Distortion: Who Holds the Key?

wcdi

Distortion plagues both engineer and heat treater alike, for no one can heat treat metal – steel in particular – without incurring distortion. When it comes to distortion, the sentiment is typically:
It is always the heat treaters fault.
The heat-treatment process must be flawed because the steel always distorts at heat treatment.
Don’t the heat treaters know what they are doing?
These are statements that the heat treater hears almost on a daily basis from either his client or the plant engineer. Whichever way one cares to look at the problem of distortion, it will always manifest itself at the treatment process.

Granted, there can be flaws in the heat-treatment process as there can be with any manufacturing process. But when the heat treater receives the steel component from the client, he has had no control over the machining and manufacturing techniques.

Any time that one manipulates a piece of steel from the mill in any way, shape or form, stresses are introduced into the steel, which will become residual stress. Any time that any one of the following machining and manufacturing practices are conducted, stress is induced:
Forging
Extrusion
Drawing
Bending
Fabrication
Turning
Milling
Grinding
Once induced stress is resident in the steel, the only effective way of removing it is by the application of heat. So, no matter how good the heat-treatment procedure is, the steel will be stress relieving itself on the ramp-up to the process temperature and manifest in the form of distortion.

Even if the heat treater applies all the care in the world to the steel, he is faced with the problem of phase changes that will occur as the steel comes up to an austenitizing temperature, for example. The phase changes that will occur are: ferrite to austenite and austenite to martensite (when the steel is cooled rapidly).

When ferrite changes to austenite, there will be a volumetric size change. This means that the steel component is no longer the pre-heat-treatment size. Once at the austenite (hardening) temperature and after the appropriate temperature soak followed by the quench (rapid cooling), a further volumetric change will occur to martensite.
Ferrite is a nine-atom structure, and it is known as a body-centered cubic (BCC) lattice structure.
The austenite structure forms as a result of the decomposition of ferrite followed by the regrouping of the atoms into a 14-atom structure, which we know as austenite. So, once again, there is a volumetric change and consequently a size change. The structure of austenite is face-centered cubic.
On the rapid cooling from the 14-atom structure of austenite, steel is cooled down rapidly to form the phase of martensite. Martensite is a nine-atom structure but now with a completely different shape – tetragonal. This lattice structure is known as body-centered tetragonal.
It can be seen that even the best shops in the world cannot heat treat without initiating a phase change and stress relieving, both of which will lead to distortion

yao0452

谁掌握关键点

hznsmx

sometimes distortion is good for production. the key is that we can forecast what distortion is going to be.

sswpyq594596

ferrite changes to austenite

microsoft

没有人翻译啊？我先翻译几句。
变形像瘟疫一样缠绕着工程师和热处理者。没有一个人能在无变形情况下热处理钢铁，当说起变形时，一下观点总是有代表性的：
变形总是热处理者的责任。
热处理过程一定是有缺陷的，因为热处理时总是产生变形。
难道热处理者不知道他们正在做什么吗？

microsoft

热处理方几乎每天能从客户或工厂工程师听到这些陈述，无论如何关心变形问题，再热处理过程总是显现出来。
热处理过程确实能产生缺陷，因为零件存在一些机加工工序。当热处理方从客户哪里接收了工件，但是热处理方无法控制加工和制造技术。

microsoft

无论何时，不论使用什么方法加工一个钢铁零件改变其形状或结构，应力被引入到工件中，就变成了残余应力。无论何时实施以下的制造和加工工序，应力将被诱导：热锻，挤压，冷拔，弯曲，组装，研磨，车削，磨加工

microsoft

一旦钢铁中产生了残余应力，最有效的方法是加热。无论怎样好的热处理过程，在温度上升过程中应力将得到释放，同时也产生了变形。

microsoft

即使热处理方用尽全力，他面对的是相变问题，当温度接近奥氏体化温度时将出现相变。例如：铁素体转变为奥氏体，奥氏体转变成马氏体（当钢的冷速做够快时）。当铁素体转变为奥氏体时，将发生一个比容的变化。这意味着钢铁的尺寸不是热处理前的尺寸，一旦达到奥氏体温度以及有足够的保温后，被淬火（快冷），将出现比容变化更大的马氏体转变。铁素体是由9个原子组成，是体心立方晶格。

microsoft

奥氏体是由铁素体分解，然后重新组合成14个原子的晶格组织，这就是我们所知道的奥氏体。因此，这里存在一个容积改变和尺寸变化。奥氏体组织结构是面心立方晶格。当有14个原子组成的奥氏体被急冷，钢铁被快速地转化为马氏体组织。马氏体组织是由9个原子组成，完全是一个不同的四方形结构，晶格结构是体心立方结构。
世界上不存在没有相变和残余应力的热处理，这两方面因素就产生了变形。

fdjzzu

顶一下，帖子很好，楼上的翻译也很到位。