Method of fabricating a steel forging, and a forging obtained thereby

Abstract

The invention provides a method of fabricating a steel part by forging, the method being characterized by the following steps: preparing and casting a steel having the following composition in percentages by weight: 0.06%<=C<=0.35%; 0.5%<=Mn<=2%; traces<=Si<=2%; traces<=Ni<=1.5%; traces<=Al<=0.1%; traces<=Cr<=1.5%; traces<=Mo<=0.30%; traces<=V<=0.5%; traces<=Cu<=1.5%; the remainder being iron and impurities that result from preparation; forging a blank for the part at a temperature in the range 110° C. to 1300° C.: cooling the blank for the part in controlled manner in still or forged air at a speed less than or equal to 3° C. / s in the range 600° C. to 300° C., thereby imparting a bainite microstructure to the blank; machining the part; and performing a mechanical reinforcing operation on the part at locations that are to be subjected to particularly high levels of stress. The invention also provides a forging obtained in this way.

Description

[0001] The invention relates to metallurgy, and more particularly to the field of steels for fabricating forgings that are to withstand high levels of stress.[0002] Such forgings are often made of cast iron, particularly of perlitic structure spheroidal graphite (SG) iron, or out of ferrito-perlitic structure forged steel, which has the reputation of providing better resistance to fatigue than cast iron. Crank shafts for internal combustion (IC) engines constitute an example of such forgings.[0003] Zones having high stress concentrations can be reinforced by various thermochemical, heat, or mechanical treatments, such as nitriding, induction quenching, burnishing, or shot blasting.[0004] Burnishing applied to a crank shaft (this application not being exclusive) consists in putting two wheels into contact with the crank pin grooves. The wheels are oriented obliquely relative to the grooves and a normal force is applied thereto. The crank shaft is set into rotation and the normal forc...

AI Technical Summary

Benefits of technology

[0030] As will be understood, the invention consists in combining a grade of steel and a method of treatment after casting that includes a step of forging the casting, with cooling being performed in still air or in forced air, and with the zones of the casting that are subjected to the highest levels of stress being subjected to mechanical reinforcement. The selected steel composition guarantees that regardless of the way in which cooling is performed, forgings made from this steel and mechanically reinforced in the zones subject to the highest levels of stress have an ability to withstand fatigue that is sufficient to satisfy the requirements of users. Fabricating crank shafts for high performance IC engines is a particularly preferred application of the invention.

[0063] In this implementation of the invention, it is also possible to implement a small amount of annealing at 300.degree. C. to 500.degree. C. for 1 h to 3 h. This transforms the residual austenite into ferrite and carbides, thereby obtaining a small increase in the yield strength without reducing tensile strength. This improves resistance to fatigue by about 10%. The annealing may be performed after machining or after cooling but before machining.

Problems solved by technology

Their method of fabrication is thus relatively inexpensive, but their lifetime in the presence of high levels of stress is limited.

Strength performance is considerably improved over the preceding examples, but the method of fabrication is more expensive.

In addition, it is not always possible to adapt the method to a manufacturing line initially designed to fabricate parts for cooling in still air.