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In terms of evaluating the effect which the carburisation process had on the alloys, regardless of the composition, it was clear that surface finish was directly related to the amount or carbon which the material can absorb. This being; that the increased diffusivity associated with a rougher (non-polished) face allows for the formation of a deeper expanded austenite layer, however results in a lower peak hardness. This suggests that the non-polished side allows for a deeper distribution of carbon throughout the modified layer. A smaller lattice expansion is also found on the non-polished side, due to the higher compressive stresses as a result of this increased interstitial content acting to oppose expansion.

 

One of the main purposes of the carburisation treatments is to harden the material. From that perspective, the alloys which were the best performing were Stainless Steel 254, Stainless Steel 832 and Stainless Steel 353. SS 254 and SS 832 appeared to have been case hardened as expected; showing the largest percentage lattice expansion, suggesting they have been hardened whilst maintain their corrosive resistant properties. Therefore SS 254 and SS 832 are recommended for use in conditions that require both a strengthened and corrosion resistant alloy.

 

Sample SS 353 appeared to have not been hardened to same extend by the formation of expanded austenite via carburisation, but through precipitation hardening caused by the formation of carbides at the surface, thus also maintaining a smoother finish. However it is expected that the corrosion resistant properties of this alloy may have been compromised.

 

It was also noted that there appears to be a direct relationship between the atomic spacing within the microstructure of the material, and the materials ability to be hardened by carbon diffusion. This relationship being; initially larger spacingatomic spacings allows for improved case hardening via carburisation. Again this stands without the presence of precipitation hardening.

With regards to the nickel based samples, both reacted far less accepting to the  carburisation treatment than the nickel samples, with the alloy with the greater nickel content (Nikrothal) showing almost no noticeable increase in hardness due to the treatments.

 

In terms of the comparing the K22 and K33 carburisation processes, it was difficult to draw definite conclusions due to the almost negligible effect which either treatment had on the Nikrothal samples. Assuming only the Inconel 617 samples produced viable and comparable results, it was seen that the K22 treatment was marginally, but consistently the superior of the two processes; producing both a deeper carburised layer and higher peak surface hardness.