Surface hardening in steels is a process in which a chemical composition is changed by thermo-chemical processes in a determined region and, accordingly, some micro-structure is changed. In order to obtain a harder layer than the inner region starting from the surface to a certain depth, it is mostly provided by diffusion of elements such as nitrogen and carbon. The process is particularly important in low and medium carbon steels in terms of increasing wear resistance, tensile strength and fatigue strength. The amount of elements used in cementation together with the duration of cementation is extremely important in terms of the harmonious change of structural differentiation. The effect of size and position of inclusions on the cracked structures which is affected from heat treatment is presented in the paper.

Takım aşınmalarının, parça üretim maliyeti ve üretim verimliliğine doğrudan etkisi mevcuttur. Takımlarda meydana gelen aşınmalar, üretim kalitesini doğrudan etkileyen bir faktördür. Bu çalışmada, farklı matkap çapları ve kesme parametrelerinin takım aşınmasına etkisi araştırılmıştır. Deneyler, farklı çapta matkaplar ve farklı kesme parametreleri ile 9 farklı kombinasyonda 20 tekrar seklinde yapılmıştır. Deneysel çalışmada imalat sektöründe sıkça kullanılan 6,8 mm, 9 mm ve 9,6 mm çapa sahip yekpare karbür matkap uçlar kullanılmıştır. İş parçası olarak üretimde sıkça kullanılmakta olan GG25 ve GGG40 standartlı dökme demir malzemeler belirlenmiştir. Taguchi tekniği kullanılarak en yüksek takım ömrü değerlerini veren optimum kontrol faktörleri belirlenmiştir. Ayrıca her deneyden sonra matkap uçlarındaki aşınma durumları ve tipleri takım ölçme ve ayarlama makinası yardımı ile tespit edilmiştir. GGG40 malzemenin GG25 standartlı malzemeye göre, matkap ucunda daha çok aşınmaya sebebiyet verdiği ve matkap ucunun çapı arttıkça aşınma miktarlarında da arttığı belirlenmiştir.

Kingpin parts are one of the critical products for heavy vehicles and are generally made of 20MnCr5 steel. A kingpin is a heavy metal cylindrical pin that is located underneath the front end of the trailer. The kingpin is the mechanism on the trailer that locks it to a road tractor. The parts in this study have undergone to case hardening heat treatment after the rough machining operation. During the operation in the assembly line, cracks have been realized on the surface of the parts. In this study, the kingpin parts are examined with optical microscopy, Scanning Electron Microscopy (SEM), hardness tests and %C potential analysis in order to determine the potential causes of the crack formations. By the help of the analysis, the presence of cracks became clear. There are no signs of hydrogen embrittlement. The exact outcome of the fracture could not be determined. It is seen that heat treatment of the parts is not homogeneously obtained throughout their surfaces or desired volume, and therefore fractures occur. With the present structure, it can be expected that even a low dynamic load can lead to breakage. As a result, the investigations suggest that the parts were exposed to inappropriate heat treatment parameters.