The effect of reduced oxygen content powder on the impact toughness of 316 steel powder joined to 316 steel by low temperature HIP

Bell

et al. 2015

With near-net shape technology becoming a more desirable route toward component manufacture due to its ability to reduce machining time and associated costs, it is important to demonstrate that components fabricated via Hot Isostatic Pressing (HIP) are able to perform to similar standards as those set by equivalent forged materials. This paper describes the results of a series of Charpy tests from HIP'd and forged 304L and 316L austenitic stainless steel, and assesses the differences in toughness values observed. The pre-test and post-test microstructures were examined to develop an understanding of the underlying reasons for the differences observed. The as-received microstructure of HIP'd material was found to contain micro-pores, which was not observed in the forged material. In tested specimens, martensite was detectable within close proximity to the fracture surface of Charpy specimens tested at 77 K (À196°C), and not detected in locations remote from the fracture surface, nor was martensite observed in specimens tested at ambient temperatures. The results suggest that the observed changes in the Charpy toughness are most likely to arise due to differences in as-received microstructures of HIP'd vs forged stainless steel.

Section: Discussionsupporting

confidence: 54%

A Microstructural Study on the Observed Differences in Charpy Impact Behavior Between Hot Isostatically Pressed and Forged 304L and 316L Austenitic Stainless Steel

Bell

et al. 2015

“…[7][8][9] Although this is a well-known metallurgical phenomenon which is not restricted to austenitic materials, as shown by the reported relationship between oxygen concentrations in weld metal and impact toughness of the resulting weld, [10,11] HIP manufacturers typically achieve a minimum oxygen levels over an order of magnitude greater than those typically found in forgings and castings, largely due to the challenges associated with reducing the oxygen levels further, since powder surface oxidation, one of the main mechanisms by which oxygen interferes, can arise during powder handling and storage.…”

Section: Introductionmentioning

confidence: 99%

“…It is well known that powder surface oxidation can reduce the impact toughness of the final HIP'd material [8,12] and it is believed that excessive surface oxidation of the powder particles prior to HIP can prevent complete interaction between neighboring particles, which can result in internal porosity in the HIP'd material. [8,9,13] However, questions remain on how the oxygen manifests itself in the matrix of the HIP'd material, either in solid solution, in the form of nonmetallic inclusions, or as microporosity, the mechanism by which oxygen operates in the fracture mechanism, and if there is a maximum oxygen concentration in the HIP'd material (and powder) that ensures the impact toughness is comparable to that of ''chemically equivalent'' forged/cast material. In addition to this, it is sometimes unclear whether quoted oxygen concentrations are those of (a) the initial powder specification, (b) the actual measured concentration in the powder, of which transportation and handling can affect significantly, or (c) the final HIP'd material.…”

Section: Introductionmentioning

confidence: 99%

Effect of Oxygen Content Upon the Microstructural and Mechanical Properties of Type 316L Austenitic Stainless Steel Manufactured by Hot Isostatic Pressing

Dhers

et al. 2016

Although hot isostatic pressing (HIP) has been shown to demonstrate significant advances over more conventional manufacture routes, it is important to appreciate and quantify the detrimental effects of oxygen involvement during the HIP manufacture process on the microstructural and material properties of the resulting component. This paper quantifies the effects of oxygen content on the microstructure and Charpy impact properties of HIP'd austenitic stainless steel, through combination of detailed metallographic examination and mechanical testing on HIP'd Type 316L steel containing different concentrations (100 to 190 ppm) of oxygen. Micron-scale pores were visible in the microstructure of the HIP'd materials postmetallographic preparation, which result from the removal of nonmetallic oxide inclusions during metallographic preparation. The area fraction of the resulting pores is shown to correlate with the oxygen concentration which influences the Charpy impact toughness over the temperature range of 77 K to 573 K (À196°C to 300°C), and demonstrates the influence of oxygen involved during the HIP manufacture process on Charpy toughness. The same test procedures and microstructural analyses were performed on commercially available forged 316L. This showed comparatively fewer inclusions and exhibited higher Charpy impact toughness over the tested temperature range.

“…While it has been reported that the effects of powder surface oxidation are detrimental to the impact properties of HIP'd steels, [20,21] the authors have recently published a series of papers [13][14][15][16] which investigate and quantify the effects of oxygen on material impact toughness and the mechanisms of fracture, in which it has been established that enhanced oxygen concentrations in HIP stainless steel result in a greater volume fraction of oxide inclusions in the microstructure, thereby facilitating the ductile fracture mechanism by acting as additional sites for the nucleation, and subsequent growth and coalescence of microvoids in the plastically deforming matrix. Because of the increased volume fraction of initiation sites in the HIP steels, and the reduced inter-void distances, void coalescence can occur at lower values of plastic strain since voids have smaller distances over which they need to grow to coalesce with adjacent voids.…”

Section: Hot Isostatic Pressing (Hip) Is a Metal Formingmentioning

confidence: 99%

Tensile Fracture Behavior of 316L Austenitic Stainless Steel Manufactured by Hot Isostatic Pressing

Brayshaw

Sherry

2018

Herein we investigate how the oxygen content in hot isostatically pressed (HIP'd) 316L stainless steel affects the mechanical properties and tensile fracture behavior. This work follows on from previous studies, which aimed to understand the effect of oxygen content on the Charpy impact toughness of HIP'd steel. We expand on the work by performing room-temperature tensile testing on different heats of 316L stainless steel, which contain different levels of interstitial elements (carbon and nitrogen) as well as oxygen in the bulk material. Throughout the work we repeat the experiments on conventionally forged 316L steel as a reference material. The analysis of the work indicates that oxygen does not contribute to a measureable solution strengthening mechanism, as is the case with carbon and nitrogen in austenitic stainless steels (Werner in Mater Sci Eng A 101:93-98, 1988). Neither does oxygen, in the form of oxide inclusions, contribute to precipitation hardening due to the size and spacing of particles. However, the oxide particles do influence fracture behavior; fractography of the failed tension test specimens indicates that the average ductile dimple size is related to the oxygen content in the bulk material, the results of which support an on-going hypothesis relating oxygen content in HIP'd steels to their fracture mechanisms by providing additional sites for the initiation of ductile damage in the form of voids.