EP1419281B1 - Method for increasing the dynamic stability under load of a toothed structural component - Google Patents

Description

The invention relates to a method for increasing the dependent on the carbon content dynamic Loading capacity of a toothed component made of case hardenable Stole.

From practice is known, the dynamic load capacity of components by increasing that at a Case hardening a hard, wear-resistant surface layer with a sufficiently tough core is generated. This is the Carburized surface layer and then hardened the component. For mechanically highly stressed components, such as in particular Gears and shafts, provides the case hardening of alloyed case steels is a solution with the highest Resilience can be achieved.

In case hardening, so far, a course of the carbon content set in the boundary layer, which at the Surface of a component the maximum carbon content having. With increasing distance from the surface in Direction of the core of the components decreases the carbon content continuously off.

JP 01052056 discloses a method for removing a carburized one Layer. The carburized layer is by means of mechanical manufacturing removed.

In ISO 6336-5, First Edition 1996-06-15, is a carbon content set at the surface, the +0.2% to -0.1% from the eutectoid point of the state diagram of case steels lies and a high dynamic load capacity guaranteed. This results in particular for chrome nickel case hardened steels a preferred carbon content of about 0.8%.

It is also known from practice that the carbon content on the surface and in the surface layer the Resilience of tooth root and tooth flank of a toothed wheel in influenced in different ways. For chrome nickel case hardened steels It is known that a maximum Zahnfußtragfähigkeit at a carbon content on the surface of a Component of 0.6% is present and with increasing carbon content decreases. A tooth flank bearing capacity decreases however, with increasing marginal carbon content of 0.6% up to 0.9% steadily.

This results for the known series heat treatment process of chromium nickel case-hardening steels the disadvantage that a setting of a maximum or optimal Tooth flange bearing capacity for a reduction of Zahnfußtragfähigkeit because there is a marginal carbon content over case hardening in the surface layer of the entire component in the is adjusted substantially evenly. If the carbon content set in the boundary layer of a gear in such a way is that there is a maximum of Zahnfußtragfähigkeit this in turn comes at the expense of the tooth flange carrying capacity.

To overcome this disadvantage with highly loaded gears Eliminate has passed to the tooth flank carrying capacity about the carbon content in the peripheral zone too maximize and the Zahnfußtragfähigkeit of gears by a subsequent solidification blasting in the area of the tooth root to increase.

The disadvantage here is that by the case hardening downstream hardening jets of the tooth root of gears considerable additional costs in the production arise.

Object of the present invention is therefore, a Method for increasing the depending on the carbon content standing dynamic load capacity of a toothed Case hardenable steel component available which is a subsequent solidification blasting unnecessary and a cheaper production of a component or increasing the dynamic load capacity.

According to the invention, this object is achieved by a method solved according to the features of claim 1.

The inventive method for increasing the depending carbon content dynamic Load capacity of a toothed component, for example a gear of case hardenable steel, wherein first the course of the carbon content in the surface layer of the Component is adjusted so that the carbon content, starting from the surface towards the core of the component, initially increasing and above a certain Layer depth decreases continuously, and in the subsequent a part of the surface layer in areas of the component, especially in the area of the tooth flanks, is removed, their dynamic load capacity with increasing carbon increases on the surface of the component offers the advantage that on the surface of the component an optimized marginal carbon content present, each having a maximum value ensures the carrying capacity of the relevant component area.

With the setting of the carbon content according to the invention and the subsequent hardworking certain Areas of the component may increase to a carrying capacity Finishing process, which is an increase in manufacturing costs has the consequence, omitted in an advantageous manner become.

In addition, due to the increased strength of the treated with the inventive method component Also a weight reduction possible, as for the same Strength less material usage is required.

Further advantages and advantageous embodiments of Article according to the invention will become apparent from the following Description, the claims and the drawing.

In the single figure of the drawing is exemplary a curve 2 of the carbon content in the boundary layer a produced by the process according to the invention Component another course 1 with one from practice known case hardening method produced carbon content another component faced.

The two courses 1, 2 of the carbon content in the Peripheral layer of a gear are from its surface too represented a core. Here is the content of carbon (in percent) above the edge distance from the surface of the gear (in millimeters).

The course 1 gives a carbon content C in the Edge layer of the gear again, which by a conventional Case hardening method was produced. This Course 1 shows a maximum of the carbon content C im immediate area of the surface of the gear on and the carbon content C increases with increasing edge distance steady and at least approximately linear.

The curve 2 of the carbon content C in the edge zone a gear is by means of the modified case hardening process set with edge carburizing and designed in such a way that the carbon content, starting from the surface of the gear, initially increases in the direction of the core and above a certain layer depth, in the present case for example, with an edge distance of about 0.15 mm, a Maximum of 0.7% carbon. Starting from this Maximum takes the carbon content in the direction of Kerns of the gear continuously. The so-called edge decarburization is due to a strong lowering of the carbon content the atmosphere of the carburettor in the last Stage of the case hardening process, for example during holding at hardening temperature, adjusted or generated.

The following is modified according to the invention Case hardening method described in more detail by way of example.

The gear made of case hardenable steel, for example 18 CrNiMo 88, is in throughout the case hardening process a heating furnace of a gaseous carburizing agent surrounded, its carbon content during the individual Phases of the case hardening process is changed.

The case hardening process includes those known per se Phases, namely a heating phase, a carburizing phase, a Cooling phase and a holding phase to hardening temperature of Component, these successive phases respectively over a certain period of time at given temperatures and different carbon content of the gas atmosphere be performed.

So it is common practice that the carbon content initially during the heating phase is smaller than during the subsequent carburisation phase. The carbon content can, for example, during the heating phase, 0.4% be.

The subsequent carburizing phase is known and preferably divided into four sections or steps, the carbon content in the first three Grades from 0.4% gradually to 0.6% and finally up a value of 1.1% is raised, and where the period of time in which a carbon content of 1.1% is set is fifteen times the period in which a carbon content adjusted by 0.6%. In a last temporal, short section of the carburizing phase this is reduced again to a value of 1.0%. The Temperatures of the heating phase and the carburizing phase are in the described embodiment approximately at 930 ° C.

During the subsequent to the carburization phase Cooling phase corresponds to the carbon content of the last Section or the last period of the carburizing phase.

The holding phase during which the gear is at hardening temperature is held, again divided into several Sections or stages, wherein the carbon content of the Atmosphere of the gaseous carburizing agent in a first Section is set to a value of 0.4%. In a second portion of the holding phase becomes the carbon content of the carburizing agent lowered to 0.3%, the second section of the halt phase being a longer one Period as the first portion of the holding phase includes, and z. B. last almost twice as long as the first section can. The temperature of the cooling phase and the holding phase corresponds to about 860 ° C.

It should be noted that the individual, numerically cited process parameters of the case hardening process and the duration of each phase depending on each are used and vary accordingly, the tendencies in the use of different furnace types stay the same.

After the holding phase, the gear is cured directly in oil and then annealed at about 170 ° C for 90 minutes.

After case hardening, the toothed component, For example, a gear, hard in the tooth flanks edited, with the removal of material of the edge zone about the Area of edge carburization corresponds. As a processing method Here, a grinding process is preferably provided, which has been used in practice to improve the Surface quality of the tooth flanks is provided. The area The tooth base is usually not subsequently processed. That is, the cost-intensive known from the prior art Solidification jets can be avoided and thus fall in comparison to conventional manufacturing processes no additional costs for solidification blasting at.

With the above case hardening method is the course 2 of the carbon content in the entire border area of the gear set. That means that both in the area the tooth flank as well as in the area of the tooth root the surface has a carbon content of about 0.5% to 0.6 % is present. At a carbon content of 0.6% of the Surface of a gear reaches a Zahnfußtragfähigkeit their maximum value. A higher carbon content for steels, especially CrNi steels, has a decrease the Zahnfußtragfähigkeit result. This is with the Case hardening method with edge carburizing for the Zahnfußtragfähigkeit reaches a maximum value.

The tooth flank carrying capacity has its maximum Value at a carbon content in a range of 0.6% up to 0.9%, with the tooth flank carrying capacity, starting of a carbon content of 0.6%, with increasing Carbon content in the border zone steadily increases. That means, that before the material removal a Zahnflanketragfähigkeit at a carbon content of 0.6% in the range the surface of the tooth flank has a value below one Maximums has. Due to the removal of material, the surface layer so removed that layer areas of the surface layer the tooth flank form the surface of the gear, which have a higher carbon content than near-surface areas of the tooth root. This will be the Tooth flange bearing capacity of conventional case hardening reached again. A maximum Zahnfußtragfähigkeit is not affected by the grinding of the tooth flank.

Thus, according to the invention modified Case hardening, which is a direct hardening with edge carburizing represents, and the subsequent hard machining the tooth flank optimized marginal carbon contents, each having a maximum load capacity of the tooth flank and effect the tooth base.

After the above-described modified case hardening method with edge carburization in the last stage of the Case hardening will be significantly higher levels of dynamic Breaking forces achieved as compared to conventional case hardening methods. In addition to an increase in fatigue strength is by the edge carburization also the course of the time stability shifted from gears to higher swinging game numbers.

With the modified direct use hardening method with Edge carburization will also increase the fatigue strength during swelling Bending stress and dynamic breaking force at sudden stress compared to gears, the cured according to known series heat treatment process be improved.

Although the inventive method in particular for increasing the Zahnfußtragfähigkeit with toothed components, especially with gears, is so the invention is not limited to this application. The above-described hardening process with subsequent hard machining is also for shafts for steering of commercial vehicles as well as for any other components, which a high exposed to dynamic stress and a hard, Wear-resistant edge layer on a sufficiently tough Core must have suitable.

reference numeral

1

Course of the carbon content in a boundary layer in a conventional case hardening process

2

Course of the carbon content in a boundary layer in a modified case hardening process according to the invention with edge decarburization

Claims (5)

1. Method for increasing the carbon-content-based dynamic loadability of a toothed component made of case-hardenable steel, with the characteristic (2) of the carbon content in the peripheral layer of the component first being set in a manner that the carbon content - starting from the surface and towards the core of the component - initially increases and then from a certain depth of the layer continuously decreases, and that subsequently part of the peripheral layer in areas of the component, especially in the area of the tooth flanks, is removed in such a way that the maximum carbon content is achieved on the surface of the component.
2. Method according to claim 1, characterized in that the characteristic (2) of the carbon content in the peripheral layer of the component is produced by means of carburization.
3. Method according to claim 2, characterized in that during carburization the component is surrounded by a gaseous carburizing agent and the carbon content of the atmosphere is variable.
4. Method according to claim 3, characterized in that the carbon content of the atmosphere towards the end of carburization, for example during a stop phase following a cooling phase to the hardening temperature of the component, is set to a much lower level than during carburization.
5. Method according to one of the claims 1 through 4, characterized in that the part of the peripheral layer on the tooth flank to be removed is removed by means of a machining process.

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