DIN 32CrMoV12-10 çeliğinden üretilen namlu ömrü ve balistik performansının farklı yüzey işlemleri ile geliştirilmesi
Improvement of barrel life and ballistic performance with different surface treatments made of DIN 32CrMoV12-10 steel
Yazar: HASAN ÇEP
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU ; PROF. DR. AYHAN ÇELİK
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Metalurji ve Malzeme Mühendisliği Bilim Dalı
Derece: Doktora, 2022
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Balistik performans = Ballistic performance ; Silah namluları = Gun barrels ; Yüzey kaplama yöntemleri = Surface coating methods ; Çelik = Steel

Özet:
Bu tez çalışmasında, DIN 32CrMoV12-10 çeliğinden imal edilmiş 7.62 mm çapındaki namluların sertliklerinin, sürtünme-aşınma, korozyon ve balistik performanslarının iyileştirilmesi amacı ile farklı yüzey işlemlerinin etkileri araştırılmıştır. Bu amaçla, DIN 32CrMoV12-10 çeliğinden imal edilen namlulara; krom kaplama, borlama, nitrokarbürleme + oksidasyon ve nitrürleme + oksidasyon işlemleri uygulanmıştır. İşlemler sonrasında, numunelerin yapısal, morfolojik ve mekanik özellikleri XRD, SEM, sertlik ve profilometre ölçümleri ile belirlenmiştir. Namlu malzemesinin tribolojik özelliklerinin belirlenmesi amacı ile pin-on-flat aşınma testleri gerçekleştirilmiştir. Elektrokimyasal korozyon ölçümlerinde ise açık devre potansiyeli (OCP) okuması ve potansiyodinamik polarizasyon deneyleri yapılmıştır. Namluların
balistik performasının belirlenmesi için gerçek koşullarda atış testleri gerçekleştirilmiştir. Krom kaplama sonrasında malzeme yüzeyinde krom tabakasının oluştuğu görülmüştür. Borlanmış numunelerde; malzeme yüzeyinde bor tabakası
oluşumu ve yüzey altında ise difüzyon tabakasının oluştuğu gözlemlenmiştir. Oluşan tabakalar Fe2B ve FeB fazlarını içermektedir. Nitrokarbürleme + oksidasyon ve nitrürleme + oksidasyon işlemi uygulanmış numunelerde ise malzeme yüzeyinde en üstte oksit tabakası olacak şekilde bunun hemen altında beyaz tabaka ve malzemenin iç kısımlarına uzanan difüzyon bölgesinin oluştuğu görülmüştür. Yüzeyde oluşan oksit tabakası Fe2O3 ve Fe3O4 fazlarından oluşmakta iken beyaz tabaka ve difüzyon tabakaları ε-Fe2-3N ve γ'-Fe4N fazlarından oluşmaktadır. Oksit tabakaları, sert nitrür fazları ve difüzyon tabakaları ile artan plastik deformasyon direnci sebebiyle en yüksek aşınma direnci nitrokarbürleme + oksidasyon ve nitrürleme + oksidasyon işlemi uygulanmış numunelerden elde edilmiştir. Krom kaplama, borlama, nitrokarbürleme + oksidasyon ve nitrürleme + oksidasyon işlemleri malzemenin korozyon direncini artırmıştır. Nitrokarbürleme + oksidasyon ve nitrürleme + oksidasyon işlemleri sonucunda yüzeyde oluşan oksit tabakası korozyon için ekstra koruma sağlamıştır. Krom kaplamalarda yapılan testlerde NATO standartlarına göre 12.000 atış ömrü sağlanabilmiştir. Nitrürleme + oksidasyon elde edilen namlularda NATO standartlara göre 10.000 atış sonrası ilk hızlarda düşme ve dağılımda açılma olduğu görülmüştür. Ancak bu namlularda yapılan kaplamalarda atışlar sonrasında kırılma gerçekleşmemiştir. Nitrokarbürleme + oksidasyon ile elde edilen sonuçlara göre 15.000 atışa göre değerler NATO standartlarına göre farklılık göstermiştir.

Abstract:
Today, the defence industry investments are being increased all around the world. Efforts to improve the service life of materials used in this sector are crucial in this respect. The barrel area, which is in the working region of firearms, is heavily exposed to deformations such as wear and corrosion that will cause material loss. Surfaces with high resistance to thermal effects and high performance against abrasion and thermal fatigue damage should be preferred in this region. In this context, necessary surface modifications are carried out to increase the service life. Surface treatments that offer solutions to these problems with lower costs are frequently applied in this area. Thermochemical surface treatments, Cr coating and post post-oxidation methods are preferred as surface modifications in the defence industry to increase the service life of many materials. Basically, the inner surface of the barrels; It should be hard,
resistant to wear and corrosion, high resistance to thermal effects and to be able to meet the effects of thermal fatigue. For this purpose, Cr coatings are frequently used in barrels. However, poor adhesion of Cr coatings to the barrel and the formation of microcracks in the structure after firing are the main problems that are frequently
observed. Two basic methods can be proposed to solve this problem: changing the barrel material or improving the properties of the existing barrel material. Changing the existing material brings huge costs in terms of both cost and required technology. Instead, improving the properties of the existing material emerges as a greater solution potential. In this respect, it is necessary to apply heat treatments in order to improve the surface properties of the barrels.
In this thesis, the effects of different surface treatments were investigated in order to improve the hardness, friction-wear, corrosion and ballistic performances of 7.62 mm diameter barrels made of DIN 32CrMoV12-10 steel. For this purpose, chrome plating, boriding, nitrocarburizing + oxidation and nitriding + oxidation processes were applied
on barrels made of DIN 32CrMoV12-10 steel. After the treatments, the structural, morphological and mechanical properties of the samples were determined by XRD, SEM, hardness and profilometer measurements. Pin-on-flat wear tests were carried out to determine the tribological properties of the barrel material. In electrochemical corrosion measurements, open circuit potential (OCP) and potentiodynamic polarization experiments were performed. Shooting tests were carried out in real conditions to determine the ballistic performance of the barrels. It was observed that a chromium layer was formed on the surface of the material after chrome plating. In boronized samples; It was observed that a boron layer was formed
on the surface of the material and a diffusion layer was formed under the surface. The resulting layers contain Fe2B and FeB phases. Nitrocarburizing + oxidation and nitriding + oxidation were applied in the samples, on the other hand, it was observed that the oxide layer on the material surface was formed at the top, a white layer just below it and a diffusion zone extending to the inner parts of the material. The oxide layer formed on the surface consists of Fe2O3 and Fe3O4 phases, while the white layer and diffusion layers consist of ε-Fe2-3N and γ'-Fe4N phases. Due to the increased
plastic deformation resistance with oxide layers, hard nitride phases and diffusion layers, the highest wear resistance was obtained from the samples treated with nitrocarburizing + oxidation and nitriding + oxidation processes. Chrome plating, boriding, nitrocarburizing + oxidation and nitriding + oxidation processes increased the corrosion resistance of the material. The oxide layer formed on the surface as a result of nitrocarburizing + oxidation and nitriding + oxidation processes provided extra protection for corrosion. In the tests performed on chrome plating, 12.000 shot
life was achieved according to NATO standards. After 10.000 shots, in the nitrided + oxidized barrels, it was observed that there was a decrease in the initial velocities and an opening in the dispersion. However, the coatings made on these barrels did not break after the shots. According to the results obtained by nitrocarburizing + oxidation,
the values differed according to NATO standards according to 15.000 shots. In the initial barrel material samples that were not subjected to any diffusion-based heat treatment and any coating application, the lowest surface roughness was measured as 0.09 m Ra, as well as a low surface hardness of 320 HV. In the chrome-plated samples, on the other hand, the surface hardness of 560 HV was obtained in the chromium layers with a thickness of 7-17 m, while a low surface roughness of 0.18 m Ra was measured. In boronizing coating, the surface roughness was obtained 0.22 µm Ra and surface hardness was measured as 1600 HV0.1. The boronized layer thickness was measured 50 µm and the phases were occurred as FeB and Fe2B. In nitriding and nitrocarburizing + post oxidation processes, Ra values were close to 0.34-0.36 µm. The thickness of Fe3O4 (magnetite oxides) on the top of the surface was 3-4 on the white layer and the thickness of the white layer was 8-10 µm and the hardness was 920-980 HV0.1. In the wear tests, the highest wear rate was measured in the untreated starting materials. The lowest wear rate was measured in the samples that underwent nitrocarburizing +
post oxidation and nitriding + post oxidation processes, respectively. While the highest corrosion rate was measured in the raw material samples, the lowest corrosion rates were obtained in the samples that underwent nitrocarburizing + post oxidation processes. Tests made on chrome plating can provide 12.000 shot life according to NATO standards. In the nitrided +oxidized barrels, according to NATO standards, after 10.000 shots, it was observed that there was a decrease in the initial velocities and an opening in the dispersion. Wear and breakage were found. According to the results
obtained with nitrocarburizing + oxidation, the values show a change according to NATO standards after 15.000 shots. It was observed that % CO2 reduction in nitrocarburization + oxidation process increased ballistic lifetimes. In the test
performed with boronizing, the ballistic requirements according to NATO standards were not successful due to the condition of providing distribution. However, there was no decrease in initial velocities. The pore diameter and densities on the surface of the white layer obtained in nitrocarburization affect the ballistic life. It was also observed that the ballistic life increased as the pore diameters decreased and the density increased. With the applied surface treatments, the wear resistance of the barrel material increased by approximately 50%. In addition, in nitriding + oxidation and nitrocarburizing + oxidation processes, wear resistance increased more than chrome plating and boriding with the effect of oxidation process. Corrosion resistance of the barrel material has increased with surface treatments. In all nitriding + oxidation and nitrocarburizing + oxidation processes, corrosion resistance increased more than other conditions with the effect of the oxide structure formed on the surface.