HIP effect on microporosity value of large-sized working cast blades sections of IN-92-5A alloy

. The formation of a dense cast billet is a basic requirement for products made of heat-resistant alloys. Based on the comparison of research results, the use of hot isostatic pressing (hereinafter – HIP) for castings made of IN-792-5A alloy. Work on selection of technological regimes to reduce the microporosity content in the body of cast billet has been carried out. The object of the research was a cast billet of the working blade of the fourth stage of a high-power gas turbine unit.


Introduction
The formation of a dense structure in blanks from the forged nickel alloy turbine blades is a topical task [1].
Microporosity is one of the key parameters to assess the quality of a cast billet; in most cases, this parameter is regulated by the specifications or technical requirements of the part drawing and is directly related to the performance properties of the product [2,3].Microporosity in the form of interdendrite porosity or single pores occurs during melt crystallisation; such defects are stress concentrators, which in turn reduce the service properties of the cast billet, as the initiation of cracking begins with this type of imperfection.
According to foreign and domestic literature, cast blades are subjected to HIP followed by heat treatment [4,5].
However, data on the use of HIP on Inconel 792-5A alloy parts in the literature is not sufficient to determine the degree of influence of the modes HIP on the structure of large blades in 792-5A alloy and the microporosity value [6 -9].
The aim of this paper is to investigate the impact of modes HIP on the microstructure and microporosity of large-sized cast blades blanks made of the nickel heat-resistant alloy Inconel 792-5A.
In order to achieve this, the following objectives were set and pursued: 1.The microstructure of the blades in the post-cast condition is investigated and determined.2. To determine the effectiveness of the impact of common production regimes HIP on the value of microporosity.3. Regimes have been proposed and tested HIP for large-sized Inconel 792-5A work blades.

Materials and methods
The process HIP is to apply high pressure at high temperature uniformly and in all directions to a part in a protective gas environment, which increases the density by "collapsing" the internal imperfections in the part, thereby reducing the porosity of the part [10].The shielding medium that is most commonly used is argon.The maximum temperature of the process is HIP The designation should be such that no phase transformation processes occur in the metal and no change in the geometric dimensions of the part, but that plastic deformation processes take place in the metal of the part.
The aim of the operation HIP is the "healing" of internal metallurgical defects in the part in the form of pores, discontinuities and a reduction in the microporosity value.
Experimental work was carried out on cast large-sized working blades made of the heatresistant nickel alloy Inconel 792-5A.In order to more fully assess the effectiveness of the HIP The microporosity of castings was examined on blade samples in the post-cast condition, after the HIP on generally accepted production regimes from the literature and newly developed regimes.
To determine rational parameters HIP Experimental work was carried out on common manufacturing regimes and experimental regimes were developed for large Inconel 792-5A work blades based on the results of microporosity studies.
The regimes commonly used in production and those developed are shown in Table 1.Ar -at least 99.995%, O2 -no more 0.0005%, Н2 -no more 0.003%, hydrocarbons -not more than 0.001%.
The cutting into templates was carried out according to the cutting pattern shown in the sketch of the sectional cut of a large-sized work blade F. Surface imaging was performed on an Altami MET 1T optical microscope at 50x magnification.Photo fixation and determination of geometric dimensions of surface defects (area) was performed using Altami Studio v.4.0 software.
The microporosity was assessed in the post-cast condition, after HIP according to conventional processing regimes and after HIP according to the developed regimes.

Results and discussion
Microporosity studies before HIP (in post-cast condition) are shown in Table 2.According to the results of pre-HIP research, internal defects were found in all sections in the form of individual pores, characterised as defects in casting processes, with pores having a complex geometric shape with typical sharp angles, concentrated and scattered microporosity.The average content of micro-porosity is -3.77%, which is generally good for the casting process.
For information: The maximum percentage of microporosity specified by the manufacturer for blades must not exceed -0,5%, for guide vanes (nozzle blades) -5%.
Analysing the results of microstructure examination of blades in the post-casting condition, it can be concluded that the microstructure of blades is characterised in all sections by the presence of numerous casting defects in the form of individual pores, up to 0,15 mm in size, with microporosity also characteristic for all the examined thin sections, from the above it follows that the microstructure results obtained can meet the requirements for guide vanes, because the requirements are generally lower than those for guide vanes.
Further experimental works were carried out on blades made, at the same technological modes of casting, HIP was carried out according to the generally accepted technological modes on manufacture, specified in table 1. Generally accepted HIP technological regimes were formed according to the results of literature analysis of HIP application on blades from heat-resistant nickel alloys.
The templates for the microstructure study were cut from similar areas of the blade as in the post-cast condition.The results of the microporosity study after HIP according to common production regimes are shown in Table 3. From the results of microdermabrasion tests, it was found that the average value of microporosity was -2.14%, the shape of pores is rounded (sharp corners of pores are rounded), indicating the beginning of plastic deformation processes during the HIP process, the presence of a large number of residual pores (microporosity) indicates the inefficiency of regimes.
In this regard, experimental HIP regimes were proposed, the process parameters are shown in Table 1.As the main process parameters of HIP are temperature, pressure, process duration, it was accepted to increase the value of each parameter.HIP process temperature, as stated earlier, must be sufficiently high and phase transformation processes must not occur, so the HIP process temperature was assumed to be at 20°С below the solidus temperature of the alloy Inconel 792-5A (Тsolidus=1240°С, Тliquidus=1318°С) and equal to 1220°С±5°С.The HIP process pressure was selected on the basis of the technical capability of the equipment (gas-stat) and assumed 165 MPa±5 MPa, it was also decided to increase the HIP run time from 3 h±15 min to 6 h±15 min, while the cooling rate remained unchanged.
The results of microporosity studies after HIP under the proposed experimental conditions are shown in Table 4. Analysing the results of the microstructure according to the proposed experimental modes it was established that the metallurgical defects of casting processes in the form of individual pores are practically absent, locally microporosity is completely eliminated, the average content of microporosity is 0.0375%, all these facts indicate the effectiveness of the proposed modes HIP and the possibility of using for work blades from the alloy Inconel 792-5A.

Conclusion
The microstructure of the blades in post-cast condition contains casting defects including micro-porosity (3.53-4.04%)and requires further "healing" by means of hot isostatic pressing as the most effective method at present.
HIP has a positive effect on the microstructure of large blades, because even at relatively low pressure and temperature the microporosity is partially eliminated.
It is established that by means of HIP it is possible to reduce microporosity content in the working large-sized blades from alloy Inconel 792-5A from average value of 3.77%, to 0.0375%, which testifies to efficiency of the offered modes HIP and application of hot isostatic pressing as technological operation in manufacturing of large-sized working blades from alloy Inconel 792-5A.

Fig. 1 .
Fig. 1.Sketch of a cut-out of a work blade for research

Table 1 .
HIP technological parameters

Table 2 .
Results of a microporosity study before HIP

Table 3 .
Microporosity results after HIP according to conventional production schedulesStatus: after HIP on production regimes

Table 4 .
Results of microporosity studies after HIP by experimental regimes