Abstract
The grain boundary network evolution of 316L austenitic steel powder during its densification by hot isostatic pressing (HIPing) was investigated. While the as-received powder contained a network of random high angle grain boundaries, the fully consolidated specimen had a large fraction of annealing twins, indicating that during densification, the microstructure evolves via recrystallization. By interrupting the HIPing process at different points in time, microstructural changes were tracked quantitatively at every stage using twin boundary fractions, distribution of different types of triple junctions, and the parameters associated with twin related domains (TRDs). Results revealed that, with increase in temperature, (i) the fraction of annealing twins increased steadily, but they mostly were not part of the grain boundary network in the fully consolidated specimen and (ii) the average number of grains within a TRD, the length of longest chain, and twinning polysynthetism increased during HIPing and (iii) the powder characteristics and the HIPing parameters have a strong influence on the development of grain boundary network. Based on the results obtained, possible alterations to the HIPing process are discussed, which could potentially allow twin induced grain boundary engineering.
Highlights
Powder hot isostatic pressing (HIPing) is a net shape manufacturing process that is used to produce fully dense components through the application of pressure (P) and temperature (T) on a powder compact for certain amount of time (t), which results in its complete consolidation [1]
The appearance of parallel sided 3 boundaries, i.e., annealing twins, in the orientation map suggests that they had formePd The fractions of as 3n a result of recrystallization during HIPing. boundaries and triple junctions in the as-received powder are shown in Table 2; shown are the statistics for the fully consolidated specimen for comparison
Evolution of CSL boundaries and triple junctions Grain boundary misorientation maps (Fig. 2) and the frequency of CSL boundaries (3 S 29) in the powder shown in Table 2, which was averaged over 3 particles of different sizes, indicate that the microstrPucture is dominated by high angle grain boundaries
Summary
Powder hot isostatic pressing (HIPing) is a net shape manufacturing process that is used to produce fully dense components through the application of pressure (P) and temperature (T) on a powder compact for certain amount of time (t), which results in its complete consolidation [1]. Advantages of powder HIPing include better chemical homogeneity, fine grain size, isotropic properties, increased materials utilization, and the ability to produce complex near net shaped components. Reduced lead time for manufacturing big near net shaped components and ease of in-service inspectability are other important advantages of HIPing. It has been demonstrated that HIPing, owing to the advantages it offers over conventional processing, is a viable manufacturing process for producing pressure retaining components made of 316L for nuclear reactors [4e6]. Annealing twins, which are a key microstructural feature of recrystallized 316L austenitic stainless steels, are observed in the microstructure of powder HIPed specimens (see for e.g., [7,8])
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