Table of Contents
4.2.3 "Consumables" which are normally replaced during maintenance and assembly
4.2.3.1 Bolts and nuts
It should be obvious that one works according to the instructions of the producer. Thus, one has to take care to closely follow the clamping torque and/or the given elongation. While the implementation of a calibrated torque wrench is seen as generally obvious, this is not necessarily valid for the regular control of the wrench.
Tightening of connections is an especially important procedure, decidedly influencing the fastness of the bolt connections. A too low bolt prestress conceals the danger of vibration fatigue, whereas a too high prestress could damage the bolt. The lubrication used is decisive for the aimed prestress in the bolt shaft ( "Ill. 4.2.3.1-1"). Often, the conditions of tourquing up are influenced through the media that does not get enough attention. The use of new bolts with preservation oil is typical, if they are to be removed prior to usage.
In addition, one has to pay attention to the coatings of the threads of the bolts and nuts (e.g., silver or dry film lubricant). From the angle of corrosion relationships, there could be an equality of two coating versions; the friction coefficient can, however, often not admit an exchange. When in doubt, a clarification should follow with an adequate near service screw measuring device. When using bolts from new suppliers or with alternatives in the case of tight corners of available goods, a measurement is recommended.
Silver coated bolts in hot parts have indeed the advantage of little aptness for galling, lower not so scattering thread friction and so are easier to loose. However a grave disadvantage of silver is, that it promots sulfidation and embrittling diffusion. Therefore, today, often in hot parts no silver coated bolts are used.
The expense involved in screw connections is not to be underestimated. That is why the question of the repeated use of bolts and nuts surfaces frequently. The answer lies in the experiences of the manufacturer, relevant to the connection at hand. E.g., in hot parts, it is not seldom that seized bolts are only releasable through very high initial breakaway torque and then the danger of an unknown impairment of the screw exists, with the risk of fracturing during reassembly.
Experience reveals that a protection against damage through critical and attentive personnel, precisely in the case of connections, is especially effective. Correspondingly, personnel should be motivated through technical informations. All deviations from usual behavior of bolt connections during torquing are important: alteration in the elongation behavior in dependence on torque.
![[[@en:4:42:423:ex_en4dot2dash7.svg|Example 4.2-7]]](/lib/exe/detail.php?id=en%3A4%3A42%3A423%3A423&media=en:4:42:423:ex_en4dot2dash7.svg "[[@en:4:42:423:ex_en4dot2dash7.svg|Example 4.2-7]]")
Example 4.2-7: A graphite containing lubricant was prescribed for the thread of a clamping element. Due to technical reasons, however, a compound containing MoS2 was used for assembly. As a consequence of the mounting of the center bolt through a center hole during assembly of a high loaded component composed of Ni alloys, some of the compound was wiped off and got locked between the centering surfaces. The compound decomposed at operation temperature under deficient air, prior to turning harmless due to oxidation. Through diffusion, cracks and fracture of the component with extreme secondary damages arose.
"Illustration 4.2.3.1-1": Bolts are an important engine element and therefore require our special attention. This is valid for the prior treatment of the bolt as well as for the actual screwing and unscrewing procedure.
Pre treatment: First of all, because of the current specifications or recommendations, one has to clarify whether one is dealing with the bolts prescribed (design, manufacturer) and if the thread has to be lubricated. Frequently, a lubrication is not planned, if the thread already shows a coating. In this case a possible preservation oil must also be removed, else there is the danger of over stressing the bolt through the prescribed clamping torque. If lubrication is required, the appropriate, i.e., specified lubricant is important.
Notice: The thread friction (nuts and bolts) is of decisive importance for the desired characteristics and the operation load of the connection (prestress, dyn. load).
False lubrication can also lead to corrosion or embrittlement and, consequently, to bolt fracture in operation. If bolts are to be reused, one has to check if the manufacturer has permitted this and if certain measures towards quality insurance are demanded. Such measures include inspection with a magnifying glass, crack inspection or recoating. With regard to self locking nuts, one has to check their reusability.
Tightening procedure: During a screwing procedure, one pays attention to the right amount of torque. If this behaves unusually, e.g., it is not attained on account of too strong bolt elongation or only after noticeably more turnings than usual, the cause has to be investigated ( "Example 4.2-7"). Questions are to be cleared concerning the calibration of the torque wrench or the right bolt material.
Unscrewing procedure: When unscrewing the connection, attention should be given to loose bolts or a shearing off of seized bolts. There can be alarm signals for bolt problems. Bolts, indicated by the break torque, feared to have experienced an overload should not to be reused.
Notice: Show bolts during tightening an unnormal behavior the case must be duobtless clarified.
If applicable, tests on a bolt testing rig are recommended. The OEM or the lab should possess such a device.
"Illustration 4.2.3.1-2": (Lit. 4.2-4): Mechanical operation loads that are the case of bolt failures are primarily dynamic. Thereby develop fatigue fractures/cracks. They can as well occur by high frequent vibrations as by low frequent loads like thermal fatigue ( "Ill. 3.3-16") or cyclic changing centrifugal forces.
Fatigue fractures appear without plastic deformations also in ductile materials. However they are not called brittle fractures because the material is not embrittled, but only looks like this due to the crack propagation mechanism. If an analysis of the fracture in the SEM is possible, in most cases a specialist is in the position for a certain identification. Enough experience enables a macroscopic evaluation ot the fracture surface. Thereby at least first hints at type and hight of the dynamic loads can be expected.
At bolts, due to stress concentration areas where forces act and at geometric notches there are regions which are predestined to be overloaded and to develop fatigue failures.
Fatigue fractures under design loads can develop from weak points and defects by production, assembly or induced by operation. To operation caused defects (upper frame) count:
- ‘Pittings’ like from corrosion and sulfidation (at Ni-alloys).
- Fretting wear.
- During assembly galling grooves at the shaft can occur
Static load produces at sufficient high operation temperature as failure cause creep failures with plasic deformation (creep). During long operation times under a low load the creep strain can be very small. It does not necessarily attract attention by ovious plastic deformation. However, also in such cases we can not come from a material embrittlement as cause of the fracture. A feature is a heavy oxidation of the fracture surface relative to the residual fracture. This aggrevates also the microscopic fracture analysis, especially of the incipient crack area. Metallographic creep voids (pores) can allow a reliable confirmation.
Forced fractures on bolts of gas turbines due to mechanical overload are very rare. They can develop as secondary failures, for example
- in an case of containment (hit by a fragment) on the bolting of casing flanges.
- At rotor bolting when extreme unbalances occur (blade fracture. rotor bow, "Ill. 2.2-2").
- As a shear failure of a centric clamp bolt after the hub fracture of a turbine wheel.
- At flange boltings in rotors by fragments or foreigen objects (e.g., tools) spinning around inside the rotor
Depending from type and direction of the overload (tension, bending, shear, torsion) a typical fracture course can exhibit (lower frame). From the macroscopic and microscopic fracture pattern, when indicated with a macro etching (damaged fracture surface) forced fractures can be identified as well as the type and the direction of the force.
"Illustration 4.2.3.1-3": (Lit. 4.2-4): By far the most fractures and cracks of bolts in gas turbines show, except forced fractures that are normally secondary failures, at least macroscopic a brittle appearance. This can have different causes:
Causative embrittlement:
Stresscorrosion cracking (SCC) is a potential threat for bolts and nuts from high strength steels („A1“, „A2“). To cracks and fractures it comes under design conform conditions only, if the structure/material deviates from the specifications. Mostly this can be proved if the hardness limits are exceeded (mostly 32 HRc). The fracture patterns seem often pronounced crystalline and show corrosion features (rust), especially at the origin. Microscopic the specialist can identify this failure mode on evaluable fracture surfaces sure and without problems. Features show the relationship of the failure mechanism with hydrogen embrittlement.
Hydrogen embrittlement („B1“, „B2“, „B3“) is caused by hydrogen which diffused into the material during production or overhaul processes, when there existed a longer time interval than specified, till the anti embrittling process occurred. This embrittlement develops over a longer time during storage or in the operation. It is irreversible and can not be proven by an impact test (falling weight test). Typical processes that cause hydrogen embrittlement can be galvanic coating, etching and the stripping of coatings.
Embrittlement by diffusion of solid foreigen metals (SMIE). This danger exists at unforseen high operation temperatures. Cracks start preferential in the thread („C1“).
Embrittlement by dipping of foreigen melts (LME). Thereby in a high speed process wetting metal melt penetrates under enough high tension stresses the material. („D1“). The origin area of the fracture surface can show an unnormal discoloration (silvery) which can not be explained by oxidation.