Claims:WE CLAIM:

1. A process for manufacturing a forged hammer from a steel of 55C4 grade steel, the process comprising steps of:-

- blanking the steel bar;
- reheating the steel in an oil-fired furnace at temperature ~1200°C for about one hour;
- vertical forging the bar in vertical axis at 1050-1150°C so as to obtain compact grain structure on a face of the steel and increase strength thereof;
- horizontal forging the bar in horizontal axis to obtain a hammer;
- piercing the hammer to make a hole therein;
- quenching by spraying water for about 120 seconds;
- tempering in a tempering furnace at 300°C for releasing the residual stress and to impart toughness by quenched microstructure modification for 3 to 4 h.

2. The process as claimed in claim 1, wherein the hammer is quenched till colour of an eye portion changes to black.

3. The process as claimed in claim 1 or 2, wherein a delay of 2-5 mins is maintained between the horizontal forging and quenching to prevent quench crack formation.

4. The process as claimed in any of the preceding claims, wherein the hammer is rough grinded to remove parting line from a surface of the hammer.

5. The process as claimed in any of the preceding claims, wherein the hammer is buffed to impart smooth surface.

6. A forged hammer made up of 55C4 grade steel, comprising:

microstructure of tempered martensite on the face upto depth of 10mm to 15mm with ferrite-perlite body microstructure.
, Description:“Improved method of manufacturing of forged hammer for mining”

FIELD OF THE INVENTION

[001] The present invention relates to improved method of manufacturing of forged hammer for mining. The hammer is made of IS: 1570 (Part 2/Sec.1-1979) 55C4 grade of carbon steel (non-alloyed) without restricting scope of the invention to the same.

BACKGROUND OF THE INVENTION

[002] Forged hammers, which are being used in mining purpose, specifically for manual stone breaking have been reported to be associated with the drawbacks of mushrooming and bulging while manufacturing the hammer from 55C4 grade of carbon steel. The forged hammer has been characterised for hardness, depth of hardness and the microstructure at different section thereof. After analysing it was found that the face hardness and the depth of hardness was low. This causes the problem of bulging and mushrooming during its use. In order to increase the hardness and depth of hardness industries are generally using alloy steel rich in Chromium and Manganese and other alloys. However, alloying increases the cost of the hammer.

[003] Therefore, to achieve the higher hardness and higher depth of hardness of the hammer manufactured from the same 55C4 carbon steel, the process of manufacturing needs to be improved so as to address the issues encountered during its use without increasing the cost of steel for alloying.

[004] In view of the above, the present invention introduces improved method of manufacturing of forged hammer for mining improving the hardness and depth of hardness of forged hammer made from 55C4 carbon steel.

OBJECTS OF THE INVENTION

[005] It is therefore an object of the present invention to provide improved method of manufacturing of forged hammer for mining so as to improve the hardness of the face and depth of hardness of the hammer made of 55C4 carbon steel.

[006] Another object of the present invention is to avoid the bulging and mushrooming problem of the hammers during use for the entire life thereof.

[007] A still another object of the present invention is to propose a method for manufacturing forged hammer for mining application without increasing the cost of steel by addition of higher alloying elements.

[008] Another object of the present invention is to devise the optimum process parameters of the forged hammer for accomplishing the maximum hardness and depth of hardness while using 55C4 grade of steel bar.

[009] A still another object of the present invention is to propose a method which improves the hardness value of the forged hammer without impairing toughness thereof.

[0010] A still further object of the present invention is to propose a method for manufacturing forged hammer or tools for similar applications with better performance, which is reliable in operation and easy to manufacture using cost effective non-alloyed grade of medium carbon steel.

SUMMARY OF THE INVENTION

[0011] Accordingly, the present invention proposes a method of manufacturing a superior medium carbon steel hammer with more depth of hardness and higher hardness level, which eliminates the occurrence of mushrooming and bulging problem during use.

[0012] The details of the process steps are explained hereinbelow.

- Blanking the steel,
- Reheating of the steel bars of 55C4 grade in oil fired conventional furnace for a defined temperature and time.
- To homogenise the pancaked microstructure of the bar, on stroke of longitudinal reduction is applied before forging it to the shape of hammer.
- The bar is forged at the optimised temperature in a closed die forging press.
- After forging the hole for inserting the handle is made in a piercing press.
- Subsequently the red hot hammer is kept in open air to set the desired temperature prior to quenching. This air cooling time reduces the formation of quench crack due to the excessive thermal deformation during quenching.

- After air cooling for a desired time it is placed in a water quenching strand to quench with water from both of its faces for a desired period.
- After quenching, the hammers are kept in open air for a day followed by placing it into a tempering furnace.
- Tempering of the hammer is carried out at a defined temperature in a continuously charging furnace for a defined time period.
- Grinding and painting of the hammers are conducted.

[0013] Solution: Average hardness value of 59.4 HRC was achieved at the face of the hammer as against the target value of 58 +2/-1 HRC in a final trial batch of forged hammer production. The depth of hardness achieved was 7 to 9 mm from the face. The hammer made by this process was used at various mining field for more than one year and no issue of bulging and mushrooming was observed.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0014] Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings of exemplary embodiments of invention and wherein:

Fig.1 shows: Use of forged hammer of the present invention for stone breaking at mining.
Fig.2 shows: Issues of Mushrooming and Bulging in forged Hammer from the end users according to prior art.
Fig.3 shows: Hardness profile of the hammer before applying the invented process with distance from surface (mm) in X-Axis and hardness (HRC) in Y-Axis.
Fig.4a shows: First stage forging
Fig.4b shows: Second stage forging.
Fig.5a & 5b shows: Water quenching of hammers.
Fig.6 shows: Grinding and buffing of Hammer.
Fig. 7 shows: Finishing operations of hammers.
Fig.8 shows: Optical Microstructure at different stages of processing indicating 3% natal etched for sample F, Q and T from left to right respectively.
Fig.9 shows: Hardness profile of other grade (alloyed carbon steel) sample with distance from surface (mm) in X-Axis and hardness (HRC) in Y-Axis.
Fig.10 shows: Microstructure of sample for Gleeble before forging (a) 100 X, (B) 500X.
Fig.11 shows: The Time temperature plot of a few Gleeble samples forged and quenched at different temp from Time, Sec in X-Axis and Temp in °C in Y-Axis.
Fig.12 shows: Stress strain curve during simulated forging at different temperature from strain in X-Axis and stress (MPa) in Y-Axis.
Fig.13 shows: Optical (x500) Microstructure after tempering of the Gleeble forged samples and the corresponding average the hardness obtained are (a) 648 HV (relatively higher prior-austenite grain size), (b) 660 HV and (c) 673 HV (relatively lower prior-austenite grain size).
Fig. 14 shows: (A) Photograph of the sample prepared from the hammer face, point ‘a’ depicted the hammer face, point ‘b’ 12 mm below the face and point ‘c’ 22 mm below the hammer face. (a) Optical microstructure at point ‘a’ x50 (Tempered Martensite), (b) optical microstructure at point ‘b’ x50 (mixed phases) and (c) shows the optical microstructure at point ‘c’.
Fig. 15 shows: SEM microstructure near hammer face (a) Fine Prior austenite grains x150, (b) Tempered Martensite structure x25000.
Fig. 16 shows: SEM microstructure below 7mm from the face shows the presence of tempered martensitic and higher depth of hardness (a) low and (b) high magnification.
Fig. 17 shows: SEM microstructure below 20 mm from the hammer faces shows ferrite-pearlite structure as a result hardness is significantly lower than the face (a) low and (b) high magnification.
Fig. 18 shows: Hardness profile of the trial hammer shows good (7 mm) amount of depth of hardness with distance from Hammer face in mm in X-Axis and hardness (HV) in Y-Axis.

BRIEF DESCRIPTION OF THE ACCOMPANYING TABLES

Table-1 illustrates: Chemical analysis of first hammer sample
Table-2 illustrates: Chemical analysis of the other grade (alloyed carbon steel) sample.
Table-3 illustrates: Chemical analysis of second hammer sample.
Table-4 illustrates: Chemistry of the sample taken for Gleeble simulation.
Table-5 illustrates: The average hardness values obtained from the different Soaking-Forging and Quenching temperature.
Table-6 illustrates: Parameters set for plant trials.
Table-7 illustrates: Chemistry of the plant trial sample.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

[0015] The present invention makes a disclosure regarding an invention pertaining to improved method of manufacturing of forged hammer for mining. In the present invention described herein, the improved hammer for mining application is produced industrially from 55C4 grade (IS: 1570 Part 2/Sec.1-1979) of carbon steel (non-alloyed). As discussed under background of the invention, hammers which perform satisfactory for the similar applications are made of alloyed steel rich in Manganese, Chromium and Silicon in addition to some micro alloying elements like Niobium, Vanadium and Molybdenum. The chemical analysis of one such hammer is illustrated in Table.2. Hence, a superior quality of hammer has been made from a low cost steel grade in accordance with the instant invention.

[0016] Referring to Fig.1 the use of forged hammer is shown for breaking the granite stone in a mining near Karnataka, India. Mushrooming and bulging problem were seen in hammers made from 55C4 grade of steel while breaking stones. The samples were taken from the final stage of production for analysis. The hardness at surface was found to be avg. 54 HRC and at eye portion of the hammer the hardness was 32 HRC. Due to low hardness at the hammer surface, bulging occurs during its use. The hardness of the hammer face is desired to be as high as 58 HRC matching with the benchmark samples. It was also observed that the hardness decreases from front surface towards core. And the depth of hardness comes out to be 3-4 mm as shown in Fig.3, compared to the desired depth of hardness of 6 mm. Therefore, to avoid such issues, a method was designed to improve the forging hammer by optimizing the industrial process parameters and by using the same grade of non-allayed steel.

[0017] The manufacturing steps for forged hammers are described as follows.

I. Blanking: The blank is cut from the Billet of 55C4 grade.
II. Reheating: Reheating of the stock is done in an oil-fired furnace at temperature ~1200 oC for about one hour.
III. Forging: First stage forging is carried out in vertical axis, which helps to obtain compact grain structure on the face of the hammer and increases the strength as shown in the Fig.4a. The Second stage of forging is undertaken in horizontal axis to get the shape of hammer as seen in Fig.4b.
IV. Piercing to make a hole in the hammer.
V. Quenching: Hammer is placed on fixture (shown in Fig.5), which does not move during quenching. Water is sprayed over the surface for a time period of 120 sec. The quenching of the hammer is done till the colour of the eye portion is changed to black. There is about 3 min delay between forging and the quenching process.
VI. Tempering: After quenching the hammers are directly sent to a tempering furnace heated at 300oC for release of the residual stress and to impart some toughness by means of quenched microstructure modification for 3 to 4 h.
VII. Grinding and Buffing: Hammer face is rough grinded to remove parting line from the surface. Owing to rough grinding the hammer face is not uniform due to manual error. Buffing is introduced to give smooth and better surface. A grinding process is followed to eliminate eye-hole burrs (as in Fig.6).
VIII. Finishing: Uniform varnishing is done throughout the face after buffing in the stand. Weighing of each individual hammer is done before painting. Eye holes are painted separately with brush for uniform painting as shown in Fig. 7.

All the time and temperatures of the above presses were set by suitable engineering steel microstructure to obtain the desired hardness from the steel bars of 55C4 grade. For that conventional method of measuring hardness and toughness was employed. Microstructures were evaluated by optical and Scanning Electron Microscope. Gleeble simulators were implemented for simulating different heating, forging and tempering conditions. Details are described by the example given below.

Example:
Following characterisation of the hammer samples were done from the samples not performing well at the field. Some of its results are as follows:

I. Chemical analysis of the hammer sample was done as shown in Table-1.
II. Hardness was measured from the hammer face. It was 55.1 HRC in average of 10 samples.
III. Hardness profile from the cross-section was measured at various depths from the face and the result is given in Fig.3.
IV. Microstructure: Optical Micro structure of samples of the hammer face after Forging, Quenching and Tempering were observed as shown in F, Q and T respectively in Fig.8. It was felt the martensitic transformation is not completed in some region of the samples after quenching.
V. Chemical analysis of the other grade (alloyed carbon steel) samples was found to be richer than the present invention in terms of Mn, Si, Cr content with the presence of slight micro alloying elements as given in Table-2.
VI. Hardness profile of the other grade (alloyed carbon steel) sample was also measured as given in Fig.9. The depth of hardness is found to be around 4-6 mm which is slightly higher than the inferior 55C4 grade of hammer.
VII. The manufacturing process of the hammer manufacturing has been critically reviewed. All the basic measurements of reheating, forging, quenching and tempering was studied at plant. It was felt that the quenching process was insufficient as the temperature of the hammer after quenching raised to more than 100oC.
VIII. Based on the above study the FIFO (First in first out) system of production and sufficiently cooling of the hammer after quenching has been adopted. Then again the hammer same samples had been analysed. Chemistry of samples processed after maintaining basic conditions are given in Table-3. Optical microstructure of the samples as shown in fig.10 reveals martensitic structure. Also, average hardness of hammer face was slightly increased to 57.2 HRC, by adopting the process of cooling the hammers after quenching to the room temperature for at least 24 h before putting it into the tempering furnace.
IX. Gleeble Simulation: Small cylindrical (10 mm dia and 15 mm barrel length) samples of the same grade were prepared from the rolled rod sample for Gleeble simulation of forging and quenching at different temperatures. The hardness, microstructure of each sample has been examined to find out the best suitable forging and quenching temperature. Chemistry (wt%) of the sample taken for Gleeble simulation is given in Table-4. Average hardness before forging: 202 HV10 ( ~ 28 HRC). Microstructure of rod sample before forging is given in Fig.-11, showing as ferrite-perlite structure. Gleeble Forging parameters were kept constant for all samples. Strain e=0.3 and strain rate e= 10 / sec. Some typical time temperature graph of the Gleeble samples are given for reference in Fig. 12.
X. Hardness Results obtained after Gleeble simulation is given in Table-5. From this table the range of optimised forging and quenching temperature was obtained.
XI. From Table-5, it has been observed that the best result (maximum hardness) was obtained when the soaking and forging temperature is in lower side (1050 and 1000 oC) and the difference between soaking and quenching temperature is on higher side (200 oC). At lower forging temperature near 1000 oC forging would be difficult as per the stress strain data plotted in Fig.13. It is observed that 1050oC is a suitable forging temperature for the hammers without significant increase in stress whereas at 1000oC the forging would be difficult and may lead to increase in die wear.
XII. The targets forging temperature is optimised as 1050oC and then let it cool down to decrease the temperature by 200°C before water quenching at 850°C. The expected average hardness in such case has been predicted as 650+/-25 HV, which is equivalent to 58 +/- 1 HRC. Based on the above optimisation a plant trial has been conducted as per the final parameters mentioned in Table-6.
XIII. Prime reason for the increase in hardness with the lowering in forging temperature can be explained by the prior-austenitic grain size before forging. This has been validated with the microstructure as qualitatively compared in Fig. 14.
XIV. The austenitic grain size of medium carbon steel is logarithmically proportional to the soaking temperature. Therefore, forging at lower temperature gives a smaller final grain size, which adds up some hardness to the transformed microstructure (tempered Martensite) on the face of the hammer.
XV. Implementation and Plant Trial: The chemical analysis of the steel is again given in Table-7 for the hammer, which has been processed with modified process parameters. Average hardness of 59.34 HRC was obtained after the plant trial.
XVI. Optical microstructure of plant trial sample has been shown in Fig.15, wherein the desired microstructure obtained the desired hardness, depth of hardness and toughness of the hammer face.
XVII. SEM Microstructure of samples Corresponding to the optical microstructure along the specified sample positions have been given in Fig. 16, 17 and 18 respectively.
XVIII. Hardness Profile as shown in Fig. 19, obtained from the hammer sample (after plant trial) was found slightly better than the alloyed steel hammer sample.
XIX. This product has been tested in the field for more than one year and no problem has been observed.
Table-1: Chemical analysis of first hammer sample

% C % Mn % S % P % Si % Cr % Al % Cu % Ti N2 (ppm)
0.562 0.61 0.017 0.014 0.186 0.017 0.0035 0.005 0.0029 70

Table-2: Chemical analysis of the other grade (alloyed carbon steel) sample

C Mn S P Si Al Ti Cr Ni Mo V Cu Nb
0.42 1.26 0.035 0.011 0.26 0.02 0.002 1.34 0.017 0.002 0.002 0.012 0.002

Table-3: Chemical analysis of second hammer sample

% C % Mn % S % P % Si % Cr % Al % Cu % Ti N2 (ppm)
0.565 0.614 0.019 0.015 0.187 0.017 0.0033 0.0046 0.0029 72

Table-4: Chemistry of the sample taken for Gleeble simulation

C Mn S P Si Al Ti Cr Ni Mo

0.59 0.60 0.01 0.02 0.18 0.00 0.00 0.02 0.02 0.01

Table-5: The average hardness values obtained from the different Soaking-Forging and Quenching temperature.
Soaking & Forging Temp. Water Quenching Temp. Delta Temp.
(Forging - Quenching) Avg. Hardness(HV) after Tempering
1100 1000 100 603
1100 900 200 648
1050 850 200 660
1000 950 50 588
1000 900 100 627
1000 800 200 673
Table-6: Parameters set for plant trials
Soaking & Forging Temp Water Quenching Temp Delta (Forging - Quenching) Expected Avg. Hardness
after Tempering
1050-1000 850-800 ~200 675 +/-25 HV 58 +/-1 HRC

Table-7: Chemistry of the sample taken for plant trial

% C % Mn % S % P % Si % Cr % Al % Cu % Ti N2 (ppm)
0.61 0.66 0.018 0.018 0.194 0.02 0.0032 0.0038 0.0023 65

Thus, the invented process of making forged hammer from 55C4 grade carbon steel involves the following process parameters:

(i) Reheating Temperature of the furnace should be kept nearly 1200°C.
(ii) Sufficient time (for about 2-5 min) between the horizontal forging and Quenching to be maintained to air cool the forged hammer up to 850°C before quenching.
(iii) Quenching of hammer to be done for about 2 min by spraying water from its faces. There should not be any water vapour coming out near the eye portion before taking out the sample from the water quenching stand.
(iv) After quenching, sample should be cooled for sufficient time (~ 8 hours) to attain the room temperature before putting the quenched hammers into the tempering furnace.
(v) First in first out (FIFO) system to be maintained during processing of hammer in each stage of production.

ADVANTAGEOUS FEATURES OF PRESENT INVENTION

Low cost unalloyed steel bar can be used as the input for making the superior quality forged hammer compared to the moderately alloyed steel as described above.

No additional processes adopted to improve the performance of the forged hammer.

The temperature of the re-heating furnace is lowered which leads to sa.ng the cost of fuel and electricity.

The normal life of the hammer is more than the alloyed steel hammer.
As the steel of the hammer is of plain carbon grade, therefore, after the normal life of the hammer, the reprocessing of the remaining steel is easy.

It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-