With some crankshaft failures such as oil starvation bearing surfaces that are otherwise kept apart by an oil film come into contact and friction occurs. This leads to the bearing journal being subjected to very high temperatures sometimes in excess of 900 Deg C. When the rotating shaft comes to a standstill it rapidly cools and the microstructure of the material is badly affected.
If such a situation happened in a controlled manner this sort of process could be used to harden a material. However in this failure situation the hardening takes place, but in an entirely random manner, leading to very hard areas (up to 650 Hb) next to soft areas (300 Hb).
These above values are well in excess of the OEM's design specification and would make the safe re-use of the crankshaft impossible. Often heat cracking is also present and this cannot be left in the shaft under any circumstances as it would lead to catastrophic failure at some point (very soon) in the future.
Machining of the journal surface can be undertaken in-situ to remove the hard layers in some circumstances. If the hardness drops to below about 450Hb it is often considered ok by the OEM to run the shaft (assuming no cracks are present).
In the event that the hardness is too deep to remove by machining then the shaft would normally be considered scrap. In this situation the shaft can often be heat treated (annealed) on site within the engine to recover the shaft and so save the stripping down and removal / replacement with a new (often very expensive) shaft.
The Heat Treatment Process
To start with, the exact material composition of the shaft is required. Typically the original hardening and tempering heat treatment that was used. This is needed to set the various parameters to be used in the annealing process. For example the maximum temperature to which the journal is to be heated, the rate of heating, the soak time I.e. the time the crankpin needs to be held at the top temperature and finally the rate of cooling to ambient temperature.
The annealing process has to be carefully controlled to keep the soak temperature below the original tempering temperature of the shaft This ensures there is no influence on the basic mechanical structure of the shaft.
From our experience the typical reduction in hardness is typically 200HB (sometimes slightly more) subject to the as found condition of the crankpin journal.
The crankshaft is checked for hardness and for bend prior to starting the treatment. The straightness of the shaft is monitored during the process to try and ensure minimum remaining bend after the heat treatment. In most cases the annealing process relieves the built up stresses and any misalignment reduces to a minimum.
In most cases pre-machining of the crankpin is used to remove any light cracking or damage and to get rid of the top heat damaged layer. Often this in itself can relieve some stresses reducing any bend present.
Any large loads or constraints will need to be removed from the shaft so as to allow the shaft to move freely during the process.
Remote heat sensors are then placed on the crankpin to allow monitoring of the temperature during the cycle. The heating mats (specially sized for the size of the journal) are fitted and then are covered carefully by thermal insulation wraps to keep the heat where it is needed and allow for a tightly controlled heating and cooling cycle.
The time taken to complete an annealing cycle is typically 1.5 to 2 days.
Note: - that crankshafts manufactured with a hardened layer (Typically over 500hB) are not suitable for this process.
A secondary benefit in heat treating a crank pin is that it reduces the amount of subsequent re-machining and makes it more controllable.