Takeshi Fujimatsu

Initiation Behavior of Crack Originated from Non-Metallic Inclusion in Rolling Contact Fatigue

Recently, it has been generally accepted that flaking life is dependent on the size of non-metallic inclusion under proper condition in bearing use. Statistics of extreme values to predict the maximum non-metallic inclusion size and ultrasonic test to assess large non-metallic inclusions in a large volume are widely used as practical methods for the evaluation of bearing steel cleanliness. Murakami’s formula is well known, which describes the relationship between non-metallic inclusion size and fatigue strength. However, the formula cannot be directly applied to rolling contact fatigue because of the difference in applied stress mode. While the equation was invented to predict the fatigue limit in principal stress mode, rolling contact fatigue is mainly caused by shear stress. It is expected that the condition of bearing use will be more critical due to the downsizing tendency in various industrial or automotive machine units. Thus the research on the flaking mechanism becomes more important from the viewpoint of global ecology because that is beneficial to the improvement in bearing life and methods for cleanliness evaluation. Due to the difficulty in experimental observation, however, the details of the flaking mechanism in rolling contact fatigue caused by non-metallic inclusion has not yet been clarified. Focusing on the relationship between non-metallic inclusion and initiation of the crack, the flaking mechanism is proposed in this paper.

Crystallographic Analyses on Cracks Initiated by Rolling Contact Fatigue in High Carbon Chromium Bearing Steel

Crack formation by a rolling contact fatigue in a high carbon chromium bearing steel has been discussed. Newly developed method for preparing specimens including pre-existing voids enabled one to observe the early stage of fatigue crack formation. Many fatigue cracks were formed around the voids. The positions of crack formation and the direction of the cracks were consistent with those forecasted by finite element analysis. Fatigue crack formation was followed by formation of the WEAs.

Crack Initiation and Propagation Behavior Around the Defect in Steel Under Rolling Contact Fatigue

Synopsis: The clarification of uncertain factors of rolling contact fatigue (RCF) life variation is expected to lead us to better understanding of the RCF mechanism and further improvement of the service life of bearings. The objective of this study is to clarify the effect of defect location on RCF life. Artificial cavities, pores, and drilled holes were introduced to the specimens as a flaking origin under RCF for simplification on the presumption that their physical properties and interfacial rubbing between cavity and matrix were ignorable. The RCF test resulted in flakings initiated from the pore located right below the center of the track, when a specimen included numerous pores. Their RCF lives were simply determined by fracture mechanical parameters, size of pore, and orthogonal shear stress range parallel to rolling direction. On the other hand, RCF life was prolonged when the drilled hole in a specimen was located near the contact edge, although the resultant flakings appear the same irrespective of defect location. Therefore, defect location is one of the important factors for RCF life variation. The following were found through a further verification experiment and finite-element analysis: (1) A crack initiates from a drilled hole surface because of principal tensile stress at early stages of RCF irrespective of location of the hole. (2) Both of the orthogonal shear stresses, parallel and perpendicular to rolling direction, lead to the threedimensional propagation of crack. (3) The weakening/damaging effect from a hole near the contact edge is less than that from a hole near the center of the contact track.

Austenite Grain Stability of Titanium-Modified Carburizing Steel

It is important to suppress grain coarsening during the carburizing treatment of automobile components such as gears and shafts so that high strength is maintained and heat treatment distortion is minimized. Two manufacturing methods are often used for the cost reduction. One is the increasing carburizing temperature. The other is the adoption of cold forging instead of hot forging. These methods are likely to result in grain coarsening in the case of conventional steels. It is well known that the key factors to control the grain growth are the initial austenite grain size and the volume and mean size of precipitates. In this study, the grain growth property of Ti-modified steel was investigated to confirm the influence of precipitates, and compared with those of Nb-modified and conventional steels. The influence of forging temperature on the grain growth property in a Ti-modified steel was also investigated.

Development of High Strength Carburizing Steel, ECOMAX4, with Improved Heat Treatment Distortion and Dimensional Change

ECOMAX4は自動車、産業機械におけるギヤ、シャフト 等に代表される駆動系部品への適用を想定して開発した浸 炭用鋼である。その開発にあたっては、駆動系部品におけ るトータルコストダウンや燃費向上のための小型・軽量化 のニーズを考慮し、高強度化、部品製造過程における熱処 理・加工コストの低減や環境負荷物質の排出抑制、ならび に駆動ノイズ低減による静粛性・快適性向上に対する具体 的寄与を念頭に置いている。駆動系部品で高強度が必要と される場合、JIS規格のクロムモリブデン鋼やニッケルク ロムモリブデン鋼、もしくはさらにニッケルやモリブデン を増量添加した開発鋼が使われるが、希少な合金元素の添 加は鋼材コストを上昇させ、また加工性を低下させる懸念 があるため、ニッケル・モリブデンフリーの高強度鋼が望 まれている。部品加工方法の観点では、熱間鍛造工法に比 べてCO2排出量削減ならびにニヤネットシェイプ化による 歩留まり向上に有利な冷間鍛造工法への適合が求められ る。ただし、汎用鋼を冷間鍛造後に焼ならしを施さずに浸 炭するとオーステナイトの異常粒成長が起こりやすいとい う課題があり、それを防止できる鋼が必要となる。また、 浸炭部品を焼入れする際には、熱処理変形が不可避的に生 じる。この変形度合いが大きければ、部品の矯正や形状修 正に時間を要する、ひいては不良率が増大するといった問 題が発生し、例え変形が小さくてもギヤ噛み合い時のノイ ズの増大につながるため、熱処理変形の抑制は重要な課題 である。このような背景のもと、当社では、部品の製造過 程における工程省略・簡略化、部品の高精度化、部品の高 強度化に寄与する優れた熱処理変形特性、耐結晶粒粗大化 特性、耐ピッチング性を有するニッケル・モリブデンフリー のECOMAX4の開発に成功し、商品化している。

Influence of Repeated Quenching After Carburizing on Prior Austenite Grain Size

High strengths and impact values are required for carburizing steels used for automotive drive train parts. It is well known that the finer-grain microstructures are favorable for these properties. In this study, with a view to effective refining of grain size with controlling the alloying elements, influence of Mn and Cr content on prior austenite grain size after carburizing and repeated quenching was investigated on Ti-modified carburizing steels with ferrite and pearlite microstructure. It was found that the increase in Mn and Cr contents in steels were effective for the grain refining. These effects were attributed to fine ferrite and pearlite microstructure caused by the lowered Ar3 point due to higher Mn content, and retarding the grain growth of austenite by the fine dispersion of carbides in the carburized layer of specimen due to higher Cr content.