This article provides actual data for testing performed to compare argon and nitrogen as cooling media in vacuum furnaces. The data includes five different highly alloyed materials.

Argon and nitrogen are the most utilized gases for cooling during thermal processing in vacuum furnaces. Nitrogen (AW = 14.0067) is approximately 2.9 times lighter than argon (AW = 39.948). Nitrogen has a faster cool rate (approximately four times faster than argon) and is much cheaper than argon (approximately eight times cheaper). However, nitrogen has a tendency to be slightly decarburizing for steels and to form nitrates at the surface of NiCo alloys at temperatures above 1450°F. 

Most of the aerospace OEMs would rather avoid the usage of nitrogen as cooling gas due to the reasons listed above.

In some specific applications, cooling in nitrogen may have a beneficial effect on mechanical properties of the processed alloys. A good example is related to the cold-forming industry. Some NiCo alloys, such as Inco 625 or Inco X-750, have a tendency of spring-back after the first forming. In order to bring the part to the desired shape, more than one cold-forming operation may be necessary. Typically, an in-process anneal is applied before two forming operations. A faster cooling rate during the annealing process may lower the hardness for some materials, such as Inco X-750. 

The purpose of this analysis was to compare the effects of cooling various alloys in argon versus nitrogen on mechanical properties and surface morphology.

Sheet-metal samples were tested in Abar-Ipsen furnaces. The furnace used could cool in either nitrogen or argon at pressures less than 1 bar (“negative” pressure). All the tests concluded that using nitrogen instead of argon for thermal processing metal sheet (less than 0.500 inch think) does not affect mechanical properties of the material, does not create any particular surface contamination (as long as the vacuum chamber is maintained very clean) and, in cases such as annealing of Inco X-750, can reduce the hardness to a desired value.

Background

Five different materials were selected to be tested. Five samples 1 inch wide x 4 inches long x 0.072 inch thick were cut from the material received for each test.

Five samples of each material were processed using argon cool, and five samples of each material were processed using the same recipe but with nitrogen cool.

All the samples were processed one batch after another in the same vacuum furnace with no other parts on that load. The vacuum-chamber pressure was maintained at 0.5 µm (microns) during soak.

Fig. 1. Incolloy X-750 solution annealed and nitrogen quenched (500X)

The samples were tested for hardness (10 readings, two on each sample). After that they were evaluated for the following mechanical properties: grain size, bend test and surface contamination (depth of intergranular attack and/or intergranular oxidation – IGA/IGO).

Test Results

Testing of Inco X-750 (AMS 5542)

The material was in-process annealed at 1800°F for 50 minutes. Results for hardness and tensile testing can be found in Table 1.

Results for bend test – 180° bend over D=2T – are shown in Table 3. Grain size and surface contamination results are also shown in Table 3. The microstructure can be seen in Fig. 1.

Testing of 321 CRES (AMS 5510)

The material was in-process annealed at 1800°F for 30 minutes. Results for hardness and tensile testing can be found in Table 1. Results for bend test – 180° bend over D=T – are shown in Table 3 as well as grain size and surface contamination. The microstructure is shown in Figs. 2 and 3.

Fig. 2. 321 CRES solution annealed and nitrogen quenched (500X)

Fig. 3. 321 CRES solution annealed and argon quenched (500X)

Testing of Inco 625 (AMS 5599)

The material was in-process annealed at 1800°F for 60 minutes. Results for hardness and tensile testing can be found in Table 2. Results for bend test – 180° bend over D=2T – are shown in Table 3. Grain size and surface contamination results are also shown in Table 3. The microstructure can be seen in Figs. 4 and 5.

Fig. 4. Inco 625 solution annealed and nitrogen quenched (500X)

Fig. 5. Inco 625 solution annealed and argon quenched (500X)

Testing of Inco 718 (AMS 5596)

The material was in-process annealed at 1800°F for 30 minutes. Results for hardness and tensile testing can be found in Table 2. Results for bend test – 180° bend over D=2T – are shown in Table 3. Grain size and surface contamination results are also shown in Table 3. The microstructure is shown in Figs. 6 and 7.

Fig. 6. Inco 718 solution annealed and nitrogen quenched (500X)

Fig. 7. Inco 718 solution annealed and argon quenched (500X)

Testing of Hastelloy X (AMS 5536)

The material was in-process annealed at 1975°F for 15 minutes. Results for hardness and tensile testing can be found in Table 2. Results for bend test – 180° bend over D=1.5T – are shown in Table 3 along with grain size and surface contamination results. The microstructure is shown in Figs. 8 and 9.

Fig. 8. Hasteloy X solution annealed and nitrogen quenched (500X)

Fig. 9. Hasteloy X solution annealed and argon quenched (500X)

Conclusions

Cooling Inco X-750 in nitrogen significantly reduced the average hardness as well as the yield and tensile strength. Ductility of the material was increased when compared with cooling in argon. There was no surface contamination and no change in microstructure, grain size and bending capability during cooling in either media.

Cooling CRES 321 in nitrogen showed no significant change when compared with cooling in argon, except a slight refining of the grain size. The surface morphology was not affected.

Cooling Inco 625 in either nitrogen or argon developed practically the same average of hardness, bending capabilities and overall mechanical properties. No surface contamination was identified after processing in nitrogen.

Cooling Inco 718 in nitrogen resulted in a significant decrease in hardness when compared with cooling in argon. No contamination was detected on the test coupon surface.

Cooling Hasteloy X in nitrogen slightly increased the hardness, but all other properties were the same when compared with cooling in argon.

In some specific applications, such as cold forming, when a faster gas cool is necessary during in-process anneal, nitrogen gas can be succesfully used to replace argon. Even though in some cases material properties remained the same, the reduction in the cost of cooling media can be very attractive for the processor.

During this analysis, no contamination and no change in surface morphology of the test coupons used was identified.

For more information: Contact Alex Pohoata, special process engineer, F&B Mfg. LLC, 4245 N. 40th Ave., Phoenix, AZ 85019; tel: 602-533-1107; e-mail: apohoata@fbmfg.com; web: www.fbmfg.com