A furnace operating with a concentration below the explosive limit of hydrogen does not need to meet NFPA requirements, which gives it distinct cost advantages. This paper compares the properties obtained for the popular MIM 316 stainless steel processed at a partial pressure of 15 mbar (below the explosive limit) and 400 mbar. A comparison of physical properties, microstructures and fracture surfaces obtained by both processing parameters was made.

There are furnaces in the market that claim they can run parts under 100% hydrogen, but they can run the hydrogen at only 15 mbar partial pressure. A partial pressure of 15 mbar of hydrogen is below the lower explosion limit of hydrogen. Hence, these furnaces do not have to meet the NFPA safety requirements for hydrogen furnaces.^[1] Because no explosion ports and other safety equipment are required, these furnaces have a distinct cost advantage over standard hydrogen furnaces that must meet NFPA regulations. So, do these furnaces perform the same and produce the same results?

Regular MIM Hydrogen Batch Furnace

What we are calling a regular MIM hydrogen batch furnace is one that may be used with 100% flowing hydrogen in any range from 1 atmosphere to vacuum below the explosive limit of hydrogen. Since continuous furnaces are incapable of running partial pressures, we will call the regular MIM batch hydrogen furnace a “400-mbar furnace” for short and the 15-mbar hydrogen batch furnace a “15-mbar furnace.”

Elnik MIM 3000 (400-mbar) series furnaces (Fig. 1) have all refractory-metal hot zones and retorts; mass flow controllers for H₂, N₂ and argon or a mixture of those; and dry screw vacuum pumps that are able to operate between a vacuum of 10^-2 mbar and 1,000 mbar pressure. This enables the furnace to produce atmospheres above the explosive limits of hydrogen (typical LEL is 40 mbar). Therefore, these furnaces must be built to NFPA 86, Standards for Ovens and Furnaces.^[1]

These standards address the requirements for class-D furnaces, describing all the safety requirements for handling flammable gases. The requirements include purging the chamber before the introduction of flammable gases; exhausting the flammable gas through the vacuum pump; diluting the exhaust from the chamber or passing it through a burner; and purging the flammable gases from the chamber via five consecutive flushes of inert gas or by pumping the chamber to a minimum vacuum level of 10^-1 mbar. Prudent chamber design also adds an explosion-relief valve, which is designed with a minimum of 0.095 m² of relief area for every 0.425 m³ of furnace volume. This is commonly called a blast port (Fig. 2).

The 15-mbar Furnace

A typical vacuum furnace (15-mbar furnace) is shown in Figure 3. This type of furnace does not usually have a retort nor does it have to fulfill any of the requirements for NFPA 86, Standards for Ovens and Furnaces for flammable gases since the use of a flammable gas is below the lower explosive limit (LEL) of hydrogen.

Experiments

ISO tensile bars were made from BASF Catamold 316 L-A feedstock. Figure 4 shows a typical ISO tensile bar. All the tensile test bars were catalytically debound together at the same time.

The tensile bars were debound and sintered at a partial pressure of 400 mbar of 100% hydrogen, while the other batch was sintered at 15 mbar of hydrogen. The gas flow for the 15-mbar run had to be reduced slightly to maintain the 15-mbar pressure for the entire cycle. All parts were sintered at 1370^oC (2500°F) for 75 minutes.

Densities were measured by a helium pycnometer, and carbon tests and microstructures were all done at DSH. An outside laboratory did the tensile tests and performed the SEM work on the fracture surface of the tensile specimen and the corrosion samples.

Results and Discussions

Physical Properties
The results of the tensile tests are not reported. There was a wide variation of the measured properties because of inconsistent molding at the center of the test bars (Fig. 5).

Corrosion Tests
Corrosion tests were done by immersing the tensile bars in a 1% saline solution at 62^oC for five days. The 1% saline corresponds to the saline content of most body fluids, whereas every 5^oC above body temperature (37^oC) corresponds to doubling the effect of the immersion time in the test. The parts were immersed for five days at 62^oC, which would correspond to the equivalent of immersion for 5 x 2⁵ = 160 days.

The parts sintered at 400 mbar showed no corrosion and a much better corrosion resistance, while the parts sintered at 15 mbar started showing signs of corrosion after one day.

For the 15-mbar corrosion samples shown in Figure 6, one of the bars showed corrosion within the first day and was removed after 24 hours. This sample was bent slightly and then cut to facilitate placing the part in the vacuum chamber of the SEM. The remaining two parts, which each show large areas of corrosion and a large amount of corrosion products after the five days of immersion, are seen in the cup with the solution.

Figure 7 shows the three parts sintered at 400 mbar placed in the solution at the same time. These three parts showed no corrosion after the five days in the solution.

Microstructures
The unetched microstructures of the 316 (Fig. 8) show fine “pores” distributed uniformly around some larger pores. The parts sintered at 400 mbar also show a skin around 200-250 µm in thickness that is almost pore-free. The unetched structures both reveal the presence of what seem to be fine spherical pores about 2-5 µm in diameter.

The etched microstructures of the parts (Fig. 9) are typical for the alloy, showing twins. Fine pores seen inside the grains are easily discernable at the higher magnifications, although they are also seen at the 100X magnification. Only the structure for the 400 mbar is shown since there are no apparent differences seen from the microstructures. The fine pores seen in the unetched structures are masked when the structures are etched.

Scanning Electron Microscopy (SEM)
Figure 10 shows the fracture surface of a tensile bar sintered at 15 mbar. The inclusions seen here have been identified as oxides as shown in the energy-dispersive X-ray analysis (EDAX) in Fig. 11.

The SEM of the bars sintered at 15 mbar (Fig. 10) shows ductile fracture as expected. EDAX results (Fig. 11) show that a large number of these cups have silicon-oxide inclusions, which is typical of ductile fractures. This is expected because the 316 L-A feedstock contains a master alloy and carbonyl iron powder, which is treated with a very fine layer of silica to prevent agglomeration and further oxidation of the iron powder.

The fine pores of the parts sintered at 400 mbar showed oxide contents about 200 times less than those seen in Figure 10.

SEM of Corrosion Samples
Figures 12 and 13 show the 15-mbar corrosion sample that was removed one day after corrosion before and after bending. The sample lost some of the rust and salt because of the bending.

Figure 14 shows a magnified view of the cracked area. Bending causes the part to further fracture along the grain boundaries. An EDAX of the region (Fig. 15) shows chlorides and sodium from the saline; oxygen and silicon from the silica inclusions; calcium, probably from the tap water; and chromium and iron. The picture probably verifies the theory that the presence of the silica-type inclusions in the presence of the chloride ions results in the formation of galvanic cells that enhance the rate of corrosion of the alloy sintered at 15 mbar. When the same alloy is sintered at 400 mbar, there are no oxide inclusions to form the galvanic cells that enhance the corrosion rate.

Discussions

A previous sintering study by the authors^[2] of 17-4 PH material in hydrogen, nitrogen, argon and vacuum also revealed the presence of oxide globules when a reducing atmosphere was absent. A hydrogen atmosphere at 400 mbar or more was necessary to reduce the silica and the chromium oxides. This resulted in the best mechanical properties and the best corrosion resistance.

Steffen Krug et al^[3] studied the effect of the furnace type (graphite and molybdenum) and the furnace atmosphere (hydrogen, nitrogen, argon and vacuum). Their conclusions are similar to those obtained in the study we did on the 17-4 PH material,^[2] except that they did not look at the unetched structures and missed the fine “pores” from the drag-out or reduction of oxides, which become indiscernible when the parts are etched. Hence, they did not look at SEM studies of the fracture surfaces and have not published anything further on the corrosion resistance. As a result, they concluded that there was little difference between sintering in hydrogen in a molybdenum furnace and sintering in vacuum in a graphite furnace, which is contradictory to our findings.

Summary

• Sintering at 400 mbar resulted in slightly better physical properties than at 15 mbar. The 15-mbar samples met the minimum requirements per the MPIF Standard 35, while the 400-mbar samples met the typical requirements.
• Fine “porosity,” 2-5 µm in diameter, is seen in the unetched structures for the parts sintered in both 400 mbar and 15 mbar. These pores get masked when the specimens are etched. MIM parts should be shown in both the unetched and etched conditions to ensure these structural conditions are not missed.
• Parts sintered at 400 mbar have a pore-free outer skin about 300 µm thick not seen in parts sintered at 15 mbar.
• SEM of the fracture surfaces shows many inclusions for the 15-mbar parts but almost none for the 400-mbar parts. The inclusions in the 15-mbar parts are oxides, while those for the 400-mbar parts seem to be prior oxides reduced by the hydrogen, based on the spectral analysis.
• The 15-mbar samples corroded within 24 hours in a 1% saline solution at 62°C. The 400-mbar samples showed none after five days.
• SEM of the bent, corroded sample strongly suggests corrosion begins at pore sites with oxide inclusions.
• The microstructures do not show a large difference between sintering at 15 mbar and 400 mbar. However, the overall effect of sintering at 400 mbar is a superior product with optimum corrosion resistance comparable to wrought products.

Acknowledgements

We would like to thank BASF Corporation, courtesy of Martin Blömacher, for supplying the feedstock and Arburg, courtesy Hartmut Walcher, for molding the tensile bars. We would also like to thank Daniel H. Herring, “The Heat Treat Doctor,” for the vacuum furnace photo and discussions on NFPA 86, Standards for Ovens and Furnaces.

References

1. NFPA 86, Standards for Ovens and Furnaces, 2015 Edition, National Fire Protection Association, Quincy, MA
2. S. Banerjee, C.J. Joens, “A Comparison of Techniques for Processing Powder Metal Injection Molded 17-4 PH Materials,” Advances in Powder Metallurgy & Particulate Materials, Part 4 Powder Injection Molding (Metals & Ceramics), MPIF, Princeton, NJ, 2008
3. Steffen Krug and Stefan Zachmann, “Influence of sintering conditions and furnace tehnology on chemical and mechanical properties of injection molded 316L,” Powder Injection Molding International, Vol. 3, No. 4, December 2009, Innovar Communications Ltd., Shrewsbury, UK, p. 66, 2009