DESC:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2016

COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
TITLE
“A Novel Method for Revealing Prior Austenitic Grain Boundaries in Precipitation Hardened Martensitic Steels”
APPLICANT
(a) Name : Mishra Dhatu Nigam Limited
(b) Nationality : Indian
(c) Address : PO Kanchanbagh, Hyderabad, Telangana – 500058, India

The following specification particularly describes the application and the manner in which it is to be performed.
PRIORITY STATEMENT
The present application hereby claims priority from Indian patent application with the application number 202341031475, filed on 3 May 2023, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD OF INVENTION
The present invention described herein, relates to the field of metallographic display technology. More particularly, the present invention relates to a technique for enhancing the visibility of Prior Austenitic Grain Boundaries (PAGBs) of Martensitic Precipitation Hardened Stainless Steel (PHSS) through a unique combination of heat treatment and etching procedures for accurate grain size measurement.

BACKGROUND OF INVENTION
In metallurgy, grain size is an important factor that influences the mechanical properties of metals. Precipitation hardened martensitic steel is widely used due to its high strength, toughness, and excellent corrosion resistance. However, it is challenging to evaluate the grain size in these steels as the grain boundaries are not clearly visible. Therefore, it is very important to identify grain boundaries in order to determine the grain size.
The Martensitic Precipitation hardened Stainless Steel (Martensitic PHSS) derives strength from fine coherent precipitates and have high specific strength, toughness, and adequate corrosion resistance at ambient and high temperatures. Martensitic PHSS consists of lathe martensitic structure. Traditional etchants used to reveal the grain boundaries in other steels are not effective for precipitation hardened martensitic steels.
Hence, Prior Austenitic Grain Boundaries (PAGB’s) are not clearly demarcated to measure the grain size for Martensitic PHSS. According to Hall-Petch equation, the grain size is inversely related to the material strength.
The available literature discloses a number of high temperature treatments to reveal Prior Austenitic Grain Boundaries (PAGBs). The materials were heated beyond the service temperature for determining the grain size. These high temperature treatments methods have their own limitations such as grain coarsening.
Therefore, in order to overcome the above challenge, a novel technique comprising of unique heat treatment cycle followed by a suitable etchant is developed to reveal the prior austenitic grain boundaries in Martensitic Precipitation Hardened Stainless Steel.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the disclosure. This summary is neither intended to identify key or essential inventive concepts of the disclosure nor is it intended for determining the scope of the disclosure.

A present invention discloses a novel technique for revealing prior austenitic grain boundaries in martensitic precipitation hardened stainless steel (Martensitic PHSS) comprising of heat treating a metallographic sample of the martensitic PHSS in an electrically heating furnace at a temperature ranging from about 485°C to about 621°C for 3.5 – 4 Hrs; grinding the heat-treated metallographic sample using Silicon Carbide Waterproof Emery Papers of grit size ranging from 60-800; Polishing the ground metallographic sample with 1 micron diamond polishing paste; Etching the polished metallographic sample using an etchant selected from Picrol or Aqua Regia for 1-2 min at room temperature; observing the etched surface of the metallographic sample under an Inverted optical microscope to reveal the prior austenitic grain boundaries.

OBJECT OF THE INVENTION
The object of the present invention is to provide a novel technique for Revealing Prior Austenitic Grain Boundaries in Martensitic Precipitation Hardened Stainless Steels.
Another object of the invention is to disclose new etching procedure to reveal Prior Austenitic Grain boundaries in Martensitic Precipitation Hardened Stainless Steels

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly explain the embodiments of the present invention or the technical solutions in the prior art, the optical micrographs are introduced below:
Figure 1 illustrates an Optical micrographs of the prior austenite grains boundaries of MDN 17-4PH and MDN 15-5PH precipitated hardened martensitic stainless steel (a) as received (b) Post Treatment
Figure 2 illustrates an Optical micrographs of the prior austenite grains boundaries of MDN 465 and MDN 250 precipitated hardened martensitic stainless steel (a) as received (b) Post Treatment
Figure 3 illustrates an Optical micrographs of the prior austenite grains boundaries of MDN 13-8Mo and MDN 11-10PH precipitated hardened martensitic stainless steel (a) as received (b) Post Treatment

DESCRIPTION OF THE INVENTION

The present invention discloses Prior Austenitic Grain Boundaries in Martensitic Precipitation hardened martensitic steel by using suitable heat treatment and etchants for accurate grain size measurement.

The present invention relates to a method for revealing prior austenitic grain boundaries in precipitation hardened martensitic steels. The method comprises a novel heat treatment cycle followed by a suitable etchant.

The martensitic precipitation hardening stainless steels are a family of corrosion resistant alloys some of which can be heat treated to provide tensile strengths of 850MPa to 1700MPa and yield strengths of 520MPa to over 1500MPa. They are used in the Landing gears, Space and Aircraft structural materials in aerospace and military hardware components where a combination of high strength, corrosion resistance and a generally low but acceptable degree of toughness is required.

In accordance with an embodiment of the present invention, the metallographic samples of Martensitic PHSS are prepared, followed by heat treatment in an electrically heating furnace at temperatures ranging from approximately 485°C to 621°C for a specified duration. This heat treatment cycle is designed to promote the formation and visibility of Prior Austenitic Grain Boundaries.

Further, the heat-treated samples undergo a grinding process using Silicon Carbide Waterproof Emery Papers of various grit sizes to achieve a smooth surface. This is followed by polishing with 1 micron diamond polishing paste to further refine the sample surface.

The important step in the method involves etching the polished samples using specific etchants namely Picrol or Aqua Regia. Picrol, comprising picric acid, methanol, and hydrochloric acid, or Aqua Regia, a mixture of hydrochloric acid and nitric acid, are applied to the sample surface at room temperature for a predetermined duration. This etching process selectively highlights the PAGBs, making them visible under an Inverted optical microscope.

The revealed PAGBs are then examined microscopically to measure grain size using standard metallographic techniques as per ASTM E 112. This measurement is crucial for assessing the material's mechanical properties and ensuring quality control in applications where Martensitic PHSS is utilized.

Herein, MDN 17-4PH, 15-5PH, 465, 250, 13-8Mo, and 11-10PH refer to specific compositions of martensitic precipitation hardened stainless steels (Martensitic PHSS) used in the present invention. They represent different alloys within the Martensitic PHSS family, each characterized by its unique composition of alloying elements. Following is the composition of alloys used herein

MDN 17-4PH: It refers to a martensitic precipitation hardened stainless steel with the following composition (in weight percentage): Chromium (Cr) - 15.5%, Nickel (Ni) - 4.5%, Copper (Cu) - 3.8%, Manganese (Mn) - 0.6%, Silicon (Si) - 0.2%, Carbon (C) - 0.04%, and the balance being Iron (Fe). According to the present invention, it is heat treated at 621°C for 4 Hrs to reveal PAGBs.

MDN 15-5PH: It refers to a martensitic precipitation hardened stainless steel with the composition: Chromium (Cr) - 15.3%, Nickel (Ni) - 5.2%, Copper (Cu) - 3.1%, Manganese (Mn) - 0.4%, Molybdenum (Mo) - 0.04%, Silicon (Si) - 0.4%, Carbon (C) - 0.04%, and the balance being Iron (Fe). According to the present invention, MDN 17-4PH is heat treated at 621°C for 4 Hrs to reveal PAGBs.

MDN 465: It refers to a martensitic precipitation hardened stainless steel alloy characterized by its composition: Chromium (Cr) - 11.6%, Nickel (Ni) - 11.2%, Titanium (Ti) - 1.6%, Manganese (Mn) - 0.01%, Molybdenum (Mo) - 1.00%, Silicon (Si) - 0.03%, Carbon (C) - 0.003%, and the balance being Iron (Fe). According to the present invention, MDN 465 is heat treated at 510°C for 4 hrs to reveal grain boundaries.

MDN 250: It is a martensitic precipitation hardened stainless steel alloy with the composition: Nickel (Ni) - 17.7%, Titanium (Ti) - 0.45%, Molybdenum (Mo) - 4.9%, Aluminum (Al) - 0.11%, Silicon (Si) - 0.01%, Carbon (C) - 0.002%, and the balance being Iron (Fe). According to the present invention, MDN 250 is heat treated at 485°C for 3.5 hrs to reveal grain boundaries.

MDN 13-8Mo: It is a martensitic precipitation hardened stainless steel alloy with the composition: Chromium (Cr) - 12.8%, Nickel (Ni) - 8.4%, Molybdenum (Mo) - 2.0%, Aluminum (Al) - 0.95%, Silicon (Si) - 0.03%, Carbon (C) - 0.025%, and the balance being Iron (Fe). According to the present invention, MDN 13-8Mo undergoes specific heat treatment at 510°C for 4 Hrs for grain boundary revelation.

MDN 11-10PH: It is a martensitic precipitation hardened stainless steel alloy with the composition: Chromium (Cr) - 10.6%, Nickel (Ni) - 9.5%, Titanium (Ti) - 1.0%, Molybdenum (Mo) - 2.0%, Aluminium (Al) - 0.14%, Silicon (Si) - 0.02%, Carbon (C) - 0.013%, and the balance being Iron (Fe). According to the present invention, MDN 11-10PH undergoes heat treatment at 530°C for 4 hrs to reveal grain boundaries.

The present invention is further described below with reference to the accompanying drawings and specific embodiments of the specification:

Illustration 1
In an embodiment, the present invention discloses martensitic precipitation hardening stainless steel, MDN 17-4PH having composition in wt % as follows: Cr - 15.5%, Ni- 4.5%, Cu – 3.8%, Mn-0.6 Si – 0.2% C – 0.04% and balance Fe. The heat treatment is carried out in electrically heating furnace. The method includes the following:
Sample Preparation – Taking the metallographic sample and heat treating it in electrically heating furnace at 620 – 625 degree C for 4 Hr.
Grinding: grinding the metallographic sample obtained with a Silicon Carbide Waterproof Emery Paper. The emery paper is selected from 60,100, 320, 600, 800 & 100 grit sizes.
Polishing - apply 1 micron diamond polishing paste to the surface of the ground metallographic sample;
Etching – The polished or scratch free mirror surface of sample is then etched in picrol for 2 min at room temperature. Herein, the picrol comprises of picric acid, 10 gm; methanol 100 ml and HCl 5 ml.
Observing – The etched surface is further examined under Inverted optical microscope.

Example 2
In another embodiment, the present invention discloses martensitic precipitation hardening stainless steel, MDN 15-5PH having composition in wt % as follows: Cr - 15.3%, Ni- 5.2%, Cu – 3.1%, Mn-0.4 , Mo – 0.04%, Si – 0.4% C – 0.04% and balance Fe. The heat treatment is carried out in electrically heating furnace. The method includes the following:
Sample Preparation – Taking the metallographic sample and heat treating it in electrically heating furnace at 620 – 625 degree C for 4 Hr.
Grinding: grinding the metallographic sample obtained with a Silicon Carbide Waterproof Emery Paper. The emery paper is selected from 60,100, 320, 600, 800 & 100 grit sizes.
Polishing - apply 1 micron diamond polishing paste to the surface of the ground metallographic sample;
Etching – The polished or scratch free mirror surface of sample is then etched in picrol for 2 min at room temperature. Herein, the picrol comprises of picric acid, 10 gm; methanol 100 ml and HCl 5 ml.
Observing – The etched surface is further examined under Inverted optical microscope.

Example 3
In another embodiment, the present invention discloses displaying of grain boundaries in martensitic precipitation hardening stainless steel, MDN 465 having composition in wt % as follows: Cr - 11.6%, Ni-11.2%, Ti 1.6%, Mn-0.01 , Mo – 1.00%, Si – 0.03% C – 0.003% and balance Fe. The heat treatment is carried out in electrically heating furnace. The method includes the following:
Sample Preparation – Taking the metallographic sample and heat treating it in electrically heating furnace at 500 – 530 degree C for 4 Hr.
Grinding: grinding the metallographic sample obtained with a Silicon Carbide Waterproof Emery Paper. The emery paper is selected from 60,100, 320, 600, 800 & 100 grit sizes.
Polishing - apply 1 micron diamond polishing paste to the surface of the ground metallographic sample;
Etching – The polished or scratch free mirror surface of sample is then etched in Aqua Regia for 1 min at room temperature. Herein, the Aqua Regia comprises of HCl: HNO3 in 3:1.
Observing – The etched surface is further examined under Inverted optical microscope.

Example 4
In another embodiment, the present invention discloses martensitic precipitation hardening stainless steel, MDN 250 having composition in wt % as follows: Cr - 0.01%, Ni-17.7%, Cu – 0.01%, Ti - 0.45%, Mn-0.01 , Mo – 4.9%, Al – 0.11% Si – 0.01% C – 0.002% and balance Fe. The heat treatment is carried out in electrically heating furnace. The method includes the following:
Sample Preparation – Taking the metallographic sample and heat treating it in electrically heating furnace at 470 – 490 degree C for 3.5 Hr.
Grinding: grinding the metallographic sample obtained with a Silicon Carbide Waterproof Emery Paper. The emery paper is selected from 60,100, 320, 600, 800 & 100 grit sizes.
Polishing - apply 1 micron diamond polishing paste to the surface of the ground metallographic sample;
Etching – The polished or scratch free mirror surface of sample is then etched in picrol for 2 min at room temperature. Herein, the picrol comprises of picric acid, 10 gm; methanol 100 ml and HCl 5 ml.
Observing – The etched surface is further examined under Inverted optical microscope.

Example 5
In another embodiment, the present invention discloses martensitic precipitation hardening stainless steel, MDN 13-8Mo having composition in wt % as follows: Cr - 12.8%, Ni-8.4%, Mn-0.01% , Mo – 2.0%, Al – 0.95% Si – 0.03% C – 0.025% and balance Fe. The heat treatment is carried out in electrically heating furnace. The method includes the following:
Sample Preparation – Taking the metallographic sample and heat treating it in electrically heating furnace at 500 – 520 degree C for 4 Hr.
Grinding: grinding the metallographic sample obtained with a Silicon Carbide Waterproof Emery Paper. The emery paper is selected from 60,100, 320, 600, 800 & 100 grit sizes.
Polishing - apply 1 micron diamond polishing paste to the surface of the ground metallographic sample;
Etching – The polished or scratch free mirror surface of sample is then etched in Aqua Regia for 2 min at room temperature. Herein, the Aqua Regia comprises of HCl: HNO3 in 3:1.
Observing – The etched surface is further examined under Inverted optical microscope.

Example 6
In another embodiment, the present invention discloses martensitic precipitation hardening stainless steel, MDN 11-10PH having composition in wt % as follows: Cr – 10.6%, Ni-9.5%, Ni – 0.01%, Ti - 1.0 %, Mn – 0.01%. Mo – 2.0%, Al – 0.14% Si – 0.02%. C – 0.013% and balance Fe. The heat treatment is carried out in electrically heating furnace. The method includes the following:
Sample Preparation – Taking the metallographic sample and heat treating it in electrically heating furnace at 520 – 540 degree C for 4 Hr.
Grinding: grinding the metallographic sample obtained with a Silicon Carbide Waterproof Emery Paper. The emery paper is selected from 60,100, 320, 600, 800 & 100 grit sizes.
Polishing - apply 1 micron diamond polishing paste to the surface of the ground metallographic sample;
Etching – The polished or scratch free mirror surface of sample is then etched in Aqua Regia for 1 min at room temperature. Herein, the Aqua Regia comprises of HCl: HNO3 in 3:1.
Observing – The etched surface is further examined under Inverted optical microscope.

Referring to Fig 1 to 3, of the present invention showcases prior austenitic grain boundaries of martensitic precipitation hardened Stainless Steel after heat treatment. The revealed prior austenitic grain boundaries are then used to measure the grain size using standard metallographic techniques. The grain size is an important quality control parameter in achieving the desired mechanical properties in precipitation hardened martensitic steels.

ADVANTAGES AND APPLICATION
The present invention offers several advantages over existing methods for measuring grain size in martensitic precipitation hardened stainless steels. The novel heat treatment cycle and etching procedure are highly effective in revealing prior austenitic grain boundaries, which are not clearly demarcated in precipitation hardened steels. This enables accurate measurement of grain size, which is critical in achieving the desired mechanical properties. The resulting fine precipitates in solid solution offer high strength, toughness, and excellent corrosion resistance. Therefore, the present invention represents a significant advancement in the field of metallurgy and offers a valuable tool for quality control in the production of precipitation hardened martensitic steels.
,CLAIMS:CLAIMS
We claim:
1. A method for revealing prior austenitic grain boundaries in martensitic precipitation hardened stainless steel (Martensitic PHSS) comprising:
a. Heat treating a metallographic sample of the martensitic PHSS in an electrically heating furnace at a temperature ranging from about 485°C to about 621°C for 3.5 – 4 Hrs;
b. Grinding the heat-treated metallographic sample using Silicon Carbide Waterproof Emery Papers of grit size ranging from 60-800;
c. Polishing the ground metallographic sample with 1 micron diamond polishing paste;
d. Etching the polished metallographic sample using an etchant selected from Picrol or Aqua Regia for 1-2 min at room temperature;
e. Observing the etched surface of the metallographic sample under an Inverted optical microscope to reveal the prior austenitic grain boundaries.

2. The method as claimed in claim 1, wherein the martensitic PHSS is selected from the group consisting of MDN 17-4PH, MDN 15-5PH, MDN 465, MDN 250, MDN 13-8Mo, and MDN 11-10PH.
3. The method as claimed in claim 1, wherein the etchant used for etching the metallographic sample is Picrol comprising 10g picric acid, 100 ml methanol, and 5 ml hydrochloric acid.
4. The method as claimed in claim 1, wherein the etchant used for etching the metallographic sample is Aqua Regia comprising hydrochloric acid and nitric acid in a ratio of 3:1.
5. The method as claimed in claim 1, wherein the heat treatment is carried out for a duration of 3.5 to 4 hours.
6. The method as claimed in claim 1, wherein the martensitic PHSS exhibits a specific tensile strength ranging from 850 MPa to 1700 MPa and a yield strength ranging from 520 MPa to over 1500 MPa.