Oxygen Content in PM HIP 625 and its Effect on Toughness

Abstract

Oxygen control during powder manufacturing and handling is crucial when manufacturing HIPed parts. The influence of elevated oxygen content on mechanical properties is something that has been debated and investigated for many years. The general consensus in the industry is that oxygen has a very detrimental effect on the toughness of the material if present in excessive amounts. The detrimental effect of oxygen content on the impact toughness of the material has resulted in HIPed specifications, both existing and under development, with limits on the oxygen content in the material. Many specify a relatively low limit on oxygen content at e.g. 120 ppm which can have adverse effects on yield in powder manufacturing which might increase costs without accomplishing the desired effect of ensuring sufficient toughness. As this study show, oxygen content and chemistry alone is not enough to describe the effect of oxygen content on the HIPed material. Setting a limit at e.g. 120 ppm will not guarantee that one gets better properties or even reaches the desired properties of the material. The study show it is important where the oxygen is located in the powder and to separate bulk oxygen content and the surface oxygen content, where the latter has a more pronounced effect on toughness. In the study four batches of alloy 625 have been investigated, all with only relatively small variations in oxygen content but with drastically different toughness and differences in how oxygen is distributed in the material. Introduction Powder Metallurgical (PM) materials are sensitive to oxygen due to the large surface area of the fine powder. In some PM processes e.g. press & sinter and Metal Injection Molding, oxygen content can be reduced in sintering by performing it in hydrogen. However, when consolidating the material using Hot Isostatic Pressing (HIP) the consolidation occurs with vacuum the capsule which has little or no effect on the oxygen content. Therefore, oxygen control throughout the manufacturing process is important as any adsorbed oxygen cannot be removed in the later stages of manufacturing. Other studies have investigated the influence of oxygen on mechanical properties on HIPed austenitic and duplex stainless steel. In general the studies show a correlation between oxygen content and impact toughness, especially at lower temperatures [16]. Usually it is toughness that is reduced by excessive oxygen in the material but also welding properties of the material can be affected. Currently there are few material specifications on HIPed material and most that exist are project or product specific. There are a few specifications and standards covering PM HIP material e.g. ASTM (A988, A989 and B834), ASME code cases (N-834 and 2840) as well a mention in API 6A. However, more specs are in the works and many of them specify maximum oxygen content in the material. There is a trend to set lower and lower maximum allowable oxygen content which in turn can have a negative effect on price of the produced parts. When Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 135-141 doi: http://dx.doi.org/10.21741/9781644900031-19 136 levels are below 120 ppm it gets much more difficult for powder and part manufacturers to meet this and it might not have the desired effect on mechanical properties. Other studies have shown that properties at levels of oxygen from 120 ppm and below is not necessarily connected to the amount of oxygen, in fact a material with higher oxygen content can have better toughness than a material having significantly lower oxygen content [7, 8]. In this study 4 different Alloy 625 materials, manufactured with Ar or N gas atomizing have been investigated with regards to microstructure and mechanical properties. Experimental Sample manufacturing. Manufacturing of the powders was done using gas atomization. Process and powder handling parameters was varied to achieve different distribution of the oxygen in the powder. The atomized powders were sieved at -250 μm prior to filling of the capsules. N and Ar atomized powder are hereafter labeled N625 and A625 respectively. The powders were filled in rectangular-shaped mild-steel capsules of outer dimensions 180x70x50mm and sheet thickness 2mm, evacuated and sealed and subsequently HIPed in a standard HIP cycle with a plateau at 1150°C temperature, 100 MPa pressure and 3 hours. Testing on all materials was performed in the as-HIPed condition. Chemical analysis. All materials were analyzed with regards to chemical composition in the as-HIPed condition. Ar-testing was done on the capsule filling pipe that was filled with 253MA material. The same procedure that is often used in the industry. Mechanical testing. Tensile testing was performed using ISO 6892-1:2009. Charpy impact toughness testing was performed per ASTM A370-17 at -46°C. Average of three tests is presented. Microstructural characterization. Was performed using Light Optical Microscopy (LOM) as well Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Results Chemical analysis. The chemical analysis for the material in the as-HIPed condition can be seen in table 1. All the material are similar but do contain some minor differences, especially when comparing N and Ar atomized powders. As expected the N-content in the N-atomized powder is significantly higher compared to the Ar-atomized powder. The later does contain higher amounts of the strong nitride forming elements Ti and Al as well as a lower amount of Fe. Microstructure. Figure 1 show LOM and backscattered SEM micrographs of the Aratomized materials. In the A625:1 material several clusters oxide particles are observed (white spots in figure a, black spots in figure c). Many of these are correlated to the surface of the prior powder particles as they form a semi-continuous network that clearly highlight the spherical shape of the prior powder particles. These so called Prior Particle Boundary particles (PPBs) are Table 1. Composition of materials in the as-HIPed condition (wt.%). Ni Cr Mo Nb Fe Ti Al C Si N S P O A625:1 Bal. 21.14 8.99 3.55 1 0.22 0.21 0.007 0.02 0.007 0.002 0.003 0.0095 A625:2 Bal. 21.43 9.07 3.71 1.11 0.29 0.27 0.018 0.06 0.006 0.001 0.003 0.0128 N625:1 Bal. 21.59 9.25 3.73 2.55 0.01 0.07 0.021 0.02 0.066 0.001 <0.003 0.0105 N625:2 Bal. 21.74 9.16 3.69 2.38 <0.02 0.03 0.015 0.02 0.1 0.001 0.003 0.0096 Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 135-141 doi: http://dx.doi.org/10.21741/9781644900031-19 137 Figure 1. LOM (a & b) and SEM (c & d) micrographs of A625:1 (a & c) and A625:2 (b & d). Table 2. Mechanical properties.