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精密模鍛的變形溫度和精度

一、精密模鍛的變形溫度
 
1、 Deformation temperature of precision forging
 
加熱是精密模鍛工藝中的重要環(huán)節(jié)。精密模鍛經(jīng)常在高溫和中溫下進行。
 
Heating is an important link in precision forging process. Precision forging is often carried out at high and medium temperatures.
 
1.高溫精密模鍛
 
1. High temperature precision die forging
 
要防止加熱時毛坯表面產(chǎn)生嚴重的氧化與脫碳，可以在少、無氧化敞焰加熱爐或其夠有效控制氣氛的加熱爐中加熱。要防止熱鍛件冷卻過程中發(fā)生二次氧化，可在有保護的裝置中進行冷卻；也可以把鍛件有次序地分放在有格子的砂箱中，當需要緩慢冷卻時，司把鍛件放在熱砂箱或石棉粉中冷卻。
 
In order to prevent serious oxidation and decarburization on the surface of the blank during heating, it can be heated in the open flame furnace with little or no oxidation or the furnace with enough effective atmosphere control. In order to prevent secondary oxidation during cooling of hot forgings, cooling can be carried out in a protected device; forgings can also be placed in a grid sand box orderly. When slow cooling is required, the forging can be placed in a hot sand box or asbestos powder for cooling.
 
2.中溫精密模鍛
 
2. Medium temperature precision die forging
 
降低加熱溫度，可以防止強烈的氧化與脫碳，只要具有足夠的塑性和適當?shù)淖冃慰沽Γ部梢赃_到精密模鍛的目的。中溫精密模鍛就是在未產(chǎn)生強烈氧化的溫度范圍內(nèi)，加熱并進行模鍛的方法。如圖8—4所示是45 8鋼的高溫強度極限和高溫氧化性能曲線。可見，強度極限從500℃開始急劇下降，到600度降為室溫時的一半。600度以上強度極限較低，都可進行模鍛成形。所以45 8鋼在600 875℃范圍內(nèi)，既有較低的變形抗力，又無強烈氧化，可使鍛件達到較高的尺寸精度和表面光潔程度。在溫鍛條件下，模具工作溫度一般在400度左右，高速鋼比較適合用作溫鍛模具材料。
 
Reducing heating temperature can prevent strong oxidation and decarburization. As long as it has enough plasticity and proper deformation resistance, it can also achieve the purpose of precision forging. Medium temperature precision die forging is a method of heating and forging in the temperature range without strong oxidation. As shown in Fig. 8-4, the high temperature strength limit and high temperature oxidation performance curve of 458 steel. It can be seen that the strength limit began to drop sharply from 500 ℃ to half of that at 600 ℃. If the strength limit is lower than 600 ℃, die forging can be carried out. Therefore, 458 steel in the range of 600875 ℃ has low deformation resistance and no strong oxidation, which can make forgings achieve high dimensional accuracy and surface finish. Under the condition of warm forging, the working temperature of the die is generally about 400 ℃. High speed steel is more suitable to be used as the material of warm forging die.
 
二、精密鍛件的精度
 
2、 Precision of precision forging
 
影響精密模鍛件精度的因素主要有：毛坯體積偏差、模具和鍛件的彈性變形、模具和毛坯(鍛件)的溫度波動、模具精度以及設備精度等。
 
The main factors affecting the precision of precision die forging are: the volume deviation of blank, the elastic deformation of die and forging, the temperature fluctuation of die and blank (forging), the precision of die and equipment, etc.
 
 
 
圖1 45 8鋼高溫強度極限及氧化層厚度
 
Fig. 1 high temperature strength limit and oxide layer thickness of 45 8 steel
 
設計鍛件圖時，只需要保證主要部位的尺寸精確，不需要也不應該要求所有的尺寸都精確。這是因為毛坯的尺寸以及成形中的許多因素不可能絕對準確地控制，而塑性變形是遵守體積不變條件的，必須利用某些部位的較寬松要求來調節(jié)這些誤差。
 
In the design of forging drawing, it is only necessary to ensure the size of main parts is accurate, and it is not necessary and should not require all dimensions to be accurate. This is because the size of the blank and many factors in the forming can not be absolutely and accurately controlled, and the plastic deformation is in accordance with the condition of constant volume, so it is necessary to use the looser requirements of some parts to adjust these errors.
 
1.毛坯體積偏差
 
1. Blank volume deviation
 
對于開式精密模鍛，毛坯體積偏差一般不影響鍛件的尺寸偏差。但在閉式精密模鍛中，假設模膛的水平尺寸不變和不產(chǎn)生飛刺，則毛坯體積偏差將引起鍛件高度尺寸的變化。
 
For open precision forging, the volume deviation of blank generally does not affect the dimensional deviation of forgings. But in closed precision die forging, if the horizontal dimension of die bore is constant and there is no spurs, the volume deviation of blank will cause the change of forging height dimension.
 
毛坯體積產(chǎn)生偏差的原因：一是下料不準確；二是加熱時各毛坯燒損程度不一致。因此，要提高鍛件精度，就要提高下料精度和改善加熱情況。目前精密下料可使毛坯的重量偏差控制在1％以內(nèi)(一般下料方法為3％～5％以上)。其次，在鍛件圖和工藝設計時，應根據(jù)毛坯體積可能的變化范圍采取適當?shù)拇胧＠缭龃箦懠骋环较虺叽绻睿换虿捎瞄_式模鍛使多余金屬流入飛邊槽；對某些帶孔的鍛件，可利用沖孔芯料來調節(jié)體積偏舞。
 
The reasons for the deviation of blank volume are as follows: first, the blanking is not accurate; second, the degree of burning loss of each blank is not the same during heating. Therefore, in order to improve the forging accuracy, it is necessary to improve the blanking accuracy and heating conditions. At present, the weight deviation of blank can be controlled within 1% by precise blanking (the general blanking method is more than 3% ~ 5%). Secondly, in forging drawing and process design, appropriate measures should be taken according to the possible change range of blank volume. For example, increase the dimensional tolerance of forging in a certain direction; or use open die forging to make the excess metal flow into the flash groove; for some forgings with holes, the punch core material can be used to adjust the volume deviation.
 
模膛的尺寸精度和使用過程中的磨損，對鍛件尺寸精度有直接影響。在模膛的不同部分金屬的流動情況和所受到的壓力不同，磨損程度也不同。
 
The dimensional accuracy of the die bore and the wear in the process of use have a direct impact on the dimensional accuracy of forgings. The flow and pressure of the metal in different parts of the die bore are different, and the wear degree is also different.
 
在開式模鍛中，模膛水平方向的磨損，會引起鍛件外徑尺寸增大和孔徑尺寸減小；模膛垂直方向的磨損，會引起鍛件高度尺寸增大。
 
In open die forging, the wear in the horizontal direction of the die cavity will cause the increase of the outer diameter size and the decrease of the aperture size of the forging; the wear in the vertical direction of the die cavity will cause the increase of the height size of the forging.
 
在閉式模鍛中，模膛磨損的影響如圖8-5所示，將引起鍛件水平方向尺寸L(D)增大。若毛坯體積不變，并且假設不產(chǎn)生飛邊、充填良好，鍛件高度尺寸H將減小。在這種情況下，鍛件高度尺寸公差△H不能由模膛高度方向的磨損來決定。在模具設計時，模具水平方向尺寸應該按照鍛件相應尺寸最小值設計，而模具高度方向尺寸應該按照鍛件相應尺寸最大值設計，這樣，當模具磨損達最大限度時，鍛件水平方向尺寸達到最大值，而高度方向尺寸達到最小值。
 
In closed die forging, the influence of die bore wear is shown in Figure 8-5, which will cause the dimension L (d) of forging to increase in horizontal direction. If the blank volume is constant, and if no flash is produced and the filling is good, the forging height dimension h will be reduced. In this case, the tolerance △ h of forging height dimension can not be determined by the wear in the height direction of die bore. In die design, the horizontal dimension of die should be designed according to the minimum value of corresponding dimension of forging, while the height dimension of die should be designed according to the maximum value of corresponding dimension of forging. In this way, when the wear of die reaches the maximum, the horizontal dimension of forging reaches the maximum value, while the height dimension reaches the minimum value.
 
 
 
圖2 閉式模鍛模膛磨損的影響
 
Fig. 2 Effect of wear on closed die
 
3.模具和鍛件的彈性變形
 
3. Elastic deformation of dies and forgings
 
精密模鍛時，模具和毛坯均會產(chǎn)生彈性變形。模膛受內(nèi)壓力作用而尺寸增大；毛坯則產(chǎn)生壓縮彈性變形。外力去除后，兩者發(fā)生彈性恢復，使鍛件尺寸變化。彈性變形引起的尺寸變化，可根據(jù)兩者材料的彈性模量、應力數(shù)值和相應部分的尺寸來計算，但最終通常是通過工藝試驗確定的。
 
In precision die forging, both die and blank will produce elastic deformation. The die bore increases in size due to internal pressure, while the blank produces compression elastic deformation. After the external force is removed, the elasticity of the two parts recovers and the forging size changes. The size change caused by elastic deformation can be calculated according to the elastic modulus, stress value and the size of the corresponding part of the two materials, but it is usually determined by process test finally.
 
4.其他因素
 
4. Other factors
 
毛坯的表面質量(指氧化、脫碳和表面粗糙度等)是實現(xiàn)精密模鍛的前提。設備的精度和剛度對鍛件的精度有重要影響。模具的影響更直接，如模膛的設計精度和冷縮量是否適當、模具溫度及其波動的預測、模具的剛度和導向精度等。有了高精度的模具，在一般設備上也可能成形出精度較高的鍛件。工藝操作是否符合規(guī)范，例如加熱溫度的偏差、潤滑情況影響到金屬充滿模膛的難易程度，從而引起鍛件尺寸的波動。
 
The surface quality of blank (including oxidation, decarburization, surface roughness, etc.) is the premise of precision forging. The accuracy and rigidity of the equipment have an important influence on the accuracy of forgings. The influence of die is more direct, such as whether the design accuracy and cold reduction of die bore are appropriate, the prediction of die temperature and its fluctuation, the rigidity and guiding accuracy of die, etc. With high precision die, forging with high precision can be formed on general equipment. Whether the process operation conforms to the specification, for example, the deviation of heating temperature and lubrication affect the difficulty of filling the die cavity with metal, thus causing the fluctuation of forging size.