FIELD OF THE INVENTION

This invention relates generally to the field of metallurgy. More particularly the invention relates to manufacture of large sized intricate forgings made up of alloy material under controlled conditions of mixing of constituent elements. More specifically, the invention relates to the manufacture of forgings of 9CrlMo grade alloy material.

BACKGROUND OF THE INVENTION

During the process of making of large sized forgings of alloys with intricate geometry and shape which find their application in the power industry, there needs to be control exercised in two aspects while carrying out the operation viz.,

a) Controlling of sulphur content in the alloy within a very narrow range, and

b) Controlling of mixing of aluminium content in appropriate quantities in the alloy for ensuring proper Nitrogen / Aluminium ratio for improved properties.

Moreover, there arises a problem during solidification of Ferritic Martensitic steels, which is the low solubility of nitrogen occurring at high temperatures in which a ferritic phase of the steel known as delta ferrite is formed. This results in the formation of gas holes in the remelt Ingots obtained, which is an undesirable occurrence affecting the mechanical properties of the forgings.

Fig 1 shows the Iron - Carbon Equilibrium diagram. The formation of the ferritic phase of steel known as 'Delta ferrite' is shown in shaded black portion which occurs at elevated temperatures. The content of nitrogen is more in this phase due to poor solubility of the gas at high temperature and this is responsible for formation of gas holes in the forged components. Hence the remelting of the forgings is done using vacuum arc remelting method and the nitrogen gas levels are controlled throughout the process for avoiding gas holes formation.

SUMMARY OF THE INVENTION

In one aspect the invention provides a method for obtaining forgings using the alloy of Grade 9CrlMo, with larger dimensions with intricate geometry which finds their application in the power generation with the forged components possessing the desired properties.

In another aspect the invention provides a method for achieving forgings of larger dimensions with appropriate nitrogen and aluminum ranges to impart improved properties to the forged components.

In another aspect the invention provides a method for obtaining forgings of larger dimensions in which the control over sulphur content is maintained at a very narrow range,

In another aspect the invention provides a method for obtaining large dimension forgings in which the levels of nitrogen is controlled throughout the operation to avoid formation of gas holes,

Accordingly the invention provides a method of making forgings with intricate geometry and shape using an alloy with molybdenum and Chromium as major constituents, the method comprising of the following steps

- upsetting of the ingots of the alloy to the required outer diameter of the forged component converting one half of the obtained upset into round forging in form of a hemisphere with the aid of punches and mandrels designed for the purpose

- forging of the top end which is the remaining half of the upset of the forged component to form sprouts with the help of dies designed for the purpose, roughly achieving the shape of a net

- shaping of the outer dish on a die with the help of mandrels designed for the purpose

- post forging treatments and softening of the forging.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure-1 illustrates the Iron-Carbon equilibrium diagram depicting the formation of various phases at different temperatures with their respective carbon contents.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

According to an embodiment of the invention, the content of sulphur and aluminium in the alloy material were carefully controlled during the process of primary melting. The following method describes the method of control of Aluminium levels during the process of formation of the alloy.

According to an embodiment of the invention Aluminium additions are made in the form of bars or shots. Since aluminium is a deoxidizer the extent to which it can be retained in steel depends upon the oxygen level in the molten steel. Instead of carrying out online measurement of oxygen levels, based on previous experience of making steel aluminium additions are done. After checking the online chemical analysis, if required, adjustments are done. But in this particular steel grade to maintain lower level of aluminium no additions are required and deoxidation of bath is carried out using other deoxidizers like ferro- silicon.

The following method describes the procedure adopted for controlling of Sulphur levels.

According to an embodiment the Sulphur levels are controlled by maintaining good reducing slag during primary melting. Online analysis of Sulphur is carried out and if required, the slag condition is improved by adding Ferro silicon, graphite and lime powder mixture, and after the desired levels of Sulphur content is achieved the molten alloy is tapped to be used for the required application.

The following table shows the content of sulphur and aluminium in the alloy material for achieving the desired properties in the forgings derived out of the alloy material,

Further, the ingots used for manufacturing the large forgings were remelted through vacuum arc remelting in a controlled atmosphere so that nitrogen levels are controlled throughout the operation to ensure that the formation of gas holes are avoided. During this operation, the drop in levels of nitrogen under particular levels of pressure obtained from empirical data helps in maintaining the nitrogen levels at appropriate levels.

The ingots of the basic alloy material with Molybdenum and Chromium as major constituents were first upset to achieve the forgings with the desired outer diameter values. Then, they were subjected to punching action using different punches and also with the aid of mandrels of varied dimensions for reducing the cross sectional dimensions to form the inner dish of the forged component and converting one half of the achieved upset in the form of a hemisphere. This was followed by carrying out the action of forging of the top end of the forged component which was earlier upset resulting in the formation of sprouts and roughly achieving the net shape. Then, the shaping of the outer dish of the forged component was carried out on a die for which mandrels designed for the specific work were used- Finally the forged component was softened to achieve the end product i.e., large forgings with intricate dimensions and shapes with desired properties.

TERMINOLOGY/ CHEMICAL FORMULAE INVOLVED:-

Cr--> Chromium

Mo -> Molybdenum

CLAIMS

What is claimed is:

1. A method of making forgings with intricate geometry and shape using an alloy with molybdenum and Chromium as major constituents, the method comprising of the following steps

- upsetting of the ingots of the alloy to the required outer diameter of the forged component

- converting one half of the obtained upset into round forging in form of a hemisphere with the aid of punches and mandrels designed for the purpose

- forging of the top end which is the remaining half of the upset of the forged component to form sprouts with the help of dies designed for the purpose, roughly achieving the shape of a net

- shaping of the outer dish on a die with the help of mandrels designed for the purpose

- post forging treatments and softening of the forging.

2. The method of making forgings as claimed in claim 1, wherein the content of sulphur in the alloy is maintained in the range of 0.005 % to 0.010 %

3. The method of making forgings as claimed in claim 1, wherein the content of aluminium in the alloy is maintained less than 0.010 %

4. The method of making forgings as claimed in claim I, wherein the levels of nitrogen is controlled throughout the operation to avoid formation of gas holes by remelting of the ingots through vacuum arc remelting in a controlled atmosphere.

5. The method of making forgings as claimed in claim 1, is carried out through a controlled thermo mechanical treatment to form the components with the required mechanical and physical properties.