接触热点浅析
发布时间：2020-06-23 09:12
<p style="margin-top: 0px; margin-bottom: 0px; padding: 0px; max-width: 100%; clear: both; min-height: 1em; color: rgb(51, 51, 51); font-family: -apple-system-font, BlinkMacSystemFont, "Helvetica Neue", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei UI", "Microsoft YaHei", Arial, sans-serif; font-size: 17px; letter-spacing: 0.544px; text-align: justify; white-space: normal; background-color: rgb(255, 255, 255); line-height: 1.75em; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; color: rgb(123, 12, 0);"><strong style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 100%; font-size: 16px; box-sizing: border-box !important; overflow-wrap: break-word !important;">一、接触热节的产生</span></strong></span></p><p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">人们较多地注意到的是冒口的补缩作用，而往往忽视对铸件热干扰的副作用。长期以来， 毫不怀疑地将冒口安放在铸件的几何热节处，如冒口安故在板件的中央、杆件的中部、轮缘和 轮辐的交接处等，使本来就大的热节更大了。以平板铸件为例，如图1-5。</span></section></p><p style="text-align: center;"><img src="https://www.luliaohui.com/uploads/home/article/5ef1564209f18.png"/></p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">设平扳壁厚为δ热节圆直径亦为δ。在平板中心安放一个直径为D高为h的圆柱形顶冒口，从几何结构上看，冒口与铸件接触处的热节圆直径为d,,则dy随冒口直径D值变化如表1-2。</span></section><p><section><br/></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">考虑到冒口颈处尖角砂被加热，该处冷却缓慢，加上冒口中金属液补缩流动带入的热量，实际上接触处的热节圆要比图1-5.表1-2所示的值还要大。冒口和铸件接触处这个被加大了的热节称为接触热节 （Junction-Effect of Neighbourhood）,如图1-5所示。它大于铸件原来的几何热节，在一定限度 内也大于冒口直径。取图1-5中平板铸件的边长为130mm,厚度δ为25mm,改变圆柱形冒口直径，在湿砂型条件下浇注灰铸铁铁液，用热电偶测蛍平板铸件几何中心、冒口几何中心以及铸件和冒口联接处（冒口根）的凝固时间如表1-3。</span></section></p><p style="text-align: center;"><img src="https://www.luliaohui.com/uploads/home/article/5ef1565e4fbfe.png"/></p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">可以看出，冒口根部的凝固时间比铸件的凝固时间长，也比冒口的凝固时间长，实际上是整个铸件一冒口系统的最后凝固单元，此处即为接触热节。在传统的冒口设计中，収冒口直径为铸件壁厚（或几何热节）的1.5倍，意在让冒口晚于铸件擬固，达到顺序擬固。实际上接触热节成为最后凝固区域。加大冒口尺寸，又会形成更大的T形接触热节.当铸件薄壁处发生收缩时，首先通过胃口根部从冒口中抽吸金属液。当冒口凝固之后，受冒口热影响而延长了凝固时间的冒口根部一方面本身收缩;另一方面还要供应相邻部位的补缩液。这就是铸铁件冒口根部常产生缩孔、缩松的原因。</span></section><p><section><br/></section><section><br/></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; color: rgb(123, 12, 0);"><strong style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 100%; font-size: 16px; box-sizing: border-box !important; overflow-wrap: break-word !important;">二、中性热节</span></strong></span></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">在铸件上安放冒口，在几何上一定会增加接触热节。另一方面，考虑到金属的导热能力大于型砂，从物理上，冒口中的金属散热，会减小接触热节，在增加和减少之间，存在这样一个冒口尺寸；在几何上增加热节的部分，正好被物理上加速散热减小热节部分所抵消，这个尺寸为中性尺寸，此时的冒口和铸件接触处的热节为中性热节，此时的冒口（或冒口颈）称为中性棒体，如图1-6,</span></section></p><p style="text-align: center;"><img src="https://www.luliaohui.com/uploads/home/article/5ef15679b7f6b.png"/></p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; color: rgb(123, 12, 0);"><strong style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 100%; font-size: 16px; box-sizing: border-box !important; overflow-wrap: break-word !important;">三、接触热节的防止和消除</span></strong></span></section><p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">接触热节是由冒口安设引起的，为了防止和消除接触热节，对冒口的位置、类型及大小提出如下几点要求：</span></section><section><br/></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">1.冒口不要放在铸件的几何热节上，冒口应该离开铸件的几何热节，以防止其对铸件的热干扰，同时又要靠近热节，保证对铸件的补缩。</span></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;"><br/></span></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">2.用侧冒口代替顶冒口，如压边冒口、飞边冒口、耳冒口等,避免采用圆柱形、方形和腰圆形顶冒口。</span></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;"><br/></span></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">3.在冒口下面、侧面的铸件处安放冷铁，冒口提供补缩，冷铁消除接触热节，如图1-7.</span></section></p><p style="text-align: center;"><img src="https://www.luliaohui.com/uploads/home/article/5ef1569568b06.png"/></p><p><br/></p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">4.将冒口颈做成中性棒体尺寸。</span></section><p><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;"><br/></span></section><section><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;">5.从中性尺寸开始进一步缩小冒口颈，加大冒口体。</span></section></p><p><span style="margin: 0px; padding: 0px; max-width: 100%; box-sizing: border-box !important; overflow-wrap: break-word !important; font-size: 16px;"><br/></span></p><p><br/></p>
