TITLE OF THE INVENTION

COMMERCIAL SCALE DEVELOPMENT OF AN ULTRA HIGH STRENGTH- HIGH FRACTURE TOUGHNESS MARAGING STEEL USING THERMO MECHANICAL TREATMENTS

FIELD OF INVENTION:

The present invention in general is related to the field of metallurgy. More specifically, the present invention belongs to maraging steel and the method of making maraging steel having high strength, toughness and ductility.

The present invention can be used for all the applications where strength, toughness and also ductility are desirable.

BACKGROUND OF THE INVENTION:

Maraging steels are high strength steels generally carbon free iron-nickel alloys with additions of cobalt, molybdenum, titanium and aluminium. The term maraging is derived from the strengthening mechanism, which refers to age hardening or aging by precipitation of intermetallic phase in martensite matrix.

Maraging steels are characterized by a combination of ultra high strength coupled with excellent fracture toughness. These steels are strengthened by inter-metallic precipitates derived essentially from addition of Molybdenum and Titanium. Addition of Cobalt in these steels not only enhances the strength of the steel by synergistic effect but also controls the transformation temperature of the steel. However, as the strength of the steel is increased through modifications in alloying additions, the toughness of the steel also decreases monotonically.

The strength-toughness combination in the steel is extremely structure sensitive and is limited by the composition and the microstructure. Hence suitable modifications to processing schedule needs to be incorporated so that the strength as well as the toughness can be substantially enhanced

The problem with conventional 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. Another issue is, as the level of alloying elements increased the steels have significant amount of retained austenite along with martensite at room temperature after solution treatment.

OBJECT OF THE INVENTION:

The object of the present invention is to produce maraging steel in a commercial scale with enhanced mechanical properties which exhibit high strength, high fracture toughness and high ductility, and can be used for various applications, by transforming the remaining austenite to martensite using thermal or thermo-mechanical treatments.

SUMMARY OF THE INVENTION

The present invention is related to method of enhancing the mechanical properties of a maraging steel having composition - Nickel (Ni) - 18%, Cobalt(Co) -15%, Molybdenum (Mo) -6.5%, Titanium (Ti) -1% and balance being Iron (Fe) with some incidental impurities (composition is in weight percent with an error margin of 10%), using thermal or thermo-mechanical treatments. The maraging steel of this composition generally contain some amount of metastable retained austenite phase at room temperature which needs to be transformed to stable martensite in order to realize superior mechanical properties. The following methods are used depending on the mill forms of the steel.

a) In case of hot worked steel products, a refrigeration treatment or cryogenic treatment in which the steel is cooled to very low temperatures around -80°C to -180°C using liquid nitrogen in order to transform the retained austenite to martensite. This treatment is essentially given to hot forged/hot rolled steels.

b) In case of cold rolled steel products, cold working in steps of 10-15% is applied after annealing, which is the driving force for transforming the metastable austenite to martensite.

Both the methods are employed in a commercial scale to achieve ultra high strength maraging steel.

This invention provides a comprehensive solution to achieve superior mechanical properties in the above mentioned steel, processed to different mill forms (hot forged/rolled or cold rolled steel). These thermo mechanical treatments can also be applied to steels having metastable austenite as one of the phases in the micro structure at room temperature.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: Flowchart illustrating the process of making high strength maraging steel with high fracture toughness and ductility using method 1.

Figure 2: Flowchart illustrating the process of making high strength maraging steel with high fracture toughness and ductility using method 2.

DETAILED DESCRIPTION OF THE INVENTION

The following description with reference to the accompanying flowchart is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely^exemplary.

The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and. consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

It is to be understood that the singular forms "a," "an," and ''the" include plural referents unless the context clearly dictates otherwise.

Features that are described and/or illustrated with respect to one embodiment may be used in the sarnie way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. The present invention will be more clearly described with reference to the flowchart showing embodiments thereof.

The present invention is related to method of enhancing the properties of a.maraging steel of composition - Ni-18%, Co-15%, Mo-6.5%, Ti-1% and balance being Iron (Fe) with some incidental impurities (composition of the maraging steel is in weight percent with an error margin of 10%), using suitable and novel thermo-mechanical treatments. These maraging steels have very high nickel, cobalt and molybdenum and very low carbon content. The microstructure mainly consists of highly alloyed low carbon martensites with some amount of retained austenite phase depending upon the composition of the steel. Steel belonging to this class of 18%-Ni has high strength and they differ from the other conventional steels. The yield strengths of these steels are in the range of l-500MPa to 2500 MPa.

The strength in maraging steel is essentially derived from additions of Mo, Ti which give rise to precipitation of inter-metallic phases during aging. Addition of Co, enhances this strengthening effect as well as controls the transformation temperatures. However with increasing additions the increase in strength is at the cost of decrease in ductility and toughness thereby limiting the application of such steels. Increasing alloying additions also increases retained austenite levels in the steel (in spite of increased cobalt additions in the alloy).

The present invention addresses this issue comprehensively by evolving suitable thermal/ thermo mechanical treatments which can be given to hot worked (forged/rolled) as well as cold rolled products. The two methods developed in this invention aims at complete transformation of the retained austenite to martensite through a cryogenic treatment or thermo mechanical treatment so as to enhance the mechanical properties of the steel. In this context, method-2 is more effective in enhancing the properties as it utilizes the TRIP phenomena to enhance strength as well as fracture toughness and ductility. It may be mentioned that such high levels of ductility and toughness at ultra high strength levels have not been reported in a commercially processed material.

In the present invention, suitable thermal or thermo mechanical treatments have been incorporated in the processing schedule with the view to obtain significant enhancement in the strength, ductility as well as fracture toughness in the product. In this context, the following methods have been applied.

Method 1:

In this method, the hot worked steel products are subjected to the solution treatment. In this treatment the forged bars of maraging steel are treated to a temperature of 850°C to 870°C and then air cooled to room temperature. The structure of the steel in this stage comprises of martensite which is around 90% and retained austenite is around 10%. The temperature of the bars allowed to comedown to room temperature by air cooling.

To make it completely martensitic cryogenic cooling, temperature around -80 to -180°C is applied which brings down the temperature of steel products rapidly. For this purpose, they are immersed in a tank containing liquid Nitrogen and kept inside the tank for 4-5 hours to ensure transformation of retained austenite to martensite.

Then, the steel bars are taken out of the tank and left in air to allow it come to room temperature. Subsequently, the bars are again reheated to around 500°C for 4-5 hours and then they are air cooled to room temperature. This treatment brings about strengthening of maraging steel by precipitation hardening process.

Method 2:

Method 2 is applicable to the cold rolled products. These cold rolled products are earlier hot rolled products which were subjected to cold rolling to become cold rolled products. At first, the cold rolled products are annealed at 850-870°C, for .1 to 2 in order to relieve all the internal stresses and make the material soft. The annealed product comprises of martensite which is around 90% and retained austenite is around 10% at room temperature.

The annealed plates are then subjected to pickling in order to remove the oxide scales on the surface which are produced during hot working. The cold rolled, annealed and pickled steel is subjected to cold working at room temperature. This cold working is applied in the manner of steps of about 10-15%, which progressively reduces the thickness of the plate.

The processed steel is subjected to aging treatment where they are reheated to temperature around 500°C for 4-5 hours and then air cooled. This treatment is given to strengthen the steel by precipitation hardening.

The transformation of retained austenite (RA) to martensite in both the processes is confirmed by X-ray diffraction measurements. The cryogenic treated steel as well as cold rolled steel after the regular aging treatment exhibited the properties as indicated in table below.

Table 1: Mechanical properties exhibited by the steel, which were subjected to different types of treatments

The processing methods for hot worked as well as cold rolled products have been adequately addressed by incorporating a suitable cryogenic treatment or a thermo mechanical treatment so as to achieve completeness in phase transformation during heat treatment, which results in significant enhancement (over 100% improvement in ductility and around 25% in fracture toughness) in the mechanical properties as indicated in Table above.

Another problem is that, steels become increasingly notch sensitive at ultra high strength levels and hence it is very important that the steel should possess very high cleanliness coupled with very low levels of carbon, sulphur, phosphorous and other trace impurities. This problem is addressed to limited extent by using very high purity raw materials during melting stage so as to control the cleanliness and impurity levels and also to control the hot working schedule. For this purpose, steel is processed through double vacuum .melting (Vacuum Induced Melting (VIM) + Vacuum Arc Remelting (VAR)), using virgin raw materials. Melts of 2500 Kg charge weight was processed in VIM and remelted in 400 mm diameter VAR crucible. The composition of liquid metal during primary melting in VIM was accurately controlled to desired levels by monitoring the analysis using spectrographic methods. Double vacuum melting ensures steel with extremely low impurities and high degree of cleanliness which is very important while developing steels with ultra high strength levels, having high fracture toughness.

Extensive trials were carried on the large scale commercial melts of 2500 Kg, processed through VIM+VAR route and both of the methods were applied later to obtain the objective of our invention. Encouraging results were obtained which made enhancements in the properties of the steel, which are cited in the table 1. The present invention provides steel having a very high level of yield strength in the range 2475-2525 MPa coupled with fracture toughness in the range 26-30 MPaVm. The cryogenic treatment as well as thermo mechanical processing treatment ensures completeness of phase transformation and consistent levels of mechanical properties in hot as well as cold rolled products processed from commercial scale melts. With such attractive combination of strength and toughness the steel is considered to be a promising engineering material for critical applications.

The invention addresses the issue of achieving ultra high strength levels with simultaneous enhancement of toughness and ductility which makes the steel a promising material for critical engineering applications.

It can be used for any engineering applications and many fields like aerospace, military, marine, and nuclear power fields, where ultrahigh strength coupled with very high toughness is desirable. The steel with suitable surface treatments such as nitriding, can be used for various tooling applications such as mandrels in extrusion press.

CLAIMS:

We claim,

1. A method for making ultra high strength and high fracture toughness maraging
steel under controlled thermal/thermo mechanical conditions, where in the said
method comprising of following steps:

a. treatment of hot worked steel products;
b. treatment of cold rolled steel products;

2. A method as claimed in claim 1, where in the treatment of hot worked steel
products further comprises the following steps:

a. Solution treatment at 850-870°C, for 1 to 2 hours, followed by cooling in
air till room temperature; and
b. Cryogenic treatment between -80 to -180°C for at least 4 hours, followed
by cooling in air till room temperature; and
c. Aging treatment at 500°C for at least 4 hours, preferably 4 to 5 hours and
followed by cooling.in air till room temperature;

3. A method as claimed in claim 1, where in the procedure of treating cold rolled
steel products comprises the following steps:

a. Annealing operation at 850-870°C, for at least 1 to 2 hours; and
b. Pickling operation and followed by cooling in air till room temperature;
and
c. Cold working operation in the steps of 10 to 15% steps; and
d. Aging treatment at 500°C for at least 4 hours, preferably 4 to 5 hours and
followed by cooling in air till room temperature;

4. A method according to claim 1, wherein the maraging steel has a composition in
weight percent with an error composition of 10%:
18% Nickel; 15% Cobalt; 6.5% Molybdenum; 1% Titanium;
Iron and incidental impurities;

5. A method according to claim 2, wherein cryogenic treatment involves, the application of liquid nitrogen.

6. A method according to claim 3, wherein pickling treatment involves of the application of sulphuric acid.

7. A method according to claim 1, wherein the maraging steel is processed through double vacuum melting, comprising of following steps:

a. primary melting of virgin raw materials using vacuum induction melting;
and
b. remelting using vacuum arc remelting;