Influence of Segregants on Creep Fracture in Powder Metallurgical Martensitic Stainless Steel

Doctoral thesis, 1996

This thesis deals with the behaviour of segregants in Powder Metallurgical (PM) 10%Cr steel and their impact on creep fracture. Particular interest is taken in determining the reaction products involved in creep cavitation and how these are connected to the powder surface reactions during the gas atomisation and consolidation (hot isostatic pressing). The techniques applied include primarily Auger electron spectroscopy (AES) as well as Electron Spectroscopy for Chemical Analysis (ESCA), Secondary Ion Mass Spectroscopy (SIMS), Scanning Electron Microscopy (SEM) and optical microscopy. To facilitate thickness determination of reaction products on non-planar surfaces such as powder, a method for AES depth profiling was established based on the knowledge of the etch rate dependence on the angle of ion incidence and a stereophotogrametric technique. Three different PM 10%Cr steels are studied; two 10.5Cr1W1MoVNb steels, with and without 0.002wt% B addition, and one 11Cr2WVNb steel. The alloys are included in a national program aimed at developing new steam turbine materials. The goal is to enable use at temperatures above 600 °C (compared to presently used 550-565 °C), whereby improved energy efficiency should result. The analyses demonstrate the relation between the surface reactions during atomisation and consolidation, leading to products on the prior particle boundaries (PPBs), and the creep cavity formation. During atomisation, surface enrichment of impurities (e.g. Sn, Zn, Cu, S and possibly Ca) and minor alloying constituents (e.g. B in the B-alloyed steel) occurs in addition to the principal surface products; Fe, Cr and Mn oxides. The enrichment of several elements (Mn, Sn, Zn, Cu and S) can be attributed to the associated lowering of the surface energy of the liquid metal droplet. Powder finer than about 40 um is oxidised to much lesser degree and has a higher surface content of Mn and Cr oxides. The higher surface content of surface active elements (e.g. Mn and S) on smaller particles is suggested to result from these particles rapidly being highly undercooled before solidifying, whereby diffusion to the surface can dominate over evaporation. The decoration of PPBs in the compacted materials can generally not be visualised by means of optical microscopy. However, this was possible using imaging SIMS, which revealed PPB oxides containing Cr, Mn, Si and V together with the impurities Ca, Na, Al, Ti and, in B-alloyed material, B. In the B-alloyed material, coarse PPB inclusions are found. These inclusions consist of mixed B nitride and oxide containing Mn, Cr and Si. It is suggested that the B on the powder surfaces enhances the coarsening of the PPB products. Correlation between MnS inclusions and PPBs is also indicated. The creep rupture characteristics differ between the alloys with and without B. In the former alloy, creep cavity coalescence and creep fracture occurs at the former austenite grain boundaries (FAGBs), irrespective of creep test conditions. In the latter alloy, creep cavities coalesce much less, and with extended creep test duration, creep fracture appear to an increasing amount at PPBs. Only about 10% of the FAGBs are contiguous with the PPBs in both alloys. However, the FAGB/PPB contiguity is a crucial factor leading to creep cavity formation. In the B-alloyed material, creep cavity formation is further facilitated by the coarse PPB inclusions present. Irrespective of alloy composition, Cr-containing carbide/nitride appear together with segregated S in creep cavities. Furthermore, Sb and Sn are enriched in creep cavities. The enrichment of Sb increases with the cavity size, whereas also Sn is encountered in larger developed cavities. It is supposed that the presence of these elements in creep cavities enhances the cavity growth. Phosphorus is only found at limited levels in smaller cavities.