Electric heating – Metal heating – By arc
Reexamination Certificate
1999-05-03
2002-04-02
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121130, C219S121120, C219S121110, C219S121160, C148S508000, C148S516000, C148S525000
Reexamination Certificate
active
06365866
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns the beam welding of engineering components made of hardenable steel. The invention is useful for all components joinable by beam welding methods, which components are subjected to mechanically, cyclically, or dynamically high loads. Because of a local wear load, these components consist at least partially of hardenable steels or which are hardened and annealed because of their high mechanical load. The invention can be used particularly advantageously for the production of extremely varied, in particular rotationally symmetric power transmission elements, pressure-exposed hollow parts, hydraulic rams, valves, etc. A preferred area of application of the invention is motor vehicle and mechanical engineering, primarily automotive engineering.
2. Description of Background Information
Carbon steels with a carbon content of C≧0.25% and low-alloy steels with carbon contents of C≧0.20% are only weldable to a limited extent by commercial means, as the increased hardening in the welding and heat affected zone caused by the carbon and by various alloy elements results in cracks. The hardening and subsequent crack formation occurs through the formation of martensite or lower bainite, which are only slightly deformable, and slightly (or not at all) self-annealed, which are incapable of plastically mitigating the high transient stresses occurring during cooling.
A method to prevent inadmissible hardening, and thus also crack formation with conventional welding methods requires the bulk preheating of the components. For carbon steels with carbon contents C of 0.3%≦C≦0.45%, preheating temperatures of 150 to 275° C. are recommended (see, e.g., J. Ruge “Manual of Welding Technology”, Vol. 1, “Materials”, 3rd ed., Springer-Verlag, Heidelberg 1991, ISBN-3-540-52697-8, p. 126, p. 144]. For low-alloy steels, the necessary preheating temperature may rise to temperatures as high as 400° C. (see, e.g., Quality and Stainless-Steels of the GDR, Leipzig, 1972, Vol. 1).
However, for many components, in particular mass-produced components, conventional welding methods have disadvantages with regard to welding speed, component distortion, unit costs, and after-treatment expense. These shortcomings are from relatively low power densities, which result in relatively low heating speeds, relatively high introduction of heat, and large welding seam volumes.
Beam welding methods such as laser or electron beam welding avoid these disadvantages by using power densities which are as many as a few orders of magnitude higher. However, such methods result in a higher hardening of the fusion heat affected zone, with a corresponding higher susceptibility to cracking of the welding seams. This shortcoming severely restricts the palette of beam weldable steels, since the limit of carbon and alloy element content which can be managed without cracking drops.
The effects of these drawbacks are increased, in that conventional methods of bulk preheating can be integrated into automated beam welding systems only with difficulty. Short cycle times are too expensive, and result in a deterioration of welding seam quality from oxidation of the joint.
This drawback is caused by extremely high quenching speed, which is clearly less than the t
8/5
-times.
According to the patent J-1-40194 entitled “Laser Beam Welding method for joining material”, a method is known for laser welding non-hardenable metal sheets to reduce the quenching speed with laser welding by process-integrated post-heating. For this, a high frequency inductor—located behind the laser welding head in terms of the feed rate and fixedly connected thereto—is guided at the laser welding rate at a distance from the surface determined by the focus distance of the laser beam and the geometric layout. Because of the use of the high frequency, a narrow strip is heated on both sides of the I-welding seam of the welded sheet, and thus the quenching speed is reduced. The object of the method is increased ductility and improvement of the workability of the metal sheet.
For this process, the metal sheet is heated to approximately 1000° C. However, because of the use of high frequency, the process is restricted in application to thin metal sheets. By changing the feed rate of the inductor (which can be changed only to the same extent as the laser welding rate defined by other criteria), the temperature, the inductor length and width, the cooling speed may be varied within a relatively narrow framework.
A drawback of this method is that it can be used only for thin metal sheets and only very limited for hardenable steel. It is therefore not usable for power transmission elements or mechanically functional components.
The nature of the above drawback is that the superimposed temperature time cycle of the inductive post-heating cannot be adjusted, or at least cannot be adjusted for all depth zones, within the welding seam to the requirements necessary to avoid hardening with beam welding of hardenable steels. Specifically, since the heating depth is not adjustable to the necessary welding seam depth, the cooling speed cannot be selected adequately small (or at least not for the entire welding seam depth). The high peak temperature of the post-heating cycle therefore destroys the defined normalized or tempered structure. Because the heating depth with the inductive application of energy is too little with a high frequency, heat energy is not introduced until after termination of the welding process. The and the heat penetration rate into the component is comparatively small relative to the laser welding speed. The limiting isotherm of adequately higher annealing temperatures reaches the deeper regions of the welding seam only after periods of time in which the temperature has already fallen below the M
S
-temperature. Consequently, hardening occurs. The high peak temperature of approximately 1000° C., which is higher than the austenitizing temperature and which results from the relative heating depth which is slight with reference to the component thickness as well as the relatively high quenching speed resulting from the high feed rate leads moreover to the danger of new hardening even in regions outside the HAZ of the welding zone.
SUMMARY OF THE INVENTION
The present invention provides a method by which hardenable steels may be effectively welded crack-free and without troublesome hardening.
The present invention uses a transient temperature field and a method to produce it which can be readily integrated into processes and which is adaptable even for hardenable steels with relatively high critical cooling times and relatively deep welding seams. The entire welding seam has has an adequately low cooling speed and the normalized or tempered basic structure of the initial state outside the welding zone and the heat affected zone is not damaged.
The short-time heat treatment is performed as the sole preheating. The heating depth before the beginning of the beam welding t
i2
is selected such that it reaches 1.0 to 5.0 times the welding seam depth. The energy exposure time itself, the induction frequency, and, to a small extent, the peak temperature T
max
of the preheating cycle serve as free parameters for the setting of the heating depth t
i2
.
The peak temperature is selected in a temperature range from 620 K≦T
max
≦T
Z
−30 K., whereby the temperature T
Z
depends on the starting structure of the materials to be joined. In the pearlitic state, T
Z
corresponds to the temperature at which a perceptible spherodizing of cementite begins within a period of 1 second to 100 seconds. In the tempered initial state, it corresponds to the temperature of the preceding annealing treatment. The selection of this temperature guarantees the best conditions for the subsequent cooling cycle without damaging the structure.
It is crucial for the welding result, and particularly advantageous for the process design, that the quenching time &tgr;
k
is adjusted by the use of the natural cooling capac
Brenner Berndt
Duschek Carsten
Gnann Rüdiger Arnold
Naunapper Dietmar
Elve M. Alexandra
Fraunhofer-Gesellschaft zur Föderung der angewandten Forschung e
Greenblum & Bernstein P.L.C.
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