Thermal Mass Effect on the Solution Cooling Rate and on HIPped Astroloy Component Properties

Abstract

Astroloy is a Ni-based superalloy with high-volume fraction of γ', which gives high temperature properties but reduces its forgeability. Therefore, powder metallurgy manufacturing processes such as Near Net Shape HIPping are the most suitable manufacturing technology for Astroloy. However, NNSHIP has its own drawbacks, such as the formation of prior particle boundaries (PPBs), which usually tend to decrease material mechanical properties. The detrimental effect of PPBs can be reduced by optimizing the entire HIP processing route. Conventional HIP cycles have very low cooling rates, especially in big components from industry, and thus a series of post-heat treatments must be applied in order to achieve desirable microstructures and improve the mechanical properties. Standard heat treatments for Astroloy are long and tedious with several steps of solutioning, stabilization and precipitation. In this work, two main studies have been performed. First, the effect of the cooling rate after the solutioning treatment, which is driven by the materials' thermal mass, on the Astroloy microstructure and mechanical properties was studied. Experimental analyses and simulation techniques have been used in the present work and it has been found that higher cooling rates after solutioning increase the density of tertiary γ' precipitates by 85%, and their size decreases by 22%, which leads to an increase in hardness from 356 to 372 HB30. This hardness difference tends to reduce after subsequent standard heat treatment (HT) that homogenizes the microstructure. The second study shows the effect of different heat treatments on the microstructure and hardness of samples with two different thermal masses (can and cube). More than double the density of γ' precipitates was found in small cubes in comparison with cans with a higher thermal mass. Therefore, the hardness in cubes is between 4 and 20 HB 30 higher than in large cans, depending on the applied HT.

Keywords: Astroloy; HIP; Ni superalloys; cooling rate effect; gamma prime coalescence; gamma prime precipitation; heat treatments; high-temperature alloys; metals and alloys; thermal mass effect.

Figure 1

Graphical abstract of the steps performed in the paper.

Figure 2

Particle size distribution of powders.

Figure 3

(a) Scheme of the can and cube dimensions, both after HIP, for study B. Additionally, the can microstructural analysis areas were pointed out, in red and green, for study A. (b) Extracted can slice for hardness mapping, study A.

Figure 4

FEG-SEM image of a typical standard HTed Astroloy microstructure taken with the secondary electron detector. γ′ are coloured with red for primary γ′, blue for secondary γ′ and green for tertiary γ′.

Figure 5

Segmentation parameters for γ′ precipitates in cans, the small change for cubes is reflected as dot points.

Figure 6

Fourth can thermocouple locations.

Figure 7

DSC of Astroloy B1 powder for (a) heating and (b) cooling.

Figure 8

Solution cooling process of the powder M5 can, experimental (E) and simulated results (S).

Figure 9

Hardness values in the inner and external part of the can.

Figure 10

Microstructure of (a) as-HIP can (B1 powder) and solutioned can (M5 powder) in two areas (b) external upper part and (c) inner part.

Figure 11

γ′ quantitative metallography after solution treatment of the samples including the general content and total area percentage for the inner and external upper part.

Figure 12

Tertiary γ′ (a) size and (b) density of precipitates after solution treatment in the inner and external upper part of the can.

Figure 13

Hardness thermal mapping after HT-A, inner (1) and external upper (6) areas are marked.

Figure 14

Temperature evolution during the solution cooling process of cube and can.

Figure 15

Hardness measurements of HT samples.

Figure 16

Astroloy microstructure revealed with kallings N°2 (a) after HIP and after HT-A for (b) can and (c) cube.

Figure 17

γ′ properties of the samples with HT-A and HT-B for the can and cubes, including (a) the general content and total area percentage and (b) relative distribution.

Figure 18

γ′ size by population of the samples with HT-A and HT-B for the can and cubes, (a) primary, (b) secondary and (c) tertiary.

Figure 19

γ′ density of precipitates by population (a) primary, (b) secondary and (c) tertiary.

Figure 20

Relationship between solutioning cooling rate and hardness after HT-A.

Figure 21

Relationship between tertiary γ′ density, size and following HTs (Sol: Solution, HT-B: Sol + P2 and HT-A: Sol + S1 + S2 + P1 + P2).

Figure 22

Relationship between (a) tertiary γ′ density—size and (b) tertiary γ′ density—hardness.