Causes of forging cracks and heat treatment cracks

锻造裂纹：钢在锻造过程中，由于钢材存在表面及内部缺陷，如发纹、砂眼、裂纹、夹杂物、皮下气泡、缩孔、白点和夹层等，都可能成为锻打开裂的原因。另外，由于锻打工艺不良或操作不当，如过热、过烧或终锻温度太低，锻后冷却速度过快等，也会造成锻件开裂。

Forging crack: during the forging process of steel, the surface and internal defects of steel, such as hair lines, sand holes, cracks, inclusions, subcutaneous bubbles, shrinkage cavities, white spots and interlayer, may be the cause of forging crack. In addition, due to poor forging process or improper operation, such as overheating, overburning or too low final forging temperature, too fast cooling speed after forging, etc., it will also cause forging cracking.

热处理裂纹：淬火裂纹是宏观裂纹，主要由宏观应力引起。在实际生产过程中，钢制工件常由于结构设计不合理，钢材选择不当、淬火温度控制不正确、淬火冷速不合适等因素，一方面加大淬火内应力，会使已形成的淬火显微裂纹扩展，形成宏观的淬火裂纹，另一方面，增加了显微裂纹的数量，降低了钢材的脆断抗力Sk,从而加大淬火裂纹的形成可能性。

Heat treatment crack: quenching crack is a macro crack, which is mainly caused by macro stress. In the actual production process, steel workpieces are often due to unreasonable structural design, improper steel selection, incorrect quenching temperature control, inappropriate quenching cooling speed and other factors. On the one hand, increasing the quenching internal stress will expand the formed quenching microcracks and form macro quenching cracks. On the other hand, it increases the number of microcracks and reduces the brittle fracture resistance sk of steel, so as to increase the possibility of quenching cracks.

影响淬裂的因素很多，这里仅将生产中常碰到的几种情况作一介绍：

There are many factors affecting quenching crack. Here, we will only introduce several situations often encountered in production:

1.原材料已有缺陷而导致的淬裂：

1. Quenching crack caused by existing defects of raw materials:

如果原材料表面和内部有裂纹或夹杂物等缺陷，在淬火之前未发现，有可能形成淬火裂纹。

If there are cracks or inclusions on the surface and inside of raw materials, which are not found before quenching, quenching cracks may be formed.

2、夹杂物导致的开裂：

2. Cracking caused by inclusions:

如果零件内部夹杂物严重，或本身因夹杂物严重已经隐藏有裂纹，淬火时将有可能产生裂纹

If the inclusions inside the parts are serious, or there are hidden cracks due to serious inclusions, cracks may occur during quenching

3、因原始组织不良而导致的淬裂

3. Quenching crack caused by poor original structure

4、淬火温度不当造成的淬裂

4. Quenching crack caused by improper quenching temperature

淬火温度不当引起零件淬裂，一般有两种情况：

There are generally two cases of quenching crack caused by improper quenching temperature:

（1）仪表指示温度低于炉子的实际温度，使淬火温度偏高，造成淬火过热，导致工件发生开裂。凡是过热 淬火开裂的金相组织均存在晶粒粗大和粗大的马氏体。

(1) The indicated temperature of the instrument is lower than the actual temperature of the furnace, which makes the quenching temperature higher, resulting in quenching overheating and workpiece cracking. There are coarse grains and coarse martensite in the metallographic structure of superheated quenching cracking.

（2）钢件实际含碳量高于钢材牌号所规定的含量，若按原牌号的正常淬火工艺淬火时，等于提高了钢的淬火温度，容易造成零件过热和晶粒长大，使淬火时应力加大而引起淬裂。

(2) The actual carbon content of steel parts is higher than that specified by the steel brand. If the steel is quenched according to the normal quenching process of the original brand, it is equivalent to increasing the quenching temperature of the steel, which is easy to cause overheating and grain growth of the parts, increase the stress during quenching and cause quenching crack.

5、淬火冷却不当造成的淬裂

5. Quenching crack caused by improper quenching cooling

淬火时由于冷却不当，也会使零件产生淬裂事故。

Due to improper cooling during quenching, quenching crack accidents will also occur to parts.

6、机加工缺陷导致的淬裂

6. Quenching crack caused by machining defect

由于机加工不良，在零件表面留下了粗而深的刀痕，尽管是很简单的零件或不是应力集中的地方，也会在淬火时造成开裂 ，或在服役过程中发生早期损坏。

Due to poor machining, rough and deep knife marks are left on the surface of parts. Although it is a very simple part or not a place with stress concentration, it will also cause cracking during quenching or early damage during service.

7、零件外形对淬火裂纹的影响

7. Influence of part shape on quenching crack

零件几何形状不合理，或截面过度区厚薄相差较大，在淬火时均易因应力集中产生裂纹

The geometric shape of the part is unreasonable, or the thickness of the section transition zone varies greatly, which is easy to produce cracks due to stress concentration during quenching

8、不及时回火导致的开裂

8. Cracking caused by untimely tempering

淬火后不及时回火，将可能因淬火残余应力过大而导致裂纹产生。

Failure to temper in time after quenching may lead to cracks due to excessive quenching residual stress.

总之，应用金相方法进行缺陷分析是一项较为复杂的工作，由于零件失效有时是由多种因素所造成的，所以在实际工作中，应做多方面调查，掌握实情，并从多方面进行分析，以确保缺陷分析的准确性。

In short, the application of metallographic method for defect analysis is a more complex work. Because the failure of parts is sometimes caused by a variety of factors, in practical work, we should investigate in many aspects, master the facts, and analyze from many aspects to ensure the accuracy of defect analysis.