Plastic strain of cobalt-based hardfacings under friction loading

Abstract

Hardfacing Plastic strain High load tribological test Work-hardening Strain-induced phase transformation a b s t r a c t Aeronautic forging dies are subjected to very high loads and temperatures for a long contact time between the pre-heated parts and dies. Cobalt-based hardfacings are commonly deposited on dies and their main wear mechanism is large plastic deformation of the die radii.This paper deals with the wear damage mechanisms of three different cobalt-based hardfacings: Stellite 21 deposited by a MIG process, Stellite 21 and Stellite 6 deposited by a LASER process. The tribological tests are carried out on a high load Ring on Disc tribometer at room temperature. The postmortem investigations are undertaken by SEM observations, micro-hardness measurements as well as by X-ray diffraction analyses.Results show that the increase of the hardness, in order to improve the wear behaviour, can be achieved by a higher carbon content and by a lesser iron dilution that depends on the deposition process. A very important work-hardening, up to 90%, is also observed under sliding conditions and a relationship is established between the increase of the micro-hardness and the plastic strain level. Two different plastic strain mechanisms are observed. For high (MIG) or low (LASER) iron dilution levels, the plastic strain causes respectively a reorientation of grains or a FCC to HCP phase transformation; the latter being associated with a lower friction coefficient.

Shear mode M3 in the first sites of ductile metallic alloys: Some considerations on the physical mechanisms leading to internal particle flows

“…The dual-tempered martensitic steel (X38CrMoV5 steel) and the cobalt-base alloy (coating of stellite 21 grade) are commonly used in forging tools. X38CrMoV5 steel is used as a bulk material in forging dies and stellite 21 is used as a thick (> 1 mm) wear-resistant coating of forging-die surfaces [30][31][32][33][34][35]. In these experiments, stellite 21 coatings on 40NiCrMo18 steel substrate were applied by two processes (a MIG welding process and a LASER cladding) in order to assess the influence of chemical composition on the plastic deformation of cobalt-base coatings (differences due to the process parameters) [30].…”

“…The microstructure of the thick coatings is dendritic and characterized by columnar grains perpendicular to the substrate surface ( Fig. 6-a) [30]. The carbides are precipitated into inter-dendritic spaces.…”

“…The tribological devices used were a pin-on-disc tribometer and a ring-on-disc tribometer. These two devices have been extensively presented in previous research works [30,32,33].…”

“…Some coatings made of cobalt-base alloys (stellite) display martensitic transformation in the sub-layers of the contact surface under friction experiments, i.e. deformation leading to transformation of the cobalt alloy from a metastable Face Centered Cubic (FCC) microstructure to a Hexagonal Closed Pack (HCP) microstructure [27][28][29][30]. Authors attribute the galling resistance of thick cobalt-base alloy coatings to a low stacking fault energy, which contributes to the deformation-induced martensitic phase transformation under friction solicitations.…”

“…The addition of alloying elements, in the Co-Base material, leads to a change in the level of the stacking fault energy. In particular, the iron-element increases this energy and thus enlarges the stability area of the metastable Face Centered Cubic microstructure [30]. The purposes of this article are to investigate some mechanisms of plasticity in Tribologically Transformed Surfaces (TTS), to define the consequences of an accumulation of plastic deformation in the material's response in terms of the "Third-Body" concept and to express the shear Mode M3, in physical terms.…”