JP7069838B2 - Welding method - Google Patents

Description

The present invention relates to a tool steel welding technique.

Conventionally, various techniques for determining welding quality (for example, mechanical strength, residual stress, welding deformation) have been proposed (for example, Patent Documents 1 to 5). In Patent Document 1, the characteristic value of the surface temperature distribution of the molten pool / heat-affected zone is extracted from the image data, and the amount of heat input forming the molten pool is estimated based on the characteristic value. Estimate the temperature change (cooling rate) of the molten pool / heat-affected zone from the amount of heat input and various databases. Then, the material physical properties based on the fine structure of the welded portion are estimated based on this temperature change, and the quality of the weld is judged from the material physical properties and the load stress database.

In Patent Document 2, a temperature change in the vicinity of a welded portion irradiated with laser light is detected, and the quality of welding of the welded portion is determined based on the temperature change. Further, in Patent Document 3, the range in the average value of the radiation temperature of the work piece, which makes the quality of the work part good, is stored, and the average value of the radiation temperature of the work part measured by the radiation thermometer is A technique for determining that the quality of the workpiece is good when it is within the stored range is disclosed.

Patent Document 4 discloses a welding analysis method for analyzing the temperature of each member when a plurality of members are welded by a computer simulation. Further, Patent Document 5 discloses a technique for numerically simulating a structure including a heat-affected zone using a finite element analysis model.

Japanese Unexamined Patent Publication No. 2016-198805 Japanese Unexamined Patent Publication No. 05-337662 Japanese Unexamined Patent Publication No. 2017-217675 Japanese Unexamined Patent Publication No. 2006-247746 Japanese Unexamined Patent Publication No. 2013-246830

Products used for automobile bodies and the like are produced by pressing steel sheets using a press die. In addition, cast products such as engine blocks are produced by casting using a mold. Since various dies such as press dies and dies are used continuously for a long time, wear or breakage may occur on the molded surface, particularly on a high load portion such as an edge portion or a high pressure portion. If the molded surface of the mold is worn or damaged, the surface will be scratched and the quality of the product will deteriorate.

In order to reduce the cost of such a mold, a technique of repairing and regenerating a damaged portion of the mold by welding is known. The mold is repaired by overlay welding a welding material to the repaired portion to form a welded portion, and then cutting by machining to return to the original shape. However, in welding, there is a problem that a softened layer is generated in HAZ (Heat Affected Zone). If a softened layer of a welded portion appears on the surface of a machined die, it may cause new causes of wear and breakage of the die after repair. Further, when creating a new mold using welding technology, there is also a concern about defects due to the appearance of a softened layer on the surface of the mold. Further, the same problem may occur when a softened layer is generated in a welded portion of a cutting tool such as a drill or a milling cutter or a striking tool such as a hammer.

Therefore, it is required to predict the occurrence of the softened layer in the welded portion and reflect it in the welding conditions. Since the generation of the softened layer is considered to be caused by the phase transformation occurring in HAZ, a method of verifying the phase transformation of the welded portion to estimate the fine structure and predicting the generation of the softened layer can be considered. In Patent Document 1, the cooling rate of the heat-affected zone is used in order to estimate the fine structure of the welded portion. However, the shape and cooling rate of each region must be verified / calculated one by one, and there is a problem that the amount of calculation is enormous and it takes time.

The present invention has been made in view of such a problem, and an object of the present invention is to predict the generation of a softened layer in a heat-affected zone of a welded portion in a short time, and to determine and change welding conditions. It is to provide a possible welding method.

The welding method according to one aspect of the present invention includes a step of acquiring an actually measured value regarding the hardness of the cross section of the welded portion and specifying a phase transformation region appearance condition from the relationship between the exposure time and the temperature at which the actually measured value is obtained. When the heat-affected zone of the welded portion is included in the phase transformation region appearance condition, the step of predicting the generation of the softened layer by assuming that the softened layer is generated in the heat-affected zone, and the generation prediction result of the softened layer. It has a step of determining the quality of welding based on the above, and a step of changing welding conditions based on the result of determining the quality of welding.

According to the present invention, it is possible to provide a welding method capable of predicting the generation of a softened layer in a heat-affected zone of a welded portion in a short time and determining / changing the welding conditions.

It is a flowchart which shows the welding method which concerns on embodiment. It is a figure explaining the method of determining a calculation area. It is a figure explaining the method of determining a calculation area. It is a figure which shows the relationship between the exposure time of a laser, and the temperature of a cross section of a weld. It is a continuous cooling transformation diagram of the tool steel.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Equivalent components in each figure are designated by the same reference numerals, and duplicate description is omitted.

The present invention relates to a welding technique for tools such as cutting tools and striking tools, and tool steel used for various dies. The outline of the welding method according to the embodiment is changed to a welding condition in which the welding conditions are initially set, the generation of a softened layer that deteriorates the mechanical properties of the welded portion is predicted, and the generation of the softened layer is suppressed. Is what you do.

For example, when repairing a mold, a welded material is built-up welded to a damaged portion of the mold that has been worn or damaged to form a welded portion, and the welded portion is cut and repaired. When repairing a damaged part of a mold by TIG welding, there is a problem that the heat affected zone (HAZ) is softened because the amount of heat input is large in TIG welding.

On the other hand, in laser clad welding, the total amount of heat and the melted portion can be easily controlled, so that it is considered that the generation of the softened layer can be reduced as compared with TIG welding, but it is difficult to eliminate the generation of the softened layer itself. Therefore, in the embodiment, the generation of the softened layer in the heat-affected zone of the welded portion is predicted simply in a short time, and the welding conditions are determined and changed.

FIG. 1 is a flowchart illustrating a welding method according to an embodiment. FIG. 1 shows a method of determining welding conditions. In addition, although FIG. 1 illustrates a method of determining welding conditions using a flowchart for convenience, a process / procedure as a "method" is shown, and whether or not the welding device for welding is automatically controlled. Welding is not limited.

With reference to FIG. 1, the overall flow of the method for determining the welding conditions of the mold will be described. As shown in FIG. 1, first, the initial setting of welding conditions is performed (S1). In the initial setting of welding conditions, known or standard welding conditions such as conventional values, empirical values, know-how values, experimental values, and simulation values are set. In the case of laser clad welding, the laser output, scanning position / pattern, material supply amount / speed, etc. are set.

Next, the calculation area, which is the target for predicting the generation of the softened layer, is determined (S2). Since it takes a very long time to verify / calculate the occurrence prediction of the softened layer one by one in all areas and feed it back to the welding conditions, the area in which the calculation is performed is limited in the embodiment. The step of determining the calculation region includes determination of the thermal energy stable region (S21) and determination of the curing region (S22). In addition, although it is described in FIG. 2 that each process is executed in the order of S21 and S22, these processes may be performed from any of them or may be executed at the same time.

First, the determination of the thermal energy stable region (S21) will be described with reference to FIG. FIG. 2 shows a cross section of a position where the amount of heat input to the welded portion is maximum. When welding is performed on the base metal 10, the temperature continuously changes from the melting boundary toward the base metal 10 from a region heated to a temperature close to the melting point of the material to a region hardly affected by heat. The place where the microscopic structure changes in this region is called the heat-affected zone 11. The welded portion 13 is composed of a weld metal (melted region) 12 that has been melted and solidified, and a heat-affected zone 11 whose structure has changed due to the heat of welding. The weld metal 12 has a composition in which the weld material and the base material 10 are mixed.

As shown in FIG. 2, the base metal 10 is formed with a welded portion 13 on which the weld metal 12 is overlaid. The region of the base metal 10 in contact with the weld metal 12 becomes the heat-affected zone 11. In laser clad welding, the thermal energy of the laser becomes unstable immediately after the start of welding and immediately before the end of welding. Therefore, of the welded portions 13 formed on the base metal 10, the regions immediately after the start of welding and immediately before the end of welding are excluded from the calculation region for predicting the generation of the softened layer, and the stable region 15 in which the thermal energy is stable is defined as the exclusion region 14. It is a calculation area.

Next, the determination of the cured region will be described with reference to FIG. Generally, the softening layer 11a is formed on the heat-affected zone 11 of the base material 10. Therefore, the molten region made of the weld metal 12 in the welded portion 13 is excluded from the calculation region for predicting the generation of the softened layer. Further, as shown in FIG. 3, in laser clad welding or the like, since the molten region has a high temperature of about 1500 ° C., the heat-affected zone 11 has a two-layer structure of a softening layer 11a and a hardening region 11b due to the quenching effect. Therefore, the hardened region 11b of the welded portion 13 is also excluded from the calculation region for predicting the generation of the softened layer.

Then, for the arithmetic region determined in S2, the phase transformation is verified, the microstructure is estimated, and the generation of the softened layer is predicted (S3). The prediction of the occurrence of the softened layer includes, for example, the prediction of the material of the softened layer and the region where the softened layer is generated. In the embodiment, in order to reduce the calculation amount, the generation of the softened layer is simply predicted by using the past measured values without using the cooling rate of the welded portion. The prediction of the occurrence of the softened layer will be described in detail later.

Then, based on the prediction of the occurrence of the softened layer obtained in S3, the quality of the welded portion is determined (S4). The quality of the weld is determined by, for example, the amount and thickness of the softened layer, the expected hardness of the welded portion based on these, and the like.

When there is no problem in the welding quality (S4, YES), the method for determining the welding condition is completed, and the initial condition set in S1 becomes the condition for the subsequent welding. On the other hand, when there is a problem in welding quality (S4, NO), the initial conditions set in S1 so as to suppress the generation of the softened layer are changed (S5).

Here, the process of predicting the generation of the softened layer of S3 will be described in detail.
Before explaining the embodiment, a comparative example of a method for predicting the occurrence of a softened layer will be described with reference to FIG. FIG. 5 is a schematic diagram in which the temperature history of the cross section of the welded portion is superimposed on the CCT (Continuous Cooling Transformation) diagram of SKD61, which is an example of tool steel. In FIG. 5, the broken line indicates SKD61, and the solid line indicates the temperature history of the cross section of the welded portion. In FIG. 5, the vertical axis indicates the temperature (° C.), and the horizontal axis indicates the cooling time (min) from the maximum ultimate temperature of 1030 ° C. In FIG. 5, B, P, and M are temperature-time domains generated by the bainite phase, the pearlite phase, and the martensite phase, respectively.

The softened layer is believed to occur due to the phase transformations that occur in HAZ. Therefore, as shown in FIG. 5, a method of obtaining the "cooling rate" from the CCT diagram, verifying the phase transformation, estimating the fine structure of the welded portion, and predicting the occurrence of the softened layer can be considered. However, the shape and cooling rate of each region must be verified / calculated one by one, and the calculation takes time.

Therefore, in the embodiment, the generation of the softened layer is simply predicted by using the past measured values without using the cooling rate of the welded portion. FIG. 4 is a diagram showing the relationship between the “exposure time” of the laser and the “temperature” of the cross section of the welded portion. In FIG. 4, the horizontal axis represents time and the vertical axis represents temperature.

For example, in laser welding using an alloy tool steel SKD as a base material, since the heat input energy is large, the pearlite phase changes from a layered structure to a granular structure at the heat-affected zone, so that a softened layer is formed. Therefore, in laser clad welding using alloy tool steel SKD as the base material, it is considered that a softened layer is generated by the appearance of the pearlite phase in the heat-affected zone.

First, the measured value regarding the hardness of the cross section of the welded portion is acquired. Then, the phase transformation region appearance condition is specified from the relationship between the exposure time at which the measured value is obtained and the temperature, and is stored in the measured database. For example, in laser clad welding using alloy tool steel SKD as the base material, the condition for the pearlite phase to appear in the heat-affected zone can be specified as the temperature of the heat-affected zone in the range of 500 to 600 ° C. By satisfying the condition for appearance of this phase transformation region, it is considered that a softened layer having a Vickers hardness of Hv60 or less is generated.

In FIG. 4, the region surrounded by the alternate long and short dash line represents the pearlite phase transformation region in which the pearlite phase appears. When predicting the generation of the heat-affected zone, it is assumed that the region of the heat-affected zone included in the temperature range of 500 to 600 ° C. specified as the condition for appearance of the phase transformation region is the softened zone with reference to the actual measurement DB. It is regarded. In the embodiment, no calculation is performed for other phase transformation regions such as the ferrite phase and the martensite phase. In this way, by suppressing the amount of calculation, it is possible to predict the occurrence of the softened layer in a short time.

In arc welding where the heat input energy is smaller than that of the laser, the influence of the structure change of ferrite in the heat-affected zone may be large with respect to the generation of the softened layer. In this case, the conditions for the appearance of the phase transformation region in which the ferrite phase appears can also be stored in the actual measurement DB based on the correlation with the measured value regarding the hardness of the cross section of the welded portion, the exposure time, and the temperature.

If it is determined in S4 that there is a problem with the welding quality due to the generation of the softened layer, the welding conditions are changed in S5 so as to suppress the generation of the softened layer. For example, in laser clad welding, in order to suppress the generation of a softened layer, it is possible to reduce the time and amount of the powder material in a molten state and increase the cooling rate of the molten region. For that purpose, welding conditions such as "narrowing down the laser irradiation diameter" and "increasing the scanning speed" can be changed.

As described above, according to the embodiment, it is possible to directly predict the generation of the softened layer from the hardness, temperature, and exposure time of the cross section of the welded portion without using the cooling rate calculation, which requires a large amount of calculation. It is possible to determine the quality of welding and change the welding conditions without taking a lot of time. Further, by limiting the calculation area for predicting the occurrence of the softened layer, it is possible to reduce the amount of calculation for predicting the occurrence of the softened layer and reduce the time required for verification / calculation.

The present invention is not limited to the above embodiment, and can be appropriately modified without departing from the spirit.

10 Base metal 11 Heat-affected zone 12 Welded metal 13 Welded part 14 Excluded area 15 Stable area 11a Softened layer 11b Hardened area

Claims (1)

A step of acquiring an actually measured value regarding the hardness of a cross section of a welded portion in the past and specifying the relationship between the exposure time and the temperature at which the actually measured value is obtained as a condition for appearance of a phase transformation region.
When the heat-affected zone of the welded portion to be predicted is included in the phase transformation region appearance condition, the step of predicting the generation of the softened layer by assuming that the softened zone is generated in the heat-affected zone.
The process of determining the quality of the weld based on the result of predicting the occurrence of the softened layer, and
The process of changing the welding conditions based on the result of the quality judgment of the welding quality, and
Have,
Welding method.

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