Bys S.

Khmelnytsky National University, Ukraine.

Ensuring of spring seal ring dimensional stability

Annotation: Rolling bits’ high durability is provided by their support protection from abrasive by means of steel rings instalment with guaranteed gap in string ring lock. Due to abrasive and hydrogen wear as well as ring’s external surface grip the gap increase takes place which causes joint uncovering and abrasive parts ingress into rolling bit support and, finally, bearing unit workability failure.

The research of friction conformity and string steel wear in different structural situation in the air and hydrogen media has been carried out in the work. An attempt has been made to determine main indicators of constructional durability and wear resistance of steel. The string steel optimal structure has been determined that provides high wear resistance, durability and reliability at work.

At present oil and gas wells are predominantly drilled with rolling bits of different types, which are able to work in different conditions. Long-term observations and exploitation experience showed that the absence of failures and workability are mainly determined by wear resistance and constructive peculiarities of support components, their production and exploitation quality [1, 2, 4, 5, 11]. In most cases (up to 70%) bit supports wear leaves behind cutting structures wear and is the major reason for bits substitution. The support components are at very complicated exploitation conditions. On the one hand they are subjected to wear and grip, and on the other they are permanently contacting with hydrogenous working media as well as ambient media (water and hydrosulphuric solutions). Encapsulation is of great importance for supports’ durability rise. For rolling bits’ open support protection from abrasive medium a string ring is used. The duration of its protective action depends, first of all, on the constructively correct choice of its geometrical dimensions – slit width in the range of 0.1…0.2 mm. The string ring contacts the milling cutter raked surface with its external surface, and slides on end surface inside journal furrow. At the same time the saturation of ring material with hydrogen takes place, which, in turn, causes hydrogen wear advent, i.e. total wear increase.

So, the aim of the research was 60C2 spring steel friction and wear regularities determination (С - 0,57...0,65%; Si - 1,5...2,00%; Mn - 0,60...0,90%; Cr £ 0,30%; Ni £ 0,25%) with different structural conditions in the air and hydrogenous medium.

Mechanical characteristics of 60С2 steel after hardening and tempering are given in table 1 [14].

Table 1 - Mechanical characteristics of 60С2 steel after hardening and tempering

Tempering temperature, °С.	Structural condition	sв, МPа.	s0,2, МPа.	d, %.	y, %.	HRC, МPа.	КІс, МPа×Öm.
200	LM+RА	2510	2230	0,53	0,41	590	22,58
300	М	2470	2110	1,28	11,9	550	29,03
400	Т	2180	1970	2,42	26,0	500	42,58
500	Т+S	1580	1380	2,95	34,1	460	54,52

LM - low-temperature tempered martensite; RA - retained austenite; M – martensite; T – troostite; S – soorbite.

The researched steel structure, obtained as a result of hardening and low-temperature tempering, is a typical lath martensite with high dislocation density, containing about 8% RA. After medium-temperature tempering – dispersible structure troostite plus soorbite.

Tribological characteristics of the researched steel was estimated on the basis of analysis of controlled variable dependencies (wear intensity, temperature in the friction zone, friction coefficient) on veriable factors (slip velocity and specific load) fig.1.

Fig. 1 - Dependencies of 60C2 steel wear intensity (1,2) and friction slip coefficient (3,4) with different structural condition at pressure of P=10MPa for different velocities: a - V = 0,1 m/s; b - 0,2 m/s; c - 0,3 m/s; d - 0,4 m/s (1, 3 – in the air; 2, 4 – in hydrogen).

Fig. 1 shows the fact that tribological characteristics of patterns being researched in the air and hydrogenous medium differ greatly. Differences can be observed at friction slide speed and pattern clamp pressure. Curves analysis, given in fig.1 showed that at certain conditions wear intensity and friction coefficients decrease for patterns in hydrogenous medium in comparison with ones, being researched in the air and vice versa. Minimal wear intensity and friction coefficient values at certain conditions of the research (friction slip speed, pattern clamp pressure, structural condition) can also be noticed. It’s necessary to note the fact that relative slide speed being rised above 0.3 m/s as well as tempering temperature being increased above 300°С make tribological situation of the researched 60C2 steel with the rider unable to work due to metal of friction surfaces grip and, as a consequence, rapid mechanical friction of contact surfaces takes place. It being noticed that the disastrous wear is observed from the very beginning of work.

Microstructure analysis of friction surfaces of the steels researched with different initial structural condition confirms grip focuses presence on their surface.

The least wear intensity value is observed at friction slip speed of 0.2 m/s and pattern clamp pressure of 10 MPa for patterns with tempering structure of 300°С, whereas minimum friction coefficient value has been noticed to be at friction slip speed of 0.3 m/s and pressure of 10 MPa for low-temperature tempered martensite. It’s necessary to be noticed that in the friction slip speed and pressures range, being researched, the patterns tempered at 300°С have the lowest wear intensity at probation in the air (fig.1, curves 1).

The results of research of 60C2 steel patterns’ wear intensity dependency in the air and with different structural condition given in fig. 1 indicate the fact that the curves have qualitatively similar character and differ in quantity only. Actually, at constant wear intensity is going down to certain minimal value with friction slip speed increase, but then is going up with further speed increase. Moreover, the tendency is typical for all patterns with different structural condition. The dependency character is unchangeable with pattern clamp pressure increase.

Patterns hydrogen pickup changes wear intensity situation, but the patterns with tempering structure of 300°С have the lowest wear intensity, practically for all conditions of research (fig. 1, curves 3).

Patterns hydrogen pickup leads not only to qualitative, but qualitative change of wear intensity character of 60C2 steel in comparison with patterns researched in the air. In addition, steel wear intensity increase in temperature range of 100-300°С for all speed and pressure ranges takes place. This can be explained by the fact that materials’ hydrogenation with low-temperature martensite structure causes greater embrittlement of the given materials and, correspondingly, their intensive wear [3, 6, 12, 15].

Analysing friction surfaces, it has been noted the secondary structures presence with residual phase and structural changes on metal surface layers which had been caused by processes, accompanying friction and wear. Such interaction products are the third phase and have a screening effect, and wear process itself is a creation and destruction alternation of the protective secondary structures [9, 10].

Fig. 2 - Dependency of contact temperature T from structural condition of 60C2 steel and testing rate (P=10MPa): а - V = 0,1 m/s; б - 0,2 m/s; в - 0,3 m/s; г - 0,4 m/s(1 – in the air; 2 – in hydrogen).

Increasing tempering temperature of 60C2 steel up to 400°С, the decrease of patterns’ wear intensity in hydrogen medium is observed, and, in some cases, the patterns’ wear intensity is found lower than those researched in the air. Particular fact is necessary to be noted that wear intensity of the patterns with tempering structure of 500°С and higher (the research of the patterns with tempering structure of 650°С was also carried out) in hydrogen medium is lower than in the air for all speed and clamp pressure ranges. In this case, obviously, hydrogen braking of moving dislocations takes place, which causes materials’ wear resistance increase. This suggestion is verified in several works [7, 8, 13, 16, 17]. Moving dislocations’ hydrogen braking occurs due to Kottrell’s atmosphere creation, which hinder dislocations’ relocation in metal crystal lattice. It’s obviously that hydrogen, which is present at tension imposition, diffuses with active slip surfaces in the result of slip friction and forms atmospheres that hinder dislocations’ migration and hardening of the material takes place.

Fig. 3 - Dependency of contact temperature T_F from slip velocity of 60C2 steel with different structural condition (P=10MPa): a – tempering temperature of 200°C; b - 300°С; c - 400°С; d - 500°С (1 – in the air; 2 – in hydrogen).

The temperatures in friction zone, depending on experiment’s condition, have been researched (fig.2, 3). For the patterns, researched in the air and hydrogen medium, temperature in the friction zone increases with simultaneous both slip friction speed and pressure increase. Moreover, low-tempered martensite, being transited to soorbite and slip friction speed being increased, the decrease of temperature in the patterns’ friction zone in hydrogen medium has been noticed. It should be also noticed that for the patterns tested both in the air and hydrogen medium with tempering structure of 400°С the lowest temperature in the friction zone for whole loading-speed range is observed, whereas for the patterns with tempering structure of 300°С it is the highest.

Analyzing 60C2 steel friction slip coefficient dependency with different structural condition for all criteria of research (fig. 1), one can notice, that the dependency character of the patterns, researched in the air and hydrogenous medium, is different. It must be noticed that friction coefficient of hydrogenated patterns is less than non-hydrogenated ones at friction slip speed of 0.2 m/s. Similar regularity is also observed at patterns clamp pressure increase.

Factual research of friction surfaces testifies the latter to be covered with protective skins of the secondary structures. Moreover, the secondary structures’ qualitative difference on the friction surfaces, researched in the air and hydrogenous medium, can be clearly visible. The secondary structures, obtained at patterns research in hydrogenous medium, are light, whereas the ones obtained at research in the air are dark.

Possibility of materials’ wear resistance prediction on the basis of standard mechanical properties characteristics and destruction tenacity criterion (K_Ic) is found to be of special interest. So, comparative evaluation of ultimate stress limit (s_В), hardness (HRC), fracture toughness (K_Ic) and wear intensity (I) dependence of 60С2 steel from it’s structural condition has been carried out (fig. 4). It has been determined that temperature being increased, ultimate stress and hardness limit decreases, whereas destruction tenacity criterion increases. Material’s wear intensity goes down at first, but then goes up. In our opinion, such a trend of wear intensity curve can be explained by the fact that fracture toughness value increase raises it’s wear resistance, though durability values decrease (up to tempering structure of 300-400°С).

Fig. 4 Dependencies of constructive durability and wear resistance values from 60C2 steel structural condition.

Further tempering temperature increase though causes destruction of material and durability characteristics decline, but wear intensity increases. On the basis of that, we can make a conclusion that depending on material’s structure there may be two ways of high stress relaxation, which takes place in friction contact zone of two surfaces: plastic deformation (which takes place in materials with low yield stress and high fracture toughness) or destruction (at considerable alloy hardening defects of submicrostructure are locked, plastic deformation is minimal and relaxation is realised due to new surfaces creation – crack growth). For 60C2 steel with tempering structure up to 400°С relaxation of tensions, occurring in surface layers of materials, being rubbed, is obviously carried out by fragile destruction. At tempering temperature increase up to 400°С relaxation mechanism change takes place. Besides, one cannot but admit cymbate dependency character of wear intensity (I) and fracture toughness (K_Ic) (fig. 4), which denotes two characteristics bond. So, to impart high wear resistant values it’s necessary not only to increase it’s yield stress, but also destruction tenacity criteria (K_Ic), where destruction tenacity’s share is obviously dominant.

Thus, the research let us recommend 60C2 spring steel with martensite tempering structure (tempering temperature of 300°С) for rolling bits’ support protection from abrasive parts ingress and their rapid wear, which provides minimal wear intensity values.

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