为什么是锻件？
Why Forgings?

　　与铸件比较，锻件阀门的阀体具有更加均匀结构，更好的密度，更好的强度完整性，更好的尺寸特性，和更小的尺寸误差。定向构造（管线）在整个强度和应力方面都比铸件具有更高的性能。（图1和2）
　　When compared with castings, forged valve bodies offer the advantages of more uniform structure, greater density, higher strength integrity, enhanced dimensional characteristics and closer dimensional tolerances. The directional structure (flowlines) is superior from an overall strength and stress standpoint against castings. (Figures 1 & 2)

高强度
High Strength

　　热锻造促进在结晶和晶粒细化，使得材料能够达到最大可能的强度和一致性，并且件与件之间的变异最小。颗粒流精密地沿着阀体轮廓流动，这些连续的流线有利于减少疲劳或常见故障的发生率。
　　Hot forgings promote recrystallization and grain refinement allowing the material to develop maximum possible strength and uniformity with a minimum variation from piece to piece. The grain flow closely follows the outline of the body and continuous flow lines decrease the susceptibility for fatigue or common failures. (Figure 2)

结构完整性
Structural Integrity

　　锻造消除了内部缺陷，产生了连贯一致的金相组织，保证了优异的性能。在应力和晶体内腐蚀问题严重的地方，锻件都能够保证较长的使用寿命和无故障服务。
　　Forging eliminates internal flaws and produces a coherent and uniform metallurgical structure assuring optimum performance. Where stress and intragranular corrosion are a problem, a forging will assure long life and trouble-free service.

可靠性
Reliability

　　能够满足设计结构要求的锻件性一直是锻件最重要的优点之一，在某种程度上位于上述特性之首。
　　The ability of forgings to meet design requirements consistently is one of the most important advantages and takes into account all the preceding characteristics to some degree.

在尺寸和金相方面的一致性
Dimensional and Metallurgical Uniformity

　　闭模锻造的尺寸一致性造成关键壁厚的完全控制，避免了铸造工艺中铁心移位造成的缺陷。
　　Dimensional uniformity of closed-die forgings results in positive control of critical wall thickness, eliminating deficiencies caused by shifted cores in castings.

　　通过优质，无分离钢锭和1万至3万吨压力机的冲击力保证了没有内部缺陷的、一致的金相结构。
　　A uniform metallurgical structure without internal flaws is assured by (a) quality, segregation-free billet and (b) high impact forces achieved on 10,000 - 30,000 ton presses.

图1：高应力弯钩区域的连续流径线。
Figure 1: Continuous flow lines in highly stressed crotch area.

锻件的质量保证
Quality Assurance of Forgings

　　通过锻件的使用，以及其一致性和高质量，对于1类铸造部件的X射线需求就消失了。美国海军在核潜艇和航空母舰上使用锻件作为部件时也持同样的态度。对于锻件成品，ASME法典的全部要求就是超声检验（U.T.），磁粉（M.T.）或液体渗透试验（P.T.）。通过U.T., M.T.,或P.T.方法发现的报废短见，是非常罕见的。部件的采购，其交货期是可以控制的，因此阀门的交货也就更可靠。
　　Through the use of forgings, with their uniformity and high quality, the radiographic requirement for comparable Class 1 cast components is eliminated. The same attitude has been taken by the United States Navy when using forgings for and other components for nuclear submarines and aircraft carriers. All that is required by the ASME Code is ultrasonic examination and magnetic particle or liquid penetrant testing in the finished condition. Rejections of forgings for inherent deficiencies found by U.T., M.T., or P.T. methods are rare. Procurement of parts, lead times can be controlled, resulting in more reliable valve deliveries.

铸件和锻件阀体的定向构造比较
Comparison of Directional Structure of Cast & Forged Bodies

图2
Figure 2

　　锻件在温度变化很大的环境下，其蠕变的抗疲劳强度比铸件高出3倍多。
　　Fatigue Resistance to Creep Under Temperature Fluctuation is More Than Three Times Better for Forgings

　　温度频繁变化的环境下，表面应力的计算公式：
　　The formula for calculating surface stress during frequent temperature fluctuations is:

　　对于100^oF的热冲击，F22和F91在T_m = 400^oF下的平均值是：
　　For a thermal shock of 100^oF, avarage values for F22 and F91 at T_m = 400^oF are: