WO2016027207A1

WO2016027207A1 - A method of hardening die surfaces

Info

Publication number: WO2016027207A1
Application number: PCT/IB2015/056199
Authority: WO
Inventors: Babasaheb Neelkanth Kalyani; Jayant Bhaurao MANE; Shreyas Shashank KIRWAI; Sachin Sitaram PATIL; Mukund Vishnu MAVALANKAR
Original assignee: Bharat Forge Limited
Priority date: 2014-08-18
Filing date: 2015-08-14
Publication date: 2016-02-25

Abstract

The present invention discloses a modified process of die manufacturing which substantially improves the wear life of the dies. The process consists of introduction of shot blasting operation after the machining operation and before the nitrocarburising operation. The process increases the case depth of the surface hardened layer thereby increasing the wear life of the hot forging dies. Gaseous Ferritic Nitrocarburising (GFN) process is carried out on die steel material for increasing the surface hardness. Performance of the surface hardening process depends on surface preparation and its treatment. Surface preparation entails modifying roughness and residual stresses on the surface. The shot blasting step of the invented process introduces uniform roughness and compressive residual stresses on the surface, which helps to increase the hardening depth during GFN.

Description

A Method of Hardening Die Surfaces

Field of invention

This invention relates to the hot forging die manufacturing process. More specifically, this invention relates to the surface hardening process carried out to improve the wear life of the dies.

Background of invention

The hot forging industry uses batch type of production for automobile and other parts. The size of the batch is restricted by the structural integrity of the die among other things.

The structural integrity of the die is affected by two different phenomena's. The first phenomenon is cracking of the dies while the other is wear of the dies. Die crack leads to complete failure of the dies and causes problems like cold shuts and extensions in the forged components. Die wear is a more gradual phenomenon and leads to dimension oversize and/or under-fill problems in the forged components. Experience has shown that the die wear is a production batch size limiting factor in most of the cases. Therefore, to increase the production batch size, which in turn will increase productivity and O.E.E (Overall equipment effectiveness); the wear resistance of the dies has to be increased. The hot forging dies have to undergo strenuous working condition in the form of relative motion with material being forged. The relative motion between hot metal and the die surface leads to abrasive die wear. The abrasive die wear, among other factors, is inversely proportional to the hardness of the surface under consideration. Thus, the wear can be reduced by increasing the surface hardness of the dies. Hence, to counter the die wear phenomenon surface hardening of the dies is done.

For the surface hardening of the dies many different methods are used. The different surface hardening processes which are currently being used in the industry are carburizing, nitriding, carbonitriding, bonding, nitrocarburising etc. Surface hardening processes involve a diffusion process which modifies the chemistry of the surface. The basic process is thermochemical in nature as energy, in the form of heat, is provided to enhance the diffusion of hardening species into the surface and subsurface regions of the part. During the surface hardening process, the hardening species is transferred from a media into the steel at elevated temperatures. Most commonly used surface hardening species are Carbon (C), Nitrogen (N) and Boron (B). These hardening species increase the hardness of the surface through two mechanisms. These species can enter the matrix in the form of solid solution and lead to hardening of the surface. They can also form precipitates (carbides/Nitrides/Borides) in the matrix which will also lead to hardening of surface. After surface hardening, a compound layer and an underlying diffusion zone (i.e. case) are formed at the surface of the steel.

The medium used to provide the hardening species can be gas, liquid, solid or plasma. This process is carried out in two temperature ranges. The first range is the one in which the part is heated up to the austenitising temperature. At this temperature the steel has greater solubility for the hardening species (i.e. B, N or C). Normally, after the completion of the diffusion process, quenching of the part is required to attain the properties. This requirement may lead to distortion of the parts during the process. Bonding, carburizing and nitro-carburizing can be carried out in this temperature range.

In the second case, the part is heated within the Ferritic region only. After the completion of the diffusion process, quenching of the part is not required. Hence, this process minimizes the distortion problem. Carburizing and Nitro-carburizing can be carried out in this temperature range also. Nitriding is carried out in this temperature range only. This invention pertains to the Gaseous Ferritic Nitrocarburising process. In the present work, the nitrocarburising case depth has been increased by imparting compressive residual stresses on the surface of the die steel. EP Patent 2484493A1 describes a method to improve the durability of a steel product by shot peening treatment of the surface. Two or more shot peening operations are done after the nitriding operation. The first operation is done to remove the compound layer from the nitride layer. The second shot peening operation is done to provide compressive residual stresses on the surface. In the said patent the inventors have used shot peening operation as against shot blasting operation being used in our invention. Moreover, in the said patent the shot peening operation is used by the inventor after the surface hardening treatment, while, our invention proposes use of shot blasting prior to the surface hardening treatment. Further, the said patent is more suited to the casting dies while our invention is related to forging dies.

US Patent 6117249 discloses a method to improve the wear life of a metallic machine part. In the said patent, the part is first roughened by shot/grit/glass bead blasting process. Then the part is hardened by any of the surface hardening process. After this a plastic coating is made on this roughened and surface hardened part. The said patent is related to machine parts, more particularly screws, which are in relative motion with plastic parts. Our invention deals with hot forging dies which are in contact with hot metal being formed. This invention does the shot/grit/glass bead blasting for the sole purpose of roughening of the surface. Our invention uses shot blasting for providing compressive residual stresses and imparting uniform surface roughness which in turn increases the surface hardening case depth. In the said patent surface hardening process is followed by coating of surface with PTFE. In our invention no such follow up process proposed.

US Patent 7621201B2 discloses a method to reduce adhesion of hot work piece to the die surface. The said patent describes a process of cleaning of the tool surface, nitrocarburising of the tool surface, removal of the white layer by grinding and/or polishing and then application of boron nitride or tungsten bisulfide particles on the surface and its subsequent heat treatment. The said patent is related to sheet metal forming dies for aluminium and magnesium alloys as against our invention which is best related to the hot bulk forging process. Further, the said patent does not use shot blasting process for the improvement of the properties of the dies.

WO Patent 2004101846 Al discloses a method to increase the life of a tool holder. The said patent proposes a manufacturing process which has following process after the machining: the tool is surface hardened using nitrocarburising treatment. This hardened surface is treated to impart further compressive residual stresses on it. This surface treatment can be in the form of shot peening. This invention is best suited to machining tools as against our invention which is best applicable to forging dies. The said patent proposes shot peening operation after the surface hardening operation while our invention proposes shot blasting prior to the surface hardening operation. Objects of Invention

It is an object of invention to improve hot forging die life by combining effect of shot blasting and surface hardening (Nitriding/Carburizing/Nitro-carburizing and applicable industry standard practices).

Another object of invention is to increase the surface hardening case depth on the hot forging dies which in turn reduces the wear and tear of the dies.

Yet another object of invention is to reduce the machine downtime required for the online repair of the die surface damaged due to wear.

Still another object of invention is to increase the production batch size of the hot forging process.

Brief description of drawings

Figure 1 shows the flow chart of the conventional process of forging die manufacturing.

Figure 2 shows the flow chart of invented process for forging die manufacturing with the addition of shot blasting process in it.

Figure 3 shows the microstructure of a cross section near the surface of the die. The darkly etched zone indicates the nitrocarburised (hardened) layer. Figure 4 shows the micro hardness traverse at the cross section shown in Figure 3. This shows the hardness variation from surface towards core.

Summary of invention

Increased productivity and reduced cost of production is achieved by increase in the wear resistance of the forging dies. The process of nitrocarburising has increased the wear resistance of the dies. However, the industry strives for further improvement of the wear resistance. The increased wear resistance translates into larger batch sizes and hence, more productivity and reduced cost of production.

The present invention discloses a modified process of die manufacturing which substantially improves the wear life of the dies. The process consists of introduction of shot blasting operation after the machining operation and before the nitrocarburising operation. The process increases the case depth of the surface hardened layer thereby increasing the wear life of the hot forging dies.

Gaseous Ferritic Nitrocarburising (GFN) process is carried out on die steel material for increasing the surface hardness. Performance of the surface hardening process depends on surface preparation and its treatment. Surface preparation entails modifying roughness and residual stresses on the surface. The shot blasting step of the invented process introduces uniform roughness and compressive residual stresses on the surface, which helps to increase the hardening depth during GFN. The increase in case depth is measured by analysing the depth of compound layer as well as diffusion layer formed during nitrocarburising process.

Detailed description of the invention

The present invention discloses a process in which a step of shot blasting is introduced before the surface hardening process. While the invention may be carried out using any of the surface hardening methods such as carburizing, nitriding, carbonitriding, boriding, nitrocarburising, the invention has been illustrated using nitrocarburising method of surface hardening. Figure 1 and 2 shows the conventional process and the invented process of die manufacturing respectively. The shot blasting operation is performed using steel shots of predefined diameter.

A surface hardening operation such as nitrocarburising increases the hardness of the surface of the dies. The hardened layer consists of nitrides, carbides and carbonitrides of iron and its alloying elements. The nitrocarburising process leads to formation of compressive residual stresses on the treated surface.

The depth of the nitrocarburised layer (henceforth "case depth") depends on the diffusion of the hardening species i.e. C and N. The case depth is measured using micro hardness traverse as shown in Figures 3 and 4. The diffusion of C and N depends on the alloying content of the steel. Also it depends on the surface cleanliness, the surface roughness and the surface residual stresses.

The present invention proposes a method to modify the surface roughness, cleanliness and the surface residual stresses. This could be achieved by blasting any materials surface using media which are categorised as metallic grit, metallic shot, sand, glass and other miscellaneous materials. Non-metallic media include sand, glass, agricultural products (such as crushed walnut or pecan shells, rice hulls, rye husks, corncobs, sawdust, etc.), plastic and nylon. For a specific application one of these media is selected based on the properties of the part and requirements of the final part.

These media are energised and propelled against the work surface using one of the three principal methods given below:

1. Airless centrifugal blast blade or vane type wheels,

2. Compressed air, direct pressure dry blast nozzle system or

3. Indirect suction wet or dry blast nozzle system.

The first of the above three methods works on the principle of centrifugal force generated by rotating blades or vanes on a wheel, whereas the second and the third methods work using pneumatic systems. The latter two methods require high power to generate the compressed air or steam which is used to accelerate the media which is then passed through a final nozzle for giving direction. The first method requires approximately 10% of power as that of second and third methods. Any of the above mentioned machines/methods can be used for blasting of the work surface with a given media.

In the preferred embodiment of the invention, steel shot blasting using an indirect suction dry blast nozzle system is used prior to nitro-carburising process.

In the present work, a number of different sizes of shots and one type of sand was used for the surface preparation of the samples. The effect of sand, shot size and blasting time on roughness, residual stress and depth of residual stress is examined. Shots made from steel up to 0.85mm size were used for shot blasting. Based on the above data the optimum shot size and blasting time are

recommended for optimum nitro carburizing case depth. In one embodiment, step of shot blasting is carried out using shots made from materials selected from a group comprising metallic grit, metallic shot, metal alloys, sand, glass, shells of crushed walnut or pecan nuts, rice hulls, rye husks, corncobs and sawdust, plastic and nylon. The sand/shot blasting operation is carried out using a pneumatic operated machine or similar equipment. The operative pressure for shot blasting is 4.7 to

5.0 bars. Blasting operation is carried out at various periods depending on the desired results. For example, durations of 10-50 seconds or more may be used. The sand/shot blasting process is a surface treatment process. It imparts residual stresses on the treated surface. The impact of this process is limited to a very small layer depth on the surface. Moreover, the surface roughness is also altered by this process.

This has the following advantageous effects on the surface of the dies:

- It cleans the surface thoroughly,

- It imparts uniform surface roughness on the die surface, and

- It imparts compressive residual stresses on the surface of dies.

The impact of this process is limited to a very small layer on the surface.

Moreover, the surface roughness is also altered by this process. These three modifications together increase the diffusion of the hardening species (C and N) in the matrix and hence increase the case depth.

The method of increasing the case depth of the present invention substantially increases the wear resistance of the die material. This reduces the wear and tear of die taking place during the hot forging operation which in turn allows for increased batch size per run. The nitro carburising case depth of the sand/shot blasted samples is observed to increase in proportion to the size of the sand or steel shots used or with the time for which the shots were propelled. The sand/shot blasting process increases the surface roughness through the combined effect of localized plastic deformation and material removal

The sand/shot blasting process imparts residual stresses on the treated surface. These stresses are compressive in nature. The surface residual stresses are typically measured using X-ray stress analysers.

The embodiments of the present invention are explained as follows.

The description that follows of this invention is illustrated with respect to forging dies made of alloy steel.

In the preferred embodiment of the invention a method is disclosed, said method comprising the following steps:

- Providing a heat treated die block, the heat treatment being in the form of hardening followed by tempering.

- Machining the heat treated die block to a required shape. - Shot blasting the die surface. The shot blasting is done using steel or sand shots. The steel/sand shots are propelled on the die surface. The shot blasting time period is dependent on the functional surface area of the die.

- Nitrocarburising of the dies.

In an embodiment of the invention, any method of surface hardening, carburizing, nitriding, carbonitriding, bonding, may be used in place of nitrocarburising.

In another embodiment of the invention, shot blasting is applied to the surface which will be in contact with the material to be forged.

In another embodiment of the present invention, a product with hardened surface is disclosed wherein the product has been produced using the heat treatment disclosed in the preferred embodiment, and which product has a martensitic microstructure.

In yet another embodiment of the invention, the steel or sand shots are propelled in a closed chamber, using an air stream coming out of a nozzle. In a further embodiment, the shot blasting is carried out using a turbine wheel type shot blasting equipment. It is evident from the foregoing discussion that the present invention takes forms as disclosed in the items listed below.

A method of hardening die surfaces incorporating the steps of machining of die surfaces, nitrocarburising, followed by hot forging, characterised in that a step of shot blasting is introduced following the step of machining of die surfaces.

A method of hardening die surfaces as disclosed in item 1 characterized in that said method comprises the steps of:

providing a heat treated die block, the heat treatment being in the form of hardening followed by tempering.

machining the heat treated die block to a required shape.

shot/sand blasting the die surface

carrying out surface hardening of the dies.

A method of hardening die surfaces as disclosed in item 2 characterized in that said surface hardening is carried out using any one of the group of methods comprising carburizing, nitriding, carbonitriding, bonding, nitrocarburising and applicable industry standard practices.

A method of hardening die surfaces as disclosed in items 2 and 3, characterized in that said shot/sand blasted surface is the surface that would be in contact with the material to be forged. A method of hardening die surfaces as disclosed in items 1 to 4, characterized in that, said shot blasting step uses metal shots or sand, wherein said shots are propelled on the die surface.

A method of hardening die surfaces as disclosed in items 1 to 5, characterized in that, said shot blasting is carried out using a pneumatic equipment, in a closed chamber, wherein air stream comes out of a nozzle. A method of hardening die surfaces as disclosed in items 1 to 6, characterised in that said step of shot blasting is carried out using metal shots.

A method of hardening die surfaces as disclosed in items 1 to 6, characterised in that said step of shot blasting is carried out using shots made from materials selected from a group comprising metallic grit, metallic shot, metal alloys, sand, glass, shells of crushed walnut or pecan nuts, rice hulls, rye husks, corncobs and sawdust, plastic and nylon.

A method of hardening die surfaces as disclosed in items 5 to 8, wherein said step of shot blasting is carried out at a pressure of 4 to 7 bars.

A method of hardening die surfaces as disclosed in items 1 to 9 characterised in that time period of said shot blasting is proportional to the functional surface area of the die.

A method of hardening die surfaces as disclosed in items 1 to 10 characterised in that said time period is up to 50 seconds/inch². 12. A method of hardening die surfaces as disclosed in items 1 to 11 characterised in that said metal shots are up to 0.85mm in size.

13. A hot forging die with a hardened surface characterised in that said hardened surface has been produced using a method disclosed in any of the methods of item 1 to 12.

It is evident from the foregoing discussion that the invention has the following embodiments. These are: 1. A method of hardening component surfaces to enhance components life incorporating the steps of machining of component surfaces, surface hardening, characterised in that a step of shot blasting is introduced following the step of machining of component surfaces.

2. A method of hardening component surfaces as disclosed in embodiment 1, characterised in that said component is forging die.

3. A method of hardening die surfaces as disclosed in embodiment 2 characterised in that said method further comprises the steps of:

- machining the heat treated die block to a required shape, shot/sand blasting the die surface

carrying out surface hardening of the dies. A method of hardening die surfaces as disclosed in embodiments 1-3 characterised in that said surface hardening is carried out using any one of the group of methods comprising carburizing, nitriding, carbonitriding, bonding, nitro-carburising and applicable industry standard practices. A method of hardening die surfaces as disclosed in embodiments 1-4, characterised in that said shot/sand blasted surface is the surface that would be in contact with the material to be forged.

A method of hardening die surfaces as claimed in claims disclosed in embodiments 1 to 5, characterised in that, said shot blasting step uses metal shots or sand, wherein said shots are propelled on the die surface. A method of hardening die surfaces as disclosed in embodimentsl to 6, characterised in that, said shot blasting is carried out using a pneumatic equipment, in a closed chamber, wherein air stream comes out of a nozzle. A method of hardening die surfaces as disclosed in embodimentsl to 7, characterised in that said step of shot blasting is carried out using metal shots.

A method of hardening die surfaces as disclosed in embodimentsl to 8, characterised in that said step of shot blasting is carried out using shots made from materials selected from a group comprising metallic grit, metallic shot, metal alloys, sand, glass, shells of crushed walnut or pecan nuts, rice hulls, rye husks, corncobs and sawdust, plastic and nylon. 10. A method of hardening die surfaces as disclosed in embodimentsl to 9, characterised in that said step of shot blasting is carried out at a pressure of 4 to 7 bars.

11. A method of hardening die surfaces as disclosed in embodimentsl to 10, characterised in that time period of said shot blasting is proportional to the functional surface area of the die.

12. A method of hardening die surfaces as disclosed in embodiments 1 to 11, characterised in that said time period is up to 50 seconds/inch².

13. A method of hardening die surfaces as disclosed in embodimentsl to 12, characterised in that said metal shots are up to 0.85mm in size.

14. A forging die with a hardened surface, characterised in that said hardened surface has been produced using a method disclosed in any of the methods of embodiments 1 to 13. 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 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.

Claims

A method of hardening component surfaces to enhance components life incorporating the steps of machining of component surfaces, surface hardening, characterised in that a step of shot blasting is introduced following the step of machining of component surfaces.

A method of hardening component surfaces as claimed in claim 1, characterised in that said component is forging die.

A method of hardening die surfaces as claimed in claim 2, characterised in that said method comprises the steps of:

machining the heat treated die block to a required shape.

shot/sand blasting the die surface

carrying out surface hardening of the dies.

A method of hardening die surfaces as claimed in claim 1-3 characterised in that said surface hardening is carried out using any one of the group of methods comprising carburizing, nitriding, carbonitriding, bonding, nitro- carburising and applicable industry standard practices.

A method of hardening die surfaces as claimed in claims 1-4, characterised in that said shot/sand blasted surface is the surface that would be in contact with the material to be forged.

6. A method of hardening die surfaces as claimed in claims 1 to 5, characterised in that, said shot blasting step uses metal shots or sand, wherein said shots are propelled on the die surface.

7. A method of hardening die surfaces as claimed in claims 1 to 6, characterised in that, said shot blasting is carried out using a pneumatic equipment, in a closed chamber, wherein air stream comes out of a nozzle.

8. A method of hardening die surfaces as claimed in claims 1 to 7, characterised in that said step of shot blasting is carried out using metal shots.

9. A method of hardening die surfaces as claimed in claims 1 to 8, characterised in that said step of shot blasting is carried out using shots made from materials selected from a group comprising metallic grit, metallic shot, metal alloys, sand, glass, shells of crushed walnut or pecan nuts, rice hulls, rye husks, corncobs and sawdust, plastic and nylon.

10. A method of hardening die surfaces as claimed in claims 1 to 9, characterised in that said step of shot blasting is carried out at a pressure of 4 to 7 bars.

11. A method of hardening die surfaces as claimed in claims 1 to 10, characterised in that time period of said shot blasting is proportional to the functional surface area of the die.

12. A method of hardening die surfaces as claimed in claims 1 to 11, characterised in that said time period is up to 50 seconds/inch².

13. A method of hardening die surfaces as claimed in claims 1 to 12, characterised in that said metal shots are up to 0.85mm in size.

14. A forging die with a hardened surface, characterised in that said hardened surface has been produced using a method disclosed in any of the methods of claims 1 to 13.