汽車引擎蓋掛耳熱處理淬裂研究

Investigation on quenching cracks for automobile hood hook in the heat treatment process

本研究針對SAE 1541鋼材生產汽車引擎蓋掛耳，在品管UT檢測後發現表面疑似熱處理淬火裂紋進行研究。由於淬裂發生的原因很多，遂進行破損分析探討其成因，並提出有效的改善建議。綜合破損分析結果可知，造成引擎蓋掛耳淬裂的原因主要有四點：1.冷卻速率過快，硬度值偏高，接近表面的拉張應力隨之偏高。2.表面（微）脫碳，使表面與次表面間的界面，因淬火相變態時的應力分布而形成不連續面。3.酸洗後表面品質不佳，缺口效應會產生應力集中現象。4.感應加熱造成過熱或升溫過快，導致晶粒成長及熱應力變化過大。根據以上的淬裂發生的機制，提供以下改善方案：1.高分子淬火液的濃度必須監控，控制淬火後的硬度在HRC 55左右，若硬度超過HRC 56以上，在局部應力集中位置，或有缺口效應的位置，容易產生淬裂。2.控制球化退火爐氛，儘量降低脫碳層厚度，尤其是避免全脫碳層發生。3.縮短酸洗時間，避免過度酸洗造成表面缺陷；折彎模具必須檢視，發現瑕疵必須修整，避免產生刮傷造成缺口效應。4.感應加熱的升溫速率過快，須調降樣品加熱速度，使樣品的升溫速率減緩，也減少過熱情形發生。後續經過以上相關製程的協調改善後，有效解決汽車引擎蓋掛耳的淬裂問題。

SAE 1541 steel was used to produce the part of automobile hood hook. It was inspected that the surface cracks was suspected the quenching cracks in the heat treatment process through the UT examination. In this study, the damage analysis of the failure part was conducted to realize what caused the quenching cracks initiation and propose the effective improvement suggestions. According to the comprehensive damage analysis results, there are four reasons for the quench cracking: 1. The cooling speed is too fast when quenching so that the hardness value is too high and the tensile stress at the surface is much higher than the interior. 2. The micro decarburization in the surface makes the discontinuous stress distribution formed in the interface between the surface and the subsurface during the quenching phase transformation. 3. The surface quality is poor after pickling, and the stress concentration occurs due to the notch effect. 4. The induction heating is overheated or the temperature rises too fast, causing the grain growth and thermal stress change too much. According to the above quenching mechanism, the improvement scheme is provided: 1. The concentration of the polymer quenching liquid must be monitored, and the hardness after quenching is controlled at about HRC 55. If the hardness value exceeds HRC 56, there are quenching cracks formed where the local stress concentration is, or the notch effect. 2. Control the spheroidizing annealing furnace atmosphere to reduce the thickness of the decarburized layer, especially to avoid the occurrence of the full decarburization layer. 3. Shorten the pickling time to avoid excessive acid washing to cause surface defects. The mold must be inspected and repaired if found the defects on its surface to avoid scratching the hooks. 4. It is advisable to reduce the induction heating rate of the sample, slow down the heating rate of the hook, and avoids the occurrence of overheating. After the subsequent improvement of the above related processes, the problem of quenching cracks of the hood hook is effectively solved.