Detecting Argon Trapped in Reference Samples Made by Hot Isostatic Pressing

Transcription

1 Manuscript refereed by Mr Andreas Foelzer (Bohler Edelstahl GmbH & Co KG, Austria) Detecting Argon Trapped in Reference Samples Made by Hot Isostatic Pressing Pelle Mellin (Swerea KIMAB, Kista, Sweden) Martin Östlund (Sandvik Materials Technology, Sandviken, Sweden) Wendy Fredriksson (Bodycote Hot Isostatic Pressing, Surahammar, Sweden) Christophe Pellegrini (Erasteel Kloster, Söderfors, Sweden) Henrik Blom (Carpenter Powder Products, Torshälla, Sweden) Otto Björnberg (SIS, Swedish Standards Institute, Stockholm, Sweden) Annika Strondl (Swerea KIMAB, Kista, Sweden) Abstract We reveal in this paper a procedure to make reference materials containing a known level of Ar. Risk assessment when detecting Ar in a production material is based on the content relative to a specified safety limit (usually 50 ng Ar/g steel). Hence we only need to know if a production material contains more or less than this limit. Now, we can produce material at this limit. To evaluate, we use two types of instruments, on the following contents: 24, 48, 71, 95 and ng Ar/g steel. We found that all instruments could distinguish a higher and a lower level, from the safety limit. These contents are close to the lowest detectable limit for both instruments. The instruments are a G8 Galileo from Bruker Elemental and an ELTRA-Werf (several units) from Takon AB. The mechanisms of Argon segregation are reflected upon as the need for a uniform distribution in reference samples is important. Especially if one would do the instrument calibration solely based on reference samples in the future. 1. Introduction Argon is commonly used as a pressing medium in PM HIP (Hot Isostatic Pressing, using powdered metals in evacuated capsules). But if Argon enters the capsule, it is trapped in the microstructure as pores. After heat treatment, the pores can grow resulting in Thermally Induced Porosity (TIP). These pores constitute severe weaknesses [2]. Even normal air contains enough Argon to create these pores. Dry air contains about 0.93 % Ar (or 1.28 percent by weight). Therefore, prior to HIP, leak-checking and evacuation is always done when sealing the capsule. The way Argon occasionally enters a capsule is believed to be, by (1) air-leakage or, more frequently, by (2) the pressing medium seeping through imperfections created during welding. These weldimperfections are sometimes only exposed during consolidation. This is since the capsule and weld material softens and undergo shape-change. In fact, the great difference in pressure forces Ar even through the smallest opening. The scale of an atomic diameter can be enough according to Hofer et al. [1]. When a leakage occurs, the level of Argon is usually very high. The capsules sometimes stay pressurized and appear swelled. This is due to a leak which later in the HIP-cycle becomes sealed again. This leads to extremely high Argon contents and an improperly consolidated material. Heat treatment can cause such capsules to burst-open. But otherwise, this high Ar-content material is easy to distinguish. In industry practice, this is a fairly infrequent problem but it occasionally happens. As such, managing the risk of Argon leaking into capsules is important. But detecting Argon is not trivial, which at low (but still dangerous levels) constitutes a small fraction by mass (ng/g or ppb level). These low levels of impurities cannot be measured by weight, density or microscopy. Instead, various analytical methods are usually used after melting some of the material. We aim to also investigate those in this paper. 2. Method This paper provides a method to make reference samples which yields uniform Ar-contents close to the target level. We intend to use this research as a basis for standardization.

2 2.1. Making reference samples Firstly, we produced capsules from low-alloyed steel pipes (outer diameter: 10 mm, inner diameter: 6 mm). One side of the pipe was closed, using a press applying 10 ton force. Welding using a FastMig 65 and low-alloyed solder was applied, to completely seal one side of the pipes. Secondly, we filled the pipes with a coarse filling pipe powder composed of 253MA. This is a corrosion resistant alloy which will dissolve at slow rate when pickling away the capsule material. Thirdly, we carried out evacuation partially. The Ar-content of air is in this case used to artificially add some to the material (see the setup in Fig. 1 a)). Reference materials with 20, 40, 60 and 80 ng Ar/g steel before correction, was produced in this way, see the corresponding pressures in Table 1. Here we have also take into account humidity vs Ar-content (see Fig. 1 b)). Dry air at 1 bar pressure causes an Argon level of about 1000 ng Ar/g steel. After complete evacuation, a valve (1) connecting the capsule with the vacuum pump was closed. An intentional leak in the connection between the closed valve and the vacuum pump slowly normalized the pressure. As the pressure was still low (but slowly rising) the valve (1) was opened just a little so that the capsule pressure could rise even more slowly. Hence we could carefully control the pressure to meet the required pressure in Table 1. a) (1) leak meter pump meter CMR 361, from Pfeiffer 250 mm ~140 mm (2) 6 mm Weld seam Closed during seal Hot pinch Fig. 1. a) Setup for partial evacuation of capsules. b) Argon content in air at varying humidity. b) Fourthly, we sealed valve (2) to the capsules and squeezed the pipe shut after first heating using a gas torch. Then, quickly after, we applied a low-alloyed solder using a FastMig 65. Now the capsules are ready for HIP, a standard program reaching 1160 C was used. Placement in the HIP chamber (vertical/horizontal) appears to have no effect according to previous experiences. Outer diameter of the capsules was measured after HIP to estimate the initial packing factor: 62% in our attempts versus an assumed 68%. Hence, the expected content was corrected to 23.8, 47.5, 71.3, 94.9 ng Ar/g steel which reflects the change in packing factor as a basis of calculation. Fifthly, we pickled the capsule material away, using 20 wt% HCl in distilled water for ~60 h. During the pickling, the capsules and the acid mixture was contained in bottles which was sonicated from outside. Refilling of the sonication bath and the acid mixture was carried out in ~12 h intervals. Finally, the long sample pieces were cut into 0.7 g pieces (and when investigating the effect of sample-weight on measured content: g) Table 1. Reference sample list Sample Target Ar-content, ngar/gsteel Remaining pressure, mbar Mass of steel powder, g See Fig. 2 for the reference samples produced by the above described method. The capsule material is completely removed as the blackened surface indicates.

3 Fig. 2. Ready reference samples for testing instruments Detecting Argon Three instruments for detecting Ar trapped in metal has been identified on the market: the Eltra-WERF manufactured by Takon AB in Sweden, the G8 Galileo manufactured by Bruker Elemental in Germany, and the PMA1000 manufactured by NCS in China. In Fig. 3, the Eltra-WERF and G8 Galileo system is shown in principle, which are the instruments investigated in this paper. ELTRA- Werf He 10 port, gas sampling valve, 2 ml GC Coloumn 1 Coloumn 2 Relayed 4 port valve Detector DID Furnace Particle filter Ion source (discharge) Ar ±x Galileo G8 He Flushback CH4, CO H2, N2, Detector (faraday cup + channeltron) Furnace Particle filter Ion source (Ir/Y) MS Quadropole Ar + Fig. 3. The principle of an Eltra-WERF vs a G8 Galileo. Both setups are identical in terms of the furnace part. Then either GC+DID or MS is used for Ar detection The sample is melted, in both instruments, in a disposable graphite crucible under He-atmosphere. The He carries liberated Ar to either a gas chromatograph (GC) or a mass spectrometer (MS). In both cases the Ar is detected in an ionized form which is carried by the He. The GC is fed with a 2 ml sample of the gas mixture, and filters-out Ar by the retention time through the first column. The MS, on the other hand, continuously measures the peak at m/z = 40 during the whole process. As such, the Ar content is measured for the whole gas stream from the furnace. In both cases, the signal from the detector is transferred to a software which integrates the signal over time. Then the integrated value is divided by another value which was measured for a gas containing a known concentration of Ar. This prior measurement is in essence the calibration and is carried out by first preparing a gas mixture (and for the MS, by also dosing a known gas-volume). The GC always samples 2 ml regardless if calibrating or measuring. 3. Results Many measurements of Ar-content were carried out, but many parameters are left to be investigated. This paper focuses on the outcome of targeting certain Ar-contents, then the sample weight, and then Ar-segregation (if any).

4 3.1. Qualitative analysis of distinguishing a high from a low level The instruments could in all cases distinguish a lower and a higher content compared to a chosen limit 48 ng/g, which is close to a typical limit of 50 ng/g. This is also the lowest measurable content for both instruments. Usually, if the measured content is lower than 50 ng/g, the material can be safely used, but if above, additional investigation into the application should be made. Fig. 4. Qualitative analysis - if the instruments ELTRA-Werf and G8 Galileo could distinguish a higher and lower content compared to a reference content of 48 ng/g Quantitative analysis Ar content When targeting a level of 24, 48, 71 and 95 ng/g, the measured content were 21, 32, 56 and 119 ng/g, as shown in Fig. 5 (measured with an ELTRA-Werf). Despite the challenges, this is considered a good outcome. Fig. 5. Effect of pressure, and in turn the target content, in all samples (1-4). The measured content (multiple values) is reported for both an Eltra Werf and a G8 Galileo. The same sample weight was used in all measurements. As written above, the instruments are calibrated versus a gas with known concentration, and can in theory quantify the content in any gas stream (provided that the response is linearly dependent on content). At the same time, quantifying accurate contents at these low levels is notoriously difficult. At the other end, there is an upper limit in content for both detectors, where the response is not linearly dependent on the content anymore. Calibration could take this into account, however the limit in linearity likely corresponds to far higher contents than present limits allow. The non-linear response in Fig. 5 is interesting and since both instruments register this non-linearity, sample making could be

5 responsible. Some additional consideration of the reference making is advisable. Shutting the capsules in a cold state will be tested and could be an easy way to improve the linearity. When using both the G8 Galileo and the ELTRA-Werf, a linear effect of the sample weight on the measured value was found (Fig. 6). This is difficult to explain. The background noise constitutes a fairly big portion of the lowest content (see Sample 1 in Fig. 7), but its proportion should diminish when the content becomes higher and the measured content vs sample weight should level out. Fig. 6. The effect of the sample weight on the measured value. Sample 3 was used here. Fig. 7. The MS Signal in the G8 Galileo for the different samples (the same weight was used) Occurrence of Ar-segregation There is no, or very slight, indication of segregation (see Fig. 8). It means the method to make reference samples was successful in producing uniform contents of Ar. Then, one can be sure that when testing several pieces from the same capsules, the same content should be expected.

6 Fig. 8. Measured with an ELTRA-Werf. Sample 2 was used here. Segregation of all gases is otherwise a common phenomenon. For segregation, non-uniform consolidation is the main reason, and it is amplified for Ar since the solubility in steel is low (even at high temperatures). According to this reason the highest content of Ar should be found in the last consolidated part. Also, the lowest level of Argon should be present in the region which first reached 92 % theoretical density, where most pores become closed. The conclusion here is that the whole powder mass consolidated at the same time, owing to the small capsule diameter (6 mm ID). 4. Conclusions Firstly, the instruments could in all cases distinguish a lower and a higher content compared to a limit 48 ng/g, the typical production limit is 50 ng/g. When targeting a level of 24, 48, 71 and 95, the measured content was, 21, 32, 56 and 119 ng/g, respectively (using an ELTRA-Werf). This indicates a fairly accurate outcome, even though it is very challenging. Secondly, a non-linear correlation was found between the air-pressure left in the capsule and the measured Argon content. Since this non-linearity was found by both instruments, the sample making is likely responsible. Finally, there was no segregation of Argon in the reference samples. Acknowledgments and Future work This research was funded through the Membership Research Consortia (MRC) at Swerea KIMAB. Supporting companies are AB Sandvik Materials Technology, Bodycote HIP AB, Erasteel Kloster AB and Carpenter Powder Products AB. In future work, shutting the capsules in a cold state will be tested. Furthermore, this research is intended to be a contribution to future standardized procedures. A first attempt to find and agree on a practice of making reference samples and determining the actual argon content, was made by interested parties in The target was to elaborate a national Swedish standard that could be used as a quality document between the manufacturer and the customer. The Swedish PM HIP industry still is interested in developing a national standard (possibly also an ISO standard) for low levels of Argon. References [1] Beat Hofer, Fabrizio Bigolin, Michael Hamentgen, Heinrich Stremming, and Christian Zühlke. Measuring the gas content in hip components and impurities in the Argon- and chemical reacted gas used to compacting near net shape parts and castings. presentation of the measurement technique. HIP [2] Chunlei Qiu. Net-shape hot isostatic pressing of a nickel-based powder superalloy. PhD thesis, University of Birmingham, December 2010.