Is Ni200 (N6) really unbearable above 315°C? ——Engineering considerations for upgrading nickel crucible to Ni201 (N4)

In the fields of chemical industry, optoelectronic crystal growth and high-purity molten salt heat treatment, nickel crucible is the core tool that can withstand high-temperature and strong alkali corrosion. When the process temperature hovers above the "life and death line" of 315°C for a long time, materials engineers must make a choice between Ni200 (national standard N6) and Ni201 (national standard N4). The room temperature mechanical properties of the two are almost the same, but after exceeding 315°C, the 0.13% carbon content difference is enough to cause a gap of several times or even dozens of times in the service life of the crucible. This is by no means a rough selection of materials that "just work", but a precise calculation related to process stability, equipment residual value rate and production safety.

The upper limit of carbon content of Ni200 (N6) is 0.15%, while Ni201 (N4) is strictly controlled below 0.02%. Nickel itself has a face-centered cubic lattice and has excellent plasticity, but the solid solubility of carbon in nickel decreases sharply as the temperature decreases. At room temperature, supersaturated carbon segregates at grain boundaries in a combined or free state.

When the crucible is in the medium-to-high temperature range of 315°C to 760°C for a long time, the nickel atoms gain enough kinetic energy to cause grain boundary slippage, and the carbon atoms at the grain boundaries will preferentially combine with nickel to precipitate worm-shaped or spherical graphitic carbon. This process is called "graphitization". Graphite itself is a brittle phase and does not have metal bonding force. Its precipitation is equivalent to "wedge" countless micro-crack sources on the originally strong grain boundaries.

More insidiously, graphitization is accompanied by volume shrinkage, causing local stress concentration at grain boundaries. As time goes by (usually starting to become apparent after more than 3,000 hours), the post-break elongation of the material will drop from the original more than 40% to less than 5%. At this point, the crucible no longer has the ability to plastically deform, and any thermal shock or slight mechanical impact will cause brittle through cracking along the grain boundaries. For crucibles loaded with thousands of kilograms of expensive molten salt or liquid metal, this kind of brittle fracture without warning often leads to huge disasters such as the scrapping of the furnace body and material leakage.

The clever thing about Ni201 is to cut off the supply at the source. After reducing the carbon content to 0.02%, even if it stays in the sensitive temperature zone of 400°C to 600°C for a long time, there is still a lack of enough carbon atoms at the grain boundaries to aggregate, precipitate and grow. This completely eliminates the thermodynamic driving force for graphitization.

After continuous heating for 10,000 hours in an environment of 550°C, no graphite precipitation was seen in the observation of the metallographic structure of Ni201. Its high-temperature durable strength remained above 120MPa, and the elongation remained at 30%. For Ni200 under the same working conditions, obvious graphite balls appeared after 3,000 hours, and the tensile strength after 5,000 hours was lower than 60% of the room temperature index. Because of this, the American ASTM B160 standard clearly defines the upper limit of the recommended continuous use temperature of Ni201 as 600°C, while Ni200 is strictly limited to 315°C. For thermal cycle conditions with repeated temperature rise and fall, the thermal fatigue resistance of Ni201 is much better than that of Ni200, because graphitization is irreversible and cannot be recovered through annealing once precipitated.