Seven Causes for Quench Cracking in Steel

  

外国人、中国人，会怎样认识？

国外的《7 Causes For Quench Cracking Of Steel》[1]是一篇“旧文”了，最初的连接大约是：https://pmpaspeakingofprecision.com/2010/08/03/7-causes-for-quench-cracking-of-steel/  7 Causes For Quench Cracking Of Steel。

国内也看到一份“英文”资料，“Causes of Quenching Cracks in Steel Castings”[2]。可以。

 

大家对比看，更有收获。

 

因为无锡汪庆华老师的缘故，看到国内资料的这一段，很有意思：

5. Quenching cracks caused by improper cooling during quenching

Due to improper cooling during quenching, quenching cracking accidents will also occur in the parts. For example, No. 45 steel tends to form quench cracks during quenching. Especially when the carbon content is at the upper limit and the diameter of the part is 7-8mm, cracking is easy to occur. Therefore, it is extremely important to choose a suitable cooling medium during quenching. In addition, the structure of some parts is more complex, and the cross-sectional size changes greatly. If the coolant is not selected properly, the thin wall parts are likely to cause stress concentration and lead to quenching cracks.

5. 淬火过程中冷却不当导致的淬火裂纹

由于淬火过程中冷却不当，零件也会发生淬火开裂事故。例如，45号钢在淬火过程中容易形成淬火裂纹。特别是当碳含量处于上限且零件直径为7-8mm时，容易发生裂纹。因此，在淬火过程中选择合适的冷却介质非常重要。此外，一些零件的结构更加复杂，横截面尺寸变化很大。如果冷却液选择不当，薄壁零件可能会导致应力集中并导致淬火裂纹。

 

 

参考资料：

[1] https://www.pmpa.org/wp-content/uploads/2020/07/PMPA_Craftsman57.pdf  PMPA

Seven Causes for Quench Cracking in Steel

Failures of steel parts in service or production occur infrequently. However, when steel parts fail, the consequences are dire.

Here are seven ways that steel can fail as a result of quench cracking from heat treatment.

1. Overheating during the austenitizing portion of the heat treatment cycle can coarsen normally fine-grained steels.

Coarse-grained steels increase hardening depth and are more prone to quench cracking than fine-grained steels.

Avoid overheating and overly long dwell times while austenitizing.

2. Improper quenchant. Yes, water, brine or caustic will get the steel harder. If the steel is an oil-hardening steel, the use of these overly aggressive quenchants will lead to cracking.

3. Making an improper selection of steel for the process.

4. Too much time between the quenching and the tempering of the heat-treated parts.

A common misconception is that quench cracks can occur only while the piece is being quenched. This is not true. If the work is not tempered right away, quench cracks can （and will） occur.

5. Improper design. This includes sharp changes of section, lack of radii, holes, sharp keyways, unbalanced sectional mass and other stress risers.

6. Improper entry of the part/delivery of the quenchant to the part. Differences in cooling rates can be created, for example, if parts are massed together in a basket. This can result in the parts along the edges cooling faster than those in the mass in the center. Part geometry can also interfere with quenchant delivery and effectiveness, especially on induction lines.

7. Failure to take sufficient stock removal from the original part during machining. This can leave remnants of seams or other surface imperfections that can act as a nucleation site for a quench crack.

Finally, materials that are heat treated to high strength levels, even though they did not quench crack, may contain localized concentrations of high residual stresses. If these stresses are acting in the same direction as the load applied in service, an instantaneous failure can occur. This will be virtually indistinguishable from a quench crack during an examination, because of its brittle failure mode, lack of decarburization on surface of the fracture or other forensic evidence of a process failure.

When looking at quench cracking failures under the microscope, cracks and crack tributaries that follow the prior austenitic grain boundaries are a pretty good clue that grain coarsening and/or its causes, such as overheating or too much time at a certain temperature, have occurred. Temper scale on the fracture surface helps the metallurgist know that the crack was present before tempering.

Decarburization may show that the crack was open prior to quenching.

This 1 1/2-inch diameter high speed steel reamer cracked because of excessively high quenching temperature.

All Craftsman’s Cribsheets are available for viewing and download at short.productionmachining.com/cribsheets.

钢中淬火开裂的七个原因

服务或生产中的钢部件故障很少发生。然而，当钢部件失效时，后果是可怕的。

以下是钢因热处理产生的淬火开裂而失效的七种方式。

1.在热处理循环的奥氏体化部分过热会使通常细粒度的钢变粗。

粗晶粒钢比细晶粒钢增加硬化深度，更容易淬火开裂。

奥氏体化时避免过热和停留时间过长。

2.淬火剂不当。是的，水、盐水或苛性碱会使钢变硬。如果钢是油硬化钢，使用这些过度腐蚀的淬火剂会导致开裂。

3.选择不当，用于该工艺的钢。

4.热处理零件淬火和回火之间的时间过长。

一个常见的误解是，淬火裂纹只能在工件淬火时发生。这不是真的。如果未立即对工件进行回火，则可能（也将）出现淬火裂纹。

5.设计不当。这包括截面的急剧变化、半径不足、孔洞、尖锐键槽、不平衡截面质量和其他应力提升。

6.零件进入不当/淬火剂输送至零件。例如，如果零件在一个篮子中聚集在一起，就会产生冷却速度的差异。这会导致沿边缘的零件冷却速度快于中心质量中的零件。零件几何形状也会影响淬火剂的输送和有效性，尤其是在感应线上。

7.在加工过程中，未从原始零件上去除足够的库存。这可能会留下焊缝或其他表面缺陷的残余，这些缺陷可能会成为淬火裂纹的成核部位。

最后，热处理到高强度水平的材料，即使没有淬火裂纹，也可能含有高残余应力的局部集中。如果这些应力的作用方向与使用中施加的载荷相同，则可能发生瞬时失效。由于其脆性失效模式、断裂表面脱碳不足或其他工艺失效的法医证据，这与检查过程中的淬火裂纹几乎无法区分。

在显微镜下观察淬火裂纹失效时，遵循先前奥氏体晶界的裂纹和裂纹分支是晶粒粗化和/或其原因（如过热或在特定温度下时间过长）发生的一个很好的线索。断裂表面上的回火刻度有助于冶金学家在回火前知道裂纹存在。

脱碳可能表明裂纹在淬火前是开放的。

这种直径为1/2英寸的高速钢铰刀因淬火温度过高而开裂。

所有Craftsman的Cribsheets均可在short.productionmachineing.com/Cribsheets上查看和下载。

 

[2] https://xjlpowertech.com/causes-of-quenching-cracks-in-steel-castings/  山东新巨龙电力科技集团有限公司

Causes of Quenching Cracks in Steel Castings

Wenchell  July 22, 2022  5:30 pm

1. Quenching cracks caused by defects in raw materials

If there are cracks on the surface and inside of the raw material, which are not found before the heat treatment, quenching cracks may form. Observed under a metallographic microscope, there are decarburized layers on both sides of the crack, and the grains of ferrite in the decarburized layer are coarse.

2. Quenching cracks caused by inclusions

If the inclusions in the parts are serious, it is easy to cause stress concentration, and cracks may occur during quenching.

3. Quenching cracks due to poor original structure

（1） If the microstructure of the steel has severe band-like segregation or severe chemical composition segregation, it will cause great structural transformation stress during quenching. Furthermore, the over-burning phenomenon is easy to occur in the carbide aggregates, which makes the parts prone to cracking.

（2） If the residual internal stress of the steel is large before quenching, it is easy to cause cracking during quenching. Parts in this situation tend to have coarse grains and Widmanners structures.

（3） If the parts need to be repaired after one quenching, and the structural stress is not eliminated before the second quenching, cracks may occur in the second quenching, and the cracks are often distributed along the first quenching layer.

4. Two kinds of quenching cracks caused by improper quenching temperature

（1） The indicated temperature of the instrument is lower than the actual temperature of the furnace, which makes the actual quenching temperature on the high side, resulting in overheated quenching and cracking of the parts. Coarse grains and coarse martensite exist in all microstructures cracked by superheat quenching, and the cracks produced mainly exist in the form of intergranular.

（2）The actual carbon content of the steel is higher than the content specified by the steel grade. If the original grade is quenched by the normal process, the quenching temperature of the steel is increased, so it is easy to cause the parts to overheat and grain grow, which makes the quenching process. The increased stress may cause quench cracking.

5. Quenching cracks caused by improper cooling during quenching

Due to improper cooling during quenching, quenching cracking accidents will also occur in the parts. For example, No. 45 steel tends to form quench cracks during quenching. Especially when the carbon content is at the upper limit and the diameter of the part is 7-8mm, cracking is easy to occur. Therefore, it is extremely important to choose a suitable cooling medium during quenching. In addition, the structure of some parts is more complex, and the cross-sectional size changes greatly. If the coolant is not selected properly, the thin wall parts are likely to cause stress concentration and lead to quenching cracks.

6. Quenching cracks caused by machining defects

Due to poor machining, deep and coarse tool marks are left on the surface of the parts, and during quenching and cooling, stress concentration is caused there and cracks are caused.

7. The influence of part shape on quenching cracks

If the geometry of the parts is unreasonable or the thickness of the transition zone of the section varies greatly, cracks are likely to occur due to stress concentration during quenching. In addition, if the forging streamlines of the parts are poorly distributed, quenching cracking defects may also be caused during quenching.

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铸钢件淬火裂纹的成因

文切尔 2022年7月22日 下午5:30

1. 原材料缺陷导致的淬火裂纹

如果在热处理前未发现原材料表面和内部有裂纹，则可能会形成淬火裂纹。在金相显微镜下观察，裂纹两侧有脱碳层，脱碳层中的铁素体晶粒粗大。

2. 夹杂物导致的淬火裂纹

如果零件中的夹杂物严重，则容易引起应力集中，淬火时可能出现裂纹。

3. 原始结构不良导致的淬火裂纹

（1） 如果钢的显微组织具有严重的带状偏析或严重的化学成分偏析，将在淬火过程中产生巨大的组织转变应力。此外，碳化物聚集体中容易发生过度燃烧现象，这使得零件容易开裂。

（2） 如果淬火前钢的残余内应力较大，则在淬火过程中容易产生裂纹。在这种情况下，零件往往具有粗晶粒和Widmannes结构。

（3） 如果零件在一次淬火后需要修复，并且在第二次淬火之前没有消除结构应力，则在第二淬火中可能会出现裂纹，并且裂纹通常沿着第一淬火层分布。

4. 淬火温度不当导致的两种淬火裂纹

（1） 仪表的指示温度低于熔炉的实际温度，这使得实际淬火温度偏高，导致零件过热淬火和开裂。过热淬火开裂的所有组织中都存在粗晶粒和粗马氏体，产生的裂纹主要以晶间形式存在。

（2） 钢的实际碳含量高于钢等级规定的含量。如果通过正常工艺对原始等级进行淬火，钢的淬火温度会升高，因此容易导致零件过热和晶粒长大，从而导致淬火过程。增加的应力可能导致淬火开裂。

5. 淬火过程中冷却不当导致的淬火裂纹

由于淬火过程中冷却不当，零件也会发生淬火开裂事故。例如，45号钢在淬火过程中容易形成淬火裂纹。特别是当碳含量处于上限且零件直径为7-8mm时，容易发生裂纹。因此，在淬火过程中选择合适的冷却介质非常重要。此外，一些零件的结构更加复杂，横截面尺寸变化很大。如果冷却液选择不当，薄壁零件可能会导致应力集中并导致淬火裂纹。

6. 加工缺陷导致的淬火裂纹

由于加工不良，零件表面会留下深而粗糙的工具痕迹，在淬火和冷却过程中，应力集中并产生裂纹。

7. 零件形状对淬火裂纹的影响

如果零件的几何形状不合理或截面过渡区的厚度变化很大，则淬火过程中可能会因应力集中而产生裂纹。此外，如果零件的锻造流线分布不良，淬火过程中也可能导致淬火裂纹缺陷。

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