EDGE HARDENING OF CHAIN SAW GUIDE BARS

Abstract: Chain saw guide bar with hardened edge surfaces, and method for edge hardening of guide bars, where induction hardening followed by quenching with a cooling medium produces a hardened layer on edge surfaces (12) and chain groove sides (13), but where side surfaces (11) remain unhardened by being protected from the cooling medium.

Description: EDGE HARDENING OF CHAIN SAW GUIDE BARS Background Guide bars for chain saws have long been made with hardened edge surfaces, produced by deposition of a surface layer of harder material than the guide bar body by welding according to patents US 2,962,812 or US 3,241,228, or by a heat treatment consisting of hardening and tempering. Since the hardened edges are considerably harder than that part ofthe saw chain links which slides along the guide bar edges, the wear is reduced and occurs mainly on the links ofthe chain, which normally has a shorter lifetime for other reasons, too. The higher hardness is, however, accompanied by higher brittleness and higher risk of cracking due to vibrations and bending. These cracks do not normally lead to fracture of the entire guide bar, and usually penetrate only the hard surface layer, but will then cause unevenness and rapid wear of the chain. After long use, the surface cracks can be starting points of deeper fatigue cracks. The largest risk of cracking occurs close to the front border of the rear part of the guide bar which is clamped between a clamping plate and the machinery part of the chain saw, since the bending moment is greatest in a narrow region in front of and behind this border, and since the guide bar in this critical region has oil supply holes and other cutouts causing stress concentration. It is thus previously known to avoid any form of hardening in this longitudinal region according to patent US 2,962,812. The risk of cracking varies over the bar thickness, and is largest at the outer corners where the edge surface meets the flat side surfaces. The present invention concerns a method of hardening the edges by heat treatment, which combines good wear resistance with low risk of cracking, and which may be continued through the critical region at the border of the clamped part ofthe guide bar. Description A guide bar hardened according to the invention is shown in cross-section in figure 1 , which shows which points and surfaces ofthe guide bar have special requirements for hardness, and in figure 3, which also shows the location of coolant nozzles and covers during the hardening treatment. The surfaces of the guide bar comprise flat side surfaces (11), edge surfaces (12), chain groove sides (13) and chain groove bottom (14). The edge surfaces (12) meet the side surfaces (11) at outer corners (15) and meet the groove sides (13) at inner corners (16). The edge surfaces (12) and the groove sides (13) are in sliding contact with the links of the saw chain, and should thus have high hardness at least in the regions adjoining the inner corners (16). The largest bending stresses occur at the outer corners (15) and the adjoining parts ofthe side surfaces (1 1) which should thus be less hard. Normal carbon steel grades such as used in guide bars are hardened by heating to a temperature above 750 deg C, preferably around 800 deg C, followed by rapid quenching to below 400 deg C. The heating is conveniently done by electric induction, and the quenching by spraying with a coolant medium such as oil emulsion or cold air. The choice of shape ofthe induction coil will influence the temperature distribution over the cross section, and the shape and location of coolant nozzles will influence the quenching speed, making it possible to choose the hardness distribution. According to the invention, heat is applied to the whole edge surfacees (12) as well as the adjoining parts of the groove sides (13) and ide surfaces (1 1). This can be acheived by letting the induction coil penetrate some Wc into the chain groove between the groove sides. For quenching, coolant (21) is sprayed into the chain groove and on part of the edge surfaces, but not at all on the flat side surfaces (11), which are protected from the coolant by protective covers (22) or by a stream of hot medium along the side surfaces. The covers may also be heated to keep the side surfaces warm. The result is that the upper part of the groove sides ( 13), the inner corners (16) and the edge surfaces (12) are fully hardened, while the side surfaces (1 1 ) and the outer corners (15) are so slowly cooled that they get low hardness. A following tempering will increase the toughness and restrict the hardness ofthe harder parts to what is most suitable for wear resistance, 55-63 HRC, while the softer parts get lower hardness than 47 HRC. A chain saw guide bar according to the invention has as high wear resistance as a bar where the higher hardness extends to the side surfaces (11), but is much less prone to crack formation, since the most highly stressed regions at the outer corners (15) and adjoining parts of the side surfaces (1 1) are much more soft and tough. The thickness ofthe hardened layer should be higher at the edge surfaces, where the wear from the chain is largest, than on the groove sides, and the border between hard and soft regions should run from the vicinity of the outer corners (15) slopingly towards the groove bottom (14) as shown by a curve (17) in figure 1.

Claims: Claims 1. Chain saw guide bar, surfaces of which comprise side surfaces (1 1 ), edge surfaces (12), chain groove sides (13) and groove bottom (14), characterized by a major part ofthe edge surfaces and the adjoining part of the groove sides being hardened by heat treatment, while the side surfaces have lower hardness. 2. Guide bar according to claim 1, characterized by the border between hardened and softer material running from the vicinity of the outer corners (15) where the side surfaces ( 1 1 ) meet the edge surfaces (12), slopingly towards the groove bottom (14). 3. Method for edge hardening of chain saw guide bars, surfaces of which comprise side surfaces (1 1 ), edge surfaces (12) and chain groove sides (13), the method comprising electric induction heating followed by rapid quenching by a cooling medium, characterized by the quenching occuring on the edge surfaces and the groove sides but not on the side surfaces. 4. Method according to claim 3, characterized by the cooling medium being hindered from reaching the side surfaces (11 ) by protective covers (22). 5. Method according to claim 4, caracterized by the covers being heated. 6. Method according to claim 3, characterized by the cooling medium being hindered from reaching the side surfaces by a flow of hot medium along the side surfaces.

Inventors: LEINI ARVO, LUNDSTROEM MATS, SUNDSTROEM ERIK, LEINI, ARVO, LUNDSTROEM, MATS, SUNDSTROEM, ERIK
Applicants: SANDVIK AB, SANDVIK AB, (PUBL)
Application year: 1996
Main classification: C21D9
IPC codes: B27B17, C21D1, C21D9