The alloy 21-6-9 is a nitrogen-strengthened austenitic stainless steel often used in aerospace applications due to its high strength, good fabrication properties, and toughness at cryogenic temperatures. However, minimal research has been conducted on alloy 21-6-9 using the additive manufacturing process laser powder-bed fusion (L-PBF). The L-PBF technique has been seen as a key to reducing production time and avoiding costly machining. Therefore, there is an interest in investigating L-PBF-processed 21-6-9 to determine the effects of L-PBF on properties at elevated and cryogenic temperatures. In this study, prior to tensile testing the alloy 21-6-9 underwent heat treatments that simulated aerospace applications and the alloy was analyzed and characterized to evaluate phase stability. The effects of elevated and cryogenic temperatures (77K) on the tensile behavior and microstructure were investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The tensile tests showed that the yield strength and ultimate tensile strength improved, while ductility varied depending on the conditions and test environment. The ultimate tensile strength was approximately 80% higher at 77K than at room temperature, although the elongation decreased by around 90%, possibly due to the formation of strain-induced martensite.

ACI HH II is an austenitic castable stainless steel for high-temperature applications such as grates in a Grate-Kiln indurator. Eight different alloy compositions have been used to evaluate the oxide formation and the microstructural evolution during an isothermal heat treatment at 800°C with a holding time of 200 h in an atmosphere containing 20% O2–N2. Residual ferrite is transformed into sigma phase during the heat treatment while secondary carbides are precipitated in the austenite. Silicon will form a thinner oxide but will reduce its adherence to the steel. Additions of Ti are beneficial for the properties of the oxide layer as the oxide is well adherent and thinner, while additions of Mn increase the growth rate of the oxide and enhance the risk of growth stresses in the oxide. It is proposed that by combing the beneficial effects of both Ti and Si it is possible to suppress the malign effects of Mn additions.

This thesis focuses on the different degradation mechanisms of the stainless steel in a travelling grate in a Grate-Kiln iron ore pellet indurator. The travelling grate is a conveyor belt that transports green-body pellets to a rotary kiln while the pellets are being dried and pre-heated to a temperature of 900-1100 °C by recycled hot air. After unloading of the pellets to the rotary-kiln for further sintering, the travelling grate is cooled in room temperature while returning to the loading zone of the wet pellets.

The steel was tested during thermal cycling in a test-rig, in order to simulate the influence of thermo mechanical fatigue and oxide spallation. The influence of erosion-deposition was investigated in a modified horizontal industrial combustion kiln at 800 °C, with slag and coal from production used as erosive media and combustion fuel, respectively. The influence of minor alloying additions of Mn, Si and Ti on the microstructure was explored by eight different casted alloy compositions. Isothermal heat treatments were performed at 800 °C during 200 hours on steel immersed in deposits recovered from a travelling grate in production.

The three main degradation mechanisms found in this work are thermal spallation, erosion-deposition and deposit induced accelerated corrosion (DIAC). Thermal spallation of the oxide layer is caused by the thermal expansion difference between the oxide and the metal during heating and cooling. It has been found that Ti improves the spallation resistance while Si reduces it. Spallation of deposits is another cause believed to increase the degradation. Erosion-deposition appears due to simultaneous erosion and deposition of particles on the travelling grate that causes erosion or deposition depending on the amount of alkali metals in the environment. The velocity of the particles also influences erosion and deposition in the way that higher velocities increase erosion. DIAC is proposed to form on the travelling grate due to the concentration of chloride- and sulphate containing alkali metals in the deposits.

 Other than these major degrading mechanisms, minor degradation mechanisms such as internal oxidation, sigma formation, carburization and sensitization towards inter-granular attack have been found inside the steel during heating. Thermo mechanical fatigue (TMF) causes intergranular cracks in the material of the travelling grate. Casting issues such as micro-segregation have also been addressed in this thesis.

A few different ways to improve degradation resistance have been proposed, such as homogenization heat treatments, optimization of process parameters and inhibitor solutions. 

A test-rig for thermal cycling has been developed to investigate the mechanisms behind some of the damages done to a grate-link in a pelletising indurator for iron ore pellets. The results from the test-rig were compared with the degradation of grate-links that had been in service for 8 months in the indurator. The damages on the grate-links induced both in the test-rig and at service were defined as internal oxidation which facilitated propagation of intergranular stress corrosion cracking (IGSCC). Internal oxidation was initiated by spallation or removal of the oxide layers. The test-rig was able to successfully perform a lab-scale simulation of the degradation experienced in the indurator. Thermal cycling was applied to plate-formed specimens by constraining their thermal elongation to form bending and thereby stimulate the mechanisms of spallation and sensibilisation to IGSCC by internal oxidation.

Austenitic stainless steels are often used in high temperature applications due to their inherent resistance to corrosion. The grate-chain in some Grate-Kiln processes for sintering of iron pellets is made of these austenitic steels to withstand the severe environment. It has been shown however that the grate-chain is affected by several degrading mechanisms in the harsh environment of the sintering process. A grate-chain that had been used for 13 months in production was investigated in order to find the mechanisms of degradation. Results show that slag products are accumulated on the grate-chain during time and interact with the steel mainly due to the content of alkali metals. The resistance towards degradation seems to decrease with time which is suggested to be caused by the depletion of chromium.