Dimensional Changes During Service
Dimensional Changes During Service
(OP)
Comments appreciated on the following:-
I am working on some failures which might be explained by several mechanisms one of which includes small dimensional changes during service.
Failures involve rotating equipment which has been in service for a number of years potentially seeing many millions of cycles of normal loads plus many starts and stops. The materials are quenched and tempered alloy steels.
Is there any information or references to these materials changing shape during their service life due to either redistribution of residual stresses or perhaps transformation of retained austenite?
I am working on some failures which might be explained by several mechanisms one of which includes small dimensional changes during service.
Failures involve rotating equipment which has been in service for a number of years potentially seeing many millions of cycles of normal loads plus many starts and stops. The materials are quenched and tempered alloy steels.
Is there any information or references to these materials changing shape during their service life due to either redistribution of residual stresses or perhaps transformation of retained austenite?





RE: Dimensional Changes During Service
I do not know to what degree, if any the material moves but I would look very closely at that theroy if I was asigned a task like this.
Years ago I was working with forged gears that warped during heat treat even when they were press quenced. We ended up having to take geat care in normalizing and tempering the raw forging in order to get all the stress' out of the material.
Good luck, David
RE: Dimensional Changes During Service
We have large induced draft (ID) fan elements on our power boilers that were fabricated by welding quenched and tempered low alloy steel. These fans operate at 900 rpm for many years in various temperature ranges and are subjected to numerous start/stop cycles. The weld detail used in the construction of the fan element was a combination of partial penetration and fillet welds.
A majority (99.9%) of the rotating equipment problems that I have seen and analyzed over the years are typically high cycle fatigue cracks caused by stress risers from fabrication (weld undercut or poor design). I have never seen any fan blade failure caused by dimensional changes from transformation of retained austenite or from excessive residual stresses. Although, local residual stresses from welding could indeed play a significant role in the initiation and propagation of fatigue cracks if a stress riser is present.
We take great care during fabrication of new rotating equipment or for repairs to existing rotating equipment to assure attention to detail like blend grinding weld toes to reduce stress concentration and stress relieving new, welded fan elements to assure increased resistance to fatigue crack initiation in service.
RE: Dimensional Changes During Service
How critical is the balance of these components, and was that affected by the dimensional changes? Have the parts been balanced after being put in service?
BK
RE: Dimensional Changes During Service
The failures occur by fatigue crack initiation and growth but it is not clear what the root cause might be. It is possible that the failures are initiated at the site of small non-metallic particles but I have so far not been able to identify the presence of any specific inclusions . The second possibility is a change in load condition due to changes in relative position or relative size. Instrumentation attached to the parts do not show any sign of relative position change so I was exploring the shape change theory.
RE: Dimensional Changes During Service
1. The transformation of retained austenite will cause small dimensinal changes, more significant in higher carbon alloys. Most application where this is a real concern are heat treated for maximum stability, using some combination of sub zero processin ( -80 to -120 F very common) and the have the final tempering temperature as high as practile with respect to the required hardness.
2. Changes in residual stress may also have an impact. If the final processes happen to be grinding processes, a significatn amount of stress can be created, sometime this is an advantage, sometimes not. Compressive stress assist in prolonging fatigue life. Tensile stress is generally considered undesirable.
3. If the final processes include grinding, it is also possible to damage the mateiral by grinding "burns", (i.e. overheating the part by aggressive grind processes.) These burns can be inter-related to varying stress patterns also.
If there are shafts associated with the fans, then some of these items may apply, as the fan shafts are most likely ground. The fan blaeds themselves may not be ground, but may have the effects of other forming/shaping processes.
Hope this provides some ideas.
RE: Dimensional Changes During Service
RE: Dimensional Changes During Service
RE: Dimensional Changes During Service
LewTam Inc.
Petrophysicist, Leading Hand, Natural Horseman, Prickle Farmer, Crack Shot, Venerable Yogi.
RE: Dimensional Changes During Service
This was an effective solution dimensionally but produced carbide networks in the grain boundaries so that metallurgically the coalesced carbide trails acted as sites for crack initation and paths for crack propagation from Hertzian stresses. Since you have not been able to identify the presence of any specific inclusions, it is probable that you would have detected carbide networks if present. But this brings into question what metallographic practice you are using since even for aircraft-bearing-quality-alloys there are allowable inclusion count and size.
Not only can grinding burn result in cracks but if LSGT (Low Stress Grinding Technique) is not employed, the burn can generate untempered and overtempered Martensite which later can act as crack initiation sites when loaded with Hertzian stresses. Does ISO 14104 nital etch procedure provide for detection of untempered and overtempered Martensite in the case surface?
RE: Dimensional Changes During Service
The ISO 14104 test procedure can identify various levels of grinding damage it is the international version of AGMA 2007
RE: Dimensional Changes During Service
Maui
RE: Dimensional Changes During Service
Have you considered creep. It causes "growth", but I believe that there has to be heat involved.
RE: Dimensional Changes During Service
Aviat - I think the temperatures are too low - less than 150C