Title of Invention:
Design improvement on afterburner diffuser casing of a 41KN thrust, twin-spool, turbojet military aero engine.
Field of Invention:
The present invention relates to mechanical engineering rejuvenation technology, specific to flange replacement of afterburner diffuser.
Background:
The diffuser casing assembly is a part of after burner which increases the engine thrust without increasing the air mass flow rate assembled downstream of the aero engine (after 2nd stage turbine rotor). The diffuser casing is designed to decrease the rate of gas flow to a value suitable for reliable ignition and combustion of fuel in the afterburner during initiation of reheat ratings, steady stabilization of flame and straightening of the gas flow at the turbine outlet. Diffuser casing consists of front flange (1); middle sheet metal casing (2) and rear flange (3) as shown in Figure-1. Front and rear flanges of the diffuser casing are meant for structural load whereas the middle sheet metal portion is meant for thermal shock loading. Diffuser casing is manufactured by two butt welded flanges (front and rear) with outer casing. Diffuser casing assembly is assembled in between 2nd stage nozzle box casing (4) and variable area jet nozzle assembly (6). The front flange (1) of the diffuser assembly is assembled to the 2nd stage nozzle box casing (4) by bolt joints and rear flange (3) is assembled with the front flange (5) of variable area jet nozzle assembly (6) by telescopic ring (7). The detail of diffuser assembly is shown in Figure-2 and Figure-3.
During service due to many factors such as high temperature, mechanical
loading, vibration etc. large no. of cracks have is generated near to front
flange and casing weld seam joint. As per original technology, it is permitted
to rework of two nos. of cracks, length up to 200 mm (at longitudinal and
circular seam) on diffuser wall outer body. If the crack length is beyond repair
limit, then the whole diffuser casing assembly is to be rejected. *

To avoid such occurrence, a detail analysis was carried out and modified front flange design was proposed. Also, the new process is developed to replace the front flange on the existing diffuser casing assembly and salvaging the diffuser casing assembly.
Brief Summary of Invention:
As per original technology, if the crack on the weld seam is beyond repair technology then whole diffuser casing assembly was rejected. In order to salvage the same a repair scheme was developed by cutting the front flange (1) from the diffuser assembly as shown in Figure-5 and welding a new front flange with increased length of 30±0.2 mm from 28+0.2 mm (length of front flange (1) is 28±0.2 mm during manufacturing stage). But the salvage scheme did not work properly as similar trend of failure also observed in the diffuser assembly repaired as per the said scheme. Hence, there is need for new / suitable salvage procedure which will improve the reliability of the component.
This invention relates to the reduction of formation of cracks in the diffuser casing during aero engine exploitation. The principle of invention is the new front flange design as per Figure-4 and its assembly with casing by removing existing front flange as shown in Figure-6. By doing so the strength is increased and work hardening near to welding area at front flange and casing joint can be minimized.
Brief Description of Drawing:
A full understanding of invention can be gained from following description of the preferred embodiments when read in conjunction with Figure-1 to 6.
Figure-1 describes the construction of diffuser casing.
Figure-2 and Figure 3 describes the assembly condition of the diffuser along with 2nd stage nozzle box casing (4) and jet nozzle casing (6).

Figure-4 describes the comparison of Existing (unmodified) front flange with New Modified front flange i.e. with increased length from 28±0.2mm to 33±0.2mm.
Figure-5 describes cutting place for front flange i.e. 30±0.2mm as per existing method.
Figure-6 describes cutting place for modified front flange i.e. 33±0.2mm as per new/ modified process.
Detail Description of the Invention:
Stress analysis is carried out by varying the length of front flange length in order to optimize the length of front flange. It is observed that, the maximum equivalent stress (Von-Mises stress) at welding zone between front flange and outer casing is evaluated by static structural analysis considering the results of thermal analysis. From the stress analysis, it is concluded that, the value of the maximum equivalent stress present at welding zone. It is quite obvious, because the whole diffuser and jet nozzle load is carried by front flange (1) of the diffuser and it acts as a cantilever at the bolt joint area as the attachment between the diffuser and jet nozzle is in the form of telescopic ring joint as shown in Figure-3. From stress analysis it is concluded that, the equivalent stress induced at the weld zone between front flange & diffuser casing is getting reduced after increasing the length of front flange. Stress analysis with increase of front flange by 5mm, 10mm, 15mm and 20mm was carried out differently. The percentage equivalent stress value at weld zone gradually reduces but the structural rigidity is increased. Considering the structural rigidity of the front flange and thermal shock taking capacity of the sheet metal middle casing the length of the front flange can be increased by 5mm. Hence, the total length of front flange will be 33±0.2 mm from 28±0.2 mm. The design of modified front flange i.e. with increased length is shown in Figure-4.
Earlier, the front flange with short length and reduced fillet radius was assembled with casing. Due to this, during welding with casing work hardening happened which causes higher residual tensile stresses. Also due

to reduced fillet radius stress concentration factor was higher. This results in generation of the higher and larger no. of cracks during exploitation leads to rejection.
In this invention, adopting the new front flange design and using new flange assembly process with casing, number of cracks generation reduced drastically. Now, requirement of replacement of full diffuser casing assembly with new one is nil.
Modified front flange with increased 5mm length i.e. with increased length of 33±0.2 mm from 28±0.2 mm is used during overhaul of aero engine in case the crack length is beyond permissible limit of repair.
Detail process for replacement of front flange (special) on diffuser outer casing is prepared as per following procedure:

Operation Number DETAIL PROCESS
1. MARK THE DIFFUSER FOR CUTTING.
2. MOUNT THE CASING ON LATHE. CUT OFF THE FRONT FLANGE MAINTAINING THE CASING HEIGHT 780.35±0.5.
3. DE-BURR THE EDGES OF THE CUTTING AREA.
4. POLISH THE CASING ON A WIDTH OF NOT LESS THAN 10MM FROM THE END FACE TILL THE METALLIC LUSTURE FOR AAW (ARGON ARC WELDING) ON BOTH SIDES & CLEAN WITH PETROL.
5. ASSEMBLE THE SPECIAL FRONT FLANGE WITH DIFFUSER CASING BY WELD JOINT MAINTAINING THE PRPER LENGTH.
6. CARRYOUT REWELDING OF DEFECTS BY MANUAL
AAW
7. CARRYOUT LOCAL HEAT TREATMENT OF WELD SEAM AREA OF FRONT FLANGE BY OXY-ACETYLENE FLAME FOR 10-15 MINUTES TO ATTAIN A TEMPERATURE OF 800 DEGREE CENTIGRADE TILL OBTAINING RASPBERRY COLOUR.
8. WASH THE COMPONENT BY CLOTH SOAKED WITH

PETROL
9. CARRY OUT CALIBRATION OF THE COMPONENT
10. CARRY OUT CHALK TEST ON FRONT FLANGE WELD SEAM AREA
11. CARRY OUT REWELDING OF DEFECTS ON FRONT FLANGE WELD SEAM
12. CARRYOUT X-RAY INSPECTION ON THE WELD SEAM
13. CARRYOUT MACHINING OF FRONT FLANGE
14. CARRY OUT DRILLING OF HOLE ON THE FRONT FLANGE FOR BOLT JOINT
15. CARRY OUT SCALLOPING THE FRONT FLANGE
16. CARRY OUT MILLING ON THE FRONT FLANGE
17. DEBURR THE MACHINED SURFACE.
18. CARRY OUT LAPPING OF FLANGE
19. CARRY OUT TOTAL INSPECTION OF DIMENSION AS PER MANUFACTURING TECHNOLOGY OF DIFFUSER CASING


We claim:
1. A process for reliability improvement of afterburner diffuser assembly of 41KN engine, comprising of the following steps:
- mark the diffuser for cutting;
- mount the casing on lathe. Cut off the front flange (1) maintaining the casing height 780.35±0.5 as per figure-6;
- de-burr the edges of the cutting area;
- polish the casing on a width of not less than 10mm from the end face till the metallic luster for AAW (argon arc welding) on both sides & clean with petrol;
- assemble the special front flange with diffuser casing by weld joint maintaining the proper length;
- carryout re-welding of defects by manual AAW;
- carryout local heat treatment of weld seam area of front flange by oxy-acetylene flame for 10-15 minutes to attain a temperature of 800°C till obtaining raspberry color;
- wash the component by cloth soaked with petrol;
- carry out calibration of the component;
- carry out chalk test on front flange weld seam area;
- carry out re-welding of defects on front flange weld seam;
- carryout x-ray inspection on the weld seam;
- carryout machining of front flange;
- carry out drilling of hole on the front flange for bolt joint;
- carry out drilling of hole on the front flange for bolt joint;
- carry out scalloping the front flange;
- carry out milling on the front flange;
- deburr the machined surface;
- carry out lapping of flange; and
- carry out total inspection of dimension as per manufacturing technology of diffuser casing.

ABSTRACT
Title: Design improvement on afterburner diffuser casing of a 41KN thrust, twin-spool, turbojet military aero engine.
The diffuser casing assembly is a part of after burner system, which increases the engine thrust without increasing the air mass flow rate by initiation of reheat ratings, steady stabilization of flame and straightening of the gas flow at the turbine outlet. It is manufactured by two butt welded flanges (front and rear) with outer casing. It is assembled in between 2nd stage nozzle box casing by bolting and variable area jet nozzle assembly by telescopic joint.
Due to its structural design and operating factors like high temperature, mechanical loading, vibration etc. crack may develop. Hence, a new design is introduced with suitable theoretical calculation to improve the reliability of the component by increasing length of front flange from 28±0.2 mm to 33±0.2 mm.