Heat-Treatment Troubleshooting

wcdi

There are many aspects to the troubleshooting of heat-treatment problems. In fact, that may not be quite a true statement as there are many factors that contribute to heat-treatment problems. The problems are not necessarily created by the heat-treatment procedure; the problems can be created prior to heat treatment.

Given below are some of the contributing factors that can cause a problem to arise at the heat-treatment operation:
Steel chemistry
Steel quality (grain size, nonmetallic inclusions, oxides)
Hot rolling or forging practice
Temperature control at forging
Mechanical design (sharp corners, sectional thickness changes, keyways, threads, holes and many other design features)
Normalizing practice
Lack of intermediate stress relieving
Machining practice
Heat-treatment practice (lack of preheating, overheating, under tempering, not tempering quickly enough)
Focusing on the potential for heat-treatment problems, remember that as a heat treater you cannot stop the phase changes from occurring, which will result in a volumetric size change of the workpiece!

When heating to an austenitizing temperature for hardening, if preheating is not considered (especially for a tool steel), there will be a great potential for cracking and certainly for aggressive distortion to occur.

There is no magic formula for the temperature ramp up to the austenitizing temperature. One would need to look at the part complexity to determine how to ramp up to the austenitizing temperature. The more complex the part geometry, the slower the ramp up to temperature should be.

Overheating is another problem often encountered with austenitizing for hardening. High austenitizing temperatures can often be chosen by the heat treater to “be sure that the steel is truly in the austenitizing temperature range.” There are very significant problems with overheating, including:
Grain growth
Potential for retained austenite
Dissolution of alloy carbides at high temperature
Potential for quench cracking along grain boundaries
Potential for service failure
Problems at heat treatment can also be caused by non-metallic inclusions. These have the potential to lead to phosphorous segregation at the austenite grain boundaries as well as cementite particle precipitation also on grain boundaries.

Non-metallic phases such as sulfides, alumina and silicates can influence the fracture toughness of the steel in question.

Furnace temperature uniformity (particularly on tempering) will strongly influence hardness profiles. So, it is necessary to ensure that the furnace temperature control is both uniform and accurate. Temperature uniformity is extremely important during the tempering procedure. Remember that the thermocouple measures temperature only at the tip of the thermocouple. We assume that the thermocouple reading is that of the furnace. We can only safely make that assumption if there is an air circulation fan in the furnace.

Heat-treatment troubleshooting is a progressive “process of elimination.” It is necessary to start at the beginning of the life of the steel (in other words, the steel as it arrived from the mill) by checking the test certificate. The test certificate contains significant information that should be checked, including:
Analysis if possible (if not, proceed with the following information)
Mill hardness
Micro-cleanliness test for sulfides, etc. (ASTM E45 Method A)
Mill scale thickness and decarburization depth
Grain size
Hardenability (Jominy end-quench test)
It is not always necessary to check the analysis. If the steel does not conform to above reported values on the test certificate, there is cause for concern about that steel.

panpandada

free so translated it into our mother language for others‘ convenience.
I'm a  freshmen   
文章中的mill 是该翻成“轧制”吗？请多指教
排除热处理问题可从很多方面着手。事实上，因为影响热处理的因素众多，你那一点可能并不是造成热处理问题的真正原因。问题可能并不来自热处理过程中，问题可能由热处理前造成。
以下是可能造成热处理问题的某些因素：
钢材化学成分
钢材质量（晶粒度、非金属夹杂物、氧化物）
热轧或锻加工
锻加工的温度控制
机械设计（尖角、截面厚度变化、键槽、螺纹、孔及许多其他设计因素）
正火
缺少中间应力释放环节
机加工
热处理（缺少预热、过热、不充分回火、回火过慢）
当您专注于潜在的热处理问题是，谨记作为一名热处理工您不能阻止导致工件体积变化的状态改变的发生。
当为硬化（淬火）而加热到奥氏体化温度后，如果不考虑预热处理（尤其对于工具钢），将潜在巨大的裂纹风险并必然发生极具变形
还不存在魔方般的奥氏体化温升曲线，将视工件的复杂程度决定如何爬升奥氏体化的温度曲线。工件越浮渣，温升要越慢。
过热也是奥氏体化经常遇到的一个问题。热处理工经常选用较高的奥氏体化温度来“确保钢确在奥氏体化温度范围内”。过热会产生一些重大问题，包括晶粒生长
潜在剩余奥氏体
合金碳化物在高温下分解
潜在沿晶界的淬火裂纹
潜在寿命失效
因非金属夹杂物也可能引起热处理问题，潜在在奥氏体晶粒边缘导致磷化偏析及晶粒边缘渗碳体颗粒析出。
非金属相如硫化物、氧化铝和硅酸盐可影响钢材的断裂韧性
炉温的一致性（尤其回火）将大大影响表面硬度。因此必须确保炉温一致并精确。回火温度的一致性机器重要。记住热电偶仅仅是在电偶的末端测量温度的。我们假定热电偶读数就是炉温，这种假设只在炉内有空气循环风扇时比较确切。
热处理疑难解决时不断向上的排除过程（类似与PDCA循环，译者注），必须从钢材的源头开始（换句话说，从钢材出厂）检查检验证明。需核对检验证明里一些重要的信息，包括：
分析报告，如可能（如无，按以下程序）
Mill hardness ？轧制硬度
硫化物微观洁净度,等. (ASTM E45 Method A)
轧制范围及脱碳层深度
晶粒度G
淬透性 (Jominy end-quench test)
    并不需总是核对分析报告。但是如果钢材和检验证明不匹配，那就得担心这些钢材了。

liangdeyu

For a heat treatment engineer, we are not only care about defect but also need to deal with this issue.

hitfengye

Thank you for share. This paper supply me a lot of useful words us well as heat treatment knowledge. In my opinion, the quality of raw material maybe mainly determine the final quality. If i was wrong, welcome you check it. THank you!

panpandada

至于说国内的材料不好，那是事实。
是我们不能做好材料么？非也，使我们的过程能力保证不足。
为什么我们的过程不稳定？因为我们没有强大的执行力。
为什么执行力不好？因为汉民族是投机的民族。
为什么我们会成为投机的民族？想想从小我们受的教育吧：教的是如何通过捷径解题，而不是传授严谨态度。
所以社会上，投机成风，难免到材料上偷工减料，鱼目混珠。追根溯源，这源于民族本性，要想改变，必须改变我们的教育，我们的社会环境

陆丰

Note：
Mill scale thickness and decarburization depth --- 一次氧化皮（轧制过程中产生的）厚度及脱碳层深度。
Jominy end-quench test--- 乔氏末端淬透性试验。

panpandada

Note：
Mill scale thickness and decarburization depth --- 一次氧化皮（轧制过程中产生的）厚度及脱碳层深度。
Jominy end-quench test--- 乔氏末端淬透性试验。
陆丰 发表于 2009-12-1 18:59

谢谢指点，学习