Technical Sciences / 8. Metal Treatment in Machine-Building

PhD in Technical Sciences, professor Boldyrev A.I.

Voronezh State Technical University, Russia

FORMATION OF SURFACE LAYER

WITH DESIRED PROPERTIES

In the process of operation workpiece surface layer is subjected to the most intense physical chemical influence (mechanical, thermal, chemical, magnetic, etc.). In most cases the service surface properties of workpiece start to deteriorate. That is why surface layer as a rule has to meet more strict requirements than the bulk of metal.

Mechanical hardening of metal items is one of the main types of endurance strength increase, it works at alternate loadings. Hardening effect appears at definite peening that can be achieved only when initial surface layer has stable characteristics (inherited effects of previous operations).

It is known that fatigue failure is the result of microdefects stacking in surface layer, particularly increase of microcracks [1]. Every material has its own structural framework, thermodynamic stability, defining its peening degree, that does not cause loss of strength. Peening degree is outer indicator of change in physical and mechanical material properties depending on the number of cycles.

Fatigue failure limit depends on test temperature. The increase of temperature activates the factor of thermoactivated deformation, defining processes of hardening and cyclic yielding. At the definite stage of cyclic influences local damages appear and then turn into micro- and macrocracks, destroying the item.

Mechanical hardening promotes elimination of microdefects. But in peening excessive inner energy can cause loss of strength, that is mentioned in many publications [1, 2 et al.]. In paper [2] it is shown that fatigue limit in alloy steels of 30ХН2МФА type can grow up to 12,3 %, that corresponds to scientific studies. At cyclic tests it was revealed that ultimate stress limit of heat-resistant alloys depends not only on the structure but also on mechanical and thermal processing technology. That is why we need data summarizing test results in discovery of optimum peening for different alloys with individual (within batch of materials) inherited effects which have maximal stability (in some cases negligibly small) after electrochemical dimensional processing.

Local stress concentrators (cuts, damages, etc.) can be taken into account through coefficients presented for instance in the paper [3]. Depending on voltage gradients coefficient values can be from 2,5 – 3,0.

Mechanical peeing decreases anisotropy of properties in grains, promoting appearance of microcracks. Previous electrochemical dimensional processing allows to get before hardening surface layer of material with non-deformated or slightly deformated grains having stable (minimal) energetic indices, this promotes stabilization of control in achievement of exact peening index. Although after anode dissolution the appearance of microetching is possible, it does not influence the process of following mechanical hardening, because micropits arise mainly on the borders of grains and become apparent only at rather high grade of grains, for example in austenitic structure of high-temperature alloys.

Peening degree of surface layer after hardening can reach 60 % [1], but the optimum index in tested materials is in diapason up to 20 %. That is why control of peening is fulfilled in patterns according to microhardening measurements, that give stable estimation of peening size. The number of cycles during testing is determined by standards and the limits of durability are normalized (lower limits). Whereby during tests structural transformations are possible (generation of martensite from austenite, etc.) [1]. This paper also shows that change in number of loading cycles can cause microhardening growth up to 42 %, that we saw after electrochemical treatment at hardening by vibroimpact method according to existing modes. Here we faced embrittlement of ruptured zone because of change in structure, this became evident in machine steel after peening degree more than 15 – 16 %.

So in case with combined electrochemical treatment with ensuring of desired peening degree there are conditions for increase in number of cycles before initiation of surface rupture hearths and shift of microcracks penetration period, that determines ultimate stress limit in materials at high-cycle loadings.

From the perspective of energetic theory the development of surface microcracks at initial structure of material can become localized in joint area, that during hardening by proposed method is mechanivally united with the grain and reduces the appearance of microcracks by increase of endurance strength up to theoretically possible level. This is an important phase of control in mechanism increasing the results of high-cycle testsensuring efficiency of items even after the appearance of stress concentrators in the form of net made by local slightly developing microcracks not leaving plastic zone (in case when peening degree exceeds optimum value). The intensity of rupture heaths development depends on phase state of material, size of grains. Just so in austenite-martensite alloys (high-temperature and heat-resistant steels in particular) there is certain decrease in index of optimum peening, that is may be connected with micro embrittlements on boreders of grains during the process of anode stock removal with inherited effects.

Cited literature

1. Suslov A.G. Quality of surface layer in machine parts / А.G. Suslov. Мoscow: Machine-Building, 2000. 320 p.

2. Smolentsev V.P. Electrochemical Treatment of Inner Surfaces / V.P. Smolentsev. Мoscow: Machine-Building, 1978. 176 p.

3. Birger I.A. Residual Stresses / I.A. Birger. Мoscow: Mashgiz, 1968. 232 p.

4. Mukhin V.S. Technological Methods of Items Service Properties / V.S. Mukhin. Ufa: Ufa Aviation Institute, 1982. 56 p.

5. Sulima A.M. Surface Layer and Service Properties of Gas Turbine Engines Items / A.M. Sulima, M.I. Evstigneev. Moscow: Machine-Building, 1980. 240 p.

6. Boldyrev A.I. Experimental Investigation of Surface Layer State after Electrochemical Mechanical Treatment / A.I. Boldyrev. Bulletin of Voronezh State Technical University, 2010. Volume 6. № 10. P. 15-20.