Deformation and Fatigue Behaviour of Porous and Dense Austenitic Stainless Steel

Sammanfattning: PM stainless steels are increasing in use and are expected to do so in the future. A lot of work has been carried out concerning production and sintering of stainless steel powders and on the resulting corrosion behaviour. The deformation behaviour of these materials is less studied. The search for new applications raises a need for an increased knowledge of the deformation behaviour of these materials.

This thesis treats the monotonic and cyclic deformation behaviour of porous and dense austenitic stainless steels, where emphasis is placed on the porous materials. By including dense steels, the understanding of the deformation behaviour of porous structures is facilitated. All materials in the study resemble the commercial grade 316L.

The monotonic yield stress of the fully dense materials studied follows the classic Hall-Petch relation. Nitrogen amplifies the effect of a fine grain size by promoting planar slip. The cyclic yield stress, determined from the cyclic stress-strain curves, does also follow a Hall-Petch type of relation, the alloying with nitrogen here having less importance. The latter finding is explained by the breakdown of the planar slip mode during cyclic straining. Small surface cracks initiate early during cyclic straining but the nucleation ceases after approximately half of the life time. The mean crack length increases continuously until final failure. The initiation is related to the formation of slip bands on the surface. Thus, high nitrogen content promotes initiation at slip bands and the following crack growth is very much related to these bands. This may cause an increased roughness induced crack closure and a more simultaneous growth of the whole crack population, which indicates a higher consumption of strain energy. Opening of slip bands may also cause a crack growth retarding effect that can be compared to crack branching. A coarser grain size causes flocculation of cracks and a transition to intergranular crack initiation. However, cracks grow exclusively in a transgranular manner independent of grain size. A finer grain size may hinder small cracks in early growth, which seems to be promoted by a low nitrogen content.

The deformation behaviour of PM austenitic stainless steels is heavily influenced by the presence of pores, where the interconnected ones have the strongest influence. During monotonic deformation, the pores cause microplasticity, which is more pronounced in thin sections. Oxides in the ppb's (previous particle boundaries) may have a stiffening effect, resulting in a more truly elastic behaviour and a more marked yield point during initial deformation. The following strain hardening is less affected by the pore morphology and ppb structure. The materials strain-harden continuously preceding fracture and hardly any necking is detectable. Hereby the uniform elongation and the tensile strength become uniquely related. During cyclic straining, pores enforce a strain gradient in the material, which causes a pronounced cyclic hardening. The distribution of strains is more inhomogeneous in materials in which interconnected pores dominate. Fatigue cracks initiate at pores and grow along ppb's, as was the case in the studied materials with >0.2% oxygen. Fatigue cracks appear more frequently at the surface of materials cyclically strained at high strain amplitudes (>0.003), and internal cracking dominates at lower strain amplitudes. The fatigue strength is believed to be less affected by weak interparticle bonding than the ductility.