TITLE OF THE INVENTION

Modified Heat Treatment Practice for Enhanced Strength and Toughness in a Martensitic Stainless Steel

FIELD OF INVENTION:

The present invention in general is related to the field of Metallurgy. More specifically, the present invention belongs to martensitic stainless steel and the method of heat treatment to obtain more strength and toughness.

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

BACKGROUND OF THE INVENTION:

High strength and high toughness materials have a growing demand and have wide variety of applications especially in Defence, Aeronautical and other high temperature & pressure applications. Hence, there is a need to improve both the strength and the toughness of steel. There are various practices for improving the said characteristics. They are solid solution strengthening, precipitation strengthening, grain size strengthening, dislocation strengthening etc. Grain size refinement is one of the best-established strengthening methods in steels, as well as in other metals. Strength of the materials improves by altering the grain size, as yield strength is dependent on the reciprocal of square root of the grain size. Where as the drawback with methods in the prior art, is that they provide either enhancement in strength or toughness but not both.

The technical problem to be solved is to overcome the deficiencies of the prior art, to enhance both the strength and toughness significantly in martensitic stainless steels and also in all kinds of said steel products including not only flat products but also heavy or thick section forgings (complex profiled forgings)

OBJECT OF THE INVENTION:

The object of the present invention is to produce martensitic stainless steel with enhanced mechanical properties which exhibit high strength as well as high toughness using modified heat treatment or thermo-mechanical treatment by refining the sub-structure.

The other objective is obtain the same result i.e., enhanced properties of said steel with high strength as well as high toughness in all range of products including flat and complex profiled forging.

SUMMARY OF THE INVENTION

The present invention is related to method of enhancing the mechanical properties of a martensitic stainless steel having composition - 0.07% < C < 0.13% - traces < Si < 0.5 % - traces < Mn < 0.8% - 15.3% < Cr < 16.8% - 3.5% < Ni < 4.5% - traces < Nb < 0.25% -traces < S < 0.015% - traces < P < 0.020% - traces < Cu < 0.3%, the balance consisting of iron and impurities resulting from the production.

The material is subjected to high temperature austenitization treatment to dissolve all the carbides and it is followed by rapid cooling to room temperature in order to retain the solutes in solution.

After that tempering treatment is applied and this precipitates the carbides uniformly in the matrix. Since the microstructure after the first phase high temperature austenitization treatment is very coarse, a series of thermal cycling treatment are subsequently given in the next phase of heat treatment schedule with the objective of refining the grain structure.

This heat treatment is particularly suitable to martensitic / precipitation hardening stainless steels containing intentional additions of grain refining elements such as Niobium, Vanadium or Titanium. Therefore the heat treatment schedule requires slight modification depending on the type of grain refiners added into the material. The present invention is adopted here for an alloy where the grain refiner is niobium.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: Flowchart illustrating the process of making high strength and high toughness martensitic steel.

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 same 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 mechanical properties of a stainless steel having composition -

Carbon (C) 0.05%-0.15%

Silicon (Si) traces - 0.5 %

Manganese (Mn) traces - 0.8%

Chromium (Cr) 15.0% -18.0%

Nickel (Ni) 3.0%-5.0%

Niobium (Nb) traces - 0.25%

Sulphur (S) traces - 0.015%

Phosphorus (P) traces - 0.020%

Copper (Cu) traces - 0.3%,

the balance consisting of iron and inevitable impurities resulting from the production. Niobium (Nb) is grain refining element in the above composition of stainless steel. The steel becomes high strength and high toughness martensitic steel when the stainless steel of above composition is subjected to the modified heat treatment process described in the present invention.

Selection of raw materials and the processing of the raw materials are the foremost steps in the process of obtaining stainless steel of said composition. In order to maintain the desired quality of steel, a rigorous quality assurance program is adopted from raw material stage to finished products. Hence the usage of 100% virgin raw materials is necessary to obtain desired properties. The raw materials chosen are as follows:

a. Low Carbon Ferro Chrome / Chromium Metal

b. Ferro Nickel / Nickel Pellets

c. Ferro Niobium / Niobium Metal

d. Ferro Manganese

e. Ferro Silicon

In the initial stage, compatible steel scrap and the chosen virgin raw materials are processed through Induction Melting route. The composition of molten steel is monitored continuously during melting process by drawing the melt samples from the molten metal and analyzing using spectrographs / chemical methods to ensure the desired quality.

Further to ensure low level of gas content, remelting route is applied to yield a chemical composition within the specified limits. The molten steel is subjected to Electro Slag Remelting (ESR) to enhance the quality of the primary melted material further. Preferential removal of. sulphur and oxygen during electro slag remelting also aids retention of nitrogen (about 250 ppm) uniformly through out the solidified material. The result is a stainless steel of said composition with enhanced homogeneity and soundness. The objective of the present invention is to enhance the strength and toughness further of the steel with said composition using suitable modification in heat treatment.

The present invention involves modified heat treatment schedule suitably in order to obtain dual benefit of precipitation strengthening and grain / sub structure refinement using suitable heat treatment cycle so that strength as well as toughness can be significantly enhanced in the product. In general, the heat treatment cycle involves hardening and tempering which can control either strength or toughness depending upon the tempering temperature but not both.

The modified heat treatment in the present invention involves two phases, also described in the flow chart. In the first phase, high temperature austenitization treatment is performed on the steel to dissolve all the carbides forming solutes. It is followed by rapid cooling to room temperature to retain the solutes in solution. Now, the cooled material is subjected to tempering treatment which precipitates the carbides uniformly in the matrix.

The microstructure after the first phase high temperature austenitization treatment is very coarse. Hence in the second phase of heat treatment schedule, a series of thermal cycling treatment are subsequently given with the objective of refining the grain structure. The series of thermal cycling treatment comprises of hardening / solution treatment followed by tempering / aging treatment, where both the solution as well as tempering (aging) temperatures are progressively lowered with the objective of refining the structure.

In first step, solution is performed to a temperature of 1150°C/AC and followed by aging treatment to 610°C/AC forms the first phase. In the second phase, the first step is heat treatment is performed to a temperature of 1040°C/AC and followed by aging treatment to 610°C/AC. Finally in the third one, solution treatment is performed to a temperature of 975°C/AC and followed by aging treatment to 320°C/AC. A fine grained microstructure with uniform distribution of precipitates is obtained with the combined effect of the heat treatment schedules in the first and second phase.

The present invention is adopted here for an alloy where the grain refiner is Niobium. Niobium is added to large number of steels that find use in high temperature application. Niobium forms carbides and nitrides of MX type that are fine and is distributed in the matrix. These type of precipitates are required in steels that find application in power sector as they enhance the creep properties of the material and thereby the life of alloy.

This heat treatment is particularly suitable for martensitic £ precipitation hardening stainless steels containing intentional additions of grain refining elements such as niobium, vanadium or titanium. It can be used for any alloy which is alloyed with carbides and nitrides formers such as niobium, vanadium or titanium. The heat treatment schedule designed requires slight modification depending on the type of grain refiners. The process annealing is carried out during intermediate stages of forging to remove surface defects whenever required or on finished hot worked material to remove transformational stresses for ease of machining. The modified heat treatment process in the present invention also eliminates the requirement of sub zero or cryo treatment.

Typical mechanical properties of the obtained martensitic steel after modified heat treatment practice.

Mechanical Properties of the modified heat treated Martensitic Steel 0.2% P. S., (Yield strength or Proof Strength), I 945 - 980 MPa (Ultimate tensile strength) UTS, 1190- 1300 MPa Impact toughness, (Charpy U-Notch) 180 - 260 Joules Grain size, ASTM No. ' 7&8

The resulted final product is a fine grained tempered martensite structure with a well distributed second phase (niobium carbides) precipitates as shown below.

Fig. 2: Fine grained tempered martensite structure with a well distributed second phase (niobium carbides) precipitates.

Thus, the method employed in the present invention enhanced both the strength and toughness significantly in martensitic stainless steels and also this method is applicable for all kinds of said steel products including not only flat products but also heavy or thick section forgings (complex profiled forgings).

CLAIMS:

We claim,

1. A high strength and high toughness martensitic stainless steel with composition, essentially consisting of, in percentage by weight -

Carbon (C) 0.05% - 0.15%

Silicon (Si) traces - 0.5 %

Manganese (Mn) traces - 0.8%

Chromium (Cr) 15.0% -18.0%

Nickel (Ni) 3.0% - 5.0%

Niobium (Nb) traces - 0.25%

Sulphur (S) traces - 0.015%

Phosphorus (P) traces - 0.020%

Copper (Cu) traces - 0.3%,

the balance consisting of iron and inevitable impurities resulting from the production, where in Niobium (Nb) is grain refining element.

2. A method of making high strength and high toughness martensitic stainless steel as claimed in claim 1, where in the said method comprising of following steps:

i. High temperature austenization treatment of the stainless steel of said composition, followed by rapid cooling to room temperature and tempering treatment; First, heat treatment is performed to a temperature of 1150°C/AC and followed by aging treatment to a temperature of 610°C/AC; and

ii. A series of thermal cycling treatment comprising of following steps:

a. Second, heat treatment is performed to a temperature of 1040°C/AC and followed by aging treatment to a temperature of 610°C/AC; and

b. Third, heat treatment is performed to a temperature of 975°C/AC and followed by aging treatment to a temperature of 320°C/AC;

3. A high strength and high toughness martensitic stainless steel with composition as claimed in claim 1, where in the grain refining element is at least one of the element from Niobium, Vanadium or Titanium.

4. A high strength and high toughness martensitic steel with composition as claimed in claim 1, where in the said martensitic stainless steel is having 0.2% P. S.? (Proof Strength) from 945 to 1050 MPa, Ultimate tensile strength from 1190 to 1300 MPa, Impact Strength from 180 to 260 CUN and Grain size with ASTM No. 6 or finer.