FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patent Rules, 2003
Complete Specification
(See section 10 and rule 13)
A Method Of Stress Relieving For Hot Forging Dies For Use In Repairing
Cracked Dies
Bharat Forge Limited
An Indian company registered under the Indian Companies Act, 1956.
Mundhwa, Pune Cantonment, Pune – 411036, Maharashtra, India
The following specification particularly describes the invention and the manner in
which it is to be performed.
2
Field of invention
This invention relates to the hot forging die (the Die) manufacturing process.
More specifically, this invention relates to the weld repair process of the dies.
Background 5 of invention
Hot forging operation is divided broadly in two categories. These categories are
Open Die Forging (ODF) and Closed Die Forging (CDF). In closed die forging
operation, hot metals in plastic state are given required complex shape using tools
made of alloyed steels. These tools are called hot forging dies (hereafter “dies”).
10 These dies have to face extreme working conditions during their use. The different
factors acting simultaneously on these dies are high temperature of the material
being forged and relative motion between the material being forged and the die
surface. Further, as the deformation process is carried out in a press by closing of
the dies (attached to the press) with high velocity, with the material in between
15 them, it leads to impact load on both dies and material being forged. These factors
together lead to thermal as well as impact fatigue of the dies. This fatigue finally
causes cracking of the dies. Cracking is considered as failure of the dies and
hence, the dies have to be taken out of production and have to be repaired.
20 A cracked die is repaired by a process of gouging, followed by welding and
thermal stress relieving of dies, and machining (Refer Figure 1). In the first step,
the cracked part of the die is gouged to completely remove the portion, containing
3
the crack, from the die. This gouged area is filled with welding material. This
welded die is subjected to stress relieving and then machined at required location.
This completes the die repair cycle and the die is ready for production.
The welding operation is further divided in two parts. The 5 first is the actual
deposition of the weld material in gouged section of the die. The welding process
is a high temperature process which leads to high temperature gradients in the die
material. The geometry of the die sections becomes non-uniform due to the
presence of die impressions, which leads to non-uniform heating as well as
10 cooling of the die material. The temperature gradient caused by the welding
process along with subsequent non-uniform cooling (caused by the presence of die
impression), leads to formation of residual stresses. These residual stresses, if
present during the machining operation, will lead to tool breakages and
dimensional instability. Further, these residual stresses can also lead to premature
15 failure of the dies during their operation. To avoid this, as a second step, Stress
Relieving of the welded dies is done. This operation relieves all the residual
stresses present in the dies and makes it ready for the next operation of machining.
The stress relieving of the hot forging dies is traditionally done using Thermal
20 Stress Relieving process (TSR) (Refer Figure 1). In TSR, the die is heated to a
high temperature (which is below its tempering temperature) and kept at that
temperature for a predefined period of time (which is dependent on the cross
4
section of the die). The TSR temperature is below the transformation temperature
(temperature above which the microstructure changes to austenite); hence, there is
no change in the metallurgical properties or mechanical properties of the die.
However, at high temperature the yield strength of the die material is reduced.
Consequently, under the combined influence of the residual 5 stresses and the
reduced yield strength at that temperature, localized yielding of the material takes
place which ultimately leads to reduction of residual stresses. It should be noted
that residual stresses are always lower than the room temperature yield strength of
a material. If the residual stresses are not higher than the reduced yield strength
10 (i.e. where yielding is not possible), in such cases the combination of high
temperature and residual stresses result in creep mechanisms which leads to
reduction of residual stresses. In this manner, these two mechanisms reduce the
residual stresses to a minimum. But, there are certain drawbacks associated with
the TSR process as given below:
15 1. Use of high temperatures during the TSR process leads to oxidation and
corrosion of the surface of the dies.
2. TSR process is a very time consuming process and has a very large cycle
time.
3. TSR process is also a very energy intensive process as all the dies have to
20 be heated and maintained at that temperature for long period of time. This
is normally done using oil fired furnaces.
5
4. Use of oil fired furnace leads to pollution and hence, the carbon footprint
of the industry is increased due to TSR.
5. As the TSR process is a high temperature process, sometimes distortion of
the dies also takes place.
5
This invention proposes an alternative method of stress relieving of the hot
forging dies. The invention recommends use of pulsation of dies instead of the
TSR method for the stress relieving of the dies (Refer Figure 2).
10 Objects of Invention:
Accordingly the object of the invention is to provide a method for stress relieving
of hot forging dies during their repair cycle using pulsations or vibrations.
Another object of the invention is to provide a method for stress relief that is cost15
effective and time-efficient.
A further object of the invention is to provide a stress relief method that avoids
deployment of large furnaces and reduces the overall carbon foot print of the
plant.
20
Yet another object of the invention is to provide an apparatus to carry out stress
relief of the hot forging dies.
6
Brief description of drawings
Figure 1 shows the flow chart of the conventional die repair process.
Figure 2 shows the flow chart of modified die repair process.
Figure 3 shows the schematic of arrangement for a pulsation device that uses an
electric shaker used in the stress relieving process 5 of the invention.
Figure 4 shows the relation between the weight of the dies and the treatment time
for stress relieving.
Summary of invention
10 The cost of tooling’s (forging dies) and that of their repair is part of the overall
cost of the forgings being made. Reduction in the die repair cost will increase the
profitability of the parts being produced. The Thermal Stress Relief (TSR) process
used during the die repair cycle is not only very costly but also a very time
consuming process. Further, it requires operation of large furnaces which increase
15 the carbon footprint of the factory and also require lot of maintenance.
In the present invention the inventors have proposed replacement of the TSR
process by the use of pulsations or vibrations for stress relieving (Refer Figures 1
& 2 for conventional and modified process flow). In the invented method of
20 repairing a cracked die, the cracked die is treated by gouging out the area
surrounding the crack and filling it with weld material. The die thus treated is
pulsated for a pre-determined time. The pulsation is done at such a rate (frequency
7
measured in Hz, for example) at which the amplitude of pulsation is highest and
hence, the stresses generated in the material are high. These stresses are cyclic in
nature. When these stresses are added to the residual stresses, the combined stress
may lead to local yielding of the material, which reduces the overall residual
stresses. Moreover, the pulsations increase the mobility of 5 the dislocations,
produced due to residual stresses. Increased mobility leads to rearrangement of
dislocations, which in turn reduces the residual stresses.
This method of stress relieving substantially reduces the time required for stress
10 relieving. Further it drastically reduces the cost of stress relieving.
The invention also provides an apparatus to carry out stress relief of the treated
dies.
15 Detailed description of the preferred embodiments
It is understood that this invention is illustrated with respect to forging dies (1)
made of alloy steel. The dies (1) which have failed during the course of forging
production run, by the mechanism of die crack come under the purview of this
invention.
20
During the die (1) repair cycle, the cracks are removed from the dies (1) by
gouging and the gouged area is filled with weld material using welding. Such die
8
is termed as a treated die (2). The welded zone of the treated die (2) is left with
some residual stresses due to the inherent nature of the welding process. The
stresses are mostly concentrated at the interface between the weld deposit and the
parent material.
5
This invention proposes use of pulsation/vibrations method for stress relieving of
the treated dies. For producing the pulsations, a mechanical, hydraulic or electric
shaker (3) can be used. Typically a Variable Frequency Drive (VFD) shaker is
used. The selection of shaker (3) will depend on the size of the treated die (2)
10 being further treated for stress relief – the larger/heavier the treated die (2), the
larger the capacity/power of the shaker (3). At least one sensor (4) is mounted on
the treated die (2) to measure the amplitude of the pulsations being transmitted by
the shaker (3) to the treated die (2).
15 Figure 3 shows a typical set-up for the process of invention. A shaker (3) is
attached to the treated die (2), preferably with the help of a rigid plate (5) which
helps transfer the pulsations to the treated die (2) over a wide area and as
uniformly as possible. The plate (5), on which the shaker (3) is mounted, may be
tightly clamped using clamps (6) on to the treated die (2). As an example, the
20 clamps (6) may be C-clamps. The location of sensor (4) is largely immaterial –
the entire treated die (2) that is being treated for stress relief pulsates, therefore the
pulsations can be sensed from virtually any location on the treated die (2). This
9
sensor (4) is connected to a control unit (7 – control unit or its connection to the
treated die not shown in figures). The rate of pulsation produced by the shaker (3)
can be changed. The sensor (4) mounted on the treated die (2) measures the
amplitude of pulsations and this is monitored on the control unit (7). For stress
relieving, the treated die (2) is pulsated with an optimum 5 rate or frequency
(measured in Hz, for example) at which the amplitude is at a maximum, for a
predetermined period or duration of time. The optimum frequency is arrived at as
a result of frequency variation and amplitude monitoring. The time duration is
decided based on weight of the treated die (2) being treated for stress relief. The
10 relation between the weight of dies and treatment time is shown in Figure 4, from
which it is evident that the predetermined duration is in the range of 20-30
minutes for dies of up to 1000kg total weight and in the range of 30-40 minutes
for the dies of between 1000-2000 kg total weight, and in the range of 40 – 80
minutes for the treated dies of a total weigh between 2000 to 3000kg.
15
More than one shaker (3) may be used. Location of all shakers selected so as to
provide pulsations as uniformly as possible. In the case there is more than one
shaker (3) and/or more than one sensor (4) are used, the combined effect on
amplitude is monitored.
20
In summary, the apparatus for repairing cracked dies disclosed in the invention
uses a variable frequency drive vibrator which is attached to a treated die using a
10
rigid plate, said rigid plate being tightly clamped on to said treated die, a vibration
amplitude measurement sensor that is mounted on said treated die, and a vibration
amplitude control unit to which said sensor is connected.
The invention will be explained using the following example. 5 An electric shaker
(3) with an electric motor, having unbalanced rotor was used in this case to
produce required pulsations. The shaker/motor is mounted on a metal plate (5).
For substantially uniform transmittal of the pulsations to the treated die (2), the
plate (5) was tightly clamped on to the treated die (2) to be stress-relieved (refer to
10 Figure 3). A sensor (4) was mounted on the die to measure the amplitude of the
pulsations being transmitted by the shaker (3) to the treated die (2) (refer to Figure
3). This sensor (4) was connected to a control unit (7) which was used to give the
required rate of pulsations to the treated die (2).
15 Total 22 die elements were used for this experiment. The dies were made of die
steels DIN 1.2714 and DIN 1.2344 which are commonly used in the Forging
industry. The weight of dies varied from 500 kg to 2000 kg.
For the invented stress relieving process a single phase electrical shaker of 750 W
20 power was used. The smaller dies (weighing up to 500 kg) were pulsated for 30
minutes while the bigger dies (weighing up to 2000 kg) were pulsated for 40
minutes.
11
The stress relieved dies were then machined and used for hot forging process. The
machining operation was studied closely and no problems were found during the
process. (If the dies are not properly stress relieved, the present residual stresses
leads to frequent tool breakage). Moreover, no issues were detected during the
forging runs, which indicate that the residual stressed were 5 removed. (If the
residual stresses are present in the dies, the die fails prematurely during the
forging run).
The method of the invention for repairing cracked dies comprises the following
10 steps:
 Gouging the cracked dies at the location of failure such that a volume of
material is gouged out of the die so that the crack is completely eliminated
from the parent material. This step gives a gouged die.
 Filling up the gouged area of the gouged die by deposition of welding
15 material using Welding method. This step gives a gouged and welded die.
 Insulating the gouged and welded dies from the shop floor by placing them
on insulating material such as wooden bars, rubber tyres etc. and
consequently reducing the amount of pulsations being transferred to the
shop floor.
20  Determining the required rate of pulsation for the die by varying the rate
from zero upwards using the VFD. The required rate of pulsation is the
value at which the amplitude is highest. Applying pulsations at that rate
12
and performing the stress relieving operation. This gives a stress relieved
die.
It is evident from the foregoing discussion that the invention comprises the
following 5 embodiments:
1. A method of stress relieving for hot forging dies, used in repairing cracked
dies (1), characterised in that the said method comprises the following
steps:
- gouging the cracked dies (1) at the location of failure such that a
10 volume of material is gouged out of the die so that the crack is
completely eliminated from the parent material, thus providing a
gouged die;
- filling up the gouged area of the gouged die by deposition of welding
material using a welding method thus producing a gouged and welded
15 die also termed as a treated die (2);
- placing said treated die on a rigid surface;
- insulating said treated dies from said surface by placing them on
insulating material selected from a group comprising wood, rubber,
and the like, and consequently reducing the amount of pulsations being
20 transferred to said surface;
- determining the required rate of pulsation at which the amplitude of
pulsation is maximum for the die by varying the rate from zero
upwards using a variable frequency drive (VFD);
13
- applying pulsations at said required rate for a predetermined duration.
2. A method as disclosed in embodiment 1, characterised in that said
predetermined duration is in the range of 20-30 minutes for dies of up to
1000kg total weight and in the range of 30-40 minutes for the dies of
between 1000-2000 kg total weight, and in the 5 range of 40 – 80 minutes
for the treated dies of a total weigh between 2000 to 3000kg.
3. A method as disclosed in embodiment 2, characterised in that said
pulsations are provided by single phase electric shaker (3) having a VFD.
4. An apparatus for stress relief of treated dies, characterised in that said
10 apparatus comprises a variable frequency drive which is attached to a
treated die using a rigid plate, said rigid plate being tightly clamped on to
said treated die, a vibration amplitude measurement sensor that is mounted
on said treated die, and a vibration amplitude control unit to which said
sensor is connected.
15
While the above description contains much specificity, these should not be
construed as limitation in the scope of the invention, but rather as an
exemplification of the preferred embodiments thereof. It must be realized that
modifications and variations are possible based on the disclosure given above
20 without departing from the spirit and scope of the invention. Accordingly, the
scope of the invention should be determined not by the embodiments illustrated,
but by the appended claims and their legal equivalents.
14
We claim:
1. A method of stress relieving for hot forging dies, used in repairing cracked
dies (1), characterised in that the said method comprises the following steps:
- gouging the cracked dies (1) at the location of failure such that a
volume of material is gouged out of the die so 5 that the crack is
completely eliminated from the parent material, thus providing a
gouged die;
- filling up the gouged area of the gouged die by deposition of welding
material using a welding method thus producing a gouged and welded
10 die also termed as a treated die (2);
- placing said treated die on a rigid surface;
- insulating said treated dies from said surface by placing them on
insulating material selected from a group comprising wood, rubber,
and the like, and consequently reducing the amount of pulsations being
15 transferred to said surface;
- determining the required rate of pulsation at which the amplitude of
pulsation is maximum for the die by varying the rate from zero
upwards using a variable frequency drive (VFD);
- applying pulsations at said required rate for a predetermined duration.
20 2. A method as claimed in claim 1, characterised in that said predetermined
duration is in the range of 20-30 minutes for dies of up to 1000kg total
weight and in the range of 30-40 minutes for the dies of between 1000-2000
15
kg total weight, and in the range of 40 – 80 minutes for the treated dies of a
total weigh between 2000 to 3000kg.
3. A method as claimed in claim 2, characterised in that said pulsations are
provided by single phase electric shaker (3) having a VFD.