THE INFLUENCE OF MICROSTRUCTURE AND HYDROGEN-CONTAINING ENVIRONMENT ON THE INTENSITY OF CAST IRON AND STEEL DAMAGE BY SLIDING FRICTION

Fig. 8 shows the generalized scheme of the surface layers structure of graphitized cast-iron friction (with the partial explanation for the influence of alloying elements on the  structure and tribotechnical properties) . This scheme can be also connected with the generalized scheme of cast-iron "behavior" (materials) during sliding friction when lubricating conditions change (see the second part of the article). Thus, the dashed lines (the figures explain this condition) indicate the limited presence or absence of lubricant on the surface of friction (see the second part of the article Fig. 2.). The  explanations on the scheme are given for all of the material.

It is possible to induce  essential changes in the properties  and microstructure of cast-iron  by complex alloying. Even the introduction of a small amount of additives up to 1 % increases  the mechanical and tribotechnical properties [21]. The introduction of carbide elements, e. g.  Cr, V, Ti, Mo, leads to the following:

- the increase of alloy hardness, due to:

·         increase of microhardness of a solid solution of metal matrix,

·         increase of microhardness of cementite carbides (Fe, Me)₃C;

·         formation  of  complex alloying carbides, for example М₂₃С₆ М₇С₃;

·         formation of the special highly rigid carbides of  VC, TiC;  V₄C (Fe, Cr, V)_xC_y which  harden the metal matrix after special thermal processing;

- change in the morphology of carbides and graphitic phases;

-   change in the morphology of parts of an alloy crystallized in the form of dendrites ;

-   change in the morphology and strength characteristics of other components, e. g. phosphide eutectic;

- change in the type of connections in metal matrix;

-   small dispersion of carbides in metal matrix, which promotes the restraint of promotion of dispositions;

-   change in the type of metal matrix (at high percentage of alloying elements), and, consequently,  a change in properties of an object in general.

The introduction of modifiers in an alloy (for example, Ca) promotes the clarification of section borders between phase components of an alloy from non-metallic inclusions that promotes the increase of crack resistance. Assuming that non-metallic inclusions can initiate  appearing and distribution of cracks during friction, then for some chemical grey-iron compounds modification is also a necessary condition for promoting the increase of wear resistance [22]. 

There are also some data proving that Cr and phosphorus are part of secondary structures that interfere into activation processes on the surfaces of friction [23, 24].

The fact that the alloy has the ability to harden during friction, increases its superficial hardness and, as a rule, the bearing ability of object material. One of the best known cast-irons, possessing the abilities specified above, is austenite manganese cast-iron [25].

In the wear conditions an object is  affected by different cyclic loads  which influence  fatigue resistance of structural materials. Adsorbed hydrogen on the object surface, released from a lubricant in the wear process, also accelerated the fatigue process of contacting materials [26 ]. The viscoplastic metal matrix  also brings an advantageous influence on the intensity of wear process, connected with the character of separation from the material.  Figure 9, image (a) shows  cast-iron with the viscous character of the destruction of pin edge, while picture (b) shows a specimen where the fragile character prevails.