Loading of the heat treat oven Heat Treat. Throughout the rugged, daily usage the crankshaft is exposed to extremely high forces. Both, crankshaft and connecting rod, are subject to a heat treat process at almost 1000°C after the initial soft machining stage. This process modifies the crystal structure of the surface, giving the component the increased wear resistance required.

What does it mean "a crankshaft is hardened"? What is the purpose? By adding carbon to the outer layer of the shaft the component is surface hardened, while the inside remains soft and therefore more ductile and tough. The result is a high wear resistance and long service life. What is the grain structure obtained after quenching the carbon-enriched steel? The grain structure depends on the quenching speed and the chemical composition of the steel. The structure obtained in our crankshafts is called "martensite", a very hard but non-brittle crystalline structure. Carbon is not the first thing that comes to mind when a layman thinks of the term "hardening". How does it work? The crankshaft is placed at almost 1000° C in an oven containing a carbon atmosphere. The carbon penetrates the surface of the part. After quenching, a harder surface structure is generated. This is the basis for low wear in
very heavy-duty operation conditions. Tempering is a complex and expensive process, taking 4 to 6 hours. Is it worth it? Yes of course, time and effort is well spent. Without surface hardness, the bearings and other parts would rapidly wear into the shaft. Only this special hardening process ensures the typical long life of this critical STIHL component.