一、精密模锻的变形温度

 

1、 Deformation temperature of precision forging

 

加热是精密模锻工艺中的重要环节。精密模锻经常在高温和中温下进行。

 

Heating is an important link in precision forging process. Precision forging is often carried out at high and medium temperatures.

 

1.高温精密模锻

 

1. High temperature precision die forging

 

要防止加热时毛坯表面产生严重的氧化与脱碳，可以在少、无氧化敞焰加热炉或其够有效控制气氛的加热炉中加热。要防止热锻件冷却过程中发生二次氧化，可在有保护的装置中进行冷却；也可以把锻件有次序地分放在有格子的砂箱中，当需要缓慢冷却时，司把锻件放在热砂箱或石棉粉中冷却。

 

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.中温精密模锻

 

2. Medium temperature precision die forging

 

降低加热温度，可以防止强烈的氧化与脱碳，只要具有足够的塑性和适当的变形抗力，也可以达到精密模锻的目的。中温精密模锻就是在未产生强烈氧化的温度范围内，加热并进行模锻的方法。如图8—4所示是45 8钢的高温强度极限和高温氧化性能曲线。可见，强度极限从500℃开始急剧下降，到600度降为室温时的一半。600度以上强度极限较低，都可进行模锻成形。所以45 8钢在600 875℃范围内，既有较低的变形抗力，又无强烈氧化，可使锻件达到较高的尺寸精度和表面光洁程度。在温锻条件下，模具工作温度一般在400度左右，高速钢比较适合用作温锻模具材料。

 

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、精密锻件的精度

 

2、 Precision of precision forging

 

影响精密模锻件精度的因素主要有：毛坯体积偏差、模具和锻件的弹性变形、模具和毛坯(锻件)的温度波动、模具精度以及设备精度等。

 

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.

 

 

 

图1 45 8钢高温强度极限及氧化层厚度

 

Fig. 1 high temperature strength limit and oxide layer thickness of 45 8 steel

 

设计锻件图时，只需要保证主要部位的尺寸精确，不需要也不应该要求所有的尺寸都精确。这是因为毛坯的尺寸以及成形中的许多因素不可能绝对准确地控制，而塑性变形是遵守体积不变条件的，必须利用某些部位的较宽松要求来调节这些误差。

 

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.毛坯体积偏差

 

1. Blank volume deviation

 

对于开式精密模锻，毛坯体积偏差一般不影响锻件的尺寸偏差。但在闭式精密模锻中，假设模膛的水平尺寸不变和不产生飞刺，则毛坯体积偏差将引起锻件高度尺寸的变化。

 

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.

 

毛坯体积产生偏差的原因：一是下料不准确；二是加热时各毛坯烧损程度不一致。因此，要提高锻件精度，就要提高下料精度和改善加热情况。目前精密下料可使毛坯的重量偏差控制在1％以内(一般下料方法为3％～5％以上)。其次，在锻件图和工艺设计时，应根据毛坯体积可能的变化范围采取适当的措施。例如增大锻件某一方向尺寸公差；或采用开式模锻使多余金属流入飞边槽；对某些带孔的锻件，可利用冲孔芯料来调节体积偏舞。

 

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过程中的磨损，对锻件尺寸精度有直接影响。在模膛的不同部分金属的流动情况和所受到的压力不同，磨损程度也不同。

 

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.

 

在开式模锻中，模膛水平方向的磨损，会引起锻件外径尺寸增大和孔径尺寸减小；模膛垂直方向的磨损，会引起锻件高度尺寸增大。

 

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.

 

在闭式模锻中，模膛磨损的影响如图8-5所示，将引起锻件水平方向尺寸L(D)增大。若毛坯体积不变，并且假设不产生飞边、充填良好，锻件高度尺寸H将减小。在这种情况下，锻件高度尺寸公差△H不能由模膛高度方向的磨损来决定。在模具设计时，模具水平方向尺寸应该按照锻件相应尺寸最小值设计，而模具高度方向尺寸应该按照锻件相应尺寸最大值设计，这样，当模具磨损达最大限度时，锻件水平方向尺寸达到最大值，而高度方向尺寸达到最小值。

 

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.

 

 

 

图2 闭式模锻模膛磨损的影响

 

Fig. 2 Effect of wear on closed die

 

3.模具和锻件的弹性变形

 

3. Elastic deformation of dies and forgings

 

精密模锻时，模具和毛坯均会产生弹性变形。模膛受内压力作用而尺寸增大；毛坯则产生压缩弹性变形。外力去除后，两者发生弹性恢复，使锻件尺寸变化。弹性变形引起的尺寸变化，可根据两者材料的弹性模量、应力数值和相应部分的尺寸来计算，但最终通常是通过工艺试验确定的。

 

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

 

毛坯的表面质量(指氧化、脱碳和表面粗糙度等)是实现精密模锻的前提。设备的精度和刚度对锻件的精度有重要影响。模具的影响更直接，如模膛的设计精度和冷缩量是否适当、模具温度及其波动的预测、模具的刚度和导向精度等。有了高精度的模具，在一般设备上也可能成形出精度较高的锻件。工艺操作是否符合规范，例如加热温度的偏差、润滑情况影响到金属充满模膛的难易程度，从而引起锻件尺寸的波动。

 

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.