DESC:PRIORITY STATEMENT
The present application hereby claims priority from Indian patent application with the application number 202341024827, filed on 31 March 2023, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD OF INVENTION
The present invention in general is related to the field of metallurgy. More specifically, the present invention relates to maraging steel 250 and the method of improving fractured toughness and ultrasonic inspectability of maraging steel 250.

BACKGROUND OF INVENTION
Maraging steels are high strength carbon free iron-nickel alloys with additions of cobalt, molybdenum, titanium and aluminium. A conventional maraging steel has very high tensile strength of about 2000 MPa, it is therefore used for making various components requiring very high strength, such as rocket part, centrifugal part, aircraft part and many more.
While maraging steel has extremely high tensile strength owing to strengthening elements molybdenum and titanium in its composition, it also includes non-metallic inclusion (hereinafter, referred to as merely "inclusion") of oxides such as Al2O3 and Al2O3 or nitrides and carbonitrides such as TiN and TiCN. These inclusions in the maraging steel causes fatigue fracture.
In order to solve the problem, various proposals have been made. For example, in JP2001214212, a method of producing Ti-containing steel is disclosed, according to which a raw material of Ti-containing steel without titanium-nitride inclusions is melted in a vacuum induction furnace (VIM) and cast to produce a Ti-containing steel material as an electrode, and the material is re-melted in a vacuum arc melting (VAR) process to refine the titanium-nitride inclusions.
Moreover, since the generation of the titanium-nitride inclusions also depends on melting conditions, the problem cannot sufficiently be solved only by the management of the raw materials.
Further, actually, in the maraging steel, oxide inclusions are also confirmed in addition to titanium and nitride inclusions. The number of existing oxide inclusions is small, but the inclusions have a comparatively large size. There is a concern about that the presence of such large oxide inclusions adversely affect mechanical characteristics of the material such as the fractured toughness.
Further, the problem with existing processing technology is that as the strength of the steel is increased through modifications in alloying additions; the ductility as well as fracture toughness becomes extremely low, thereby limiting its application.
With this background, the present invention proposes an improved method which can remarkably reduce the fracture toughness of MDN 250.

OBJECT OF THE INVENTION
The primary object of the present invention is to reduce the size of residual non-metallic inclusions viz. oxides and nitrides in Maraging Steel 250.
Another objective of the present invention is to disclose an improved method of manufacturing Maraging Steel 250 with fractured toughness of approx. 100.
Another objective of the present invention is to produce Maraging Steel 250 with improved ultrasonic inspectability.

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 method of producing Maraging Steel Alloy, MDN 250 with improved fractured toughness of 90 - 100 MPavm, the method comprising the steps of melting a consumable electrode in a Vacuum Induction Melting (VIM) furnace; monitoring the liquid metal composition during said melting in the VIM furnace using spectrographic methods to ensure precise control to desired levels; subjecting the ingot obtained from Vacuum Induction Melting furnace to Vacuum Arc Remelting (VAR) for refinement, thereby producing a refined ingot; re-melting the said refined ingot obtained through a second Vacuum Arc Remelting (VAR) process, thereby producing a doubly refined ingot; subjecting the doubly refined ingot obtained to Heat Treatment and Thermal Mechanical Processing; and yielding Maraging Steel Alloy, MDN 250 of desired fractured toughness.

DETAILED DESCRIPTION OF THE INVENTION
The present invention is related to method of producing Maraging Steel Alloy, MDN 250 having fractured toughness of 90 - 100 MPavm. The maraging steel composition includes Nickel (Ni) – 18%, Cobalt (Co) -15%, Molybdenum (Mo) - 6.5%, Titanium (Ti) -1%, Aluminium (Al) and balance being Iron (Fe) with some incidental impurities of C, N and O (composition is in weight percent with an error margin of 10%).
Maraging Steel 250 become increasingly notch sensitive at ultra-high strength levels and hence it is very important that the MDN 250 should possess very high cleanliness coupled with very low levels of carbon and other trace impurities. For this purpose, steel is processed through double vacuum arc melting.
In accordance with an embodiment of the present invention, the consumable electrode for Vacuum Arc Remelting is produced in Vacuum Induction Melting using virgin raw materials. Spectrographic methods were utilized to monitor the analysis of the liquid metal composition during primary melting in VIM, thereby enabling accurate control to desired levels. The ingot is subjected to Vacuum Arc Remelting for refinement. It has been observed that there are still trace of oxides and other impurities which affect the fracture toughness of Maraging Steel 250.
To achieve the objective of high fracture toughness and improved ultrasonic inspectability, MIDHANI conducted extensive trials. After continuous efforts, it was observed that Maraging Steel 250 showcased significant improvement in fracture toughness which were processed using novel methodology of using the electrode produced from Vacuum Arc Remelting to again Vacuum Arc Remelting route. This novel methodology of double vacuum arc remelting was subsequently adopted in the production followed by Heat Treatment and Thermal Mechanical Processing. This method significantly improves the fractured toughness and ultrasonic inspectability of Maraging Steel 250.
In accordance with an embodiment, the maraging steel of the present invention can be produced from a large steel ingot and has excellent fatigue strength. The MDN 250 with high fractured toughness achieved using this methodology allows it to be used in critical engineering applications such as structural materials for spacecraft and aircraft.
ADVANTAGES AND APPLICATION
The present invention offers an advantage of improving the fractured toughness of Maraging Steel significantly as a High Fracture toughness of 90 - 100MPavm is achieved using this methodology. The disclosed methodology improves the ultrasonic inspectability of Maraging Steel 250 which further helps in Quality Control, material characterization and detection of flaws i.e. internal defects thereby ensuring the quality, reliability, and safety of metal components made. MDN 250 obtained is optimal in applications where the thickness of the product is thin.
,CLAIMS:Claims
We claim
1. A method of producing Maraging Steel Alloy, MDN 250 characterized by a fractured toughness ranging from 90 to 100 MPavm, the method comprising the steps of
a. Melting a consumable electrode in a Vacuum Induction Melting (VIM) furnace;
b. Monitoring the liquid metal composition during said melting in the VIM furnace using spectrographic methods to ensure precise control to desired levels;
c. Subjecting the ingot obtained from Vacuum Induction Melting furnace to Vacuum Arc Remelting (VAR) for refinement, thereby producing a refined ingot;
d. Re-melting the said refined ingot obtained through a second Vacuum Arc Remelting (VAR) process, thereby producing a doubly refined ingot;
e. Subjecting the doubly refined ingot obtained to Heat Treatment and Thermal Mechanical Processing; and
f. Yielding Maraging Steel Alloy, MDN 250 of desired fractured toughness.

2. A method of producing Maraging Steel Alloy, MDN 250, as claimed in claim 1, wherein the said consumable electrode comprises Nickel (Ni) in an amount of about 18%, Cobalt (Co) in an amount of about 15%, Molybdenum (Mo) in an amount of about 6.5%, Titanium (Ti) in an amount of about 1%, Aluminium (Al), and the balance being Iron (Fe) with incidental impurities of Carbon (C), Nitrogen (N), and Oxygen (O), wherein the composition is in weight percent with an error margin of 10%;
3. A method of producing Maraging Steel Alloy, MDN 250, as claimed in claim 1, wherein the produced MDN 250 displays improved ultrasonic inspectability.
4. A method of producing Maraging Steel Alloy, MDN 250, as claimed in claim 1, wherein the produced MDN 250 is suitable for critical engineering applications such as structural materials for spacecraft and aircraft.