淬火裂紋與回火脆性

　    很多對力學(xué)性能與表面硬度請求高的大鍛件，鍛后要經(jīng)粗加工，再進(jìn)行調(diào)質(zhì)熱處理或表面淬火。在熱處理時(shí)，由于溫度急巨變更，將產(chǎn)生很大的溫度應(yīng)力。由于相變還產(chǎn)生構(gòu)造應(yīng)力，和鍛件存在的殘存應(yīng)力疊加，合成的拉應(yīng)力值要是跨越質(zhì)料的抗拉強(qiáng)度，而且沒有塑性變形松弛，將會(huì)產(chǎn)生種種形式的開裂和裂紋。例如縱向、橫向、表面和中間裂紋，表面龜裂和上層剝離等。由于大鍛件截面尺寸大，加熱、冷卻時(shí)溫度漫衍不勻稱，相變歷程復(fù)雜，殘存應(yīng)力大，而且程度不同地存在著種種宏觀和微觀缺點(diǎn)，塑性差，韌性低，這都能加劇裂紋萌發(fā)與擴(kuò)大的歷程，往往造成即時(shí)的或延時(shí)的開裂破壞，乃至炸裂與天然置裂等，造成龐大經(jīng)濟(jì)損失。

   　 對策是：

    　1）接納合理的熱處理規(guī)范，控制加熱速率與冷卻歷程，削減加熱缺點(diǎn)與溫度應(yīng)力；

 　   2）幸免鍛件中存在緊張的冶金缺點(diǎn)與殘存應(yīng)力；

 　   3）淬火后及時(shí)回火。

  　  回火脆性系碳化物析出或磷、錫、銻、砷等有害微量元素沿晶界聚集而惹起的脆性增大的傾向。

　    防止回火脆性的對策是：

  　  1）削減鋼中有害元素的含量；

  　  2）削減鋼中偏析；

  　  3）幸免在回火脆性溫度區(qū)熱處理，適當(dāng)快冷，防止有害組元富集。

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Quenching crack and temper brittleness

Many large forgings with high requirements on mechanical properties and surface hardness are subjected to roughing after forging, and then quenched and tempered or surface hardened. At the time of heat treatment, a large temperature stress is generated due to a sudden change in temperature. Since the phase transformation also produces tectonic stress, and the residual stress existing in the forging is superimposed, the resultant tensile stress value is the tensile strength across the material, and there is no plastic deformation relaxation, which will produce various forms of cracking and cracking. For example, longitudinal, transverse, surface and intermediate cracks, surface cracks and upper peeling. Due to the large cross-section size of large forgings, the temperature is unevenly distributed during heating and cooling, the phase transition process is complicated, the residual stress is large, and various macro and micro defects are present in different degrees. The plasticity is poor and the toughness is low, which can aggravate crack germination. With the expansion process, it often causes immediate or delayed cracking damage, and even bursting and natural cracking, resulting in huge economic losses.

   The countermeasures are:

    1) Accept reasonable heat treatment specifications, control heating rate and cooling history, and reduce heating defects and temperature stress;

 2) Excessive metallurgical defects and residual stresses in forgings;

 3) Tempering in time after quenching.

  The temper brittleness carbide precipitation or the tendency of the harmful trace elements such as phosphorus, tin, antimony, and arsenic to aggregate along the grain boundary tend to increase.

The countermeasures against temper brittleness are:

  1) reduce the content of harmful elements in steel;

  2) reducing segregation in steel;

  3) It is spared from heat treatment in the temper brittle temperature zone, and it is suitable for rapid cooling to prevent harmful component enrichment.