Causes of forging cracks and heat treatment cracks

鍛造裂紋：鋼在鍛造過程中，由于鋼材存在表面及內部缺陷，如發紋、砂眼、裂紋、夾雜物、皮下氣泡、縮孔、白點和夾層等，都可能成為鍛打開裂的原因。另外，由于鍛打工藝不良或操作不當，如過熱、過燒或終鍛溫度太低，鍛后冷卻速度過快等，也會造成鍛件開裂。

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.

熱處理裂紋：淬火裂紋是宏觀裂紋，主要由宏觀應力引起。在實際生產過程中，鋼制工件常由于結構設計不合理，鋼材選擇不當、淬火溫度控制不正確、淬火冷速不合適等因素，一方面加大淬火內應力，會使已形成的淬火顯微裂紋擴展，形成宏觀的淬火裂紋，另一方面，增加了顯微裂紋的數量，降低了鋼材的脆斷抗力Sk,從而加大淬火裂紋的形成可能性。

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.

影響淬裂的因素很多，這里僅將生產中常碰到的幾種情況作一介紹：

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

1.原材料已有缺陷而導致的淬裂：

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

如果原材料表面和內部有裂紋或夾雜物等缺陷，在淬火之前未發現，有可能形成淬火裂紋。

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、夾雜物導致的開裂：

2. Cracking caused by inclusions:

如果零件內部夾雜物嚴重，或本身因夾雜物嚴重已經隱藏有裂紋，淬火時將有可能產生裂紋

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

3、因原始組織不良而導致的淬裂

3. Quenching crack caused by poor original structure

4、淬火溫度不當造成的淬裂

4. Quenching crack caused by improper quenching temperature

淬火溫度不當引起零件淬裂，一般有兩種情況：

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

（1）儀表指示溫度低于爐子的實際溫度，使淬火溫度偏高，造成淬火過熱，導致工件發生開裂。凡是過熱 淬火開裂的金相組織均存在晶粒粗大和粗大的馬氏體。

(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）鋼件實際含碳量高于鋼材牌號所規定的含量，若按原牌號的正常淬火工藝淬火時，等于提高了鋼的淬火溫度，容易造成零件過熱和晶粒長大，使淬火時應力加大而引起淬裂。

(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、淬火冷卻不當造成的淬裂

5. Quenching crack caused by improper quenching cooling

淬火時由于冷卻不當，也會使零件產生淬裂事故。

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

6、機加工缺陷導致的淬裂

6. Quenching crack caused by machining defect

由于機加工不良，在零件表面留下了粗而深的刀痕，盡管是很簡單的零件或不是應力集中的地方，也會在淬火時造成開裂 ，或在服役過程中發生早期損壞。

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、零件外形對淬火裂紋的影響

7. Influence of part shape on quenching crack

零件幾何形狀不合理，或截面過度區厚薄相差較大，在淬火時均易因應力集中產生裂紋

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、不及時回火導致的開裂

8. Cracking caused by untimely tempering

淬火后不及時回火，將可能因淬火殘余應力過大而導致裂紋產生。

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

總之，應用金相方法進行缺陷分析是一項較為復雜的工作，由于零件失效有時是由多種因素所造成的，所以在實際工作中，應做多方面調查，掌握實情，并從多方面進行分析，以確保缺陷分析的準確性。

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.