FORM 2
THE PATENTS ACT-1970
(39 of 1970)
Complete Specification of project
Development of Oxy-Parkerizing technology for Ultra high strength steel 40KHN2SMA
Aircraft Upgrade Research & Design Centre,
Hindustan Aeronautics Limited,
Nasik
The following specification particularly describes the nature of invention and the manner in which it is to
be performed.

1) Background of the invention:
A case of failure of Main Landing Gear Brace bolt part No. MYE7-4100-140 was reported from 12FBSU on T-96 aircraft C-1608. On 16th June 2007 there was a ground incident after installing Main Landing Gear. Investigation of failed brace bolt was carried out by National Aerospace Laboratory, Bangalore. NAL concluded, the brace bolt failed by hydrogen embrittlement. The Brace Bolt is fabricated from stamping material grade 40KHN2SMA (EI-643VD) CHMTU1-607-68 and heat treated for Ultimate Tensile strength 190-215 Kg/mm2. The brace bolt had picked up hydrogen at the time of surface reconditioning. The surface reconditioning treatment carried out on brace bolt is oxy-parkerizing. The steel can pick up hydrogen during this process. This invention relates to developing oxy-parkerizing technology which will avoid failure of parts manufactured from 40KHN2SMA due to hydrogen de-embrittlement.
2) Objective of the invention
The objective of the invention was to develop oxy-parkerizing technology which will avoid failure of parts manufactured from 40KHN2SMA due to hydrogen de-embrittlement.
3) Detailed description of the invention:
The principle of oxy-parkerizing is that the surface of a steel part becomes coated with a ferrous phosphate layer after immersion in a phosphoric acid solution. All oxy-parkerizing baths are acidic and attack the metal to some extent. Hydrogen evolution is certain but the risk of embrittlement is small which is mainly due to the dilute nature of the phosphoric acid solutions used during the process & oxy-Parkerizing coating is porous to hydrogen. Though risk of hydrogen embrittlement is less, but it cannot be ruled out in such high strength steel like 40KHN2SMA. High strength steel is most vulnerable to this type of hydrogen embrittlement. The detail of the process carried out on brace bolt is as per Instruction No. 593-65 and placed at Appendix-A. In this instruction hydrogen de-embrittlement treatment after oxy-parkerizing process is not mentioned.
Other cause for hydrogen de-embrittlement is traced to pre-oxyparkerizing cleaning acid pickling operation. On study of the process it can be observed that there is no acid cleaning operation as called in oxy-parkerizing process with hydro sulphuric acid.
The hydrogen damage can be controlled in two ways, Lowering or eliminating hydrogen generation. Removing damage by de-embrittlement treatment immediately after coating. Lowering or eliminating hydrogen evolution in oxy-parkerizing process is not possible. Removal of hydrogen by de-embrittlment is the only choice to ensure the risk is minimized. The oxy-parkerizing treatment generally does not pose major embrittlement problems but hydrogen evolution is certain. Also there is no acidic cleaning done in present technology as done in modern treatments. Hence the process given by Russian ensures lower or no generation of hydrogen. But the risk of hydrogen embrittlement cannot be ruled out in very high strength steel.
The de-embrittlement treatment for steel is effective only when it is performed within 2 to 4 hr after the steel has picked up hydrogen. If delayed, the hydrogen damage is irreversible. The dehydrogenation treatment (at temperature 180-2000C for a duration of 5-6 hours) was added in oxy-parkerizing technology for parts manufactured from high strength steel 40KHN2SMA.

OXY-PARKERIZING PROCESS
Steel parts after machining should undergo the following subsequent operations:
a) Assembly to the fixture.
b) Degreasing
c) Washing in hot water
d) Washing in cold water
e) Oxy-Parkerizing
f) Washing in cold water
g) Washing in hot water
h) Treating in potassium bichromate solution
i) Washing in hot water
i) Drying
k) Inspection of oxy-parkerized film
I) Additional treatment (painting or oiling)
The details of the above-mentioned operations are as follows:
a) Assembly: Assemble parts in the special suspension of the fixtures or hand them on a wire. Parts should be assembled in such a way so as to avoid the formation of air pockets.
b) Degreasing: Degreasing of parts is conducted in phosphate-alkali solution, containing the following constituent (gms/litre):

Triosodium phosphate 30-70
Sodium or potassium carbonate 20-26
Caustic soda 05-15
Sodium silacate solution or OP-7, OP-10 10-20 2-3
Schedule:
Temperature: 80 to 90°C
Degreasing period: Up to full degreasing
c) Washing in hot water: After degreasing, parts are washed in hot running water at 50°C.
d) Washing in cold water: Wash the parts in cold running water. Surface of parts after washing should be fully wetted with water, if not, degrees the parts again.

e) Oxy Parkerizing: Conducted in a bath containing the following constituents
(gms/litre):

Barium nitrate 30-40
Zinc nitrate 20-10
Zinc monphosphate 8-12
Schedule:
Temperature: 80 to 85°C
Period of treatment: 5 to 10 minutes
f) Washing in coid water: Wash the parts in cold running water.
g) Washing in hot water: Wash in hot running water at 50 °C. Washing water should be clean. Take special care so that it does not contain scales
h) Treating in potassium bichromate solution: Treating in a solution of potassium bichromate is carried out for increasing the protection properties of film formed during oxy-parkerizing. A solution containing 50-80 gms/litre of potassium bichromate is recommended. Treatment is carried out at a temperature of 70-80 °C for 3-5 minutes.
i) Washing in hot water: After treating the parts in potassium bichromate solution, parts are washed in hot running water.
j) Drying: Drying of parts is done by blowing with clean compressed hot air. Final drying is carried out at a temperature of 110-120 °C.
k) Inspection of oxy-parkerized film: Quality inspection of oxy-parkerised film is done visually as well as by checking the protection properties. The film should be fine crystalled, its colour depends on the composition of steel.
I) Additional Treatment: Parts after oxy-parkerizing, depending upon their purpose should be subjected to painting or if painting is not possible, water repellancy treatment is carried out. Oiling is also permitted.

Claims:
We claim
i) Development of oxy-parkerizing technology for Ultra high strength steel 40KHN2SMA is unique in nature & nobody has done this invention globally.
Future Scope:
This technology will be used to avoid failure of parts manufactured from 40KHN2SMA due to hydrogen de-embrittlement.