I am interested to know what is the information (in terms of microstructural parameters), if any, that gets transfered from Hot Forming to final fatigue life of any component, considering the fact that it has undergone various intermediate Heat Treatments operations like Normalizing, Carburizing, Quenching and Tempering between Hot Forming stage and final fatigue testing stage. In other words, how can I bring in the effect of Hot Forming of a Gear blank, on to the fatigue life of final finished gear, from a modelling perspective? I am a bit confused in this regard, as i think the recrysatallised microstructure must be getting effected during various heating and cooling cycles involved in various heat treatment processes and not much of the modellable-information must be getting transferred which can effect its final fatigue life. It would be highly appreciated, if any one can throw more light on this.
Dear Mr. Khan,
this is a typical processing question. i guess a direct observation route is dificult but a statistic modelling might be possible. Please refer to Box and Hunter "Statistics for experimenters". in the second part the autors discuss the method for factorial design. This method is helpful to get information about influencing parameters. if you know them a more detailed investigations becomes possible but now focussed on the identified effects. This second step might be a second working package.
my regards
The major advantage of hot forming (forging) is the production of elongated directionally oriented grains, which provides greater strength at critical locations, such as root of gear teeth. Any subsequent heat treatment, such as pack carburizing, will result in grain growth due to recrystallization. However, there are methods, which provide surface modification at low temperature. Also, if the steel is hardenable, due to its composition, the surface treatment like flame, induction, laser beam hardening will leave the bulk of the microstructure in the as-forged form and thus the directional improvement in properties provided by forging will be retained. Besides there are some other factors (such as porosity in casting, if that process is used, especially for large gears) where forging is advantageous. In a nutshell you have to consider the complete sequence of manufacturing process, for which the study of manufacturing process followed by leading manufacturer of gears will be of great help.
What controls fatigue life? Do fracture surfaces indicate that inclusions are the fatigue failure initiators? If so, you can relate maximum inclusion size to fatigue life and incorporate the effects of heat treatment by using yield stress in your model. That is where I would start.
I believe the questioner is more interested in processing of the forging (gear blank) from the microstructure perspective and not in the analysis of the actual fatigue failure of a forging when it has already occurred. From this angle the following information may be useful.
Inclusion (slag inclusion) is a defect commonly observed in castings. In forging the common defects are cold shut (fold), other types of cracks arising out of improper die design or working at low temperature. The main feature of forging is the fibrous structure (elongation of grains), which lends higher strength along the longitudinal direction of the grains. This also results in lower strength transverse to fibers. Proper forging die design requires that the fibers should be oriented along the direction of higher stresses.
In rare cases of large forgings where the cast billet is used for forging the inclusions present in the billet get elongated and are present as fibers. As stated in my previous answer only local heat treatment should be used to preserve the elongated grain structure. Thus any analysis of fatigue strength should invariably take into account the complete processing history of the part without which the analysis will be superfluous only.
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