JPH0130913B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a processing method for improving the mechanical characteristics of a crankshaft for an internal combustion engine. In order to achieve higher output from internal combustion engines, it is necessary to improve the strength of the crankshaft to withstand the higher output. Conventionally, methods such as hardening and tempering the entire crankshaft, or surface hardening treatments such as induction hardening and nitriding have been adopted, but in the former case, machinability during subsequent machining The deterioration of the surface roughness causes a decrease in productivity, and the deterioration of the surface roughness causes a decrease in strength. On the other hand, in the latter case, processing time becomes longer, resulting in lower productivity and higher production costs. An object of the present invention is to provide the above-mentioned improved processing method capable of eliminating the above-mentioned problems.The present invention aims to provide an improved treatment method capable of eliminating the above-mentioned problems. It is characterized in that it is cold-pressed so that it becomes more than one-fold, and then the entire crankshaft is subjected to nitrocarburizing treatment. The table below shows various steel types.

【table】

[Table] Examples of steel types to which the present invention can be applied to obtain the desired effect are carbon steels for mechanical structures such as JIS S43C to S48C, new carbon steels, and spheroidal graphite cast iron represented by JIS FCD50. New carbon steels C to F have the following essential chemical components:
C0.30~0.60%, Cr0.50% or less, Al0.20% or less,
It is necessary to contain V0.30% or less. C is an essential component for the core strength of the crankshaft and surface hardening during nitrocarburizing, and if it is less than 0.30%, the hardness will be insufficient and sufficient strength cannot be achieved.
If it exceeds 0.60%, the core hardness increases and machinability decreases. Cr and Al have the property of improving soft-nitriding properties, but when Cr exceeds 0.50% and Al exceeds 0.20%, the hardening depth increases and the straightening ability such as strain relief decreases. V is added to the medium carbon steel base and causes uniform precipitation hardening on the inside and outside through appropriate air cooling treatment after hot forging, and achieves hardness equivalent to that obtained without heat treatment such as quenching and tempering. It is an effective ingredient in obtaining Therefore, when roll processing is applied as a cold pressing process to the stress concentration area of the axial portion of the crankshaft, further work hardening can be obtained, and the material itself has excellent resistance to temper softening during subsequent nitrocarburizing treatment. Due to its nature, if the processing temperature is 600°C or less, the hardness of the entire crankshaft including the core will not decrease. However, when V exceeds 0.30%, toughness decreases. In addition, the above essential chemical components include the following groups (a) ~
Chemical components selected from (c) may be added alone or in combination. (a) If one or two chemical components selected from Ti, Zr, and Nb+Ta are added at a concentration of 0.30%, strength can be improved by utilizing precipitation hardening, but if the content exceeds 0.30% Soft-nitriding property decreases. (b) 1.5% each of Si, Mn, Cu, Mo, W, and Co
If less than 1% is added, the base can be strengthened, but if it exceeds 1.5%, the nitrocarburizability will decrease. (c) S0.15% or less, Pb0.30% or less, Te 0.10% or less,
Adding less than 0.30% Se improves machinability, but
When each chemical component exceeds the above value, toughness decreases. Comparison in the table Carbon steel is inferior in either strength or machinability and is therefore unsuitable as a constituent material for crankshafts. Using gray cast iron such as FC35 and spheroidal graphite cast iron such as FCD50 is not only advantageous in terms of forming method and material cost compared to forming the crankshaft by forging using carbon steel, but also has advantages in cold forming. It was confirmed that a cast iron crankshaft that had been subjected to pressing and nitrocarburizing was superior in terms of strength compared to a forged crankshaft that had only been subjected to cold pressing. This is because in cast iron, graphite is agglomerated at the same time as work hardening due to cold pressing, thereby improving the nitriding quality during the subsequent soft nitriding treatment and reducing the notch sensitivity due to graphite. However, if the tensile strength is less than 40Kg/ ^mm2 , the strength of the cast iron itself is too low even if it is low-priced, and it cannot meet the required characteristics for the high output of internal combustion engines. It must be 40Kg/mm2 or ^more . For this reason, spheroidal graphite cast iron such as FCD50 is optimal. Figure 1 shows a crankshaft 1 manufactured using steel type A in the table.The crankshaft 1 is made by heating a steel bar with a diameter of 80 mm obtained by rolling to 1200°C for forging, and then forging it on a conveyor. Air-cooled,
Furthermore, in order to make the internal and external hardness and structure uniform, the material was normalized at 900°C for 60 minutes. Stress concentration points in the axial portion of the crankshaft 1, namely the crank main shaft portion 2 and the crank pin portion 3
An annular recessed portion 5 is formed in the fillet portion of the pulley shaft portion 4 by roll processing as cold pressing processing. In this roll processing, both ends of the crankshaft 1 are centered and supported on a support member, and then each fillet is pressed into contact with a roll and recessed from the state shown by the chain line in Fig. 2 to the state shown by the solid line. It is something to do. Figure 3 shows the hardness distribution in the depth direction of the fillet part after roll processing, where line x ₁ indicates the pressure force of the roll when no roll processing is performed, lines x ₂ to _{x 4} indicate the pressure force of the roll of 430, 600, 600, 820 Kg/mm ² , and as is clear from FIG. 3, by performing roll processing, the hardness increases significantly from the surface to a predetermined depth. In this case, if the pressing force of the roll increases, the hardness will also increase, but if the pressing force exceeds 800 kg/ ^mm2 , abnormal surface pressure will cause "peeling" and the strength will vary, which is not desirable. . The table below shows the relationship between the machining depth of the fillet portion, surface roughness, and crankshaft curvature. For example, in the case of the crankshaft 2, each machining depth is expressed as the ratio t/D of the machining amount t (Fig. 2) to its diameter D. This is expressed by the deflection of the crank main shaft section 2 at the center in the supported state.

[Table] As is clear from the table, the machining depth is 1/100~
If it is in the range of 1/30, the surface roughness of the roll will be transferred and a good surface roughness of about 0.8μ can be obtained, but if the machining depth is 1/150, the surface roughness of the fillet part will be transferred. It was found that some parts remained and the strength improvement effect was poor. On the other hand, the bending of the crankshaft tends to increase as the machining depth increases.
If it is set to 30, a runout of about 0.16 mm will occur, and the polishing allowance will increase partially in the next polishing process.
This causes problems such as the need for special consideration when removing bends after soft nitriding, which is disadvantageous in terms of productivity and quality assurance. Therefore, the appropriate machining depth is in the range of 1/100 or more and less than 1/30. The table below shows that a crankshaft similar to that shown in Figure 1 was manufactured using steel type D in the table through hot forging and forging air cooling treatment on a conveyor, and the fillet portion of the crankshaft was manufactured with a pressing force of 150 and 200, respectively. ,400,800,
Each crankshaft was rolled at a rate of 900Kg/mm ² and then subjected to nitrocarburizing treatment at 580°C for 60 minutes using a salt bath, and the strength improvement ratio and surface flaws of each crankshaft were investigated. The strength was evaluated using a servohydraulic fatigue tester to apply a repeated bending load to each crankshaft at a load level of N=1.0×10 ⁷ times. In addition, the strength improvement ratio is steel type A or B in the table.
A crankshaft similar to the one shown in Fig. 1 manufactured using the same method as shown in Fig. 1 was subjected to only the same nitrocarburizing treatment as described above without being subjected to roll processing, and was then subjected to the same fatigue test as described above, and the load level obtained from this was used as a reference. Calculated.

[Table] As is clear from the table above, the pressing force is 200
Below Kg/ ^mm2 , for example 150Kg/ ^mm2, the strength improvement ratio is as low as 5% and the effect is poor, while at 800Kg/ ^mm2
If it exceeds 900 kg/mm ² , for example, "peeling" will occur, and the notch effect of this "peeling" will cause variations in strength, resulting in a low strength improvement ratio. Therefore, if the pressing force is in the range of 200 to 800 Kg/mm ² , the strength improvement ratio will increase to 20 to 40%, and there will be no surface scratches, so a high quality product can be obtained. A pressing force of 400 kg/kg is applied to the fillet part of the as-cast crankshaft, which is cast using steel type K in the table above.
When roll processing of mm ² was performed and then soft nitriding treatment similar to the above was performed, a strength improvement ratio of 20% was observed, but no change in strength was observed when only roll processing was performed. In addition, in the case of steel type J, no change in strength was observed between when roll processing and nitrocarburizing were performed and when only roll processing was performed. Figure 4 shows the case in which the fillet portion of a crankshaft obtained through the same process as above using steel type D in the table above is subjected to roll processing, or is subjected to the same nitrocarburizing treatment in the next step without roll processing. shows the hardness distribution in the depth direction of the fillet part, where line y ₁ is when no roll processing is applied, and lines y ₂ to _{y 4} are when the pressing force of the roll is 250, 430, and 600 kg/mm ² , respectively. , FIG. 4 shows that when rolling and nitrocarburizing were combined, the hardness was improved compared to the case without rolling. In addition, when cold pressing and soft nitriding are combined, it is generally believed that the residual stress obtained by cold pressing disappears during the soft nitriding, which can also be called heat treatment, resulting in a decrease in strength. The reason why the strength is improved in the invention depends on the characteristics of the material itself, but when nitrogen diffuses into the part that has been subjected to cold pressing, recrystallization of that part is hindered, which reduces residual stress. It is thought that one reason is that it becomes difficult to disappear. As described above, according to the present invention, by applying cold pressing to the stress concentrated portion of the shaft-like portion of the crankshaft, and then performing nitrocarburizing treatment to the entire part,
It is possible to obtain a crankshaft with high strength and excellent wear resistance that can sufficiently withstand the high output of an internal combustion engine. Furthermore, by employing cold pressing and soft nitriding, the time required for these processes can be shortened, productivity can be improved, and production costs can be reduced.

[Brief explanation of drawings]

Figure 1 is a front view of the crankshaft, Figure 2 is an enlarged sectional view of the part indicated by the arrow in Figure 1, Figure 3 is a graph showing the relationship between the distance from the fillet surface after cold pressing and hardness, and Figure 2 is a graph showing the relationship between hardness and distance from the fillet surface after cold pressing. FIG. 4 is a graph showing the relationship between the distance from the fillet surface and hardness when nitrocarburizing treatment is performed after cold pressing. 1... Crankshaft, 2, 3... Crank main shaft portion as a shaft-shaped portion, crank pin portion.

Claims (1)

[Scope of Claims] 1. A cold pressing process is applied to a stress concentration area of a shaft-like part of a crankshaft formed from iron material so that the machining depth is 1/100 or more of the diameter of said shaft-like part. 1. A method for improving the mechanical properties of a crankshaft for an internal combustion engine, the method comprising: applying nitriding to the entire crankshaft. 2. The method for improving mechanical properties of a crankshaft for an internal combustion engine according to claim 1, wherein the cold pressing process is roll processing performed under conditions of a pressing force of 200 kg/mm ² or more. 3. The method according to claim 1 or 2, wherein the iron-based material is carbon steel containing 0.30% or more of C, 0.50% or less of Cr, 0.20% or less of Al, and 0.30% or less of V. A treatment method for improving the mechanical properties of a crankshaft for an internal combustion engine.