A review of the degradation mechanisms of the hot forging tools

archives of civil and mechanical engineering 14 (2014) 528–539

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Review

A review of the degradation mechanisms of the hot forging tools Z. Gronostajski, M. Kaszuba *, M. Hawryluk, M. Zwierzchowski University of Technology Wroclaw, Institute of Production Engineering and Automation, Łukasiweicza 5 Street, 50-371 Wrocław, Poland

article info

abstract

Article history:

The mechanisms of the degradation of hot forging tools and several mathematical models

Received 12 June 2014

for the theoretical evaluation of them are described. Examples of abrasive wear, oxidization,

Accepted 28 July 2014

thermomechanical fatigue and plastic deformation and the interdependences between

Available online 30 August 2014

them, based on the authors' research, are provided. According to the presented research the commonly accepted view that abrasive wear is the dominant mechanism in the

Keywords:

degradation of the dies in hot forging is highly dubious. The effect of each of the above

Wear

phenomena on the life of forging dies is generally considered separately and there is no

Forging

holistic description of the physical wear process, which would cover all the phenomena

Tool durability

simultaneously. In reality, the degradation phenomena occur simultaneously and interact with each other. # 2014 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

1.

Introduction

Die forging is currently the most advanced forging technique used in the mass production of critical parts. It has several obvious benefits, but is not devoid of drawbacks, the most serious of which is the low durability of the forming tools [1,5– 8]. It is estimated that the costs of the tools may amount to as much as 8–15% of the total production costs. Actually, if the time needed to replace the worn out tooling and the cases when the tools unexpectedly fail are taken into account, the costs may increase by as much as 30–50%. Moreover, tool wear significantly contributes to deterioration in the quality of the produced forgings. The most common forging defects caused by tool wear are die cavity filling errors, i.e. under-filling, laps, burrs, distortions, scratches, delamination and micro- and

macrocracks. The defects affect the functionality of the end product made out of the forging. Because of the high market competition, manufacturers of die forged products continually reduce their costs and improve the quality of the forgings, whereby they are very much interested in the low tool durability problem [3,8]. Tool durability (life) is defined in several ways. In production terms, the durability of a tool is usually expressed by the number of forgings which can be performed with this tool to obtain products of desired quality. According to this definition the average tools durability can change in very wide range from 2000 to 20,000 pieces. In tool terms, durability is associated with degradation and so it is defined as the ability to withstand degradation phenomena [9]. In the paper mainly second definition is considered.

* Corresponding author. Tel.: +48 713202164. E-mail address: [email protected] (M. Kaszuba). http://dx.doi.org/10.1016/j.acme.2014.07.002 1644-9665/# 2014 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

archives of civil and mechanical engineering 14 (2014) 528–539

The great number and variety of degrading factors having a bearing on the life of forging tools, and their mutual interactions make an analysis of the problem very difficult [8,10,11,15]. In the literature on the subject one can find much information on degradation phenomena, which are variously classified. According to the statistics provided by many authors, forging dies are taken out of service because of: the loss of their dimensions due to abrasive wear – 70% [1,7,16,19– 21] and plastic deformation – 25% of the forging dies, fatigue cracks and for other reasons (structural and material defects or faulty heat and thermochemical treatment) – only 5%. Many of the phenomena often occur simultaneously and the correlations between them depend mainly on the design of the tools, the conditions in which they are forged and made, the heat treatment of the tool material, the shape of the preform and the slug, etc. [1,8,10].

2.

Fig. 3.1 – Schematic showing places where dominant types of wear occur in forging die cross section.

Forging die operating conditions

During hot die forging the tools are subjected to the action of the three main degrading factors: intensive thermal shocks, cyclically variable mechanical loads and very high pressures at high temperature [15,22]. In order to reduce yield stress during the hot forging of steel products the formed material is heated up to a temperature of 1000–1200 8C. At the instant when the material is being deformed the temperature of the tools in their surface layer may reach 800 8C, which is followed by intensive cooling, whereby the tools are exposed to large temperature gradients. In the die cross-sectional plane the difference between the temperature on the surface and the temperature in the nearsurface region may amount to a few hundred degrees. Warm die forging is conducted at lower temperatures than hot forging, e.g. the temperature of the steel being formed amounts to about 900 8C. This means that the tool heat loads resulting from the cyclic heating and cooling of the tool surface are not so large as in the case of hot forging. Despite this, the tools used in semi-hot forging processes have a rather short life due to the combination of cyclically variable tool temperatures and large mechanical loads resulting from the deformation of the cooler, and more hard, material [2,4].

3.

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Degradation mechanisms in forging tools

The service life of forging tools depends mainly on their design, and make, the heat treatment of the tool material, the conditions in which they are forged, the shape of the preform and the slug, etc. [8]. In the literature on the subject one can find much information on the degradation phenomena involved. These mechanisms are variously classified [8,23]. Research has shown that the following wear mechanisms occur in the surface layer of forging tools: abrasive wear, thermomechanical fatigue, plastic deformation, fatigue cracking, adhesive wear and oxidation [24]. The shape of the tool working impression, determining the contact time, the pressures, the friction path and the changes in temperature, has a bearing on the rate of occurrence of the

particular degradation mechanisms. Fig. 3.1 shows the places in the cross section in which the particular mechanisms dominate. In the flat areas where the duration of contact between the tool and the hot material being shaped is the longest and where the greatest pressures occur, thermomechanical fatigue is the dominant degradation mechanism. The inner radii of the roundings are the places where cyclic tensile stresses produced by the external loads arising as the forging is being formed tend to concentrate. As a result, fatigue microcracks, developing into large cracks in the course of tool service, appear in these places. The outer radii in the die impression and the areas where the die impression passes into the flash bridge are the places where due to material weakening under high temperatures the yield point of the material is lowered, which results in plastic deformation. The intensive flow of the formed material in these areas causes abrasive wear, which is further intensified by the hard oxides (acting as an abrasive material) formed during the high-temperature oxidation of the surfaces of the tool and the forging [12,13].

3.1.

Abrasive wear

Abrasive wear is the result of material loss, mainly through material separation from the surface. This occurs when there are loose or fixed abrasive particles or protruding irregularities (formed of harder material) on the surfaces of the interacting parts (Fig. 3.2a) [1,17,18,23,27]. In the case of forging tools, which are much harder than the material being formed, such a mechanism occurs when abrasive particles are present in the areas of contact between the tool and the material being formed (Fig. 3.2b). Abrasive wear can be intensified by the hard oxides formed during the high-temperature oxidation of the surfaces of the forging and the die and small particles torn from the surface of the die (Fig. 3.3a). As a result of this mechanism, grooves form along the direction in which the material being formed shifts. Their depth and shape depend mainly on the forging conditions. The protrusions (groove ridges) (Fig. 3.3) are particularly susceptible

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Fig. 3.2 – Mechanism of abrasive wear [27].

Fig. 3.3 – Typical abrasive wear of forging die: (a) intensifying particle abrasive wear, (b) area where die impression passes into flash bridge after 4300 forging cycles.

to abrasion and are quickly removed from the tool's surface in the course of its further service, which results in material loss and in a change in the tool geometry [23,27,29]. Particularly susceptible to this kind of wear are the places where the longest sliding distance of the material being formed occurs. Most often these are the outer radii in the die impression and the areas where the die impression passes into the flash bridge.

3.1.1.

Abrasive wear models

For years experiments have been conducted on special test stands (attempting to model most accurately the actual forging conditions) in order to understand and describe the wear phenomena involved. Moreover, several mathematical models for the theoretical evaluation of wear have been formulated. One of the most popular such models (constituting the basis for most of the relevant equations) is the Archard model (3.1) [14]. Most of the research concentrates on developing and improving this model. The main investigative method consists in conducting tribometer tests. The model describes the abrasive wear resulting from the contact between two bodies sliding against each other: W ¼K

FS H

(3.1)

where W – wear, K – a coefficient of wear, F – the normal force, S – the sliding distance, H – hardness.

The model assumes that the wear of a given element is directly proportional to the normal force and the sliding distance and inversely proportional to the hardness of the material of which it is made. Dimensionless coefficient K is an experimentally determined quantity characteristic of each of the materials. Theoretically its value is constant, but because of the kind of the investigated process, it is in a range of 7  103  7  106 (1.3  104 for tool steel) depending on the type of materials being in contact and the presence of a lubricant [28,30]. The Shaw model represents another approach to the description of abrasive wear. In this model the magnitude of wear is correlated with the amount of energy dissipated as a result of friction: B m ¼ PL u

(3.2)

where B – a wear modulus, P – the normal force, L – the sliding distance, m – a coefficient of friction, u – specific wear energy. The above equation describes the amount of energy needed to produce a particle resulting from wear under the given pressures, sliding distance and the magnitude of friction between the sliding elements [28]. There also exist Archard model expansions specifically developed to describe the wear of dies in the forging process. Such equations as the ones formulated by Kang or Bahrens

archives of civil and mechanical engineering 14 (2014) 528–539

531

Fig. 3.4 – Adhesive wear mechanism [27].

define the magnitude of wear after each forging process instance, taking into account not only the effect of temperature on the hardness of the material, but also the decrease in tool hardness depending on temperature impact duration [1,9,14,16,37,44].

3.2.

Adhesive wear

Adhesive wear occurs in surface layer plastic deformation microareas, especially where there are surface irregularities. It usually occurs under high pressures and at relatively low velocities during interaction between similar materials or materials showing chemical affinity (typical forging process conditions). Under high pressures the material being formed shifts on the tool surface, removing the oxide coating covering the surface of the forging and that of the tool and exposing clean surfaces. This happens mainly in areas of projecting surface irregularities (surface roughness peaks). When the materials in these places are brought close together so that interatomic forces begin to act, local metallic bonds form between the surfaces. Then as a result of the further mutual displacement of the surfaces the bonds are destroyed. The plastic deformations of the surface layer in the places with

such bonds contribute to this process. The breaking of the bonds results in the separation of metal particles which tend to be smeared on the surfaces. The material loss accompanying adhesive wear is often ascribed to abrasive wear. The adhesive wear mechanism is shown in Fig. 3.4. The size of the broken off particles depends mainly on the forging process parameters and the properties of the surface layer of the tool and that of the forging. An example of adhesive wear of the die used in the second operation of the forging of the CV universal joint housing is shown in Fig. 3.5. The temperature of the material formed in this process amounts to about 900 8C, i.e. it is much lower than that in typical hot forging processes, favouring this type of wear. Adhesive wear can manifest itself in material sticking to and being smeared on the tool surface below the cross section reduction radius (Fig. 3.5c).

3.3.

Oxidation

Oxidative wear consists in the degradation of the surface layer of metal parts due to the separation of oxide coatings formed as a result of oxygen absorption. Oxygen diffusion takes place in metal microvolumes being elastically and plastically

Fig. 3.5 – Example of adhesive wear: (a) view of forging die working surfaces, (b) microarea, (c) scan of working surface.

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Fig. 3.6 – Chips in oxide coating on forging die surface after 1850 forging cycles.

deformed while films of solid solutions form. Oxidative wear is considered to occur when the intensity with which oxide coatings form is higher than that with which the surface is degraded by abrasion (Fig. 3.6) [16,23]. In hot forging processes high-temperature oxidation often occurs on the surfaces of the tool and the forging, whereby scale forms on them. In the hot forging temperature range the oxide coating (scale) is formed by three kinds of iron oxides: wustite FeO, magnetite Fe3O4 and haematite Fe2O3 [20,39].

3.3.1.

Effect of oxides on wear

The way in which the surface of the forging and that of the tool interact depends much on the degree of their oxidation and the properties of the scale. Depending on the oxidation temperature, different kinds of oxides with different properties form [16,32]. At temperatures below 560 8C mainly hard haematite (Fe2O3) and magnetite (Fe3O4) form, wustite FeO being unstable at these temperatures. When during forging the scale separates from the surface of the tool or that of the forging it acts as an abrasive material on the tool surface, accelerating its wear [32]. Above 570 8C multilayered scale (consisting of all the three iron oxides) forms (Fig. 3.7). Evidence of this, the analysis of the chemical composition of the scale showed that its Fe and O content ranges from 55/55% to 5/95% (Fig. 3.8) [39]. As the temperature rises to 700 8C, the percentage of soft wustite FeO in the scale increases at the expense of the other oxides. As a result, in a temperature range of 700–910 8C the scale is made up of mainly wustite. Such scale can function as a wear reducing solid lubricant. Above 910 8C haematite and magnetite reappear in the scale [31–34]. The hardness of the particular iron oxides is shown in Table 3.1. Regardless of the properties of the oxides in the scale, the oxide coating contributes to tool surface cracking. The kinetics

Fig. 3.7 – Cross section of scale taken from material of forging, steel 20 HG [43].

Fig. 3.8 – Linear distribution of oxygen and iron content in sample shown in Fig. 3.7 [43].

of the oxidation of the surface affect the rate of cracking and the morphology of the cracks. Cracks in the oxide coating often propagate into the surface layer of the tool (Fig. 3.9). Moreover, the cracks appearing on the surface become filled with oxides which act as a wedge and accelerate the development of the cracks (Fig. 3.10).

3.4.

Mechanical fatigue cracking

Mechanical fatigue cracking of the forging tools occurs due to the accumulation of strains in the surface layer, caused by the cyclic stresses generated by the external loads arising as the forging is being formed. As a result, fatigue cracks appear in places of strain concentrations and develop in the course of further tool service (Fig. 3.11) [9,27].

Table 3.1 – Hardness of iron oxides [33]. Oxide Hardness (HV)

FeO

Fe3O4

Fe2O3

270–300

420–500

1030

archives of civil and mechanical engineering 14 (2014) 528–539

533

Fig. 3.9 – Network of cracks in oxide coating, which propagate into material.

Crack resistance depends on the amplitude of the strains – a higher amplitude will result in life reduction (Fig. 3.12), as described by the Coffin–Manson equation (3.4).

3.5.

Thermal and thermomechanical fatigue

Thermomechanical fatigue is a kind of wear in which a local loss of cohesion and the resulting material loss are caused by material fatigue due to the cyclic action of stresses (generated by high pressures, i.e. mechanical loads and temperature gradients) in the forging die surface layer [9,15,22,25,26,34,35]. Due to the intensive thermal cyclic loads, produced by the alternate heating and cooling of the tool surfaces, the material is alternately tensioned and compressed whereby thermal stresses arise resulting in a network of cracks. This form of degradation is referred to as thermal fatigue (it can be treated similarly as mechanical fatigue). In addition, the presence of

Fig. 3.10 – Crack filled oxides after 7044 forging units.

cyclically variable mechanical loads leads to fatigue processes, which intensify as the network of cracks caused by thermal fatigue appears, resulting in macrocracks. Since the mechanisms of thermal fatigue and those of mechanical fatigue are mutually dependent, they considered jointly as thermomechanical fatigue [15] (Fig. 3.13).

3.5.1.

Thermal fatigue model

The thermo-mechanical fatigue has their source in mechanical fatigue. Even though there exist theoretical models of thermal fatigue, so far they have not been successfully applied to predict the lifetime of forging dies. Most of the thermal fatigue theories are based on the case of a beam with fixed ends, whose length during heating and cooling remains constant [38,41,42]. In the course of alternate heating and cooling the beam undergoes successive cycles of deformation (Fig. 3.14).

Fig. 3.11 – Fatigue crack in forging die corner.

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Fig. 3.12 – Schematic hysteresis loop (a) and fatigue life depending on amplitude of total strain ea, plastic strain eapl and elastic strain eas (b, logarithmic scale) [9].

Fig. 3.13 – Network of thermomechanical cracks after 550 forging units.

In the case of an ideal thermal cycle, the whole thermal strain is converted into elastic or plastic deformation (3.3). ð1  #ÞDs e p ¼ aDT  E

(3.3)

Plastic deformation can be used to determine the number of cycles until failure from the Coffin–Manson formula (3.4). Nnf e p ¼ Ce f

(3.4)

where Nf – the number of cycles until failure, ep – plastic deformation, ef – a coefficient of resistance to fatigue, defined as material deformation needed for failure to occur in a single cycle, C and n – material constants. In the real process, not the whole thermal strain is converted into mechanical deformation. Some of the thermal strain is converted into free deformation of the tool working surface (3.5). eT ¼ aDT  ð 2 e  2 p Þ ee ¼

ð1  #ÞDs E

Fig. 3.14 – Schematic diagram of thermal fatigue [42].

(3.5) (3.6)

Hence ep ¼

aDT  ð1  #ÞDs E  eT

(3.7)

where ep – plastic deformation, aDT – total thermal strain, (1  y)Ds/E – elastic deformation, eT – free deformation of the working surface. After a substitution into the Coffin–Manson formula one gets:  1=n Ce f (3.8) Nf ¼ aDT  ð1  #ÞDs=ðE  eT Þ In the case of forging, the situation is even more complex since one should take into account the plastic deformation resulting from the mechanical loading of the die. The great pressures occurring on the surface affect the

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Table 3.2 – Yield points of selected grades of hot work tool steel at different temperatures [15]. Grade of steel (steel symbol acc. to PS) 1.2343 1.2344 1.2365

Yield point Re (MPa) 25 8C

450 8C

600 8C

1570 1750 1570

1080 1040 1080

470 540 640

plastic deformation of the material, reducing or intensifying fatigue (depending on the area) in each cycle. The thermal fatigue cycle of forging tools can be divided into two stages. The first thermal shock occurs when the material is being formed. Not only the surface layer material is compressed, but also the deformation is intensified by the mechanical loading (high pressures). The second thermal shock occurs during cooling. The material in the surface layer is tensioned, which under the thermomechanical loading quickly results in a network of cracks. In the case of the hot forging at temperature over 1100 8C, which is very common for steel forging with lubrication of tools every step, the authors observed the primary network of thermomechanical cracks after forging 500 pieces (Fig. 3.13) After next 2000 pieces the intensive growth of primary network occurred and new secondary smaller network appeared. The material near to the fracture is softer and is more prone to abrasive wear and plastic deformation (Fig. 4.1).

3.6.

Plastic deformation

The weakening of the material caused by high temperatures results in the lowering of the material's yield point, which combined with mechanical interactions leads to the plastic deformation of the die impression in the particularly susceptible areas. If plastic deformation does not occur, stress cannot exceed the yield point of the tool material in any point of the tool (3.9). s < Re

(3.9)

where s – stress (MPa, MN/m2), Re – the yield point (MPa).

535

Table 3.2 shows the yield point values at the temperatures of 25 8C, 450 8C and 600 8C for popular hot work tool steels used for forging dies [15]. In hot forging processes the temperature of the surface layer of the tools may reach 800 8C while the stresses generated by external loads can be as high as 1000 MPa, which indicates that plastic deformations are highly likely to occur in forging dies (Fig. 3.15) [15].

3.6.1.

Change in tool hardness during operation

The susceptibility of the tool's surface layer to plastic deformations increases with its service life and depends on its hardness [1,9,16]. Therefore data on how tool hardness changes as a result of the spontaneous tempering which occurs during contact with the hot material are very useful for predicting the possible occurrence of plastic deformations [36,37,41]. The tempering mechanism depends on the diffusion of carbon and alloying elements and it is closely connected with temperature and time. The effect of tempering temperature and time on the hardness of steel is expressed by the Holloman–Jaffe parameter M [39,40] (10): M ¼ TðC þ logtÞ

(3.10)

where T – tempering temperature on the Kelvin scale, K; t – tempering time in s; C – a constant dependent on carbon concentration. The tempering is usually correlated with hardness and tempering temperature at a constant tempering time, determined in laboratory conditions. Diagrams showing the dependences for the particular grades of tool steel can be found in the technical literature and in material specification charts. In real conditions forging tools under high temperature are exposed to the action of high forging pressures, which can additionally contribute to the decrease in hardness due tempering. The present authors made an attempt to determine the change in hardness of tools' steel (Unimax) in industrial

Fig. 3.15 – Plastic deformation in surface layer of die during hot forging after forging 1850 pieces.

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Fig. 3.16 – Change in die hardness as function of forging temperature and time.

process conditions of forging constant velocity joints (initial temperature of billet is 940 8C with lubrication at every step). Using tool microhardness measurements and temperature distribution and contact time data, a relation describing the change in tool hardness as a function of industrial forging process parameters was determined (Fig. 3.16). The diagram was plotted for the forging die area in which the largest changes in hardness were observed. The relation shown in Fig. 3.16 describes the change in hardness as a function of forging temperature and time in a zone extending down to 4 mm from the working surface of the studied dies. The temperature was measured at a distance of 4 mm from the working surface by means of a thermocouple placed in a specially prepared channel and was found to amount to 350 8C. The temperature on the working surface and 2.5 mm down from this surface was determined by numerical modelling. The respective temperatures were 650 8C and 450 8C. The forging time was determined on the basis of the single-cycle forging time (0.117 s) multiplied by the number of cycles worked by the investigated dies. An analysis of the changes in hardness as a function of forging temperature and time has shown that temperature has a greater influence on the change in hardness than time. The decrease in hardness with tool service life is slight. On the surface where the temperature reaches 650 8C, the most marked decrease takes place in the 1500–2000 s interval of the forging time, amounting to about 250 HV. At 450 8C and 350 8C hardness remains practically at the same level in the whole range of the analyzed tool service lifetime.

4.

development. The decrease in hardness of the tool surface layer caused by the long contact with the hot material lowers its abrasive wear resistance and increases its susceptibility to plastic deformations and thermomechanical cracking. Moreover, the rate of occurrence of the particular degradation mechanisms changes as the process parameters change and varies between different places on the tool, which determines the contact time and the changes in temperature [14,43]. Fig. 4.1 shows the hot forging die working surface with a visible network of thermomechanical cracks and abrasive wear traces in the form of grooves. Also plastic deformations of the surface layer are visible. This is evidence of the interdependence of the degradation mechanisms. Most of the reported research on the mechanisms of the degradation of hot forging tools determine abrasive wear as main mechanism, whereas the authors' studies and their long

Recapitulation of degradation mechanisms

The effect of each of the above phenomena on the life of forging dies is generally considered separately and there is no holistic description of the physical wear process, which would cover all the phenomena simultaneously. In reality, the degradation phenomena occur simultaneously and interact with each other. Abrasive wear is intensified by the presence of the hard oxides formed as a result of the oxidation of the tool surface. Moreover, the oxides by filling fatigue cracks accelerate their

Fig. 4.1 – Forging die working surface with network of thermomechanical cracks and abrasive wear traces after forging 1850 pieces.

archives of civil and mechanical engineering 14 (2014) 528–539

537

Fig. 4.2 – Diagram of development of the various degradation mechanisms.

experience relating to the durability of forging tools and degradation mechanisms show that the mutual interactions between the all phenomena abrasive wear, thermomechanical fatigue, plastic deformations, fatigue cracking, adhesive wear and oxidation make determination of dominant mechanism too difficult. There are a lot of parameters including shape of die, hardness, contact time, sliding distance, material die, billet temperature, lubrication, etc. which intensify or limit the degradation mechanisms; however, in authors' opinion the cooling by lubrication and temperature are the most crucial. In the most popular forging of steel in the closed die carried out at temperature over 1100 8C with additional cooling by lubrication, the thermomechanical fatigue is the most adverse factor. It results very quickly in fine cracks (Fig. 4.2a). The further development of the cracks depends on the process parameters. The interaction between the die and the forging and the rate of material flow result in a secondary network of cracks extending over the whole contact surface (Fig. 4.2b). Moreover, small pieces of crushed scale separated from the forging and the tools clearly intensify the wear (Fig. 4.3), whereby grooves consistent with the direction of material flow form in the area of the original network of cracks (Fig. 4.2c). Another factor having a significant bearing on the intensity of the particular degradation mechanisms is the decrease in the hardness of the tool surface layer as a result of its contact with the hot material.

In the case of forging at temperature over 1100 8C without additional cooling by lubrication (this is very popular method of open die forging for preparing preform for closed die forging) the thermal shock is much lower and then the weakening of the material caused by high temperatures results in the lowering of the material's yield point, which combined with mechanical interactions leads to the plastic deformation of the die impression in the particularly susceptible areas as well as lager abrasive wear due to lower hardness of tempered material (Fig. 4.4). If the forging temperature is decreased the amount of plastic deformation diminishes and the abrasive wear becomes more important which could be intensified by thermomechanical fatigue in the case of lubrication application. This is also in warm forging process in which the forging temperature of 900 8C (Fig. 4.5). In the light of the research results the commonly accepted view that abrasive wear is the dominant mechanism in the degradation of the dies in hot forging is highly dubious.

Fig. 4.3 – Small pieces of crushed scale separated from the tools.

Fig. 4.4 – Plastic deformation and abrasive wear after forging 6900 pieces.

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deformations occur concurrently from the very beginning of the forging process and in the given conditions they can only be more or less intensive. There are a lot of parameters including shape of die, hardness, contact time, sliding distance, material die, billet temperature, lubrication etc. which intensify or limit the degradation mechanisms; however, in authors' opinion the cooling by lubrication and temperature are the most crucial. The schemas of degradation mechanisms of the die are proposed for different temperatures and lubrication.

Acknowledgment This research was carried out as part of project NCN 2011/01/B/ STB/02056

references

Fig. 4.5 – Network of thermomechanical cracks after forging 8700 pieces.

In the paper mainly definition of die durability associated with degradation mechanisms is considered. The problem of durability in production terms (the number of operations which can be performed with this tool to obtain products of desired quality) is also connected with the degradation mechanism but there is no clear correlation between them. The die hard worn out can be used if the final product is correct.

5.

Summary

The authors' studies and long experience relating to the durability of forging tools and their degradation mechanisms show that a large number of different factors have a bearing on the durability of the tools. Because of the mutual interactions between the factors the problem is very difficult to analyze. The research done so far has shown that the main mechanisms responsible for the degradation of the tools in warm and hot forging processes are: abrasive wear, thermomechanical fatigue, plastic deformations, fatigue cracking, adhesive wear and oxidation. Most of the reported research on wear deals with abrasive wear, whereas our studies have clearly shown that in forging processes thermomechanical fatigue, very quickly resulting in fine cracks, is the most adverse factor. The further development of the cracks depends on the process parameters, the interaction between the die and the forging, the rate of material flow and in most cases results in a secondary network of cracks extending over the whole contact surface. The commonly accepted view that abrasive wear is the dominant mechanism in the degradation of the dies in hot forging is highly dubious. All the mechanisms, such as thermomechanical cracking, abrasive wear and plastic

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