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Heat-Treatment of Crankshafts

crankshaftsThe article last month covered the subject of materials used for Formula One crankshafts. This feature looks at the subject of heat treatments currently used in Formula One.

As we saw last month, the materials in current Formula One use are mainly nitriding steels. By nitriding steels, we mean those with a composition containing elements which are strong nitride-formers, chief among these being chromium, aluminium, molybdenum and vanadium.

As you can see from looking at the tables in last month’s article, both of the steels shown contained chromium and molybdenum, with 3S132 also containing some vanadium. EN41 is a steel which contains significant amounts of aluminium (>1%), although this doesn’t seem a popular choice for crankshafts.

However, before we come to the important subject of nitriding, we will look at the other heat-treatment processes which are used prior to nitriding. In general, the racing crankshaft, certainly in Formula One, is machined from a solid cylindrical billet of material. This material, unless specifically requested to be delivered in a hardened condition, will be delivered with a standard heat-treatment from the mill, and will typically be in a softer condition than you will want to specify for the core hardness of a racing crankshaft. This is not seen to be a problem as there will be several stages of heat-treatment before the nitriding treatment is undertaken. After initial rough machining, the crankshaft has had a lot of material quickly removed and there are machining stresses in the steel, which can lead to unexpected distortion further along the production route. The advantages of having a softer material at this stage are (a) the material removal can often be quicker and (b) owing to the lower shear strength of the material, the machining stresses are lower. The traditional method of crankshaft manufacture, by offset turning using big-stiff lathes, was quite a violent method of rough machining thereby putting a lot of stress into the material. Whilst it is impressive to see the old methods used, the shallower cuts and higher speed machining methods used on CNC machining centres mean less machining stress.

At this stage, with a reasonable amount of stock left on the rough crankshaft, it undergoes a stabilisation treatment, often called stress-relieving or stress-relief annealing. This is done at a reasonably high temperature, and is made quicker by using increased temperatures. The relationship between the degree of stress-relief and time / temperature is a complex one; there is a parameter which contains the time and temperature variables, and increasing the parameter leads to a greater degree of stress relief. Let us, for example, say that a given steel will have 90% of its stress relieved by five hours at 600°C. The same steel would need almost 100 days at 500°C to achieve the same effect. 600°C is a typical temperature for this type of treatment, and is high enough to cause significant temper-softening in materials which are hard enough to use for racing crankshafts. It would therefore be of little use to order pre-hardened material from the mill. Sometimes there is more than one stabilisation treatment used before the hardening and tempering treatment which will define the final core hardness of the crankshaft.

Next month we shall continue to examine the heat-treatment processes to which a crankshaft is subjected before it is finished.

Written by Wayne Ward.

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