The forging die is the tooling to achieve the die-forging process, which is one of the key factors in carrying out die-forging production.
Die belongs to consumable accessories, the failure of the die refers to the phenomenon of losing the use function during the specified life.
The service life of the die refers to the number of parts produced during the period from the time that is put into use until the normal consumption loss.
Premature failure of molds will not only cause production pauses, but also increase costs, affect the market competitiveness of products, and reduce the economic benefits of enterprises.
Therefore, how to give full play to the performance of the die material, how to improve the quality and service life of the forging die, and how to reduce the production cost of the die is a key concern of the forging industry.
This article mainly discusses the main forms of forging die failure and its causes, and find some effective ways to improve the service life of the forging die.
The manifestation of hot forging die failure
Hammer forging dies and machine forging dies are hot forming dies used on free forging hammers, die forging hammers and presses, which are typical hot work dies that bear both mechanical and thermal loads during the working process.
The mechanical load is mainly impact and friction, and the thermal load is mainly alternate heating and cooling.
Forging dies work in the above complex conditions, its failure form is also complex: wear and cracking of the cavity part, thermal fatigue (thermal cracking) and plastic deformation of the cavity surface.
Figure 1 shows the various failure modes prone to die at different positions of the forging die cavity.
Figure 1 Different positions of forging die failure in the cavity
According to the data in Fig. 2, among the main failure modes, the probability of wear is about 68%, cracking is about 24%, plastic deformation (collapse) is about 3%, and thermal cracking is about 2%.
Figure 2 Proportion of various main failure modes of forging die
Various typical failure form and causes analysis of hot forging die
Wear and tear
The surface characteristics of hot forging die when wear occurs are shown in Figure 3.
Figure 3 Surface wear morphology of forging die
Under the dual action of mechanical load and thermal load, on the one hand, the blank and the cavity surface produce impact contact stress; on the other hand, the high-speed flow of the blank and its oxide skin and the cavity surface produce strong friction.
As a result, wear tends to occur on the projecting rounded corners and the flash groove bridge of the die as shown in Figure 1.
Wear is related to factors such as die material, blank type and forging process.
Lowering the forging temperature (which increases the deformation resistance of the blank) will dramatically increase die wear, and the explosion caused by the combustion of the oil-based lubricant confined in the gap between the die and the blank will cause a kind of corrosive wear.
Therefore, hot forging die wear is usually associated with the following nine factors:
1) The overheating caused by the long contact time between the blank and the cavity surface under high-pressure load.
(2) Incorrect heat treatment to obtain low-strength metallurgical organization.
3) Insufficient cooling lubrication.
4) Excessively low die hardness.
(5) Excessively low forging (the blank) temperature.
(6) Insufficient forging steps.
(7) The die does not have sufficient venting holes.
(8) The complexity of the cavity.
(9) Surface treatment of the die.
The corresponding countermeasures to improve the hot wear problem for the above reasons are as follows.
(1) Minimize the contact time between the blank and the cavity surface under high-pressure load.
2) The correct heat treatment process, such as suitable austenitizing temperature, high quenching cooling rate as possible, and avoiding surface decarburization etc.
(3) Appropriate increase in die hardness while ensuring toughness.
(4) During the forging process, it ensures that the temperature of the blank is within the forging temperature range, and in particular, to avoid the temperature of the blank being lower than the final forging temperature.
(5) Reasonable arrangement of forging stations.
(6) The die should be arranged with exhaust holes.
(7) Nitriding treatment should be done on the die surface.
The morphological characteristics of forging die cracking are shown in Figure 4.
Figure 4 Morphological characteristics of forging die cracking
According to the nature of the cracking, it can be divided into early brittle cracking and mechanical fatigue cracking.
Early brittle cracking of die generally occurs when the die is first used.
It happens when the number of hammers is small, sometimes fracture occurs after only one hammer blow.
The morphological characteristics of the fracture are that it starts from the source of the fracture, and the crack expands outward in a herringbone pattern.
The mechanical fatigue cracking of the die is the fracture that occurs after the mold has been subjected to multiple forging strokes.
Its macro and micro-fractures also have the characteristics of general fatigue fractures, but the crack extension zone on macroscopic fractures is generally smaller.
The causes of die cracking are summarized in seven main categories, shown as followed:
(1) Die overloading (such as the too low temperature of the processed material).
(2) Mold preheating temperature is too low or not preheated.
(3) Excessive coolant/lubricant.
(4) Improper structural design of the mold causing stress concentration.
(5) Low metallurgical quality of the die material or defective forging quality.
(6) Heat treatment defects and die machining defects.
(7) Incorrect installation of the tooling, etc.
All the above factors can induce crack initiation and lead to early fracture and mechanical fatigue fracture.
Figure 5 The influence of different heat treatment processes on the structure and properties of the die (the die steel grade is ASSAB 8407, high-grade H13 steel)
Figure 5 shows the effect of different cooling rates on the impact toughness and microstructure of hot work steel during vacuum quenching.
When the cooling rate is insufficient, on the one hand, the martensite content is reduced.
On the other hand, a large number of carbides are precipitated on the grain boundaries, which sharply reduces the impact toughness of the material and increases the risk of die cracking.
In order to prevent die cracking, it should be prevented as shown in Figure 6, the appearance of the white layer of electrical discharge machining (EDM).
The ductility of the EDM white layer is very poor, which can lead to cracking.
When nitriding, the presence of an overly thick nitride layer and veined nitrides can also severely reduce the toughness of the die.
The effect of nitride layer depth on toughness and the microstructure characteristics of vein-shaped nitrides are shown in Figure 7 and Figure 8, respectively.
In summary, the corresponding countermeasures to improve the die cracking problem are as follows：
1) To avoid die overload, for example, the blank temperature should be within a reasonable range to ensure a low enough deformation resistance.
2) Die should be correct preheating (150 ~ 200 ℃), which can improve the die toughness and reduce the thermal stress of the die.
Figure 6 EDM white layer morphology
Figure 7 Depth of the nitriding layer on the impact of steel impact toughness die
Fig. 8 Microstructural features of the vein-like nitrides of the nitriding layer
(3) Reasonable die design to increase the radius of round corners as much as possible, reasonable arrangement of porosity and flash and the use of inserts structure etc.
(4) Take correct and effective cooling measures to avoid excessive thermal stress on the surface.
(5) Select high quality and high toughness of the mold material.
(6) Correct quenching and tempering heat treatment and surface treatment, especially to avoid over-nitriding.
(7) Avoid EDM white layer residues and rough tool surfaces (e.g. deep tool marks).
Thermal fatigue cracks (cracking)
The morphological characteristics of the die cavity surface thermal fatigue cracks (cracking) is shown in Figure 9.
Figure 9 Thermal fatigue crack morphology characteristics on die cavity surface
The so-called “thermal fatigue” refers to the fatigue cracks and failures produced by the die under the repeated action of cyclic thermal stress as shown in Figure 10.
There are 7 main causes of thermal fatigue (cracking), shown as followed:
1) Over-cooling on the mold cavity surface.
2) Improper cooling.
Figure 10 Working temperature and thermal stress distribution on the cavity surface.
(3) Improper selection of coolant/lubricant type.
(4) The mold cavity surface temperature is too high.
(5) Inadequate preheating of the mold.
(6) Improper selection of mold material.
(7) Heat treatment defects and surface treatment defects.
The corresponding countermeasures to improve thermal fatigue (cracking) are as follows.
(1) Avoid surface tempering and softening caused by excessively high cavity surface temperature, thereby avoiding the decline of thermal fatigue resistance of the die.
2) Take correct and effective cooling measures, on the one hand to avoid excessive surface thermal stress, on the other hand to avoid surface tempering softening.
3) Correct selection of mold preheating temperature, which is generally recommended 150 ~ 200 ℃, avoids too high or too low preheating temperature.
4) Choose die material with high quality and excellent toughness.
(5) Correct heat treatment process (such as suitable austenitizing temperature and quenching cooling rate as high as possible, and full tempering) to avoid excessive thick nitride layer and vein nitride when nitriding.
Plastic deformation (collapse)
Figure 11 Topographic features of plastic deformation in hot forging die
When the forging die is locally subjected to working stress that exceeds the yield strength of the die material, plastic deformation occurs.
Figure 11 is a typical morphological feature of plastic deformation caused by severe tempering and softening of the surface due to excessively high die cavity surface temperature.
Plastic deformation often occurs in the die cavity in the force and heat temperature rise parts, such as ribs, camber and other protruding parts.
Under the action of the high temperature of the blank and the temperature rise caused by friction during the deformation process of the mold cavity (higher than the tempering temperature of the mold), the yield strength of the mold material is reduced, and a softened layer is formed on the surface.
In the deeper part of the softened layer, plastic deformation phenomena such as the collapse of edges and corners or depressions in the deep cavity will occur.
The main causes of plastic deformation of the forging die occur as followed:
1) Excessive low blank temperature (resulting in excessive flow stress of the workpiece material).
2) Improper selection of die steel materials, for example, die steel thermal strength is insufficient.
3) The die temperature is too high.
4) The improper heat treatment process, for example, die hardness is too low.
The corresponding countermeasures for improving plastic deformation are as follows:
1) Blanks should be heated to a suitable starting forging temperature and ensure that the temperature of the blank is not lower than the final forging temperature during the forging process.
2) Select the die material with higher high-temperature strength and temper resistance.
3) Die should avoid excessive preheating temperatures and cavity surface temperatures during forging.
4) Use the correct heat treatment process to increase the die hardness appropriately.
The main failure modes of forging dies include wearing and cracking of the cavity part, thermal fatigue (thermal cracking) and plastic deformation of the cavity surface.
By discussing the main forms of failure of forging dies and analyzing the causes of failure, this article proposes some solutions to prevent forging die failure and hopes that it will serve as some reference for forging manufacturers.