ABSTRACT

In tribosystems to control friction and wear is necessary. Several solutions have been developed over the last few years, such as heat treatments, thermo-chemical treatments, thin films deposition and duplex coatings. The purpose of the present work is to understand how the microstructure, mechanical and tribological properties of AISI 440B vary after being subjected to the following treatments i) quenching, ii) plasma nitriding. The approach to study AISI 440B microstructure consisted of XRD (X-Ray Diffraction), optical microscopy and SEM (Scanning electron microscopy). The approach to study AISI 440B mechanical properties consisted of Rockwell C hardness and Vickers microhardness. Finally, the approach to study AISI 440B tribological performance was: a) surface and roughness observations through contact rugosimetry and AFM, b) friction coefficient vs. time curves through unlubricated reciprocating tribological test (ASTM G133-05), c) disc and sphere mass loss, d) wear track and wear cap observations through SEM-FEG/EDS, e) surface profile and wear track depth through optical interferomety. The nitrided layer presented tempered martensite as well as γ', ε and CrN nitrides. The maximum surface hardness was 1340 HV 0,1 and a case depth of ~100 µm was found. Regarding the wear mechanisms, in the "dry sliding of AISI 52100 sphere against as-quenched AISI 440B discs" tribosystem, the wear mechanisms acting in the body were: severe wear with i) severe plastic deformation, ii) 3-body abrasion at the ends of the wear tracks, iii) oxidation, and iv) mild delamination (oxide pull-out). Regarding the counterbody, it was found: i) mild abrasion and ii) oxide adhesion. In the "dry sliding plasma nitrided AISI 440B against AISI 52100 spheres" tribosystem the body suffered mild wear with i) oxidation and ii) delamination. The counterbody suffered i) strong abrasion and ii) oxide adhesion.

ResumoEsse trabalho avaliou a microestrutura e a microdureza do aço inoxidável martensítico AISI 440B nitretado a plasma em reator de fonte DC pulsada, a 480ºC, por 6 horas e pressão da câmara de 3,3 Torr. Foram utilizadas técnicas de difração de raios-X, microscopia ótica, microscopia eletrônica de varredura (MEV) e microdureza. A camada nitretada apresentou aproximadamente 24 µm de espessura, não tendo sido observada a formação de camada branca, constituída exclusivamente por nitretos de ferro ou de cromo. Entretanto, notou-se, na zona de difusão, a presença de nitretos nos contornos de grão, possivelmente, de cromo. O endurecimento superficial do aço AISI 440B, atingiu valores entre 1417 -1677 HV0,025, ao passo que o a matriz metálica do aço temperado apresentou durezas de aproximadamente 54HRc. Palavras-chave: Nitretação a plasma; Aços inoxidáveis martensíticos; Engenharia de superfície. MICROSTRUCTURAL AND MICROHARDNESS EVALUATION OF AISI 440BPLASMA NITRIDED Abstract This study has evaluated the microstructure and microhardness of martensitic stainless steel AISI 440B plasma nitrided at 480ºC for 6h and 3,3 Torr using a DC Pulsed reactor. Techniques such as X-Ray diffraction, optical microscopy, scanning electron microscopy (SEM) and microhardness were used. The resulting microstructure was a nitrided layer of about 24 µm thick, white layer was not present, but it was noticed in the diffusion zone the presence of nitrides at the grain boundaries, possibly chrome ones. The magnitude of microhardness improvement on AISI 440B was also measured, the surface hardness after nitriding stayed between 1417 to 1677 HV0,025 whereas the pre-nitrided material hardness was approximately 54HRc.

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