Posted by Ken in CO on Friday, January 19, 2001 at 12:09PM :
In Reply to: Nitriding my crankshaft posted by Matt Wilson on Thursday, January 18, 2001 at 10:46PM :
I don't believe it's "standard", atleast in a rebuild situation. It may be standard in the manufacturing process of crankshafts.
First, I doubt he's referring to the process used to coat drillbits, but it is possible. It more likely he is referring to either the carbonitriding or nitrocarburizing process. Both are thermal treatments with specific atmospheres.
First, carbonitriding: It's done in a salt bath, granulate or gas atmosphere where the carbon content is controlled. The part gets heated to 850 - 1000 degrees C. The hardening then occurs when the part is quenched(direct hardening), when it cools to room temp via convection(single hardening), or it is cooled to an intermediate temp then reheated to the hardening temp, creating a martensitic surface layer. Specific temps are used to elicit higher carbon contents in the upper layers(case refining) or for the core(core refining). Too much carbon and you get residual austentite, not desirable. Since the carbon gradient is very important and is controlled through temp, atmosphere, time, and the metallurgy of the part, the process is not easily done correctly. With carbonitriding, the nitrogen helps with the tempering properties resulting in better wear resistance. The main reason I don't believe this is a process used on cranks is the temps the process requires and the volumetric changes that occur with microstructure transformations, do not allow one to hold tolerances required of crank journals.
Nitrocarburizing(nitriding) is a thermal treament process that occurs at 500 - 600 degress C. It is used to enrich the surface layer with nitrogen, while some carbon also diffuses into the surface. The nitrided region of the part is made up of layers, an outer layer, a white layer, and a diffusion layer. The layers are of different hardnesses and correspond to hardness gradient from the surface to the core not unlike that of surface or case hardening. A nitrided part will show increased resistance to wear, corrosion, and cyclic stress.
Due to the low temps involved with the nitrocarburizing process, no volumetric changes occur in the part which occur with the carbonitriding process which, due to transformations in the microstructures does experience volumetric change. Also, with the nitrocarburizing process changes in dimensions and shape are extremely small. So, as a final process on a crankshaft, which must maintain dimensions inside several thousandths of an inch, I would presume nitrocarburizing to be the process used.
For an engine with a known problem of soft journals or some such problem, I suppose that treating could help. But there are plenty of high performance engines which do not use a treated crank (excepting what was done in original manufacture). For an engine in a truck? Are the cranks in the 230s having problems with the journals wearing before the bearings? If the bearings are already softer(which they should be), do you really need to make the crank journals even harder? Especially in a truck engine which isn't seeing particularly fantastic loads on the crank bearings, main or rod. Since journals on a crank can pretty much be repaired no matter what, I imagine the main purpose may be to increase the durability of the crank as whole, not perticularly the journals, but their increased resistance to wear is a bonus. This would lead me to believe that if the crank's resistance to cyclic stress can be increased through nitriding, then the rods should also be considered. I myself have a set of '58 forged steel rods for a Pontiac V8 that I'm looking to have nitrided in the future. It will dramatically improve their longevity.
So, to succinctly answer some of your questions:
I don't think it is standard.
I don't know what specifics for your crank are needed.
It would NOT be carbonitriding, it would be nitrocarburizing, very different processes.
I'm not an expert, just have had a enough schooling to make me dangerous.
Ken
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