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Die Life and Die Failure
Proper selection of the de material and of the die manufacturing technique determines, to a large extent, the useful life of forming des. Dies may have to be replaced for a number of reasons, such as changes n dimensions due to wear or plastic deformation, deterioration of the surface finish, breakdown of lubrication, and cracking or breakage. In hot impression die forging, the principal modes of die failure are erosion, thermal fatigue, mechanical fatigue and permanent (plastic) deformation.
In erosion, also commonly called die wear, material is actually removed from the die surface by pressure and sliding of the deforming material, wear resistance of the die material, die surface temperature, relative sliding speed at the die/material interface and the nature of the interface layer are the most significant factors influencing abrasive die wear. Thermal fatigue occurs on the surface of the die impression in hot forming and results in “heat checking”. Thermal fatigue results from cyclic yelling of the de surface due to contact with the hot deforming material. This contact causes the surface layers to expend, and, because of the very steep temperature gradients, the surface layers are subject to compressive stresses. At sufficiently high temperatures, these compressive stresses may cause the surface layers to deform. When the de surface cools, a stress reversal may occur and the surface layers will then be n tension. After repeated cycling in this manner, fatigue will cause formation of a crack pattern that s recognized as heat checking. Die breakage or cracking is due to mechanical fatigue and occurs in cases where the dies are overloaded and local stresses are high. The dies are subject to alternating stresses due to loading and unloading during the deformation process and this causes crack initiation and eventual failure.
Die life and de failure are greatly affected by the mechanical properties of the die materials under the conditions that exist in a given deformation process. Generally, the properties that are most significant depend on the process temperature. Thus, die materials used in cold forming processes are quite different from those used in hot forming.
The design and manufacture of dies and the selection of die materials are very important in the production of discrete parts by use of metal forming processes. The dies must be made by modern manufacturing methods from appropriate die materials in order to provide acceptable die life at a reasonable cost. Often the economy success of a forming process depends on die life and de costs per piece produced. For a given application, selection of the appropriate die material depends on three types of variables:
(a)Variables related to the process itself, including factors such as size of the die cavity, type of machine used and deformation speed, initial stock size and temperature, die temperature to be use, lubrication, production rata and number of parts to be produced.
(b)Variables related to the type of die loading, including speed of loading, i.e. impact of gradual contact time between dies and deforming metal (this contact time is especially important in hot forming), maximum load and pressure on the dies, maximum and minimum die temperatures, and number of loading cycles to which the dies will be subjected.
(c)Mechanical properties of the die material, including harden ability, impact strength, hot strength(if hot forming is considered)and resistance to thermal and mechanical fatigue.
譯文二：
模具的壽命與失效
正確的選擇模具材料和模具的制造技術(shù)，在很大程度上決定著成形模具的使用壽命。為著某些原因，模具可能不得不更換。例如，由于磨損或塑性變性而使尺寸發(fā)生改變、表面損壞、光潔度降低、潤滑故障和裂紋即破裂。在熱壓模緞中，模具失效的主要模式是腐蝕作用、熱疲勞、機械疲勞和永久性即塑性變形。
腐蝕，通常也叫做模具磨損，實際上模具由于受到壓力后模具表面上的材料發(fā)生剝落。變形材料的滑移、模具材料的抗磨性，模具表面溫度、模具和材料接觸表面的相對滑動速度以及接觸層的性質(zhì)，都是影響模具磨損的最主要的因素。]
熱成形加工中會發(fā)生熱裂效應(yīng)，熱疲勞都發(fā)生在模具模腔的表面。由于跟熱變形材料接觸，就在周期性屈服的模具表面引起了熱疲勞。由于溫度梯度的急劇變化，這種接觸引起的表面層的膨脹，而且表面層受到壓應(yīng)力的影響。在溫度足夠高的時刻，這些壓應(yīng)力可引起表面層的破壞。當模具表面冷卻時，可發(fā)生反向應(yīng)力，因而表面層將處于拉應(yīng)力狀態(tài)。這種狀態(tài)循環(huán)往復(fù)將引起形成龜裂的模面，那就是作為識別熱裂紋的特征。
模具破裂或產(chǎn)生裂紋是由于機械疲勞，并且是在模具過載和局部應(yīng)力高等情況下發(fā)生的。在變形加工過程中，由于加載、減載、模具承受交變應(yīng)力作用，這就將引起開裂并發(fā)生重大破壞。
在給定的成形工藝條件下，模具材料的機械性能對模具壽命和模具的損壞影響很大。一般而言，最具影響的性能是取決于加工過程的溫度。這樣，用于冷卻盛開加工工藝的模具材料與用于熱成形加工的材料有著極大的區(qū)別。
對于金屬成形加工工藝的小批、單件生產(chǎn)，模具的設(shè)計、制造和模具材料的選擇是非常重要的。為著提供成本合理和具有令人滿意的壽命的模具，必須用合適的模具材料和用現(xiàn)代的制造方法來制造模具。成形加工的經(jīng)濟效益常常是取決于模具壽命和所制造的每件模具的成本。根據(jù)上述應(yīng)用，合適的模具材料的選擇取決于以下三個方面的因素：
(a)與加工工藝本身有關(guān)的因素，包括模腔尺寸、所用機器形式和變形速度，毛坯尺寸和溫度，要用的模具溫度、潤滑、生產(chǎn)率和要生產(chǎn)的零件數(shù)量。
(b)與模具加載形式相關(guān)的因素，包括加載速度，即模具與正在變形的金屬之間的沖擊時間或逐漸接觸的時間（在熱變形加工中，這種接觸時間顯得特別重要），在模具上的最大載荷和壓力，最大和最小的模具溫度以及模具將要承受的加載周期的數(shù)目。
(c)模具材料的機械性能，包括硬度、沖擊強度、熱強度（如果考慮成形加工的話）和機械疲勞的性能。
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