This invention is related to design novel method on integrating vane segments of axial diffuser of gas turbine engine.

Background of invention:

In some of the gas turbine engine configuration, the axial diffuser vanes are casted in light alloy, are fitted on casing as a segments. This is mainly due to ease of manufacturing sophisticated vane profile and ease of assembly. In the current practice, the Vane Segments are fitted on main casing with the help of "Cadmium Plated Carbon Steel Screws", where the tail portion of the screw is trimmed to match the curved segment profile to avoid obstruction in flow path.

It is observed that the gas turbine engines which are operated in saline or corrosive atmosphere, the segment attachment screws are getting sheared off near the neck, resulting in separation of head and tail portion of attachment screw (refer Figure i). Once sheared off, the tail portion of the attachment screw is getting unscrewed due to engine vibration, entering into engine gas flow path, damaging the downstream components like nozzle guide vanes, turbine blades, exhaust cone. Such damage to engine components is of catastrophic in nature and may lead to failure of engine and may cause fatal accident.

Around 30 cases were investigated for failure of these screws. Mechanical properties of the failed screws checked for chemical composition and hardness found satisfactory confirming to designed values. Micro examination of the cracked surface of the screw carried out. Significant oxide products were found present on the facture surface of the failed screw. Detailed fracto-graphic studies revealed presence of inter-granular features on the facture surface of the screw. Fracture features confirmed the screw has failed in brittle manner. Micro examination of the cracked screw revealed that the crack has initiated from the surface and had propagated almost throughout the thickness of the head of the screw. The crack had propagated in an inter-granular manner. Micro examination also revealed branching of the crack.

SEM-EDX analysis revealed presence of, "Cadmium" all along the path of crack propagation, including the crack tip in the screw. At some locations within the crack, oxides of iron were also detected in additional to cadmium. Also, the entire fracture surface of the failed screws of the engine revealed presence of, "Cadmium" in additional to oxides of iron.

Presence of cadmium within the crack including the crack tip, presence of cadmium on the entire fracture of the screws and the presence of brittle inter-granular fracture features indicate towards failure ofthe screws by, "Liquid Metal Inducted Embrittlement(LMlE)"."

LMIE is a catastrophic brittle failure of a normally ductile metal when coated with a thin film of a liquid metal and subsequently stressed in tension. The failed screws were designed for having cadmium plating. From the observation made on the failed screws shows that the screws were embrittled by molten cadmium viz. the cadmium plating on the screw had apparently melted (melting point of cadmium is 321 C) probably due to high service temperature.

The most critical and mandatory condition for LMIE is the liquid metal should be in immediate contact with the surface of the solid to initiate embrittlement and the liquid metal shall subsequently be present at the tip of the propagating crack to cause brittle failure. Other prerequisite condition that shall be fulfilled for to initiate in the solid metal are presence of tensile stress and barrier (such as grain boundary) that serves as a stress concentrator and crack nucleating site. Also some of the literatures suggest that stress is not a prerequisite for this type of LMIE failures.

In the present case of failure of attachment screws, all the necessary and favorable conditions for LMIE Failure appear have met. SEM-EDX analysis revealed presence of all along the path of crack propagating (that is inter-granular in nature) including the crack tip and all over the fracture surface of the screws. Grain Boundary penetration of a solid metal by a specific liquid metal causes the solid metal to eventually disintegrate resulting in LMIE failures. Hence based on Microscopic and facto-graphic studies it is concluded that the attachment screws has failed by Liquid Metal Induced Embrittlement (LMIE).

Summary of invention:

A novel method is introduced that comprises of the following features:

1) Invention of novel method of positive locking mechanism, by punch peening at 3 spots on the bottom of tail portion of screw on the curved portion of diffuser segment profile ( refer Figure 3) This novel locking mechanism will ensure that the tail portion of the screw is kept attached to diffuser segment even after the screw is sheared off.

2) Novel modification on attachment screw material comprising of use of "corrosion and heat resistance stainless steel with silver plating" in place of existing screw material with "plain carbon structural steel, having cadmium plating" on the screw surface. This novel method will eliminate failure of screw by Liquid Metal Induced Embrittlement (LMIE) which is occurring at operating temperature of 321 C and above, as the existing "cadmium plating" on attachment screw surface is changed to "silver plating". Silver doesn't oxidize significantly at these operating temperatures, will serves as corrosion deterrent and dry lubricant with low-shear strength on screw thread surface. Corrosion and heat resistance stainless steel material of the novel attachment screw will eliminate its failure due to corrosion and stress corrosion cracking.

Exhaustive structural and dynamic analysis has been carried out to assess the structural integrity and dynamic characteristic of the gas turbine engine under prevailing operating conditions. The basic design of the gas turbine engine has been kept undisturbed, with no additional weight / mass penalty. The life of the components is increased as corrosion and heat resistance material has been used. Reliability of the gas turbine engine has been increased as the failure of screw is controlled.

Objective of invention

1. Main objective is to design and develop a novel positive locking mechanism by punch peening at 3 spots on the bottom of tail portion of screw, on the curved segment profile to prevent separation of tail portion of screw from the diffuser assembly.

2. To innovate suitable material and plating on the attachment screw for gas turbine engines operated in corrosive or saline atmosphere.

3. To bring out novel technological facts on failure of gas turbine engine components and design aspects needs to be considered during selection of engineering materials on the gas turbine engine components operated in corrosive or saline atmosphere.

4. To innovate simple solution, where basic design of the gas turbine engine is kept undisturbed, with no additional weight / mass penalty.

Detailed description of the invention

The three vane segment sectors ofdiffuser assembly are integrated on turbine casing with the 12 numbers modified corrosion and heat resistance Stainless Steel attachment screws with silver plating. The tail portion of the attachment screws are trimmed on the bottom to match curved profile of the segments. Three spots of punch peening are formed on the interface of screw bottom and segment (refer Figure 3) with a special peening punch. Due to peening operation the screw material will flow sidewise into segment material, thereby developing positive locking on the bottom of attachmentscrew.

The invention with its advantages is illustrated with reference to the Figure 2 and Figure 3. Figure 2 represents the integration of, "modified heat and corrosion resistance attachment Screw having silver plating", Diffuser segments and Engine casing.

Figure 3 represent the detail configuration of DifTuser Segment. Detailed Cross Section A-A shows curved surface and Detail B indicates, "the novel locking position by peeing 3 spots on the curved surface".

Analysis carried out

Modal and detailed design analysis on assembly of Axial diffuser using novel locking method is carried out using 3D modeling Unigraphic Software. Florescent dye penetration test carried out at the punch peeing location. Exhaustive static and dynamic analysis of the novel assembly is carried out on test rig and on gas turbine engine, under the prevailing conditions to assess the structural integrity of the assembly. The assembly was inspected after exhaustive test and found satisfactory.

Claims

1 claim

1. Innovative integration of vane segments of Axial diffuser of gas turbine by novel method of locking the attachment screw and use of suitable material, has resulted in increase in reliability and durability of gas turbine engine and its components.

2. The method claimed in claim 1 has resulted in elimination of failure of attachment screw of Axial diffuser assembly of gas turbine engine.
3. The method claimed in claim 1 has resulted in enormous saving and taken care of flight safety concerns. This is achieved by eliminating failure of attachment screw, which otherwise use to turn out as 10D (Internal Object Detect) enters into the engine flow path, further damaging the engines major downstream components like nozzle guide vanes, turbine blades, exhaust cone.