistance than their wrought counterparts due to porosity and processing conditions [5, 8]. Typically these highly pre-alloyed powders exhibit very limited compressibility after die pressing. Consequently liquid phase sintering is an attractive method for obtaining improved performance provided limited shrinkage to prevent loss of dimensional tolerances and avoid distortion is achieved. Boron is an effective sintering enhancer for obtaining high density iron and iron-alloys [9, 10] but typically offers narrow sintering windows since even small additions of this element produce a noticeable increase on the densification rate upon formation of a liquid phase [11-13]. The permanent liquid forms at relatively low temperatures exhibiting excellent wetting characteristics and an extremely low solubility in the solid [14, 15]. Due to these attractive characteristics the use of boron has been actively investigated for sintering stainless steels [15-27] although detrimental effects on mechanical properties have also been reported [28, 29]. In the present work high density sintered stainless steels are presented showing outstanding combinations of mechanical properties and improved corrosion resistance in comparison to previously used PM materials. Under the alloy design used here industrial trials carried out on specific components for the exhaust system show possibility of simultaneously achieving high density and the required dimensional tolerances. 1

2 EXPERIMENTAL PROCEDURE The stainless steel powders used in this work are the grades 316LHC, 409LE and FeCr30 containing different amounts of boron between 0.14 and 0.24% and whose chemical compositions are summarised in Table 1. TABLE 1. Chemical composition (wt%) of the iron alloy powders used in this work. Steel grade Cr Ni Mo Si Mn O C B Nb Fe 316LHC Bal. 409LE Bal. FeCr Bal. Cylindrical green compacts (16mm in diameter x 5mm height) manufactured from these powders were obtained after cold die pressing at 700 MPa. Green densities achieved for the pressed compacts were 6.76 g/cm 3, 6.64 g/cm 3, 5.81 g/cm 3 for the grades 316LHC, 409LE and FeCr30 respectively. Sintering was carried out at a heating rate of 20 ºC/min in a tubular furnace for 1 h at selected temperatures between 1210 and 1280ºC, under a hydrogen atmosphere with a dew point lower than -40ºC. As an additional support experiments using Differential Scan Calorimetry (DSC) were carried out in hydrogen using 6mm diameter x 12mm height pins. The microstructures were characterised by Scanning Electron Microscopy (SEM) using a Philips XL 30 instrument fitted with an EDS detector and by Transmission Electron Microscopy (TEM) using a JEOL JSM-2100 instrument at an electron accelerating voltage of 200kV. Hardness measurements and tensile tests carried out under MPIF standards were used as a means of evaluation of the mechanical properties developed after sintering. The corrosion resistance of the materials in the as-sintered state was measured in an acid media by potentiodynamic polarisation tests, at a speed of 1 mv/s, using a reference electrode of Ag/AgCl and a Pt counter-electrode, submerged in a solution of HCl. A set of industrially processed components were also produced and subjected to corrosion tests in a salt bath for variable time periods. RESULTS AND DISCUSSION Figure 1 shows the density attained with the experimental stainless steels, containing varying amounts of boron, after sintering in hydrogen for 1 h at different temperatures up to 1280ºC. For the sake of comparison plots corresponding to the stainless steel powders without boron and sintered under exactly the same conditions, are also shown. The Figure includes the DSC traces recorded up to 1300ºC for selected boroncontaining green compacts. 2

3 Figure 1.- Curves representing sintered density as a function of temperature for : a) 316LHC, b) 409LE, c) FeCr30 and d) DSC traces for those three steels. Comparing against the sintering behaviour of the base powders it may be realised that boron enhances densification during sintering. Without boron only limited densification is observed in all cases. As realised from the DSC traces, two endothermal peaks at around 1220 and 1240ºC for the 409 and 316 steel grades respectively, indicate the formation of a liquid phase. From the density vs. temperature plots (Figures 1a and b) a marked increase in density is observed for those steels at these temperatures and the final density achieved is dependent on the concentration of boron, the sintering temperature and chemical composition of the base powder. It is important to underline that for all boron concentrations a constant density is achieved between 1230 and 1250ºC for both steels thus resulting in a sintering window of about 20ºC. As seen from Figures 1a and b such sintering window is actually wider for the 316 grade than for the 409 ferritic steel. Referring to the corresponding DSC trace from the Fe30Cr powder grade (Figure 1d), it may be realised that although exhibiting an overall endothermic behaviour with temperature, there is not any sharp peak revealing formation of a liquid phase, consequently densification is rather limited (Figure 1c) since sintering mechanisms are entirely based on solid state events. 3

4 Figure 2.- SEM images using BSE detector for specimens of 316LHC containing 0.18 %B, after sintering for 1 h in H 2 at: a) 1210ºC, b) 1230ºC. c) 1250ºC, d) 1280ºC. The formation of a liquid phase as a function of temperature and the chemical composition of these Fe powders was interpreted on thermodynamic grounds after calculation of the Fe-Cr-B phase diagram. However, before discussing this aspect of the work it is convenient to analyse the microstructures observed after sintering at different temperatures. For the 316LHC austenitic stainless steel Figure 2 reveals that an intergranular phase (labelled as liquid), corresponding to the solidification of the liquid phase, is observed for sintering temperatures above 1210ºC. At 1210ºC (Figure 2a) and lower temperatures the microstructure is constituted by austenitic grains and a dispersion of precipitates (arrowed on the micrograph) which were identified as Ironrich borides also containing Cr and lesser amounts of Mo. After 1 h necks between Feparticles are clearly formed but accompanied by a limited elimination and spherodisation of porosity. As noticed by Figures 2b to d this process more obviously takes place as the sintering temperature is increased because of the activation of mass transport mechanisms based on Liquid Phase Sintering (LPS). A similar trend is observed for the behaviour of the 409LE ferritic stainless steel (Figure 3). Although with differences in chemical composition and size of the constituents a common feature in both steel grades after sintering in the presence of a liquid is that the intergranular phase formed on cooling exhibits an eutectic morphology. Figure 3.- SEM images using BSE detector for specimens of 409LE containing 0.24 %B, after sintering for 1 h in H 2 at: a) 1210ºC, b) 1250ºC. Figure 4 illustrates, in contrast to the behaviour observed for the steel grades mentioned above, that sintering of the FeCr30 alloy under exactly the same conditions does not lead to the formation of a liquid within the whole range of temperatures and even at the maximum boron concentration used in this work. This observation is consequently in 4

5 total agreement with the characteristics of the DSC trace corresponding to this material (Figure 1d) up to 1300ºC. Figure 4.- SEM images obtained with BSE detector for specimens of FeCr30 containing 0.24 %B, after sintering for 1 hr in H 2 at: a) 1210ºC, b) 1250ºC. As illustrated in these Figures a common feature to all those alloys is the formation of mixes borides prior to the generation of the liquid for the 316 and 409 grades and in the whole temperature range for the FeCr30 alloy. The formation of these borides has been demonstrated [30, 31] to be a pre-requisite for the generation of the liquid through a eutectic reaction being that the reason for the eutectic morphology of the intergranular phase observed after cooling. All these borides contain Cr but formed with a different Fe/Cr ratio depending on the chemical composition of the Fe-base powder used. Inspection of these borides under the TEM revealed that they belong to the tetragonal crystal system being of the M 2 B type where M represents a combination of Fe-Cr-Mo with varying relative concentration ratios before and after liquid formation. Characterisation by TEM of the borides for the 316LHC stainless steel grade is illustrated in Figure 5. The diffraction patterns obtained from the intergranular phase allow characterisation of two M 2 B-borides which may be generically identified as Cr 2 B (dark constituent in Figure 3d) and Mo 2 B (in white contrast in Figure 3d) with lattice parameters given by a=5.617nm, b=4.180nm and a=5.674nm, b=4.11nm respectively. For the alloy systems studied here and previous to the formation of the liquid the borides are (Fe-Cr-Mo) 2 B for 316, (Fe-Cr-Nb) 2 B for the 409 and (Fe-Cr) 2 B for the FeCr30 grade. 5

6 Figure 5.- TEM image and diffraction patterns from a specimen of 316LHC %B after sintering at 1250ºC for 1h. Based on the experimental evidence presented above the formation of the liquid and the microstructural development of the studied alloy systems is discussed considering the calculated ternary phase diagram in Figure 6 and previous information on the binary phase diagram of the Fe B system. For this latter system it is well established that a eutectic reaction (Fe + Fe 2 B L) takes place at 1175ºC [15], however, the microstructure in the Fe Cr B alloys exhibits an important amount of clearly resolved precipitates at even higher temperatures and a liquid has not yet been generated. In order to understand the influence of Cr in modifying the Fe B binary system, an analysis of the ternary phase diagram was carried out based on the calculation of two relevant isothermal sections (Figure 6). The calculations were carried out using the Thermocalc software and assuming that all possible combinations of M 2 B mixed borides from Fe 2 B to Cr 2 B could be well described as ideal solutions ( Hmix = 0). This approximation must however be very close to reality since an extremely high value of the heat of solution ( Hmix) would be required to allow direct equilibrium between Cr 2 B and a Fe-rich Fe Cr solid solution. The analysis of the isothermal section at 1210ºC for the Fe Cr B ternary system indicates that the solid phases (in weight percent) able to be in equilibrium with the liquid (87.25%Fe-9.74%Cr-3.01%B) are a BCC Fe-rich solid solution, containing 6 % Cr and %B, and a Fe-rich (Fe,Cr) 2 B boride containing 21.1 % Cr and 8.9 % B. Clearly, as indicated by the blue and red dots in the diagram, the overall chemical composition of the ferritic 409 and FeCr30 grades (assuming constituted only by Fe-Cr-B) lie in the region where the stable phases are α- ferrite (BCC) solid solution and M 2 B borides. Hence a liquid cannot be expected under these conditions as confirmed by the corresponding micrographs. 6

7 Figure 6.- Fe-rich corner of two Thermocalc calculated isothermal sections for the ternary Fe-Cr-B alloy system. Included micrographs correspond to specimens of the indicated grades containing 0.24%B and sintered at those temperatures for 1h followed by normal cooling. Increasing the temperature to 1250ºC produces a noticeable shift and tilt of the α+m 2 B+L ternary region towards higher Cr-contents thus enabling larger fields of stability for the regions containing a liquid. This means that such increase in temperature allows Cr-richer α and (Fe, Cr) 2 B to establish thermodynamic equilibrium with a liquid. As a consequence now the blue dot representing the grade 434LCB+0.24%B lies in the α+liquid field. The corresponding micrograph shows evidence of the intergranular phase therefore indicating the formation of a liquid at the sintering temperature in excellent agreement with the prediction of the calculated diagram. In contrast, again in total agreement with the calculations, the microstructure of the FeCr %B alloy still shows borides embedded in the BCC matrix without any traces of liquid. As realised by the arguments exposed above the formation of a liquid in the alloy systems studied here obeys thermodynamic principles based on a eutectic reaction between borides formed during heating and the surrounding matrix. The chemical composition of, both, borides and matrix are controlling factors determining the possibility of generating a liquid. The isothermal sections of the ternary phase diagram calculated in this work obviously represent a simplified version of the real chemical composition of the alloys since other alloying elements have been neglected in the calculation. Nevertheless, it is clear that for a fixed concentration of boron, the key element is chromium. Consequently the temperature at which the eutectic reaction may lead to the formation of a liquid is very much dependent not only on the concentration of chromium in the matrix but also the amount of chromium in the precipitates formed. It is also apparent that the amount of chromium in these complex borides may increase as the initial concentration of chromium increases in the iron base powder. This may 7

8 also be the reason why, as noticed by the DSC traces in Figure 1d, the generation of a liquid in the 316 grade takes place at a slightly higher temperature than for the 409 ferritic steel. Besides, for the FeCr30 alloy, sintering trials carried out up to 1375ºC revealed that the formation of the liquid is completely inhibited even at temperatures close to the melting point of this material. Figure 7.- Tensile curves for 316LHC with varying concentrations of boron and SEM image illustrating the morphology of the intergranular phase of 316LHC %B sintered at 1250ºC for 1h. Figure 8.- SEM image of the fracture surface and EDS analysis of the oxide particles for a specimen of 316LHC containing 0.18%B after tensile testing. Several authors [28, 29] have reported detrimental effects of boron on the mechanical properties of sintered steels due to the formation of a continuous network of borides that leads to embrittlement and intergranular fracture. Nevertheless, sintering stainless steels to high density using boron may result in effective benefits in terms of mechanical properties and corrosion resistance [26, 27]. This is illustrated in Figure 7 where the tensile curves correspond to specimens of the austenitic 316LHC grade sintered at 1250ºC for 1h reaching a density of 7.4 g/cm 3. Although a gain is noticed in the yield stress of the materials containing boron, the main improvements are noticed in UTS, fracture stress and certainly elongation. This gain in ductility is related to the design of the alloy which renders an intergranular phase constituted by a mixture of Cr-containing borides, Mo-containing borides and austenite. This may be realised by reference to the SEM image in Figure 7 which shows that the ternary eutectic results in the formation of a continuous metallic phase thus favouring improvement of mechanical properties by preventing the formation of a boride network. 8

9 Figure 9.- Industrially produced components from stainless steel 316LHC containing 0.18%B after plastic deformation under flexure. The formation of strong metal to metal contacts is furthermore enhanced by proper deoxidation and spherodisation of oxides. As noticed (Figure 8) the fracture surface of a specimen of the austenitic stainless steel after tensile testing clearly exhibits a ductile behaviour and dimples allocating the Si-rich oxides. This behaviour is consistent with the corresponding 30% elongation this specimen reached during testing. As a result of this performance and in practical terms Figure 9 shows a set of industrially processed components exhibiting outstanding ductility for PM standards. Figure 10.- Potentiodynamic curves for the austenitic stainless steel 316LHC as a function of boron content and industrially processed components after salt bath testing for (top) 240 and (bottom) 500 hours. The corrosion resistance of the as-sintered austenitic stainless steel was evaluated in the laboratory through the use of potentiodynamic testing in an acid media. The results obtained as a function of boron content are summarised in Figure 10. The polarisation curves for the steels containing boron show a lower corrosion current and higher potential than those exhibited by the base powder thus indicating an improved corrosion resistance for the materials containing boron. Additionally, a set of industrially processed components were subjected to salt bath testing. As noticed by the illustration in Figure 10 the components remain unstained after 240 h and show only small areas affected after 500 h. 9

10 CONCLUSIONS The densification by Liquid Phase Sintering of stainless steels, both, ferritic and austenitic may be controlled so to have sufficiently wide sintering windows to reach predetermined density values thus avoiding excessive shrinkage leading to distortion. The liquid phase is formed through an eutectic reaction between complex Cr-containing borides and the stainless steel matrix. Such borides are formed during heating having different Fe/Cr ratios depending on the chemical composition of the iron based powder. Cr/B ratio in the steel is one of the critical parameters for controlling densification since liquid formation can even be inhibited for too high Cr/B ratios. On the other hand, too high B concentrations lead to increased amounts of liquid hence excessive shrinkage and narrow sintering windows. It is shown that adequate design of the alloy and proper choice of sintering atmosphere allow obtaining extremely attractive mechanical properties, especially ductility and corrosion resistance, since high density and strong metallic bonds between particles are guaranteed after solidification of the liquid. REFERENCES 1. M. C. Baran, et al. Evaluation of P/M ferritic stainless steel alloys for automotive exhaust applications. in Adv. in Powder Met. and Particulate Materials Chicago IL: Metal Powder Industries Federation. 2. E. Klar and P.K. Samal, Sintering of stainless steel. Powder Metal Technologies and applications, (ASM Handbook), : p C. M. Sonsino, et al. Development of corrosion resistant diffusion-bonded pump impellers from stainless PM steels. in PM World Congress Granada. 4. R. M. German, Sintering of 304L Stainless Steel Powder. Metallurgical Transactions A, (12): p H. S. Nayar and B. Wasiczko, Nitrogen absorption control during sintering of stainless steel parts. Metal Powder Report, (9): p K. T. Kim and Y. C. Jeon, Densification behavior of 316L stainless steel powder under high temperature. Materials Science and Engineering A, : p Capus, J.M., Stainless steel at the 2000 powder metallurgy conference. Advanced Materials and Processes, (3): p P. K. Samal, Factors affecting corrosion resistance of Powder Metal (PM) Stainless Steels. Key Engineering Materials, : p R. M. German and K. A. D'Angelo, Enhanced sintering treatments for ferrous powders. Int. Met. Rev., (4): p P. E. Zovas, et al., Activated and liquid phase sintering-progress and problems. J. of Metals, 1983: p K. Koichu, N. Mitsuru, and H. Hiroshi, Effect of boron and silicon additions on liquid-phase sintering behavior and corrosion resistance of P/M ferrite type stainless steels. Funtai Oyobi Fummatsu Yakin/J. of the Jpn. Soc. of Powder and Powder Met., (10): p K. S. Narasimhan, Sintering of powder mixtures and the growth of ferrous powder metallurgy. Materials Chemistry and Physics, (1-3Jan):p D. S. Madan, Enhanced sintering and property improvement in ferrous P/M compacts. The Int. J. of Powder Met., (4): p

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