Fadiga térmica de ferros fundidos brancos multicomponentes. / Thermal fatigue of multicomponent white cast iron.

AUTOR(ES)

Claudia Regina Serantoni da Silva

DATA DE PUBLICAÇÃO

2003

RESUMO

The effects of the volume fraction of eutectic carbides and of the matrix hardness on the thermal fatigue resistance of multicomponent white cast iron were investigated. Alloys Fe-4Cr-V-2Mo-2W-2C, V ranging from 5 to 8 wt% and Fe-4Cr-8V-Mo-2W-2C, Mo ranging from 2 to 5 wt % were used. Disc shaped samples were quench and tempered for obtaining two matrix microhardness levels: 450 HV and 650 HV. Thermal fatigue tests were carried out for 100 and 500 cycles. Each cycle involved high frequency induction heating of the surface to 600°C and subsequent cooling in water during 45 seconds (equalization of the bulk and surface temperature). The test specimens were characterized before and after the thermal fatigue tests. Before the tests, eutectic carbide (type, morphology, volume fraction, syze, shape and distribution of carbides) and matrix microhardness were characterized. After the tests, the macroscopic and microscopic thermal fatigue cracks (number and depth) and matrix microhardness were characterized. The nucleation of the thermal fatigue cracks takes place mostly at the specimen surface, induced by mechanical and metallurgical stress risers. The crack nucleates at the matrix (roughness as mechanical stress risers as well as at carbides (at the carbide/matrix interface or at the carbide itself). The nucleation rate is influenced by the volume fraction of eutectic carbide (the higher the volume fraction, the higher the nucleation rate) and by the matrix microhardness (the higher the microhardness, the lower the nucleation rate). The crack propagation mostly takes place at the carbide/matrix interface or through the carbide. The propagation rate is affected by the carbide distribution. The higher the “carbide continuity/carbide free path” ratio, the higher the propagation rate. The propagation rate decreases with increasing test time, regardless the eutectic carbide volume fraction and the matrix microhardness. The propagation behaviour during the first 100 cycles is characterized by instable crack propagation controlled by the fracture toughness of the material; from 100 to 500 cycles, the propagation is controlled by the stress magnitude. The syze of the test specimen also influenced the tests results: the larger the specimen syze, the higher the nucleation and propagation rates. This is attributed to the effect of increasing thermal gradient across the specimen with increasing specimen diameter.

ASSUNTO(S)

thermal fatigue cilindro de laminação high speed steel fadiga térmica rolling mill aço rápido

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