Field of invention

This invention pertains to manufacture of complex parts using a forging process. More specifically, this invention pertains to the hot forging of complex parts in automotive industry such as crankshafts using continuous as cast billets.

Background of invention

Hot Forging Process is used to produce metal parts for variety of applications, one of the applications being production of automobile parts. Forging is a manufacturing process where metal is shaped by plastic deformation under high pressures resulting into high strength parts. This process gives directional properties to the final product. The forged parts are designed in such a way that these directional properties help to achieve the functional requirements of the part.

This invention relates to the manufacturing of complex parts typically used in automotive industry, although complex parts used in any other industry may be manufactured using the process of invention. The description that follows has been explained using the example of a crankshaft which is used in an automobile engine. The crankshaft is generally manufactured through closed die (hot) forging process. The typical flow diagram of conventional manufacturing process of a crankshaft is shown in Figure 1. The raw material for the hot forging process is, generally, in the form of metal billets. The production methods of these billets, generally, consist of following steps as shown in Figures 2 and 3 :

1. Casting of metal by

Ingot route or

- Continuous Casting (ConCast) route

2. Cogging and/or rolling of the cast material.

The cast material has some inherent defects. During the solidification process, the material in contact with the mould walls cools first and this solidification continues inwards. During the solidification the material is also shrinking. Due to this shrinking, porosities and centre looseness (known as shrinkage defects) are formed in the cast material near its centreline. Moreover, some gases are also formed/evolved during the solidification process which may get trapped in the cast material in the form of porosities. To remove these defects the cross section of the cast material is reduced using forming operations.

As shown in Figure 2, when the billet is cast through the ingot route, the material is, generally, first cogged and then it is rolled. On the other hand, as is shown in Figure 3, the ConCast billets are, generally, directly rolled to the required cross section.

The cogging and/or rolling of the cast material are done for following reasons: 1. The cross section of the cast material is reduced to a size which can be economically used for the hot forging process.

2. The cast structure of the metal consists of dendrites near the walls of the cast mould. The cogging and/or rolling process breaks the dendritic structure. Thus, the output of these processes is a more homogenous and equiaxed microstructure.

3. Defects like shrinkage porosity, cracks and segregation are associated with the cast products. The cogging and/or rolling process helps in closing of the shrinkage porosity and cracks.

4. The cogging and/or rolling process also produces longitudinal grain elongation in the metal. This leads to a grain flow pattern which produces directional properties in the metal.

US Patent 0169856 Al 2007 discloses a method of producing stainless steel parts using a hybrid casting and forging process. The steps proposed in this invention are casting of the billets in required shape and size. Then these billets are forged to required shape. Finally, annealing treatment is done to achieve the required properties. The said patent is specifically targeted to stainless steel. On the contrary our invention is most suitable for alloy steels used for making the automobile parts. The patent also specifies that the billets are to be cast separately for each forging and also that the shape of the billet roughly confirms with the final shape of the product. Thus, the patent does not use costly bar shaped billets which are produced by the conventional method. On the contrary, our invention proposes the use of bar shaped billets produced using the conventional ConCast method of casting. Thus, our invention does not require any special casting process for production. The said patent specifically defines forging temperature band of 1000 °C to 1100 °C while our invention considers forging operation at temperature in the excess of 1100 °C. Further, the said patent requires a solution treatment operation after the completion of the forging operation while our invention does not require solution treatment after the forging operation. EP Patent 2329899 Al discloses an alternative method of producing a light alloy vehicle wheel. The said patent modifies the conventional process by changing the cross section of the cast metal from round to rectangular and using a modified upsetting operation for changing the rectangular billet to thin cylindrical preform. The said patent pertains to forging of light alloys while our invention is related to the forging of the alloy steel. The said patent targets vehicle wheel specifically, while, our invention is more suitable for steel forgings used in automotive sector. The said patent is more related to change in the casting process as against our invention which is more related to change in forging operation. US Patent 7070735 B2 discloses a method of continuous casting of aluminium alloy so that the surface quality of the cast material is very good and segregation is minimum. The said patent allows the producer to produce continuous cast material of such quality that it can be used directly for forging operation. The said patent is specifically targeting aluminium alloy while our invention is most suitable for alloy steel components. The said patent is more concerned with the casting process and does not consider the forging of the components. Our invention concentrates on the forging of the components and achieving the required mechanical and metallurgical properties of the forged components. Moreover, the said patent requires a change in the chemistry of the raw material by addition of Ca or Be while our invention does not require any chemistry modification in the raw material.

JP Patent 60082257 discloses a method for producing a billet without central segregation. The said patent discloses forging operation which is carried out continuously at the end where the solidified hot metal is coming out of the continuous casting system. The forging method considered in the said patent is open die forging method. As against this, our invention pertains to using continuous cast billet for closed die forging. Thus, our invention tries to avoid the open die forging method discussed in the said patent.

US Patent 5983481 A discloses a method for producing a forged steel bar with refined and/or compacted grain structure. The said patent uses a combination of rolling operations after the continuous casting of the metal. The said patent is specifically related to forged steel bars and the forging process considered is rolling. Our invention is more suited to the closed die forging method of complex shaped products. Moreover, our invention tries to avoid the rolling process described in the said patent. CA Patent 1131879 Al relates to production of continuous cast steel bar and process for its manufacturing, with improved surface quality. The said patent focuses on the casting process for metals and does not consider forging process. Our invention is related to the forging process and achieving the required mechanical, metallurgical and fatigue properties for the same.

If the rolling and/or cogging operation is not used during the billet manufacturing process then the billets will have the casting defects mentioned earlier. The closed die forging process reduce these defects, but, some defects remain with reduced intensity, which travel to locations near surface due to the inherent properties of the closed die forging process. These defects when present near the surface can significantly reduce the life of the final product.

There is therefore a need to provide a process that eliminates the rolling step but still eliminating the drawbacks mentioned above.

Brief description of drawings Figure 1 shows the flow chart of the conventional process for the crankshaft manufacturing.

Figure 1 A shows a conventional crankshaft forging process

Figure 2 shows conventional process of billet manufacturing through ingot route. Figure 3 shows the conventional process of billet manufacturing through Continuous Cast (ConCast) route.

Figure 4 shows the invented process of billet manufacturing.

Figure 5 shows the flow chart of the invented process for the crankshaft manufacturing.

Figure 6 shows the porosities and centre looseness (defects) in the transverse cross section of the ConCast material.

Figure 7 shows the porosities in the longitudinal section of the ConCast material.

Summary of invention

The cost of a crankshaft made using the forging - machining route comprises of the cost of all the processes used for its manufacturing. The current production process consists of steel casting - rolling/cogging - closed die forging - machining. Reduction in the cost of manufacturing will result into an economical product. Moreover, forging process has high carbon footprint and is an energy intensive process. The present invention eliminates one of the steps from the crankshaft manufacturing process. This will achieve the purpose of reduction in the cost of production as well as carbon footprint of the whole process.

The present invention eliminates the rolling and/or cogging process from the complete crankshaft manufacturing cycle. The modified manufacturing process is as shown in Figures 4 and 5.

The rolling and/or cogging process reduces the cross section of the cast material to a section which can be used by the closed die forging process. This invention accomplishes the said requirement by using ConCast billets of the size required by the forging process, thus, eliminating the requirement of cross section reduction through rolling and/or cogging. Moreover, proper forging process design is done to take care of closure of internal porosities and centre looseness, breaking of as cast structure and required grain flow during the forging operation. Thus, the crankshaft produced by the invented process has properties (mechanical, metallurgical, fatigue) equivalent to those produced by the conventional process.

Detailed description of the invention

It is understood that, this invention is illustrated with respect to a crankshaft made of alloy steel, although it is applicable to any part of complex geometry. These parts are made by the manufacturing process of casting - forging - machining. Porosities and center looseness are the inherent problems of the casting process. In the conventional process, these defects are removed using the cogging and/or rolling processes. These processes reduce the cross section of the cast material thus closing these defects. Normally, this is followed by the forging operation which is done to achieve the final shape of the product.

The inventors have found that, the forging operation in itself is a cross section reducing operations and has the potential of removing all the defects mentioned above. The casting defects like center looseness and porosities are removed by the deformation of the material at high temperature which eliminates these defects/discontinuities thus, leading to continuous material. To completely remove the defects, a certain level of deformation is required below which the defects will not be eliminated totally. Moreover, in the cast material, the size of defects (porosities and center looseness) depends on the size of casting. Bigger the casting, bigger are the defects. This is particularly so in the case of the materials those are cast using ingots.

As shown in Figures 6 and 7 the defects such as undesirably high porosity and looseness are present throughout the cast material as evident from the transverse and longitudinal cross sections taken from these materials. In the present invention, a smaller size of cast material was used which had smaller defects. This material was forged by a multistep process which assured the closing of the defects. The conventional forging process had 3 steps (See Figure 1 A). The cross section of the cast material was successively reduced in each step. It was found that the defects were closed in the first step itself. The next steps have further consolidated the material.

The analysis was done on a ConCast material of section RCS 160. During this, invention shows that the size of the center looseness observed was around 6-8 mm. FEM analysis and its correlation with actual plant level trials lead to the fact that for closing a center looseness of 6-8 mm, minimum 1.35 strain value (deformation) is required at its position. The forging process design has to take care of this criterion in order to produce perfectly consolidated structure in the crankshaft.

It is also found that the more complex the shapes, the better is the effect of closed die forging after eliminating the rolling step. As seen from the example of a crankshaft, the closed die forging is carried out in two stages - the blocker stage and the finisher stage. The defects are reduced further with each stage.

In the case of a complex part such as a crankshaft, a number of passes are required in the step of reduced roll preform making. The reduced roll preform is put through the blocker die. The output of the blocker die is subjected to the finisher die. At each stage the level of pressure applied and the time of the forging operation ensures that all defects are removed at the end operation.

Accordingly, the manufacturing process of invention uses a combination of casting and forging and by completely eliminating the step of rolling when the casting is of continuous type. In the case of continuously cast materials there's no need of cogging operation either, when using the process of invention. In the case where the casting is of ingot type, the step of cogging is retained but the step of rolling is eliminated.

The steel used for the manufacturing of the crankshaft is cast preferably using the continuous casting technology (hereafter referred to as ConCast material). The cross section of this ConCast material is maintained as per the requirement of the closed die forging operation.

Next, the ConCast material is cut into billets of required length. These cut billet are then forged using the process of invention to required shape and size.

These billets are used as an input to the hot forging process. In the closed die forging process itself, the material is deformed above re-crystallisation temperature. This operation has the same three effects on the material, i.e. breaking of dendrites, closing of porosity/distribution of segregation and grain flow.

Further to this, the forged part is then either heat treated or control cooled based on the material from which it is made. Generally, two types of steel are used for manufacturing of crankshaft namely heat treated steels and micro alloyed steel. Traditionally, machine and vehicle parts, requiring high strength and good fatigue resistance are produced by forging and then are heat treated to produce martensitic microstructures. These hardened microstructures are then tempered at different temperature as per strength and toughness requirement. These are called heat treated steels. But, nowadays for moderate high strength products, micro alloyed steels are used. The use of small amount of micro alloying elements such as vanadium, titanium, niobium etc., in micro alloyed steel, provides the comparable strength but at the expense of toughness. Any further heat treatment after forging is eliminated with the use of micro alloyed steel. Only cooling of the final part, under controlled conditions, give the desired properties.

Next, the forged part is machined. This gives the final part. In one embodiment the invented method thus comprises of the steps:

- Casting of metal using continuous casting technology (As shown in Figure 4)

- Hot forging of the as cast material consisting of - Cutting to required length

- Reduce rolling (Shown in Figure 1 A)

- Blocker operation (Shown in Figure 1 A)

- Finisher operation (Shown in Figure 1 A)

- Trimming

- Hot calibration

Heat treatment/control cooling.

Machining of the part consisting of

- Facing/centering.

- Rough turning.

- Finish machining.

- Oil hole drilling.

- Induction hardening.

- Final grinding/super finishing.

Fatigue testing and validation

The manufacturing of products through the hot forging route is an energy intensive process. This route requires the heating and processing of the metal multiple time (e.g. during casting, rolling, cogging, forging, heat treatment etc.). This makes it a "high carbon footprint" process. The technical advancement and economic significance of this invention lies in the following aspects. 1. The elimination of the rolling and or cogging process eliminates multiple heats required during this process. This drastically reduces the carbon footprint of the process.

2. The elimination of rolling process results into cost effective manufacturing.

3. It also reduces the overall cycle time of the complete manufacturing process i.e. casting to finished product.

4. This also eliminates the chances of the material getting rejected due to the rolling and/or cogging defect.

Example:

This investigation is aimed at testing the validity of the assumption that cast billet can be used directly as an input to forging operation without affecting fatigue properties of a crankshaft. The main difference between the regular forging process and this invention is the input material condition. Hence, the main objective of this Design Of Experiment (DOE) was to study the evolution of the casting defects in the cast billet through the whole forging process - machining process and finally its effect on the fatigue properties of the crankshaft. The following steps were decided for the DOE:

1. Selection of a product.

2. Manufacturing of the "As Cast" raw material. 3. Raw material testing of As Cast material and regular material.

4. Die and tooling design modification for changed raw material section.

5. Die and tooling manufacturing.

6. Forging trial.

7. Processing of parts (post forging operation).

8. Section analysis of RR preform, Blocker and Finisher.

9. Mechanical and metallurgical testing + grain flow of forged crankshafts.

10. Ultrasonic testing of forged parts.

11. Machining of crankshafts.

12. Quality check of machined crankshafts

13. Fatigue testing of the parts and failure analysis.

A six cylinder crankshaft was selected for this experiment. An 'As Cast billet of the type RCS 160' was finalized for experimental purpose. The tests that were carried out on the raw material were: Ultrasonic testing, Chemical composition, Gas analysis, Y segregation, Jominy end quench test, Step down test, Macro etching for centre looseness and Microstructure analysis. A four pass roll was used as a part of the reduce roll modification process. Blocker and finisher dies were modified to suit the selected part. A number of pieces were cut from the billet and a number of forging trials were carried out. The forged parts were subjected to the same tests as raw material. It was noted that the test results for forged parts were superior to results on the raw material. It is evident from the foregoing discussion that the present invention has the following embodiments.

1. A method of forging complex parts from continuously as cast billets, characterised in that said method comprises continuous casting of metal, using as cast billet as input to hot forging, heat treating or controlled cooling of forged material, machining into final shape.

2. A method of forging as disclosed in embodiment 1, characterised in that said hot forging step comprises:

- cutting the continuously cast billets to required length,

- carrying out a reduce rolling operation,

- carrying out a blocker operation,

- carrying out a finisher operation,

- carrying out a trimming operation,

- carrying out a hot calibration.

3. A method of forging as disclosed in embodiments 1 and 2, characterised in that said heat treating comprises of hardening and tempering while controlled cooling step comprises cooling the final products in controlled conditions.

4. A method of forging as disclosed in embodiments 1 to 3, characterised in that said complex part is a crankshaft. 5. A component manufactured using a method of forging as disclosed in any of embodiments 1 to 4, characterised in that said forging is carried out from as received continuously cast billets having inherent defects. 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.