Internal oxidation

Precipitation of oxides at a more or less large distance from the surface of a
casting/workpiece due to diffusing oxygen (according to EN 10052 – oxidation, internal, term 84). Internal oxidation which may occur during carburization is often referred to as “edge oxidation”.

In cast iron materials, oxidation already starts at 250°C but remains so insignificant up to temperatures of approx. 400°C that it can be neglected However, any further rise in temperature then results in a multiplication of the oxidation rate (Fig. 1) which increases very rapidly at even higher temperatures.

The scaling resistance (see Scaling) of unalloyed nodular graphite cast iron is slightly higher than that of gray cast iron as the latter is subject to additional oxidation attacks along its graphite flakes which do not occur at the graphite nodules with their favorable volume/surface area ratio. Thus, a GJL 250 will exhibit double the amount of scaling compared to unalloyed nodular graphite cast iron when subject to a temperature in the range of 550°C for 32 weeks. However, this difference becomes smaller as the temperature increases.

At temperatures of 500°C, ferritic nodular graphite cast iron has a slightly better scaling resistance than pearlitic grades, but here too, the difference disappears as the temperature rises.

The oxidation resistance can be improved by means of alloy elements. Tin (Fig. 1) and, in particular, silicon (Fig. 2) each have a beneficial influence.

Scaling-resistant nodular graphite cast iron is obtained with > 4% silicon (K. Röhrig) and can be used at temperatures of up to 800°C. When using a 5% silicon alloy, the operating temperature can be about 900°C, while a silicon content of 6% permits 950°C.

When exceeding the temperature of ferrite/austenite conversion, this results in volume changes causing microcracks, considerably accelerating the oxidation attack.

Generally, in gray cast iron, it is nearly only iron that is oxidized at temperatures of less than 700°C, whereas the graphite in the structure is burnt at temperatures above 700°C.

Therefore, an oxide film is formed along the graphite flakes in the low temperature range (Figs. 3 and 4); this increases the propensity to cast irongrowth as oxide formation is connected to an increase in volume. The zone of internal oxidation is also called “subscale”.

  • Fig. 1: Oxidation (GJS scaling at between 550 and 650°C (according to K. Röhrig)
  • Fig. 2:  Influence of the silicon content of ferritic GJS on the oxidation by air at 650°C (according to I.C. Hughes, BCIRA)
  • Fig. 3: Oxide film along the graphite flake
  • Fig. 4: EDX analysis of the oxide film shown in Fig. 3
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