Graphitized steels may offer a beneficial microstructure after hot rolling and annealing, either used as a medium-strength steel with improved mechanical properties due to solution strengthened ferrite replacing pearlite (similar to the three Si-solution strengthened ferritic ductile iron grades in ) and improved machinability due to the small graphite spheroids, or as a precursor with good machinability before various hardening heat treatments.
However, in the austenitization step in the final heat treatment, the graphite dissolves and small voids are formed, again due to the several orders of magnitude higher diffusion rate for interstitial carbon compared to diffusion rates for iron and substitutional solutes. Figure 8 actually shows an ausferritic matrix with small voids at the former graphite positions.
The depth of focus of the SEM is ideally suited for investigating the resulting voids, see Fig. 9a and b. In 8b, residuals of graphite-nucleating hard inclusions (nitrides) are shown.
Already in 1937, Bonte  observed the void formation: “Even though the free graphite is later dissolved…voids remain in the finished product.”
Void formation has been observed now and then for almost 80 years, but is in recent literature more or less overlooked. In the former Soviet Union, there were in the early 1960s a large number of observations of and discussion on the void formation, both in graphite containing irons and in steels [32,33,34,35].
One odd application of the phenomena is shown in a US patent dated 1959 , on an oil-permeable steel. The inventor states: “I have discovered that its porosity increases when it is subjected to repeated heating and cooling above and below its A1…” The idea of the patent is to repeat this cycling 40 times and thereby produce steel with 5 vol.% of voids/porosity.
One more recent paper  by Miura et al. utilized the void mechanism in heat-treated graphitic steel to create a model material for studying deformation at elevated temperatures.
The present authors are puzzled by the lack of recently reported observations of voids after the final heat treatments from groups working today with hardening heat treatments of ferritic ductile irons or graphitized steels.
The voids formed are not, from our experience, possible to heal during conventional austenitization although one early article by Hughes and Cutton  claimed the opposite: “…after the solution of the graphite spheroids, no porosity or voids were evident in the steel matrix.”
There is an influence from voids on mechanical properties in ausferritic steels. We have so far only performed comparing Charpy V impact energy testing on the same rolled and austempered steel containing 0.5 wt.% C and 3.8 wt.% Si, without or with prior graphitization. The steels were austempered at two different salt bath temperatures with the following mechanical properties for austempering without prior graphitization:
The Charpy V values for rolled and austempered samples without versus with prior graphitization are as follows:
Figure 10a and b shows a Charpy V fracture surface in two magnifications. Note the plastically expanded voids marked by arrows in Fig. 10b.
When spheroidal graphite cast iron is pre-quenched before austempering the ausferrite is refined by the secondary graphite spheroids, as earlier described. Voids formed during austenitization of graphitic steels do not refine the ausferrite during austempering. The structure in Fig. 8 is not of the refined “acicular ferrite”-type in Fig. 3. The difference is probably due to the finer dispersion of graphite from excess carbon (thus not dissolving forming voids) in the first case, compared to the voids formed from dissolved graphite in steel.
The only way of efficiently healing these voids (or closed casting porosity) is by subjecting the material to hot isostatic pressing (HIP), where the isostatic argon gas pressure in the range of 100–200 MPa is acting on the outer surfaces at temperatures where the hot strength of the metal alloy is less than the applied pressure, thus causing local “superplasticity,” creep, and diffusion bonding.
This method is currently extensively used for castings in expensive metal alloys that are difficult to cast without porosity such as Ti-6Al-4 V and Ni-based superalloys, to improve mechanical properties especially in fatigue. The process has usually been too expensive for most steels and cast irons if no other benefits can be concurrently obtained.
However, recent development of HIP equipment where the cooling rate of dense argon gas (with a density similar to water) can be increased from < 100 K/min to > 1000 K/min enables quenching of workpieces after prior austenitization (whereby closed porosity is eliminated). Residual stresses are also much lower than after conventional quenching, since any residual stresses from prior process steps are eliminated by yielding and creep during austenitization, while new quenching stresses cannot be created until the workpiece has been cooled sufficiently for the material to become stronger than the compressive stresses from the isostatic pressure. Further the freedom to vary process temperature and alternate between promotion of nucleation at a lower and growth at a higher temperature can give better combinations of strength and ductility/toughness through creation of improved microstructures. These concurrent processes make hardening heat treatments in a HIP equipped with Uniform Rapid Quenching (URQ™) a cost-efficient method.
The current authors have investigated the combination of casting porosity elimination and creation of improved microstructures during austempering of ADI and ausferritic steels . Recent use of this process with the aim to reach even higher ductility levels for the previously described rolled steel resulted in the following mechanical properties: Rp0.2 = 1112 ± 20 MPa; Rm = 1445 ± 24 MPa; A5 = 20.7 ± 1.3%.
The Charpy V values for samples rolled and austempered in HIP without or with prior graphitization are as follows: 40.8 ± 4.4 J resp. 38.0 ± 1.6 J (− 7%), indicating that the influence from previous voids have been eliminated by power-law creep followed by diffusion bonding under the isostatic pressure of 170 MPa during austenitization before quench.